Micro-mesoporous N-doped brookite-rutile TiO2 as efficient catalysts for water remediation under UV-free visible LED radiation

Journal of Catalysis 346 (2017) 109–116

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Micro-mesoporous N-doped brookite-rutile TiO2 as efficient catalysts for water remediation under UV-free visible LED radiation Oluwadamilola Pikuda, Corrado Garlisi, Gabriele Scandura, Giovanni Palmisano ⇑ Department of Chemical and Environmental Engineering, Masdar Institute of Science and Technology, PO BOX 54224, Abu Dhabi, United Arab Emirates

a r t i c l e

i n f o

Article history: Received 27 November 2016 Revised 16 December 2016 Accepted 18 December 2016

Keywords: N-doped brookite-rutile Water remediation Visible photocatalysis

a b s t r a c t N-doped brookite-rutile catalysts were prepared using the sol-gel method with different nitrogen precursors, namely urea (CH4N2O), propionitrile (C3H5N), ammonium hydroxide (NH4OH), ammonium nitrate (NH4NO3) and ethylene diamine (C2H8N2). Testing the photoactivity of the prepared catalysts with 4nitrophenol revealed that ammonium nitrate was the best doping agent with a nominal N-content of 0.8% (w/w), yielding a 5-fold increase in the pseudo-first order constant of 4-nitrophenol disappearance and a 3-fold increase in the pseudo-first order constant of TOC disappearance, with respect to undoped TiO2, when irradiated with LED visible light (>425 nm). In the same experimental conditions, commercial catalysts such as Evonik P25 and mixtures of commercial rutile and brookite failed to work. XRD allowed to identify two crystal phases, i.e. brookite and rutile, and to show that the most active catalyst had the highest brookite and lowest amorphous content, along with the largest rutile crystallites. Through HRTEM, the morphology and crystallinity were further investigated: brookite particles were much smaller and roundish with respect to rutile and the intimate contact between the two phases was also well highlighted. N-doping did not produce oxygen vacancies as shown by Raman spectroscopy; thus, the doping can be considered interstitial rather than substitutional. Surface hydroxylation did not promote oxidation ability, as revealed by TGA-DTA: the most reacting catalyst is the least hydroxylated one. BET revealed that the samples are partially mesoporous (type IV hysteresis), although no template/surfactant was used, and the pore size and volume seemed to affect their activity. UV-vis DRS allowed to extrapolate the band gaps, only slightly narrower for N-doped samples, which, however, showed a pronounced absorption of visible radiation compared to undoped TiO2. Photoluminescence showed that the emission due to electron-hole recombination decreases with the N-loading, eventually reaching a minimum plateau for doping amounts just above the optimal one. Ó 2016 Elsevier Inc. All rights reserved.

1. Introduction In recent years, the use of Titanium dioxide (TiO2) in photocatalysis has become of great environmental impact, especially in air and water remediation. TiO2 is a highly photoactive, stable, non-toxic and cheap semiconductor, as both powder and thin film, being able to degrade adsorbed contaminants. During light irradiation, electron-hole pairs are generated which diffuse to the surface of the catalysts and initiate oxidation reactions [1]. Under ultraviolet (UV) irradiation, TiO2 possesses a strong oxidation power that allows various organic pollutants to be broken down and converted into CO2 and H2O [2–4]. Many techniques have been reported in the literature for the modification of TiO2 to trigger its photoactivity under visible light, ⇑ Corresponding author. E-mail address: [email protected] (G. Palmisano). http://dx.doi.org/10.1016/j.jcat.2016.12.010 0021-9517/Ó 2016 Elsevier Inc. All rights reserved.

which is widely available and safe compared to UV. Heteroatom doping is among the most popular ones [5]. Doping with nitrogen may encourage the formation of oxygen vacancies, promoting the activity under visible light by acting via electron traps, upon excitation of electrons from the valence band to the energy levels of oxygen vacancy sites. Nitrogen can replace oxygen in the TiO2 crystal lattice resulting in N-doped TiO2 [4,5]. Electrons interact with atmospheric oxygen while the vacant sites interact with water or other available radicals to generate reactive oxygen species, which in turn activate oxidation reactions [6–8]. On the other hand, the introduction of interstitial rather than substitutional sites, can be also generated by N impurities, in this case without resulting in the formation of any oxygen vacancy, yet ending up again with a great ability to absorb visible light. Interstitial doping can be differentiated from substitutional with the aid of X-Ray photoelectron spectroscopy, if the doping occurs close enough to the surface of TiO2 nanoparticles rather than in their bulk: specifically, N 1s peaks

