Microporous and Mesoporous Materials 132 (2010) 80–86

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Microporous and Mesoporous Materials

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Synthesis and structural characteristics of nitrogen doped TiO2 aerogels

M. Popa a,*, D. Macovei b, E. Indrea c, I. Mercioniu b, I.C. Popescu a, V. Danciu a

a Babes-Bolyai University, Faculty of Chemistry and Chemical Engineering, 400028 Cluj-Napoca, Romaniab National Institute of Materials Physics, Bucharest-Magurele, P.O. Box MG-7, 077125, Romaniac National Institute for Research and Development of Isotopic and Molecular Technologies, 400293 Cluj-Napoca, Romania

a r t i c l e i n f o

Article history:Received 17 June 2009Received in revised form 19 November 2009Accepted 24 December 2009Available online 4 January 2010

Keywords:N-doped TiO2 aerogelsSalicylic acidPhotocatalysis

1387-1811/$ - see front matter � 2009 Elsevier Inc. Adoi:10.1016/j.micromeso.2009.12.024

* Corresponding author. Tel.: +40 264 593833; fax:E-mail address: mip3pim@yahoo.com (M. Popa).

a b s t r a c t

Nitrogen doped TiO2 aerogels were obtained by sol–gel process followed by supercritical drying withliquid CO2. Urea and NH3 (25% aqueous solution) were used as nitrogen sources. The as-prepared aerogelswere submitted to thermal treatments performed under different experimental conditions (temperatureand heating time). Increasing the temperature from 450 �C to 550 �C a decrease about 32% and 15.4% ofthe particle size and surface OH groups’ concentration, respectively and an increase about 21.4% of theincorporated nitrogen in TiO2 lattice were observed. The immersion of the TiO2 aerogels in NH3 solutioninduced a decreasing by 48.86% and by 80% of the SBET and the aerogel porosity, respectively. The photo-catalytic activity of the obtained N-doped TiO2 was found to depend mainly on the nitrogen incorporationin the TiO2 lattice.

� 2009 Elsevier Inc. All rights reserved.

1. Introduction

Regarding the green world objectives TiO2 is considered as apromising material. It is non-toxic, inexpensive, presents a notableUV light absorption, as well as photo-corrosion resistance and oxi-dative properties. In the last few years TiO2 was widely used in so-lar cell [1,2], photo(electro)catalysis [3–5], etc. However, due to itslarge band gap (3.2 eV), TiO2 can absorb only �3% of the total solarradiation. In order to extend the light absorption to the visible re-gion of spectrum, TiO2 aerogels doped with various chemical spe-cies were prepared [6,7]. Nevertheless, the doping with transitionmetals has a controversial effect on TiO2, inducing a decrease ofits photocatalytic activity, due to either the thermal instability[7] or to an increase of the photogenerated electron–hole recombi-nation rate [8].

The beneficial effect of nitrogen on the photocatalytic activity ofTiO2 was described for the first time by Asahi et al. [7]. Subsequentstudies reported that nitrogen doped TiO2 exhibit absorption in thevisible region and showed an enhanced photocatalytic activity un-der visible light irradiation [9–11]. In the last years the chemicalnature of nitrogen doping centers and their influence on the bandstructure of TiO2 were also thoroughly investigated [12,13]. All re-sults lead to the conclusion that nitrogen effect on TiO2 depends onmany factors: the synthesis procedure (sol–gel, ion implantation,magnetron sputtering, oxidation of titanium nitride, etc.), the loca-tion of nitrogen species (NOx, substitutional N or NHx) in the TiO2

structure, the interaction between the N centers and oxygen

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+40 264 590818.

vacancies [12]. Usually, doping with NH3 was performed by usinggaseous NH3–Ar mixture [6,13–15] or/and by adding NH3 solutionin the reaction mixture during synthesis [16,17].

