Advanced Powder Technology 22 (2011) 443–448

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Advanced Powder Technology

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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 Koreab Department of Materials Science and Engineering, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul 151-742, Republic of Koreac Department of Industrial Materials Research, Korea Institute of Geoscience and Minerals Resources, 30 Kajungdong, Yuseong-gu, Daejeon 305-350, Republic of Korea

a r t i c l e i n f o a b s t r a c t

Article history:Received 30 March 2010Received in revised form 28 June 2010Accepted 30 June 2010Available online 13 July 2010

Keywords:PhotocatalystsN-doped TiO2/ZrO2

Visible light absorptionNOx

0921-8831/$ - see front matter � 2010 The Society ofdoi:10.1016/j.apt.2010.06.014

* Corresponding author. Tel.: +82 54 478 7634; faxE-mail address: kim@kumoh.ac.kr (T.O. Kim).

Zirconia and nitrogen-doped TiO2 powder was synthesized using a polymer complex solution method forthe preparation of an enhanced visible light photocatalyst. The produced catalysts were characterized viathe Brunauer, Emmett, and Teller method (BET), X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectra, and UV–Vis spectrophotometry anal-yses. 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 latticeand 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 photocatalyticactivities of the prepared catalysts were evaluated for the decomposition of gaseous NOx under UV andvisible light irradiations. The photocatalytic activities of N-doped TiO2/ZrO2 were much greater thanthose of commercial Degussa P25 in both the UV and visible light regions. The high photocatalytic activitycan be attributed to stronger absorption in the visible light region, a greater specific surface area, smallercrystal sizes, more surface OH groups, and to the effect of N-doping, which resulted in a lower band gapenergy.� 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 effectivephotocatalyst for the photodegradation of organic pollutants inwater and air because of its low cost, non-toxicity, strong oxidizingpower, and long-term photostability without secondary pollution[1–5]. The only disadvantage is the large band gap energy of ana-tase TiO2 (3.2 eV). As a result, this compound can be activated onlyunder UV light irradiation. Many attempts have been made to ob-tain 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 inthe visible light range [8]. To more efficiently utilize solar energy,non-metal doping of TiO2 was performed to prepare a visiblelight-sensitive photocatalyst via band gap reduction of the photo-catalyst [2,9–12].

The introduction of a second metal oxide proved to be an effec-tive route to improve the thermal stabilization and UV light photo-catalytic activity of TiO2. Mixed ZrO2–TiO2 has been widelyinvestigated in the photocatalysis field [12,13]. The addition ofsmall amounts of ZrO2 into TiO2 can prevent the anatase-to-rutilephase transformation. Therefore, the addition of ZrO2 enhances the

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thermal stability of the phase transformation of pure TiO2. Further-more, ZrO2 improves the thermal stabilization and increases thesurface area, thus promoting photocatalytic activity [13].

Sato reported the synthesis of nitrogen-doped TiO2 using vari-ous nitrogen sources such as urea, ammonia, ammonium chloride,and nitric acid to dope nitrogen into the TiO2 structure [15]. Theimprovement in the visible light photocatalytic activity of nitro-gen-doped TiO2 is mainly due to the reduction of the band gapthrough hybridization of the N 2p states with O 2p states on thetop of the valence band [2]. Current studies of nitrogen-dopedTiO2 are concerned with explaining the N states, which are typi-cally examined using X-ray photoelectron spectroscopy (XPS).Most reports agree that the N 1s peak at 396–398 eV is the charac-teristic peak of the Ti–N linkages, indicating that nitrogen atomsare doped into the TiO2 lattice and are responsible for the en-hanced photocatalytic activity [2,15–18].

To prepare a visible light enhanced photocatalyst for the pres-ent study, N-doped TiO2/ZrO2 powder was prepared using a poly-mer complex solution method (PCSM). Titania catalysts with highvisible light activities were successfully prepared via PCSM in ourprevious study [19]. The doping of TiO2 with ZrO2 as a transitionmetal improved the photocatalytic efficiency but did not improvethe weak absorption in the visible light region. By doping withnon-metal N, the band gap was narrowed, allowing enhanced pho-tocatalytic activity in the visible light region. Furthermore, it was

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444 J.Y. Kim et al. / Advanced Powder Technology 22 (2011) 443–448

possible to utilize solar energy for the photocatalysis. The surfaceareas, crystal sizes, chemical structures, and visible light responsesof the prepared catalysts were characterized using BET, XRD, TEM,FT-IR, XPS, and UV–Vis. In addition, the photocatalytic activities ofthe catalysts were investigated under both UV and visible light bymonitoring the decomposition of NOx as a typical indoor pollutant.

