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ZrO2 visible light photocatalysts
Advanced Powder Technology 22 (2011) 443–448
Contents lists available at ScienceDirect
Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Synthesis and characterization of N-doped TiO2/ZrO2 visible light photocatalysts Ji Young Kim a, Chan Soo Kim b, Han Kwon Chang c, Tae Oh Kim a,* a
Department of Environmental Engineering, Kumoh National Institute of Technology, 1 Yang ho-dong, Gumi, Gyeongbuk 730-701, Republic of Korea Department of Materials Science and Engineering, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul 151-742, Republic of Korea c Department of Industrial Materials Research, Korea Institute of Geoscience and Minerals Resources, 30 Kajungdong, Yuseong-gu, Daejeon 305-350, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 30 March 2010 Received in revised form 28 June 2010 Accepted 30 June 2010 Available online 13 July 2010 Keywords: Photocatalysts N-doped TiO2/ZrO2 Visible light absorption NOx
a b s t r a c t Zirconia and nitrogen-doped TiO2 powder was synthesized using a polymer complex solution method for the preparation of an enhanced visible light photocatalyst. The produced catalysts were characterized via the Brunauer, Emmett, and Teller method (BET), X-ray diffraction, transmission electron microscopy, Xray photoelectron spectroscopy, Fourier transform infrared spectra, and UV–Vis spectrophotometry analyses. The N-doped TiO2/ZrO2 photocatalyst showed a high specific surface area and small crystal sizes. The XPS spectra of the N-doped TiO2/ZrO2 sample indicated that nitrogen was doped into the TiO2 lattice and enhanced the photocatalytic activity. The UV–Vis absorption spectra of the N-doped TiO2/ZrO2 sample noticeably shifted to the visible light region compared to that of the TiO2. The photocatalytic activities of the prepared catalysts were evaluated for the decomposition of gaseous NOx under UV and visible light irradiations. The photocatalytic activities of N-doped TiO2/ZrO2 were much greater than those of commercial Degussa P25 in both the UV and visible light regions. The high photocatalytic activity can be attributed to stronger absorption in the visible light region, a greater specific surface area, smaller crystal sizes, more surface OH groups, and to the effect of N-doping, which resulted in a lower band gap energy. Ó 2010 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Titanium dioxide (TiO2) has been widely used as an effective photocatalyst for the photodegradation of organic pollutants in water and air because of its low cost, non-toxicity, strong oxidizing power, and long-term photostability without secondary pollution [1–5]. The only disadvantage is the large band gap energy of anatase TiO2 (3.2 eV). As a result, this compound can be activated only under UV light irradiation. Many attempts have been made to obtain higher photocatalytic reactivity under visible light irradiation. Previous investigations utilized metal ion doping of TiO2 [6,7]; however, the drawback of cationic dopants is weak absorption in the visible light range [8]. To more efficiently utilize solar energy, non-metal doping of TiO2 was performed to prepare a visible light-sensitive photocatalyst via band gap reduction of the photocatalyst [2,9–12]. The introduction of a second metal oxide proved to be an effective route to improve the thermal stabilization and UV light photocatalytic activity of TiO2. Mixed ZrO2–TiO2 has been widely investigated in the photocatalysis field [12,13]. The addition of small amounts of ZrO2 into TiO2 can prevent the anatase-to-rutile phase transformation. Therefore, the addition of ZrO2 enhances the * Corresponding author. Tel.: +82 54 478 7634; fax: +82 54 478 7641. E-mail address: [email protected] (T.O. Kim).
