Influence of calcination parameters on the synthesis of N-doped TiO2

by the polymeric precursors method

Margaret Dawson a, Gabriela Byzynski Soares b,c,1, Caue Ribeiro c,n,2

a Department of Materials Engineering, Universidade Federal de São Carlos, Rodovia Washington Luis KM 235 SP-310, São Carlos CEP 13565-905,São Paulo, Brazilb Department of Chemistry, Universidade Federal de São Carlos, Rodovia Washington Luis KM 235 SP-310, São Carlos CEP 13565-905, São Paulo, Brazilc Embrapa Instrumentação, Rua XV de Novembro 1452, São Carlos CEP 13560-970, São Paulo, Brazil

a r t i c l e i n f o

Article history:Received 8 January 2014Received in revised form21 March 2014Accepted 31 March 2014Available online 13 April 2014

Keywords:PhotocatalysisTitanium dioxideNitrogen dopingDye degradationHerbicide degradation

a b s t r a c t

In this paper, the influence of calcination parameters on the synthesis of N:TiO2 catalysts obtainedthrough the polymeric precursors method was evaluated. The powders were prepared by annealing Ti4þ

precursor resins at different temperature-time conditions in air, resulting in powders with differentdegrees of crystallinity for N doping, which was done by adding urea to the as-prepared powders andcalcining in N2 atmosphere. The N doping process resulted in band gap narrowing of TiO2 and, varyingannealing temperature and time, can be an alternative method for preferential formation of substitu-tional N or interstitial N. It was found that the percentage of interstitial N increased with an increase inannealing temperature, resulting in the complete absence of substitutional N at 400 1C. The photo-catalytic performance of the powders was evaluated using Rhodamine-B and Atrazine solutions underultraviolet and visible irradiations. The coefficients revealed that interstitial N had a positive correlationto both ultraviolet and visible photoactivity. In contrast, substitutional N showed a negative correlation.Further, the ratio of substitutional N to interstitial N indicated a strong negative correlation to ultravioletlight photoactivity and no correlation to visible light photoactivity. However, substitutional N should becontrolled for better photocatalytic properties.

& 2014 Elsevier Inc. All rights reserved.

1. Introduction

The intrinsic properties of TiO2 such as chemical and thermalstability, low cost and non-toxicity have contributed to its popularuse for photocatalytic degradation of organic substances [1–3].Despite having all these properties, it also has a band gap of 3.2 eVwhich creates a limitation to the efficient use of the catalyst in thesolar spectrum, as solar light accounts for only 3% of UV light [4].For this reason, various attempts to extend the absorption spec-trum of TiO2 into the visible region have been investigated.

Impurity doping of TiO2 with transition metals, Fe [5], Au [6],Cu [7] and non-metals C [8], N [9,10] and S [11] has been successfulin improving photocatalytic activity under visible light irradiation.Although doping with metals has often been promoted, it presentsmajor drawbacks such as thermal instability and high recombina-tion centers that reduce photocatalytic activity [12,13]. The nonmetal doping approach especially with nitrogen (N), has since

attracted much attention after Asahi et al. [14] obtained N:TiO2

films by sputtering and discovered that substitutional N causesband gap narrowing of TiO2. After this discovery, a range of N:TiO2

synthesis methods has emerged: sol gel [15,16], ion implantation[17], pulsed laser deposition [18], hydrothermal synthesis [19], andplasma [20]. It is therefore evident that a key issue of concern ishow preparation methods and precursors can be manipulated toobtain an optimum dopant position (substitutional or interstitial)and N doping levels that will effectively extend the absorption ofTiO2 into the visible region and also increase photocatalyticactivity. Substitutional N is widely cited as responsible for bandgap narrowing and thereby enhancing visible photoactivity [21–26]. In contrast, studies have indicated that band gap narrowing isa necessary condition; however, it might not be sufficient toimprove visible photocatalytic activity. Several factors need to beconsidered such as N concentration, type of analyte (dye orcolorless compound), crystallinity of TiO2 and oxygen vacancies[27–30]. In fact, the N state responsible for visible-light photo-activity is still intriguing and open to debate [31]. Recent findingsindicate that interstitial N is a promising state for band gapnarrowing as well as visible photoactivity. Peng et al. [32]prepared N:TiO2 catalysts containing both interstitial and substi-tutional N states. They revealed that interstitial N showed better

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Journal of Solid State Chemistry

http://dx.doi.org/10.1016/j.jssc.2014.03.0440022-4596/& 2014 Elsevier Inc. All rights reserved.

n Corresponding author.E-mail addresses: missmargaretdawson@gmail.com (M. Dawson),

gabi.byzynski@gmail.com (G.B. Soares), caue.ribeiro@embrapa.br (C. Ribeiro).1 Tel.: þ55 16 2107 2800; fax: þ55 16 2107 2902.2 Tel.: þ55 16 2107 2902.