2. Experimental

2.1. Materials

Titanium tetraisopropoxide (TTIP, Junsei, 98%), hexadecyltrimethylammonium bromide (HDTMA, Aldrich, 99%), zirconiumoxynitrate 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 synthe-sized using PCSM [19]. A mixture of 0.1 M TTIP and 100 ml IPAwas stirred for about 30 min at 60 �C. Then, 2 g PEG dissolved inIPA was added via magnetic stirring for 1 h. HDTMA and the re-quired amount of ZrO(NO3)2�2H2O were added to this premixedsolvent under vigorous stirring conditions for another hour andcalcined for 2 h at 500 �C. All syntheses were performed using sim-ilar steps to obtain samples with Zr/(Zr + Ti) molar ratios in therange of 0–0.9. A TiO2 photocatalyst without the addition of ZrO(-NO3)2�2H2O and HDTMA was also prepared to compare to the N-doped TiO2/ZrO2 photocatalyst. In this study, the N-doped TiO2/ZrO2 sample with a Zr/(Zr + Ti) molar ratio of 0.1 was primarilyused.

2.3. Characterization

The specific surface areas of all samples were calculated usingthe BET method. The morphologies and sizes of the photocatalystswere observed using TEM (JEM-2010, JEOL). The crystal phases ofthe samples were analyzed using XRD (RTP 300 RC, Rigaku Co.)with Cu-Ka radiation at a generator voltage of 40 kV and a tubecurrent of 100 mA. FT-IR (Spectrum GX & AutoImage, USA) spectrawere recorded in a Perkin–Elmer Spectrum One system, and XPS(ESCALAB 250, VG Scientifics) was performed to measure the sur-face properties. A general C 1s, O 1s, N 1s, and Ti 2p core spectrumscan 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 surfaceplane) of 55�. All of the binding energies were referenced to the C1s peak of the surface adventitious carbon at 284 eV. The UV–Visabsorbance spectra were obtained using a UV–Vis spectrophotom-eter (S-4100, Sinco).

2.4. Evaluation of photocatalytic activity

Nitrogen oxide was chosen for the evaluation of the photocata-lytic activity of the prepared samples, as NOx is a typical indoorpollutant [20,21]. The photocatalytic activity experiments of thecommercial Degussa P25 and N-doped TiO2/ZrO2 samples for thedegradation of NOx gas were performed at room temperature usinga NOx analyzer (Teledyne MODEL-200E, USA). These catalysts wereprepared by coating an ethanol suspension of the synthesized sam-ples 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 glassplates with the catalyst coatings were dried at 25 �C for 1 h to

evaporate the ethanol and were then maintained at room temper-ature (25 �C) before being used. After coating, the glass plates wereplaced in a quartz reactor, which was maintained in the dark atroom temperature. The analysis of NOx concentration in the reactorwas conducted with a NOx analyzer. The NOx vapor was allowed toreach adsorption equilibrium with the catalysts prior to irradia-tion. First, NO gas (100 ppm) was diluted to 5 ppm with N2 gasat a rate of 550 mL/min. As a result, the initial concentration ofNOx after establishing adsorption equilibrium was 5 ppm. Next,F8T5BLB (8W) and G8T5 (8W) lamps were placed above the reactoras the UV and visible light sources, respectively. Then, the samplesin 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 thelamps were stabilized for 20 min to obtain a constant intensity, thephotocatalytic activities of the catalyst samples were evaluated bycomparing the apparent reaction rate constants over 10 min. Thephotocatalytic degradation of NOx is a pseudo-first-order reaction,and its kinetics may be expressed as ln(C0/C) = kt, where k is theapparent reaction rate constant, and C0 and C are the concentra-tions 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 sampleswere 29.354 and 187.352 m2/g, respectively. The high surface areaof the N-doped TiO2/ZrO2 sample can be attributed to the additionof ZrO2, which decreased the particle size of the TiO2 and, thus, in-creased the total surface area [13,14].