thermal stability of the phase transformation of pure TiO2. Furthermore, ZrO2 improves the thermal stabilization and increases the surface area, thus promoting photocatalytic activity [13]. Sato reported the synthesis of nitrogen-doped TiO2 using various nitrogen sources such as urea, ammonia, ammonium chloride, and nitric acid to dope nitrogen into the TiO2 structure [15]. The improvement in the visible light photocatalytic activity of nitrogen-doped TiO2 is mainly due to the reduction of the band gap through hybridization of the N 2p states with O 2p states on the top of the valence band [2]. Current studies of nitrogen-doped TiO2 are concerned with explaining the N states, which are typically examined using X-ray photoelectron spectroscopy (XPS). Most reports agree that the N 1s peak at 396–398 eV is the characteristic peak of the Ti–N linkages, indicating that nitrogen atoms are doped into the TiO2 lattice and are responsible for the enhanced photocatalytic activity [2,15–18]. To prepare a visible light enhanced photocatalyst for the present study, N-doped TiO2/ZrO2 powder was prepared using a polymer complex solution method (PCSM). Titania catalysts with high visible light activities were successfully prepared via PCSM in our previous study [19]. The doping of TiO2 with ZrO2 as a transition metal improved the photocatalytic efficiency but did not improve the weak absorption in the visible light region. By doping with non-metal N, the band gap was narrowed, allowing enhanced photocatalytic activity in the visible light region. Furthermore, it was
0921-8831/$ - see front matter Ó 2010 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. doi:10.1016/j.apt.2010.06.014
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possible to utilize solar energy for the photocatalysis. The surface areas, crystal sizes, chemical structures, and visible light responses of the prepared catalysts were characterized using BET, XRD, TEM, FT-IR, XPS, and UV–Vis. In addition, the photocatalytic activities of the catalysts were investigated under both UV and visible light by monitoring the decomposition of NOx as a typical indoor pollutant. 2. Experimental 2.1. Materials Titanium tetraisopropoxide (TTIP, Junsei, 98%), hexadecyl trimethylammonium bromide (HDTMA, Aldrich, 99%), zirconium oxynitrate 2-hydrate (Kisita Chemical, 99%), isopropyl alcohol (IPA, Daejung Chemical, 99%), and polyethylene glycol (PEG, Fluka, M.W. = 20,000) were used without further purification. Distilled, deionized water was used for the preparation of the catalysts. 2.2. Preparation of catalysts The TiO2 and N-doped TiO2/ZrO2 photocatalysts were synthesized using PCSM [19]. A mixture of 0.1 M TTIP and 100 ml IPA was stirred for about 30 min at 60 °C. Then, 2 g PEG dissolved in IPA was added via magnetic stirring for 1 h. HDTMA and the required amount of ZrO(NO3)22H2O were added to this premixed solvent under vigorous stirring conditions for another hour and calcined for 2 h at 500 °C. All syntheses were performed using similar steps to obtain samples with Zr/(Zr + Ti) molar ratios in the range of 0–0.9. A TiO2 photocatalyst without the addition of ZrO(NO3)22H2O and HDTMA was also prepared to compare to the Ndoped TiO2/ZrO2 photocatalyst. In this study, the N-doped TiO2/ ZrO2 sample with a Zr/(Zr + Ti) molar ratio of 0.1 was primarily used. 2.3. Characterization The specific surface areas of all samples were calculated using the BET method. The morphologies and sizes of the photocatalysts were observed using TEM (JEM-2010, JEOL). The crystal phases of the samples were analyzed using XRD (RTP 300 RC, Rigaku Co.) with Cu-Ka radiation at a generator voltage of 40 kV and a tube current of 100 mA. FT-IR (Spectrum GX & AutoImage, USA) spectra were recorded in a Perkin–Elmer Spectrum One system, and XPS (ESCALAB 250, VG Scientifics) was performed to measure the surface properties. A general C 1s, O 1s, N 1s, and Ti 2p core spectrum scan was recorded with non-monochromatized Mg-Ka radiation (1253.6 eV) at a pass energy of 50 eV and an electron takeoff angle (the angle between the electron emission direction and the surface plane) of 55°. All of the binding energies were referenced to the C 1s peak of the surface adventitious carbon at 284 eV. The UV–Vis absorbance spectra were obtained using a UV–Vis spectrophotometer (S-4100, Sinco). 2.4. Evaluation of photocatalytic activity Nitrogen oxide was chosen for the evaluation of the photocatalytic activity of the prepared samples, as NOx is a typical indoor pollutant [20,21]. The photocatalytic activity experiments of the commercial Degussa P25 and N-doped TiO2/ZrO2 samples for the degradation of NOx gas were performed at room temperature using a NOx analyzer (Teledyne MODEL-200E, USA). These catalysts were prepared by coating an ethanol suspension of the synthesized samples onto six square glass plates with dimensions of 4.5 4.5 cm. The masses of the catalysts used for each test were 0.6 g. The glass plates with the catalyst coatings were dried at 25 °C for 1 h to
evaporate the ethanol and were then maintained at room temperature (25 °C) before being used. After coating, the glass plates were placed in a quartz reactor, which was maintained in the dark at room temperature. The analysis of NOx concentration in the reactor was conducted with a NOx analyzer. The NOx vapor was allowed to reach adsorption equilibrium with the catalysts prior to irradiation. First, NO gas (100 ppm) was diluted to 5 ppm with N2 gas at a rate of 550 mL/min. As a result, the initial concentration of NOx after establishing adsorption equilibrium was 5 ppm. Next, F8T5BLB (8W) and G8T5 (8W) lamps were placed above the reactor as the UV and visible light sources, respectively. Then, the samples in the reactor were irradiated with UV at an intensity of 5500 lW/ cm2 and with visible light at an intensity of 210 lW/cm2. After the lamps were stabilized for 20 min to obtain a constant intensity, the photocatalytic activities of the catalyst samples were evaluated by comparing the apparent reaction rate constants over 10 min. The photocatalytic degradation of NOx is a pseudo-first-order reaction, and its kinetics may be expressed as ln(C0/C) = kt, where k is the apparent reaction rate constant, and C0 and C are the concentrations of initial NOx and NOx after time t (min), respectively [22].
