Carbon Self-Doped Carbon Nitride Nanosheets with Enhanced Visible-Light Photocatalytic Hydrogen Production

Hongwei Wang 1, Guiqing Huang 1, Zhiwei Chen 2 and Weibing Li 2,* 1 School of Environmental Science and Engineering, Qingdao University, Qingdao 266071, China; hwwang6688@126.com (H.W.); hgq_ll2513@163.com (G.H.) 2 School of Environment and Safety Engineering, Qingdao University of Science and Technology, 53 Zhengzhou Road, Qingdao 266042, China; chenzhiwei1204@163.com * Correspondence: lwbing@qust.edu.cn; Tel.: +86-532-8402-2617


Introduction
Environmental pollution and energy crises have been seriously threatening the survival of mankind and the sustainable development of society.The photocatalysis technique is considered as an environmentally friendly technology to solve these problems because of its characteristics of being green, its low energy consumption and sustainable recycling [1].
In 2009, Wang et al. [2] reported graphene-like carbon nitride (g-C 3 N 4 ) as a novel metal-free polymer semiconductor photocatalyst in water splitting for the first time.g-C 3 N 4 has attracted a lot of attention because of its suitable bandgap, abundant composition elements and visible-light response.However, g-C 3 N 4 is limited in practical applications due to its small specific surface area and serious photocarrier recombination.Therefore, it is urgent to increase the specific surface area and the photocarrier transport rate of g-C 3 N 4 .
Traditional g-C 3 N 4 generally shows a layered structure that is formed by stacking a large number of two-dimensional triazine ring nanosheets [3].Modification of the micromorphology can effectively increase the specific surface area and improve the photocatalytic performance.Ultrathin g-C 3 N 4 flake (UF-CN) was prepared through a facile wet mechanical grind method [4].With a yield rate of 90%, the prepared UF-CN showed higher photocatalytic hydrogen production and photocatalytic degradation rate.Han et al. [5] treated "seaweed" architecture g-C 3 N 4 by direct calcination of the freeze-dried precursor.The as-prepared samples showed increased active sites, improved electron transfer ability and increased visible-light response, which resulted in the enhancement of the photocatalytic performance.
Design and construction of g-C 3 N 4 -based heterojunctions can effectively accelerate the transport and transfer of photocarriers, greatly reducing the recombination rate of photocarriers and improving the photocatalytic performance.Various materials, such as CdS [6,7], MoS 2 [8], ZnO [9] and TiO 2 [10], have been used for constructing g-C 3 N 4 -based heterojunctions.However, the heterostructure would reduce the redox ability of the photocarriers, which weakened the energy driving the photocatalytic process.Another commonly used modification method is doping, which can modulate the electronic structure of g-C 3 N 4 to improve the photocatalytic performance.Lan et al. [11] proposed a Br-modified g-C 3 N 4 through thermal polycondensation of the mixture of urea and NH 4 Br.The introduction of bromine modulated the electronic structure, optical absorption, conductivity and charge-carrier transfer rate of g-C 3 N 4 without changing the polymer architecture of the g-C 3 N 4 , leading to enhanced photocatalytic performance.Zhang et al. [12] prepared I-modified g-C 3 N 4 by a simple in-situ surface modification method.The modification enhanced the visible-light absorption, accelerated the transfer rate of photocarriers, and finally increased the photocatalytic hydrogen production rate.Many other dopant atoms, such as S [13], P [14], B [15] and F [16], have been adopted to dope in the triazine structure of g-C 3 N 4 .However, the introduction of heterogeneous atoms is likely to cause asymmetric doping or impurities, which can serve as recombination centers for the photogenerated electrons and holes, therefore reducing the photocatalytic performance.Carbon materials such as graphene, graphdiyne, carbon nanotube and carbon quantum dots have been considered as potential materials to couple with photocatalysts to construct effective composite photocatalysts due to their unique physical properties and optical properties [17].Moreover, the introduction of carbon into g-C 3 N 4 could effectively avoid the generation of impurities.Therefore, morphology control and element doping may effectively improve the photocatalytic performance of g-C 3 N 4 .
Inspired by these works, we prepared a carbon self-doped g-C 3 N 4 nanosheet (C-C 3 N 4 NS) by heating the mixture of bulk g-C 3 N 4 and glucose, expecting to improve the photocatalytic performance of g-C 3 N 4 .The bulk g-C 3 N 4 was obtained by the traditional thermal polymerization, while the secondary heat treatment can effectively exfoliate the layered g-C 3 N 4 into the C 3 N 4 nanosheet (C 3 N 4 NS) and introduce the C element into the triazine structure.Therefore, the obtained C-C 3 N 4 NS showed enhanced visible-light photocatalytic hydrogen production.Design and construction of g-C3N4-based heterojunctions can effectively accelerate the transport and transfer of photocarriers, greatly reducing the recombination rate of photocarriers and improving the photocatalytic performance.Various materials, such as CdS [6,7], MoS2 [8], ZnO [9] and TiO2 [10], have been used for constructing g-C3N4-based heterojunctions.However, the heterostructure would reduce the redox ability of the photocarriers, which weakened the energy driving the photocatalytic process.Another commonly used modification method is doping, which can modulate the electronic structure of g-C3N4 to improve the photocatalytic performance.Lan et al. [11] proposed a Br-modified g-C3N4 through thermal polycondensation of the mixture of urea and NH4Br.The introduction of bromine modulated the electronic structure, optical absorption, conductivity and charge-carrier transfer rate of g-C3N4 without changing the polymer architecture of the g-C3N4, leading to enhanced photocatalytic performance.Zhang et al. [12] prepared I-modified g-C3N4 by a simple in-situ surface modification method.The modification enhanced the visible-light absorption, accelerated the transfer rate of photocarriers, and finally increased the photocatalytic hydrogen production rate.Many other dopant atoms, such as S [13], P [14], B [15] and F [16], have been adopted to dope in the triazine structure of g-C3N4.However, the introduction of heterogeneous atoms is likely to cause asymmetric doping or impurities, which can serve as recombination centers for the photogenerated electrons and holes, therefore reducing the photocatalytic performance.Carbon materials such as graphene, graphdiyne, carbon nanotube and carbon quantum dots have been considered as potential materials to couple with photocatalysts to construct effective composite photocatalysts due to their unique physical properties and optical properties [17].Moreover, the introduction of carbon into g-C3N4 could effectively avoid the generation of impurities.Therefore, morphology control and element doping may effectively improve the photocatalytic performance of g-C3N4.

