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Influence of calcination parameters on the synthesis of N-doped TiO2 by the polymeric precursors method
Journal of Solid State Chemistry 215 (2014) 211–218
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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, Brazil b Department of Chemistry, Universidade Federal de São Carlos, Rodovia Washington Luis KM 235 SP-310, São Carlos CEP 13565-905, São Paulo, Brazil c Embrapa Instrumentação, Rua XV de Novembro 1452, São Carlos CEP 13560-970, São Paulo, Brazil
art ic l e i nf o
a b s t r a c t
Article history: Received 8 January 2014 Received in revised form 21 March 2014 Accepted 31 March 2014 Available online 13 April 2014
In this paper, the influence of calcination parameters on the synthesis of N:TiO2 catalysts obtained through the polymeric precursors method was evaluated. The powders were prepared by annealing Ti4 þ precursor resins at different temperature-time conditions in air, resulting in powders with different degrees of crystallinity for N doping, which was done by adding urea to the as-prepared powders and calcining in N2 atmosphere. The N doping process resulted in band gap narrowing of TiO2 and, varying annealing temperature and time, can be an alternative method for preferential formation of substitutional N or interstitial N. It was found that the percentage of interstitial N increased with an increase in annealing temperature, resulting in the complete absence of substitutional N at 400 1C. The photocatalytic performance of the powders was evaluated using Rhodamine-B and Atrazine solutions under ultraviolet and visible irradiations. The coefficients revealed that interstitial N had a positive correlation to 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 ultraviolet light photoactivity and no correlation to visible light photoactivity. However, substitutional N should be controlled for better photocatalytic properties. & 2014 Elsevier Inc. All rights reserved.
Keywords: Photocatalysis Titanium dioxide Nitrogen doping Dye degradation Herbicide degradation
1. Introduction The intrinsic properties of TiO2 such as chemical and thermal stability, low cost and non-toxicity have contributed to its popular use for photocatalytic degradation of organic substances [1–3]. Despite having all these properties, it also has a band gap of 3.2 eV which creates a limitation to the efficient use of the catalyst in the solar spectrum, as solar light accounts for only 3% of UV light [4]. For this reason, various attempts to extend the absorption spectrum 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 successful in improving photocatalytic activity under visible light irradiation. Although doping with metals has often been promoted, it presents major drawbacks such as thermal instability and high recombination centers that reduce photocatalytic activity [12,13]. The non metal doping approach especially with nitrogen (N), has since
n
Corresponding author. E-mail addresses: [email protected] (M. Dawson), [email protected] (G.B. Soares), [email protected] (C. Ribeiro). 1 Tel.: þ55 16 2107 2800; fax: þ55 16 2107 2902. 2 Tel.: þ55 16 2107 2902. http://dx.doi.org/10.1016/j.jssc.2014.03.044 0022-4596/& 2014 Elsevier Inc. All rights reserved.
attracted much attention after Asahi et al. [14] obtained N:TiO2 films by sputtering and discovered that substitutional N causes band 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], and plasma [20]. It is therefore evident that a key issue of concern is how preparation methods and precursors can be manipulated to obtain an optimum dopant position (substitutional or interstitial) and N doping levels that will effectively extend the absorption of TiO2 into the visible region and also increase photocatalytic activity. Substitutional N is widely cited as responsible for band gap narrowing and thereby enhancing visible photoactivity [21– 26]. In contrast, studies have indicated that band gap narrowing is a necessary condition; however, it might not be sufficient to improve visible photocatalytic activity. Several factors need to be considered such as N concentration, type of analyte (dye or colorless compound), crystallinity of TiO2 and oxygen vacancies [27–30]. In fact, the N state responsible for visible-light photoactivity is still intriguing and open to debate [31]. Recent findings indicate that interstitial N is a promising state for band gap narrowing as well as visible photoactivity. Peng et al. [32] prepared N:TiO2 catalysts containing both interstitial and substitutional N states. They revealed that interstitial N showed better
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photoactivity in the visible region than substitutional N. Ai et al. [33] also reported interstitial N as a photoactive state. Despite these findings, there is limited information on how photocatalytic activity is affected when both nitrogen states are present in a sample. Sol gel methods offer flexibility in terms of modification of precursors 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 catalysts using titanium tetraisopropoxide, hydrogen peroxide and ethylmethylamine. The as-prepared TiO2 sol (5 at%) was calcined at 300 1C for 24 h in air. Another method with different dopant precursors (diethylamine, triethylamine and urea) was employed to synthesize N:TiO2 catalysts which were calcined at 800 1C in N2 atmosphere [36]. In the above mentioned sol gel methods, the N precursor is added to the sol and then calcined in air or additional calcination in N2 atmosphere is carried out. Calcination plays an important role in the incorporation of N as it defines the crystallinity and phase of TiO2 nanoparticles [37]. Few authors have reported preferential formation of substitutional and interstitial N through calcination of TiO2 powders and TiO2(1 1 0) single crystals respectively in NH3 atmosphere at elevated temperatures [21,38]. However, much attention has not been drawn to the possibility of also controlling the N states formed in TiO2 lattice by altering the annealing conditions (temperature and time) in air before subsequent treatment in N2 or NH3 atmosphere. Hence, the effect of TiO2 crystallinity on the N state formed during doping deserves more attention. In a recent paper, Soares et al. [27] prepared N: TiO2 catalysts using the polymeric precursors method, a modified sol gel process. In this method, urea ranging from 0.1% to 2% (weight of equivalent TiO2) was added to a TiO2 resin and calcined at 450 1C for 2 h in air, obtaining N doping by diffusion. This method is simple compared to previously reported methods being that it does not require expensive reactants or special calcination conditions, but then controlling the doping process (i.e., preferential N incorporation—interstitial or substitutional) is complicated. Therefore, the main focus of this work is to investigate the influence of calcination parameters on nitrogen doping of TiO2 and to associate the N states (interstitial or substitutional) found in the N:TiO2 catalysts with photocatalytic activity. In order to reach this goal, the N:TiO2 powders were characterized by Scanning electron microscopy, Diffuse reflectance spectroscopy, X-ray diffraction and Raman spectroscopy. The N state formed was investigated by X-ray photoelectronic spectroscopy and the N:TiO2 powders were tested by photocatalytic oxidation of Rhodamine-B aqueous solution under Ultraviolet C (UVC) and visible (vis) irradiation. The respective photocatalytic activity constants were used to evaluate the effect of the N states in the samples on photocatalytic activity.
2. Experimental The resin synthesis procedure was based on the polymeric precursor method [27]. Briefly, the as-prepared Ti4 þ resin was divided into five parts, each part was annealed in air at a specific temperature 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 at 450 1C for 2 h was chosen as a reference sample and was not submitted to N doping. For the N doping process, 2 wt% urea (equivalent to TiO2 weight) was added to each resultant powder and calcined in N2 atmosphere at 450 1C for 2 h. The N doped samples were identified according to their annealing temperature and 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 reference sample was designated as TiO2.
