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Journal of Hazardous Materials 184 (2010) 855–863

Contents lists available at ScienceDirect

Journal of Hazardous

Materials

j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /j h a z m a

Nitrogen-doped TiO 2nanotube array ?lms with enhanced photocatalytic activity under various light sources

Yue-Kun Lai a ,b ,Jian-Ying Huang c ,Hui-Fang Zhang a ,Vishnu-Priya Subramaniam b ,Yu-Xin Tang b ,Dang-Guo Gong b ,Latha Sundar b ,Lan Sun a ,Zhong Chen b ,Chang-Jian Lin a ,?

State Key Laboratory of Physical Chemistry of Solid Surfaces,and College of Chemistry and Chemical Engineering,Xiamen University,Xiamen 361005,China b

School of Materials Science and Engineering,Nanyang Technological University,50Nanyang Avenue,Singapore 639798,Singapore c

Fujian Institute of Research on the Structure of Matter,Chinese Academy of Sciences,Fuzhou 350002,China

a r t i c l e i n f o Article history:

Received 17March 2010

Received in revised form 28August 2010Accepted 31August 2010

Available online 15 September 2010Keywords:

TiO 2nanotube array Nitrogen-doping Visible light activity Methyl orange Intermediate

a b s t r a c t

Highly ordered nitrogen-doped titanium dioxide (N-doped TiO 2)nanotube array ?lms with enhanced photocatalytic activity were fabricated by electrochemical anodization,followed by a wet immersion and annealing post-treatment.The morphology,structure and compostition of the N-doped TiO 2nan-otube array ?lms were investigated by FESEM,XPS,UV-vis and XRD.The effect of annealing temperature on the morphology,structures,photoelectrochemical property and photo-absorption of the N-doped TiO 2nanotube array ?lms was investigated.Liquid chromatography and mass spectrometry were applied to the analysis of the intermediates coming from the photocatalytic degradation of MO.The experimen-tal results showed that there were four primary intermediates existing in the photocatalytic https://www.wendangku.net/doc/5d9881002.html,pared with the pure TiO 2nanotube array ?lm,the N-doped TiO 2nanotubes exhibited higher pho-tocatalytic activity in degradating methyl orange into non-toxic inorganic products under both UV and simulated sunlight irradiation.

1.Introduction

Since the discovery of water photolysis on TiO 2electrode by Fujishima and Honda [1]in 1972,TiO 2became one of the most widely researched materials for use in solar cells [2,3],pollutant degradation [4–6],photolysis of water [7,8],gas sensor [9,10]and bio-applications [11,12]due to its unique and favorable physio-chemical properties.However,the anatase TiO 2material cannot ef?ciently utilize visible light ( >380nm)of the solar energy because of its comparably large band gap.To overcome this prob-lem,considerable efforts have been taken to narrow the band gap.Doping with different types of transition metal cations [13–15],surface modi?cation with noble metal [16–19],as well as doping with nonmetal anions [20–41]have been explored in an effort to increase the visible light absorption or suppress the recombination of photogenerated carries.

Asahi et al.reported a visible light active TiO 2?x N x ?lm by sput-tering the TiO 2target in a N 2/Ar gas mixture,which attracted a great attention of N-doped TiO 2as a visible light photocatalyst [20].The common approaches to form N-doped TiO 2photocatalyst include sputtering of TiO 2targets in N 2mixture gas [31,32],annealing

?Corresponding author.

E-mail address:cjlin@https://www.wendangku.net/doc/5d9881002.html, (C.-J.Lin).

TiO 2or Ti-compounds under ammonia gas [33–37],ion implanta-tion and thermal treatment [38,39],and hydrolysis of N-containing solutions [40,41].