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at ca. 397 eV and 400 eV are generally attributed to substitutional and interstitial nitrogen, respectively [9]. It is well known that TiO2 occurs in nature in 3 common polymorphs, i.e. anatase, rutile and brookite, and these three have different properties influencing photoactivity. While the rutile phase is the most thermodynamically stable, photocatalytically active titania is available predominantly in the anatase phase. Undoped anatase and brookite have a band gap of ca 3.2 eV, which makes them active under UV light only. The band gap of rutile is slightly smaller (ca. 3.0 eV) [9]. Nevertheless, rutile crystals are usually less photoactive, whereas anatase-rutile composites can result in a better activity in comparison with pure anatase thanks to an improved electron-hole separation at the crystals interface due to their different electronic properties [10]. The photoactivity of pure anatase, doped anatase and doped anatase-rutile has been extensively reported in the literature. Jang et al. [11] showed that the anatase phase is the most active phase of TiO2 and Porter et al. [12] suggested further that the high photoreactivity of anatase is due to grain morphology and the peculiar crystal structure of this phase. Brookite is structurally similar to anatase but, in addition, it possesses highly active junction sites. According to Kominami et al. [13], this results in it having better electrochemical potentials to trigger the oxidation of pollutants. Investigations on mixed phases of titanium dioxide revealed that the synergic effect of a mixture improves the interfacial charge transfer reactions with respect to pure phase catalysts, and this significantly promotes photoactivity [14–20]. Zhang et al. [17] showed that anatase-brookite mixed phases exhibit higher photoreactivity than pure phases. Moreover, the contact between rutile and brookite fosters the interfacial charge transfer thanks to the different energy levels of their valence and conduction bands [14]. Although S,N,C-codoping of triphasic anatase-rutile-brookite has been recently investigated [17], N-doping of biphasic brookite-rutile composites has never been assessed so far, and this is the aim of the present work, where the samples were prepared by using a sol-gel technique with the aid of hydrochloric acid. Benchmark Evonik P25 and mixtures of commercial rutile and brookite were used for comparison purposes. The photocatalytic activity of the prepared catalysts (undoped and nitrogen doped) was tested by using 4-nitrophenol as model pollutant, under irradiation with UV-free visible LED sources. The decrease in total organic carbon was also monitored to understand the extent of mineralization. A deep characterization of the prepared catalysts has been performed by means of XRD, Raman spectroscopy, XPS, HRTEM/SAED (selected area electron diffraction), TGA-DTA, BET porosimetry, UV/vis DRS, and photoluminescence (PL).

2. Experimental A sol-gel method was used to prepare TiO2 by using Titanium butoxide (TBOT) as the precursor. TBOT, 2-propanol and HCl (4 M) in a volumetric ratio of 1:5.461:1.283, respectively, were mixed and the obtained clear solution (total volume: 116.24 mL) was kept under stirring (1200 rpm) in a sealed 250 mL Pyrex bottle at 70 °C for 20 h to allow for hydrolysis and condensation. The formed 1-butanol, the solvent (2-propanol) and residual water in the resulting suspension were evaporated in a rotary evaporator at 70 °C and the resulting powders were annealed at 450 °C for 4 h. N-doped samples were prepared by using the appropriate mass of urea (CH4N2O), propionitrile (C3H5N), ammonium hydroxide (NH4OH), ammonium nitrate (NH4NO3) and ethylenediamine (C2H8N2), dissolved in the aqueous HCl solution prior to the mixing with the TBOT alcoholic solution. Only the cationic moiety was considered as able to dope in NH4NO3. Undoped TiO2 was also prepared to serve as a control sample during the experiments. The