The phase composition of the samples was identified through X-ray diffraction patterns recorded by a diffractometer (Shimadzu XDR 6000) with a Cu anode (λCu-Kα, 87¼ 0.154 nm), from 2θ ¼10–801, at 20 min 1. The average crystallite size was calculated by the Scherrer's formula [39] and the theoretical surface area (S.A) was estimated using the Braunuer–Emmet–Teller (BET) method [40], through N adsorption isotherms at 77 K in micrometrics ASAP 2000 equipment. The morphology and size of the as-prepared powders were characterized by a JSM-6701F/JEOL Field emission scanning electron microscopy (FESEM). The optical properties of the nanoparticles 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, Raman spectra were collected with a FT Raman Bruker RFS100/S equipment, using the 1064 nm line of a 450 W YAG 89 laser. The change in the bonding state and surface chemical composition related to N doping 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 rhodamine solution. In a separate beaker, a solution with 2 mg of SAM 2 was prepared. The same procedure was carried out for the other 3 samples (SAM 3, SAM 4, TiO2). In a separate test, Atrazine was used following the same procedure. The solutions were then irradiated with six UVC lamps (TUV Philips, 15 W, with maximum intensity at 254 nm). During the experiment, the solutions were homogenized by magnetic stirring and maintained at a constant temperature of 19 1C. A UV–vis spectrophotometer (Shimadzu-UV1601 PC spectrophotometer) was used to monitor changes in the spectral intensity of the analytes before and after irradiation. At intervals of 30 min or 60 min, 3 mL of each irradiated solution was homogenized and analyzed and the aliquot was recovered for further irradiation. The same procedure was carried out using a visible light source with six fluorescent lamps (Quality, 15 W, and maximum 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 anatase phase is present in all sample and its intensity increased as annealing temperature increased. The absence of rutile and TiN phase 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 effect on the final phase of the samples after N doping. However, the average crystallite size of the samples estimated by the Scherrer's formula, in Table 1, revealed that the crystallite size could depend on the temperature–time conditions of the annealed powders. For example, SAM 4 and SAM 2 show that a small change in temperature can promote crystallite growth. Further, SAM 3, SAM 2 and SAM 1, show that annealing at a lower temperature for long periods appear to have the same effect on the crystallite growth as a short annealing time at a slightly higher temperature. Since the amount of N added in each synthesis (from urea) was the same for all samples, it is assumed that residual carbon from the Ti4 þ polymeric resin could have also contributed to retardation of crystallite growth during N incorporation [41]. Raman measurements were carried out on the samples and the spectra are shown in Fig. 1B. All samples confirm anatase phase with corresponding bands located at 144 cm 1, 399 cm 1, 515 cm 1 and 639 cm 1. Surprisingly, SAM 4 with a band narrowing of 0.32 eV, did not show any clear active mode of anatase
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0
200
600 400 Raman Shift/ cm-1
800
1000
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: Raman spectra between 0 cm 1 and 1000 cm 1 of (a) SAM 1; (b) SAM 2; (c) SAM 3 and (e) SAM 4.
Table 1 Crystallite size, surface area and band gap value of N:TiO2 nanoparticles.
BET (SA)/m2 g‐1 Crystal Size/nm Band gap/eV
SAM 01
SAM 02
SAM 03
SAM 04
104.57 0.47 7 3.05
104.07 0.84 8 3.15
116.7 7 0.69 7 3.13
210.9 7 0.82 5 2.88
phase, even though anatase was confirmed in this sample by XRD results. The observed discrepancy can be associated with the sensibility of the Raman technique to sample color. In particular, this sample was very dark due to residual carbon. Once again, the presence of rutile phase is not confirmed in the samples. The BET surface area results are presented in Table 1. As can be seen, the samples annealed at 350 1C have higher surface areas than the others. In fact, the agglomeration of fine particles at high annealing temperatures (SAM 1 and SAM 2) as observed in FESEM images (Fig. 2) results in the reduction of surface area. The same effect is evident in samples annealed for a long period of time. The micrographs (Fig. 2) demonstrate a predominate morphology of almost spherical nanoparticles in the form of aggregates.
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They suggest that N doping did not modify particle or surface morphology, but rather particle size. The particle size estimated from the micrographs is not in agreement with XRD measurements due to the presence of large aggregates made of fine crystallites. The light adsorption spectra of the doped samples are represented by Kubelka–Munk function F(R1) vs. wavelength plots (Fig. 3). These plots directly evaluate band gap energy of semiconductors when certain conditions are satisfied. First, TiO2 is assumed to have an indirect band gap and second, the Kubelka– Munk function should be calculated using a diffuse reflectance data. Satisfying these conditions, the band gap energies can be extrapolated (inset of Fig. 3), and are summarized in Table 1. In general, the doped samples present lower band gap energies than pristine TiO2 (3.2 eV) varying from 2.88 eV to 3.15 eV, which can be attributed to the incorporation of N into the lattice of TiO2. SAM 4 exhibited a remarkable N induced band gap narrowing than the rest of the samples; however it has an additional adsorption shoulder at 380 nm to 420 nm (Fig. 3). It is assumed that this additional adsorption shoulder originated from residual carbon due to the incomplete pyrolysis of the Ti4 þ resin at a low annealing time and temperature. The influence of the synthesis method on the chemical composition of the doped samples was investigated through XPS spectra of Ti 2p and N 1s (Fig. 4). It can be inferred from the Ti 2p spectra (Fig. 4A), that all doped samples, including pristine TiO2, presented two principal peaks assigned to Ti (2p1/2) and Ti (2p3/2) transitions, with binding 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 with reference to pristine TiO2, indicating the presence of TiON bonds [44]. The formation of TiON bonds suggests a surface modification of TiO2 by N species. On the other hand, no Ti signal shift was observed for SAM 4 annealed at a lower temperature. It reflects that this annealing condition maintains the bulk structure and the Ti bonds of the doped sample similar to that of pristine TiO2. However, the presence or absence of Ti3 þ ions is influenced by the extent of nitrogen incorporation [45]. The N 1s core spectra of the doped samples are shown in Fig. 4C. The appearance of N 1s peak at 400 eV, accepted as interstitial N doping, is evident in all samples [46], while substitutional N signal at 398 eV is present in all samples except SAM 1 [47]. In order to investigate the effect of temperature on the chemical characteristics of the samples, XPS analysis was used to estimate the proportion of interstitial and substitutional N states presented in Table 2 The proportion of interstitial and substitutional N was estimated by deconvoluting the N peaks, as shown for SAM 4 in Fig. 4B. It is interesting to note that the amount of interstitial and substitutional N in a sample depends on the annealing temperature and time prior to doping in N2 atmosphere. When annealing is done at temperatures close to anatase crystallization temperature, i.e., 450 1C, the formation of interstitial N can be favored than substitutional N (SAM 3). In some cases, substitutional N can be completely absent (SAM 4). This suggests that during the N doping process, if the starting material is already crystalline (stronger Ti–O bonds), the substitution of oxygen with N can be prevented and consequently the concentration of substitutional N is lower. In contrast, low annealing temperature facilitates the formation of substitutional N due to the amorphous structure of the precursor powder characterized by weak Ti– O bonds. Substitutional N is reported as the principal N state that contributes to band gap narrowing due to the merging of N 2p states and O 2p states in the valence band [14]. It is important to note that SAM 4 has more substitutional than interstitial N states, which may have contributed to its significant band gap narrowing.
