Cite this: DOI: 10.1039/c3ra43403f

Simultaneous N-doping of reduced graphene oxide andTiO2 in the composite for visible light photodegradationof methylene blue with enhanced performance3

Received 4th July 2013,Accepted 31st July 2013

DOI: 10.1039/c3ra43403f

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Xiong Yin,{a Hailong Zhang,{ab Peng Xu,a Jing Han,c Jianye Li*b and Meng He*a

The nitrogen-doped P90 TiO2 (N-P90), nitrogen-doped reduced graphene oxide (N-RGO) and their

composite were synthesized via a one-step annealing treatment process under NH3 atmosphere using

commercial P90 TiO2 and GO as starting materials. The as-prepared N-P90, N-RGO and N-P90/N-RGO

composite were characterized by field emission scanning electron microscopy (FESEM), X-ray diffraction

(XRD), X-ray photoelectron spectroscopy (XPS) and ultraviolet–visible diffuse reflectance spectroscopy

(DRS). The results indicated that both the reduction of graphene oxide and the incorporation of nitrogen

into both RGO and TiO2 matrices were accomplished simultaneously in the facile process. The

photocatalytic activity of the as-prepared samples was evaluated using the degradation of methylene

blue (MB) under visible light irradiation. N-P90/N-RGO composites showed a significantly enhanced

photocatalytic performance compared with P90 TiO2, N-P90 and N-P90/RGO composites. The higher

photocatalytic activity of N-P90/N-RGO composites can be ascribed to the more efficient separation of the

photogenerated charges resulting from the improved electrical conductivity of the N-RGO sheets, as well

as the enhanced absorption in the visible light region. Overall, this work demonstrated a facile approach of

incorporating nitrogen into commercial TiO2 and RGO simultaneously and a novel strategy of fabricating a

visible light-active photocatalyst with improved efficiency for mass application.

Introduction

Heterogeneous photocatalysis is one of the promising tech-nologies in the elimination of organic pollutants and water-splitting, as well as artificial photosynthesis.1 TiO2 is the mostwidely used inorganic photocatalyst due to its low-cost,nontoxicity, outstanding photo-electric properties, long-termstability and commercial availability.2,3 The band gap of TiO2

is about 3.05 eV and 3.20 eV for rutile and anatase phases,respectively. Therefore, TiO2 can only absorb the UV light ofthe solar spectrum, which carries about 4% of the incidentsolar energy. This largely limits the overall photocatalyticefficiency.4,5 Under UV light illumination, electrons are excitedfrom the valence band of TiO2 to the conduction band,forming the electron-hole pairs, which are responsible forphotocatalytic activity. Unfortunately, the photogenerated

electrons and holes can easily recombine before they migrateto the photocatalyst surface, resulting in the low photocatalyticefficiency of TiO2 and subsequently limiting its photocatalyticapplications.3,6 Therefore, extending the photoresponse ofTiO2 from the UV region to the visible light range andsuppressing the recombination of photogenerated chargecarriers are crucial for the enhancement of TiO2 photocatalyticactivity.

In an attempt to extend the optical absorption of TiO2 tothe visible light region, various strategies have been developedto modify TiO2, including transition metal ion doping,7

nonmetal ion doping8 and coupling with organic dyesensitizers.9 In the case of nonmetal ion doping, incorporatingnitrogen into TiO2 using various methods has been frequentlyinvestigated. The reported methods for incorporating nitrogeninto TiO2 include ion implantation,10 sputtering,11 hydrother-mal synthesis,12 sol–gel synthesis13 and thermal treatment.14

Nitrogen is a close neighbor of oxygen in the periodic table,and theoretical calculations suggest that the p orbitals ofnitrogen overlap significantly with the valence band O2porbitals, which facilitates the generation of charge carriersunder visible light illumination.15 It should be noticed that, atleast in some cases, N-doped TiO2 (N-TiO2) prepared byvarious methods showed somewhat different properties.Among the above-mentioned methods, the annealing of TiO2

aNational Center for Nanoscience and Technology, Beijing, China.

E-mail: mhe@nanoctr.cn; Fax: +86-10-62656765; Tel: +86-10-82545555bUniversity of Science & Technology Beijing, China. E-mail: jyli@ustb.edu.cn;

Fax: +86-10-82376349; Tel: +86-10-82376349cInstitute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China

3 Electronic supplementary information (ESI) available: SEM images and TGAcurves of the selected samples, a table to compare the photocatalyticperformance of the samples of this work with those reported in a previous study.See DOI: 10.1039/c3ra43403f{ These authors contributed equally.

