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www.rsc.org/materials

0959-9428(2010)20:1;1-A

ISSN2050-7488

Materials for energy and sustainability

Journal of

Materials Chemistry A

www.rsc.org/MaterialsA Volume1 | Number 1 | January 2013 | Pages 0000–0000

Journal of

Materials Chemistry A

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This article can be cited before page numbers have been issued, to do this please use: J. Xu, L. Zhang, R. Shi and Y. Zhu, J.

Mater. Chem. A, 2013, DOI: 10.1039/C3TA13188B.

http://pubs.rsc.org/en/journals/journal/TA

http://dx.doi.org/10.1039/c3ta13188b

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1

1

Chemical exfoliation of graphitic carbon nitride for efficient heterogeneous2

photocatalysis34

Jing Xu,† Liwu Zhang, † Rui Shi, Yongfa Zhu* 5

Department of Chemistry, Tsinghua University, Beijing, 100084, PR China67

ABSTRACT. Single atomic layer nanosheet materials show great application potential in8

many fields due to their enhanced intrinsic properties from their counterparts and newly born9

properties. Herein, g-C3 N4 nanosheets with single atomic layer structure is prepared by a10

simple chemical exfoliation method. The as-prepared nanosheets show single atomic11

thickness of 0.4 nm and lateral size of micrometers. The structure and photocatalytic12

properties of the as-prepared single layer g-C3 N4 are then studied. Compared with the bulk g-13

C3 N4, single layer g-C3 N4 nanosheets show great superiority in photogenerated charge carriers14

transfer and separation. Accordingly, the photocatalytic H2 production and pollutant15

decomposition activities, and photocurrent generation of single layer g-C3 N4 nanosheets are16

much higher than that of the bulk g-C3 N4, indicating the great application potential of single17

layer g-C3 N4 nanosheets in photocatalysis and photosynthesis.18

19202122232425262728

2930313233

Keywords: Carbon nitride; Single atomic layer; Nanosheet; Photocatalysis3435363738

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1. INTRODUCTION34

Single atomic layer nanosheets have attracted extensive attention due to their unique5

structural and physiochemical properties.1 Since the discovery of the two-dimensional atomic6

crystal graphene,2 which represents many intriguing properties different from bulk graphite7

and potentials in diverse applications, great efforts have been performed to synthesize8

nanosheets with single atomic layer structure.3-7 By exfoliating layered materials into9

nanosheets, exotic electronic properties and high specific surface areas accompanying with10

greatly enhanced host capabilities have been obtained and show great potential in application11

in electronics, energy storage, solar energy conversion, etc.8, 9 12

Graphite-like carbon nitride (g-C3 N4), a metal-free polymer semiconductor, has attracted13

plenty of attention due to its potential application in solar energy conversion, photosynthesis,14electrocatalysis, bioimaging and biomedical application.10-21 The g-C3 N4 has been shown to be15

able to produce H2 by water splitting and decompose organic pollutants under visible light16

irradiation by pioneering works of Wang and coworkers.22 More potential applications of g-17

C3 N4 are emerging as researches carry on in structure engineering and activity improving. 12,1823-31 19

Motivated by graphene, researchers have tried to prepare the nanosheet form of g-C 3 N4 to20

fully explore the potential of g-C3 N4. Compared with graphite, the layer of g-C3 N4 is21

composed by C-N bonds instead of C-C bonds, while weak van der Waals force exists22 between the layers. Encourage by the exfoliation of graphite into graphene layers by the23

widely used Hummers method,32 Liu and coworkers employed the Hummers method to24

exfoliate bulk g-C3 N4.33 However, instead of nanosheets, particles with several hundred25

nanometers thickness were obtained and the planar atomic structure of g-C3 N4 has been26

destroyed, indicating that the Hummers method may destroy the intrinsic structure of g-C 3 N4.27

The failure is attributed to the not strong enough hydrogen bonding between the layers of28

strands of polymeric melon units with NH/NH2 groups proposed by Lostsch et al.,34 which is29

different from the planar pure covalent bonding cohesion within the graphite. Liu and30coworkers thus developed a direct thermal oxidation “etching” process of bulk g-C3 N4 to get31

g-C3 N4 nanosheets with a thickness of around 2 nm, implying a multilayer of g-C 3 N4 (6-732

layers).33 Ultrathin g-C3 N4 nanosheets were also prepared by ultrasound exfoliation or liquid33

exfoliation routes from bulk g-C3 N4.11, 35 The resulted nanosheets possess thickness of >2 nm.34