Results and Discussion
Inspired by these works, we prepared a carbon self-doped g-C3N4 nanosheet (C-C3N4 NS) by heating the mixture of bulk g-C3N4 and glucose, expecting to improve the photocatalytic performance of g-C3N4.The bulk g-C3N4 was obtained by the traditional thermal polymerization, while the secondary heat treatment can effectively exfoliate the layered g-C3N4 into the C3N4 nanosheet (C3N4 NS) and introduce the C element into the triazine structure.Therefore, the obtained C-C3N4 NS showed enhanced visible-light photocatalytic hydrogen production.

Results and Discussion
Figure 1A,B presents typical SEM images of C3N4 NS and 0.4 C-C3N4 NS.The C3N4 NS (Figure 1A) exhibits a simple typical layered structure, which is similar to that of bulk C3N4.The layered structure of the C3N4 NS was exfoliated by a thermal oxidation etching process [18].
However, the layered structures of 0.4 C-C3N4 NS (Figure 1B) are corroded, which might be ascribed to the introduction of C atoms.To investigate the state of C, high-resolution transmission electron microscope (HRTEM) was tested.As shown in Figure 1C,D  Figure 2A shows the FTIR spectra of C3N4 NS and C-C3N4 NS.The peaks at 1200-1600 and 805 cm −1 are corresponding to the characteristics of C-N heterocycle stretches and tri-s-triazine ring modes, respectively.This phenomenon indicates that the original C3N4 backbones are not damaged after the introduction of carbon doping.The peaks between 3000-3300 cm −1 are assigned to the N-H stretching vibrations, and the peaks for the C-C3N4 NS are weaker than those for the C3N4 NS, which can be ascribed to the decrease of N atoms caused by the doping of C atoms at the N sites [19,20].The solid-state 13 C-magic-angle spinning (MAS)-NMR spectra of C3N4 NS and 0.4 C-C3N4 NS (Figure 3A) showed two strong peaks at 158.4 and 166.4 ppm, which correspond to C3N (1) and C2N-NHx (2) in the triazine ring units, respectively.No other peak is observed for 0.4 C-C3N4 NS, indicating that the introduction of carbon does not cause the open-loop reaction of the triazine ring, and that the C-C3N4 NS remained in the original C3N4 triazine ring structure, which is similar to the above Fouriertransform infrared (FTIR) results [21].The maintenance of these 2D conjugated backbone structures provides the prerequisite for the π-delocalized electronic systems [12].
The atomic distributions of C and N are further revealed by the electron-energy loss spectroscopy (EELS) elemental mapping.Figure 3B shows the EELS mapping of 0.4 C-C3N4 NS, the distribution of C and N elements being shown in Figure 3C,D.C and N elements are evenly distributed throughout all the 0.4 C-C3N4 NS material, and no more carbon is observed to be coated on the edge of 0.4 C-C3N4 NS, indicating that the C is not clustered on the C3N4 surface in the form of a coated surface.Moreover, there are no carbon particles adhered to the surface of the 0.4 C-C3N4 NS, as shown in the HRTEM results.Therefore, traces of introduced C atoms might be embedded into the structure of the tri-s-triazine ring.The NMR results exhibit that the introduced C atoms do not cause the open-loop reaction of the triazine ring, and the FTIR results show that the N-H stretching vibrations get weaker after the C doping.It can be concluded that the N atoms in the tri-s-triazine ring were substituted by the introduced C atoms. Figure 2A shows the FTIR spectra of C 3 N 4 NS and C-C 3 N 4 NS.The peaks at 1200-1600 and 805 cm −1 are corresponding to the characteristics of C-N heterocycle stretches and tri-s-triazine ring modes, respectively.This phenomenon indicates that the original C 3 N 4 backbones are not damaged after the introduction of carbon doping.The peaks between 3000-3300 cm −1 are assigned to the N-H stretching vibrations, and the peaks for the C-C 3 N 4 NS are weaker than those for the C 3 N 4 NS, which can be ascribed to the decrease of N atoms caused by the doping of C atoms at the N sites [19,20].Figure 2A shows the FTIR spectra of C3N4 NS and C-C3N4 NS.The peaks at 1200-1600 and 805 cm −1 are corresponding to the characteristics of C-N heterocycle stretches and tri-s-triazine ring modes, respectively.This phenomenon indicates that the original C3N4 backbones are not damaged after the introduction of carbon doping.The peaks between 3000-3300 cm −1 are assigned to the N-H stretching vibrations, and the peaks for the C-C3N4 NS are weaker than those for the C3N4 NS, which can be ascribed to the decrease of N atoms caused by the doping of C atoms at the N sites [19,20].The solid-state 13 C-magic-angle spinning (MAS)-NMR spectra of C3N4 NS and 0.4 C-C3N4 NS (Figure 3A) showed two strong peaks at 158.4 and 166.4 ppm, which correspond to C3N (1) and C2N-NHx (2) in the triazine ring units, respectively.No other peak is observed for 0.4 C-C3N4 NS, indicating that the introduction of carbon does not cause the open-loop reaction of the triazine ring, and that the C-C3N4 NS remained in the original C3N4 triazine ring structure, which is similar to the above Fouriertransform infrared (FTIR) results [21].The maintenance of these 2D conjugated backbone structures provides the prerequisite for the π-delocalized electronic systems [12].
The atomic distributions of C and N are further revealed by the electron-energy loss spectroscopy (EELS) elemental mapping.Figure 3B shows the EELS mapping of 0.4 C-C3N4 NS, the distribution of C and N elements being shown in Figure 3C,D.C and N elements are evenly distributed throughout all the 0.4 C-C3N4 NS material, and no more carbon is observed to be coated on the edge of 0.4 C-C3N4 NS, indicating that the C is not clustered on the C3N4 surface in the form of a coated surface.Moreover, there are no carbon particles adhered to the surface of the 0.4 C-C3N4 NS, as shown in the HRTEM results.Therefore, traces of introduced C atoms might be embedded into the structure of the tri-s-triazine ring.The NMR results exhibit that the introduced C atoms do not cause the open-loop reaction of the triazine ring, and the FTIR results show that the N-H stretching vibrations get weaker after the C doping.It can be concluded that the N atoms in the tri-s-triazine ring were substituted by the introduced C atoms.The solid-state 13 C-magic-angle spinning (MAS)-NMR spectra of C 3 N 4 NS and 0.4 C-C 3 N 4 NS (Figure 3A) showed two strong peaks at 158.4 and 166.4 ppm, which correspond to C 3N (1) and C 2N -NHx (2) in the triazine ring units, respectively.No other peak is observed for 0.4 C-C 3 N 4 NS, indicating that the introduction of carbon does not cause the open-loop reaction of the triazine ring, and that the C-C 3 N 4 NS remained in the original C 3 N 4 triazine ring structure, which is similar to the above Fourier-transform infrared (FTIR) results [21].The maintenance of these 2D conjugated backbone structures provides the prerequisite for the π-delocalized electronic systems [12].