samples were labeled as TiO2_X_Y% where X indicates the Nsource and Y the precursor weight% of nitrogen with respect to TiO2. The irradiation system employed for the reactivity study consisted of a 33 W flat visible LED source, under which a 400 mL beaker was placed containing 250 mL suspension. Fig. S1 (Supporting Information) reports the lamp emission spectrum showing a 425 nm irradiation threshold. A magnetic stirrer operating at 1800 rpm was used during the tests. The radiation impinging the free surface of the suspension was 160 W/m2 (in the 450–950 nm range, evaluated with a radiometer Delta Ohm 9721 and the matching probe) and a Pyrex glass was placed over the suspension to prevent evaporation. Before performing the reactivity tests, each catalyst sample was dispersed (by 5 min ultrasound treatment) into 250 mL DI water under irradiation, in gradually increased amounts, to obtain a final transmitted radiation intensity from the bottom of the beaker of ca. 10% with respect to the transmitted one in the presence of only DI water; this amount was the one used during the subsequent photocatalytic tests so as to ensure that all the particles were irradiated during the runs. Table S1 reports the used amount of catalyst for each sample. Reactivity tests were run in the presence of 5 mg L
1 of 4-nitrophenol and 10 mg L
1 of phenol (only for selected samples). Oxygen was bubbled into the suspension for 30 min in the dark before turning the lamp on, in order to get a saturated solution and to induce the adsorption/desorption thermodynamic equilibrium, and it was kept on during all the runs. Samples were withdrawn periodically from the reactor by using a syringe and catalyst was immediately removed with a 0.2 lm PTFE filter. The total irradiation time was 6 h. The extent of degradation of 4-nitrophenol was measured as a function of the visible absorbance of each liquid sample at a wavelength of 315 nm by using a Perkin Elmer UV-VisNIR Spectrophotometer (Lambda 1050). The extent of Total Organic Carbon (TOC) decrease was measured with a Shimadzu TOC-L Total Organic Carbon analyzer fed with a zero-air gas cylinder. The quality of DI water was good enough (15 MX) to ensure a TOC blank value always below 0.1 ppm. The preparation of TiO2_NH4NO3 samples with doping percentage between 0.6% and 1.0% was repeated three times from new batches of chemicals. The light absorbance and photoreactivity tests were carried out with the same experimental setup as described earlier. However, to measure more accurate concentrations, the decrease of 4-nitrophenol was analyzed also by using a Thermo Scientific HPLC equipped with Dionex UltiMate 3000 Photodiode Array Detector. An Acclaim-120 C18 Reversed-phase LC column was used for all analyses with a mobile phase consisting of 33% (v/v) distilled water, 33% (v/v) acetonitrile and 34% (v/v) methanol with a isocratic flow rate as 0.2 mL min
1. The same conditions were used to analyze phenol. X-Ray diffraction was used to determine the crystalline structure of the prepared catalysts using XRD PANalytical Empyrean diffractometer, a Cu Ka radiation of 1.54 Å, scan step-size 0.0167° and a 2h scan range of 10–90°. The software, HighScore Plus, with standard diffractograms 01-087-0920 for rutile and 00-016-0617 for brookite was used to identify the crystal phases [18]. The crystallite size was determined by using the DebyeScherrer equation with respect to the rutile and brookite major peaks at 27.52° and 30.86°, respectively, after the diffractograms background had been subtracted using the default background subtraction mode on the HighScore Plus software. Raman spectroscopy was run on catalyst powders to confirm the XRD identification and to investigate the occurrence of shifts due to oxygen vacancies. The analyses were carried out in a confocal Witec Alpha 300R Raman spectroscope. Samples were excited with a 532 nm laser having a single mode output power of 52 mW and the scattered light was collected in a back scattering

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configuration with the aid of a 50 objective lens. All Raman analyses were carried out at room temperature, with an integration time of 0.5 s and 100 accumulations. The signals were registered in the range 50–700 cm
1. TEM was performed to understand the morphology and dimensions of the particle crystals, and to identify rutile and brookite particles through selected area electron diffraction (SAED) analysis. TEM samples were prepared by suspending 1 g L
1 of the catalyst in 2-propanol, treating by ultrasounds for 5 min, and dropping 2 L for three times on a Forvar/Carbon 300-mesh Cu grid (purchased from Tedpella); finally, the solvent was let evaporate at room temperature. The analysis was performed by using a Tecnai G2 transmission electron microscope, operating at 200 kV. The plot-profile function of ImageJ was used to evaluate the lattice dspacing, while SingleCrystal software was used to analyze the electron diffraction patterns of rutile. TGA/DTA analyses were performed by using a Netzsch STA 449 F3 thermal analysis equipment, in nitrogen flow. Samples of ca. 80 mg were used and the following temperature program was applied: the sample was first kept at 30 °C for 10 min, then temperature was increased from 30 °C to 120 °C at a rate of 5 °C min
1 and the sample was kept at 120 °C for 15 min. Afterward, temperature was increased from 120 °C to 750 °C at a rate of 10 °C min
1. The temperature was then decreased from 750 °C back to 30 °C at a rate of 50 °C min
1. UV-vis DRS was performed by using a Perkin Elmer UV-Vis-NIR Spectrophotometer (Lambda 1050) equipped with an integration sphere. The adsorption/desorption isotherms of the catalysts were determined by using a Quantachrome NOVA 2000e surface area and pore size analyzer by using N2 as adsorbent. Before the analysis, degassing of the samples was achieved in static conditions under vacuum at 400 °C for 4 h. Specific surface area was determined by using the multipoint BET method, as an average of adsorption and desorption values in the low pressure range (P/ P0 = 0–0.35). The adsorption curve in the whole range of pressures was fitted with two different models (Barrett-Joyner-Halenda, BJH and Density Functional Theory, DFT) for the determination of the pore size distribution. Photoluminescence emission spectra (PL) of the samples were measured using a Perkin Elmer LS 55 Fluorescence Spectrometer equipped with a front surface sample holder, where the powders were positioned. PL measurements were then recorded on dry powders with the following parameters: excitation wavelength: 300 nm; scanning speed of 300 nm min
1; excitation slit width: 2.5 nm; emission slit: 5.0 nm. All the chemicals used were purchased from Sigma-Aldrich, including the benchmarks Evonik P25, Rutile and Brookite powders.

