Hydrothermal N-doped TiO2: Explaining photocatalytic properties by electronic and magnetic identification of N active sites

Applied Catalysis B: Environmental 93 (2009) 149–155

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Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb

Hydrothermal N-doped TiO2: Explaining photocatalytic properties by electronic and magnetic identification of N active sites Massimiliano D’Arienzo a,*, Roberto Scotti a, Laura Wahba a, Chiara Battocchio b, Edoardo Bemporad b, Angeloclaudio Nale a, Franca Morazzoni a a b

Department of Materials Science, INSTM, University of Milano-Bicocca, Via R.Cozzi 53, I-20125 Milano, Italy Department of Physics, INSTM and CISDiC, University of Rome ‘‘Roma Tre’’, Via della Vasca Navale 84, I-00146 Rome, Italy

A R T I C L E I N F O

A B S T R A C T

Article history: Received 30 July 2009 Received in revised form 9 September 2009 Accepted 14 September 2009 Available online 30 September 2009

N-doped TiO2 nanocrystals with high photoactivity in the visible range, were successfully synthesized by hydrothermal method, followed by thermal annealing at different temperatures (350–600 8C), in order to allow differential nitrogen diffusion into the TiO2 lattice. Optical and magnetic properties, studied by diffuse reflectance spectroscopy, electron paramagnetic resonance and X-ray photoelectron spectroscopy analysis, revealed that TiO2 was effectively doped. The thermal treatment induces insertion of nitrogen into TiO2 lattice in the form of nitride anion N, detected as N by EPR, whose ionic character varies with the temperature of annealing. The amount of N increases till 450 8C, then it decreases. Similar trend was observed for the photomineralization of phenol under visible light irradiation (l > 385 nm): the photoactivity of N-doped samples becomes maximum for N–TiO2 annealed at 450 8C. The overall results suggest that the efficacy of the catalyst depends on the ability of N centers to trap photogenerated holes. This effect lowers the rate of electron–hole recombination and allows the N (N + h+) center acts as strong oxidizing agent. ß 2009 Elsevier B.V. All rights reserved.

Keywords: Nitrogen-doped titanium dioxide Photocatalytic activity under visible light Optical absorption EPR spectroscopy

1. Introduction In recent years we focused our interest on the relations between the TiO2 crystal phases rutile, anatase and mixed phase, obtained by sol–gel or hydrothermal procedures, and their photocatalytic properties under UV irradiation [1–4]. It was suggested that the photoactivity depends both on the crystal phase and on the employed oxidizing agent, O2 or H2O2. Ultimately in all the studied cases the photocatalytic activity resulted opposite to the rate of electron–hole recombination. As a matter of fact the number of holes and electron traps, O and Ti3+centers respectively, generated under UV irradiation of the catalysts, suggests that higher the amount of these charge carriers higher the electron– hole recombination rate and the photoactivity of the catalyst in the phenol degradation reaction. Due to the need of carrying out the oxidative processes under visible light irradiation, that are experimental conditions easier to be scaled-up for practical purposes, and being the energy gap of pure oxide too wide (3.0–3.2 eV for rutile and anatase respectively) to allow electron excitation in the region between 400 and 800 nm, we further operated by doping hydrothermal nanocrystalline TiO2 with nitrogen atoms.

* Corresponding author. Tel.: +39 02 64485123; fax: +39 02 64485400. E-mail address: [email protected] (M. D’Arienzo). 0926-3373/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2009.09.024

Several investigations recently reported both on the preparation and the catalytic activity, under visible light irradiation, of Ndoped TiO2 [4–9]. The electronic properties of N doping centers and their role in improving the photoactivity of TiO2 under visible light irradiation were also deeply investigated [10–19]. In spite of the great number of data reported in the literature, the visible light induced electron transfer processes associated to the improve of catalytic effects are still under debate, mainly as regards the type and the number of photoactive centers, their location in the energy gap and their ability to favour the electron– hole charge separation. The present paper reports about the hydrothermal preparation of N-doped TiO2, further annealed in O2 at different temperatures. It is expected that the annealing process drives the dopant diffusion into TiO2 lattice, thus causing modification on the electronic properties of N centers located in different sites of the oxide, and, consequently, on the photocatalytic activity. With the aim of relating the nitrogen chemical state to the catalytic properties, doped and undoped TiO2 annealed under the same conditions and with similar morphology and structure, were investigated. The electronic centers responsible for the light absorption were studied by UV–Vis diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR). The photoactivity was tested in the oxidative phenol degradation, by using gaseous oxygen as oxidizing agent.