In this paper, TiO2 aerogels were obtained by sol–gel process fol-lowed by supercritical drying with liquid CO2, and, subsequently,were doped with nitrogen by using, as nitrogen sources, urea(added in the synthesis mixture) and NH3 aqueous solution (fordoping TiO2 aerogels). Taking into account that the structure ofaerogels obtained by supercritical drying technique is characterizedby a high surface area and porosity [18,19], it was consideredinteresting to correlate the synthesis conditions (especially thedoping procedure, the temperature and time of thermal treat-ments) with the morpho-structural properties and photocatalyticactivity of the resulted N-doped aerogels. X-ray diffraction (XRD),Fourier transformed infrared spectroscopy (FTIR), diffuse reflec-tance UV–Vis spectrophotometry (DRS), X-ray photoelectronspectroscopy (XPS), determination of the surface OH groups’concentration, and N2 adsorption–desorption isotherms wereemployed to achieve the structural characterization of the obtainedaerogel samples. The photocatalytic activity of the as-preparedN-doped TiO2 aerogels was estimated for salicylic acidphotooxidation.

2. Experimental

2.1. Samples preparation

The TiO2 samples were prepared by sol–gel process in acidcatalysis. Urea (99%; Aldrich, St. Louis, MO, USA) and NH3 inaqueous solution (25%; Viromet SA, Victoria, Romania) were used

Table 1Doping methods and thermal treatments of the obtained aerogels.

Sample Doping method Thermaltreatment (�C)

Time(min)

A Undoped TiO2 aerogel 530 10B Urea doped TiO2 aerogel 530 10A530-10 Sample A immersed in NH3 530 10A550-10 550 10A450-10 450 10A450-60 450 60B530-10 Sample B immersed in NH3 530 10G530-10 Undoped TiO2 gel immersed in NH3 530 10

M. Popa et al. / Microporous and Mesoporous Materials 132 (2010) 80–86 81

as nitrogen sources. For TiO2 gels synthesis the following reagentswere used: titanium (IV) isopropoxide (TIP) (>98%; Merck-Schuc-hardt, Darmstadt, Germany), anhydrous ethanol (EtOH; analyticalgrade, Aldrich), deionised H2O and HNO3 (analytical grade, 70%;Primexchim, Bucharest, Romania). Generally, the molar ratios wereused as follows: H2O/TIP = 3.75, EtOH/TIP = 21 and HNO3/TIP = 0.1.In one case, in order to obtain N-doped TiO2, 5% urea ethanolicsolution (using the molar ratio urea/TIP = 0.052) was added duringthe TiO2 gel synthesis. The synthesis was carried out at room tem-perature, under continuous stirring. TiO2 aerogels were dried usinga liquid CO2 dryer (SAMDRI-PVT 3D, Tousimis, Maryland, USA),operated in the following conditions: 0.1 kg/min fill rate; 3.5 hfor purging; 1 h for supercritical conditions (100 at, 40 �C); 1 hfor depressurizing.

The obtained undoped TiO2 gel/aerogel, and N-TiO2 aerogelswere immersed for three days in 25% NH3 solution (2.5 g/10 mLNH3 solution). In a first step, all samples were thermally treatedat 450 �C for 10 min (temperature was raised with a 60 �C/minslope, by using a CARBOLITE furnace, Sheffield, UK) Then, aerogelsamples were treated at different temperatures for different times(Table 1). All N-doped TiO2 aerogels, by immersion in NH3 aqueoussolution, exhibited an intense yellow color.

2.2. Morpho-structural characterization

X-ray diffraction (XRD) measurements were performed using anX-ray diffractometer (BRUKER D8 Advance, Karlsruhe, Germany)working at 45 kV and 30 mA. The Cu Ka radiation, Ni filtered,was collimated with Soller slits. The microstructural informationobtained by single X-ray profile Fourier analysis of the TiO2 anatasenanoparticles were the effective crystallite mean size (D) and theroot mean square (rms) of the microstrains, averaged along the[hkl] direction, he2i1=2

hkl [20,21]. The Warren–Averbach X-ray profileFourier analysis of the (1 0 1), (2 0 0) and (2 1 5) anatase peak pro-files were processed by the XRLINE [22] computer program. Theunit cell parameters were refined using the PowderCell software[23].