Journal of Solid State Chemistry 215 (2014) 211–218

photoactivity in the visible region than substitutional N. Ai et al.[33] also reported interstitial N as a photoactive state. Despitethese findings, there is limited information on how photocatalyticactivity is affected when both nitrogen states are present in asample.

Sol gel methods offer flexibility in terms of modification ofprecursors and doping processes for powders and films. Further,stochiometry as well as particle size of powders can be controlled[34]. For example, Jagadale et al. [35] synthesized N:TiO2 catalystsusing titanium tetraisopropoxide, hydrogen peroxide and ethyl-methylamine. The as-prepared TiO2 sol (5 at%) was calcined at300 1C for 24 h in air. Another method with different dopantprecursors (diethylamine, triethylamine and urea) was employedto synthesize N:TiO2 catalysts which were calcined at 800 1C in N2

atmosphere [36]. In the above mentioned sol gel methods, the Nprecursor is added to the sol and then calcined in air or additionalcalcination in N2 atmosphere is carried out. Calcination plays animportant role in the incorporation of N as it defines the crystal-linity and phase of TiO2 nanoparticles [37]. Few authors havereported preferential formation of substitutional and interstitial Nthrough calcination of TiO2 powders and TiO2(1 1 0) single crystalsrespectively in NH3 atmosphere at elevated temperatures [21,38].However, much attention has not been drawn to the possibility ofalso controlling the N states formed in TiO2 lattice by altering theannealing conditions (temperature and time) in air before sub-sequent treatment in N2 or NH3 atmosphere. Hence, the effect ofTiO2 crystallinity on the N state formed during doping deservesmore attention. In a recent paper, Soares et al. [27] prepared N:TiO2 catalysts using the polymeric precursors method, a modifiedsol gel process. In this method, urea ranging from 0.1% to 2%(weight of equivalent TiO2) was added to a TiO2 resin and calcinedat 450 1C for 2 h in air, obtaining N doping by diffusion. Thismethod is simple compared to previously reported methodsbeing that it does not require expensive reactants or specialcalcination conditions, but then controlling the doping process(i.e., preferential N incorporation—interstitial or substitutional) iscomplicated.

Therefore, the main focus of this work is to investigate theinfluence of calcination parameters on nitrogen doping of TiO2 andto associate the N states (interstitial or substitutional) found in theN:TiO2 catalysts with photocatalytic activity. In order to reach thisgoal, the N:TiO2 powders were characterized by Scanning electronmicroscopy, Diffuse reflectance spectroscopy, X-ray diffraction andRaman spectroscopy. The N state formed was investigated by X-rayphotoelectronic spectroscopy and the N:TiO2 powders were testedby photocatalytic oxidation of Rhodamine-B aqueous solutionunder Ultraviolet C (UVC) and visible (vis) irradiation. The respec-tive photocatalytic activity constants were used to evaluate theeffect of the N states in the samples on photocatalytic activity.

2. Experimental

The resin synthesis procedure was based on the polymericprecursor method [27]. Briefly, the as-prepared Ti4þ resin wasdivided into five parts, each part was annealed in air at a specifictemperature and time as follows: 450 1C for 2 h, 400 1C for 2 h,380 1C for 6 h, 350 1C for 12 h, 350 1C for 6 h. The resin annealed at450 1C for 2 h was chosen as a reference sample and was notsubmitted to N doping. For the N doping process, 2 wt% urea(equivalent to TiO2 weight) was added to each resultant powderand calcined in N2 atmosphere at 450 1C for 2 h. The N dopedsamples were identified according to their annealing temperatureand time in air: 400 1C for 2 h (SAM 1), 380 1C for 6 h (SAM 2),350 1C for 12 h (SAM 3), 350 1C for 6 h (SAM 4). The referencesample was designated as TiO2.