The atomic distributions of C and N are further revealed by the electron-energy loss spectroscopy (EELS) elemental mapping.Figure 3B shows the EELS mapping of 0. Figure 4A shows the XRD results of the C3N4 NS and C-C3N4 NS.All the samples exhibit two characteristic peaks at 13° and 27.4° corresponding to the (100) in-planar peak and the (002) interlayer-stacking peak of g-C3N4, respectively [2].Enlarged XRD patterns (Figure 4B) show the gradual shift of the (002) peak to a higher angle with the introduction of carbon, indicating disturbance of the packing and undulations of the single layers.The introduction of the carbon dopant could form delocalized π-bonds that complete the π-delocalized electronic systems [22].High-resolution C 1s XPS spectra (Figure 4C) shows that the C3N4 NSs were deconvoluted into two peaks at 284.9 eV and 288.4 eV, which can be ascribed to graphitic carbon atoms and sp 2 -bonded carbon atoms in the heterocycle (N-C=C) of aromatic g-C3N4, respectively [23].For the 0.4 C-C3N4 NS, the peak corresponding to sp 2 -bonded carbon slightly shifted to 288.3 eV, which indicated that the binding energies of carbon atoms were affected by the doped carbon.High-resolution N 1s XPS spectra (Figure 4D) shows that all peaks of the 0.4 C-C3N4 NS shifted to lower binding energy compared to those of C3N4 NS.Therefore, the assumption that the N atoms in tri-s-triazine ring were substituted by C atoms, as shown in Figure 5, is further confirmed.The doped C atoms substituting N atoms maintained the original 2D conjugated backbone structures and reduced the number of N-H bonds, which resulted in the abovementioned results [24].Figure 4A shows the XRD results of the C 3 N 4 NS and C-C 3 N 4 NS.All the samples exhibit two characteristic peaks at 13 • and 27.4 • corresponding to the (100) in-planar peak and the (002) interlayer-stacking peak of g-C 3 N 4 , respectively [2].Enlarged XRD patterns (Figure 4B) show the gradual shift of the (002) peak to a higher angle with the introduction of carbon, indicating disturbance of the packing and undulations of the single layers.The introduction of the carbon dopant could form delocalized π-bonds that complete the π-delocalized electronic systems [22].High-resolution C 1s XPS spectra (Figure 4C) shows that the C 3 N 4 NSs were deconvoluted into two peaks at 284.9 eV and 288.4 eV, which can be ascribed to graphitic carbon atoms and sp 2 -bonded carbon atoms in the heterocycle (N-C=C) of aromatic g-C 3 N 4 , respectively [23].For the 0.4 C-C 3 N 4 NS, the peak corresponding to sp 2 -bonded carbon slightly shifted to 288.3 eV, which indicated that the binding energies of carbon atoms were affected by the doped carbon.High-resolution N 1s XPS spectra (Figure 4D) shows that all peaks of the 0.4 C-C 3 N 4 NS shifted to lower binding energy compared to those of C 3 N 4 NS.Therefore, the assumption that the N atoms in tri-s-triazine ring were substituted by C atoms, as shown in Figure 5, is further confirmed.The doped C atoms substituting N atoms maintained the original 2D conjugated backbone structures and reduced the number of N-H bonds, which resulted in the abovementioned results [24].
Figure 6A shows the photocatalytic hydrogen production performance of C 3 N 4 NS and C-C 3 N 4 NS.For C 3 N 4 NS, the H 2 evolution rate reaches 2200.6 ± 210.4 µmol/g at 4 h, while all the C-C 3 N 4 NS samples exhibit higher evolution rate.Among them, 0.4 C-C 3 N 4 NS reaches the highest H 2 evolution rate of 6545.8 ± 260.3 µmol/g at 4 h, which is almost three-times higher than that of C 3 N 4 NS; in order to rule out the contingency of the experiment, we did three sets of parallel experiments for each set of data.Corresponding error bars are given in Figure 6A, and the error ranges of the H 2 evolution rates are within reasonable tolerances.The corresponding volume histogram of hydrogen production is shown in Figure 6B, and the 0. binding energies of carbon atoms were affected by the doped carbon.High-resolution N 1s XPS spectra (Figure 4D) shows that all peaks of the 0.4 C-C3N4 NS shifted to lower binding energy compared to those of C3N4 NS.Therefore, the assumption that the N atoms in tri-s-triazine ring were substituted by C atoms, as shown in Figure 5, is further confirmed.The doped C atoms substituting N atoms maintained the original 2D conjugated backbone structures and reduced the number of N-H bonds, which resulted in the abovementioned results [24].Figure 6A shows the photocatalytic hydrogen production performance of C3N4 NS and C-C3N4 NS.For C3N4 NS, the H2 evolution rate reaches 2200.6 ± 210.4 μmol/g at 4 h, while all the C-C3N4 NS samples exhibit higher evolution rate.Among them, 0.4 C-C3N4 NS reaches the highest H2 evolution rate of 6545.8 ± 260.3 μmol/g at 4 h, which is almost three-times higher than that of C3N4 NS; in order to rule out the contingency of the experiment, we did three sets of parallel experiments for each set of data.Corresponding error bars are given in Figure 6A, and the error ranges of the H2 evolution rates are within reasonable tolerances.The corresponding volume histogram of hydrogen production is shown in Figure 6B, and the 0.4 C-C3N4 NS exhibits the highest hydrogen production performance as well (1636.45± 65.075 μmol/g/h).Additionally, the photocatalysis stability of the 0.4 C-C3N4 NS was tested further, as shown in Figure 6C.After four cycles, the hydrogen evolution performance of the 0.4 C-C3N4 NS sample only decreased a little, and stabilized in the third and fourth cycles, indicating the stable photocatalysis performance of 0.4 C-C3N4 NS.