Although N-doped TiO 2has been widely fabricated and stud-ied for the photocatalysis under visible light irradiation,most of the work uses either powders or thin compact ?lms.In the present work,we investigate the N-doped TiO 2nanotube array ?lm prepared by treating TiO 2nanotube array ?lm with ammo-nia solution.This method avoided the use of hazardous ammonia gas,or laborious ion implantation process.Moreover,to our knowl-edge,there are so far few reports about the effect of N-doping and annealing temperature on the photocatalytic performance of TiO 2photocatalyst under different light sources.In the current work,high-pressure mercury lamp and tungsten–halogen lamp light sources have been used.Photocatalytic activity of doped and undoped TiO 2nanotube ?lms was investigated.2.Experimental

2.1.Preparation of N-doped TiO 2nanotube arrays

Highly ordered TiO 2nanotube arrays with a tube length about 350nm were grown from Ti sheets (>99.6%purity)via electrochem-ical anodization in 0.5%HF electrolyte with Pt counter electrode under 20V for 20min as previously described in the literature

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184 (2010) 855–863

Fig.1.SEM top-view images of (a)TiO 2nanotube array annealed at 450?C,N-doped TiO 2nanotube arrays annealed at various temperatures (b)450?C,(c)600?C,and (d)700?C.

[42,43].The as-prepared samples were immersed in 1M NH 3·H 2O solution for 10h and annealed in a muf?e furnace under ambient atmosphere for 2h to obtain N-doped TiO 2nanotube array elec-trode with crystalline phase.

2.2.Characterization of N-doped TiO 2nanotube arrays

The morphologies of the prepared samples were observed using a ?eld emission scanning electron microscope (FESEM,LEO-1530)and their crystalline phase was identi?ed using an X-ray diffrac-tometer (Philips,Panalytical X’pert,Cu K ?radiation).The surface chemical composition of samples was analyzed by X-ray photoelec-tron spectroscopy (XPS,PHI Quantum 2000)with Al K ?radiation source.All the binding energies were referenced to the C1s peak at 284.8eV of surface adventitious carbon.The absorption prop-erties of the samples were recorded using a diffuse re?ectance UV-vis spectrometer (Varian,Cary 5000)with wavelength range of 300–650nm.

2.3.Photoelectrochemical and photocatalytic measurements Photoelectrochemical measurements were carried out in 0.1M Na 2SO 4solution using an LHX 150Xe lamp,a SBP 300grating spec-trometer,and an electrochemical cell with a quartz window.The generated photocurrent signal was collected by a lock-in ampli?er (5210,EG and G,PAR Co.,USA)with a light chopper at zero bias with a step of 5nm in the range of 300–600nm.The reproducibil-ity was checked by repeating the measurement at least three times and the average value is taken as the reported photocurrent.Elec-trochemical impedance spectroscopy (EIS)spectra were measured by applying an AC voltage of 10mV amplitude within the frequency range of 105–10?2Hz in 0.1mol L ?1Na 2SO 4solution.

For the photocatalytic degradation experiments,methyl orange (MO)was chosen as a target compound.The detail processes are similar with the ones previously described in the literature [44,45].The initial concentration of the dye was 20mg/L and the pH value of the MO solution (pH =3.0)was adjusted with H 2SO 4.The quartz

glass reactor was equipped with a water jacket to control the temperature.The photo-irradiation was performed with a 200W high-pressure mercury lamp emitting at a wavelength of 365nm as the UV light source and 500W tungsten–halogen lamp was used to produce the simulated sunlight.Before the photocatalytic degra-dation,the photocatalyst (1.0cm ×1.5cm)was soaked in 30mL MO solution for 30min to establish the adsorption/desorption equilib-rium.The solution periodically taken from the reactor was analyzed with a UV-vis spectrophotometer (Shimadzu UV-2100,Japan).The analytical wavelength selected for optical absorbance measure-ment was 508nm.The blank test was also carried out by irradiating MO homogeneous solution without TiO 2photocatalyst for checking the self-photolysis of MO.