Fig. 1 presents TEM images of the prepared TiO2 and N-dopedTiO2/ZrO2 samples. The TiO2 sample consisted of irregularly shapedparticles with a wide range of sizes (12–50 nm), as shown inFig. 1a. The N-doped TiO2/ZrO2 catalyst shown in Fig. 1b, in con-trast, consisted of uniformly sized particles (5–7 nm) in a honey-comb structure smaller than that of the TiO2 sample. Fig. 1cshows a high magnification image of Fig. 1b, where a few disor-dered agglomerated structures were observed. However, in someareas of the sample, there were randomly oriented nanocrystal-lites, indicating that the addition of ZrO2 can effectively suppressthe growth of TiO2 crystals and stabilizes their structure.

The XRD patterns of the TiO2 and N-doped TiO2/ZrO2 samplesare shown in Fig. 2. The N-doped TiO2/ZrO2 sample was 100% ana-tase after heat treatment at 500 �C, and the TiO2 sample was a mix-ture of anatase and rutile phases. This result indicates that theaddition of ZrO2 into TiO2 can effectively suppress the anatase-to-rutile phase transformation as well as the crystal growth ofthe N-doped TiO2/ZrO2 samples [12,23,24].

Neppolian et al. reported that the addition of ZrO2 can increasethe surface acidity of TiO2/ZrO2 catalysts due to formation of stron-ger surface OH groups [13]. The OH groups act as traps for theholes and suppress the electron–hole recombination process [13].Other researchers have stated that the OH groups are formed asa result of dissociative adsorption of H2O molecules, which occursin order to reduce the coordinative desaturation of the surface sites[25]. The FT-IR spectra of the TiO2 and N-doped TiO2/ZrO2 samplesare shown in Fig. 3. Peaks at 1631 and 3423 cm�1 were observed inthe spectra of both samples, corresponding to OH groups [12,13].The peak intensity of the N-doped TiO2/ZrO2 sample was muchstronger than that of the TiO2 sample, indicating that the additionof 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 andN-doped TiO2/ZrO2 samples, XPS measurements were performed.The XPS spectra (Fig. 4a) show that the TiO2 sample contained onlyTi, O, and C elements. However, the N-doped TiO2/ZrO2 sample hadnew peaks at 397.45 and 403.1 eV, which could be due to the bind-

Fig. 1. TEM images of the (a) TiO2 and (b and c) N-doped TiO2/ZrO2

(Zr/(Zr + Ti) = 0.1) samples.

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.

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ing energy of N 1s. The N 1s, Ti 2p, and O 1s regions of the N-dopedTiO2/ZrO2 sample are illustrated in Fig. 4b–d, respectively. In the N

1s spectra of the N-doped TiO2/ZrO2 sample (Fig. 4b), two peaksappeared at 397.45 and 403.1 eV due to the addition of nitrogeninto TiO2, which is typically observed at 396–404 eV [16]. For theother samples, the peak at 397.45 eV was attributed to the pres-ence of substituted nitrogen in the O–Ti–N structure and was as-signed to the atomic b-N state [2,9,25]. The peak at 403.1 eV mayalso be due to molecularly adsorbed nitrogen-containing com-pounds on the surfaces of the samples [9,26].

For the Ti 2p region (Fig. 4c), the peak for the TiO2 sample wasobserved at 459.25 eV, while the peak for the N-doped TiO2/ZrO2

sample appeared at 456.45 eV, corresponding to the lower bindingenergy of Ti 2p. The typical binding energy of the Ti 2p peak in aTiO2 crystal is 458.5–459.7 eV [18]. Therefore, nitrogen was suc-cessfully incorporated into the TiO2 lattice. The measured bindingenergy of Ti 2p for the N-doped TiO2/ZrO2 sample was due to theformation of O–Ti–N bonds by partial replacement of the O atomswith 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 N-doped TiO2/ZrO2 samples were observed at 530.5 and 527.85 eV,respectively, illustrating a significantly lower binding energy for

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 theTiO2 and N-doped TiO2/ZrO2 (Zr/(Zr + Ti) = 0.1) samples.