The phase composition of the samples was identified throughX-ray diffraction patterns recorded by a diffractometer (ShimadzuXDR 6000) with a Cu anode (λCu-Kα, 87¼0.154 nm), from2θ¼10–801, at 20 min�1. The average crystallite size was calcu-lated by the Scherrer's formula [39] and the theoretical surfacearea (S.A) was estimated using the Braunuer–Emmet–Teller (BET)method [40], through N adsorption isotherms at 77 K in micro-metrics ASAP 2000 equipment.

The morphology and size of the as-prepared powders werecharacterized by a JSM-6701F/JEOL Field emission scanning elec-tron microscopy (FESEM). The optical properties of the nanopar-ticles were studied by an Ultraviolet–visible–near infrared (UV–vis–NIR Cary 5G spectrophotometer) in diffuse reflectance mode.In order to verify the phase composition of the samples, Ramanspectra were collected with a FT Raman Bruker RFS100/S equip-ment, using the 1064 nm line of a 450 W YAG 89 laser. The changein the bonding state and surface chemical composition related to Ndoping were identified by X-ray photoelectron spectroscopy (XPS)using a UNI-SPECS UHV XPS system.

Rhodamine-B (2.5 mg L�1) solution and Atrazine (2.5 mg L�1)solution were used to study the photoactivity of the N:TiO2

powders. 2 mg of SAM 1 was dissolved in 20 mL of rhodaminesolution. In a separate beaker, a solution with 2 mg of SAM 2 wasprepared. The same procedure was carried out for the other3 samples (SAM 3, SAM 4, TiO2). In a separate test, Atrazine wasused following the same procedure. The solutions were thenirradiated with six UVC lamps (TUV Philips, 15 W, with maximumintensity at 254 nm). During the experiment, the solutions werehomogenized by magnetic stirring and maintained at a constanttemperature of 19 1C. A UV–vis spectrophotometer (Shimadzu-UV-1601 PC spectrophotometer) was used to monitor changes in thespectral intensity of the analytes before and after irradiation. Atintervals of 30 min or 60 min, 3 mL of each irradiated solution washomogenized and analyzed and the aliquot was recovered forfurther irradiation. The same procedure was carried out using avisible light source with six fluorescent lamps (Quality, 15 W, andmaximum intensity at 440 nm).

3. Results and discussion

Fig. 1A shows the X-ray diffraction patterns of the N:TiO2

powders. The characteristic peak (1 0 1) corresponding to anatasephase is present in all sample and its intensity increased asannealing temperature increased. The absence of rutile and TiNphase is observed, indicating that the synthesis method and N2

atmosphere did not induce anatase–rutile transformation. Also,the temperature–time conditions did not have a significant effecton the final phase of the samples after N doping.

However, the average crystallite size of the samples estimatedby the Scherrer's formula, in Table 1, revealed that the crystallitesize could depend on the temperature–time conditions of theannealed powders. For example, SAM 4 and SAM 2 show that asmall change in temperature can promote crystallite growth.Further, SAM 3, SAM 2 and SAM 1, show that annealing at a lowertemperature for long periods appear to have the same effect on thecrystallite growth as a short annealing time at a slightly highertemperature. Since the amount of N added in each synthesis (fromurea) was the same for all samples, it is assumed that residualcarbon from the Ti4þ polymeric resin could have also contributedto retardation of crystallite growth during N incorporation [41].

Raman measurements were carried out on the samples and thespectra are shown in Fig. 1B. All samples confirm anatase phasewith corresponding bands located at 144 cm�1, 399 cm�1,515 cm�1 and 639 cm�1. Surprisingly, SAM 4 with a band narrow-ing of 0.32 eV, did not show any clear active mode of anatase

M. Dawson et al. / Journal of Solid State Chemistry 215 (2014) 211–218212

phase, even though anatase was confirmed in this sample by XRDresults. The observed discrepancy can be associated with thesensibility of the Raman technique to sample color. In particular,this sample was very dark due to residual carbon. Once again, thepresence of rutile phase is not confirmed in the samples.

The BET surface area results are presented in Table 1. As can beseen, the samples annealed at 350 1C have higher surface areasthan the others. In fact, the agglomeration of fine particles at highannealing temperatures (SAM 1 and SAM 2) as observed in FESEMimages (Fig. 2) results in the reduction of surface area. The sameeffect is evident in samples annealed for a long period of time.