Pseudo-first order rate constants obtained from the exponential regression on the decrease of 4-nitrophenol and TOC concentrations vs. irradiation time are shown in Tables 1 and 2. Indeed, as usual in photocatalytic oxidation, the exponential fitting gave a good R2 (> 0.9). The rate constants were normalized to the total surface area of the suspended powder, by assuming that all the surface is irradiated and active (as specified in the experimental section). Table 1 shows that TiO2_NH4NO3 gave the highest catalytic degradation of 4-nitrophenol and TOC decrease compared to other N-doped catalysts at 0.8% doping, with a good reproducibility (all reactivity tests were carried out at least three times). Other nitrogen sources did not produce such a significant effect, pointing to a probable ineffectiveness of the doping process. The reasons for the failure of the other dopants have not been investigated, but it is worth noting that the best dopant is also the only inorganic salt among all the used N-sources, and this could help in the incorporation of its nitrogen atoms into the crystal lattice of TiO2. Variation of the percentage of N-doping for TiO2_NH4NO3 further revealed that 0.8% is the optimal weight percentage of nitrogen doping for photoreactivity, as shown in Table 2. Notably the prepared catalysts, especially the most active one (TiO2_NH4NO3_0.8%), possess a strong ability to abate total organic carbon, thus being able to mineralize the organic pollutant to produce carbon dioxide and resulting in water remediation under pure visible light. The results show that the kinetic constant for TOC degradation is about half than that of 4-nitrophenol degradation, and this can be considered a satisfactory result, especially by taking into account the low energy of visible photons with wavelengths greater than 425 nm. The undoped sample was also active, and this can be tentatively explained by hypothesizing (i) an efficient electron exchange among large rutile (with narrower band gap) and small brookite particles, in intimate contact given their sol-gel simultaneous synthesis (see TEM images in the following and in Fig. S7, Supporting Information), and (ii) a carbon sensitization due to residuals coming from the titanium alkoxide that was used as precursor during the catalysts preparation, as highlighted by the XPS spectra of undoped TiO2 (Fig. S2, Supporting Information).

Table 1 Pseudo-first order constant for the conversion of 4-nitrophenol per unit surface area of catalysts for bare and 0.8% weight N-doped TiO2 during 6 h irradiation. Sample

k, 4-nitrophenol (10
3 h
1 m
2)

k, TOC (10
3 h
1 m
2)

Undoped TiO2 TiO2_C3H5N_0.8% TiO2_NH4OH_0.8% TiO2_C2H8N2_0.8% TiO2_CH4N2O_0.8% TiO2_NH4NO3_0.8%

2.69 ± 0.15 1.23 ± 0.15 3.53 ± 0.01 2.37 ± 0.01 4.07 ± 0.12 12.2 ± 0.10

2.00 ± 0.01 0.31 ± 0.01 1.77 ± 0.21 1.68 ± 0.02 0.81 ± 0.01 6.10 ± 0.01

3. Results and discussion The photocatalytic activity of the prepared samples was assessed by degrading 4-nitrophenol, used as a model water pollutant, being extremely stable in the absence of either irradiation or catalyst, and very resistant to conventional wastewater treatment. Dark adsorption of 4-nitrophenol on the catalysts was negligible, whereas irradiation with a UV-free visible LED source produced 4-nitrophenol oxidation for all the home-prepared catalysts. Evonik P25, one among the most widespread TiO2 samples, was used as a benchmark reference, and it showed to be totally inactive toward the oxidation of 4-nitrophenol by irradiating with wavelengths greater than 425 nm. Rutile and brookite from SigmaAldrich were also tested in a 50:50w/w mechanical mixture, and no reactivity was found either.