Instead of the “top-down” fabrication of g-C3 N4 nanosheets, Wang and Coworkers found that35

a “bottom-up” method by modifying polycondensation of g-C3 N4 precursors could result in36

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thin C3 N4 nanosheets with thickness of around 3.6 nm.36 Although different methods have1

successfully been developed to get g-C3 N4 nanosheets, the as-prepared g-C3 N4 nanosheets are2

still thicker than 2nm and composed of more than 6 C-N layers. It is still a great challenge to3

prepare real graphene-like g-C3 N4 material with single atomic layer.4

Herein, we have for the first time obtained the single atomic layer g-C3 N4 nanosheets5

(Monolayer-C3 N4) by a simple chemical exfoliation method. Since g-C3 N4 is not strong6

enough against the strong oxidation effect exerted by KMnO4 oxidant used in the Hummers7

method for graphite exfoliation, KMnO4 is no more used in the chemical exfoliation method.8

The obtained Monolayer-C3 N4 is then characterized and performances in photosynthesis and9

photocatalysis under visible light irradiation are investigated.10

2.

EXPERIMENTAL SECTION11

Synthesis of single atomic layer g-C3N4 nanosheets. g-C3 N4 was prepared by heating12dicyandiamide (Tianjin Fuchen Chemical Reagents factory, China) in a muffle furnace for 4 h13

to 550 °C and keep at this temperature for another 4 h in air. g-C3 N4 monolayer nanosheets14

were synthesized via the chemical exfoliation method. The as-prepared g-C3 N4 (1 g) was15

mixed with 10mL of H2SO4 (98 wt %) in a 50mL flask and stirred for 8 h at room temperature.16

Then the mixture was slowly poured into 100 mL of deionized water and sonicated for17

exfoliation. The temperature of the suspension increased rapidly, and the color changed from18

yellow to light yellow. The obtained suspension was then subjected to 10 min of centrifugation19

at 3,000 r.p.m. to remove any unexfoliated g-C3 N4. The obtained light yellow suspension was20centrifuged, washed thoroughly with deionized water to remove the residual acid, and finally21

drying at 80 °C in air overnight. In order to eliminate the structural defects, the obtained22

powders (0.3 g) were put into a flask with 150 mL of methanol and refluxed on a mantle heater23

at 65 °C for 6 h. After centrifuged and dried at 80 °C, the obtained products were g-C3 N4 24

nanosheets.25

Characterization of the Samples. The samples were characterized by powder X-ray26

diffraction (XRD) on a Bruker D8-advance X-ray diffractometer at 40 kV and 40 mA for27

monochromatized Cu Kα (λ = 1.5406 Å) radiation. The Brunauer-Emmett-Teller (BET)28

specific surface area and pore volume of the samples were characterized by nitrogen29

adsorption at 77 K with Micromeritics 3020 instrument. Transmission electron microscopy30

(TEM) images were obtained by Hitachi HT-7700 electron microscope operated at an31

accelerating voltage of 100 kV. Atomic Force Microscope (AFM) images were acquired in32

phase mode in air using a Digital Instrument Shimadzu SPM-9600 (The samples were33

prepared by drop-casting corresponding dilute dispersions onto freshly cleaved mica surface).34


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Fourier transformed infrared (FTIR) spectra were recorded on a Bruker VERTEX 7001

spectrometer between 4000 and 600 cm-1. Diffuse reflection spectra (DRS) were obtained on a2

Hitachi U-3010 UV-vis spectrophotometer. Photoluminescence spectra (PL) of the samples3

were obtained at room temperature excited by incident light of 318 nm using a fluorescence4

spectrometer (JASCO FP-6500). Elemental analysis results were collected from an EA 30005analyzer. 6