Samples were taken from the reaction vessels and ?ltered through a 0.2?m cellulose membrane ?lter.HPLC analysis was carried out using Agilent 1200series HPLC with UV DAD detec-tor.RP-C18column (Agilent Zorbax XDB C-18,250mm ×4.6mm;5mm particles)was used to separate the degradation products present in the reaction mixture.Acetonitrile/Ammonium acetate (10mM,pH =6.6)were used as a mobile phase with 24/76(v/v)and 0.8ml min ?1as a ?ow rate.The products were detected by UV-vis diode array detector.MALDI-TOF mass spectra were recorded on a Shimadzu Biotech Axima ToF 2MS instrument equipped with the delayed extraction option.Ionisation (negative)was achieved using a N 2laser source (337nm).The mass spectrometer was oper-ated in a re?ectron negative mode,and for this operation mode the instrument was calibrated using small-cal-mix samples.The analyses were done without aid of any matrix.

3.Results and discussion

3.1.Characterization of N-doped TiO 2nanotube array ?lms

Fig.1shows top-view SEM images of the as-prepared TiO 2nan-otube array annealed at 450?C (Fig.1(a))and the N-doped TiO 2nanotube array (Fig.1(b)–(d))annealed at 450,600and 700?C for

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857

Fig.2.XRD patterns of the pure TiO2nanotube array?lm and N-doped TiO2nan-otube array?lms annealed at temperatures ranging from300to700?C.Pure TiO2 nanotube samples:(a)before annealing,(b)450?C;N-doped TiO2nanotube sam-ples:(c)300?C,(d)450?C,(e)500?C,(f)600?C,and(g)700?C.A,R,and T represent anatase,rutile and titanium,respectively.

2h.Both N-doped and pure TiO2?lm annealed at450?C show similar morphology to the as-prepared sample.This indicates low temperatures have no great effect on surface morphology and architecture of the TiO2nanotube array.The nanotube arrays have an average tube diameter around80nm and a wall thickness of about15nm.When annealed at600?C,the diameter of the N-doped TiO2nanotube slightly decreases and the wall thickness increases, indicating the obvious anatase crystal growth and rutile phase tran-sition of TiO2nanotube.When annealing temperature increased to 700?C,some part of the nanotube array architecture collapsed,this is ascribed to high temperature and phase transition heat lead to the rapid grain growth within the thin tube wall and in the underlying titanium foil[46–48].

It is well known that surface morphology change is close related to crystal growth and phase transformation.Therefore,XRD was investigated to analyze the effect phase transformation on the change of surface morphology and TiO2nanotube structures.Fig.2 shows the XRD patterns for the pure titania nanotube?lm and N-doped titania nanotube?lms under various annealing tempera-tures.The as-grown TiO2thin?lm(curve a)exhibits an amorphous structure except for the existence of typical diffraction peaks of metallic titanium[49,50].Hence,the annealing process is neces-sary to transfer the amorphous TiO2?lm into a well-crystallized anatase phase.For samples annealed at300?C for2h(curve c), two weak diffraction peaks appeared at25.4?and48.1?,in well accordance with the(101)and(200)peaks of anatase titania, indicating the initial formation of tiny crystalline anatase phase, whereas no evidence of the existence of rutile phase is observed. Further increasing the annealing temperature,the strength of these two anatase peaks also increases.Moreover,the pure TiO2nan-otube samples have more obvious rutile peaks(curve b)than that of N-TiO2sample(curve d)under450?C heat treatment.This indi-cates that the incorporation N-dopants into the TiO2lattice via O–Ti–N bonding can suppress the anatase-rutile phase transition to enhance the thermal stablility of anatase owing to the strong crystal distortion force[51,52].In addition,the presence of porous nan-otube structures and the N-doping might inhibit the migration and the arrangement of Ti and O atoms to form rutile during high tem-perature calcinations[48,53].When annealed at500?C(curve e),a small peak appeared at27.61?,indicating the starting transition of anatase phase to more stable rutile phase.As the temperature rose (curves f and g),the diffraction peak of rutile phase became

stronger Fig.3.The high-resolution spectra of N1s region of the before(dotted line)and after 450?C annealing for2h(solid line).

and the diffraction peaks of anatase phase nearly disappeared at 700?C.

