Materials Research Bulletin 48 (2013) 3485–3491

Facile synthesis of phosphorus doped graphitic carbon nitride polymerswith enhanced visible-light photocatalytic activity

Ligang Zhang a,b,1, Xiufang Chen a,1, Jing Guan a, Yijun Jiang a, Tonggang Hou a, Xindong Mu a,*a Key Laboratory of Biobased Materials, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, PR Chinab University of Chinese Academy of Sciences, Beijing 100049, PR China

A R T I C L E I N F O

Article history:

Received 28 January 2013

Received in revised form 7 May 2013

Accepted 10 May 2013

Available online 18 May 2013

Keywords:

A. Nitrides

A. Polymers

A. Semiconductors

D. Catalytic properties

D. Optical properties

A B S T R A C T

Phosphorus-doped carbon nitride materials were prepared by a one-pot green synthetic approach using

dicyandiamide monomer and a phosphorus containing ionic liquid as precursors. The as-prepared

materials were subjected to several characterizations and investigated as metal-free photocatalysts for

the degradation of organic pollutants (dyes like Rhodamine B, Methyl orange) in aqueous solution under

visible light. Results revealed that phosphorus-doped carbon nitride have a higher photocatalytic

activity for decomposing Rhodamine B and Methyl orange in aqueous solution than undoped g-C3N4,

which was attributed to the favorable textural, optical and electronic properties caused by doping with

phosphorus heteroatoms into carbon nitride host. A facile postannealing treatment further improved the

activity of the photocatalytic system, due to the higher surface area and smaller structural size in the

postcalcined catalysts. The phosphorus-doped carbon nitride showed high visible-light photocatalytic

activity, making them promising materials for a wide range of potential applications in photochemistry.

� 2013 Elsevier Ltd. All rights reserved.

Contents lists available at SciVerse ScienceDirect

Materials Research Bulletin

jo u rn al h om ep age: ww w.els evier .c o m/lo c ate /mat res b u

1. Introduction

Semiconductor-mediated photocatalysis has become the cur-rent topic of intensive interest because of its potential applicationsin solar energy conversion and environmental purification [1,2].Much effort has been devoted to the development of photo-catalysts capable of utilizing visible light, which accounts for about43% of the incoming solar energy [3–6]. Among various semicon-ductor materials in present research, graphitic carbon nitride (g-C3N4), a metal-free organic semiconductor material, is recentlyrecognized as a very promising candidate for visible-lightphotocatalyst. The metal-free material possesses a very highthermal (up to 873 K) and chemical (e.g. acid and base) stabilityand an appealing electronic structure with appropriate bandpositions and gaps (2.7 eV), which is suitable for a variety ofrelevant chemical reactions [7,8]. As a result of its unique surfaceand electronic structures, carbon nitride can catalyze Friedel–Crafts reactions [9] and CO2 reduction [10]. Moreover, somestudies have highlighted that the defectuous, polymeric version ofg-C3N4 was able to split water photochemically into hydrogen oroxygen in the presence of external redox agents and decomposeorganic dyes like Rhodamine B (RhB) to CO2 under visible light

* Corresponding author. Tel.: +86 532 8066 2725; fax: +86 532 8066 2724.

E-mail address: muxd@qibebt.ac.cn (X. Mu).1 Co-first-author of this work.

0025-5408/$ – see front matter � 2013 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.materresbull.2013.05.040

illumination [11,12]. However, the photocatalysis quantumefficiency of pure carbon nitride remains to be improved forlarge-scale industrial application. So far, some methods have beenreported to improve the photocatalytic performance of g-C3N4,such as, by introducing an appropriate textural porosity [13,14],dye-sensitization [15], doping [16–19], transition metal-modifica-tion [20,21], combination of carbon nitride with metals [22], orcoupling with inorganic semiconductor (ZnO) [23,24].