Fig. 5. UV absorption spectra of the TiO2 and N-doped TiO2/ZrO2 (Zr/(Zr + Ti) = 0.1)samples.

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the N-doped TiO2/ZrO2 sample than for the TiO2 sample. This indi-cates that nitrogen was incorporated to form the O–Ti–N structuredue to surface strain and lattice distortion during the doping pro-cess and that the O–Ti–N structure shifted to a higher infrared fea-ture than did the O–Ti–O structure [18,23]. Furthermore, the XPSresults not only showed that the nitrogen was structurally compat-ible in the formation of O–Ti–N, but also that it led to enhancedphotocatalytic activity in the visible light region due to the substi-tutional 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 investigatedand are shown in Fig. 5. A noticeable shift in the absorption spectrato the lower energy region was observed for the N-doped TiO2/ZrO2

sample compared to that of the TiO2 sample. The absorption in thevisible light range is related to the newly formed N 1s state be-cause of the incorporation of nitrogen into the TiO2 lattice[2,9,26,28,29]. As direct evidence, the band gap energies of thesamples were determined from the energy intercept of a plot of(ahm)1/2 versus the photon energy (hm) [30]. From these plots, theband gap energies of TiO2 and the N-doped TiO2/ZrO2 sampleswere approximately 3.01 and 2.81 eV, respectively. The band gapwas narrowed because the N 1s band formed above the O 1s va-lence band in the N-doped TiO2/ZrO2 sample, as shown in Fig. 4b.Furthermore, this result indicates that the N-doped TiO2/ZrO2 sam-ple 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

monitored under UV and visible light after irradiation for120 min, as shown in Fig. 6a and b, respectively. The photocatalyticdegradation behavior of commercial Degussa P25 was also mea-sured as a reference.

As shown in Fig. 6a, the N-doped TiO2/ZrO2 sample showed themaximum activity among all samples when the amount of NOx

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.

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degradation was 98.8% (k = 0.05036 min�1) for UV light, while thedegradation with TiO2 was 11.6% (k = 0.00138 min�1). In Fig. 6b,the N-doped TiO2/ZrO2 and TiO2 samples had degradation levelsof 63.5% (k = 0.0114 min�1) and 3.8% (k = 0.00043 min�1) undervisible light, respectively. On the other hand, the degradation ofNOx with commercial Degussa P25 was only 18.2%(k = 0.00223 min�1) with UV light and 4.0% (k = 0.00045 min�1) un-der visible light. These results indicate that the photocatalyticactivity of N-doped TiO2/ZrO2 is greatly improved in both the UVand visible light regions. The high photocatalytic activities can beattributed to the large specific surface area, strong absorption inthe visible light region, increased numbers of surface OH groups,and the effect of N-doping, all of which resulted in the loweringof the band gap energy.

4. Conclusion

A visible light responsive N-doped TiO2/ZrO2 photocatalyst wassuccessfully synthesized using PCSM by doping TiO2 with ZrO2 as atransition metal and with nitrogen as a non-metal. The addition ofsmall amounts of ZrO2 effectively suppressed the TiO2 phasetransformation from anatase-to-rutile and promoted the thermalstability of the N-doped TiO2/ZrO2 photocatalyst. The XPS spectraof N-doped TiO2/ZrO2 indicated that the O atoms in the TiO2 latticewere replaced with N atoms, and that the powders were composedof TiO2 including the atomic b-N state. Compared with TiO2, the

absorption spectra of the N-doped TiO2/ZrO2 sample significantlyshifted to the visible light region in the UV–Vis spectra. The visiblelight 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 inhibitedelectron–hole recombination. The photocatalytic efficiency wasimproved because the material retained its photocatalytic activityfor a longer time. Nitrogen doping was carried out to enhance thestrong absorption in the visible light region by further lowering theband gap energy. Consequently, a visible light responsive, highlyeffective TiO2 photocatalyst was developed through the doping ofTiO2 with ZrO2 and nitrogen.

Acknowledgments

We wish to express our appreciation to Prof. Kikuo Okuyama forhis support of this work. This paper was supported by ResearchFund, Kumoh National Institute of Technology.

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