The high resolution spectrum of XPS was used to identify the nitrogen elements present on the TiO2nanotube arrays.Fig.3 shows the high resolution XPS N1s core level spectra of ammonia treated TiO2nanotube array?lm before and after annealing.It is cleared that only a strong N species peak at around402.0±0.2eV can be observed before heat treatment(dotted line).The inten-sity became weak after annealing,indicating the N-state is just molecularly chemisorbed on surface of TiO2in the soaking process. However,a new and strong peak appeared at395.9±0.2eV can be assigned to N3?substituting for O2?at anion site[54].This indi-cates that the heat treatment has led to the change from absorbed state to the substituting state in TiO2.

The UV-vis diffuse re?ection spectra of pure TiO2nanotube array?lm and N-doped TiO2nanotube array?lms by the different annealing temperature are displayed in Fig.4.It is apparent that the absorbance of all the N-doped TiO2nanotube array?lms are stronger than that of the pure TiO2nanotube array?lm at wave-lengths greater than400nm.Moreover,the absorption edges of the N-doped TiO2nanotube array?lms show a slight red-shift. This red-shift of the absorption edge is related to the N-doping in the intrinsic band gap of TiO2and the interaction between N 2p and O2p orbit.The broad absorption peaks were

identi?ed

Fig.4.UV-vis absorption spectra of TiO2nanotube arrays under different annealing temperatures.

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184 (2010) 855–863

Fig.5.Photocurrent spectra of as-prepared pure TiO2nanotube array?lm without annealing(a),pure TiO2nanotube array annealed at450?C(b)and N-doped TiO2 nanotube array?lms annealed at different temperatures.(c)300?C;(d)450?C;(e) 500?C;(f)600?C.The inset shows the(I ph×hv)1/2versus hv plots of the450?C annealed pure and N-doped TiO2nanotube array?lm.

to be the sub-band gap states of the TiO2nanotube array due to its special nanotube structures[55,56].The doped TiO2nanotube array?rstly increases the UV-vis absorption below450?C.It shows that the calcined N-doped TiO2nanotube array achieved the great absorbance at450?C.The absorbance decreases with the further increasing annealing temperature from450to700?C.The reason may be due to the phase transformation from the anatase to rutile which combined with the rapid growth of crystallites and decrease of the surface area of the TiO2nanotube array.

3.2.Photoelectrochemical property

Fig.5shows the photocurrent versus wavelength plots for the different annealing temperature of N-doped TiO2nanotube array?lms.The photocurrent?rst increased signi?cantly with the annealing temperature of the N-doped nanotube array?lm,and a maximum was observed for the sample annealed at450?C.This is due to the phase change from amorphous to anatase accord-ing to the XRD results.The increasing anatase phase ratio of the N-doped TiO2nanotube array?lms may enhance the separation and transferring ef?cient of the photo-generated carriers,resulting in an increase in photogenerated current[57,58].Further increas-ing the annealing temperature leads to a decline of photocurrent intensity,and a shift of photocurrent peak to higher wavelength. When annealed at temperatures higher than450?C,the

anatase Fig.6.Nyquist plots of the450?C annealed TiO2nanotube array electrodes in dark and under UV light illumination.

phase started to change to the more stable rutile phase with a lower band gap.This transition process is accompanied by thicker nan-otube wall or even the destruction of the uniform nanotube array structures.These situations would deteriorate the separation and transferring of photogenerated carries.For instance,comparing the photocurrent of sample annealed by450?C(curve d)and600?C (curve f),a difference of about six times was observed.

The inset?gure shows the corresponding(I ph×hv)1/2versus hv plots of the pure TiO2and N-doped TiO2samples which were used for the determination of the indirect band gap energy of the TiO2?lms.Clearly,the band gap energy of the450?C annealed N-doped TiO2nanotube array sample is approximately3.07±0.05eV, which is lower than that of the pure TiO2nanotube array sample (3.16±0.05eV)and the typical value reported for anatase phase [59,60].This is also in accordance with the above UV-vis absorption result.