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Fig. 2. Field emission scanning electronic microscopy images (A) SAM 3, (B) SAM 4 (C) SAM 2 and (D) SAM 1.
F(R)/u.a.
F(R)/u.a.
where [AS] represents the photocatalyst's active sites. By substituting k0 ¼k [AS] into Eq. (1) and integrating, we have 0
lnð½Rhod B=½Rhod B0 Þ ¼ k t
λ 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 extrapolation of band gap energy,
Interstitial N, on the other hand, introduces deep energy levels above the valence band gap (0.41–1.44 eV) which may not contribute significantly to band gap narrowing [48]. The performance of the annealed powders after N incorporation is shown in Fig. 5 for photo-oxidation of Rhodamine-B (RhodB) and Atrazine (Atraz). From the photocatalytic efficiency plots of the N:TiO2 catalysts, the pseudo rate order of the photodegradation reaction was calculated. The reaction should be of the pseudo first-order with respect to Rhod-B, and must obey Eq. (1) v ¼ d ½Rhod B=dt ¼ k½Rhod B½AS
ð1Þ
ð2Þ
A plot of ln[Rhod-B]/[Rhod-B]0 as a function of t, forms a straight line whose slope is k. A plot of ln[Atraz]/[Atraz]0 as a function of t also forms a straight line whose slope is k. Rhod-B and Atraz present the same photocatalytic trend for UVC and UV–vis irradiation. In general, the kinetic rates for RhodB (dye) degradation for both visible and ultraviolet regions are accentuated compared to Atraz (colorless). Soares et al. [27] reported that both compounds have a different degradation mechanism, which can influence the final photocatalytic efficiency of N doped photocatalysts. The photocatalytic degradation of Rhod-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 is associated with interstitial N whiles O–Ti–N bonds are associated with substitutional N. The nitrogen bonds induce oxygen vacancies and/or Ti3 þ defects as a means of maintaining charge neutrality of TiO2 [49–51]. Apart from maintaining charge neutrality, they can act as color centers [52]. The electronic structure of TiO2 is also modified by nitrogen doping. The modified electronic structure and 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. According to Asahi et al. [14], the mixing of N 2p states with O 2p states in TiO2 lowers the band gap, allowing the photoactivation of the catalyst with visible light. However, density functional theory (DFT) calculation models by Valentin et al. [25] predict occupied
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Table 2 Total N content, quantity of interstitial N (%), quantity of substitutional N (%) and ratio of substitutional N to interstitial N for the N:TiO2 samples.
TiO2 SAM SAM SAM SAM
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 the deconvolution process (a) total N, (b) substitutional N and (c) interstitial N; (C) XPS spectra of N 1s of (a) TiO2, (b) SAM 1, (c) SAM 2, (d) SAM 3, (e) SAM 4 normalized.
N 2p localized states about 0.14 eV above the valence band edge. Therefore, visible light irradiation can excite electrons from the impurity energy levels to the conduction band, where the formation of superoxides anion radicals is possible through adsorbed oxygen. The holes in the nitrogen energy state can also react with OH ions to form hydroxide radical ions. Superoxide anion and hydroxide anions are widely accepted as responsible for the degradation of organic and inorganic compounds.
01 02 03 04
Total N
Interstitial N (%)
Substitutional N (%)
Substitutional N/interstitial N
1.00 1.49 0.27 3.12 24.19
100 100 89 50 32
0 0 11 50 71
0 0 0.12 1.01 2.24
Another mechanism proposed for visible photoactivity is through oxygen vacancies [53,30]. It is argued that band gap narrowing does not induce visible light activity but rather color centers or oxygen vacancies [11,52,54,55]. Oxygen vacancies can be formed due to substitutional nitrogen doping [56,57]. The vacancies can form localized energy levels about 0.8 eV below the conduction band shifting the absorption edge to lower energies [58]. Exited electrons can then be easily trapped by the empty oxygen states resulting in prolonged life of photogenerated charges [49]. Electron spin resonance test carried on NH3-heattreated TiO2 samples showed that oxygen vacancies can serve as trapping sites for adsorbed O2 for the production of superoxide anions. It is also reported that the quantum efficiency for photocatalytic reaction could be higher as the affinity of O2 molecules to oxygen vacancies is high [59]. Oxygen vacancies can induce Ti3 þ defects in TiO2 or vice versa. Ti3 þ defects have been also observed as possible species for visible photoactivity [60]. A model proposed by Weyl and Forland [61] shows that Ti4 þ cation is reduced to Ti3 þ when TiO2 crystal loses an oxygen atom. The loss of oxygen atoms is observed during substitutional N doping and consequently introduces Ti3 þ defects in TiO2. The surface defect Ti3 þ forms donor band just below the conduction band of TiO2 which contribute to visible photoabsorption in doped TiO2 [62]. Besides, there can be a reduction of surface-absorbed oxygen on TiO2 active sites leading to the formation of superoxide ions. Sun et al. [63] reported that the syngertic effect between nitrogen species and Ti3 þ 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 interstitial N was proposed to be 0.73 eV above the valence band, which is high as compared to substitutional N [65]. It then requires less energy to excite photo-induced electrons from these impurity levels to the conduction band. Defects such as oxygen vacancies and Ti3 þ can also be formed with interstitial N doping [66,67] and their roles in visible photocatalytic activity are similar to that of substitutional 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 visible activity photocatalytic is still an open question since substitutional and interstitial nitrogen can equally improve visible light activity if the photogeneration of charges and redox processes are dominant than the recombination of photogenerated charges. Therefore, an optimum quantity of interstitial N and substitutional N is required to increase photoactivity and at the same time reduce the recombination rate of charges [68]. The same applies to oxygen vacancies and Ti3 þ centers. Furthermore, the properties of the anatase phase such as surface area, particle size and phase crystallinity 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 doping results in narrow band gap and defects (Ti3 þ and oxygen vacancies). Therefore during UV irradiation, excitation occurs in both the
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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 as compared to the doped samples. As such, the lower photoactivity of the doped samples is attributed to greater recombination and reduced mobility of photogenerated charges [69]. However, the gap between SAM 1 and TiO2 presented in Figs. 5 and 6 draws attention to the possibility of obtaining N:TiO2 catalysts without significantly altering the u.v.a properties of TiO2. The effect of annealing on the ultraviolet photocatalytic activity (u.v.a) is evident in the doped samples prepared at higher temperatures (SAM 1 and SAM 2). The crystallinity of the samples could improve photoactivity. The same effect was reported for N-doped TiO2 prepared by a sol–gel method using NH4Cl followed by calcination at 300 1C, 400 1C and 500 1C, for UV photodegradation. The photocatalytic activity increased with the increase of calcination temperature from 300 1C to 500 1C assigned to anatase phase crystallinity [70]. According to Tryba et al. [71] crystallinity may improve OH radical generation of anatase samples. The analysis of the N states distribution from Table 2 and the rate constants presented in Fig. 6, show that SAM 1 (0% substitutional N) presents the highest u.v.a. in both substrates (Rhod and Atraz). For SAM 2 with substitutional N as low as 11%, u.v.a of this sample is almost half the value of SAM 1. It is an indication that interstitial N enhances u.v.a and the amount of substitutional N needs to be controlled for better u.v.a. It is important to notice that
0
30
90
60
120
time / min
0
50
100
150
200
250
time / min Fig.6. Photodegradation profiles of the Atrazine solution, using N:TiO2 nanoparticles with UVC light (A) and visible light (B) irradiation.