Generally, chemical doping with foreign anion elements is aneffective strategy to modify the electronic structures of semi-conductors as well as their surface properties, thus improving theirphotocatalytic performance. Chemical doping strategy to enhancephotocatalytic efficiency, for example, doping of anion elementswith sulfur, boron or fluorine, has been proven recently for g-C3N4

[16–19] and also for conventional photocatalysts (e.g. TiO2) byothers [25,26]. Recently, it was reported that phosphorus can beintroduced into the structural framework of graphitic carbonnitride, leading to tuned electronic properties and enhanced ionicconductivity [27]. This suggested that introducing phosphorus intothe structural framework of g-C3N4 might favor the photocatalyticefficiency. However, to the best of our knowledge, there are noreports on the application of phosphorus doped g-C3N4 forphotocatalytic degradation of organic pollutants.

Phosphorus-doped g-C3N4 catalysts could be synthesized byusing dicyandiamide and 1-butyl-3-methylimidazolium hexa-fluorophosphate (BmimPF6) as precursors, in combination witha thermal-induced polycondensation at 550 8C [27]. This one-pot

10 20 30 40 50 60

0.3CN P

0.1CN P

0.05 CNP

CN

Inte

nsity

(a.u

.)

2 theta (degree)

13.0o

27.3o

Fig. 1. XRD patterns of as-prepared xCNP and CN.

L. Zhang et al. / Materials Research Bulletin 48 (2013) 3485–34913486

synthetic approach, successfully making phosphorus homo-geneously distributed in the carbon nitride host, is not onlylow-cost, but also easy to scale-up. Thus, this abundant, stable, andmetal-free polymeric solid could fulfill the requirements for use asan industrial catalyst. An inherent disadvantage to such syntheticapproach is that the high degree of polycondensation of monomersduring the synthesis at 550 8C renders the carbon nitride materialswith low surface area (�10 m2 g�1 for pure g-C3N4). It is wellknown that a large surface area would be favorable for photo-catalytic reaction by providing more possible reaction sites,enhancing mass transfer, and improving light-harvesting. Toobtain carbon nitride materials with large surface area, somesynthetic strategies have been used. Hard templates such as silicaparticles [13,14], anodic aluminum oxides [28] and TiO2 spheres[29] have been used to improve the surface area of g-C3N4.However, these methods required additional strong acid (NH4HF2

or HF) or strong base (NaOH) treatment to remove hard templates,which are not only complicate but also not environmentallybenign. Thus, it is highly desirable to develop a simple andtemplate-free strategy to improve the surface area of phosphorus-doped carbon nitride material.

In this paper, phosphorus-doped g-C3N4 was synthesized via afacile heating method with a phosphorus containing ionic liquid asprecursors. The obtained doped materials showed enhancedvisible light photocatalytic activity than pure g-C3N4. The effectof phosphorus contents on the efficiency of photocatalyticdegradation of dyes is investigated and discussed in detail.Moreover, in attempts to further improve the physicochemicalproperty, the as-prepared materials were further annealed at550 8C in sufficient air. This work clearly presents a promisingmetal-free photocatalyst for environmental purification.

2. Experimental

2.1. Catalyst preparation

Phosphorus-doped g-C3N4 samples were synthesized by mixingdicyandiamide (3 g) and different amounts (0.05 g, 0.1 g, 0.3 g) ofBmimPF6 in 15 mL water with stirring at 100 8C to remove water.The white mixtures were put into a crucible with cover and heatedat 550 8C for 4 h in air. The obtained samples were denoted as xCNP(x = the initial amount of BmimPF6). The resultant xCNP sampleswere milled into powder and put into a crucible without cover, andthen annealed at 550 8C for 4 h in air atmosphere. The CNPcatalysts postannealed at 550 8C were denoted as xCNP550. Thepure carbon nitride sample without and with post-treatment weredesignated CN and CN550, respectively.