3. Results and discussion The surface areas of the TiO2 and N-doped TiO2/ZrO2 samples were 29.354 and 187.352 m2/g, respectively. The high surface area of the N-doped TiO2/ZrO2 sample can be attributed to the addition of ZrO2, which decreased the particle size of the TiO2 and, thus, increased the total surface area [13,14]. Fig. 1 presents TEM images of the prepared TiO2 and N-doped TiO2/ZrO2 samples. The TiO2 sample consisted of irregularly shaped particles with a wide range of sizes (12–50 nm), as shown in Fig. 1a. The N-doped TiO2/ZrO2 catalyst shown in Fig. 1b, in contrast, consisted of uniformly sized particles (5–7 nm) in a honeycomb structure smaller than that of the TiO2 sample. Fig. 1c shows a high magnification image of Fig. 1b, where a few disordered agglomerated structures were observed. However, in some areas of the sample, there were randomly oriented nanocrystallites, indicating that the addition of ZrO2 can effectively suppress the growth of TiO2 crystals and stabilizes their structure. The XRD patterns of the TiO2 and N-doped TiO2/ZrO2 samples are shown in Fig. 2. The N-doped TiO2/ZrO2 sample was 100% anatase after heat treatment at 500 °C, and the TiO2 sample was a mixture of anatase and rutile phases. This result indicates that the addition of ZrO2 into TiO2 can effectively suppress the anataseto-rutile phase transformation as well as the crystal growth of the N-doped TiO2/ZrO2 samples [12,23,24]. Neppolian et al. reported that the addition of ZrO2 can increase the surface acidity of TiO2/ZrO2 catalysts due to formation of stronger surface OH groups [13]. The OH groups act as traps for the holes and suppress the electron–hole recombination process [13]. Other researchers have stated that the OH groups are formed as a result of dissociative adsorption of H2O molecules, which occurs in order to reduce the coordinative desaturation of the surface sites [25]. The FT-IR spectra of the TiO2 and N-doped TiO2/ZrO2 samples are shown in Fig. 3. Peaks at 1631 and 3423 cm 1 were observed in the spectra of both samples, corresponding to OH groups [12,13]. The peak intensity of the N-doped TiO2/ZrO2 sample was much stronger than that of the TiO2 sample, indicating that the addition of ZrO2 increased the concentration of surface hydroxyl groups, which may enhance the photocatalytic degradation performance. For further analyses of the chemical structures of the TiO2 and N-doped TiO2/ZrO2 samples, XPS measurements were performed. The XPS spectra (Fig. 4a) show that the TiO2 sample contained only Ti, O, and C elements. However, the N-doped TiO2/ZrO2 sample had new peaks at 397.45 and 403.1 eV, which could be due to the bind-
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Fig. 2. XRD patterns of the TiO2 and N-doped TiO2/ZrO2 samples.
Fig. 3. FT-IR spectra of the (a) TiO2 and (b) N-doped TiO2/ZrO2 (Zr/(Zr + Ti) = 0.1) samples.
Fig. 1. TEM images of the (a) TiO2 and (b and c) N-doped TiO2/ZrO2 (Zr/(Zr + Ti) = 0.1) samples.
ing energy of N 1s. The N 1s, Ti 2p, and O 1s regions of the N-doped TiO2/ZrO2 sample are illustrated in Fig. 4b–d, respectively. In the N
1s spectra of the N-doped TiO2/ZrO2 sample (Fig. 4b), two peaks appeared at 397.45 and 403.1 eV due to the addition of nitrogen into TiO2, which is typically observed at 396–404 eV [16]. For the other samples, the peak at 397.45 eV was attributed to the presence of substituted nitrogen in the O–Ti–N structure and was assigned to the atomic b-N state [2,9,25]. The peak at 403.1 eV may also be due to molecularly adsorbed nitrogen-containing compounds on the surfaces of the samples [9,26]. For the Ti 2p region (Fig. 4c), the peak for the TiO2 sample was observed at 459.25 eV, while the peak for the N-doped TiO2/ZrO2 sample appeared at 456.45 eV, corresponding to the lower binding energy of Ti 2p. The typical binding energy of the Ti 2p peak in a TiO2 crystal is 458.5–459.7 eV [18]. Therefore, nitrogen was successfully incorporated into the TiO2 lattice. The measured binding energy of Ti 2p for the N-doped TiO2/ZrO2 sample was due to the formation of O–Ti–N bonds by partial replacement of the O atoms with N atoms in the TiO2 lattice [18,27]. Fig. 4d presents the O 1s spectra for the TiO2 and N-doped TiO2/ ZrO2 samples. The peaks of the O 1s region for the TiO2 and Ndoped TiO2/ZrO2 samples were observed at 530.5 and 527.85 eV, respectively, illustrating a significantly lower binding energy for
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Fig. 4. XPS spectra of (a) TiO2 and N-doped TiO2/ZrO2, (b) N 1s peaks of TiO2 and N-doped TiO2/ZrO2, (c) Ti 2p peaks of TiO2 and N-doped TiO2/ZrO2, and (d) O1 s peaks of the TiO2 and N-doped TiO2/ZrO2 (Zr/(Zr + Ti) = 0.1) samples.