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powders under NH3 atmosphere is an efficient, facile and mostapplicable technique.14 The resultant N-doped TiO2 showedenhanced performance in the photodegradation of organicpollutants under visible light irradiation.14,15

Suppressing the recombination of photogenerated elec-trons and holes is another approach to enhance the photo-catalytic activity of TiO2. For this purpose, TiO2 was combinedwith other materials to fabricate composites.16 Variouscomposites including TiO2/inorganic semiconductor,17 TiO2/polymer,18 TiO2/carbon nano-materials19 have been exploredin the past decades. Recently, considerable attentions havebeen devoted to incorporating graphene sheets into TiO2-based composite materials (TiO2/G) because of the uniqueproperties of graphene, including excellent electrical conduc-tivity, high electronic mobility and relatively good opticaltransparency.20 In addition, these TiO2/G composites havebeen synthesized by various methods, including hydrothermalapproach,21 solvothermal method,22 sol–gel approach,23 elec-trospinning24 and UV-light assisted photoreduction25 as wellas atomic layer deposition.26 Incorporating graphene sheetswith N-doped TiO2,27 facet-controlled TiO2

28 and TiO2 nano-crystals of various shapes29 have been demonstrated andenhanced photocatalytic activity resulted because the gra-phene sheets in the composite facilitate the separation ofphotogenerated electrons and holes in TiO2.

Reduced graphene oxides (RGO) instead of graphene areoften used to prepare composites due to their low cost.30

Recent investigations reveal that doping RGO with N willfurther improve the electrical conductivity.31 Therefore it isexpected that N-doped RGO (N-RGO) will be more effectivethan RGO in facilitating the separation of photogeneratedelectrons and holes of TiO2 in the composites. Nevertheless,N-TiO2/N-RGO composites were seldom reported.

In the present work, we fabricated N-TiO2/N-RGO compo-sites by annealing commercial TiO2 (P90) and GO in NH3

atmosphere. In this facile one-step process, the reduction ofGO and the N-doping of TiO2 and RGO are achievedsimultaneously. The N-TiO2/N-RGO composites exhibitedvisible light activity for the photodegradation of methyleneblue about 9 times higher than unmodified P90 TiO2. Inaddition, the commercial availability of the starting materialsfacilitates the mass application of the N-TiO2/N-RGO compo-site as photocatalysts.

Experimental

Materials

P90 TiO2 powder (ca. 99% anatase; BET area: 90 m2 g21;particle size: ca. 14 nm) was kindly supplied by DegussaCompany. Graphene oxide (GO) dispersion was purchasedfrom Nanjing XFNano Material Tech Co. Ltd. Tetrabutyltitanate was obtained from Sigma-Aldrich. All chemicalreagents were used as received without further purification.Deionized water with a resistance of 18.2 MV was used in allthe experiments.

Synthesis of nitrogen-doped P90 TiO2 (N-P90)

A schematic diagram for the synthesis of samples is shown inFig. 1a. N-doped P90 TiO2 was prepared by thermal treatmentof commercial P90 TiO2 under NH3 (11%)/Ar flow at 500, 600,700 and 800 uC using a horizontal tubular furnace (STF-1200X,Kejing Mater. Tech. Co. LTD.). Typically, about 300 mg of P90-TiO2 was placed in a quartz boat and heated in the horizontaltubular furnace. The furnace was purged with NH3 for 30 minto remove air before annealing. The samples were annealed atthe desired temperature for 1 h, and then cooled to roomtemperature. The resultant products were denoted as N-P90-500, N-P90-600, N-P90-700, N-P90-800, respectively, accordingto the annealing temperatures.

Synthesis of nitrogen-doped reduced graphene oxide (N-RGO)

The as-received GO dispersion was first drop cast onto asilicon substrate with a 300 nm amorphous oxide layer, andthen heated at 80 uC in a vacuum oven. Subsequently, the GO/Si samples were transferred into the horizontal tubularfurnace, and then annealed in NH3 (11%)/Ar flow at 500 uCor 600 uC for 1 h, respectively (Fig. 1b), which were denoted asN-RGO-500 and N-RGO-600 in terms of the annealingtemperature.