Fig. 5. Photodegradation profiles of the Rhod-B solution, using N:TiO2 nanoparticles with UVC light (A) and visible light (B) irradiation.
the substrate propriety (color) does not interfere in the photocatalytic trend of the doped samples. For photocatalytic reactions under visible light irradiation, SAM 3 and SAM 1 were identified as most active in the visible region after N incorporation for both Rhod and Atraz degradation. The analysis of the nitrogen states in the samples suggest that photocatalytic efficiency in the visible region may not necessarily depend only on band narrowing but on several factors such as quantity and type of N state present in a sample. A similar behavior was reported by Kafizas et al. [72] for a film with gradating substitutional (Ns) and interstitial (Ni) nitrogen dopant concentrations across an anatase TiO2 thin-film. The film was used for the degradation of methylene blue and stearic acid under visible activity. The results showed that interstitial N dominant sections showed better visible light photocatalytic activity than substitutional N sections. Peng et al. [32] produced nitrogen doped TiO2 samples containing both substitutional and interstitial for the degradation of Methyl Orange and phenol degradation under visible light irradiation. They reported that the samples with interstitial N presented better activity than the substitutional N and could be related to the easy excitation of electrons from the interstitial N energy states to the conduction band due to its location in the mid band gap (0.73 eV). According to Wu et al. [73], photocatalytic activity of heavily doped N:TiO2 catalysts are enhanced by band gap narrowing and
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broadening of valence band. Isolated impurity states are however responsible 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 degradation of Atraz and Rhod for samples with greater percentage of interstitial 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 believed to be above the optimum amount for a good v.p.a. In this case, the nitrogen states may be acting as recombination centers. The photocatalytic trend of the samples reflects the optimum amount of N for better visible photocatalytic. It is also observed that the undoped sample presented identical v.p.a to that of SAM 4 for Rhod degradation (Fig. 7). This behavior may be associated with the structure and the Ti bond similarity (Ti4 þ ions). In fact, this association was not observed when the photocatalytic reactions were done under UVC light. In this case, surface modification through Ti–O–N bonds and Ti3 þ ions may play an important role in visible photocatalytic activity. The result shows that Ti3 þ defects (color center) may be important for the adsorption of visible light. The rate constant values in Fig. 7 and the proportion of interstitial and substitutional N in Table 2 reveal that, the sample with equal amount of substitutional and interstitial N (SAM 3) has the highest v.p.a for Rhod and Amet. It implies that band gap narrowing by substitutional N coupled with the interstitial mid gaps states can result in more photogenerated electrons and holes for the production of superoxide and hydroxyl radicals for photocatalytic activity. For samples with interstitial N above 50%, the increase in Atraz photocatalytic degradation is almost identical.
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Table 3 Statistical Pearson correlation (P) for band gap and for k values with interstitial N, substitutional N and ratio of substitutional to interstitial N.
Interstitial N Substitutional N Substitutional N/interstitial N
UV Rhod
Vis Rhod
Band gap
0.926 0.922 0.843
0.656 0.678 0.172
0.574 0.597 0.777
However, the significant decrease in photocatalytic activity observed for SAM 4, with greater amount of substitutional N than interstitial N, indicates that photocatalytic activity in the visible region is optimized when the quantity of interstitial N is equal to substitutional 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 correlation between these values and the N species (substitutional and interstitial N) detected in the samples (Table 3). Like any other correlation coefficient, p varies from 1 to þ 1, with 1 or þ1 implying a strong linear relationship and 0 implying no linear relationship. In addition, positive p values indicate a positive association between the variables while negative values indicate a negative association. As Rhod and Atraz k values present the same photocatalytic degradation trend for the doped samples, the Pearson test was carried out only for Rhod. On this note, interstitial N has a positive effect on u.v.a (Table 3). As the quantity of interstitial N increases, photocatalytic activity increases. This observation is consistent with the photocatalytic results of SAM 1 and SAM 2. However, the v.p.a analysis showed that the Pearson correlation was moderately negative and positive for substitutional and interstitial N, respectively. The ratio of substitutional N to interstitial N presented no correlation. Since there is no strong linear relationship, we cannot affirm which N species is responsible for v.p.a using the Pearson test. The statistical analysis performed on the band gap narrowing values (Table 3) revealed that interstitial and substitutional N showed a weak positive and negative correlation, respectively, but the ratio of both species presents a moderate linear relationship. Since the correlation is not strong, it is possible that the combined effect of the overlapping of 2p orbital of O with N caused by substitutional N and the formation of defects between the valence band and conduction band by interstitial N accounts for band gap narrowing. In addition, SAM 4 confirms the fact that heavy doping also contributes to band gap narrowing.
4. Conclusion
Fig. 7. Photodegradation constants of Rhod-B (A) and Atraz (B) solution degradation using N:TiO2 nanoparticles.
Polymeric precursors method is a simple synthesis route for producing N:TiO2 catalysts. The Ti4 þ resin calcination parameters in air before N doping affect the type of N formed (interstitial and substitutional 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 (Ti4 þ ) structure even after N doping. Therefore, manipulating calcination parameters can be an alternative method for selective N doping. The kinetic rates of the catalysts and statistical analysis show that interstitial N improves ultraviolet and visible light degradation of color and colorless substrates (Rhod and Atraz). However, the ratio of interstitial N to substitutional N must be controlled to ensure that the contribution of interstitial N to photocatalytic activity is not impaired. In fact, visible photocatalytic activity is better for the sample with equal quantity of interstitial and substitutional N. Also, unmodified Ti (Ti4 þ ) structure after doping can result in
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lower visible photoactivity activity. Thus, visible photoactivity may not depend only on band gap narrowing but on several factors such as quantity and type of N state present.