the N-doped TiO2/ZrO2 sample than for the TiO2 sample. This indicates that nitrogen was incorporated to form the O–Ti–N structure due to surface strain and lattice distortion during the doping process and that the O–Ti–N structure shifted to a higher infrared feature than did the O–Ti–O structure [18,23]. Furthermore, the XPS results not only showed that the nitrogen was structurally compatible in the formation of O–Ti–N, but also that it led to enhanced photocatalytic activity in the visible light region due to the substitutional doping of nitrogen for oxygen in N-doped TiO2. To compare the optical properties of the TiO2 and N-doped TiO2/ ZrO2 samples, their UV–Vis absorption spectra were investigated and are shown in Fig. 5. A noticeable shift in the absorption spectra to the lower energy region was observed for the N-doped TiO2/ZrO2 sample compared to that of the TiO2 sample. The absorption in the visible light range is related to the newly formed N 1s state because of the incorporation of nitrogen into the TiO2 lattice [2,9,26,28,29]. As direct evidence, the band gap energies of the samples were determined from the energy intercept of a plot of (ahm)1/2 versus the photon energy (hm) [30]. From these plots, the band gap energies of TiO2 and the N-doped TiO2/ZrO2 samples were approximately 3.01 and 2.81 eV, respectively. The band gap was narrowed because the N 1s band formed above the O 1s valence band in the N-doped TiO2/ZrO2 sample, as shown in Fig. 4b. Furthermore, this result indicates that the N-doped TiO2/ZrO2 sample has high photocatalytic activity in the visible light region. To evaluate and compare the photocatalytic activity of the TiO2 and N-doped TiO2/ZrO2 samples, the degradations of NOx were
Fig. 5. UV absorption spectra of the TiO2 and N-doped TiO2/ZrO2 (Zr/(Zr + Ti) = 0.1) samples.
monitored under UV and visible light after irradiation for 120 min, as shown in Fig. 6a and b, respectively. The photocatalytic degradation behavior of commercial Degussa P25 was also measured as a reference. As shown in Fig. 6a, the N-doped TiO2/ZrO2 sample showed the maximum activity among all samples when the amount of NOx
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absorption spectra of the N-doped TiO2/ZrO2 sample significantly shifted to the visible light region in the UV–Vis spectra. The visible light enhanced photocatalytic activity of the N-doped TiO2/ZrO2 sample was much greater than that of commercial Degussa P25. The addition of ZrO2 to the TiO2 resulted in a higher surface area, smaller crystal sizes, and more surface OH groups, which inhibited electron–hole recombination. The photocatalytic efficiency was improved because the material retained its photocatalytic activity for a longer time. Nitrogen doping was carried out to enhance the strong absorption in the visible light region by further lowering the band gap energy. Consequently, a visible light responsive, highly effective TiO2 photocatalyst was developed through the doping of TiO2 with ZrO2 and nitrogen.
Acknowledgments We wish to express our appreciation to Prof. Kikuo Okuyama for his support of this work. This paper was supported by Research Fund, Kumoh National Institute of Technology.
References
Fig. 6. Photocatalytic degradation of NOx under (a) UV and (b) visible light in the (.) TiO2, (d) P25, and (j) N-doped TiO2/ZrO2 (Zr/(Zr + Ti) = 0.1) samples.