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2. Experimental 2.1. Preparation of the catalyst In a typical preparation of nitrogen-doped TiO2, 1 g of hydrothermally obtained anatase TiO2 powder [1] was suspended in water; then a proper amount of urea was added (Ti/urea = 1:1). In order to disperse it homogeneously, the mixture was exposed to high-intensity ultrasound irradiation for 15 min. After that, the resultant slurry was transferred into a Teflon-lined autoclave and heated at 220 8C for 4 h. Then it was cooled to room temperature and washed several times by de-ionized water and acetone. The precipitate was centrifuged, dried at 60 8C for 10 h and finally annealed at 350 8C (sample N350) in air for 1 h. N350 was used as prepared or further annealed at 450, 500 and 600 8C for 1 h in O2 (samples N450, N550, and N600 respectively). Undoped TiO2 samples were obtained by subjecting bare anatase powder to the same hydrothermal treatment and to identical thermal annealing (Blank350–Blank600 samples). 2.2. Catalyst characterization The X-ray diffraction (XRD) patterns of all the TiO2 powders, collected with a Bruker D8 Advance diffractometer (Cu Ka radiation) operating in the range 20–408 2u (2u step 0.0208, counting time 2 s per step), were used to determine the crystalline phase and the average crystallite size. The average dimension of TiO2 particles was estimated by means of the Scherrer equation from the broadening of the XRD (1 0 1) anatase peak; the rutile content in powders was calculated using the equation XR ¼

1 ð1 þ 0:8IA =IR Þ

(1)

where XR is the mass fraction of rutile, and IA and IR are the integrated intensities of (1 0 1) reflection of anatase and (1 1 0) reflection of rutile. XPS analysis was performed by an instrument of our own design and construction. It consists of a preparation and analysis UHV chamber, equipped with a 150 mm mean radius hemispherical electron analyser having a four-elements lens system and a 16-channel detector. This provides a total instrumental resolution of 1.0 eV, as measured at the Ag 3d5/2 core level. Mg Ka nonmonochromatised X-ray radiation (hn = 1253.6 eV) was used for acquiring core level spectra of all samples (C 1s, N 1s, O 1s and Ti 2p). The spectra were energy referenced to the C 1s signal of aliphatic C atoms having a binding energy BE = 285.00 eV. Atomic ratios were calculated from peak intensities by using Scofield’s cross-section values and the reported l factors [20]. Curve-fitting analysis of the C 1s, N 1s, O 1s and Ti 2p spectra was performed using Gaussian fitting functions, after subtraction of a Shirley-type background [21]. Transmission electron microscopy (TEM) and electron diffraction (ED) measurements were performed using a Jeol 3010 apparatus operated at 300 kV with a high-resolution pole piece (0.17 nm pointto-point resolution) and equipped with a Gatan slow-scan 794 CCD camera. Powders were suspended in isopropanol, and a 5 mL drop of this suspension was deposited on a holey carbon film supported on 3-mm copper grid for the investigation. The UV–Vis diffuse reflectance spectra were obtained on dry pressed disk samples using a PerkinElmer Lambda 35 spectrophotometer equipped with an integrating sphere assembly, using Spectralon1 as reflectance sample. The spectra were recorded at room temperature in air ranging from 250 to 800 nm. Nitrogen physisorption measurements were carried out on a Quantachrome Autosorb-1 apparatus. The specific surface area (SSA, BET method),