Figure 6D shows the photocatalytic rhodamine B (RhB) degradation results of the series of samples under visible-light illumination.Three sets of parallel experiments were presented to rule out the contingency of the experiment.All data errors are within reasonable tolerances, which might result from the difference in degree of centrifugation.As shown in Figure 6D, after visible-light illumination, for the C3N4 NS photocatalyst, only 50% RhB could be degraded in 60 min.However, all C-C3N4 NS samples exhibited higher photocatalytic degradation rates.Among them, the 0.4 C-C3N4 NS showed the highest degradation performance, which reached approximately 95% RhB degradation in only 40 min.Figure 6A shows the photocatalytic hydrogen production performance of C3N4 NS and C-C3N4 NS.For C3N4 NS, the H2 evolution rate reaches 2200.6 ± 210.4 μmol/g at 4 h, while all the C-C3N4 NS samples exhibit higher evolution rate.Among them, 0.4 C-C3N4 NS reaches the highest H2 evolution rate of 6545.8 ± 260.3 μmol/g at 4 h, which is almost three-times higher than that of C3N4 NS; in order to rule out the contingency of the experiment, we did three sets of parallel experiments for each set of data.Corresponding error bars are given in Figure 6A, and the error ranges of the H2 evolution rates are within reasonable tolerances.The corresponding volume histogram of hydrogen production is shown in Figure 6B, and the 0.4 C-C3N4 NS exhibits the highest hydrogen production performance as well (1636.45± 65.075 μmol/g/h).Additionally, the photocatalysis stability of the 0.4 C-C3N4 NS was tested further, as shown in Figure 6C.After four cycles, the hydrogen evolution performance of the 0.4 C-C3N4 NS sample only decreased a little, and stabilized in the third and fourth cycles, indicating the stable photocatalysis performance of 0.4 C-C3N4 NS.
Figure 6D shows the photocatalytic rhodamine B (RhB) degradation results of the series of samples under visible-light illumination.Three sets of parallel experiments were presented to rule out the contingency of the experiment.All data errors are within reasonable tolerances, which might result from the difference in degree of centrifugation.As shown in Figure 6D, after visible-light illumination, for the C3N4 NS photocatalyst, only 50% RhB could be degraded in 60 min.However, all C-C3N4 NS samples exhibited higher photocatalytic degradation rates.Among them, the 0.4 C-C3N4 NS showed the highest degradation performance, which reached approximately 95% RhB degradation in only 40 min.Figure 7A shows the photoinduced current with time i-t curve of the as-prepared photoelectrodes.As shown in Figure 7A, the photoinduced currents of all C-C3N4 NS photoelectrodes are much higher than those of C3N4 NS.Among all C-C3N4 NS samples, 0.4 C-C3N4 NS exhibits the highest photocurrent of 1.2 μA/cm 2 .The electrochemical impedance spectroscopy (EIS) measurement result is shown in Figure 7B; the resistance arc radius of the 0.4 C-C3N4 NS was smaller than that of the C3N4 NS, suggesting that the introduction of C could facilitate the charge transport of C3N4 NS [25].
In order to detect the recombination rate of photocarriers in the samples, Photoluminescence (PL) spectra were tested.As shown in Figure 7C, all C-C3N4 NS samples show decreased PL emission intensity in comparison with C3N4 NS ones, indicating that the recombination rate of charge carriers is suppressed by the carbon doping.Photocurrent and PL results show that the introduction of carbon improves the separation of photocarriers and inhibits the recombination rate, which promotes the photocatalytic performance [26].Time-resolved PL spectra also support the above assumption.It is clear that the fluorescence lifetime of 0.4 C-C3N4 NS samples is shorter than that of C3N4 NS (Figure 7D), and the decreased lifetime manifests more efficient separation of electrons and holes.Therefore, the doped carbon can accelerate the separation and transfer of the photogenerated carriers [27].Figure 7A shows the photoinduced current with time i-t curve of the as-prepared photoelectrodes.As shown in Figure 7A, the photoinduced currents of all C-C 3 N 4 NS photoelectrodes are much higher than those of C 3 N 4 NS.Among all C-C 3 N 4 NS samples, 0.4 C-C 3 N 4 NS exhibits the highest photocurrent of 1.2 µA/cm 2 .The electrochemical impedance spectroscopy (EIS) measurement result is shown in Figure 7B; the resistance arc radius of the 0.4 C-C 3 N 4 NS was smaller than that of the C 3 N 4 NS, suggesting that the introduction of C could facilitate the charge transport of C 3 N 4 NS [25].Figure 7A shows the photoinduced current with time i-t curve of the as-prepared photoelectrodes.As shown in Figure 7A, the photoinduced currents of all C-C3N4 NS photoelectrodes are much higher than those of C3N4 NS.Among all C-C3N4 NS samples, 0.4 C-C3N4 NS exhibits the highest photocurrent of 1.2 μA/cm 2 .The electrochemical impedance spectroscopy (EIS) measurement result is shown in Figure 7B; the resistance arc radius of the 0.4 C-C3N4 NS was smaller than that of the C3N4 NS, suggesting that the introduction of C could facilitate the charge transport of C3N4 NS [25].