Photocatalytic tests and Photoelectrochemical Performance. The photocatalytic7

activities for H2 production were evaluated under visible light irradiation (λ > 420 nm). Visible8

irradiation was provided by a 500 W xenon lamp with a 420 nm cutoff filter and the average9

visible light intensity was 40.0 mW cm-2. 20 mg of the photocatalyst powder was dispersed in10

an aqueous solution (20 mL) containing triethanolamine (10 vol. %) as a sacrificial electron11

donor. 3 wt. % Pt was loaded on the surface of the photocatalyst sample by the in situ12

photodeposition method using H2PtCl6. The suspension solution was then purged with N2 for13at least 30 min to remove air and then sealed with a rubber septum. The temperature of the14

reaction solution was maintained at room temperature by a flow of cooling water during the15

photocatalytic reaction. The amount of evolved hydrogen was analyzed by a Shimadzu GC-16

2014 gas chromatography equipped with a thermal conductive (TCD) detector and a 5 Å17

molecular sieve column, using N2 as the carrier gas. 18

The photocatalytic activities were further evaluated by the degradation of phenol and19

methylene blue (MB) under visible light irradiation. In each run, 50 mL of MB solution20

(2.0×10−5 M) or phenol solution (2.0 mg L-1) containing 25 mg of as-synthesized photocatalyst21

were placed in a glass beaker. Before irradiation, the suspensions were magnetically stirred in22

the dark for 2 h to ensure equilibrium between the photocatalyst and the model molecules. At23

certain time intervals, 2 mL of the suspensions were taken and centrifuged to remove the24

particles. The concentration of phenol was analyzed using a HPLC with a UV absorbance25

detector operated at 270 nm coupled to a Venusil XBP-C18 column. The concentration of the26

MB was analyzed by recording the absorption intensity at 663 nm using a Hitachi U-3010 UV-27

vis spectrophotometer.28

The photoelectrochemical properties were performed on a CHI 660B electrochemical29

system (Shanghai, China) using a standard three-electrode cell with a working electrode, a30

platinum wire counter electrode, and a standard calomel electrode (SCE) reference electrode.31

0.1 M Na2SO4 was used electrolyte. The working electrodes were prepared as follows: 2 mg of32

as-prepared photocatalyst was suspended in 1 mL of water to produce slurry, which was then33

dip-coated on a 20 mm × 40 mm indium-tin oxide (ITO) glass electrode. Electrodes were34


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exposed in air for 1 day to eliminate water and subsequently heated at 200 °C for 1 day. All1

investigated working electrodes were of similar thickness. The electrochemical impedance2

spectroscopy (EIS) was carried out at the open circuit potential. A sinusoidal ac perturbation of3

5 mV was applied to the electrode over the frequency range 0.05-105 Hz.4

53. RESULTS AND DISCUSSION6

7

Figure 1. Illustration of synthetic strategy. (a) Intercalation of H2SO4 (98 wt %) in the interplanar space of g-8

C3 N4. (b) Intercalated g-C3 N4 was exfoliated into single layer due to the rapid heating effect of H2SO4 (98 wt %)9

mixing with water.10

20 22 24 26 28 30 32 34

Bulk C3N

4

H2SO4 intercalated

I n t e n s i t y / a . u .

2Theta / degree

11

Figure 2. XRD patterns of the bulk g-C3 N4 before and after H2SO4 intercalation.12

The synthetic strategy is illustrated in Figure 1. First, the bulk g-C3 N4 was mixed with13

H2SO4 (98 wt %) and stirred for 8 hours at room temperature to allow the intercalation of14

H2SO4 in the interlayer space of g-C3 N4. The intercalation mechanism is generally similar15

with the widely reported H2SO4 intercalated graphite.37 The intercalation of H2SO4 into bulk16

g-C3 N4 is confirmed by X-ray diffraction (XRD) (Figure 2). It is found that the (002)17

diffraction at around 27.4 o relating to the characteristic interlayer stacking structure is shifted18

to 25.0 o, indicating that the interlayer distance is expanded from 0.325 nm to 0.356 nm. Then19

the mixture was slowly poured into deionized water and sonicated, due to the sonication and20

rapid heating effect when concentrated H2SO4 is mixed with water, the intercalated bulk g-21


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C3 N4 was exfoliated into single layer. The obtained dispersion was then subjected to1

centrifugation to remove any unexfoliated and aggregated g-C3 N4. The exfoliation is found to2

significantly depend on the concentration of sulfuric acid. H2SO4 with lower concentrations3