A JEOL 200 CX transmission electron microscope (TEM) (Wis-consin, USA) operating at an accelerating voltage of 200 kV as wellas the electron diffraction (SAED) was employed. Particle sizeswere measured from bright and dark field images whereas thephase content was investigated by using electron diffraction pat-terns. Samples were prepared by dipping a 3 mm holey carbon gridinto ultrasonic dispersed oxide powder in ethanol.

Diffuse reflectance UV–Vis spectra of the investigated powderswere recorded using a spectrophotometer with integrating sphereattachment (Jasco V-650, Tokyo, Japan). The relation between theabsorption coefficient a and incident photon energy hm for the indi-rect transition can be written as:

ða� hmÞ ¼ Aðhm� EgÞ2 ð1Þ

where h is the Planck’s constant, m is the radiation frequency, A andEg are a constant and the indirect band gap energy, respectively.

Band gap energy Eg was evaluated by extrapolating the linear partof the (a � hm)1/2 vs hm curve to the energy axis [24].

The X-ray photoelectron spectra (XPS) were recorded usingan ultrahigh resolution system (<7 meV) (PHOIBOS 150, Berlin,Germany). The X-ray source was a dual anode Al/Mg or Al/Ag usinga Focus 500 monochromator. Prior to analysis, the sample(100 mg) was palletized under a pressure of 100 kg/cm2, in orderto form a disk of 7 mm diameter.

The adsorption–desorption isotherms of the samples were re-corded using a surface area analyzer (Sorptomatic 1990, Thermo,Waltham, USA). Prior to the adsorption–desorption isothermsmeasurements, the investigated samples were heated at 120 �Cfor 2 h. The specific surface area (SBET) of the samples was calcu-lated by BET method using the N2 adsorption. The mesopore sizedistribution and mesopore volume were determined using theBarrett, Joyner, Halenda method.

Fourier transform infrared spectroscopy (FTIR) was performed,using an infrared spectrophotometer (Jasco 615, Cork, Ireland).

The determination of the surface OH groups’ concentrationwas spectrophotometrically done (at k = 354 nm), by measuringthe decreasing of initial concentration of iron acetilacetonate[Fe(acac)3, dissolved in toluene] due to its complexation by OHgroups ((S–O)3�xFe(acac)x), existing on the sample surface, accord-ing to the equation [25]:

ð3� xÞS—OHþ FeðacacÞ3 ! ðS—OÞ3�xFeðacacÞx þ ð3� xÞacacH

ð2Þ

where S stands for TiO2 surface (0.01 g S–OH/4 mL of 10�3 M Fe(a-cac)3) and acacH stands for the resulted acetylacetone.

2.3. Photoactivity measurements

The photocatalytic activity of the doped and undoped TiO2 aero-gels was determined by monitoring the photodegradation of sali-cylic acid (SA) (5 � 10�4 M), taken as standard pollutant.Photodecomposition experiments were performed in a Teflon cell,equipped with a quartz window for UV–Vis illumination, using ahigh-pressure Hg lamp (250 W, HBO OSRAM), having a sharp max-imum at 365 nm and a flat one at 477–530 nm.

The photodegradation profile was obtained by using spectro-photometric determination (k = 295 nm) of the SA concentration.The working temperature was 20 �C and the solution pH was 5.3.The ratio between N-TiO2 photocatalyst and SA solution was of16.6 g aerogel/1 L salicylic acid. Before UV–Vis irradiation, the or-ganic compound was allowed to adsorb on the catalyst surfacefor 30 min in the dark, till the equilibrium was reached. The de-crease of the salicylic acid concentration after adsorption on thecatalyst was about 4% from its initial concentration.