Fig.6gives the typical Nyquist plots of EIS spectra for TiO2nan-otube array electrodes with or without UV light irradiation.The impedance arc radius of electrodes in the dark were much bigger than that under UV light irradiation,which indicated that there were few electrons across the TiO2-electrolyte interfaces in dark. While under the UV light illumination,the arc radius of the N-doped TiO2nanotube array electrode is smaller than that of the un-doped electrode.This demonstrated that the N-doped nanotube array electrode displayed greater separation ef?ciency of photogener-ated electron–hole pairs and faster charge transfer than that of the pure TiO2nanotube?lm at the solid–liquid interface.

Therefore,

https://www.wendangku.net/doc/5d9881002.html,parison of photocatalytic degradation rates of MO for pure TiO2and N-doped TiO2nanotube array?lms with different annealing temperatures under high-pressure mercury lamp illumination(a)and tungsten–halogen lamp illumination(b).

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Table1

Effect of the annealing temperature of N-doped TiO2nanotube array?lm on the photocatalytic degradation?rst-order rate constant k under different light sources. Photocatalyst Self-photolysis TiO2N-TiO2N-TiO2N-TiO2

450?C300?C450?C500?C

Under high pressure mercury lamp illumination

Apparent rate constant k(min?1)0.01000.09670.07320.16900.0704 Correlation coef?cient R20.99230.99520.99270.99660.9926

Under tungsten–halogen lamp illumination

Apparent rate constant k(min?1)–0.01780.02410.0270.0200

Correlation coef?cient R2–0.99830.99840.9986

0.9998

Fig.8.UV-visible spectra of MO at different time intervals under high-pressure mercury lamp illumination(a),under tungsten–halogen lamp illumination(b).

heat treatment of TiO2nanotube soaked with high concentrated ammonia is a promising way to improve the ef?ciency of photocat-alyst.

https://www.wendangku.net/doc/5d9881002.html,parison of photocatalytic activity

Fig.7demonstrates the kinetic behaviors of the MO pho-todegradation by the TiO2nanotube array catalyst calcined at different temperatures for2h under the high-pressure mercury lamp illumination.The MO photodegradation clearly obeyed the ?rst-order reaction kinetics.The photolysis experiments,in the absence of TiO2photocatalyst,revealed that the self-degradation of MO was almost negligible under UV illumination.Table1shows the effect of annealing temperature of the TiO2nanotube array photocatalysts on the?rst-order rate constant k of photocatalytic degradation under different light sources.The apparent rate con-stant of photocatalytic degradation with the presence of TiO2 nanotube photocatalyst was signi?cantly higher than that of MO self-photofading under both light sources,indicating the TiO2play an important role in the photocatalytic process.From the plot of absorption vs.wavelength under different light source irradiation (Fig.8),it can be seen that a rapid decrease in the absorbance peak at508nm re?ects the degradation of MO on the N-doped TiO2nan-otube array photocatalyst.The color removal of MO pollutant was almost completely in15min under high-pressure mercury light irradiation(Fig.8(a)),and more than80%under tungsten–halogen lamp illumination after60min(Fig.8(b)).Under the high-pressure mercury lamp illumination,the photocatalytic degradation rate of MO initially increased with increasing of the annealing tempera-ture of N-doped TiO2nanotube array?lm between300and450?C (0.071–0.169min?1),and then decreased.The optimized ef?ciency was obtained for the one annealed at450?C.This is attributed to the higher content in the crystalline anatase phase for the sam-ples calcined at450?C than those at300and600?C.According to the XRD results,the phase transformation from amorphous TiO2to anatase was not incomplete at300?C.At600?C,some of anatase phase was transformed to the more stable but photocatalytically less active rutile phase.Moreover,the rapid TiO2crystallite growth by sintering resulted in a thickening of tube walls,and decrease in surface area.