Table 2 Pseudo-first order constant for the conversion of 4-nitrophenol per unit surface area of catalysts for varying doping percentages of NH4NO3 during 6 h irradiation. Sample

k, 4-nitrophenol (10
3 h
1 m
2)

k, TOC (10
3 h
1 m
2)

Undoped TiO2 TiO2_NH4NO3_0.4% TiO2_NH4NO3_0.6% TiO2_NH4NO3_0.7% TiO2_NH4NO3_0.8% TiO2_NH4NO3_0.9% TiO2_NH4NO3_1.0% TiO2_NH4NO3_1.2% TiO2_NH4NO3_1.6%

2.69 ± 0.15 2.32 ± 0.48 2.16 ± 0.17 3.19 ± 0.01 12.2 ± 0.10 3.88 ± 0.58 2.37 ± 0.19 2.57 ± 0.21 3.85 ± 0.13

2.00 ± 0.01 2.56 ± 0.29 1.27 ± 0.01 2.00 ± 0.15 6.10 ± 0.01 2.57 ± 0.71 1.48 ± 0.19 1.74 ± 0.01 1.65 ± 0.33

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Given the significant reactivity difference among TiO2_NH4NO3_0.7%, TiO2_NH4NO3_0.8%, and TiO2_NH4NO3_0.9%, other catalysts batches were prepared and re-tested. Results derived from this further investigation confirmed the initial ones, as shown in Supporting Information (Fig. S3) where the comparison among the rate constants obtained from two different batches of powders is reported. Three independent analytical techniques (HPLC, UV-vis absorbance and TOC) were used. Furthermore, the same catalysts were tested by using a different pollutant, i.e. phenol, doubling the concentration to 10 mg L
1 under same experimental conditions. The results shown in Table S2 (Supporting Information) fully confirm the superior performance of TiO2_NH4NO3_0.8%. The XRD patterns of representative catalysts are shown in Fig. 1, while patterns for all prepared samples are presented in Figs. S4–5 (Supporting Information). The major diffraction peaks that were detected are at 2h 25.63°, 27.52°, 30.86°, 36.17° and 54.46°. Some of the characteristic peaks of anatase (such as 75.12°) are absent while all major peaks of rutile and brookite are present. In particular, the peaks at 2h = 27.52° and 2h = 30.86° are characteristic of rutile (1 1 0) and brookite (1 2 1). Other observed characteristic diffraction peaks of rutile are at 2h = 41.34°, 54.46°, 56.77° and 69.18°, while the main peaks of brookite are at 2h = 25.63° (resulting from the overlapping of two diffraction peaks), 30.86° and 48.11°. It can be observed that no peaks can be seen in the doped catalysts, which are not present in bare TiO2. This confirms that the crystal structure of the doped samples is similar to that of the undoped TiO2 sample. Changing the nitrogen source did not affect the crystal phases (see Fig. S4, Supporting Information). As qualitatively shown by the relative intensity of the main peaks of rutile and brookite, doping seems to foster the formation of latter one, probably because the nucleation and growth mechanisms of rutile (the most stable thermodynamic phase of TiO2) are detrimentally affected by the introduction of a heteroatom in the crystalline

Fig. 1. XRD patterns of representative catalyst samples.

Fig. 2. Raman Spectra of representative catalyst samples obtained by excitation at 532 nm.

lattice. The sharper peaks of rutile suggest a high crystallinity with larger crystals compared to brookite, for both the undoped and doped samples. Using full-width at half-maximum (FWHM), Debye-Scherrer’s equation [19] was applied to the rutile peak at 2h = 27.52° and to the brookite peak at 2h = 30.86°. The calculated values are reported alongside other particle properties for representative samples in Table 3, where the average crystallite size for rutile was 30 nm, while that of brookite was 12 nm (Table S3, in Supporting Information, shows the properties of all the prepared catalysts). The average crystallite size for rutile in the sample TiO2_NH4NO3_0.8% (which was shown to be the most reactive) was the highest (33.8 nm), whereas the brookite size for the same sample was among the lowest ones. This significant difference can facilitate charge separation at rutile/brookite boundaries, where electron can be exchanged between the conduction bands of the two crystal phases, finally boosting reactivity. The crystal phases found in this work are in agreement with previous studies, such as Reyes-Coronado et al. [16] showing that the use of HCl for the hydrothermal preparation of catalysts from Titanium (IV) butoxide results in a fraction of the TiO2 being brookite, whereas anatase is generally prepared using Titanium (IV) isopropoxide [9,21] or TiCl4 [9] as precursors. The percentages of brookite and rutile, obtained from the very recent method proposed by Bellardita [22] and Jensen’s method [23], respectively, show that the most reactive catalyst has the highest brookite and the lowest amorphous contents (Table 3). Raman bands for representative catalysts, shown in Fig. 2, were clearly observed for brookite at 154 (A1g), 251 (A1g), 327 (B1g), 371 (B2g), 416 (A1g), 454 (B2g), and 640 (A1g) cm
1 [23]. Also, the signals corresponding to rutile, i.e. A1g, Eg and B1g bands at 145, 447 and 612 cm
1, can be seen in the positions suggested in the literature [24]. The high number of the signals characteristic for brookite, however, hampers a rigorous identification of the exact