The intensity of the used UV–Vis light source was determinedby actinometry, using potassium ferrioxalate (K3Fe(C2O4)3)(absorption domain from 250 nm to 577 nm), according to the fol-lowing equation [26]:

FeðC2O4Þ3�3 þ hm! Fe2þ þ 2C2O2�4 þ C2O��4 ð3Þ

where Fe2+, C2O2�4 , C2O��4 stand for ferrous, oxalate and oxalate rad-

ical ions.The quantity of formed Fe2+ was spectrophotometrically esti-

mated, using phenanthroline (k = 510 nm). The intensity ofthe light absorbed by the system was estimated at 2.26 � 10�6

Einstein/min. The photonic efficiency (n) of SA was determinedusing the equation [26]:

n ¼ SA moles reacted per time unit=Einstein absorbed per time unit ð4Þ

82 M. Popa et al. / Microporous and Mesoporous Materials 132 (2010) 80–86

3. Results and discussion

3.1. XRD measurements

The XRD patterns (Fig. 1A) support the conclusion that all sam-ples obtained under our synthesis conditions have mainly a crys-talline structure corresponding to tetragonal anatase. Thetetragonal rutile (1 1 0) diffraction line at 2h = 27.43� was only evi-denced by the deconvolution of the (1 0 1) anatase diffraction pro-file (Fig. 1B). This fact indicates the low crystallinity of the rutilephase and makes uncertain its quantitative evaluation, using thePowderCell program.

TEM images of the specimen A530-10 exhibit anatase nanopar-ticles having diameters between 10 and 15 nm and some rutileparticles (average size 40 nm) having a lamellar morphology(Fig. 2A). The structural information given by SAED is in good

Fig. 1. XRD spectra of the obtained aerogel samples (A); deconvolution of 1 0 1anatase peak (A-anatase, R-rutile) (B).

Fig. 2. TEM and SAED images of the sample A530-10.

agreement with X-ray diffraction data. The presence of rutile phasewas only seen by some isolated (1 1 0) spots observed at d = 2.50 Åbetween the successive (1 0 1) and (2 0 0) anatase rings (Fig. 2B).

The values of the anatase particles size are relatively high due tothe short time of the thermal treatments. When the temperature isincreased from 450 �C to 550 �C a decrease with about 32% of thegrain sizes is induced (Fig. 3). This phenomenon contradicts otherprevious results, where the increasing of the temperature favorsthe increasing of the grain size [21]. A possible reason could bethe starting process of transformation of the anatase particles torutile at the surface of the aerogel. The thermal treatment of about1 h at 450 �C decreases the mean size of the anatase particles withabout 55%, the sample (A450-60) having the smallest anatase par-ticle size among the investigated ones. Generally, a high sinteringtemperature causes the loss of dopants from the N-doped TiO2

nanoparticles [27]. Consequently, a short time of the thermal treat-ments was preferred in order to preserve the nitrogen on the TiO2

surface. The microstrain values hei1=2hkl could be identified as a mea-

sure of the lattice defects. They are strongly dependent on the tem-perature and the particle size (D), respectively. The microstrainincreases as the particles size decreases (Fig. 4). There are many

Fig. 3. Particles size and surface OH groups’ concentration evolution as function oftemperature.

Fig. 4. Microstrain evolution as function of particles size.

Table 2Lattice parameters a = b, c, unit cell volume V, the effective crystallite mean size, D(nm), and the root mean square (rms) of the microstrains, he2i1=2

hkl , of the anatasestructure phase of the aerogel analyzed samples.

Sample D (nm) Unit cell parameters Unit cellvolume V (Å3)

he2i1=2klh � 103

a = b (Å) c (Å)

A 17 3.79 9.50 136.4 8.1B 17 3.78 9.50 136.4 8.0A530-10 17 3.79 9.49 136.1 7.8A550-10 15 3.79 9.49 136.0 9.2A450-10 22 3.79 9.50 136.4 5.6A450-60 10 3.78 9.48 135.7 13.8B530-10 18 3.79 9.49 136.1 7.0G530-10 11 3.78 9.47 135.8 14.1

XRD maximum errors: 12% (D), 10% (microstrain), ±0.01 Å (cell parameters), 0.5%(cell volume).