(75 wt% and 50 wt%) were also studied, however the results show that lower concentration4

H2SO4 failed to exfoliate bulk g-C3 N4 into single layer (Figure S1 and S2 in Supporting5Information). Compared with the reported thermal oxidation “etching” process33, ultrasound6

exfoliation11 or “bottom-up” method36 for the fabrication of g-C3 N4 nanosheets, the chemical7

exfoliation method allows intercalation of H2SO4 in the interlayer space of g-C3 N4, thus8

higher possibility to obtain single layer nanosheet after exfoliation. Moreover, the chemical9

exfoliation is also more cost-efficient than the reported methods mentioned above.10

11

12

13

14

15

16

17

18

19

20

21

22

Figure 3. Typical AFM images (a) and TEM images (b) of the as-prepared Monolayer-C3 N4 nanosheets. The23

tables and curves in the right panel show the height and width information of different nanosheets from the AFM24

images.25

26

Figure 3a shows the typical atomic force microscope (AFM) image of the exfoliated g-27

C3 N4 nanosheets. The lateral size of these sheets is ranging from tens of nanometers to several28

micrometers. The thickness analysis of the nanosheets by AFM reveal an average thickness of29

about 0.4 nm, which is very close to the theoretical thickness of monolayer g-C3 N4 (about30

0.325 nm). Statistical studies of the thickness of single layer g-C3 N4 nanosheets by measuring31

different samples confirm this observation (Figure S3 in the supporting information). Some32

multilayered g-C3 N4 nanosheets with thickness from 1 nm to 4 nm are also observed in the33

AFM studies, which comprise ca. 40% of the sample, indicating the current chemical34

aa

b


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exfoliation can produce g-C3 N4 nanosheets with 60% single layer C3 N4. TEM image (Figure1

3b) illustrates the very transparent feature of the nanosheets, indicating the ultrathin thickness2

of the nanosheets. The darker part in the TEM image can be attributed to the overlap of3

several g-C3 N4 nanosheets or a multilayer nanosheet. The bulk g-C3 N4 before exfoliation4

shows a thickness of more than 10 nm (Figure S4 and S5), indicating the effectiveness of the5current chemical exfoliation method. The EDX studies (Figure S6) confirmed the elemental6

composition of the nanosheets, which are shown to be composed of C and N elements, while7

the Cu detected can be attributed to the copper substrate for TEM investigation. Notably,8

oxygen is not detected in the element analysis of the nanosheets, implying the g-C3 N4 is not9

oxidized by concentrated H2SO4 employed for exfoliation.10

11

12

1314

15

16

17

18

19

20

21

22

23

24

25

Figure 4. a). XRD patterns, b). FTIR spectra, c). Enlarged FTIR spectra of the bulk g-C3 N4 26

and Monolayer-C3 N4.27

28

The structure, composition and chemical states of the Monolayer-C3 N4 is further29

investigated by XRD, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared30

(FTIR) etc. In the XRD patterns (Figure 4a), the (002) diffraction at around 27.4 o relates to31

the characteristic interlayer stacking structure, while the (100) diffraction at 13.0 o indicates32

the interplanar structural packing. It has been reported in some other monolayer structures that33

due to the formation of agglomerates and crumples of the monolayer nanosheets by van der34

Waals force during preparing sample for XRD measurment, the XRD pattern of the35

10 20 30 40 50 60 70

Bulk g-C3N

4


2 Theta / degree

Monolayer-C3N

4

a)

4000 3000 2000 1000

Monolayer-C3N

4

Bulk g-C3N

4


Wavenumber / cm-1

C-N triazine

b)

1800 1600 1400 1200 1000 800

12531320

14051572

Monolayer-C3N

4

Bulk g-C3N

4

I n t e n s i t y /

a . u .