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Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
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, Brazil b Department of Chemistry, Universidade Federal de São Carlos, Rodovia Washington Luis KM 235 SP-310, São Carlos CEP 13565-905, São Paulo, Brazil c Embrapa Instrumentação, Rua XV de Novembro 1452, São Carlos CEP 13560-970, São Paulo, Brazil
art ic l e i nf o
a b s t r a c t
Article history: Received 8 January 2014 Received in revised form 21 March 2014 Accepted 31 March 2014 Available online 13 April 2014
In this paper, the influence of calcination parameters on the synthesis of N:TiO2 catalysts obtained through the polymeric precursors method was evaluated. The powders were prepared by annealing Ti4 þ precursor resins at different temperature-time conditions in air, resulting in powders with different degrees of crystallinity for N doping, which was done by adding urea to the as-prepared powders and calcining in N2 atmosphere. The N doping process resulted in band gap narrowing of TiO2 and, varying annealing temperature and time, can be an alternative method for preferential formation of substitutional N or interstitial N. It was found that the percentage of interstitial N increased with an increase in annealing temperature, resulting in the complete absence of substitutional N at 400 1C. The photocatalytic performance of the powders was evaluated using Rhodamine-B and Atrazine solutions under ultraviolet and visible irradiations. The coefficients revealed that interstitial N had a positive correlation to 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 ultraviolet light photoactivity and no correlation to visible light photoactivity. However, substitutional N should be controlled for better photocatalytic properties. & 2014 Elsevier Inc. All rights reserved.
Keywords: Photocatalysis Titanium dioxide Nitrogen doping Dye degradation Herbicide degradation
1. Introduction The intrinsic properties of TiO2 such as chemical and thermal stability, low cost and non-toxicity have contributed to its popular use for photocatalytic degradation of organic substances [1–3]. Despite having all these properties, it also has a band gap of 3.2 eV which creates a limitation to the efficient use of the catalyst in the solar spectrum, as solar light accounts for only 3% of UV light [4]. For this reason, various attempts to extend the absorption spectrum 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 successful in improving photocatalytic activity under visible light irradiation. Although doping with metals has often been promoted, it presents major drawbacks such as thermal instability and high recombination centers that reduce photocatalytic activity [12,13]. The non metal doping approach especially with nitrogen (N), has since
n
Corresponding author. E-mail addresses: [email protected] (M. Dawson), [email protected] (G.B. Soares), [email protected] (C. Ribeiro). 1 Tel.: þ55 16 2107 2800; fax: þ55 16 2107 2902. 2 Tel.: þ55 16 2107 2902. http://dx.doi.org/10.1016/j.jssc.2014.03.044 0022-4596/& 2014 Elsevier Inc. All rights reserved.
attracted much attention after Asahi et al. [14] obtained N:TiO2 films by sputtering and discovered that substitutional N causes band 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], and plasma [20]. It is therefore evident that a key issue of concern is how preparation methods and precursors can be manipulated to obtain an optimum dopant position (substitutional or interstitial) and N doping levels that will effectively extend the absorption of TiO2 into the visible region and also increase photocatalytic activity. Substitutional N is widely cited as responsible for band gap narrowing and thereby enhancing visible photoactivity [21– 26]. In contrast, studies have indicated that band gap narrowing is a necessary condition; however, it might not be sufficient to improve visible photocatalytic activity. Several factors need to be considered such as N concentration, type of analyte (dye or colorless compound), crystallinity of TiO2 and oxygen vacancies [27–30]. In fact, the N state responsible for visible-light photoactivity is still intriguing and open to debate [31]. Recent findings indicate that interstitial N is a promising state for band gap narrowing as well as visible photoactivity. Peng et al. [32] prepared N:TiO2 catalysts containing both interstitial and substitutional N states. They revealed that interstitial N showed better
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photoactivity in the visible region than substitutional N. Ai et al. [33] also reported interstitial N as a photoactive state. Despite these findings, there is limited information on how photocatalytic activity is affected when both nitrogen states are present in a sample. Sol gel methods offer flexibility in terms of modification of precursors 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 catalysts using titanium tetraisopropoxide, hydrogen peroxide and ethylmethylamine. The as-prepared TiO2 sol (5 at%) was calcined at 300 1C for 24 h in air. Another method with different dopant precursors (diethylamine, triethylamine and urea) was employed to synthesize N:TiO2 catalysts which were calcined at 800 1C in N2 atmosphere [36]. In the above mentioned sol gel methods, the N precursor is added to the sol and then calcined in air or additional calcination in N2 atmosphere is carried out. Calcination plays an important role in the incorporation of N as it defines the crystallinity and phase of TiO2 nanoparticles [37]. Few authors have reported preferential formation of substitutional and interstitial N through calcination of TiO2 powders and TiO2(1 1 0) single crystals respectively in NH3 atmosphere at elevated temperatures [21,38]. However, much attention has not been drawn to the possibility of also controlling the N states formed in TiO2 lattice by altering the annealing conditions (temperature and time) in air before subsequent treatment in N2 or NH3 atmosphere. Hence, the effect of TiO2 crystallinity on the N state formed during doping deserves more attention. In a recent paper, Soares et al. [27] prepared N: TiO2 catalysts using the polymeric precursors method, a modified sol gel process. In this method, urea ranging from 0.1% to 2% (weight of equivalent TiO2) was added to a TiO2 resin and calcined at 450 1C for 2 h in air, obtaining N doping by diffusion. This method is simple compared to previously reported methods being that it does not require expensive reactants or special calcination conditions, but then controlling the doping process (i.e., preferential N incorporation—interstitial or substitutional) is complicated. Therefore, the main focus of this work is to investigate the influence of calcination parameters on nitrogen doping of TiO2 and to associate the N states (interstitial or substitutional) found in the N:TiO2 catalysts with photocatalytic activity. In order to reach this goal, the N:TiO2 powders were characterized by Scanning electron microscopy, Diffuse reflectance spectroscopy, X-ray diffraction and Raman spectroscopy. The N state formed was investigated by X-ray photoelectronic spectroscopy and the N:TiO2 powders were tested by photocatalytic oxidation of Rhodamine-B aqueous solution under Ultraviolet C (UVC) and visible (vis) irradiation. The respective photocatalytic activity constants were used to evaluate the effect of the N states in the samples on photocatalytic activity.
2. Experimental The resin synthesis procedure was based on the polymeric precursor method [27]. Briefly, the as-prepared Ti4 þ resin was divided into five parts, each part was annealed in air at a specific temperature 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 at 450 1C for 2 h was chosen as a reference sample and was not submitted to N doping. For the N doping process, 2 wt% urea (equivalent to TiO2 weight) was added to each resultant powder and calcined in N2 atmosphere at 450 1C for 2 h. The N doped samples were identified according to their annealing temperature and 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 reference sample was designated as TiO2.