In order to detect the recombination rate of photocarriers in the samples, Photoluminescence (PL) spectra were tested.As shown in Figure 7C, all C-C3N4 NS samples show decreased PL emission intensity in comparison with C3N4 NS ones, indicating that the recombination rate of charge carriers is suppressed by the carbon doping.Photocurrent and PL results show that the introduction of carbon improves the separation of photocarriers and inhibits the recombination rate, which promotes the photocatalytic performance [26].Time-resolved PL spectra also support the above assumption.It is clear that the fluorescence lifetime of 0.4 C-C3N4 NS samples is shorter than that of C3N4 NS (Figure 7D), and the decreased lifetime manifests more efficient separation of electrons and holes.Therefore, the doped carbon can accelerate the separation and transfer of the photogenerated carriers [27].Diffuse reflectance spectra of the series of samples are shown in Figure 8A.It is clear that the C-C3N4 samples show a progressive redshift with increasing C content, confirming that the bandgap can be narrowed by introducing C. The estimated bandgaps (Figure 8B) of the series of samples are 2.04 eV, 2.18 eV, 2.36 eV and 2.44 eV, corresponding to the 0.6 C-C3N4 NS, 0.4 C-C3N4 NS, 0.2 C-C3N4 NS and C3N4 NS, respectively.To further investigate the influence of C on the band position of the C3N4 NS, Valence band X-ray photoelectron spectroscopy (VB-XPS) was tested for the C3N4 NS and 0.4 C-C3N4 NS.As shown in Figure 8C, the VB maximum was similar for both the C3N4 NS and 0.4 C-C3N4 NS located at 2.07 eV, which was estimated to be 1.51 V versus normal hydrogen electrode at pH = 7 through formula: ENHE/V = Ф + 2.07 eV − 4.44 (Ф = 3.88 eV: the electron work function of the analyzer) [28,29].
According to the above analysis, the VB maximum potential of C3N4 NS and 0.4 C-C3N4 NS samples were both located at 1.51 V (vs.Normal Hydrogen Electrode NHE), indicating that the valence band positions of the samples were unaffected by the introduction of C into g-C3N4, and the estimated band gaps of the C3N4 NS and 0.4 C-C3N4 NS were 2.44 eV and 2.18 eV (Figure 8B), respectively.According to Eg = Ev − Ec, the calculated conduction band minimum potentials of C3N4 NS and 0.4 C-C3N4 NS were −0.93 V and −0.67 V (vs.NHE), respectively.The positive shift of the conduction band for the 0.4 C-C3N4 NS was caused by the formation of doped energy level which comes from the C. The estimated band structure schematic is exhibited in Figure 9.
Figure 10 shows the mechanism for the improvement of the photocatalysis performance under visible light.The substitution of C atoms for N atoms in the tri-s-triazine ring maintained the 2D conjugated system of g-C3N4, while the introduction of doped carbon atoms accelerates the charge mobility, thus enhancing the photocatalytic performance.In order to detect the recombination rate of photocarriers in the samples, Photoluminescence (PL) spectra were tested.As shown in Figure 7C, all C-C 3 N 4 NS samples show decreased PL emission intensity in comparison with C 3 N 4 NS ones, indicating that the recombination rate of charge carriers is suppressed by the carbon doping.Photocurrent and PL results show that the introduction of carbon improves the separation of photocarriers and inhibits the recombination rate, which promotes the photocatalytic performance [26].Time-resolved PL spectra also support the above assumption.It is clear that the fluorescence lifetime of 0.4 C-C 3 N 4 NS samples is shorter than that of C 3 N 4 NS (Figure 7D), and the decreased lifetime manifests more efficient separation of electrons and holes.Therefore, the doped carbon can accelerate the separation and transfer of the photogenerated carriers [27].
Diffuse reflectance spectra of the series of samples are shown in Figure 8A.It is clear that the C-C 3 N 4 samples show a progressive redshift with increasing C content, confirming that the bandgap can be narrowed by introducing C. The estimated bandgaps (Figure 8B) of the series of samples are 2.04 eV, 2.18 eV, 2.36 eV and 2.44 eV, corresponding to the 0.6 C-C 3 N 4 NS, 0.4 C-C 3 N 4 NS, 0.2 C-C 3 N 4 NS and C 3 N 4 NS, respectively.To further investigate the influence of C on the band position of the C 3 N 4 NS, Valence band X-ray photoelectron spectroscopy (VB-XPS) was tested for the C 3 N 4 NS and 0.4 C-C 3 N 4 NS.As shown in Figure 8C, the VB maximum was similar for both the C 3 N 4 NS and 0.4 C-C 3 N 4 NS located at 2.07 eV, which was estimated to be 1.51 V versus normal hydrogen electrode at pH = 7 through formula: E NHE /V = φ + 2.07 eV − 4.44 (φ = 3.88 eV: the electron work function of the analyzer) [28,29].Diffuse reflectance spectra of the series of samples are shown in Figure 8A.It is clear that the C-C3N4 samples show a progressive redshift with increasing C content, confirming that the bandgap can be narrowed by introducing C. The estimated bandgaps (Figure 8B) of the series of samples are 2.04 eV, 2.18 eV, 2.36 eV and 2.44 eV, corresponding to the 0.6 C-C3N4 NS, 0.4 C-C3N4 NS, 0.2 C-C3N4 NS and C3N4 NS, respectively.To further investigate the influence of C on the band position of the C3N4 NS, Valence band X-ray photoelectron spectroscopy (VB-XPS) was tested for the C3N4 NS and 0.4 C-C3N4 NS.As shown in Figure 8C, the VB maximum was similar for both the C3N4 NS and 0.4 C-C3N4 NS located at 2.07 eV, which was estimated to be 1.51 V versus normal hydrogen electrode at pH = 7 through formula: ENHE/V = Ф + 2.07 eV − 4.44 (Ф = 3.88 eV: the electron work function of the analyzer) [28,29].
According to the above analysis, the VB maximum potential of C3N4 NS and 0.4 C-C3N4 NS samples were both located at 1.51 V (vs.Normal Hydrogen Electrode NHE), indicating that the valence band positions of the samples were unaffected by the introduction of C into g-C3N4, and the estimated band gaps of the C3N4 NS and 0.4 C-C3N4 NS were 2.44 eV and 2.18 eV (Figure 8B), respectively.According to Eg = Ev − Ec, the calculated conduction band minimum potentials of C3N4 NS and 0.4 C-C3N4 NS were −0.93 V and −0.67 V (vs.NHE), respectively.The positive shift of the conduction band for the 0.4 C-C3N4 NS was caused by the formation of doped energy level which comes from the C. The estimated band structure schematic is exhibited in Figure 9.
Figure 10 shows the mechanism for the improvement of the photocatalysis performance under visible light.The substitution of C atoms for N atoms in the tri-s-triazine ring maintained the 2D conjugated system of g-C3N4, while the introduction of doped carbon atoms accelerates the charge mobility, thus enhancing the photocatalytic performance.