pore volume (desorption cumulative pore volume, DCPV), and pore size distribution (BJH method) of selected samples were measured after evacuation at 473 K for 12 h. The EPR investigation was performed by a Bruker EMX spectrometer working at the X-band frequency, equipped with an Oxford cryostat operating in the range of temperature 4–298 K. Spectra were recorded on powder samples under helium atmosphere at 123 K before and after 10 min of irradiation at this temperature inside the EPR cavity. Modulation frequency was 100 kHz, modulation amplitude 3–10 gauss, microwave power 5– 10 mW. Irradiation was performed by UV 125 W Hg high pressure lamp equipped with a cut-off filter for the UV region (wavelength l < 385 nm) and with the output radiation focused on the samples in the cavity by using an optical fiber (50 cm length, 1 cm diameter). The lamp has intense emissions lines in the far UV region and at 385, 400 and 420 nm. By using the cut-off filter, the line at 385 nm (near UV) is removed and the suitable emissions lie only in the visible region. The g values were calculated by standardization with a,a0 diphenyl-b-picryl hydrazyl (DPPH). The spin concentration was obtained by double integration of the resonance lines, referring the area to that of the standard Bruker weak pitch (9.7  1012  5% spins cm1). Care was taken in order that the most sensitive part of the EPR cavity (1 cm length) was always filled. 2.3. Photoinduced degradation of phenol Photodegradation experiments were carried out in a 600 mL discontinuous batch reactor with an external cooling jacket, and equipped with a UV 125 W Hg high pressure lamp with a cut-off optical filter for the UV radiation (l < 385 nm). The same excitation source was used for both photomineralization experiments and EPR studies in order to obtain homogeneous and comparable data. TiO2 (100  5 mg) was suspended by sonication in 600 mL of water containing 121  2 ppm of phenol (PhOH) (93  2 ppm as C). The temperature was kept at 25  2 8C. Photodegradation was carried out by using O2 as oxidative agent. The suspension was recirculated by a peristaltic pump (14 mL s1) in the dark and saturated in an online chamber by continuously bubbling oxygen (100 mL min1). Oxygen content was monitored by on-line sensors and the excess of gas was eliminated through a non return check valve. When the oxygen content in the suspension became maximum and constant (after about 10 min) the Vis source was turned on. Experiments were performed also on the Blank samples as reference materials. In order to monitor the photodegradation of phenol, aliquots (6 mL) of the reaction solution were drawn out at regular intervals and analyzed for the total organic carbon (TOC) by a Shimadzu TOC-V CSH analyzer after TiO2 powder was separated by centrifugation. 3. Results and discussion 3.1. Structure and morphology of the catalysts Fig. 1 reports the X-ray diffraction patterns of N-doped TiO2 and the reference Blank samples annealed under the same conditions, by thermal treatment at 350 8C (lines a and c) and at 600 8C (lines b and d). It is worth noting that the anatase phase has been retained as major component; however a small fraction of rutile arises in Ndoped powders (95% anatase and 5% rutile). The anatase average particle sizes of doped and N-doped powders, estimated by Scherrer equation, do not change after thermal annealing, even at the highest temperature (Table 1). On the other hand, nitrogen doping reaction causes slight decrease of the mean crystallite sizes [15].

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Fig. 3. Adsorption/desorption isotherm at liquid nitrogen for sample N600. Curves correspond to type IV isotherm with capillary condensation in the mesopores. Inset: pore size distribution.

Fig. 1. XRD patterns of (a) Blank350 (b) Blank600 (c) N350 and (d) N600 TiO2 samples.

Table 1 Phase composition, particle size, specific surface area and pore volume of selected N-doped and bare TiO2 nanocrystals. Sample

N350 N600 Blank350 Blank600

Anatase mean Crystal size (nm)

Anatase fraction (wt.%)

Specific surface area (SBET, m2 g1)

BJH pore volume (cm3 g1)

10.5 10.5 15.9 15.9

95 95 100 100

78.75 77.80 81.09 81.18

0.262 0.257 0.221 0.250

Selected TEM micrographs of bare and N-doped TiO2 nanocrystals are reported in Fig. 2. No internal pores or amorphous surface layers are detectable. Undoped titania (Blank600, Fig. 2a) shows aggregates of almost cubic nanoparticles with average sizes 15 nm. N-doped sample (N600, Fig. 2b) presents more compact aggregates of square ended anatase nanoparticles with mean size 10–15 nm. Nitrogen physisorption analyses were performed on selected doped and undoped samples. All powders were mesoporous,

showing type IV Brunauer isotherm and a monomodal pore size distribution in the range 10–15 nm (as showed by N600 sample, Fig. 3). According to the t-plot, no micropores were detected in both series. Values of the specific surface areas (SBET) and BJH pore volumes for selected samples are reported in Table 1. It is interesting to note that the thermal treatment at different temperatures has negligible effects on surface area, pore volume and pore size distribution. 3.2. Optical properties Diffuse reflectance spectra of N-doped TiO2 showed an absorption tail in the 400–520 nm region (3.25–2.50 eV) due to the presence of nitrogen. The reflectance data were converted into absorption coefficients F(R1) according to the Kubelka–Munk equation:

FðR1 Þ ¼

ð1  R1 Þ2 2R1

(2)

Fig. 4a shows the Kubelka–Munk plot over the 2.50–3.25 eV energy region. In this range the absorption intensity changes with the annealing temperature, the highest values occurring for N450 and N500.