The phase composition of the samples was identified through X-ray diffraction patterns recorded by a diffractometer (Shimadzu XDR 6000) with a Cu anode (λCu-Kα, 87¼ 0.154 nm), from 2θ ¼10–801, at 20 min 1. The average crystallite size was calculated by the Scherrer's formula [39] and the theoretical surface area (S.A) was estimated using the Braunuer–Emmet–Teller (BET) method [40], through N adsorption isotherms at 77 K in micrometrics ASAP 2000 equipment. The morphology and size of the as-prepared powders were characterized by a JSM-6701F/JEOL Field emission scanning electron microscopy (FESEM). The optical properties of the nanoparticles 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, Raman spectra were collected with a FT Raman Bruker RFS100/S equipment, using the 1064 nm line of a 450 W YAG 89 laser. The change in the bonding state and surface chemical composition related to N doping 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 rhodamine solution. In a separate beaker, a solution with 2 mg of SAM 2 was prepared. The same procedure was carried out for the other 3 samples (SAM 3, SAM 4, TiO2). In a separate test, Atrazine was used following the same procedure. The solutions were then irradiated with six UVC lamps (TUV Philips, 15 W, with maximum intensity at 254 nm). During the experiment, the solutions were homogenized by magnetic stirring and maintained at a constant temperature of 19 1C. A UV–vis spectrophotometer (Shimadzu-UV1601 PC spectrophotometer) was used to monitor changes in the spectral intensity of the analytes before and after irradiation. At intervals of 30 min or 60 min, 3 mL of each irradiated solution was homogenized and analyzed and the aliquot was recovered for further irradiation. The same procedure was carried out using a visible light source with six fluorescent lamps (Quality, 15 W, and maximum 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 anatase phase is present in all sample and its intensity increased as annealing temperature increased. The absence of rutile and TiN phase 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 effect on the final phase of the samples after N doping. However, the average crystallite size of the samples estimated by the Scherrer's formula, in Table 1, revealed that the crystallite size could depend on the temperature–time conditions of the annealed powders. For example, SAM 4 and SAM 2 show that a small change in temperature can promote crystallite growth. Further, SAM 3, SAM 2 and SAM 1, show that annealing at a lower temperature for long periods appear to have the same effect on the crystallite growth as a short annealing time at a slightly higher temperature. Since the amount of N added in each synthesis (from urea) was the same for all samples, it is assumed that residual carbon from the Ti4 þ polymeric resin could have also contributed to retardation of crystallite growth during N incorporation [41]. Raman measurements were carried out on the samples and the spectra are shown in Fig. 1B. All samples confirm anatase phase with corresponding bands located at 144 cm 1, 399 cm 1, 515 cm 1 and 639 cm 1. Surprisingly, SAM 4 with a band narrowing of 0.32 eV, did not show any clear active mode of anatase
M. Dawson et al. / Journal of Solid State Chemistry 215 (2014) 211–218
0
200
600 400 Raman Shift/ cm-1
800
1000
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: Raman spectra between 0 cm 1 and 1000 cm 1 of (a) SAM 1; (b) SAM 2; (c) SAM 3 and (e) SAM 4.
Table 1 Crystallite size, surface area and band gap value of N:TiO2 nanoparticles.
BET (SA)/m2 g‐1 Crystal Size/nm Band gap/eV
SAM 01
SAM 02
SAM 03
SAM 04
104.57 0.47 7 3.05
104.07 0.84 8 3.15
116.7 7 0.69 7 3.13
210.9 7 0.82 5 2.88
phase, even though anatase was confirmed in this sample by XRD results. The observed discrepancy can be associated with the sensibility of the Raman technique to sample color. In particular, this sample was very dark due to residual carbon. Once again, the presence of rutile phase is not confirmed in the samples. The BET surface area results are presented in Table 1. As can be seen, the samples annealed at 350 1C have higher surface areas than the others. In fact, the agglomeration of fine particles at high annealing temperatures (SAM 1 and SAM 2) as observed in FESEM images (Fig. 2) results in the reduction of surface area. The same effect is evident in samples annealed for a long period of time. The micrographs (Fig. 2) demonstrate a predominate morphology of almost spherical nanoparticles in the form of aggregates.
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They suggest that N doping did not modify particle or surface morphology, but rather particle size. The particle size estimated from the micrographs is not in agreement with XRD measurements due to the presence of large aggregates made of fine crystallites. The light adsorption spectra of the doped samples are represented by Kubelka–Munk function F(R1) vs. wavelength plots (Fig. 3). These plots directly evaluate band gap energy of semiconductors when certain conditions are satisfied. First, TiO2 is assumed to have an indirect band gap and second, the Kubelka– Munk function should be calculated using a diffuse reflectance data. Satisfying these conditions, the band gap energies can be extrapolated (inset of Fig. 3), and are summarized in Table 1. In general, the doped samples present lower band gap energies than pristine TiO2 (3.2 eV) varying from 2.88 eV to 3.15 eV, which can be attributed to the incorporation of N into the lattice of TiO2. SAM 4 exhibited a remarkable N induced band gap narrowing than the rest of the samples; however it has an additional adsorption shoulder at 380 nm to 420 nm (Fig. 3). It is assumed that this additional adsorption shoulder originated from residual carbon due to the incomplete pyrolysis of the Ti4 þ resin at a low annealing time and temperature. The influence of the synthesis method on the chemical composition of the doped samples was investigated through XPS spectra of Ti 2p and N 1s (Fig. 4). It can be inferred from the Ti 2p spectra (Fig. 4A), that all doped samples, including pristine TiO2, presented two principal peaks assigned to Ti (2p1/2) and Ti (2p3/2) transitions, with binding 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 with reference to pristine TiO2, indicating the presence of TiON bonds [44]. The formation of TiON bonds suggests a surface modification of TiO2 by N species. On the other hand, no Ti signal shift was observed for SAM 4 annealed at a lower temperature. It reflects that this annealing condition maintains the bulk structure and the Ti bonds of the doped sample similar to that of pristine TiO2. However, the presence or absence of Ti3 þ ions is influenced by the extent of nitrogen incorporation [45]. The N 1s core spectra of the doped samples are shown in Fig. 4C. The appearance of N 1s peak at 400 eV, accepted as interstitial N doping, is evident in all samples [46], while substitutional N signal at 398 eV is present in all samples except SAM 1 [47]. In order to investigate the effect of temperature on the chemical characteristics of the samples, XPS analysis was used to estimate the proportion of interstitial and substitutional N states presented in Table 2 The proportion of interstitial and substitutional N was estimated by deconvoluting the N peaks, as shown for SAM 4 in Fig. 4B. It is interesting to note that the amount of interstitial and substitutional N in a sample depends on the annealing temperature and time prior to doping in N2 atmosphere. When annealing is done at temperatures close to anatase crystallization temperature, i.e., 450 1C, the formation of interstitial N can be favored than substitutional N (SAM 3). In some cases, substitutional N can be completely absent (SAM 4). This suggests that during the N doping process, if the starting material is already crystalline (stronger Ti–O bonds), the substitution of oxygen with N can be prevented and consequently the concentration of substitutional N is lower. In contrast, low annealing temperature facilitates the formation of substitutional N due to the amorphous structure of the precursor powder characterized by weak Ti– O bonds. Substitutional N is reported as the principal N state that contributes to band gap narrowing due to the merging of N 2p states and O 2p states in the valence band [14]. It is important to note that SAM 4 has more substitutional than interstitial N states, which may have contributed to its significant band gap narrowing.