degradation was 98.8% (k = 0.05036 min 1) for UV light, while the degradation with TiO2 was 11.6% (k = 0.00138 min 1). In Fig. 6b, the N-doped TiO2/ZrO2 and TiO2 samples had degradation levels of 63.5% (k = 0.0114 min 1) and 3.8% (k = 0.00043 min 1) under visible light, respectively. On the other hand, the degradation of NOx with commercial Degussa P25 was only 18.2% (k = 0.00223 min 1) with UV light and 4.0% (k = 0.00045 min 1) under visible light. These results indicate that the photocatalytic activity of N-doped TiO2/ZrO2 is greatly improved in both the UV and visible light regions. The high photocatalytic activities can be attributed to the large specific surface area, strong absorption in the visible light region, increased numbers of surface OH groups, and the effect of N-doping, all of which resulted in the lowering of the band gap energy. 4. Conclusion A visible light responsive N-doped TiO2/ZrO2 photocatalyst was successfully synthesized using PCSM by doping TiO2 with ZrO2 as a transition metal and with nitrogen as a non-metal. The addition of small amounts of ZrO2 effectively suppressed the TiO2 phase transformation from anatase-to-rutile and promoted the thermal stability of the N-doped TiO2/ZrO2 photocatalyst. The XPS spectra of N-doped TiO2/ZrO2 indicated that the O atoms in the TiO2 lattice were replaced with N atoms, and that the powders were composed of TiO2 including the atomic b-N state. Compared with TiO2, the
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Contents lists available at ScienceDirect
Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Synthesis and characterization of N-doped TiO2/ZrO2 visible light photocatalysts Ji Young Kim a, Chan Soo Kim b, Han Kwon Chang c, Tae Oh Kim a,* a
Department of Environmental Engineering, Kumoh National Institute of Technology, 1 Yang ho-dong, Gumi, Gyeongbuk 730-701, Republic of Korea Department of Materials Science and Engineering, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul 151-742, Republic of Korea c Department of Industrial Materials Research, Korea Institute of Geoscience and Minerals Resources, 30 Kajungdong, Yuseong-gu, Daejeon 305-350, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 30 March 2010 Received in revised form 28 June 2010 Accepted 30 June 2010 Available online 13 July 2010 Keywords: Photocatalysts N-doped TiO2/ZrO2 Visible light absorption NOx
a b s t r a c t Zirconia and nitrogen-doped TiO2 powder was synthesized using a polymer complex solution method for the preparation of an enhanced visible light photocatalyst. The produced catalysts were characterized via the Brunauer, Emmett, and Teller method (BET), X-ray diffraction, transmission electron microscopy, Xray photoelectron spectroscopy, Fourier transform infrared spectra, and UV–Vis spectrophotometry analyses. The N-doped TiO2/ZrO2 photocatalyst showed a high specific surface area and small crystal sizes. The XPS spectra of the N-doped TiO2/ZrO2 sample indicated that nitrogen was doped into the TiO2 lattice and enhanced the photocatalytic activity. The UV–Vis absorption spectra of the N-doped TiO2/ZrO2 sample noticeably shifted to the visible light region compared to that of the TiO2. The photocatalytic activities of the prepared catalysts were evaluated for the decomposition of gaseous NOx under UV and visible light irradiations. The photocatalytic activities of N-doped TiO2/ZrO2 were much greater than those of commercial Degussa P25 in both the UV and visible light regions. The high photocatalytic activity can be attributed to stronger absorption in the visible light region, a greater specific surface area, smaller crystal sizes, more surface OH groups, and to the effect of N-doping, which resulted in a lower band gap energy. Ó 2010 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Titanium dioxide (TiO2) has been widely used as an effective photocatalyst for the photodegradation of organic pollutants in water and air because of its low cost, non-toxicity, strong oxidizing power, and long-term photostability without secondary pollution [1–5]. The only disadvantage is the large band gap energy of anatase TiO2 (3.2 eV). As a result, this compound can be activated only under UV light irradiation. Many attempts have been made to obtain higher photocatalytic reactivity under visible light irradiation. Previous investigations utilized metal ion doping of TiO2 [6,7]; however, the drawback of cationic dopants is weak absorption in the visible light range [8]. To more efficiently utilize solar energy, non-metal doping of TiO2 was performed to prepare a visible light-sensitive photocatalyst via band gap reduction of the photocatalyst [2,9–12]. The introduction of a second metal oxide proved to be an effective route to improve the thermal stabilization and UV light photocatalytic activity of TiO2. Mixed ZrO2–TiO2 has been widely investigated in the photocatalysis field [12,13]. The addition of small amounts of ZrO2 into TiO2 can prevent the anatase-to-rutile phase transformation. Therefore, the addition of ZrO2 enhances the * Corresponding author. Tel.: +82 54 478 7634; fax: +82 54 478 7641. E-mail address: [email protected] (T.O. Kim).