Synthesis of N-TiO2/N-RGO composites

As illustrated in Fig. 1c, 1.5 mL GO (1 mg mL21) was firstdispersed in 13.5 mL deionized water with magnetic stirringfor 10 min to obtain a well-dispersed GO suspension (0.1 mgmL21). Then, about 300 mg of P90 TiO2 was added into the GOsuspension with magnetic stirring for 3 h. To facilitate thecombination of the GO and P90 TiO2 particles, a few drops oftetrabutyl titanate were added into the suspensions withstrong magnetic stirring. Secondly, the obtained dispersionwas then subjected to centrifugation (Hitachi, CT15E) at 1000rpm for 10 min to get solid samples. The solid mixture wastransferred and dried in a vacuum oven at 80 uC for 12 h to getthe P90 TiO2/GO nanocomposites (ca. 0.5 wt.% GO). Thenitrogen-doped composites were obtained by annealing thesolid mixtures under NH3 (11%)/Ar atmosphere at 500 uC or600 uC for 1 h, respectively. Accordingly, the resulting sampleswere labeled as N-P90/N-RGO-500 and N-P90/N-RGO-600,respectively. A composite consisting of N-TiO2 and RGO,

Fig. 1 Schematic diagram for the synthesis of N-doped P90 TiO2, N-doped RGOand N-P90/N-RGO composites.

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which was named N-P90/RGO-600, was also prepared forcomparison with N-TiO2/N-RGO composites. This sample wassynthesized under the same conditions as those used toprepare N-P90/N-RGO-600 except that N-P90-600 instead of P90was used as starting material and annealing was performed inpure Ar flow rather than under NH3 (11%)/Ar atmosphere.

Characterizations

The morphology of the samples was observed with a fieldemission scanning electron microscope (FESEM, HitachiS-4800). The XRD patterns were acquired using a Rigaku D/max TTRIII X-ray diffractometer (Cu-Ka radiation, l = 1.5418 Å)operated at 40 kV and 150 mA. X-ray photoelectron spectro-scopy (XPS) measurements were carried out with an ESCA Lab250xi spectrometer using Al Ka (1486.6 eV) irradiation as X-raysource. All the spectra were calibrated to the binding energy ofthe adventitious C1s peak at 284.8 eV. Diffuse reflectancespectra (DRS) were recorded in the range from 300 to 800 nmusing a Perkin Elmer Lambda-950 instrument with BaSO4 as areference.

Photocatalytic experiments

The photocatalytic activity of the as-prepared catalysts wasevaluated by the photodegradation of a methylene blue (MB)dye solution under the visible light irradiation at ambienttemperature. The photodegradation was carried out in a self-designed 150 mL reactor fitted with a water bath coolingsystem to maintain constant temperature. A xenon lamp(NBeT, Solar-500, 500 W) was used as the light source, with allthe UV light below 400 nm being removed by a filter. Thedistance between the strip lamp and fluid level was kept as 15cm. The initial concentration of model dye solution was 2.0 61025 M. 50 mg of the as-prepared catalyst and 100 mL of MBdye aqueous solution were added into the reaction system withultrasonic mixing for 30 min. Before illumination, thesuspension was magnetically stirred for 1 h in the dark toensure that the adsorption–desorption equilibrium had beenreached. During the photodegradation process, 2 mL of thesolution was withdrawn at given intervals, and the photo-catalyst was separated from the solution by centrifugation. Theconcentration of the remaining clean transparent solution wasdetermined by measuring the absorbance of the solution at664 nm using a UV-visible spectrophotometer (Perkin ElmerLambda-950).

Results and discussion

The representative micrographs of the samples are shown inFig. 2. All the samples show similar morphology. The size ofthe P90 TiO2 particle is about 15 nm (Fig. 2a). No significantsize changes caused by annealing were observed. As shown inFig. 2d and 2e, RGO sheets are well dispersed in the aggregateof TiO2 particles.

Photographs of the as-received and N-doped P90 were givenin Fig. 3 together with the XRD patterns. The colors of thesamples prepared at 500 uC, 600 uC, 700 uC and 800 uC arewhite, pale yellow, dark blue and black, respectively. As

revealed by the powder patterns, the as-received P90 sampleconsists of anatase as the dominating phase and rutile as theminor phase. Powder patterns of the samples annealed at 500uC and 600 uC closely resemble that of the as-received P90 andno reflections of any new phase were observed. A weakreflection of TiN was observed in the pattern of the sampleannealed at 700 uC, indicating the emergence of TiN in thissample. The pattern of the sample heated at 800 uC wasdominated by the reflections of cubic TiN and no peaks ofanatase phase could be observed any more. On the other hand,reflections of rutile, the more stable phase of TiO2, werepresent in all powder patterns. Because of the emergence ofTiN in the samples annealed at 700 uC and above, we limitedthe temperature of N-doping to 600 uC and below. AnnealingP90 in this temperature range did not increase the size of TiO2

crystallites significantly, as evidenced by the fact that noobvious sharpening of reflections was observed. This was alsoconfirmed by SEM observations. Nevertheless, the content ofrutile in the samples increased slightly after annealing,indicated by the slight increase of the intensity of 110reflection of the rutile phase.