Wavenumber / cm-1

1632 807

c)


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monolayer nanosheets could resemble that of bulk form.38-40 However, it is observed that the1

intensity of (002) diffraction related to the interlayer stacking is significantly weakened after2

exfoliation as expected. The multilayer components in the sample also contribute to the3

observed (002) diffraction.4

The strong FTIR absorption bands of Monolayer-C3 N4 in Figure 4b and 4c further revealed5a typical molecular structure of graphitic carbon nitride. The absorption band near 1572 and6

1632 cm-1 are attributed to C=N stretching, while the three bands at 1253, 1320 and 1425 cm-1 7

correspond to aromatic C-N stretching. The peak at 807 cm -1 belongs to triazine ring mode,8

which correspond to condensed CN heterocycles. The broad band at 3000 – 3500 cm-1 9

corresponds to uncondensed terminal amino groups (-NH2 or =NH groups). The FTIR results10

are in fairly good accordance with the reports of g-C3 N4 material prepared by11

polycondensation and polymerization reactions.25,26 The Raman spectra are also invesitigated12

to confirm the molecular structure of g-C3 N4 (Figure S7 in supporting information). In the13Raman spectra for both bulk and monolayer g-C3 N4, there are two characteristic bands related14

to the disordered D (around 1300 cm-1) and graphitic G (around 1500 cm-1) bands, indicating15

the formation of graphitic structure. The Raman bands at 600 – 800 cm-1 can be attributed to16

the in-plane rotation of sixfold rings in a graphitic carbon nitride layer.41 17

18

19

20

21

22

23

24

25

Figure 5 .

a) C1s XPS spectra; b) N1s XPS spectra of the bulk g-C3 N4 and Manolayer-C3 N4 nanosheets.2627

The chemical states of the Monolayer-C3 N4 and the bulk g-C3 N4 were studied by XPS28

(Figure 5). The C1s spectra show two peaks locating at about 284.6 eV and 288.0 eV. The29

288.0 eV peak is further deconvoluted into two Gaussian-Lorenzian peaks. The main30

contribution peak at 287.9 eV is attibuted to the sp2 hybridized carbon bonded to N inside the31

triazine rings, while the peak at 288.6 is assigned to the sp2 hybridized carbon in the triazine32

296 292 288 284

a)


Binding energy/eV

284.6

C 1s

288.0

Monolayer-C3N

4

bulk g-C3N

4

408 404 400 396

b)


Binding energy/eV

398.6

N 1s

Monolayer-C3N

4

bulk g-C3N

4


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ring bonded to the –NH2 group. 35 The peak at 284.6 eV is typically ascribed to sp2 C=C1

bonds. The peak of 398.6 eV in the N1s spectra can also be deconvoluted into two Gaussian-2

Lorenzian peaks. The dominant peak at 398.7 eV corresponds to the sp2-bonded N involved in3

the triazine rings (C-N=C), while the peak at 399.7 eV is attributed to the bridging N atomes4

in N-(C)3 or N bonded with hydrogen.35

56

7

8

9

10

11

12

1314

Figure 6. a). DRS spectra and b). PL spectra of the bulk g-C3 N4 and Monolayer-C3 N4.15

16

The optical properties have been investigated by UV-Vis diffuse reflectance spectra (DRS)17

and photoluminenscence (PL) measurements (Figure 6a and 6b). It is found that the18

absorption edge of Monolayer-C3 N4 shows a remarkable blue shift from 470 to 425 nm,19

corresponding to an increase in the band gap from 2.64 eV to 2.92 eV compared with that of20

bulk g-C3 N4. This blue shift performance can be presumably ascribed to the decrease of21conjugation length and the strong quantum confinement effect due to the single layer structure22

of g-C3 N4 nanosheets. The PL spectra demonstrate that under photoexcitation at 318 nm, the23

emission peak of the bulk g-C3 N4 locates at around 464 nm, while the position of the emission24

peak is blue shifted to to 438nm in the case of Monolayer-C3 N4. The blue shift of the25

emission peak by 26 nm is in well agreement with the larger bandgap by 0.28 eV of26

Monolayer-C3 N4 nanosheets.27

The photocatalytic H2 production from water in the presence of the bulk g-C3 N4 or28

Monolayer-C3 N4 were studied under visible-light irradiation (λ > 420 nm), as shown in Figure297a. 3 wt.% Pt was loaded on the surface of the photocatalyst samples by the in situ30

photodeposition method from H2PtCl6 solution. It is found that Monolayer-C3 N4 shows a31

significant improvement over the reference bulk g-C3 N4. The rate of H2 evolution on32

Monolayer-C3 N4 reaches 230 µmolg-1h-1, which is about 2.6 times the rate of the bulk g-C 3 N4 33

(90 µmolg-1h-1).34

35

350 400 450 500 550 600

Monolayer-C3N

4

b)


Wavelength / nm

Bulk g-C

3N

4

200 300 400 500 600 700 800

Monolayer-C3N

4

a)

Bulk g-C3N

4

A b s .