thermal stability of the phase transformation of pure TiO2. Furthermore, ZrO2 improves the thermal stabilization and increases the surface area, thus promoting photocatalytic activity [13]. Sato reported the synthesis of nitrogen-doped TiO2 using various nitrogen sources such as urea, ammonia, ammonium chloride, and nitric acid to dope nitrogen into the TiO2 structure [15]. The improvement in the visible light photocatalytic activity of nitrogen-doped TiO2 is mainly due to the reduction of the band gap through hybridization of the N 2p states with O 2p states on the top of the valence band [2]. Current studies of nitrogen-doped TiO2 are concerned with explaining the N states, which are typically examined using X-ray photoelectron spectroscopy (XPS). Most reports agree that the N 1s peak at 396–398 eV is the characteristic peak of the Ti–N linkages, indicating that nitrogen atoms are doped into the TiO2 lattice and are responsible for the enhanced photocatalytic activity [2,15–18]. To prepare a visible light enhanced photocatalyst for the present study, N-doped TiO2/ZrO2 powder was prepared using a polymer complex solution method (PCSM). Titania catalysts with high visible light activities were successfully prepared via PCSM in our previous study [19]. The doping of TiO2 with ZrO2 as a transition metal improved the photocatalytic efficiency but did not improve the weak absorption in the visible light region. By doping with non-metal N, the band gap was narrowed, allowing enhanced photocatalytic activity in the visible light region. Furthermore, it was
0921-8831/$ - see front matter Ó 2010 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. doi:10.1016/j.apt.2010.06.014
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possible to utilize solar energy for the photocatalysis. The surface areas, crystal sizes, chemical structures, and visible light responses of the prepared catalysts were characterized using BET, XRD, TEM, FT-IR, XPS, and UV–Vis. In addition, the photocatalytic activities of the catalysts were investigated under both UV and visible light by monitoring the decomposition of NOx as a typical indoor pollutant. 2. Experimental 2.1. Materials Titanium tetraisopropoxide (TTIP, Junsei, 98%), hexadecyl trimethylammonium bromide (HDTMA, Aldrich, 99%), zirconium oxynitrate 2-hydrate (Kisita Chemical, 99%), isopropyl alcohol (IPA, Daejung Chemical, 99%), and polyethylene glycol (PEG, Fluka, M.W. = 20,000) were used without further purification. Distilled, deionized water was used for the preparation of the catalysts. 2.2. Preparation of catalysts The TiO2 and N-doped TiO2/ZrO2 photocatalysts were synthesized using PCSM [19]. A mixture of 0.1 M TTIP and 100 ml IPA was stirred for about 30 min at 60 °C. Then, 2 g PEG dissolved in IPA was added via magnetic stirring for 1 h. HDTMA and the required amount of ZrO(NO3)22H2O were added to this premixed solvent under vigorous stirring conditions for another hour and calcined for 2 h at 500 °C. All syntheses were performed using similar steps to obtain samples with Zr/(Zr + Ti) molar ratios in the range of 0–0.9. A TiO2 photocatalyst without the addition of ZrO(NO3)22H2O and HDTMA was also prepared to compare to the Ndoped TiO2/ZrO2 photocatalyst. In this study, the N-doped TiO2/ ZrO2 sample with a Zr/(Zr + Ti) molar ratio of 0.1 was primarily used. 2.3. Characterization The specific surface areas of all samples were calculated using the BET method. The morphologies and sizes of the photocatalysts were observed using TEM (JEM-2010, JEOL). The crystal phases of the samples were analyzed using XRD (RTP 300 RC, Rigaku Co.) with Cu-Ka radiation at a generator voltage of 40 kV and a tube current of 100 mA. FT-IR (Spectrum GX & AutoImage, USA) spectra were recorded in a Perkin–Elmer Spectrum One system, and XPS (ESCALAB 250, VG Scientifics) was performed to measure the surface properties. A general C 1s, O 1s, N 1s, and Ti 2p core spectrum scan was recorded with non-monochromatized Mg-Ka radiation (1253.6 eV) at a pass energy of 50 eV and an electron takeoff angle (the angle between the electron emission direction and the surface plane) of 55°. All of the binding energies were referenced to the C 1s peak of the surface adventitious carbon at 284 eV. The UV–Vis absorbance spectra were obtained using a UV–Vis spectrophotometer (S-4100, Sinco). 2.4. Evaluation of photocatalytic activity Nitrogen oxide was chosen for the evaluation of the photocatalytic activity of the prepared samples, as NOx is a typical indoor pollutant [20,21]. The photocatalytic activity experiments of the commercial Degussa P25 and N-doped TiO2/ZrO2 samples for the degradation of NOx gas were performed at room temperature using a NOx analyzer (Teledyne MODEL-200E, USA). These catalysts were prepared by coating an ethanol suspension of the synthesized samples onto six square glass plates with dimensions of 4.5 4.5 cm. The masses of the catalysts used for each test were 0.6 g. The glass plates with the catalyst coatings were dried at 25 °C for 1 h to
evaporate the ethanol and were then maintained at room temperature (25 °C) before being used. After coating, the glass plates were placed in a quartz reactor, which was maintained in the dark at room temperature. The analysis of NOx concentration in the reactor was conducted with a NOx analyzer. The NOx vapor was allowed to reach adsorption equilibrium with the catalysts prior to irradiation. First, NO gas (100 ppm) was diluted to 5 ppm with N2 gas at a rate of 550 mL/min. As a result, the initial concentration of NOx after establishing adsorption equilibrium was 5 ppm. Next, F8T5BLB (8W) and G8T5 (8W) lamps were placed above the reactor as the UV and visible light sources, respectively. Then, the samples in the reactor were irradiated with UV at an intensity of 5500 lW/ cm2 and with visible light at an intensity of 210 lW/cm2. After the lamps were stabilized for 20 min to obtain a constant intensity, the photocatalytic activities of the catalyst samples were evaluated by comparing the apparent reaction rate constants over 10 min. The photocatalytic degradation of NOx is a pseudo-first-order reaction, and its kinetics may be expressed as ln(C0/C) = kt, where k is the apparent reaction rate constant, and C0 and C are the concentrations of initial NOx and NOx after time t (min), respectively [22].
