Journal of Alloys and Compounds 540 (2012) 228–235

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Journal of Alloys and Compounds

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Nitrogen-doped TiO2 nanoparticles by using EDTA as nitrogen source and softtemplate: Simple preparation, mesoporous structure, and photocatalyticactivity under visible light

Douga Nassoko a, Yan-Fang Li a, Hui Wang a, Jia-Lin Li a, Yuan-Zhi Li b, Ying Yu a,⇑a Institute of Nanoscience and Nanotechnology, Central China Normal University, Wuhan 430079, Chinab National Key Laboratory of Silicate Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 6 February 2012Received in revised form 14 June 2012Accepted 16 June 2012Available online 27 June 2012

Keywords:PhotocatalysisTiO2

Visible lightNitrogen-dopingEDTABenzene degradation

0925-8388/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.jallcom.2012.06.085

⇑ Corresponding author. Tel.: +86 27 67867037; faxE-mail address: yuying@phy.ccnu.edu.cn (Y. Yu).

A new way to dope TiO2 nanoparticles by using ethylene-diaminetetraacetic acid (EDTA) as N source isintroduced here. The nitrogen doped titanium oxide was successfully synthesized through an EDTA mod-ified sol–gel process. The prepared samples were characterized by scanning electronic microscopy, X-raydiffractometer, Raman spectroscopy, UV–vis diffuse reflectance spectroscopy, X-ray photoelectron spec-troscopy, and Brunauer–Emmett–Teller measurement. Their photocatalytic activities were examined bythe degradation of organic pollutants such as rhodamine-B and benzene in comparison with those of N-doped TiO2 with NH3 flow as N source under visible light (k > 420 nm) irradiation. The results show thatthrough the simple sol–gel process, nitrogen can be incorporated into crystal lattice of TiO2, leading to itsresponse to visible light. With the presence of EDTA, the surface area of the prepared samples becomeslarger, pore size smaller, and pore size distribution narrower. Additionally, the prepared N-doped TiO2

samples has better photocatalytic activity for organic compound degradation than that prepared withNH3 flow and there is an optimized N doping amount. The reason for the enhanced photocatalytic activityis discussed in detail. To the best of our knowledge, it is first time to report that N-doped TiO2 nanopar-ticles can have high enough activity for benzene degradation.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

Photocatalysis allows the use of sunlight for the destruction ofhighly toxic molecules and remediation of pollutants with selec-tive, synthetically useful redox transformations in specific organiccompounds [1]. The most widely used material in heterogeneousphotocatalysis is titanium dioxide (TiO2). It is inexpensive, non-toxic, reusable, and resistant to photo-corrosion, easy available cat-alyst and has high oxidative power. It is well known that TiO2 is themost effective photocatalyst and is widely applied in the purifica-tion of air and water because it can decompose different organicpollutants [2,3].

However, TiO2 is active only under ultraviolet light irradiationbecause of its band gap of 3.2 eV (anatase). The large band gap lim-its the use of wide energy window of sunlight because solar energyis the most important alternative energy source in the future. Forcontaminants removal by using solar energy, many efforts havebeen made to modify TiO2. The methods of photosensitizer modi-fication [4] and metal ion doping [5] have been used based on crys-tal TiO2. For example, TiO2 can be modified by dye for visible light

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harvesting [4]. Moreover, metal doping can gain the same effectthrough Ag [5], Fe [6], Mo [7], etc. ion doping. Nevertheless, non-metal (N, S, C, etc.) doping is also regarded as an effective way tosensitize TiO2 [8]. Among non-metal doping, nitrogen doping wasfirstly reported [9] and is one of the most important methods forpromising application [10,11] since the band gap narrowing ofTiO2 can be achieved by O 2p mixed with N 2p states.

There are many ways such as ammonia vapor deposition [9],N2-plasma [10], ion implantation [12] and wet chemistry [13] toachieve nitrogen doping in TiO2. Since wet chemistry method issimple and easy to prepare samples with N doping evenly not onlyin the surface but in the bulk, it is used widely. So far, triethyl-amine, [14] urea, thiourea, and hydrazine hydrate [15], ammoniac[16], guanidine [17], etc. have been used as nitrogen sources toprepare N-doped TiO2 through wet chemistry. It has been foundthat the N doped TiO2 derived from different donors would havedifferent properties and N doped TiO2 using triethylamine as thenitrogen source showed the highest photocatalytic activity [15].Therefore, it is important to select suitable nitrogen source toincorporate nitrogen into TiO2 and investigate the nature ofband-gap narrowing and visible response.