Wavelength / nm


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1

2

3

4

56

7

Figure 7. a) Photocatalytic H2 production on bulk-C3 N4 and monolayer-C3 N4. b) Photocatalytic decomposition of8

phenol under visible light irradiation (λ > 420 nm). c) Stability measurement.9

10

The superiority of Monolayer-C3 N4 for photocatalysis application is further confirmed by11

the photodegradation of phenol under visible light irradiation. A first-order linear relationship12

was revealed by the plots of ln(C /C 0) versus irradiation time (Figure 7b). The determined13reaction-rate constants, k , were 0.0633 and 0.2589 h-1, respectively, for bulk g-C3 N4 and14

Monolayer-C3 N4. The photocatalytic decomposition rate over the monolayer nanosheet was15

more than four times as fast as the bulk g-C3 N4. The Monolayer-C3 N4 also shows a more than16

threefold enhancement in the photocatalytic degradation of methylene blue under visible light17

irradiation, as shown in Figure S8 in supporting information. The stability of the Monolayer-18

C3 N4 as a photocatalyst is also evaluated under visible light irradiation (Figure 7c). It is found19

that the photocatalytic activity only very slightly decreased after five successive cycles of20

photocatalysis degradation tests, indicating the Monolayer-C3 N4 is fairly stable under the21studied conditions.22

It is well known that the photocatalytic activity is greatly related to surface area, charge23

transporting, light absorption and so on. The properties and photocatalytic activity of the24

single atomic layer g-C3 N4 is different with multilayer g-C3 N4 mainly in three aspects: i) The25

electron and hole mobility in the monolayer is significantly greater than that in the multilayer,26

which has been proved in the study of graphene. ii) The surface area of monolayer is bigger27

than that of multilayer, which is also very important for a photocatalyst. iii) The light28

absorption is different due to the quantum size effect, the absorption edge is expected to be29more blue-shifted in the case of monolayer, which could affect the photocatalytic activity by30

shifting the position of conduction and valence bands of g-C3 N4. Since the Monolayer-C3 N4 31

was obtained from bulk g-C3 N4 by chemical exfoliation, the crystallinity should be similar in32

both materials, which is also confirmed by the XRD patterns. Therefore, the greatly enhanced33

photocatalytic activity of Monolayer-C3 N4 than the bulk g-C3 N4 in both H2 production and34

phenol degradation can be attributed to several reasons: First, the increased surface area, the35

0

50

100

150

200

250

300

Monolayer-C3N

4

H 2

e v

o l v e d / u m o l g - 1 h - 1

Sample

Bulk g-C3N

4

a)

0 1 2 3 4-1.2

-0.9

-0.6

-0.3

0.0

Blank

Bulk g-C3N4

Monolayer-C3N4

l n ( C / C

o )

Time / h

b)

0.30

0.45

0.60

0.75

0.90

1.05

c)

C / C

0

Time

1st 3rd 5th2nd 4th


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0 30 60 90 120

0

4

8

12

16

20

C u r r e n t / µ A

Time/s

bulk g-C3N

4

of f

Light on

Monolayer-C3N

4

a)

0 200 400 600

0

1k

2k (iv)

(iii)(ii)

b)

- Z " /

o h m

Z'/ohm

(i)

measured surface area of Monolayer-C3 N4 is 205.8 m2/g, which is almost 50 times larger than1

that of bulk-C3 N4 (4.3 m2/g), suggesting more active sites in the Monolayer-C3 N4 for2

photocatalytic reaction. Secondly, the improved photogenerated charge carriers transportation3

and separation due to the monolayer nanosheet structure. The monolayer nanosheet structure4

can significantly reduce the distance for the charge carriers to transfer from the place5generated to the solid-liquid interface due to the ultrathin structure. Finally, the negatively6

shifted conduction band due to the quantum size effect can increase the activity of7

photogenerated electrons.8

The improved charge carrier separation in Monolayer-C3 N4 is investigated by the9

photocurrent and electrochemical impedance spectroscopy (EIS) measurements (Figure 9).10