2.2. Characterization

X-ray diffraction (XRD) measurements were performed on aBruker D8 Advance diffractometer with CuKa radiation(l = 1.5147 A). Elemental analysis was performed with a vario ELcube from Elementar Analysensysteme GmbH. FTIR spectrawere recorded on a Thermo Nicolet FTIR Spectrometer. X-rayphotoelectron spectra (XPS) data were obtained on a ThermoESCALAB250 instrument with a monochromatized Al Ka line source.The binding energies were calibrated to the C 1s peak at 284.6 eV.UV–vis spectra were recorded on a HITACHI U-4000 Spectropho-tometer equipped with a Labsphere diffuse reflectance accessory.Photoluminescence (PL) spectra were measured on the Fluoro-max-4 spectrofluorometer. All the measurements were performed atroom temperature. Nitrogen adsorption-desorption isothermswere collected at 77 K on a micromeritics ASAP 2020 m+csorptometer. Before measurement, the samples were degassed ina vacuum at 150 8C for 6 h. The Brunauer–Emmett–Teller method

was utilized to calculate the specific surface areas using adsorptiondata in a relative pressure range from 0.05 to 0.4. Transmissionelectron microscope (TEM) images were taken using a field emissionH-7600 electron microscope. Energy dispersive X-ray spectroscopy(EDS) and elemental mapping were measured on a Hitachi S-4800instrument.

2.3. Photocatalytic experiments

The photocatalytic activity was evaluated by the degradation ofRhB and MO under visible light irradiation. In each experiment,50 mg of catalyst was initially suspended in a RhB (or MO) aqueoussolution (50 mL, 10 ppm). A 500 W Xe lamp together with a420 nm cut-off filter was used as a visible-light source for theirradiation of the reaction system. Prior to irradiation, thesuspensions were stirred in the dark for 12 h to assure adsorptionequilibrium. At a given reaction time, the solution (1.5 mL) wassampled and centrifuged, and then the UV–vis spectra wereobtained using a Lambda 25 spectrometer. The concentration ofRhB and MO was determined by monitoring the changes in theabsorbance maximized at ca. 554 nm and ca. 464 nm, respectively.

3. Results and discussion

3.1. Characterizations of phosphorus-doped g-C3N4

The structures of phosphorus-doped g-C3N4 hybrid materialswere investigated by XRD. Fig. 1 shows the XRD patterns of thebulk g-C3N4 and phosphorus-doped g-C3N4 samples. The XRDpattern supports the preservation of the graphite-like packing ofpractically all phosphorus-doped products, showing the typical(0 0 2) interlayer-stacking peak around 27.38 (d = 0.321 nm) andin-plane structural packing motif (1 0 0) peak at 138 (d = 0.669 nm)[8–11]. No peaks for phosphorus species were observed in all thephosphorus-doped g-C3N4 materials. In addition, the phosphorus-doped g-C3N4 samples showed a slight broad of (0 0 2) peak and adecrease of their relative intensities with increasing phosphoruscontent, indicating that the crystal growth of graphitic carbonnitride is inhibited by introduction of BmimPF6 as precursoraccording to the Scherrer’s formula [9].

The morphology and microstructures of 0.1CNP were studiedby TEM and nitrogen adsorption. Fig. 2 shows the typical TEMimages of 0.1CNP and CN. The TEM picture of 0.1CNP clearlyreflects a two-dimensional structure consisting of small flat sheetswith wrinkles and irregular shape, which is different from largesheets for bulk g-C3N4. The results of the BET surface area analysis

Fig. 2. Typical TEM images of CN (A) and 0.1CNP (B).

L. Zhang et al. / Materials Research Bulletin 48 (2013) 3485–3491 3487

(Fig. S1) showed that the BET surface area and pore volume of0.1CNP was 15 m2 g�1 and 0.104 cm3 g�1, both larger than those ofCN (10 m2 g�1, 0.045 cm3 g�1). This indicates that the phosphorusplay an important role in the processing of carbon nitridecondensation and enlarging the surface area of g-C3N4, presumablydue to the formation of gaseous species such as hydrocarbons andammoniumfluoride acting as inflating medium by BmimPF6

reacting with amine groups during carbon nitride polymerization[30].