Table 3 Structural properties of representative catalysts.

a b c d

Sample

Rutile (%)a

Brookite (%)b

Amorphous (%)c

Brookite size (nm)d

Rutile size (nm)d

Undoped TiO2 TiO2_NH4NO3_0.7% TiO2_NH4NO3_0.8% TiO2_NH4NO3_0.9% TiO2_NH4NO3_1.6%

14.6 13.2 18.5 20.2 18.9

15.0 9.8 20.0 11.0 17.6

70.4 77.0 61.5 68.8 63.5

12.7 12.2 12.2 10.8 12.6

32.3 33.2 33.8 27.4 28.7

Calculated by using Jensen’s method [23]. Calculated by using Bellardita’s method [22]. Calculated by difference. Estimated by using the Debye-Scherrer’s equation [19].

O. Pikuda et al. / Journal of Catalysis 346 (2017) 109–116

frequencies of rutile bands. For instance, the shoulder before the strong and broad band at 153 cm
1 and its asymmetric shape are due to the overlapping of different rutile and brookite signals [23,25]. Importantly, the characteristic band of anatase at 515 cm
1 is absent in all the spectra, thus confirming that this phase is not a constituent of the prepared samples. Thus, Raman spectroscopy results are in agreement with XRD results as far as the identification of the crystal phases is concerned (see also Fig. S6, Supporting Information). No significant shift was observed in the frequency of all the peaks in the spectra, indicating that no oxygen vacancies were introduced during the doping process. This lack of shifts suggests that the N-doping is interstitial rather than substitutional [26]. Thus, it can be concluded that the formation of Ti–O–N bonds is more likely than N–Ti–N for the prepared catalysts, based on previous studies on N-doping effects [27]. However, it must be stressed that during thermal annealing, a significant part of the nitrogen source can be degraded leaving the catalyst structure, and preliminary surface characterization carried out with XPS (Fig. S2, Supporting Information) showed that the nitrogen% is as low as 0.25% independent of the nitrogen loading present in the catalyst precursor and supposed to be due mainly to atmospheric nitrogen. The color of the samples, on the other hand, is very different and increasingly darker with nitrogen loading. On the basis of this, bulk doping is deemed more probable than surface doping, since XPS depth sensitivity is as low as 10 nm, whereas Raman laser radiation penetrates through the samples with depths of microns, so the latter method in some cases can be more effective in distinguishing between interstitial and substitutional doping and in assessing oxygen vacancies. All the samples appeared characterized by two kinds of particles: (i) rod-like, sized between 20 and 200 nm and (ii) roundish, with a size of 5–20 nm (Fig. S7, Supporting Information). TEM pointed to a good contact between the two kinds of crystals; thus,