Herein, we report a new type of nitrogen doping by using eth-ylene-diaminetetraacetic acid (EDTA) as nitrogen source. EDTA

D. Nassoko et al. / Journal of Alloys and Compounds 540 (2012) 228–235 229

was selected as a TiO2 dopant because EDTA could function as softtemplate for the preparation of mesoporous TiO2 with large surfacearea to accumulate organic pollutant on the surface of semicon-ductor [18]. Moreover, the process to achieve nitrogen dopingthrough EDTA was simple. The incorporation of doping elementwas made during sol gel process. The characteristics and propertiesof these prepared photocatalysts were investigated as well. Thephotocatalytic activity was detected with dye rhodamine-B (RhB)and benzene as probe molecules under visible light. The photoca-talysis mechanism of N-doped TiO2 was also discussed. To the bestof our knowledge, it is first time to report that N-doped TiO2 candegrade benzene under visible light.

2. Experimental details

2.1. Materials

Titanium tetrachloride (TiCl4) used as a precursor for TiO2, EDTA (C10H16N2O8)as nitrogen source, sodium hydroxide (NaOH), chloride acid (HCl), benzene, RhBwere all purchased from Shanghai Guoyao Chemical Co. They were analytical gradeand used without further purification. The purified water used in the experimentwas deionized water for the preparation of solutions.

Fig. 1. (A) XRD patterns of pure TiO2 (a) N-doped TiO2 with EDTA as N source(Ti:N = 13:1) (b), and N-doped TiO2 with NH3 flow as N source (c). The enlargedpeak at (121) for the three samples are shown in (B). ‘‘�’’and ‘‘#’’ stand for anataseand brookite phase, respectively.

2.2. Experimental procedure

N-doped TiO2 was prepared by sol–gel method. First, 5 mL TiCl4 was dilutedwith 200 mL icy water (0 �C). Then, certain amount of EDTA such as 0.5 g(1.3 mM) for N doped TiO2 with the ratio of Ti:N = 13:1 was added to the abovesolution under vigorous stirring. To make sure that hydrolysis was completed;NaOH (aq) (10%) was added dropwisely into the above transparent TiCl4 aqueoussolution to obtain white precipitate. To remove residues Na+ and OH� ions, the pre-cipitate was rinsed by distilled water until the pH was slightly alkaline (pH <7.5).Then, the acquired amorphous EDTA–TiO2 was well dispersed into water by theaddition of chloride acid solution (20%) with corresponding amount. The above sus-pended solution was controlled to reach pH 1.5 and had been stirred for 4 h at roomtemperature. Subsequently, it had been aged at 70 �C for 48 h. Finally, EDTA modi-fied TiO2 sol was generated with the uniform, stable, and semitransparent attributeand it could keep such a homogenous distribution state for a long time. The EDTAmodified titanium xerogel powder was prepared by aging, gelation, and vacuumdrying treatment of sol sample at 70 �C for 8 h. The same process was implementedto synthesize pure TiO2 sol without the addition of EDTA. The obtained xerogelswere annealed in air at 400 �C for 3 h and finally N-doped TiO2 and pure TiO2 sam-ples were obtained. For comparison, N-doped TiO2 was also prepared by annealingpure TiO2 xerogel with the presence of NH3 flow at 400 �C for 3 h.

The experiment of liquid photocatalytic reaction was conducted in a cylindricalglass photoreaction with an effective volume of 100 mL. N-doped TiO2 and pureTiO2 were acted as photocatalysts. Xenon lamp (Lap Pu, XQ) of 350 W was usedas a light source with a 420 nm cut-off filter right above the reactor to provide onlyvisible light. The experiments were performed in an self-constructed beaker-likeglassware with double walls for cooling water circulation. The samples (0.1 g) weredispersed in 10 lM RhB aqueous solution (100 mL). Prior to visible-light irradiation,the solution was stirred in dark for 0.5 h to reach adsorption/desorption equilib-rium. Then, the lamp was turned on and oxygen aeration was performed with anair pump to ensure constant supply of oxygen. With an interval of 30 min, 2 mL ali-quots were sampled out and centrifuged to remove particles. The collected super-natant was investigated by Shimadzu UV-1700 UV–vis spectrophotometer,yielding the residual concentration of RhB (kmax = 554 nm).