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Fig. 2. Field emission scanning electronic microscopy images (A) SAM 3, (B) SAM 4 (C) SAM 2 and (D) SAM 1.
F(R)/u.a.
F(R)/u.a.
where [AS] represents the photocatalyst's active sites. By substituting k0 ¼k [AS] into Eq. (1) and integrating, we have 0
lnð½Rhod B=½Rhod B0 Þ ¼ k t
λ 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 extrapolation of band gap energy,
Interstitial N, on the other hand, introduces deep energy levels above the valence band gap (0.41–1.44 eV) which may not contribute significantly to band gap narrowing [48]. The performance of the annealed powders after N incorporation is shown in Fig. 5 for photo-oxidation of Rhodamine-B (RhodB) and Atrazine (Atraz). From the photocatalytic efficiency plots of the N:TiO2 catalysts, the pseudo rate order of the photodegradation reaction was calculated. The reaction should be of the pseudo first-order with respect to Rhod-B, and must obey Eq. (1) v ¼ d ½Rhod B=dt ¼ k½Rhod B½AS
ð1Þ
ð2Þ
A plot of ln[Rhod-B]/[Rhod-B]0 as a function of t, forms a straight line whose slope is k. A plot of ln[Atraz]/[Atraz]0 as a function of t also forms a straight line whose slope is k. Rhod-B and Atraz present the same photocatalytic trend for UVC and UV–vis irradiation. In general, the kinetic rates for RhodB (dye) degradation for both visible and ultraviolet regions are accentuated compared to Atraz (colorless). Soares et al. [27] reported that both compounds have a different degradation mechanism, which can influence the final photocatalytic efficiency of N doped photocatalysts. The photocatalytic degradation of Rhod-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 is associated with interstitial N whiles O–Ti–N bonds are associated with substitutional N. The nitrogen bonds induce oxygen vacancies and/or Ti3 þ defects as a means of maintaining charge neutrality of TiO2 [49–51]. Apart from maintaining charge neutrality, they can act as color centers [52]. The electronic structure of TiO2 is also modified by nitrogen doping. The modified electronic structure and 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. According to Asahi et al. [14], the mixing of N 2p states with O 2p states in TiO2 lowers the band gap, allowing the photoactivation of the catalyst with visible light. However, density functional theory (DFT) calculation models by Valentin et al. [25] predict occupied
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Table 2 Total N content, quantity of interstitial N (%), quantity of substitutional N (%) and ratio of substitutional N to interstitial N for the N:TiO2 samples.
TiO2 SAM SAM SAM SAM
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 the deconvolution process (a) total N, (b) substitutional N and (c) interstitial N; (C) XPS spectra of N 1s of (a) TiO2, (b) SAM 1, (c) SAM 2, (d) SAM 3, (e) SAM 4 normalized.
N 2p localized states about 0.14 eV above the valence band edge. Therefore, visible light irradiation can excite electrons from the impurity energy levels to the conduction band, where the formation of superoxides anion radicals is possible through adsorbed oxygen. The holes in the nitrogen energy state can also react with OH ions to form hydroxide radical ions. Superoxide anion and hydroxide anions are widely accepted as responsible for the degradation of organic and inorganic compounds.
01 02 03 04
Total N
Interstitial N (%)
Substitutional N (%)
Substitutional N/interstitial N
1.00 1.49 0.27 3.12 24.19
100 100 89 50 32
0 0 11 50 71
0 0 0.12 1.01 2.24
Another mechanism proposed for visible photoactivity is through oxygen vacancies [53,30]. It is argued that band gap narrowing does not induce visible light activity but rather color centers or oxygen vacancies [11,52,54,55]. Oxygen vacancies can be formed due to substitutional nitrogen doping [56,57]. The vacancies can form localized energy levels about 0.8 eV below the conduction band shifting the absorption edge to lower energies [58]. Exited electrons can then be easily trapped by the empty oxygen states resulting in prolonged life of photogenerated charges [49]. Electron spin resonance test carried on NH3-heattreated TiO2 samples showed that oxygen vacancies can serve as trapping sites for adsorbed O2 for the production of superoxide anions. It is also reported that the quantum efficiency for photocatalytic reaction could be higher as the affinity of O2 molecules to oxygen vacancies is high [59]. Oxygen vacancies can induce Ti3 þ defects in TiO2 or vice versa. Ti3 þ defects have been also observed as possible species for visible photoactivity [60]. A model proposed by Weyl and Forland [61] shows that Ti4 þ cation is reduced to Ti3 þ when TiO2 crystal loses an oxygen atom. The loss of oxygen atoms is observed during substitutional N doping and consequently introduces Ti3 þ defects in TiO2. The surface defect Ti3 þ forms donor band just below the conduction band of TiO2 which contribute to visible photoabsorption in doped TiO2 [62]. Besides, there can be a reduction of surface-absorbed oxygen on TiO2 active sites leading to the formation of superoxide ions. Sun et al. [63] reported that the syngertic effect between nitrogen species and Ti3 þ 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 interstitial N was proposed to be 0.73 eV above the valence band, which is high as compared to substitutional N [65]. It then requires less energy to excite photo-induced electrons from these impurity levels to the conduction band. Defects such as oxygen vacancies and Ti3 þ can also be formed with interstitial N doping [66,67] and their roles in visible photocatalytic activity are similar to that of substitutional 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 visible activity photocatalytic is still an open question since substitutional and interstitial nitrogen can equally improve visible light activity if the photogeneration of charges and redox processes are dominant than the recombination of photogenerated charges. Therefore, an optimum quantity of interstitial N and substitutional N is required to increase photoactivity and at the same time reduce the recombination rate of charges [68]. The same applies to oxygen vacancies and Ti3 þ centers. Furthermore, the properties of the anatase phase such as surface area, particle size and phase crystallinity 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 doping results in narrow band gap and defects (Ti3 þ and oxygen vacancies). Therefore during UV irradiation, excitation occurs in both the
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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 as compared to the doped samples. As such, the lower photoactivity of the doped samples is attributed to greater recombination and reduced mobility of photogenerated charges [69]. However, the gap between SAM 1 and TiO2 presented in Figs. 5 and 6 draws attention to the possibility of obtaining N:TiO2 catalysts without significantly altering the u.v.a properties of TiO2. The effect of annealing on the ultraviolet photocatalytic activity (u.v.a) is evident in the doped samples prepared at higher temperatures (SAM 1 and SAM 2). The crystallinity of the samples could improve photoactivity. The same effect was reported for N-doped TiO2 prepared by a sol–gel method using NH4Cl followed by calcination at 300 1C, 400 1C and 500 1C, for UV photodegradation. The photocatalytic activity increased with the increase of calcination temperature from 300 1C to 500 1C assigned to anatase phase crystallinity [70]. According to Tryba et al. [71] crystallinity may improve OH radical generation of anatase samples. The analysis of the N states distribution from Table 2 and the rate constants presented in Fig. 6, show that SAM 1 (0% substitutional N) presents the highest u.v.a. in both substrates (Rhod and Atraz). For SAM 2 with substitutional N as low as 11%, u.v.a of this sample is almost half the value of SAM 1. It is an indication that interstitial N enhances u.v.a and the amount of substitutional N needs to be controlled for better u.v.a. It is important to notice that
0
30
90
60
120
time / min
0
50
100
150
200
250
time / min Fig.6. Photodegradation profiles of the Atrazine solution, using N:TiO2 nanoparticles with UVC light (A) and visible light (B) irradiation.