394 396 398 400 402 404 406

0

1000

2000

3000

A450-10

Binding Energy (eV)

coun

ts/s

ec

394 396 398 400 402 404 406

0

1000

2000 A530-10

Binding Energy (eV)

coun

ts/s

ec

394 396 398 400 402 404 406

0

1000

2000

3000B530-10

Binding Energy (eV)

coun

ts/s

ec

Fig. 5. N 1s core-level spectra of

M. Popa et al. / Microporous and Mesoporous Materials 132 (2010) 80–86 83

mechanisms that can induce lattice microstrains, including vacan-cies, lattice bending and dislocations [21].

The changes of the c lattice parameter (Table 2) would demon-strate distortions of the TiO2 lattice possibly due to the nitrogenincorporation. The values of the c parameter show relatively unno-ticeable variations comparing with the undoped sample (A). Somedecreases could be observed for the samples (A450-60) and (G530-10) which could suggest that the presence of nitrogen is more pres-ent as inside positions of TiO2 lattice.

3.2. XPS analysis

The N 1s spectra of the N-doped TiO2 catalysts (Fig. 5) exhibit amain peak at about 400 eV (Table 3), initially attributed by Asahiet al. [7] to molecularly chemisorbed dinitrogen (c-N2). However,the N2 chemisorption on TiO2 at room temperature was questioned

394 396 398 400 402 404 406

0

1000

2000 A450-60

Binding Energy (eV)

coun

ts/s

ec

394 396 398 400 402 404 406

0

500

1000A550-10

Binding Energy (eV)

coun

ts/s

ec

394 396 398 400 402 404 406

0

1000

2000

3000

G530-10

Binding Energy (eV)

coun

ts/s

ec

the N-doped TiO2 catalysts.

Table 3Binding energies (eV) of the N 1s core-level in the investigated catalysts and relativeweights (%) of the nitrogen species (NS: substitutional nitrogen; NI: interstitialnitrogen; Na: adsorbed nitrogen species).

Sample NS (O–Ti–N) Na (NOx, NH3) NI (Ti–O–N)

A450-10 398.17.8%

400.092.2%

–

A450-60 398.110.5%

400.089.5%

–

A530-10 398.215.9%

400.184.1%

–

A550-10 398.413.6%

400.170.8%

401.215.6%

B530-10 398.06.6%

399.987.1%

401.06.3%

G530-10 398.05.8%

399.982.8%

401.611.4%

Fig. 6. Diffusive reflectance spectra of the obtained samples.

84 M. Popa et al. / Microporous and Mesoporous Materials 132 (2010) 80–86

by Chen et al. [28] and Sato et al. [10]. They, as well as otherauthors [29–31], related the 400-eV peak to adsorbed NOx species,resulted by decomposition and oxidation of the nitrogen precur-sors, during the catalyst calcination. In the case of ammonia-basedprecursors, this peak could also be contributed by NH3 moleculesadsorbed on the surface [29,32].

All spectra presented in Fig. 5 show a low-energy component atabout 398 eV, in the proximity of the typical binding energies of396–397 eV in TiN [33–35]. Most of the previous XPS studies ofN-doped TiO2 ascribed the N 1s peak in the range 396–398 eV tothe Ti–N bonds resulted by atomic incorporation of nitrogen inthe TiO2 lattice, on oxygen sites [7,15,29,34,35]. This substitutionalconfiguration of nitrogen leads to O–Ti–N linkages, with Ti and Oas nearest and next-nearest neighbors, respectively, of the Natoms. The charge transfer from nitrogen to the more electronega-tive O2� ions, intermediated by Ti, shifts the N 1s peak to slightlyhigher energies with respect to the Ti–N bonds in TiN.

For the samples A550-10, B530-10 and G530-10, a high-energycomponent is present at 401.3 eV. It corresponds to oxidized nitro-gen in a Ti–O–N structure, with the N atoms accommodated in theinterstices of the TiO2 lattice and directly bonded to one or moreO2� ions [36–38]. The charge loss at the interstitial N site is largerthan that taking place in the substitutional nitride configuration(O–Ti–N), which moves the corresponding N 1s peak to higherenergies.