The photocatalytic activity of the samples for the gas-phase oxidation of ben-zene was tested on a closed cylindrical stainless gas-phase batch reactor with a vol-ume of 7.2 L. An UV lamp was set on the inner wall of the reactor. In order to getvisible light, a filter glass was used to cut off UV light, which only allowed the lightwith the wavelength of above 420 nm to penetrate. The reactor was connected to aGC9560 gas chromatograph equipped with a flame ionization detector (FID), amethane converter, a Porapak R column and PEG20M column through an automat-ically sampling 10-way valve (VALCO) with an air actuator. The photocatalysts werepretreated by coating an aqueous suspension of sample powders onto three disheswith diameter of 7.0 cm. The weight of the photocatalyst used for each experimentwas kept at 0.5 g. The photocatalysts dispersed in ethanol on the dishes were pre-treated in an oven at 70 �C for about 2 h and then cooled to 25 �C before use. Afterthe dishes coated with these powder photocatalysts were placed in the reactor, aknown amount of benzene (2 mL) was injected in the reactor. Then, the lamp wasturned on. The concentration of CO2 produced was obtained by subtracting the ini-tial concentration of CO2 in air. For testing the photocatalytic oxidation, the reactorwas put in an ice-water bath to maintain the reaction temperature at 40 �C. Thecoated photocatalyst was irradiated under visible light for 85 min. The whole pro-cess was similar to that shown in [19].

2.3. Analytic methods

X-ray diffraction (XRD) measurements were carried out by Y-2000 diffractom-eter (D/max 30 kV) with Cu Ka radiation (k = 0.154178 nm) at a scan rate of 0.06�2h s�1 for the characterization of crystalline phase, phase composition, and crystal-lite size. To probe structural changes at micrometer scale, Raman spectra were re-corded at room temperature by Jobin Yvon LabRAM HR 800UV micro-Ramansystem under Ar+ (514.5 nm) laser excitation. UV–vis diffuse reflectance spectra(DRS) were recorded with PerkinElmer Lambda35 Spectrophotometer. For surfacechemical analysis, X-ray photoelectron spectra (XPS) measurements were per-formed by PHI Quantum 2000 XPS system with monochromatic Al Ka source andcharge neutralizer. Nitrogen adsorption/desorption isotherms and Brunauer–Em-mett–Teller (BET) specific surface area (SBET) were measured by Bel minisorp ana-lyzer (accelerated surface area and porosimetry system), in which all the sampleswere degassed at 100 �C prior to BET measurements. Barret–Joyner–Halender(BJH) method was used to determine the pore size.

The photoluminescence (PL) spectra of the prepared samples were measured bya photoluminescence spectrometer (PerkinElmer Lambda 55). The formation of hy-droxyl radicals (�OH) on the surface of the prepared samples in water under visiblelight was detected by terephthalic acid (TA) probe method [20]. The PL spectra ofgenerated 2-hydroxyterephtalic acid (TAOH) were measured by using the samefluorescence photoluminescence spectrometer.

3. Results and discussion

3.1. XRD analysis

The XRD patterns of N-doped TiO2 prepared by two methodsand pure TiO2 samples are shown in Fig. 1A. It can be seen that al-most all of the samples had peaks present at 25.5�, 31.89�, 38.33�,45.5�, 48.35�, and 54.5�, which was attributed to TiO2 crystallitewith anatase and brookite mixed phases (JPCD card number 21–1272 and 15–875, respectively). Since the XRD pattern for N-dopedTiO2 with Ti:N = 23:1 was similar to that of the sample with

Fig. 2. (A) Raman spectra of pure TiO2 (a) N-doped TiO2 with EDTA as N source(Ti:N = 23:1) (b), and N-doped TiO2 with EDTA as N source (Ti:N = 13:1) (c). (B) Theenlarged peak at 143.46 cm�1.

Fig. 3. UV–vis diffuse reflectance spectra of pure TiO2 (a), N-doped TiO2 with EDTAas N source (Ti:N = 23:1) (b), N-doped TiO2 with EDTA as N source (Ti:N = 13:1) (c),N-doped TiO2 with EDTA as N source (Ti:N = 8:1) (d), and N-doped TiO2 with EDTAas N source (Ti:N = 65:1) (e).