To evaluate the chemical states of the incorporatednitrogen in the synthesized samples, XPS measurements werecarried out, and the corresponding spectra are shown in Fig. 4.Fig. 4a displays the N1s core-level XPS spectra of the as-prepared samples. In comparison with raw P90 TiO2, thesamples annealed at 500 uC and 600 uC present a new peakcentered at 399.9 eV. According to previous reports, the peak is

Fig. 2 SEM images of the samples: (a) P90 TiO2, (b) N-P90-500, (c) N-P90-600 (d)N-P90/N-RGO-600 composites, N-RGO sheets are highlighted with red lines; (e)TEM image of N-P90/N-RGO-600 composite.

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ascribed to the nitrogen incorporated in TiO2 in O–Ti–Nlinkages or nitrogen species bound to various surface oxygensites.31a However, two peaks of N1s were observed for thesample prepared at 700 uC, one centered at 399.4 eV and theother at 395.6 eV. The new peak at about 396 eV is attributed toTi–N-like nitrogen species in the lattice.32 For the sampleprepared at 800 uC, only the peak resulted from Ti–N-likenitrogen species was significant. Taking the XRD results intoaccount, we believe that nitrogen has been doped into thelattice of TiO2 by annealing P90 under NH3 atmosphere at 500uC and 600 uC and no TiN phase was formed in these cases.

To further investigate the influence of nitrogen doping onthe state of titanium, Ti2p core level spectra were recorded andpresented in Fig. 4b. No significant changes were observed forthe spectra of the samples annealed at 500 uC and 600 uC incomparison with that of the raw P90 TiO2. Two peaks centeredat 458.6 eV and 464.3 eV respectively were assigned to thedistinct Ti2p3/2 and Ti2p1/2 signals in the Ti4+ chemicalstate.13,32 For the sample prepared at 800 uC, obvious featuresappeared at lower binding energies, which were commonlyattributed to Ti3+. This was in agreement with the presence ofthe dominating phase of TiN in the sample.32 Features of Ti3+

were also observed in the spectrum of the sample prepared at

700 uC, in accordance with the existence of TiN in this sampleas a minor phase.

XRD patterns of GO annealed at 500 uC and 600 uC wereshown in Fig. 5a in comparison with that of GO. Thecharacteristic peak of GO is located at 12.1u in 2h, whichcorresponded to a layer spacing of 0.731 nm. After beingannealed under Ar atmosphere at 500 uC or 600 uC, GO wasreduced and the layer spacing decreased to 0.363 nm,indicated by the characteristic peak located at 24.5u in 2h.When GO was heated in NH3 (11%)/Ar flow at 500 uC or 600 uC,the resulting layer spacing was even smaller, 0.339 nm, whichis very close to the d-spacing of graphite (001) planes, 0.335nm. This may imply that the oxygen-containing groups of GOwere removed more effectively with the presence of NH3. Asignificant broadening of the reflection was observed for RGOsamples in comparison with that of the raw material GO,indicating that the thickness of RGO sheets is much smallerthan that of GO. We also noticed that the sample prepared at600 uC showed a much more significant reflection broadeningthan the one annealed at 500 uC, implying that thinner RGOsheets were obtained with higher annealing temperature.

Fig. 3 XRD patterns (a) and photographs (b) of P90 TiO2 and N-doped P90prepared at various temperatures.

Fig. 4 Representative XPS spectra of P90 TiO2 and N-doped P90 prepared atvarious temperatures: (a) core level spectra of N 1s; (b) core level spectra of Ti 2p.