Therefore, for all the investigated samples, a fraction of nitrogen(NS) is incorporated into the TiO2 lattice by substituting O2� ions.For samples treated at higher temperatures (A550-10, B530-10,G530-10), N also locates in interstitial sites (NI). The total weightof the incorporated nitrogen (Ninc) increases with the treatmenttemperature (see Table 3). For the same time of treatment(10 min), the Ninc fraction (NS or NS + NI) increases by about twotimes at a temperature increase from 450 �C (sample A450-10) to530 �C (A530-10, B530-10, G530-10). Rather surprisingly, a furtherslight increase to 550 �C (A550-10) strongly enhances the Ninc frac-tion, which almost doubles with respect to the sample A530-10.The time of treatment less influences the incorporated N fraction.An extension from 10 min (A450-10) to 60 min (A450-60) promptsan increase of only 35% of the NS fraction (from 7.8% to 10.5%).

However, it is difficult to state by XPS if the larger Ninc fractionat higher temperatures really describes increased amounts ofnitrogen in the TiO2 lattice, or it is an effect of the desorption ofthe molecular species (NH3, NOx) and the corresponding loweringof the 400-eV peak, as previously observed by Sato et al. [10].

The behaviour in the range 530–550 �C of the sample preparedfrom TiO2 aerogel is rather peculiar. Up to 530 �C (samples A450-10, A450-60, A530-10), N occupies substitutional sites only, whilea slight temperature increase to 550 �C (A550-10) compels an

important amount of nitrogen to interstitial sites, with almostequal NS and NI weights. The above results point out that the inter-stitial location seems to be favoured by higher temperatures (530–550 �C), possibly due to the partial TiO2 transition to the rutilesymmetry and achievement of a local structure around Ti moresuitable to the NI location. The substitutional configuration is pres-ent in a wider range of temperatures, between 450 �C and 550 �C,apparently without specific requirements.

This disagrees with the study of Peng et al. [39], relating thesubstitutional doping to high annealing temperatures (600 �C, un-der NH3 flow), while the interstitial location is thought to beachieved by treatments at low temperatures or even without anyannealing. However, the N 1s peak at 399.9 eV, assigned by theauthors to the interstitial N, could actually include contributionsfrom molecularly adsorbed N-containing species.

These contradictory results once more demonstrate the depen-dence of the spectral features on the specific preparation condi-tions, making difficult unequivocal conclusions with regard tothe N preference for a certain structural location.

3.3. UV–Vis diffuse reflectance measurement

Diffusive reflectance spectra measurements, performed ondoped and undoped samples (Fig. 6), showed that all N-doped sam-ples exhibit absorbance in the visible region (�430 nm), in corre-spondence with their intense yellow color. The spectrum of pureanatase TiO2 Degussa P25 (385 nm–3.23 eV) was considered as ref-erence for the doped and undoped TiO2 aerogels. The band gap val-ues are given in Table 4. The band gaps of the N-doped TiO2

samples are lower than those of the undoped samples. Qiu andBurda [27] underlined that the absorption in the visible region ofthe N-doped samples could be due to the color centers obtainedby doping rather than by narrowing the band gap. Localized Nstates within the band gap and Ti3+ defects can also induce thesame absorption red shift [26]. Di Valentin et al. [40] consider thatoxygen vacancies (Vos) (experimentally estimated at 0.75–1.18 eVbelow the conduction band of TiO2) contribute to the overall visi-ble-light photoactivity of the N-TiO2 samples (Vos traps electronsand produce F-type color centers). The removal of an oxygen atomfrom TiO2 lattice leaves two electrons to form neutral F centers, or

Table 4Morpho-structural characteristics and photocatalytic activity of the aerogel samples.