230 D. Nassoko et al. / Journal of Alloys and Compounds 540 (2012) 228–235

Ti:N = 13:1, the data are not shown here. Substantially, for all ofthe samples, the peak belonging to brookite (121) became strongerthan that attributed to anatase (101). In our experiment, the addi-tion of NaOH to reaction solution increased pH value, leading to theincrease of OH� concentration. This might promote the formationof brookite phase because its complex consumed the amount ofOH� twice as much as that for rutile phase. However, too muchNaOH could result in the preferred formation of amorphous TiO2

which contained much more OH� than the brookite complex[21–24]. From the amplified peaks corresponding to (121) crystalplane (Fig. 1B), it can be obviously seen that a slight shift to lowerangle was observed for N-doped TiO2 samples compared with theundoped one. It indicates that nitrogen from both EDTA and NH3

may be incorporated into the lattice of TiO2. The average crystallitesize can be calculated by Scherrer’s formula: D = Kk/bcosh, where Dis the crystallite size, K the shape factor (about 0.9), k the X-rayradiation wavelength, b the full line width at half-maximum heightof the main intensity peak after subtraction of equipment linebroadening and h the diffraction angle. The calculated crystallitesize using the full width at half-maximum of (121) peak of brook-ite phase was 18.4, 16.3, 16.1, and 16.1 nm for pure TiO2 nanopar-ticles, N-doped TiO2 with EDTA as N source of Ti:N = 23:1, N-dopedTiO2 with EDTA of Ti:N = 13:1 and N-doped TiO2 prepared withNH3 flow, respectively. Therefore, there was no big difference ofthe crystallite size for all of the samples, which was further verifiedby SEM images shown in Fig. S1 (Supporting information). It is wellknown that the anatase phase of TiO2 has photocatalytic activity.Its synergy with brookite phase may result in good activity as well.

3.2. Raman spectra

The comparison of Raman spectra of pure TiO2 and N-dopedTiO2 is shown in Fig. 2A. It can be seen that four resolved Ramanpeaks were observed at about 143.73, 396.26, 514.66, 640.80cm�1 for all of the samples. These peaks were attributed to Eg,B1g, A1g, and Eg modes of anatase phase for TiO2, respectively[24]. Additionally, there were two weak peaks at 240.8 and321.6 cm�1 attributed to A1g and B1g vibration modes for brookitephase of TiO2. Besides, no additional peak about nitrogen was pres-ent. The amplified figure for the peak at about 143.73 cm�1 isshown in Fig. 2B. It can be found that there was a slight shift tohigh wave numbers after N doping, which is also an evidence toverify that N may be introduced into the lattice of titania [25]. Asfor the reason why there were strong peaks for brookite phase inXRD data while Raman spectra only had small signals, it is wellknown that Raman spectroscopy as a surface sensitive techniqueis often employed to confirm phase composition and surfacehomogeneity [26] and XRD usually reveals average structuralinformation within several unit cells. For the prepared samples,the ratio of anatase and brookite phase in TiO2 could be differentin bulk and surface. It was lower in bulk while higher in surfacearea, leading to the strong peaks for brookite phase in XRD datawhile weak ones in Raman spectra.

3.3. UV–vis absorption spectra

The optical absorption spectra of pure and N-doped samplesTiO2 are shown in Fig. 3. It can be seen that there was a significantshift of the absorption spectra towards visible light (k > 420 nm)for N-doped TiO2 samples with EDTA as nitrogen source whencompared with pure TiO2. This enhanced absorption in visible re-gion is the typical feature of nitrogen-doped TiO2 [10]. The shiftof the absorption edges for the doped TiO2 samples indicates thatoptical band gap decreased in the order of pure TiO2 and N-dopedTiO2. From the plot of the square root of (ahm)1/2 versus the photonenergy, the optical band gap for materials can be obtained. The

band gap values estimated from Fig. S2 (Supporting information)were 2.29, 2.56, and 3.11 eV for N-doped TiO2 with Ti:N = 13:1,N-doped TiO2 with Ti:N = 23:1 and pure TiO2, respectively. There-fore, N doping definitely decreased the band gap of TiO2. Addition-ally, with the increase of N doping amount, the band gap graduallybecame small. So, the band gap values for the samples highly de-

D. Nassoko et al. / Journal of Alloys and Compounds 540 (2012) 228–235 231

pended on the amount of dopant. When the doping amount wastoo large, especially for the sample with Ti:N = 8:1, the absorptionability for visible light became even higher than any other samplesshown in Fig. 3 and it is hard to calculate its band gap. It was pre-viously reported that the nitrogen doping can lead to a mixing of N2p orbital with O 2p orbital to form intermediate energy levels andshift the absorption edge toward visible light region [27] or the for-mation of the isolated N 2p narrow band above the O 2p valenceband for visible light response [28–30]. In this case, since therewas no distinct peak corresponding to doping level shown in theUV–vis spectra, nitrogen doping from EDTA resulted in the combi-nation of N 2p with O 2p orbital for the formation of intermediateenergy level. This promoted electronic excitation from valenceband to the intermediate energy level by the absorption of visiblelight.