The molecular structure information of the phosphorus-dopedg-C3N4 was further provided by FT-IR spectra and elementalanalyses. The IR spectra of the as-prepared xCNP samples (Fig. S2)showed typical C–N heterocycle stretches in the region of ca.�1100–1600 cm�1 and the breathing mode of the tri-s-triazineunits at 800 cm�1 [8–11], similar to the results of undoped g-C3N4,evidencing that the original graphitic C–N network remained. Thevibrations of P-related group were hardly observed in doped g-C3N4 as phosphorus content was low and its vibration wasoverlapped by that of C–N. Elemental analyses of all the xCNPsamples (Table S1) revealed that the average C/N ratio value of�0.66 for xCNP is similar to that of undoped g-C3N4 (�0.66).

Fig. 3. Typical SEM images of 0.1CNP and correspon

Elemental analyses and IR were in good agreement and indicatedthat incorporation of phosphorus does not alter the bulk structureor the core chemical skeleton of g-C3N4 too much.

The existence of phosphorus within the framework ofphosphorus-doped g-C3N4 is clearly evidenced in the energydispersive X-ray microanalysis. The phosphorus content asdetermined by EDS is ca. 0.5 at% for 0.1CNP, implying thatphosphorus from BmimPF6 has been introduced into the g-C3N4

host. This result, together with the elemental mapping images(shown in Fig. 3), indicates that phosphorus is homogenouslydistributed in the whole host of doped g-C3N4 solid. The electronicstate of phosphorus-doped g-C3N4 was further studied by XPS.Fig. 4 showed that the P 2p binding energy peak of 0.1CNP iscentered at ca. 133.5 eV. The binding energy of P 2p in P–N and P–Cbonds was found at �133.5 eV and 131.5–132.5 eV, respectively[27]. Thus, the peak at 133.5 eV is reasonably considered to beoriginated from P–N bonds formed in phosphorus-doped g-C3N4.In addition, no fluorine could be detected in the doped materials.The XPS results support the view that phosphorus atoms have beenincorporated into the CN matrix. Therefore, the ionic liquidtemplate not only influences textural features but also enters the

ding elemental mapping images of C, N, and P.

130 13 5 14 0 14 5

CN

Inte

nsity

(a.u

.)

Binding energ y (eV)

0.1CN P

P 2p

Fig. 4. High-resolution XPS spectrum of P recorded from pristine surface of 0.1CNP.

0 30 60 90 120 150 1800.0

0.2

0.4

0.6

0.8

1.0

C/C

o

Time (min)

CN 0.05 CNP 0.1CN P 0.3CN P RhB sel f-degradation 0.1CNP-dark

A

450 50 0 55 0 60 0 65 0 70 0 75 0

0.1CN P

Inte

nsity

(a.u

.)

Wavelength ( nm)

CN

Fig. 6. PL spectra of CN and 0.1CNP at 400 nm excitation at 298 K.

L. Zhang et al. / Materials Research Bulletin 48 (2013) 3485–34913488

materials. Most probably, the phosphorus atoms replace the corneror bay carbon in polymeric C–N structures to form P–N bonding[27].

The incorporation of a low percentage of phosphorusheteroatoms into g-C3N4 changes structural and electronicaspects and consequently the optical/electronic properties ofthe resulting phosphorus-doped g-C3N4 polymers. The opticalabsorption spectra were used to investigate the effect ofphosphorus doping on the electronic structure of g-C3N4. Asshown in Fig. 5, pure CN exhibited the typical absorption patternof a semiconductor with a band gap of 2.73 eV. The spectra of thephosphorus-doped g-C3N4 showed obvious red shifts in the band–gap transition with increasing phosphorus content, implying theelectronic integration of the phosphorus-heteroatoms in thelattice of g-C3N4. A further observation indicated that thephosphorus-doped samples extend the absorbance in the visibleregion from 470 to 650 nm with increasing P content, inagreement with the color changing from pale yellow to brown(inset in Fig. 5).