Fig. 2. TEM micrograph of (a) Blank600 (b) N600.

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Fig. 4. (a) Diffuse reflectance spectra for N-doped and a representative Blank samples according to the Kubelka–Munk equation; (b) transformed Kubelka–Munk function vs. excitation energy.

The transformed Kubelka–Munk function can be constructed by plotting [F(R1)]0.5 values against excitation energy and allows to evaluate the band gap energy (Fig. 4b) [22]. It turns out that samples annealed at different temperatures have band gap energies between 3.06 and 3.10 eV and can be distinguished in two groups: Blank and N350 samples, having higher energy gap value, N450, N500, N600 samples with lower values. The shift of the gap to lower energy observed in N-doped TiO2, can be ascribed to the formation of intra gap states located above the valence band, due to substitution of oxide centers by nitride centers and/or to the interstitial introduction of nitride into the oxide lattice [10,12]. The consequent Ti–N bond formation is favoured at the highest annealing temperatures (T > 350 8C) as evidenced by the most relevant variation of the energy gap values for N450–N600 samples, in spite of the unvaried N amount.

the reaction: N þ Ti3þ ¼ N þ Ti4þ :

(3)

The relative energies in the gap of these paramagnetic centers were calculated by Di Valentin and co-workers and support the previous hypothesis [10]. Alternatively and more simply at higher temperatures the annealing in air oxidizes Ti3+ centers. As observed for the amount of N centers in samples annealed at increasing temperatures (Table 2), it seems to us that nitrogen gradually diffuses into the lattice either substituting oxide in the titanium coordination or binding to oxide, bridged to two Ti centers. At 500 8C, N begins to segregate from the lattice of TiO2 as described by XPS analysis (see later). Consequently the amount of

3.3. Electron spin resonance investigation Fig. 5 reports the EPR spectra of N350–N600 samples. At the lowest annealing temperature, N350 sample (Fig. 5 line a) shows strong and broad asymmetrical resonance lines at g = 1.97, while sharp lines at g = 2.003. Increasing the annealing temperature up to 450 8C, the resonance lines at g = 2.003 become more intense and reveal the existence of a triplet of lines at distance of 32.3 G; the signal at g = 1.97, though less intense, is still well detectable (Fig. 5 line b). After annealing at 500 8C, the resonances at g = 1.97 disappear and those at g = 2.003 slightly decrease in intensity (Fig. 5 line c). Finally, when the thermal treatment was carried out at 600 8C, a new signal with rhombic g tensor (gxx = 2.001, gyy = 1.998, gzz = 1.927) occurs (Fig. 5 line d). Signals centered at g = 1.97 are attributable to Ti3+ centers, with a d1 electronic configuration [23], more likely caused by the anatase reduction due to N inclusion into the oxide lattice; those centered at g = 2.003 and showing a triplet structure, due to interaction of the unpaired electron with the 14N nucleus, may be associated to the species N. This is the paramagnetic center, firstly described in detail by Giamello et al. [10] originated from the ionization of N precursor defects. The resonance lines observed in samples annealed at 600 8C are those of molecular NO, presumably included in the cavities of bulk oxide [24]. A relation may be suggested between Ti3+ and N centers, basing on the intensity of the respective resonance lines: increasing the temperature from 350 to 450 8C the number of N centers increases (Table 2) while that of Ti3+ centers decreases. The decrease of Ti3+centers is probably due to electron transfer by

Fig. 5. EPR spectra recorded at 123 K after 10 min of Vis irradiation: (a) N350; (b) N450; (c) N500; (d) N600.

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Table 2 Binding energy (BE) and elemental atomic ratios for a representative Blank and N-doped TiO2 samples. Sample

Ti 2p3/2 BE (eV)

N 1s BE (eV)

N(b)/N(a) Atomic ratio

Ntot/Ti Atomic ratio

N paramagnetic species Spin/g (1015)

Nparam./Ti Atomic ratio (106)

Blank N350

459.9 459.4

– 0.13

– 0.31

– 5.8

– 0.78

N450

459.3

0.12

0.35

9.8

1.6

N500

(i)459.6 (ii) 456.6 (i)458.8 (ii) 457.2

400.2 (a)400.0 (b) 397.8 (a)400.1 (b) 397.1 (a)400.3 (b) 397.1 (a)400.2 (b) 397.9

0.23

0.22

8.4

1.4

N600 a

1.08

0.81

a

4.5

0.74a

NO species.