3.4. XPS analysis

XPS technique is used to investigate chemical component at thesurface of a sample. High resolution XPS spectrum comparison of Ti2p for the samples of pure TiO2 and N-doped TiO2 with Ti:N = 13:1is shown in Fig. 4. The two peaks of pure TiO2 at 465.57 and459.54 eV were typical binding energy of Ti4+ 2p1/2 and Ti4+ 2p3/

2, respectively. However, for the N doped titania, there was a broadpeak in the range from 463 to 458 eV. Through Gaussian fitting,two peaks at 460.56 and 458.66 eV were obtained.

According to the report by [31] the Ti 2p3/2 core level decreasesfrom 459.05 to 458.25 eV after nitrogen doping. Therefore, thepeak at 458.66 eV in our case could be arisen due to N doping. Itis well known that the electronegativity of nitrogen is lower thanthat of oxygen. When nitrogen was doped into TiO2 lattice, partialTi4+ might have electronic interaction with nitrogen. The lowerelectronegativity of nitrogen resulted in the decrease of percenticonicity and electron density around N anion, and thus electrondensity on Ti increased [30]. So, the binding energy of Ti 2p3/2 de-creased after nitrogen doping. From the XPS spectrum of Ti 2p, wecan conclude that nitrogen is incorporated into TiO2 lattice. Thenitrogen incorporated into titania lattice can exist in two forms:interstitial nitrogen species (Ti–O–N) or substitution nitrogen spe-cies (O–Ti–N). It was reported that when only Ti–O–N like specieswere detected by XPS, no band gap narrowing was obtained by cal-culations [32]. In our case, there was an obvious decrease of the

Fig. 4. High resolution XPS spectra of Ti2p for pure TiO2 (a) and for N-doped TiO2

with EDTA as N source (Ti:N = 13:1) (b).

band gap for nitrogen doped TiO2 from the calculation of theUV–Vis spectra ((from 3.11 to 2.29 eV shown in Fig. S2 in Support-ing information) so that the nitrogen was the substitution of nitro-gen species.

3.5. Surface area measurements

Brunauer–Emmet–Teller (BET) gas adsorption method has be-come the most widely used standard procedure for the determina-tion of surface area of finely-divided and porous materials. FromN2 adsorption/desorption measurements, two important parame-ters, namely surface area and porosity, can be determined. Bothparameters are crucial from the point of view of catalysis becauselarge surface area often results in the increase of catalytic activity.The surface area for pure TiO2 and N-doped TiO2 with Ti:N of 23:1,13:1, 8:1 was 72.46, 77.12, 97.93, and 83.11 m2/g, respectively.The presence of EDTA during sol preparation was responsible forthe increase of surface area because it functioned as soft templatefor the formation of pore structure. However, more EDTA (Ti:N =8:1) led to the decrease of the surface area. The reason is unknownnow. It might be that more EDTA contributed to surface area de-crease through particle agglomeration and/or pore closing. Theadsorption/desorption isotherms for pure TiO2 and N-doped TiO2

samples is shown in Fig. 5. In accordance with the IUPAC classifica-tion, the N2 adsorption/desorption isotherm for pure TiO2 (Fig. 5a)was type IV with H2 hysteresis-loop, which is associated with capil-lary condensation taking place in mesopores [32]. The isotherms forthe N-doped TiO2 samples displayed type IV with H3 type hysteresisloop. This indicates that the powders had mesopores with narrowslit like shape [33]. So, they had more uniform pore distribution thanpure TiO2. The pore size distribution for all of the samples is shown inFig. S3 (Supporting information). Generally speaking, by using thesol–gel method with TiCl4 as a precursor, all of the prepared sampleshad small pore size with narrow distribution. For pure TiO2, the poresize distributed in the range of 1.2–8.0 nm and most of the pores hadthe size of 3.5 nm. When nitrogen from EDTA was incorporated intoTiO2, pore size distribution became much narrower and part of thepores was attributed to micropore, especially for the sample withTi:N = 13:1. The pore size distribution for this sample was much nar-rower than that for other N-doped samples. The pore size was lessthan 4 nm and most of pores had the size of 1.6 nm. Additionally,the N-doped TiO2 with Ti:N = 23:1 had narrower pore size distribu-tion than the sample with Ti:N = 8:1. Substantially, nitrogen dopingfrom EDTA results in the great change of pore distribution of N-doped TiO2. The mesoporous structure of these samples was formedby the agglomeration of TiO2 nanoparticles through sol–gel process.Slow hydrolysis under long time aging promoted the formation ofmono-dispersed TiO2 sol particles. Thus, the mesoporous TiO2 witha relatively narrow pore size distribution was produced by theagglomeration of the mono-dispersed TiO2 sol particles. The evapo-ration of uniformly dispersed EDTA at annealing temperaturecaused the formation of more mono-dispersed pores, in which EDTAonce was present. So, the functions of EDTA were not only as nitro-gen source but also as a soft template [24].