The effect of incorporation of phosphorus into carbon nitride onthe electronic properties was also checked by PL experiments.Fig. 6 depicts the PL spectra of 0.1CNP and CN under 400 nmexcitation at room temperature. A broad visible PL band centeredat approximately 470 nm is observed for CN, which can beattributed to the band-band PL phenomenon with the energy of

Fig. 5. UV–vis spectra of as-prepared xCNP and CN. Inset: photographs for as-

prepared xCNP and CN samples.

light approximately equal to the band gap energy of g-C3N4.Doping with phosphorus heteroatoms of g-C3N4 led to obviousquenching of PL. It is well known that the PL spectra are closelyrelated to the recombination of photo-induced electron and holes,free excitons, and self-trapped excitons. The lower intensity of the

0 60 120 180 240 300 3600.2

0.4

0.6

0.8

1.0

C/C

o

Time (min)

CN

0.1CN P

MO sel f-degradation

B

Fig. 7. (A) The photocatalytic degradation of RhB over as-prepared xCNP and CN

photocatalysts under visible light irradiation. (B) The photocatalytic degradation of

MO over 0.1CNP and CN as a function of reaction time.

0

20

40

60

80

100

0.3CNP0.1CNP0.05CNP

Con

vers

ion

of R

hB (%

)

CNP CNP550

CN

A

L. Zhang et al. / Materials Research Bulletin 48 (2013) 3485–3491 3489

PL band for the 0.1CNP probably suggested that phosphorus dopingresulted in a decrease in the recombination of electron–hole pairs,which would be favorable for the photocatalytic process.

3.2. Photocatalytic activity of phosphorus-doped g-C3N4

Since the manipulation of textural and electronic structuredemonstrated above has important consequences for the photo-catalytic activity of the phosphorus-doped g-C3N4, the degradationof RhB in aqueous solution under visible light was investigated byusing phosphorus-doped g-C3N4 with varying phosphorus contentas metal-free catalysts. Fig. 7A shows comparison of photocatalyticactivity of as-prepared xCNP and CN. xCNP samples show higheractivity than pure CN. The degradation rate of RhB increased with x

to a maximum at 0.1, beyond which the activity of the samplesbegan to decrease. Control experiments indicated that no obviousactivity was detected in the absence of either irradiation orphotocatalyst, suggesting that RhB was decomposed via photo-catalytic reactions. There is no measurable alteration in surface andlocal X-ray structures of the sample before and after the reactions,as reflected by XRD and FT-IR measurements (see Fig. S3). Theseresults demonstrated that the presented 0.1CNP photocatalyst wasstable under the experimental reaction conditions employed here.

Other organic pollutant like MO was also tested and found to beeffectively degraded by phosphorus-doped g-C3N4 sample. Asshown in Fig. 7B, 0.1CNP sample exhibited much higher photo-catalytic performance than undoped g-C3N4 towards the degrada-tion of MO. This fact further proved that doping with a lowpercentage of phosphorus heteroatoms indeed could enhance thephotocatalytic performance of the resulting carbon nitridepolymers. The enhanced activity of phosphorus-doped samplesis considered to reflect a reduction in the probability ofrecombiniation between photogenerated carriers, due to thefavorable textural, optical properties and electronic structurecaused by doping with phosphorus heteroatoms into carbonnitride host.

3.3. Effect of postsintering on phosphorus-doped g-C3N4

In order to further enhance the activity of phosphorus-doped g-C3N4, the phosphorus-doped samples were further annealed at550 8C in sufficient air, with CN annealed at similar condition as acomparison. The product yields of 0.1CNP and CN after calcinationat 550 8C were near 54 wt% and 51 wt%, respectively, indicatingthat carbon nitride was partly decomposed during thermal

10 20 30 40 50 60

0.1CNP550

0.1CN P

CN550

CN

Inte

nsity

(a.u

.)