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a charge separation occurring along their boundaries can be hypothesized (see for instance Fig. S7(b), Supporting Information), promoted by the different sizes of the two crystal phases. Nevertheless, the determination of the position of the valence and conduction bands is not easy for this complex system (N-doped brookite-rutile with amorphous phase). The morphology of brookite and rutile crystal phases was investigated by TEM micrographs, in the sporadic areas where the two kinds of particles were not in contact. Fig. 3a shows the Selected Area Electron Diffraction (SAED) pattern – identified as zone axis [1 1 0] of rutile – corresponding to the 80 nm-long rod-like particle represented in Fig. 3b. The plot profile function on ImageJ software was used to draw the gray-intensity plot in Fig. 3c and evaluate the d-spacing of the crystal fringes. The averaged d-spacing is 3.29 nm and it can be identified as (1 1 0) rutile plane [27,28]. The smaller more roundish particles shown in Fig. 4b are crystalline as highlighted by the SAED spots shown in Fig. 4a. The identification of these particles through SAED was possible despite their small sizes, by drawing the circles corresponding to the main diffraction plans of brookite; a very good fitting between the circles and the electron diffraction spots can be noticed. Moreover, the average d-spacing value of 3.62 nm ascribable to (2 1 0) or (1 1 1) planes [29] of brookite allows to further confirm the identification of the small roundish particles (Fig. 4 (c-d)). Notably, no d-spacing corresponding to the main crystal fringe of rutile was detected in any of the observed small particles. Fig. 5 shows the adsorption-desorption isotherm of TiO2_NH4NO3_0.8%, while textural features of representative samples are shown in Table 4 (a complete overview is available in Table S4, Supporting Information). The isotherm shape was similar for all the catalysts (Figs. S8–12, Supporting Information for the isotherms of representative samples), and they exhibited a hysteresis of type IV at relative pressures greater than 0.65, highlighting a predominant mesoporous structures with capillary condensation

Fig. 3. TEM characterization of rutile particles for TiO2_NH4NO3_0.8%: (a) SAED, (b) Single Particle TEM micrograph, (c) plot profile of atomic fringes, (d) HRTEM micrograph.

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Fig. 4. TEM characterization of brookite particles for TiO2_NH4NO3_0.8%: (a) SAED (highlighting brookite diffraction planes), (b) single particle TEM micrograph, (c) plot profile of atomic fringes, (d) HRTEM micrograph.

Fig. 5. Adsorption-desorption isotherms for undoped TiO2 (inset: pore radius distribution by using BJH model).

as confirmed by the pore size distribution where pores always exceed 2 nm. Table 4 reveals that the pore size and the total pore volume of the sample TiO2_NH4NO3_0.8% are among the highest ones. Thus, porosity plays a relevant role in determining the sample reactivity through a more favorable adsorption on the surface area available in bigger pores. The specific surface areas are in the range of 45–64 m2 g
1. Shown in the inset of Fig. 5, the pore

volume distribution of the catalyst is presented by using the Barrett-Joyner-Halenda (BJH) model. Also, the Density Functional Theory (DFT) model allowed a rough evaluation of the distribution in the microporosity range (Figs. S8–12, Supporting Information). Of course, both models are not specifically developed for nanoparticles made of TiO2, but they allow to assess the pore sizes by getting a tendency trend at increasing loadings of nitrogen and for different nitrogen sources. Table 5 reports the weight percentage loss due to humidity, as well as surface OH groups, corresponding to different ranges of temperatures, for representative catalyst samples (a complete overview is available in Table S5 and Figs. S13–14, Supporting Information). For all the samples, the weight loss due to humidity is relatively small and in the range of 1.6–3.2% and this is not a significant parameter in determining the catalyst reactivity. The extent of hydroxyl groups weakly bound to the catalyst surface is very similar for all the catalysts at various loadings of dopant. On the other hand, TiO2_NH4NO3_0.8% showed the lowest extent of hydroxyl groups strongly bound to surface. This suggests that extent of surface hydroxyl groups is, in the present case, not correlated with the catalyst oxidation power, as highlighted in other studies [30,31]. Moreover, the oxidizing radicals are not generated exclusively from surface OH groups, but rather from the reaction of an oxygen molecule or water with an electron in the conduction

Table 4 Textural properties of catalysts from N2 adsorption-desorption isotherms. Sample

Specific surface area (m2 g
1)

Pore half-width (DFT) (nm)

Pore radius (BJH) (nm)

Average pore size (nm)

Total pore volume (cc g
1)

Undoped TiO2 TiO2_NH4NO3_0.7% TiO2_NH4NO3_0.8% TiO2_NH4NO3_0.9% TiO2_NH4NO3_1.6%

54.9 63.5 57.6 56.3 49.1

4.65 4.65 4.66 3.90 4.67

7.07 6.96 7.11 5.15 7.09

7.16 6.47 7.27 6.80 7.89

0.194 0.219 0.216 0.194 0.199

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O. Pikuda et al. / Journal of Catalysis 346 (2017) 109–116 Table 5 Humidity and surface OH groups’ extent determined by TGA. Sample

Humidity (%) [25–120 °C]

OHweak (%) [120–300 °C]

OHstrong (%) [300–600 °C]

OHtotal (%) [120–600 °C]