3.6. Photocatalytic activities

The photocatalytic activities of the samples were evaluated byusing the degradation of RhB and benzene under visible-light irra-diation. The degradation efficiency for RhB is shown in Fig. 6. It canbe seen that when RhB solution was irradiated in the absence ofany photocatalyst, there was a slight self-degradation after 210min. With the presence of a photocatalyst, obvious degradationwas observed. Moreover, N-doped TiO2 with EDTA as nitrogensource with Ti:N = 13:1 and Ti:N = 23:1 had better activity forRhB degradation than that prepared under NH3 flow and the sam-

Fig. 5. N2 adsorption/desorption isotherms of pure TiO2 (a), N-doped TiO2 with EDTA as N source (Ti:N = 13:1) (b), N-doped TiO2 with EDTA as N source (Ti:N = 23:1) (c), andN-doped TiO2 with EDTA as N source (Ti:N = 8:1) (d).

Fig. 6. Photocatalytic degradation of Rh-B under visible light (300 W Xenon lampwith a cutoff filter) with different conditions: (a) self-degradation, (b) over pureTiO2, (c) over N-doped TiO2 with Ti:N = 23:1, (d) over N-doped TiO2 withTi:N = 13:1, (e) over N-doped TiO2 with Ti:N = 8:1, and (f) over N-doped TiO2 withNH3 flow as N source.

232 D. Nassoko et al. / Journal of Alloys and Compounds 540 (2012) 228–235

ple with Ti:N = 13:1 was the best one. Meanwhile, pure TiO2 couldalso degrade RhB although the activity was not high. According toRhB degradation results, it can be seen that more amount of N

doping for the sample with Ti:N = 8:1 led to the lowest degradationefficiency of RhB.

To make clear the decomposition mechanism of organic pollu-tants RhB, different degradation pathways has been investigatedduring the degradation process such as N-deethylation, chromo-phore cleavage, and mineralization. From the data shown inFig. S4 (Supporting information), we can notice the spectralchanges of RhB during visible-light irradiation. RhB underwentan efficient N-deethylation process with the presence of the pre-pared samples under visible light [34], while chromophore struc-ture cleavage and even mineralization could be also achievedunder identical conditions. Both conjugated chromophore struc-ture cleavage and N-deethylation occurred simultaneously here.Among all the samples, for the sample with low photocatalyticactivity, the wavelength of the maximum peak kept around553.5 nm so that it mainly followed N-deethylation process. Forthe N-doped sample with EDTA as nitrogen source of Ti:N = 13:1,the maximum absorption peak shifted the most and the color ofthe suspension changed gradually, indicating the RhB chromo-phore structure was destroyed at the same time [35].

The degradation efficiency for benzene over different photocat-alyts is demonstrated in Fig. 7A. In order to confirm benzenemolecular is destroyed, the concentration of the final product ofbenzene degradation, CO2, was also recorded with the increase oftime and the data are shown in Fig. 7B. Substantially, N-dopedTiO2 with EDTA as nitrogen source had higher photocatalytic activ-ities. Although from benzene degradation plot (Fig. 7A), it is hard todistinguish the activity of the two N-doped TiO2 samples with dif-ferent N amount from EDTA, the answer can be obviously pre-sented in Fig. 7B. There was more CO2 evolution after 20 min for

Fig. 7. (A) Photocatalytic degradation of benzene under visible light (300 W Hglamp with a cutoff filter) over pure TiO2 (a), N-doped TiO2 with Ti:N = 23:1 (b), N-doped TiO2 with Ti:N = 13:1 (c), and N-doped TiO2 with NH3 flow as N source,respectively (d); (B) the amount of CO2 produced from the photocatalyticdegradation of benzene over pure TiO2 (a) N-doped TiO2 with Ti:N = 23:1, (b) N-doped TiO2 with Ti:N = 13:1 (c), and N-doped TiO2 with NH3 flow as N source (d).