Fabrication of g-C3N4
The bulk g-C3N4 was prepared through traditional thermal condensation [4].10 g dicyandiamide (Aladdin Co., Shanghai, China) was annealed at 550 °C with a ramping rate of 10 °C•min −1 in air in a muffle furnace for 3 h to obtain a yellow powder.

Fabrication of C-Modified g-C3N4
In a typical synthesis, 1 g g-C3N4 was dissolved in 50 mL deionized water and sonicated for a few minutes.Then, a different mass of glucose (0.2 g, 0.4 g, 0.6 g, Aladdin Co., Shanghai, China) was added with stirring and then kept stirring for 1 h.The product was centrifuged at 8000 r/min for 2 min and rinsed with water and ethanol for three times, and then dried at 60 °C for 12 h.The mixture was annealed at 550 °C with a ramping rate of 10 °C/min in air in a muffle furnace for 3 h, the samples being denoted as 0.2 C-C3N4 NS, 0.4 C-C3N4 NS, 0.6 C-C3N4 NS, respectively.For comparison, C3N4 NS was prepared at the same condition in the absence of glucose.The procedure is shown as Figure 11.

Fabrication of g-C3N4
The bulk g-C3N4 was prepared through traditional thermal condensation [4].10 g dicyandiamide (Aladdin Co., Shanghai, China) was annealed at 550 °C with a ramping rate of 10 °C•min −1 in air in a muffle furnace for 3 h to obtain a yellow powder.

Fabrication of C-Modified g-C3N4
In a typical synthesis, 1 g g-C3N4 was dissolved in 50 mL deionized water and sonicated for a few minutes.Then, a different mass of glucose (0.2 g, 0.4 g, 0.6 g, Aladdin Co., Shanghai, China) was added with stirring and then kept stirring for 1 h.The product was centrifuged at 8000 r/min for 2 min and rinsed with water and ethanol for three times, and then dried at 60 °C for 12 h.The mixture

Fabrication of g-C 3 N 4
The bulk g-C 3 N 4 was prepared through traditional thermal condensation [4].10 g dicyandiamide (Aladdin Co., Shanghai, China) was annealed at 550 • C with a ramping rate of 10 • C•min −1 in air in a muffle furnace for 3 h to obtain a yellow powder.

Fabrication of C-Modified g-C 3 N 4
In a typical synthesis, 1 g g-C 3 N 4 was dissolved in 50 mL deionized water and sonicated for a few minutes.Then, a different mass of glucose (0.2 g, 0.4 g, 0.6 g, Aladdin Co., Shanghai, China) was added with stirring and then kept stirring for 1 h.The product was centrifuged at 8000 r/min for 2 min and rinsed with water and ethanol for three times, and then dried at 60 • C for 12 h.The mixture was annealed at 550 • C with a ramping rate of 10 • C/min in air in a muffle furnace for 3 h, the samples being denoted as 0.2 C-C 3 N 4 NS, 0.4 C-C 3 N 4 NS, 0.6 C-C 3 N 4 NS, respectively.For comparison, C 3 N 4 NS was prepared at the same condition in the absence of glucose.The procedure is shown as Figure 11.