The photocatalytic process under tungsten–halogen lamp illu-mination has a similar trend in the reaction rate constant with that of high-pressure mercury lamp,except for the450?C heat treatment of pure TiO2nanotube sample.The apparent rate con-stant k increases from0.0241to0.0270min?1as the annealing temperature increases from300and450?C,and then decreases to0.0200min?1at500?C,indicating that the N-doped TiO2nan-otube array?lms do have a good visible light photocatalytic activity.Although the larger percentage of anatase and has a better UV light photocatalytic activity(0.0967min?1),the vis-ible light photocatalytic activity of450?C annealing pure TiO2 sample(～0.0178min?1)is lower than that of N-doped TiO2nan-otube array?lms calcined under300?C(0.0241min?1)by using tungsten–halogen lamp.This is ascribed to the fact that

visible

Fig.9.Recycling test of the N-doped TiO2nanotube arrays?lm on the MO removal rate under different light sources.The inset shows the SEM image of the N-doped TiO2nanotube arrays after ten repeated cycles test.

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Fig.10.HPLC pro?les of methyl orange dye at different times during photocatalytic reaction under UV(a)or simulated solar lights(b)by the TiO2nanotube array photocatalyst with or without N-doping.

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861

Fig.11.MALDI-TOF mass spectrometry pro?les of methyl orange and its oxidative intermediates corresponding to the solutions after being degraded under UV for different times(a)or simulated solar lights for3h(b)at pH3.0.

light can more effectively excite the valence band electrons in the N-doped TiO2sample than pure TiO2sample.Therefore,the main contribution from visible light absorption has bene?cial effect for the enhancement of photocatalytic activity.

Beside the excellent photocatalytic degradation performance, the N-doped TiO2nanotube arrays also show good stability of pho-tocatalytic activity during the acidic liquid solution of MO.As shown in Fig.9(a),the N-doped TiO2nanotube array structures kept with-out collapse and showed no apparent change in surface morphology after10cycles of repeat use in the photocatalytic process.In addi-tion,the MO removal rate had only a slightly reduction within8% under different light sources(Fig.9(b)).However,the photocatalyst can recover its original activity as it was re-annealed at450?C for 2h.This may be due to the absorption of some intermediates not being fully removed during the photocatalytic experiment.These results indicated that the N-doped TiO2nanotube array?lm can remain active for long-term service without much degradation of its activity.

3.4.Preliminary discussion on degradation intermediates of

methyl orange

The determination of intermediates and/or the photodegrada-tion mechanism of azo dyes have been reported using GC/MS or HPLC/MS techniques[61–65].However,there are few studies on the intermediates and photodegradation pathway of the quinonoid MO(pH3.0).Fig.10(a)reports the chromatogram of the solution before and after photocatalytic degradation for different times. The intensity changes indicate the transversion of the degrada-tion products.Before UV light irradiation,it can be seen that there exists only one peak corresponding to the MO with a negative ion at m/z=304and appeared at the retention time of22.5min in HPLC spectrum.After10min of UV light irradiation,the main absorp-tion peak of MO decreased a lot and four additional fragment peaks corresponding to new main intermediate by-production initially appeared at m/z=320at28.5min,m/z=306at10.3min,m/z=290 at8.6min and m/z=276at7.9min.Increasing the UV irradiation time,intermediate peaks gradually decreased except the peak at m/z=306at10.3min?rst increased but was still with low inten-sity at20min.At last,the peaks of methyl orange and intermediate products all almost disappear in HPLC after the30min UV irradia-tion,indicating that the photocatalytic degradation in the presence of anatase TiO2nanotube array can effectively mineralize many organic pollutants.

In the case of simulated solar light irradiation(Fig.10(b)),the MO and intermediate peaks are eventually disappeared after1h photocatalytic reaction with the N-doped TiO2nanotube array catalyst.However,MO and main intermediate peaks were

still Fig.12.Proposed degradation products and mechanism during the photodegradation process of MO.

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observed with pure TiO2nanotube array catalyst under identical experimental conditions.Hence,under the arti?cial solar light,the MO dye showed a clearer trend to be degraded into low molecular weight byproducts by the photocatalyst with N-doping.