D. Nassoko et al. / Journal of Alloys and Compounds 540 (2012) 228–235 233

the sample with Ti:N = 13:1 than that with Ti:N = 23:1. Addition-ally, CO2 evolution rate was not six times of benzene removal rate.It indicates that in this case, benzene was not destroyed to becomeCO2 directly and there were other organic intermediates. The detailneeds further investigation. On the other hand, although pure TiO2

was more active for benzene concentration decrease than N-dopedTiO2 with NH3 flow, its corresponding CO2 formation amount wasthe lowest under the same irradiation time.

From the kinetic studies on photocatalytic reactions of RhB andbenzene over all of the prepared samples in the studied concentra-tion range, it is found that the reactions can be well explained by apseudo-first-order pattern with the following equation:

ln C=C0 ¼ �kappt

where C0 is the initial concentration, C is concentration after reac-tion time t, and kapp is the first order apparent rate constant. The va-lue of kapp was determined from the slope of the graphs plottedbetween �ln C/C0 and the reaction time t. The calculated kapp results

are summarized in Table S1 (Supporting information). By the com-parison of kapp values, the overall photocatalytic (RhB and benzene)degradation efficiency over different photocatalysts was observedto decrease in the order of N-doped TiO2 with Ti:N = 13:1, N-dopedTiO2 with Ti:N = 23:1, pure TiO2 and N-doped TiO2 with NH3 as Nsource and kapp value for the N-doped TiO2 with Ti:N = 13:1 wasmuch higher than that for other samples.

It is known that RhB can be excited by visible light and injectselectrons to the conduction band of TiO2. The injected electronsreacted with O2 molecules that adsorbed on TiO2 surface to yieldO�2 radical anion and subsequently OH radical by protonation[32]. So, in our case, RhB can be degraded in even pure TiO2 systemalthough its photodegradation efficiency is not high. Additionally,the filter used to remove the radiation with the wavelength lessthan 420 nm may not screen out all of UV light. So, small amountof UV light still penetrated and hit TiO2 nanoparticles, leading tothe formation of photogenerated electrons and holes over pureTiO2 for RhB and benzene degradation.

The general rule for semiconductor photocatalysis is that theactivity is mainly influenced by three factors: (a) capacity for elec-tron–hole generation, (b) electron transfer route and efficiency and(c) separation efficiency of photogenerated electron–hole pairs[36,37]. Since pure TiO2 has wide band gap, it is difficult to produceelectrons and holes under visible light. It is reasonable for pureTiO2 to have low or no activity for RhB and benzene degradation.For the sample of N-doped TiO2 with NH3 as N source, it can beseen from its UV–vis diffuse reflectance spectrum (Fig. S5 in Sup-porting information) that this sample had best absorption abilityfor UV–visible light and there was a board peak corresponding toa doping level. This doping level may function as a recombinationcenter of photogenerated hole and electron, which hampers thedegradation of organic compounds. So, although electron and holecan be easily produced under visible light for this sample, electrontransfer efficiency may be lower due to the lower energy the elec-tron has [24]. Generally speaking, the electron photo-generatedfrom long wavelength absorbance has low energy and is easy torecombine with hole again. So, the presence of the obvious dopinglevel may result in the low photocatalytic activity of the sample ofN-doped TiO2 with NH3 as N source.

To explore the electron–hole recombination characteristic forthe prepared samples, PL studies have been investigated. It isknown that the PL emission results from the recombination of ex-cited electrons and holes. Thus the higher PL intensity indicates ahigher recombination rate [38–40]. Fig. 8 shows the PL spectra ofN-doped TiO2 with Ti:N = 23:1, Ti:N = 13:1 and N doped TiO2 underNH3 flow. It can be observed that the PL intensity of N doped TiO2

with Ti:N = 13:1 is significantly lower than that of the other twosamples. This result indicates that the recombination of charge car-riers effectively decreased after incorporating N in the TiO2 lattice.So, more photogenerated charge carriers can participate in organiccompound degradation, resulting in the enhancement of photocat-alytic activity. The presence of N might improve the migration andseparation of photogenerated electrons and holes [41,42].