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The reduction of GO was also confirmed using C1s core-level XPS spectra (Fig. 5b). GO showed two distinct peaks, oneof which was located at 284.8 eV and the other at 286.9eV.31a,33 The peak at higher binding energy is related to sp3

carbon with C–O, CLO, and O–CLO bonds.31a After GO wasreduced at 500 uC or 600 uC, the peak at higher binding energyis nearly negligible. The significant decrease in intensity of thepeak at 286.9 eV indicates the reduction of GO during thethermal treatment process. This is probably due to the removalof the functional groups and sp3 carbon during the thermal

process.31a Moreover, the N-doping of RGO was evidenced bythe N1s core-level XPS spectra, which was shown in Fig. 5c. Nosignals of N were detected in the spectrum of GO while adistinct peak at around 398.5 eV, which corresponds to thepyridinic-type nitrogen,31a,33 was identified in the spectra ofthe samples annealed in NH3 (11%)/Ar flow at 500 uC or 600uC. The appearance of the N1s peak confirms that the nitrogenhas been doped into RGO. The N-doping process is attributedto the reaction between certain oxygen-containing groups inGO with NH3 to form C–N bonds. The content of nitrogen inthe sample decreased with the annealing temperature, whichis consistent with previous reports.31a These results demon-strate that the reduction and N-doping of GO can besimultaneously achieved by annealing GO under NH3 atmo-sphere at certain temperatures.

According to the above-mentioned results, the N-doped P90TiO2 and N-doped RGO samples can be individually obtainedusing thermal treatment under NH3 atmosphere at 500 uC and600 uC. Therefore, N-P90 and N-RGO can be preparedsimultaneously to form N-P90/N-RGO composite using thisfacile method. The XRD patterns and XPS spectra of N-P90/N-RGO composites are shown in Fig. 6a and 6b, respectively. Asshown in Fig. 6a, no characteristic diffraction peaks for GO orRGO are observed in all the as-prepared composites. Eachcomposite shows a diffraction pattern similar to that of P90TiO2. This is possibly due to the low content and weakscattering power of N-RGO in the composites. Moreover, the

Fig. 5 XRD patterns (a) and XPS core level spectra of C1s (b) for GO, RGO andN-RGO prepared at 500 uC and 600 uC; (c) XPS core level spectra of N1s ofN-RGO prepared at 500 uC and 600 uC.

Fig. 6 (a) XRD patterns of P90 TiO2, N-P90/N-RGO-500 and N-P90/N-RGO-600composites; (b) XPS spectra of GO, N-RGO-600, P90 TiO2, N-P90-600 and N-P90/N-RGO-500 as well as N-P90/N-RGO-600 composites.

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characteristic reflection of N-RGO could be largely overlappedwith the 101 reflection of the anatase phase. XPS measurement(Fig. 6b) revealed that the N-doping process can occur in thesynthesized composites at a temperature of 500 or 600 uC. Thequantitative estimates of the ratios between N atoms and Tiatoms in the synthesized composites were also made usingXPS measurement. The N/Ti ratio in N-doped compositesprepared at 500 uC and 600 uC is about 0.05, which is largerthan that in N-doped TiO2 (0.03). The increase in N/Ti ratiomay also indicate that the nitrogen has been simultaneouslyincorporated into RGO and P90 TiO2.

N-doping has extended the absorption range of P90 to thevisible part of the spectrum, as revealed by the UV-visiblediffused reflectance spectra (DRS) shown in Fig. 7a. Consistentwith the improved visible light absorption, the color of N-P90-600 has changed to pale yellow from white, the color of the as-received P90 (Fig. 7b). We also noticed that although thecarbon content is rather less, the color of the compositeschanges significantly in comparison with pure P90 and theN-doped P90. This is in good agreement with the drasticdecrease of the reflectance in the visible part of the UV-visspectrum with reference to that of pure P90 and the N-dopedP90.

The photocatalytic performance of the selected sampleswas presented in Fig. 8. A blank experiment was also carriedout to check the stability of MB under visible light irradiation.As shown in Fig. 8a, MB degraded very little without thepresence of any catalyst in the solution. Similar results wereobtained when RGO or N-RGO was used individually as thecatalyst (not shown). In the presence of P90, about 20% MBwas degraded within 160 min of visible light irradiation. Ingeneral, TiO2 is not a visible light-active photocatalyst becauseof its large band gap. The photocatalytic effect of P90 underthe visible light irradiation observed here may result from thesensitizing effect of MB on TiO2, which can slightly shift theabsorption to the visible light region. N-doping improved thephotocatalytic performance of P90 significantly, as evidence bythe fact that about 65% MB was degraded under the sameconditions when N-P90-600 was used as catalyst. IncorporatingRGO into the aggregates of N-doped P90 further improved itsphotocatalytic properties although RGO itself did not showphotocatalytic effect on the degradation of MB. Among all theselected samples, N-P90/N-RGO-600 showed the best photo-catalytic performance. In the presence of this sample, nearly80% MB was degraded within 160 min of visible lightirradiation. As pointed out in the previous reports, thephotodegradation of organic pollutants usually follows thepseudo-first-order kinetics.34 The kinetics of the photodegra-

Fig. 7 (a) Diffuse reflectance spectra of the samples; (b) photographs of P90TiO2, N-P90-600, N-P90/RGO-600 and N-P90/N-RGO-600.