Fig.11(a)shows the MALDI-TOF mass spectrometry pro?le recorded for MO solution after UV irradiation for different times. MO exhibited a clear MS signal corresponding to a negative ion of m/z304.After10min UV irradiation,it is found that four new peaks(m/z=320,306,290and276)are clear observed,indicating MO quickly changed to other chromophore groups.The compound of m/z=320is attributed to the monohydoxylated product of MO. The compound of m/z=306is the oxidation in the aromatic ring and loses one methyl group from nitrogen atom of amino group. The peak of m/z=290and276can be attributed to the loss of one or two methyl groups from MO(see Fig.12).

Further increasing the irradiation time,these chromophore groups can decompose to lower molecular weight by-production by the attack of oxidative species(·OH and·O2?)through two pri-mary processes of dealkylation and hydroxylation.The compound of m/z=320yields the intermediate by-products corresponding to[M–H–SO2]?at m/z=256and[M–H–N2C6H3(OH)N(CH3)2]?at m/z=156.The compound of m/z=306yields the interme-diate by-products corresponding to[M–H–SO2]?at m/z=242 and[M–H–N2C6H3(OH)NHCH3]?at m/z=156.The compound of m/z=290yields the intermediate by-products correspond-ing to[M–H–SO2]?at m/z=226,[M–H–CH3]?at m/z=275and [M–H–N2C6H4NHCH3]?at m/z=156.The compound of m/z=276 yields the intermediate by-products corresponding to[M–H–SO2]?at m/z=212,and[M–H–N2C6H4NH2]?at m/z=156.

Finally,the photogenerated oxidative species forming over TiO2 nanotube array catalyst surface further decompose these interme-diates to the?nal carbon dioxide and some non-toxic inorganic products(SO42?,NO3?and NH4+)as shown in following equation:·OH/·O2?+MO→intermediates→→CO2+H2O+SO42?

+NO3?+NH4+

The MS results(Fig.11(b))of MO solution by TiO2nanotube array catalyst with or without N-doping under tungsten–halogen lamp irradiation for3h showed that the N-doped photocatalyst exhibits better visible light activity than pure TiO2nanotube array to mineralize parent pollutant into non-toxic inorganic molecular.

On the basis of the preceding experimental results,we pro-pose the four main intermediates were found during the initial photocatalytic degradation process of quinonoid MO followed by decomposing into smaller molecular weight by-products and ?nally mineralized.The possible degradation pathway of quinonoid MO is shown in Fig.12.In addition,there were other fragment ions engendered in the degradation process which were not showed here.Those peaks were at m/z=171and143.N-doped TiO2nan-otube array catalyst shows enhanced photocatalytic activity than pure TiO2nanotube array catalyst to mineralize MO into non-toxic inorganic products under both high-pressure mercury lamp and tungsten–halogen lamp irradiation.

4.Conclusions

A simple method was developed for the fabrication of highly visible light active and stable nanocrystalline N-doped TiO2pho-tocatalysts by ambient heat treatment of the TiO2nanotube array ?lm pre-soaked in ammonia solution.The effect of annealing tem-perature on the photocatalytic activity of pure TiO2nanotube array?lms and N-doped TiO2nanotube array?lms under UV or visible light sources were investigated.It was found that photo-catalytic activity of N-doped TiO2array strongly depended on the calcination temperature and the light source used for the degrada-tion experiment.The N-doped TiO2array?lms calcined at450?C have the highest photocatalytic activity to degrade MO pollu-tant under visible(0.027min?1)or UV light(0.169min?1)sources. With combined advantages of large absorption area of the nan-otube structures and the extended light absorption of N-doping, N-doped TiO2nanotube arrays shows good potential for sus-tainably photocatalytic degradation of environmental hazardous substances.

Acknowledgments

The authors thank the National Research Foundation of the Singapore Government(Grant MEWR651/06/160),the National Nature Science Foundation of China(20773100,51072170, 21021002)and National Basic Research Program of China(973Pro-gram)(2007CB935603).

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