Catalyst Bandgap(eV)

Mesoporesmean radius(nm)

Porevolume(cm3/g)

[OHsurface](mmol/g)

SBET

(m2/g)n(104)

A 3.04 4.5 0.5 0.70 131 1.01B 2.97 4.9 0.4 0.69 103 1.06A530-10 2.80 2.1 0.1 0.57 67 1.26A550-10 2.84 2.1 0.1 0.55 71 1.08A450-10 2.80 1.9 0.2 0.64 91 0.99A450-60 2.82 2.1 0.1 0.65 99 1.22B530-10 2.86 2.0 0.1 0.38 78 1.32G530-10 3.02 4.6 0.3 0.71 105 1.26TiO2 Degussa (P25) 3.23 6.9 [48] – 0.42 42 0.50

SBET errors: ±5 m2/g.

M. Popa et al. / Microporous and Mesoporous Materials 132 (2010) 80–86 85

they may be trapped by neighboring Ti4+ species to form Ti3+colorcenters [41].

3.4. FTIR studies

FTIR studies were performed in order to determine the possiblepresence of some functional groups on the catalyst surface. Thepresence of OH groups and water on the surface are evidencedby the bands around 3400 cm�1, 3162 cm�1 and 1623 cm�1

(Fig. 7). The spectral features are more visible for the undopedsample (A) and the doped sample (B). The surface OH peak corre-sponding to 3408 cm�1, attributed to bridging hydroxyl groups[42], is drastically diminished after TiO2 immersion in NH3 solu-tion. An interaction between acid surface OH groups and NH3 solu-tion could be possible. However, no adsorbed NH3 or its relatedions were detected in the FTIR spectra, possible as a result of thethermal treatment applied after sample immersion in NH3 solu-tion. According to Lin et al. [43], the peak around 1391 cm�1 wasattributed to vibrational mode of hyponitrite. In our case the peakposition was found around 1396 cm�1. The presence of 1396 cm�1

peak in the bare TiO2 as well makes implausible this assumption,considering NH3 as a generating source. The nitric acid, used asacid catalyst during the synthesis process, could be a more reliablesource for hyponitrite formation.

3.5. Determination of surface OH groups concentration

The hydroxyl groups directly participate in the photodegrada-tion reaction mechanism by trapping the photogenerated holesthat reach the catalyst surface (producing very reactive surfaceHO�), by being as active sites for pollutant adsorption, and by cov-

Fig. 7. FTIR spectra of the obtained samples.

ering the sites which trap electrons by the adsorbed oxygen. Thisprocess hinders the electron–hole recombination [44].

The concentration of total surface OH groups (acid and basic)[OHsurface], determined by their complexation with iron acetilacet-onate, is presented in Table 4. A high amount of surface OH concen-tration was found in the undoped sample (A), urea doped sample(B) (in accord with FTIR results) and NH3 immersed TiO2 gel sam-ple (G530-10). The TiO2 wet gel immersion in NH3 solution (sam-ple G530-10) seems to be more favorable for the surface OHgroup formation. Comparing with the undoped sample (A), sample(B) and sample (G530-10), a slight decreasing of [OHsurface] can beremarked for the rest of all NH3 solution immersed TiO2 aerogels.

As expected, the concentration of surface OH groups stronglydecreases when the temperature of thermal treatment increases(Fig. 3).

3.6. Pores characterization

Adsorption–desorption isotherms of the investigated sampleswere determined in order to obtain the specific surface area andthe pores size distribution (Fig. 8). According to the IUPAC classifi-cation (1985), the physisorption isotherms of doped and undopedTiO2 aerogels correspond to type IV with the H2 hysteresis loop. Inthis case, the pore structures are complex and tend to be made upof interconnected networks of pores of different sizes and shapes[45].

Mesopores mean radius, pore volume and specific surface arepresented in Table 4. A decreasing of the porosity and SBET of theNH3 immersed TiO2 aerogels can be observed. A possible reasoncould be the collapsing effect of NH3 solution on TiO2 aerogel struc-ture. A higher SBET of the sample G530-10, comparing with the restof the NH3 impregnated samples, could be explained by the factthat the TiO2 wet gel was immersed in NH3 solution and thus,the collapsing effect on aerogel structure was diminished.