The micrographs (Fig. 2) demonstrate a predominate morphol-ogy of almost spherical nanoparticles in the form of aggregates.

They suggest that N doping did not modify particle or surfacemorphology, but rather particle size. The particle size estimatedfrom the micrographs is not in agreement with XRD measure-ments due to the presence of large aggregates made of finecrystallites.

The light adsorption spectra of the doped samples are repre-sented by Kubelka–Munk function F(R1) vs. wavelength plots(Fig. 3). These plots directly evaluate band gap energy of semi-conductors when certain conditions are satisfied. First, TiO2 isassumed to have an indirect band gap and second, the Kubelka–Munk function should be calculated using a diffuse reflectancedata. Satisfying these conditions, the band gap energies can beextrapolated (inset of Fig. 3), and are summarized in Table 1. Ingeneral, the doped samples present lower band gap energies thanpristine TiO2 (3.2 eV) varying from 2.88 eV to 3.15 eV, which canbe attributed to the incorporation of N into the lattice of TiO2. SAM4 exhibited a remarkable N induced band gap narrowing than therest of the samples; however it has an additional adsorptionshoulder at 380 nm to 420 nm (Fig. 3). It is assumed that thisadditional adsorption shoulder originated from residual carbondue to the incomplete pyrolysis of the Ti4þ resin at a lowannealing time and temperature.

The influence of the synthesis method on the chemical composi-tion of the doped samples was investigated through XPS spectra of Ti2p and N 1s (Fig. 4). It can be inferred from the Ti 2p spectra (Fig. 4A),that all doped samples, including pristine TiO2, presented twoprincipal peaks assigned to Ti (2p1/2) and Ti (2p3/2) transitions, withbinding energies shifted towards 464 eV and 458 eV, respectively[42,43]. It is possible to notice that at a higher annealing temperature(SAM 1), the Ti signal is shifted towards lower energies withreference to pristine TiO2, indicating the presence of TiON bonds[44]. The formation of TiON bonds suggests a surface modification ofTiO2 by N species. On the other hand, no Ti signal shift was observedfor SAM 4 annealed at a lower temperature. It reflects that thisannealing condition maintains the bulk structure and the Ti bonds ofthe doped sample similar to that of pristine TiO2. However, thepresence or absence of Ti3þ ions is influenced by the extent ofnitrogen incorporation [45].

The N 1s core spectra of the doped samples are shown inFig. 4C. The appearance of N 1s peak at 400 eV, accepted asinterstitial N doping, is evident in all samples [46], while substitu-tional N signal at 398 eV is present in all samples except SAM 1[47]. In order to investigate the effect of temperature on thechemical characteristics of the samples, XPS analysis was used toestimate the proportion of interstitial and substitutional N statespresented in Table 2 The proportion of interstitial and substitu-tional N was estimated by deconvoluting the N peaks, as shown forSAM 4 in Fig. 4B. It is interesting to note that the amount ofinterstitial and substitutional N in a sample depends on theannealing temperature and time prior to doping in N2 atmosphere.When annealing is done at temperatures close to anatase crystal-lization temperature, i.e., 450 1C, the formation of interstitial N canbe favored than substitutional N (SAM 3). In some cases, substitu-tional N can be completely absent (SAM 4). This suggests thatduring the N doping process, if the starting material is alreadycrystalline (stronger Ti–O bonds), the substitution of oxygen withN can be prevented and consequently the concentration of sub-stitutional N is lower. In contrast, low annealing temperaturefacilitates the formation of substitutional N due to the amorphousstructure of the precursor powder characterized by weak Ti–O bonds.

Substitutional N is reported as the principal N state thatcontributes to band gap narrowing due to the merging of N 2pstates and O 2p states in the valence band [14]. It is important tonote that SAM 4 has more substitutional than interstitial N states,which may have contributed to its significant band gap narrowing.

0 200 400 600 800 1000Raman Shift/ cm-1

Fig. 1. (A) The XRD powder patterns of (a) SAM 4; (b) SAM 3; (c) SAM 2 and(d) SAM 1, (B) Raman spectra between 250 cm�1 and 800 cm�1. Inset: Ramanspectra between 0 cm�1 and 1000 cm�1 of (a) SAM 1; (b) SAM 2; (c) SAM 3 and(e) SAM 4.