N decreases and, at 600 8C, the presence of molecular NO trapped in bulk oxide becomes evident. In a parallel way the major change in the energy gap, observed from the reflectance spectra, appears in N450, N500 and N600. The shift in the energy gap value, calculated from the reflectance data, is not exclusively associated to the presence of N centers, which in fact are a very low percent of the total N amount (see Table 2). N centers, precursors of N, should reasonably have the major responsibility of the gap shift. Irradiation with visible light leads to an increase of 30% in the intensity of N resonances, the light inducing the one electron excitation [10]: N þ hn ¼ N þ e

(4)

Besides, it cannot be excluded that also the transition N þ hn ¼ N þ e

(5)

may be involved in the process [10]. The highest N signal intensity is reached by the sample annealed at 450 8C (Table 2). Basing on previous observation, it could be suggested that

In N500 and N600 samples, a second less intense Ti 2p doublet becomes evident, having Ti 2p3/2 BE around 457 eV. This is indicative of a stronger decrease of ionic character of the Ti4+ center, possibly related to an increase in the number of nitrides in the coordination sphere of titanium centers at the surface of the catalyst. This effect is also evidenced by the increase of Ntot/Ti atomic ratios (Table 2) which ranges from 0.2–0.3 to 0.8, probably due to surface segregation of nitrogen at the highest annealing temperature. However, the formation of nitride phase can be excluded [19]. Fig. 7 reports the low intensity spectra in the N 1s region for N350–N600 samples (Table 2). Also Blank TiO2 shows N 1s peak at 400.2 eV (Table 2—not reported in Fig. 7), due to traces of ammonium chemisorbed at the surface, residual from the hydrothermal synthesis. All the spectra of N-doped samples show a broad band which can be fitted by two contributions. The peak at about 400 eV is attributable to nitrogen included into TiO2 lattice, interacting with both Ti and O centers [6,15,19]. The second peak at lower BE, around 397 eV, is attributable to N mainly interacting with Ti centers; this result is complementary to the behaviour observed in Ti 2p3/2 spectra [6,15,19]. The ratio between the two nitrogen species changes with the annealing temperature (Table 2); specifically, the intensity of signal

(i) the irradiation primarily involves N species, whose energy is located some tenth of eV over the TiO2 valence band [10]. This allows the use of higher wavelengths for the photoactivation of the catalyst. (ii) N species, the center sensitive to light irradiation, is present in all samples and presumably promotes the hole trapping, under irradiation. This contributes to decrease the electron– hole recombination rate after irradiation and is in turn responsible for the enhanced photocatalytic activity of the oxide (see later). (iii) The annealing temperature affects the amount of the sensitive centers, being the N (N) increase in competition with the nitrogen segregation as molecular NO. 3.4. X-ray photoelectron spectroscopy investigation XPS measurements allow to determine both the amount and the electronic state of Ti and N at the surface of the catalysts. Binding energy (BE) and elemental atomic ratios are reported in Table 2. Fig. 6 shows the Ti 2p core level of Blank (a), of N450 (b) and N600 (c), as representative samples. Blank sample shows an intense signal splitted in two spin-orbit coupling components, whose the Ti 2p3/2 lies at 459.9 eV; the energy is in accordance with that of Ti 2p3/2 in bare TiO2 [5,6,15,17]. The BE of Ti 2p3/2 in N-doped TiO2 decreases with respect to Blank (Table 2 and Fig. 6). This can be attributed to the lower ionicity of Ti4+ centers, interacting with nitride anions, less electron acceptor than oxide anions.

Fig. 6. Ti 2p core level XPS spectra of (a) Blank TiO2 and representative N-doped TiO2, (b) N450 and (c) N600.