To further verify the mechanism, the production of major reac-tive species, hydroxyl radicals (�OH), was monitored by a PL tech-nique using TA as probe molecule. TA easily reacts with hydroxylradicals to generate highly fluorescent hydroxyl terephthalic acid.Thus, the amount of hydroxyl radicals can be quantified by mea-suring the fluorescence of TAOH. The ready reaction of photogener-ated �OH with TA gave rise to highly fluorescence TAOH withtypical PL signal centered at 435 nm. From Fig. 9, it can be seen thatthe amount of the hydroxyl radicals produced over N-doped TiO2

with the ratio Ti:N = 13:1 were larger than those over N dopedTiO2 under NH3 flow and pure TiO2, which is consistent with thephotocatalytic activity result shown in Figs. 6 and 7. N-dopedTiO2 with EDTA as N source shows higher hydroxyl radicals

Fig. 8. PL spectra of N doped TiO2 with EDTA as N source (Ti:N = 23:1) (a), N dopedTiO2 with EDTA as N source (Ti:N = 13:1) (b), and N-doped TiO2 with NH3 flow as Nsource (c).

Fig. 9. Plots of the induced PL intensity at 435 nm against irradiation time forterephthalic acid over pure TiO2 (a), over N-doped TiO2 with NH3 flow as N source(b), and over N-doped TiO2 with EDTA as N source (Ti:N = 13:1) (c).

234 D. Nassoko et al. / Journal of Alloys and Compounds 540 (2012) 228–235

production than the one doped with NH3 flow as N source and pureTiO2. Consequently EDTA as N source may be a suitable precursorfor the preparation of N doped TiO2 photocatalysts with high activ-ity under visible light.

Above all, for the samples of N-doped TiO2 with EDTA as Nsource, since they have suitable band gap for electron–hole pairformation and electron transfer to surface for photocatalysis, Nincorporated into the lattice of TiO2 for the improvement of elec-tron–hole separation and hydroxide radical formation, and porousstructure for organic pollutant adsorbed onto the surface, it isindubitable for the samples to have better photocatalytic activityfor even benzene degradation. Herein, there is a suitable nitrogendoping amount for N-doped TiO2 samples based on photocatalyticactivity. The ratio of Ti:N = 13:1 is the best (Fig. 7). The photocata-lytic activity was the lowest for the N-doped TiO2 with the ratio ofTi:N = 8:1. The highest photocatalytic activity of this N-doped TiO2

sample might be partially due to its more intense absorption in the

visible light range (Fig. 3), more red shift in the band gap transitionof the doped TiO2, highest surface area and mesoporous structurecompared with other N-doped samples prepared from EDTA,which resulted in more photogenerated electrons and holes partic-ipating in the photocatalytic reactions under visible light and morepollutants adsorbed on the surface of the photocatalyst for degra-dation. Larger surface area provided more active sites on the sam-ple surface for the organic pollutant to be adsorbed and degraded.XPS analysis shows that nitrogen has been incorporated into TiO2

lattice (Fig. 4). The photocatalytic activity of N–TiO2 is improvedby the doped nitrogen from EDTA, because the nitrogen replacesthe oxygen in the crystal lattice of TiO2, which results in the reduc-tion of band gap for TiO2 [43,44], the improvement of photogener-ated electron–hole separation and the formation of hydroxideradicals for degradation. However, there was a suitable dopingamount. If there was less or more nitrogen incorporated intoTiO2, nitrogen doping could not have good effect and the samplescould not have good activity under visible light either.

4. Conclusions

EDTA as nitrogen source and soft template is successfully usedto dope TiO2 through sol–gel method. The prepared N-doped TiO2

with EDTA is found to be an active photocatalyst for organic pollu-tant degradation under visible light compared with that preparedunder NH3 flow. Substantially, the sample can degrade stable andtoxic benzene. Additionally, the amount of nitrogen in TiO2 has af-fected the photocatalytic activity of the prepared samples. In thiscase, the preparation ratio of Ti:N = 13:1 is the best. Nitrogenincorporation into TiO2 lattice leads to the decrease of band gapfor visible light response, the improvement of photogeneratedelectron–hole separation and the more amount of the formationof hydroxide radicals. Meanwhile, N-doped TiO2 from EDTA hashigher surface area and narrower pore distribution. Therefore,the sample of N-doped TiO2 from EDTA may be a promising photo-catalyst for future application in practice.

Acknowledgements

This work was financially supported by the National NaturalScience Foundation of China (No. 20973070), the National Basic Re-search Program of China (No. 2009CB939704), the Key Project ofNatural Science Foundation of Hubei Province (No. 2011CDA092)and self-determined research funds of CCNU from the colleges’ ba-sic research and operation of MOE.

Appendix A. Supplementary data

Supplementary data associated with this article can be found,in the online version, at http://dx.doi.org/10.1016/j.jallcom.2012.06.085.

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