Table 1Crystallite size, surface area and band gap value of N:TiO2 nanoparticles.

SAM 01 SAM 02 SAM 03 SAM 04

BET (SA)/m2 g‐1 104.570.47 104.070.84 116.770.69 210.970.82Crystal Size/nm 7 8 7 5Band gap/eV 3.05 3.15 3.13 2.88

M. Dawson et al. / Journal of Solid State Chemistry 215 (2014) 211–218 213

Interstitial N, on the other hand, introduces deep energy levelsabove the valence band gap (0.41–1.44 eV) which may not con-tribute significantly to band gap narrowing [48].

The performance of the annealed powders after N incorpora-tion is shown in Fig. 5 for photo-oxidation of Rhodamine-B (Rhod-B) and Atrazine (Atraz). From the photocatalytic efficiency plots ofthe N:TiO2 catalysts, the pseudo rate order of the photodegrada-tion reaction was calculated. The reaction should be of the pseudofirst-order with respect to Rhod-B, and must obey Eq. (1)

v¼ �d ½Rhod� B�=dt ¼ k½Rhod� B�½AS� ð1Þ

where [AS] represents the photocatalyst's active sites. By substi-tuting k0 ¼k [AS] into Eq. (1) and integrating, we have

lnð½Rhod� B�=½Rhod� B�0Þ ¼ k0t ð2Þ

A plot of ln[Rhod-B]/[Rhod-B]0 as a function of t, forms astraight line whose slope is k. A plot of ln[Atraz]/[Atraz]0 as afunction of t also forms a straight line whose slope is k.

Rhod-B and Atraz present the same photocatalytic trend forUVC and UV–vis irradiation. In general, the kinetic rates for Rhod-B (dye) degradation for both visible and ultraviolet regions areaccentuated compared to Atraz (colorless). Soares et al. [27]reported that both compounds have a different degradationmechanism, which can influence the final photocatalytic efficiencyof N doped photocatalysts. The photocatalytic degradation ofRhod-B unlike Atraz, can undergo dye-sensitizing mechanisms,as such its k values are expected to be higher.

For nitrogen doping of TiO2, the formation of Ti–O–N bonds isassociated with interstitial N whiles O–Ti–N bonds are associatedwith substitutional N. The nitrogen bonds induce oxygen vacanciesand/or Ti3þ defects as a means of maintaining charge neutrality ofTiO2 [49–51]. Apart from maintaining charge neutrality, they canact as color centers [52]. The electronic structure of TiO2 is alsomodified by nitrogen doping. The modified electronic structureand defects contribute to the photocatalytic property of TiO2

especially under visible light irradiation.Several authors have reported the mechanism and contribution

of substitutional nitrogen to visible light photoactivity. Accordingto Asahi et al. [14], the mixing of N 2p states with O 2p states inTiO2 lowers the band gap, allowing the photoactivation of thecatalyst with visible light. However, density functional theory(DFT) calculation models by Valentin et al. [25] predict occupied

Fig. 2. Field emission scanning electronic microscopy images (A) SAM 3, (B) SAM 4 (C) SAM 2 and (D) SAM 1.

λ

F(R

)/u.a

. F(R

)/u.a

.

Fig. 3. UV–vis diffuse reflectance spectra (modified Kubelka–Munk function [F(R1)E]1/2 vs absorbed light energy of (E) of (a) SAM 1; (b) SAM 2; (c) SAM 4 and(d) SAM 3. Inset: UV–vis diffuse reflectance spectra of SAM 2 showing extrapolationof band gap energy,

M. Dawson et al. / Journal of Solid State Chemistry 215 (2014) 211–218214

N 2p localized states about 0.14 eV above the valence band edge.Therefore, visible light irradiation can excite electrons from theimpurity energy levels to the conduction band, where the forma-tion of superoxides anion radicals is possible through adsorbedoxygen. The holes in the nitrogen energy state can also react withOH� ions to form hydroxide radical ions. Superoxide anion andhydroxide anions are widely accepted as responsible for thedegradation of organic and inorganic compounds.