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Fig. 7. N 1s core level XPS spectra of N-doped TiO2 annealed at different temperatures: (a) N350; (b) N450; (c) N500; (d) N600.

at 397 eV increases with the temperature and is predominant at 600 8C, where it was extimated to be more than 50% of the total N. 3.5. Photocatalytic activity The photocatalytic activity of the N-doped TiO2 samples was measured in the phenol photomineralization by using O2 as oxidizing agent, under Vis irradiation (see Section 2). The catalyst performances were evaluated by fitting the experimental data (TOC vs. time) with the function CðtÞ ¼ C 0  A

Z 0

x

! ðt  t 0 Þ2 exp  dt s

(6)

where C(t) is the TOC amount measured at the t time and C0, A, t0, s are fitting parameters (C0 is the total organic carbon in solution before the phenol degradation and t0 the time value at which the mineralization of the organic substrate begins) [1–4]. In order to compare the mineralization kinetics for the different catalysts, the maximum degradation rate (dC/dt)max, which

Fig. 8. (a) Mineralization curves of phenol (given as TOC %) in the presence of Blanks and N-doped TiO2 catalysts using O2 as oxidative agents: ^ N350; ~ N450; & N500; * N600; ^ Blank350; ~ Blank450; & Blank500; * Blank600. dashed lines are the best fit to the experimental data according to Eq. (6). (b) Related plots of dC/dtmax for the same samples are reported vs. annealing temperatures (dotted bars: Blank samples; lined bars: N-doped samples).

correspond to the maximum slope point of the fitted curves, and the half transformation time t1/2, were taken as representative parameters. Fig. 8a shows the mineralization curves for all N–TiO2 samples and the Blank samples. The results in the histogram of Fig. 8b indicate that in N450 and N500 samples the photoefficiency is greatly improved by nitrogen doping, while it decreases and becomes comparable or slightly lower than Blank references in the remaining samples. The small decrease of the surface area resulting from nitrogen doping does not produce a decrease of photocatalytic activity, indicating that the different photoactivity is not due to surface limited process. For this reason the reactivity data were not normalized for the sample surface.

Fig. 9. Comparison between the trend of the absorption intensity at 440 nm (blue circles), the photoactivity, expressed as maximum degradation rate (dC/dt)max (histogram) and the amount of paramagnetic N species (, dotted line) for the N-doped samples annealed at different temperatures. For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.

M. D’Arienzo et al. / Applied Catalysis B: Environmental 93 (2009) 149–155

Fig. 9 describes the relation between photoactivity, optical properties in visible region and amount of N centers detected by EPR under Vis irradiation. The highest photoactivity of N450 and N500 corresponds to the maximum adsorption intensity in the visible range and to the highest concentration of N centers, indicating that both the properties promote the photocatalytic activity. On the other hand the lower photoactivity of N600, if compared with the other doped samples, can be associated to the absence of N centers, as revealed by EPR spectra. In a parallel way relevant amount of differently bound N centers was rather present (see Section 3.4).

155

titania. Thus the trapping of holes onto N centers shows to really hinder the hole–electron recombination and favours the catalytic efficacy. N600 contains a relevant amount of differently bound N centers, if compared with the other doped samples, whose binding energy reveals less ionic character than those attributed to N centers. This may be the reason of the deactivation of N600 sample, where less negative nitrogen centers have lower ability to trap photoinduced holes.

References 4. Conclusions The investigation carried out on N-doped TiO2 obtained by hydrothermal treatment of TiO2 in the presence of urea and finally annealed at different temperature shows that: (i) The increase in the annealing temperature favours the N insertion into the oxide lattice, may be either as substitutional or interstitial sites. The detection by EPR of Ti3+ centers, oxygen vacancies, suggests that the inclusion as substitutional centers is surely an active process. XPS data indicate the formation of a direct Ti–N bond, but exclude the presence of the Ti–N phase. (ii) Once included, N centers are able to modify the energy gap value, shifting the absorption of samples annealed at 450, 500, 600 8C towards the visible wavelength region. (iii) N dopant is almost full present as N centers, provided few percent of N detected by EPR which act as probe of the N presence. (iv) Under visible light excitation, N centers behave as hole traps, causing that the recombination rate of the hole–electron couple become slower. Thus the presence of nitride centers N (detected as N after light excitation), should be taken as a probe of the catalyst photoactivity. (v) The photoactivity of the catalysts, measured in the oxidative degradation of phenol by O2 in comparison with the Blank TiO2 samples, suggests that N450 and N500 are more active than the respective Blanks. As well as they show the maximum adsorption intensity in the visible range and the highest concentration of N centers. The trend analogy of EPR and optical absorption intensities suggests that both N and N are responsible of the photocatalytic efficiency in N containing

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