3.7. Photocatalytic activity

Photonic efficiency was determined in order to evaluate thephotocatalytic activity of the samples (Table 4).

The photocatalytic activities of the samples A530-10, B530-10,A450-60, G530-10, A550-10 are higher with respect to the un-

Fig. 8. The adsorption–desorption isotherm and pore size distribution of thesample B530-10.

86 M. Popa et al. / Microporous and Mesoporous Materials 132 (2010) 80–86

doped TiO2 (sample A). The correlation of modified TiO2 structureby nitrogen presence and photocatalytic activity is a complex issue.The anatase structure is generally known as favoring the photocat-alytic activity [46,47]. Small anatase particles size and a high con-centration of surface OH groups could also favor the photocatalyticactivity. The presence of nitrogen in TiO2 lattice sensitizes andmakes N-doped TiO2 samples more photoactive in the visible range(UV–Vis determinations). The substitutional (N–Ti–O) (398 eV)and interstitial positions (�400 eV) of the nitrogen could also con-tribute at the increasing of photocatalytic activity in the visible re-gion [27,33,34].

The samples A530-10, G530-10 and B530-10 exhibit the highestphotocatalytic activity among all the investigated samples. A highamount of nitrogen incorporated in substitutional and interstitialpositions of TiO2 lattice, a higher SBET led to the highest photocat-alytic activity of sample B530-10. Sample A530-10 exhibits aslightly smaller photoactivity than sample B530-10, in contrastwith its higher number of surface OH groups, smaller anatase par-ticle size and higher microstrain, respectively. Sample G530-10 is aparticular case. In spite of many favorable factors (�1.63 timessmaller particle size, higher porosity and surface area than sampleB530-10, the highest surface OH concentration among of all theinvestigated samples), it did not show the highest photocatalyticactivity. Urea addition in TiO2 synthesis (sample B) showed unno-ticeable increase of the photocatalytic activity, with respect of un-doped TiO2. The photocatalytic activity of the sample B increasesabout 1.3 times after its immersion in NH3 solution (sampleB530-10), reported to the undoped TiO2. A maximum of photocat-alytic activity was obtained by increasing the temperature from450 �C (10 min) to 530 �C (10 min). The lowest photocatalyticactivity of the sample A450-10 might be due to its highest particlesize comparing with the rest of the samples.

4. Conclusions

N-doped aerogels, prepared by sol–gel technique and supercrit-ical drying, were compared with undoped TiO2 regarding theirmorpho-structural properties and the photocatalytic activity forsalicylic acid photodegradation.

XPS measurements revealed the presence of nitrogen incorpo-rated in the TiO2 lattice, in substitutional (O–Ti–N) and interstitial(O–N–Ti) locations, with the latter sites seemingly favored byhigh-temperature treatments. The temperature and the time ofthe thermal treatment had a crucial effect on the particles sizeand microstrains, as well as on nitrogen incorporation and surfaceOH groups’ concentration. Increasing the temperature from 450 �Cto 550 �C a decrease about 32% and 15.4% of the particle size andsurface OH groups’ concentration, respectively and an increaseabout 21.4% of the incorporated nitrogen in TiO2 lattice were ob-served. The immersion of the TiO2 aerogels in NH3 solution in-duced a decreasing by 48.86% and by 80% of the SBET and theaerogel porosity, respectively.

The samples treated at 530 �C, obtained by N-doping of TiO2

with urea and NH3, had the highest photocatalytic activity(�n = 1.32 � 10�4), although had moderately values of surface OHgroups’ concentration, SBET and porosity. The photocatalytic activ-ity of the obtained N-doped TiO2 was found to depend mainly onthe nitrogen incorporation in the TiO2 lattice.

Acknowledgment

This research was financed by Romanian Research Ministry (PNII 71-136).

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