Another mechanism proposed for visible photoactivity isthrough oxygen vacancies [53,30]. It is argued that band gapnarrowing does not induce visible light activity but rather colorcenters or oxygen vacancies [11,52,54,55]. Oxygen vacancies canbe formed due to substitutional nitrogen doping [56,57]. Thevacancies can form localized energy levels about 0.8 eV belowthe conduction band shifting the absorption edge to lowerenergies [58]. Exited electrons can then be easily trapped by theempty oxygen states resulting in prolonged life of photogeneratedcharges [49]. Electron spin resonance test carried on NH3-heat-treated TiO2 samples showed that oxygen vacancies can serve astrapping sites for adsorbed O2 for the production of superoxideanions. It is also reported that the quantum efficiency for photo-catalytic reaction could be higher as the affinity of O2 molecules tooxygen vacancies is high [59]. Oxygen vacancies can induce Ti3þ

defects in TiO2 or vice versa. Ti3þ defects have been also observedas possible species for visible photoactivity [60]. A model proposedbyWeyl and Forland [61] shows that Ti4þ cation is reduced to Ti3þ

when TiO2 crystal loses an oxygen atom. The loss of oxygen atomsis observed during substitutional N doping and consequentlyintroduces Ti3þ defects in TiO2. The surface defect Ti3þ formsdonor band just below the conduction band of TiO2 whichcontribute to visible photoabsorption in doped TiO2 [62]. Besides,there can be a reduction of surface-absorbed oxygen on TiO2 activesites leading to the formation of superoxide ions. Sun et al. [63]reported that the syngertic effect between nitrogen species andTi3þ species leads to visible light activity of nitrogen doped TiO2

samples with low concentration of nitrogen.For interstitial N doping, the impurity levels are formed by the

mixing of O 2p and Ti 3d states [64]. The energy state of interstitialN was proposed to be 0.73 eV above the valence band, which ishigh as compared to substitutional N [65]. It then requires lessenergy to excite photo-induced electrons from these impuritylevels to the conduction band. Defects such as oxygen vacanciesand Ti3þ can also be formed with interstitial N doping [66,67] andtheir roles in visible photocatalytic activity are similar to that ofsubstitutional N. However, the position of the energy level(0.73 eV) gives interstitial N an advantage.

As to whether which nitrogen state is good or bad for visibleactivity photocatalytic is still an open question since substitutionaland interstitial nitrogen can equally improve visible light activity ifthe photogeneration of charges and redox processes are dominantthan the recombination of photogenerated charges. Therefore, anoptimum quantity of interstitial N and substitutional N is requiredto increase photoactivity and at the same time reduce therecombination rate of charges [68]. The same applies to oxygenvacancies and Ti3þ centers. Furthermore, the properties of theanatase phase such as surface area, particle size and phasecrystallinity could affect the performance of nitrogen doped TiO2

catalyst.TiO2 presents superior u.v.a than the doped samples for both

Atraz and Rhod degradation. As discussed above, nitrogen dopingresults in narrow band gap and defects (Ti3þ and oxygen vacan-cies). Therefore during UV irradiation, excitation occurs in both the

Fig. 4. High-resolution XPS spectra of (A) Ti 2p of (a) TiO2, (b) SAM 1, (c) SAM 2(d) SAM 4 and (e) SAM 3 and (B) XPS spectra of N 1s of SAM 4 showing thedeconvolution process (a) total N, (b) substitutional N and (c) interstitial N; (C) XPSspectra of N 1s of (a) TiO2, (b) SAM 1, (c) SAM 2, (d) SAM 3, (e) SAM 4 normalized.

Table 2Total N content, quantity of interstitial N (%), quantity of substitutional N (%) andratio of substitutional N to interstitial N for the N:TiO2 samples.

TotalN

InterstitialN (%)

SubstitutionalN (%)