2 theta (degree)

Fig. 8. XRD patterns of 0.1CNP and CN photocatalysts with and without

postannealing treatment.

treatment in the presence of oxygen. The XRD pattern of0.1CNP550 and CN550 (Fig. 8) presented two feature diffractionpeaks at 27.38 and 138, indicating the original graphitic C–Nnetwork remained mostly unchanged after calcination at 550 8C.However, the postcalcined samples showed a lower intensity of(0 0 2) peak compared to as-prepared samples without postcalci-nation. This indicated that post-treatment could result in adecrease in particle size of carbon nitride induced by a reducedstructural correlation length. The result is supported by theincreased specific surface area from 10 m2 g�1 for CN to 21 m2 g�1

for CN550 and from 15 m2 g�1 for 0.1CNP to 31 m2 g�1 for0.1CNP550, respectively (Fig. S1). This result suggested that aslight thermal decomposition of carbon nitride would result insmall particle sizes and large surface area by self-introducing thenanostructure during the thermal treatment, which was inagreement with the earlier observations by others [31–33].

Fig. 9 shows that RhB degradation under visible light over theas-prepared xCNP and the same materials after calcinations at550 8C. It was found that postannealing has a great effect on thephotocatalytic performance of carbon nitride for photodegradationof RhB in aqueous solution. All the postcalcined samples exhibitedmuch higher activity than the as-prepared materials withoutpostcalcination. Even for pure carbon nitride, the activity of CN550can reach 64% after 1 h reaction, about 1.1 times higher than as-prepared CN sample. Similar to the trend of the activity of the as-prepared samples without postcalcination, the rates of RhB

0 30 60 90 120 150 1800.0

0.2

0.4

0.6

0.8

1.0

C/C

o

Time (min)

CN 0.1CNP CN550 0.1CNP550 RhB-self-degradation 0.1CNP550-dark

B

Fig. 9. (A) Comparison of photocatalytic activities for RhB degradation by xCNP and

CN samples without and with postannealing treatment under visible light

irradiation for 1 h. (B) Photocatalytic activity of 0.1CNP and CN photocatalysts

with and without postannealing treatment for RhB degradation as a function of

time.

0 30 60 90 120 150 1800.0

0.2

0.4

0.6

0.8

1.0

Time (min)

C/C

o

Blank CN550 CN550+TEOA CN550+TBA

A

0 30 60 90 120 150 1800.0

0.2

0.4

0.6

0.8

1.0B

Time (min)

C/C

o

Blank 0.1CNP550 0.1CNP550+TEOA 0.1CNP550+TBA

Fig. 10. Comparison of photocatalytic activities for RhB degradation by CN550 (A)

and 0.1CNP550 (B) in different photocatalytic systems under visible light

irradiation.

L. Zhang et al. / Materials Research Bulletin 48 (2013) 3485–34913490

degradation over postcalcined samples also increased withincreasing x to a maximum at 0.1. Among all the catalysts,0.1CNP550 gave the best conversion of up to 98% for 1 h, about 0.8times higher than that of as-prepared 0.1CNP. These results clearlyindicate that postannealing can effectively enhance the photo-catalytic activity of carbon nitride for RhB degradation undervisible light, and phosphorus doping play an important role inenhancement of photocatalytic activity in the phosphorus-dopedg-C3N4 photocatalysts. The high activity of postcalcined samplescould be attributed to the small particle sizes and large surface areaby self-introducing the nanostructure during the thermal treat-ment. Moreover, thermal decomposition would create surface sitesto facilitate catalytic sorption and to promote the localization oflight-induced electrons in the conjugated systems, which alsofavors for photocatalysis.