Fig. 5. Photodegradation profiles of the Rhod-B solution, using N:TiO2 nanoparticles with UVC light (A) and visible light (B) irradiation.
the substrate propriety (color) does not interfere in the photocatalytic trend of the doped samples. For photocatalytic reactions under visible light irradiation, SAM 3 and SAM 1 were identified as most active in the visible region after N incorporation for both Rhod and Atraz degradation. The analysis of the nitrogen states in the samples suggest that photocatalytic efficiency in the visible region may not necessarily depend only on band narrowing but on several factors such as quantity and type of N state present in a sample. A similar behavior was reported by Kafizas et al. [72] for a film with gradating substitutional (Ns) and interstitial (Ni) nitrogen dopant concentrations across an anatase TiO2 thin-film. The film was used for the degradation of methylene blue and stearic acid under visible activity. The results showed that interstitial N dominant sections showed better visible light photocatalytic activity than substitutional N sections. Peng et al. [32] produced nitrogen doped TiO2 samples containing both substitutional and interstitial for the degradation of Methyl Orange and phenol degradation under visible light irradiation. They reported that the samples with interstitial N presented better activity than the substitutional N and could be related to the easy excitation of electrons from the interstitial N energy states to the conduction band due to its location in the mid band gap (0.73 eV). According to Wu et al. [73], photocatalytic activity of heavily doped N:TiO2 catalysts are enhanced by band gap narrowing and
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broadening of valence band. Isolated impurity states are however responsible 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 degradation of Atraz and Rhod for samples with greater percentage of interstitial 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 believed to be above the optimum amount for a good v.p.a. In this case, the nitrogen states may be acting as recombination centers. The photocatalytic trend of the samples reflects the optimum amount of N for better visible photocatalytic. It is also observed that the undoped sample presented identical v.p.a to that of SAM 4 for Rhod degradation (Fig. 7). This behavior may be associated with the structure and the Ti bond similarity (Ti4 þ ions). In fact, this association was not observed when the photocatalytic reactions were done under UVC light. In this case, surface modification through Ti–O–N bonds and Ti3 þ ions may play an important role in visible photocatalytic activity. The result shows that Ti3 þ defects (color center) may be important for the adsorption of visible light. The rate constant values in Fig. 7 and the proportion of interstitial and substitutional N in Table 2 reveal that, the sample with equal amount of substitutional and interstitial N (SAM 3) has the highest v.p.a for Rhod and Amet. It implies that band gap narrowing by substitutional N coupled with the interstitial mid gaps states can result in more photogenerated electrons and holes for the production of superoxide and hydroxyl radicals for photocatalytic activity. For samples with interstitial N above 50%, the increase in Atraz photocatalytic degradation is almost identical.
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Table 3 Statistical Pearson correlation (P) for band gap and for k values with interstitial N, substitutional N and ratio of substitutional to interstitial N.
Interstitial N Substitutional N Substitutional N/interstitial N
UV Rhod
Vis Rhod
Band gap
0.926 0.922 0.843
0.656 0.678 0.172
0.574 0.597 0.777
However, the significant decrease in photocatalytic activity observed for SAM 4, with greater amount of substitutional N than interstitial N, indicates that photocatalytic activity in the visible region is optimized when the quantity of interstitial N is equal to substitutional 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 correlation between these values and the N species (substitutional and interstitial N) detected in the samples (Table 3). Like any other correlation coefficient, p varies from 1 to þ 1, with 1 or þ1 implying a strong linear relationship and 0 implying no linear relationship. In addition, positive p values indicate a positive association between the variables while negative values indicate a negative association. As Rhod and Atraz k values present the same photocatalytic degradation trend for the doped samples, the Pearson test was carried out only for Rhod. On this note, interstitial N has a positive effect on u.v.a (Table 3). As the quantity of interstitial N increases, photocatalytic activity increases. This observation is consistent with the photocatalytic results of SAM 1 and SAM 2. However, the v.p.a analysis showed that the Pearson correlation was moderately negative and positive for substitutional and interstitial N, respectively. The ratio of substitutional N to interstitial N presented no correlation. Since there is no strong linear relationship, we cannot affirm which N species is responsible for v.p.a using the Pearson test. The statistical analysis performed on the band gap narrowing values (Table 3) revealed that interstitial and substitutional N showed a weak positive and negative correlation, respectively, but the ratio of both species presents a moderate linear relationship. Since the correlation is not strong, it is possible that the combined effect of the overlapping of 2p orbital of O with N caused by substitutional N and the formation of defects between the valence band and conduction band by interstitial N accounts for band gap narrowing. In addition, SAM 4 confirms the fact that heavy doping also contributes to band gap narrowing.
4. Conclusion
Fig. 7. Photodegradation constants of Rhod-B (A) and Atraz (B) solution degradation using N:TiO2 nanoparticles.
Polymeric precursors method is a simple synthesis route for producing N:TiO2 catalysts. The Ti4 þ resin calcination parameters in air before N doping affect the type of N formed (interstitial and substitutional 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 (Ti4 þ ) structure even after N doping. Therefore, manipulating calcination parameters can be an alternative method for selective N doping. The kinetic rates of the catalysts and statistical analysis show that interstitial N improves ultraviolet and visible light degradation of color and colorless substrates (Rhod and Atraz). However, the ratio of interstitial N to substitutional N must be controlled to ensure that the contribution of interstitial N to photocatalytic activity is not impaired. In fact, visible photocatalytic activity is better for the sample with equal quantity of interstitial and substitutional N. Also, unmodified Ti (Ti4 þ ) structure after doping can result in
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lower visible photoactivity activity. Thus, visible photoactivity may not depend only on band gap narrowing but on several factors such as quantity and type of N state present.
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