SubstitutionalN/interstitial N

TiO2 1.00 100 0 0SAM 01 1.49 100 0 0SAM 02 0.27 89 11 0.12SAM 03 3.12 50 50 1.01SAM 04 24.19 32 71 2.24

M. Dawson et al. / Journal of Solid State Chemistry 215 (2014) 211–218 215

valence band and the localized N 2p levels for doped samples,while excitation occurs only in the valence band for pristine TiO2.Then, the quantity of holes in pristine TiO2 will be less ascompared to the doped samples. As such, the lower photoactivityof the doped samples is attributed to greater recombination andreduced mobility of photogenerated charges [69]. However, thegap between SAM 1 and TiO2 presented in Figs. 5 and 6 drawsattention to the possibility of obtaining N:TiO2 catalysts withoutsignificantly altering the u.v.a properties of TiO2. The effect ofannealing on the ultraviolet photocatalytic activity (u.v.a) isevident in the doped samples prepared at higher temperatures(SAM 1 and SAM 2). The crystallinity of the samples could improvephotoactivity. The same effect was reported for N-doped TiO2

prepared by a sol–gel method using NH4Cl followed by calcinationat 300 1C, 400 1C and 500 1C, for UV photodegradation. Thephotocatalytic activity increased with the increase of calcinationtemperature from 300 1C to 500 1C assigned to anatase phasecrystallinity [70]. According to Tryba et al. [71] crystallinity mayimprove OH radical generation of anatase samples.

The analysis of the N states distribution from Table 2 and therate constants presented in Fig. 6, show that SAM 1 (0% substitu-tional N) presents the highest u.v.a. in both substrates (Rhod andAtraz). For SAM 2 with substitutional N as low as 11%, u.v.a of thissample is almost half the value of SAM 1. It is an indication thatinterstitial N enhances u.v.a and the amount of substitutional Nneeds to be controlled for better u.v.a. It is important to notice that

the substrate propriety (color) does not interfere in the photo-catalytic trend of the doped samples.

For photocatalytic reactions under visible light irradiation, SAM3 and SAM 1 were identified as most active in the visible regionafter N incorporation for both Rhod and Atraz degradation. Theanalysis of the nitrogen states in the samples suggest that photo-catalytic efficiency in the visible region may not necessarilydepend only on band narrowing but on several factors such asquantity and type of N state present in a sample. A similarbehavior was reported by Kafizas et al. [72] for a film withgradating substitutional (Ns) and interstitial (Ni) nitrogen dopantconcentrations across an anatase TiO2 thin-film. The film was usedfor the degradation of methylene blue and stearic acid undervisible activity. The results showed that interstitial N dominantsections showed better visible light photocatalytic activity thansubstitutional N sections.

Peng et al. [32] produced nitrogen doped TiO2 samples contain-ing both substitutional and interstitial for the degradation ofMethyl Orange and phenol degradation under visible light irradia-tion. They reported that the samples with interstitial N presentedbetter activity than the substitutional N and could be related to theeasy excitation of electrons from the interstitial N energy states tothe conduction band due to its location in the mid band gap(0.73 eV).

According to Wu et al. [73], photocatalytic activity of heavilydoped N:TiO2 catalysts are enhanced by band gap narrowing and

1200 30 60 90

0 50 100 150 200 250time / min

time / min

Fig.6. Photodegradation profiles of the Atrazine solution, using N:TiO2 nanoparti-cles with UVC light (A) and visible light (B) irradiation.

Fig. 5. Photodegradation profiles of the Rhod-B solution, using N:TiO2 nanoparti-cles with UVC light (A) and visible light (B) irradiation.

M. Dawson et al. / Journal of Solid State Chemistry 215 (2014) 211–218216

broadening of valence band. Isolated impurity states are howeverresponsible for photoactivity of catalyst with low N concentration.It is believed that photoactivity related to isolated nitrogen band(interstitial N) may be the dominant mechanism for the degrada-tion of Atraz and Rhod for samples with greater percentage ofinterstitial N.

SAM 4 showed a significant band gap narrowing but the least v.p.a for both Atraz and Rhod degradation, its N content is believedto be above the optimum amount for a good v.p.a. In this case, thenitrogen states may be acting as recombination centers. Thephotocatalytic trend of the samples reflects the optimum amountof N for better visible photocatalytic. It is also observed that theundoped sample presented identical v.p.a to that of SAM 4 forRhod degradation (Fig. 7). This behavior may be associated withthe structure and the Ti bond similarity (Ti4þ ions). In fact, thisassociation was not observed when the photocatalytic reactionswere done under UVC light. In this case, surface modificationthrough Ti–O–N bonds and Ti3þ ions may play an important rolein visible photocatalytic activity. The result shows that Ti3þ

defects (color center) may be important for the adsorption ofvisible light.