The photocatalytic stability of 0.1CNP550 was also checked byrepeating the reaction five times under similar conditions (Fig. S4).The results showed that the catalyst can be reused for severalcycles without loss of activity, reflecting sufficient stability of thecarbon nitride based materials in decomposition of dyes, which isprerequisite for practical application. The excellent stability ofcatalytic activity during irradiation can be attributed to the strongbinding of N in the covalent carbon nitride, which can also besupported by the unchanged structure and surface properties ofthese carbon nitride based photocatalysts after reaction (see Fig.S5).

3.4. Mechanism of photoactivity

It is important to detect the main oxidative species in thephotocatalytic process for revealing the photocatalytic mecha-nism. The main oxidative species in the photocatalytic processcould be detected through the trapping experiments of holes andradicals by using triethanolamine (TEOA, hole scavenger) and tert-butyl alcohol (TBA, radical scavenger), respectively. As shown inFig. 10, in the CN550 system, the degradation of RhB was almostcompletely inhibited when the TEOA (10 vol%) was added into thereaction solution. On the contrary, the degradation of RhB wasonly partly depressed with the addition of TBA (10 vol%). Theseresults suggested that free radicals were involved but notexclusively, and the photogenerated holes were the mainoxidative species in CN550 system. During the photocatalyticprocess, the photogenerated hole can be directly transferred to theRhB dye, and then oxidize the dye. In the system of 0.1CNP550system, the photocatalytic activity was also greatly suppressed byTEOA, while the degradation of RhB was partly depressed by theTBA, indicating the main oxidative species is the same as that ofpure g-C3N4.

As mentioned above, the phosphorus-doped g-C3N4 exhibitedthe higher activity for decomposing RhB than undoped g-C3N4.Density functional theory calculations showed that the visiblelight response of carbon nitride originated from the electrontransfer from the HOMO populated by N 2p orbitals to the LUMOformed by C 2p orbitals. As observed by UV–vis spectra, thephosphorus doping gives rise to a small decrease in the bandgap. As the phosphorus is incorporated into the C sites by theform of P–N in g-C3N4, the small decrease in band gap byphosphorus doping originated from a decrease in the bottom ofconduction band. This means that the oxidation ability ofphotogenerated hole was not decreased in the phosphorus-doped g-C3N4, while light absorption was increased in the visiblelight region, which is beneficial for the photodegradationreaction. Additionally, it is known that heterogeneous photo-catalysis is a surface-based process. A large surface area not onlyfacilitates mass transfer but also provides more activity sites forsurface-dependent reactions, which would be favorable for

photocatalytic reaction. Phosphorus heteroatoms doping intocarbon nitride increased the surface area and decreased thestructural size of g-C3N4, which also contribute to the improvedactivity.

4. Conclusions

In this paper, phosphorus was doped into polymeric g-C3N4 bya co-condensation strategy between dicyandiamide and aphosphorus-containing ionic liquid. The incorporation of phos-phorus has a significant effect on the properties of the dopedsamples, including morphology, optical, and electronic nature ascompared with undoped carbon nitride. Our results showed thatthe doped materials exhibit higher photocatalytic performancesfor decomposing dyes under visible light irradiation, which can beascribed to the favorable textural, optical properties andelectronic structure caused by doping with phosphorus heteroa-toms into carbon nitride host. Moreover, the activity can befurther improved by a facile postannealing treatment. This workdemonstrates that the inexpensive metal-free phosphorus-dopedgraphitic C3N4 material is a very promising candidate forenvironmental application.

Acknowledgements

This work was supported by the National Natural ScienceFoundation of China (no. 20803038, no. 21003146 and no.21201174), the Basic Research Project of the Qingdao Science

L. Zhang et al. / Materials Research Bulletin 48 (2013) 3485–3491 3491

and Technology Program (12-1-4-9-(6)-jch), the KnowledgeInnovation Program of the Chinese Academy of Sciences (no.KSCX2-EW-J-10).

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.materresbull.2013.

05.040.

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