Undoped TiO2 TiO2_NH4NO3_0.7% TiO2_NH4NO3_0.8% TiO2_NH4NO3_0.9% TiO2_NH4NO3_1.6%

1.64 1.85 1.75 1.98 3.19

0.56 0.51 0.43 0.48 0.48

0.52 0.44 0.28 0.40 0.37

1.08 0.95 0.71 0.88 0.85

Fig. 6. Diffuse reflectance spectra (a) and digital photograph (b) of undoped and doped TiO2 samples. The legend in (a) indicates the doping %.

band or a hole in the valence band, respectively, producing superoxide radicals, H2O2, and hydroxyl radicals. Notably, as usual for TiO2 samples, the lower extent of surface hydroxylation of TiO2_NH4NO3_0.8% corresponds to a higher degree of crystallinity and, hence, of oxidizing strength. UV-visible diffuse reflectance spectra recorded in the 250– 800 nm range are shown in Fig. 6 (see also Figs. S15–16, Supporting Information). Doping with NH4NO3 shows that the best reacting catalyst (doped with 0.8%) has the lowest absorbance at wavelengths >425 nm – whereas samples doped with a lower or higher amount of nitrogen have a stronger absorbance. A large fraction of the absorbed radiation, thus, does not necessarily imply the gener-

ation of electron-hole couples able to produce a reactive event, because recombination can also take place, giving rise to photoluminescence phenomena with thermal energy emission. Optical band gap values were calculated through the onsets of the plots of the modified Kubelka-Munk function, that is F(R10 ) hv½ for indirect semiconductors, versus the energy of the exciting light, as shown in Figs. S17–18, Supporting Information. It should be stressed that the total ability of absorbing visible radiation was extremely improved in N-doped samples (Fig. 6), although the optical band gaps did not differ much due to similar reflectance thresholds. The calculated band gaps were all in the range 2.95– 2.99 eV (Table S6, Supporting Information).

Fig. 7. (a) Photoluminescence emission spectra; (b) emission maxima as a function of N-doping. N-source: NH4NO3.

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Fig. 7a shows photoluminescence (PL) spectra of undoped TiO2 and representative N-doped samples with increasing doping percentages, after background subtraction. The main emission peak was found around 420–425 nm and it was attributed to the band-to-band transition occurring due to the migration of electrons from the conduction band back to the valence band of the semiconductor. Emission at lower energy is mainly due to excitonic PL resulting from lattice defects of the crystal, especially present in nanostructured materials, as in our case. N-doping did not introduce new PL bands, consistently with previous studies [32,33], and the main emission band is reduced by increasing the dopant loading above 0.4% and up to 0.9% (see emission maxima in Fig. 7b). A loading amount smaller than 0.4% is not enough to produce any appreciable change in electron-hole recombination. Interestingly, the optimal doping amount, showed to be 0.8% in the previous sections, has an emission maximum just before the plateau, as illustrated in Fig. 7b. An excess of nitrogen source in the catalyst precursor did not change the PL but, as previously showed, was detrimental for the activity performance for the reasons given above, mainly related to crystal structure and textural properties. 4. Conclusions All the prepared rutile-brookite samples showed micro- and mesoporosity. Doping with nitrogen fostered the visible absorption and the photocatalytic reactivity under UV-free visible radiation, although oxygen vacancies were not introduced, thus suggesting an interstitial rather than a substitutional doping. The best working nitrogen dopant among the investigated ones is NH4NO3 and the optimal weight percentage of N was found to be 0.8% (w/w). This nitrogen-doped photocatalyst demonstrated strong abatement of TOC (total organic carbon) and, thus, a possible application in water remediation. XRD showed that, while all samples contain brookite-rutile mixed phases, the morphology of brookite is very different compared to rutile and – by considering the intimate contact between the two phases – this can improve the effectiveness of charge transfer. Notably, the best performing catalyst was the one with the highest brookite and lowest amorphous contents among all the samples. Bigger pore sizes and total pore volumes are deemed to promote reactivity, whereas higher hydroxylation of sample surface was not beneficial at all, since the least hydroxylated sample was the most reactive one. An investigation on PL emission highlighted that N-doping induces a decrease in the emission maxima, up to a given dopant loading which is ca. 0.9%. Further investigation will be carried out in the future to assess the electronic properties of the catalysts by using specific techniques such as ultraviolet photoelectron spectroscopy and X-ray emission spectroscopy. Finally, investigation on the active radical species responsible for the degradation is another point worth of future studies that can be done through electron paramagnetic resonance. Acknowledgments Dr. Mustapha Jouiad is gratefully acknowledged for the help provided in the interpretation of TEM results. Cyril Aubry, Thomas

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