3. Results and discussion The surface areas of the TiO2 and N-doped TiO2/ZrO2 samples were 29.354 and 187.352 m2/g, respectively. The high surface area of the N-doped TiO2/ZrO2 sample can be attributed to the addition of ZrO2, which decreased the particle size of the TiO2 and, thus, increased the total surface area [13,14]. Fig. 1 presents TEM images of the prepared TiO2 and N-doped TiO2/ZrO2 samples. The TiO2 sample consisted of irregularly shaped particles with a wide range of sizes (12–50 nm), as shown in Fig. 1a. The N-doped TiO2/ZrO2 catalyst shown in Fig. 1b, in contrast, consisted of uniformly sized particles (5–7 nm) in a honeycomb structure smaller than that of the TiO2 sample. Fig. 1c shows a high magnification image of Fig. 1b, where a few disordered agglomerated structures were observed. However, in some areas of the sample, there were randomly oriented nanocrystallites, indicating that the addition of ZrO2 can effectively suppress the growth of TiO2 crystals and stabilizes their structure. The XRD patterns of the TiO2 and N-doped TiO2/ZrO2 samples are shown in Fig. 2. The N-doped TiO2/ZrO2 sample was 100% anatase after heat treatment at 500 °C, and the TiO2 sample was a mixture of anatase and rutile phases. This result indicates that the addition of ZrO2 into TiO2 can effectively suppress the anataseto-rutile phase transformation as well as the crystal growth of the N-doped TiO2/ZrO2 samples [12,23,24]. Neppolian et al. reported that the addition of ZrO2 can increase the surface acidity of TiO2/ZrO2 catalysts due to formation of stronger surface OH groups [13]. The OH groups act as traps for the holes and suppress the electron–hole recombination process [13]. Other researchers have stated that the OH groups are formed as a result of dissociative adsorption of H2O molecules, which occurs in order to reduce the coordinative desaturation of the surface sites [25]. The FT-IR spectra of the TiO2 and N-doped TiO2/ZrO2 samples are shown in Fig. 3. Peaks at 1631 and 3423 cm 1 were observed in the spectra of both samples, corresponding to OH groups [12,13]. The peak intensity of the N-doped TiO2/ZrO2 sample was much stronger than that of the TiO2 sample, indicating that the addition of ZrO2 increased the concentration of surface hydroxyl groups, which may enhance the photocatalytic degradation performance. For further analyses of the chemical structures of the TiO2 and N-doped TiO2/ZrO2 samples, XPS measurements were performed. The XPS spectra (Fig. 4a) show that the TiO2 sample contained only Ti, O, and C elements. However, the N-doped TiO2/ZrO2 sample had new peaks at 397.45 and 403.1 eV, which could be due to the bind-
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Fig. 2. XRD patterns of the TiO2 and N-doped TiO2/ZrO2 samples.
Fig. 3. FT-IR spectra of the (a) TiO2 and (b) N-doped TiO2/ZrO2 (Zr/(Zr + Ti) = 0.1) samples.
Fig. 1. TEM images of the (a) TiO2 and (b and c) N-doped TiO2/ZrO2 (Zr/(Zr + Ti) = 0.1) samples.
ing energy of N 1s. The N 1s, Ti 2p, and O 1s regions of the N-doped TiO2/ZrO2 sample are illustrated in Fig. 4b–d, respectively. In the N
1s spectra of the N-doped TiO2/ZrO2 sample (Fig. 4b), two peaks appeared at 397.45 and 403.1 eV due to the addition of nitrogen into TiO2, which is typically observed at 396–404 eV [16]. For the other samples, the peak at 397.45 eV was attributed to the presence of substituted nitrogen in the O–Ti–N structure and was assigned to the atomic b-N state [2,9,25]. The peak at 403.1 eV may also be due to molecularly adsorbed nitrogen-containing compounds on the surfaces of the samples [9,26]. For the Ti 2p region (Fig. 4c), the peak for the TiO2 sample was observed at 459.25 eV, while the peak for the N-doped TiO2/ZrO2 sample appeared at 456.45 eV, corresponding to the lower binding energy of Ti 2p. The typical binding energy of the Ti 2p peak in a TiO2 crystal is 458.5–459.7 eV [18]. Therefore, nitrogen was successfully incorporated into the TiO2 lattice. The measured binding energy of Ti 2p for the N-doped TiO2/ZrO2 sample was due to the formation of O–Ti–N bonds by partial replacement of the O atoms with N atoms in the TiO2 lattice [18,27]. Fig. 4d presents the O 1s spectra for the TiO2 and N-doped TiO2/ ZrO2 samples. The peaks of the O 1s region for the TiO2 and Ndoped TiO2/ZrO2 samples were observed at 530.5 and 527.85 eV, respectively, illustrating a significantly lower binding energy for
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Fig. 4. XPS spectra of (a) TiO2 and N-doped TiO2/ZrO2, (b) N 1s peaks of TiO2 and N-doped TiO2/ZrO2, (c) Ti 2p peaks of TiO2 and N-doped TiO2/ZrO2, and (d) O1 s peaks of the TiO2 and N-doped TiO2/ZrO2 (Zr/(Zr + Ti) = 0.1) samples.