The rate constant values in Fig. 7 and the proportion ofinterstitial and substitutional N in Table 2 reveal that, the samplewith equal amount of substitutional and interstitial N (SAM 3) hasthe highest v.p.a for Rhod and Amet. It implies that band gapnarrowing by substitutional N coupled with the interstitial midgaps states can result in more photogenerated electrons and holesfor the production of superoxide and hydroxyl radicals for photo-catalytic activity. For samples with interstitial N above 50%, theincrease in Atraz photocatalytic degradation is almost identical.

However, the significant decrease in photocatalytic activityobserved for SAM 4, with greater amount of substitutional N thaninterstitial N, indicates that photocatalytic activity in the visibleregion is optimized when the quantity of interstitial N is equal tosubstitutional N or greater than 50%.

A statistical test (Pearson test) was performed on the results(k and band gap values) to determine the degree of correlationbetween these values and the N species (substitutional andinterstitial N) detected in the samples (Table 3). Like any othercorrelation coefficient, p varies from �1 to þ1, with �1 or þ1implying a strong linear relationship and 0 implying no linearrelationship. In addition, positive p values indicate a positiveassociation between the variables while negative values indicatea negative association. As Rhod and Atraz k values present thesame photocatalytic degradation trend for the doped samples, thePearson test was carried out only for Rhod. On this note, inter-stitial N has a positive effect on u.v.a (Table 3). As the quantity ofinterstitial N increases, photocatalytic activity increases. Thisobservation is consistent with the photocatalytic results of SAM1 and SAM 2. However, the v.p.a analysis showed that the Pearsoncorrelation was moderately negative and positive for substitu-tional and interstitial N, respectively. The ratio of substitutional Nto interstitial N presented no correlation. Since there is no stronglinear relationship, we cannot affirm which N species is respon-sible for v.p.a using the Pearson test.

The statistical analysis performed on the band gap narrowingvalues (Table 3) revealed that interstitial and substitutional Nshowed a weak positive and negative correlation, respectively, butthe ratio of both species presents a moderate linear relationship.Since the correlation is not strong, it is possible that the combinedeffect of the overlapping of 2p orbital of O with N caused bysubstitutional N and the formation of defects between the valenceband and conduction band by interstitial N accounts for band gapnarrowing. In addition, SAM 4 confirms the fact that heavy dopingalso contributes to band gap narrowing.

4. Conclusion

Polymeric precursors method is a simple synthesis route forproducing N:TiO2 catalysts. The Ti4þ resin calcination parametersin air before N doping affect the type of N formed (interstitial andsubstitutional N) and their respective amount. From XPS data,annealing close to anatase transformation temperature (450 1C)contributes to the formation of interstitial N. On the other hand,low temperatures favor substitutional N and can maintain Ti (Ti4þ)structure even after N doping. Therefore, manipulating calcinationparameters can be an alternative method for selective N doping.The kinetic rates of the catalysts and statistical analysis show thatinterstitial N improves ultraviolet and visible light degradation ofcolor and colorless substrates (Rhod and Atraz). However, the ratioof interstitial N to substitutional N must be controlled to ensurethat the contribution of interstitial N to photocatalytic activity isnot impaired. In fact, visible photocatalytic activity is better for thesample with equal quantity of interstitial and substitutional N.Also, unmodified Ti (Ti4þ) structure after doping can result in

Fig. 7. Photodegradation constants of Rhod-B (A) and Atraz (B) solution degrada-tion using N:TiO2 nanoparticles.

Table 3Statistical Pearson correlation (P) for band gap and for k values with interstitial N,substitutional N and ratio of substitutional to interstitial N.

UV Rhod Vis Rhod Band gap

Interstitial N 0.926 0.656 0.574Substitutional N �0.922 �0.678 �0.597Substitutional N/interstitial N �0.843 0.172 �0.777

M. Dawson et al. / Journal of Solid State Chemistry 215 (2014) 211–218 217

lower visible photoactivity activity. Thus, visible photoactivity maynot depend only on band gap narrowing but on several factorssuch as quantity and type of N state present.

Acknowledgments

We appreciate the financial supports from CAPES, FINEP,FAPESP (2009/10998-3), CNPQ-PIBIC (123768/2012-8) andEMBRAPA. We also acknowledge Prof. Richard Lander (IFGW/Unicamp) for his valuable support (XPS experiments), and LIEC-UFSCar for the Diffuse reflectance UV–vis spectroscopy facility.

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