The potential of the electrodes against a Pt counter electrode was set at 0.0 V vs saturated11

calomel electrode (SCE). As shown in Figure 9a, a fast and uniform photocurrent response12

was observed for each switch-on and switch-off event in both bulk g-C3 N4 and Monolayer-13C3 N4 electrodes. This photoresponsive phenomenon was entirely reversible. The photocurrent14

of the Monolayer-C3 N4 is about four times as high as that of the bulk g-C3 N4 electrode, which15

indicates that the separation efficiency of photoinduced electrons and holes is significantly16

improved in the Monolayer-C3 N4 nanosheet.17

18

19

20

21

22

23

24

25

26

Figure 8. a) Photocurrent response under chopped illumination. b) The EIS response of bulk g-C 3 N4 ((i) dark,27

(ii) under visible light irradiation) and Monolayer-C3 N4 electrodes ((iii) dark, (iv) under visible light irradiation).28

29The effect of monolayer nanosheet structure on the kinetics of interfacial charge transfer30

was confirmed by EIS. Figure 8b shows the EIS Nyquist plots of the bulk g-C3 N4 and31

Monolayer-C3 N4 electrodes in dark or under visible light irradiation. The Nyquist plot was32

also fitted to the equivalent Randle circuit (Figure 8b, inset), where Rs is the electrolyte33

solution resistance, CPE is the constant phase element for the electrode and electrolyte34

interface, and Rct is the interfacial charge transfer resistance across the electrode/electrolyte. A35


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lower value of Rct suggests more efficient charge transfer across the electrode/electrolyte1

interface, reducing the possibility of charge recombination and thus enhancing the2

photocurrent response.42 The fitted Rct values for the bulk g-C3 N4 and Monolayer-C3 N4 under3

visible light irradiation are 2570.7 and 980.2 Ω, respectively, indicating that the Monolayer-4

C3 N4 possesses more efficient charge transfer than that of the bulk g-C3 N4. One possible5reason for the more efficient charge transport in nanosheets than the bulk g-C3 N4 could be the6

electron-phonon interaction in the material. The lattice photons are different in the low7

dimension material, which could affect the conductivity of the material. The efficient charge8

mobility has been observed in low dimension materials, one typical example is graphene,9

which shows very high electron mobility.2 However, the exact mechanism is still not very10

clear.11

12

4.

CONCLUSION13

In summary, we have successfully obtained single atomic layer g-C3 N4 nanosheets by a14

simple chemical exfoliation of the bulk g-C3 N4. Although there are still 40% of multilayer15

nanosheets existing in the product, it is for the first time single layer g-C3 N4 nanosheets are16

produced. Compared with the bulk g-C3 N4, single layer g-C3 N4 nanosheets show much larger17

surface area and great superiority in photogenerated charge carriers transport. Accordingly, the18

photocatalytic activity and photocurrent of single layer g-C3 N4 nanosheets are significantly19

enhanced. A threefold enhancement in photocatalytic H2 production is observed in the single20

layer nanosheet as photocatalyst. Experiments trying to seperate the single layer nanosheets21

from the multilayer ones are undergway. The single layer g-C3 N4 shows great potential in the22

application both in environment purification and solar energy conversion.23

24

ASSOCIATED CONTENT25

Supporting Information.26

This material is available free of charge via the Internet.27

AUTHOR INFORMATION28

Corresponding Author29

E-mail: [email protected]

E-mail: [email protected]


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Present address of Liwu Zhang: Cavendish Laboratory, University of Cambridge, JJ1

Thomson Ave, Cambridge, CB3 0HE, UK2

Author Contributions3

† These authors have contributed equally to this work.4

5

ACKNOWLEDGMENT6

The project was supported by the National Basic Research Program of China (973)7

(2013CB632403), National High Technology Research and Development Program of China8

(863) (2012AA062701), Special Project on Innovative Method from the Ministry of Science9

and Technology of China (No.2009IM030500), and Atmospheric Environment Monitoring &10Pollution Control Program of Jiangsu Province (AEMPC201103). 11

12

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DOI: 10.1039/C3TA13188B


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By a simple chemical exfoliation method, for the first time the single atomic layer g-C3 N4

nanosheets are prepared.


DOI: 10.1039/C3TA13188B


xu2013

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