Fig. 8 (a) Photodegradation of methylene blue under the irradiation of visiblelight over P90 TiO2, N-P90-600, N-P90/RGO-600, N-P90/N-RGO-600 andwithout photocatalyst; (b) relationship between irradiation time and ln(C0/C)deduced from the photodegradation data shown in (a).

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dation reaction can be described using a simplified Langmuir–Hinshelwood model when C0 is very small:

ln(C0/C) = kt

where C0 and C are the initial concentration and the concentrationof MB at time t, respectively, and k is the apparent first-orderkinetics rate constant. Fig. 8b shows the linear relationship ofln(C0/C) and the irradiation time for the various photocatalysts.The good linear relationship implied that all the photocatalyticprocesses could be well described by the pseudo-first-orderkinetics. In addition, the apparent rate constants for catalystsP90 TiO2, N-P90-600, N-P90/RGO-600 and N-P90/N-RGO-600 are0.001, 0.006, 0.0085 and 0.0095 min21, respectively. The N-P90/N-RGO-600 composite presented a rate constant about 9 timeshigher than that of P90 TiO2. Its high photocatalytic activity undervisible light illumination can be explained as follows. Electron-hole pairs are created by N-doped TiO2 under the irradiation ofvisible light. These charge carriers can migrate to the surface andreact with water to form reactive oxygen species. The resultantreactive oxygen species exhibit strong oxidizing capability and candegrade the dye. The N-doping improves the visible lightabsorption of TiO2. On the other hand, RGO or N-RGO in thecomposite facilitates the separation of electrons and holes byacting as electron traps and/or providing the fast migrationchannels for photo-excited electrons. The effect of RGO can befurther improved by N-doping. In this way, the electron-holerecombination in the TiO2 can be effectively reduced, resulting ina significantly enhanced photocatalytic performance of the N-P90/N-RGO composites.

A precisely quantitative comparison of photocatalyticperformance between our samples and previously reportedsimilar materials can hardly be made since the details ofphotocatalytic experiments vary from case to case. However,such a comparison is still meaningful, especially when theexperimental details of photocatalytic measurements in bothcases are very similar. Here we compare the photocatalyticperformance of our samples with that of similar materialsreported in an extensively cited literature.35 The results ofcomparison are given in Table S1, ESI.3 As revealed by thecomparison, the photocatalytic performance of P90 seems tobe inferior to that of P25. However, the composite N-P90/RGO-600 showed a better performance than that of a compositeP25/RGO with similar carbon content. This further confirmedthe effect of N-doping of TiO2 on the photocatalytic perfor-mance. It is interesting to notice that the performance ofN-P90/N-RGO-600 is comparable with the best one of thesamples reported in this literature.

Conclusions

In summary, we developed a facile approach of dopingnitrogen into commercial P90 TiO2 and graphene oxidesimultaneously. The N-P90/N-RGO composite prepared withthis method showed significantly enhanced catalytic perfor-mance in the photodegradation of methylene blue under the

visible light irradiation in comparison with P90, N-P90 and theN-P90/RGO composite. The N-doping improves the visiblelight absorption of TiO2, and on the other hand, RGO orN-RGO in the composite may facilitate the separation ofelectrons and holes generated in TiO2 under irradiation. Thisone-step thermal nitridation method may be further developedto prepare other nitrogen-doped composites consisting of TiO2

and other carbonaceous materials. Furthermore, the utiliza-tion of commercially available products as starting materialsfacilitates the mass application of the N-doped composites aspromising photocatalysts active in the visible light range.

Acknowledgements

The work was financially supported by the Major State BasicResearch Development Program of China (973 Program) (No.2011CB932702) and the National Natural Science Foundationof China (Grant Nos. 21103032, 51272049, 21071016). We alsothank Dr Yanjun Guo and Dr Bin Wang for their help in DRScharacterization.

Notes and references

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