Fabrication of C-Modified g-C3N4
In a typical synthesis, 1 g g-C3N4 was dissolved in 50 mL deionized water and sonicated for a few minutes.Then, a different mass of glucose (0.2 g, 0.4 g, 0.6 g, Aladdin Co., Shanghai, China) was added with stirring and then kept stirring for 1 h.The product was centrifuged at 8000 r/min for 2 min and rinsed with water and ethanol for three times, and then dried at 60 °C for 12 h.The mixture was annealed at 550 °C with a ramping rate of 10 °C/min in air in a muffle furnace for 3 h, the samples being denoted as 0.2 C-C3N4 NS, 0.4 C-C3N4 NS, 0.6 C-C3N4 NS, respectively.For comparison, C3N4 NS was prepared at the same condition in the absence of glucose.The procedure is shown as Figure 11.

Characterization
The crystalline structures of the samples were identified through X-ray diffraction (XRD, D/MAX-2500/PC, Rigaku Co., Tokyo, Japan).The micro-morphologies of the samples were observed using a field-emission scanning electron microscope (SEM, JSM-6700F, JEOL, Tokyo, Japan) and a high-resolution transmission electron microscope (HRTEM, Tecnai G2 F20, FEI, Hillsboro, OR, USA).The atomic distributions of samples were further revealed by the electron energy loss spectroscopy (EELS, GIF Quantum 965, Gatan, Pleasanton, CA, USA) elemental mapping.The C atomic chemical states of samples were conducted by solid-state 13 C magic-angle spinning (MAS) NMR (DiscoveryMR750 3.0T, General Electric Company, Boston, MA, USA) measurements at room temperature.The elementary compositions and bonding information of the samples were analyzed by using X-ray photoelectron spectroscopy (XPS, Axis Ultra, Kratos Analytical Ltd., Manchester, England, UK).The optical absorptions of the solid-state samples were tested by a UV-visible diffuse reflectance spectrophotometer (UV-vis DRS, U-41000, HITACHI, Tokyo, Japan).The photoluminescence and fluorescence decay times of the as-prepared materials were characterized using a fluorescence spectrometer (PL, Fluoro Max-4, HORIBA Jobin Yvon, Paris, France) with excitation wavelength at 350 nm.The molecular structures of the samples were identified through Fourier-transform infrared (FT-IR) spectra which used a Nicolet IS10 infrared spectrophotometer (Thermo Fisher Scientific, Waltham, MA, USA) in the transmission mode using a KBr pellet method.

Photocatalytic Reaction
The photocatalytic activities of the samples were tested by photocatalytic hydrogen production and photocatalytic dye degradation under visible light.In photocatalytic hydrogen production test, 50 mg catalyst powder was dispersed in 100 mL aqueous solution which contained 10 vol % triethanolamine as sacrificial electron donor and 3 wt % H 2 PtCl 6 •6H 2 O (Aladdin Co., Shanghai, China) as a co-catalyst [30].Before the test, the photoreaction system was vacuumed until the pressure gage was stable.Externally illuminating 150 W Xe lamp (PLS-SXE300, Beijing Changtuo Co., Ltd., Beijing, China) was used as light source, and we added a 420 nm cut-off filter to achieve visible light, and adjusted the photo density to 250 mW/cm 2 .The illuminated area of the photocatalytic reaction is 28.3 cm 2 (light spot diameter is 6 cm).The entire reaction system was kept at a constant 5 • C with condensed water.In photocatalytic dye degradation test, rhodamine B (RhB, Aladdin Co., Shanghai, China) was selected as the target degradant.The specific experimental steps are as follows: 0.05 g photocatalyst was dispersed in 100 mL RhB with a concentration of 10 mg•L −1 .Before illumination, the mixture kept stirring for 30 min in the dark to achieve adsorption-desorption equilibrium.The light source and photo density were kept the same with the photocatalytic hydrogen production test.The entire reaction system was maintained at 25 • C with circulating water.The light absorptivity of solutions were decided by the UV-visible absorption spectrophotometer (U-41000, HITACHI, Tokyo, Japan) every 10 min after centrifugation with the speed of 8000 r/min for 2 min.

Photoelectrode Preparation and Photoelectrochemical Reaction
The photoelectrodes were prepared through electrophoretic deposition method.In total, 20 mg photocatalyst was dispersed in 50 mL water containing 5 mg I 2 , the mixture was stirred for 30 min to completely dissolve.Then, two Fluorine-doped Tin Oxide (FTO) conductive glasses with the deposition area of 1 cm × 1 cm and a distance 1 cm were immersed into the solution, maintaining a bias voltage of 20 V and a deposition time of 5 min.The obtained photoelectrodes were annealed at 300 • C for 30 min to improve adhesion.The photoelectrochemical systems were measured in a three-electrode system.The as-prepared photoelectrodes, and platinum and Ag/AgCl electrodes, were used as the working, counter and reference electrodes, respectively.0.1 M Na 2 SO 4 was used as electrolyte.All photoelectrochemical tests were measured using a CHI-660D (Chenhua Instrument Co., Shanghai, China) electrochemical workstation.The photoinduced current density-time (i-t) curves were measured at 1 V (vs.Ag/AgCl) bias under visible light with the photo density at 100 mW/cm 2 .The electrochemical impedance spectroscopy (EIS) was measured at 0 V (vs.Ag/AgCl) bias with the frequency range from 10 5 -10 −1 Hz.

Conclusions
In summary, we developed C self-doped g-C 3 N 4 by a simple co-condensation method, and carbon atoms were successfully introduced into the tri-s-triazine structure of g-C 3 N 4 .The secondary heat treatment exfoliated the bulk g-C 3 N 4 to nanosheets, which endows materials with higher specific surface area.Compared with pure g-C 3 N 4 , the modified C substitutes N atoms in the triazine ring, which broadens the range of light absorption, accelerates the transfer rate of charge carriers as well as increases the photocatalysis performance.Among all the C-C 3 N 4 NS samples, the optimal 0.4 C-C 3 N 4 NS unfolds the superior photocatalytic hydrogen rate of 1636.45 µmol/g/h, which is almost three-times higher than that of pure C 3 N 4 NS.Simultaneously, the 0.4 C-C 3 N 4 NS shows a better photocatalytic RhB degradation rate than that of the C 3 N 4 NS under visible light illumination, the former of which could achieve approximately 95% degradation in 40 min.The photoelectrochemical test results verify that the introduction of C accelerates the transfer rate and inhibits the combination of photo-induced carriers, while also enhancing the photocatalytic performance.This work highlights how carbon modification incorporated into g-C 3 N 4 has improved the photocatalytic performance without destroying the 2D conjugated backbone structures.