the N-doped TiO2/ZrO2 sample than for the TiO2 sample. This indicates that nitrogen was incorporated to form the O–Ti–N structure due to surface strain and lattice distortion during the doping process and that the O–Ti–N structure shifted to a higher infrared feature than did the O–Ti–O structure [18,23]. Furthermore, the XPS results not only showed that the nitrogen was structurally compatible in the formation of O–Ti–N, but also that it led to enhanced photocatalytic activity in the visible light region due to the substitutional doping of nitrogen for oxygen in N-doped TiO2. To compare the optical properties of the TiO2 and N-doped TiO2/ ZrO2 samples, their UV–Vis absorption spectra were investigated and are shown in Fig. 5. A noticeable shift in the absorption spectra to the lower energy region was observed for the N-doped TiO2/ZrO2 sample compared to that of the TiO2 sample. The absorption in the visible light range is related to the newly formed N 1s state because of the incorporation of nitrogen into the TiO2 lattice [2,9,26,28,29]. As direct evidence, the band gap energies of the samples were determined from the energy intercept of a plot of (ahm)1/2 versus the photon energy (hm) [30]. From these plots, the band gap energies of TiO2 and the N-doped TiO2/ZrO2 samples were approximately 3.01 and 2.81 eV, respectively. The band gap was narrowed because the N 1s band formed above the O 1s valence band in the N-doped TiO2/ZrO2 sample, as shown in Fig. 4b. Furthermore, this result indicates that the N-doped TiO2/ZrO2 sample has high photocatalytic activity in the visible light region. To evaluate and compare the photocatalytic activity of the TiO2 and N-doped TiO2/ZrO2 samples, the degradations of NOx were
Fig. 5. UV absorption spectra of the TiO2 and N-doped TiO2/ZrO2 (Zr/(Zr + Ti) = 0.1) samples.
monitored under UV and visible light after irradiation for 120 min, as shown in Fig. 6a and b, respectively. The photocatalytic degradation behavior of commercial Degussa P25 was also measured as a reference. As shown in Fig. 6a, the N-doped TiO2/ZrO2 sample showed the maximum activity among all samples when the amount of NOx
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absorption spectra of the N-doped TiO2/ZrO2 sample significantly shifted to the visible light region in the UV–Vis spectra. The visible light enhanced photocatalytic activity of the N-doped TiO2/ZrO2 sample was much greater than that of commercial Degussa P25. The addition of ZrO2 to the TiO2 resulted in a higher surface area, smaller crystal sizes, and more surface OH groups, which inhibited electron–hole recombination. The photocatalytic efficiency was improved because the material retained its photocatalytic activity for a longer time. Nitrogen doping was carried out to enhance the strong absorption in the visible light region by further lowering the band gap energy. Consequently, a visible light responsive, highly effective TiO2 photocatalyst was developed through the doping of TiO2 with ZrO2 and nitrogen.
Acknowledgments We wish to express our appreciation to Prof. Kikuo Okuyama for his support of this work. This paper was supported by Research Fund, Kumoh National Institute of Technology.
References
Fig. 6. Photocatalytic degradation of NOx under (a) UV and (b) visible light in the (.) TiO2, (d) P25, and (j) N-doped TiO2/ZrO2 (Zr/(Zr + Ti) = 0.1) samples.
degradation was 98.8% (k = 0.05036 min 1) for UV light, while the degradation with TiO2 was 11.6% (k = 0.00138 min 1). In Fig. 6b, the N-doped TiO2/ZrO2 and TiO2 samples had degradation levels of 63.5% (k = 0.0114 min 1) and 3.8% (k = 0.00043 min 1) under visible light, respectively. On the other hand, the degradation of NOx with commercial Degussa P25 was only 18.2% (k = 0.00223 min 1) with UV light and 4.0% (k = 0.00045 min 1) under visible light. These results indicate that the photocatalytic activity of N-doped TiO2/ZrO2 is greatly improved in both the UV and visible light regions. The high photocatalytic activities can be attributed to the large specific surface area, strong absorption in the visible light region, increased numbers of surface OH groups, and the effect of N-doping, all of which resulted in the lowering of the band gap energy. 4. Conclusion A visible light responsive N-doped TiO2/ZrO2 photocatalyst was successfully synthesized using PCSM by doping TiO2 with ZrO2 as a transition metal and with nitrogen as a non-metal. The addition of small amounts of ZrO2 effectively suppressed the TiO2 phase transformation from anatase-to-rutile and promoted the thermal stability of the N-doped TiO2/ZrO2 photocatalyst. The XPS spectra of N-doped TiO2/ZrO2 indicated that the O atoms in the TiO2 lattice were replaced with N atoms, and that the powders were composed of TiO2 including the atomic b-N state. Compared with TiO2, the
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