Figure
Figure 1A,B presents typical SEM images of C 3 N 4 NS and 0.4 C-C 3 N 4 NS.The C 3 N 4 NS (Figure 1A) exhibits a simple typical layered structure, which is similar to that of bulk C 3 N 4 .The layered structure of the C 3 N 4 NS was exfoliated by a thermal oxidation etching process [18].However, the layered structures of 0.4 C-C 3 N 4 NS (Figure 1B) are corroded, which might be ascribed to the introduction of C atoms.To investigate the state of C, high-resolution transmission electron microscope (HRTEM) was tested.As shown in Figure 1C,D, C 3 N 4 NS and 0.4 C-C 3 N 4 NS samples exhibit ultrathin nanoflakes with layered structure, and no obvious carbon particles adhered to the C 3 N 4 NS surface are observed in the 0.4 C-C 3 N 4 NS, indicating that the C might be embedded in the structure of the tri-s-triazine ring.
Figure 1A,B presents typical SEM images of C3N4 NS and 0.4 C-C3N4 NS.The C3N4 NS (Figure 1A) exhibits a simple typical layered structure, which is similar to that of bulk C3N4.The layered structure of the C3N4 NS was exfoliated by a thermal oxidation etching process [18].However, the layered structures of 0.4 C-C3N4 NS (Figure 1B) are corroded, which might be ascribed to the introduction of C atoms.To investigate the state of C, high-resolution transmission electron microscope (HRTEM) was tested.As shown in Figure 1C,D, C3N4 NS and 0.4 C-C3N4 NS samples exhibit ultrathin nanoflakes with layered structure, and no obvious carbon particles adhered to the C3N4 NS surface are observed in the 0.4 C-C3N4 NS, indicating that the C might be embedded in the structure of the tri-s-triazine ring.

Figure 2 .
Figure 2. FTIR spectra of C 3 N 4 NS and C-C 3 N 4 NS samples.
Figure4Ashows the XRD results of the C 3 N 4 NS and C-C 3 N 4 NS.All the samples exhibit two characteristic peaks at 13 • and 27.4 • corresponding to the (100) in-planar peak and the (002) interlayer-stacking peak of g-C 3 N 4 , respectively[2].Enlarged XRD patterns (Figure4B) show the gradual shift of the (002) peak to a higher angle with the introduction of carbon, indicating disturbance of the packing and undulations of the single layers.The introduction of the carbon dopant could form delocalized π-bonds that complete the π-delocalized electronic systems[22].High-resolution C 1s XPS spectra (Figure4C) shows that the C 3 N 4 NSs were deconvoluted into two peaks at 284.9 eV and 288.4 eV, which can be ascribed to graphitic carbon atoms and sp 2 -bonded carbon atoms in the heterocycle (N-C=C) of aromatic g-C 3 N 4 , respectively[23].For the 0.4 C-C 3 N 4 NS, the peak corresponding to sp 2 -bonded carbon slightly shifted to 288.3 eV, which indicated that the binding energies of carbon atoms were affected by the doped carbon.High-resolution N 1s XPS spectra (Figure4D) shows that all peaks of the 0.4 C-C 3 N 4 NS shifted to lower binding energy compared to those of C 3 N 4 NS.Therefore, the assumption that the N atoms in tri-s-triazine ring were substituted by C atoms, as shown in Figure5, is further confirmed.The doped C atoms substituting N atoms maintained the original 2D conjugated backbone structures and reduced the number of N-H bonds, which resulted in the abovementioned results[24].Figure6Ashows the photocatalytic hydrogen production performance of C 3 N 4 NS and C-C 3 N 4 NS.For C 3 N 4 NS, the H 2 evolution rate reaches 2200.6 ± 210.4 µmol/g at 4 h, while all the C-C 3 N 4 NS samples exhibit higher evolution rate.Among them, 0.4 C-C 3 N 4 NS reaches the highest H 2 evolution rate of 6545.8 ± 260.3 µmol/g at 4 h, which is almost three-times higher than that of C 3 N 4 NS; in order to rule out the contingency of the experiment, we did three sets of parallel experiments for each set of data.Corresponding error bars are given in Figure6A, and the error ranges of the H 2 evolution rates are within reasonable tolerances.The corresponding volume histogram of hydrogen production is shown in Figure6B, and the 0.4 C-C 3 N 4 NS exhibits the highest hydrogen production performance as well (1636.45± 65.075 µmol/g/h).Additionally, the photocatalysis stability of the 0.4 C-C 3 N 4 NS was tested further, as shown in Figure 6C.After four cycles, the hydrogen evolution performance of the 0.4 C-C 3 N 4 NS sample only decreased a little, and stabilized in the third and fourth cycles, indicating the stable photocatalysis performance of 0.4 C-C 3 N 4 NS.

Figure 4 .
Figure 4. (A) XRD patterns and (B) enlarged XRD patterns of series samples.High-resolution XPS spectra of (C) C 1s and (D) N 1s of C3N4 NS and 0.4 C-C3N4 NS.

Figure 4 .Figure 4 .
Figure 4. (A) XRD patterns and (B) enlarged XRD patterns of series samples.High-resolution XPS spectra of (C) C 1s and (D) N 1s of C 3 N 4 NS and 0.4 C-C 3 N 4 NS.

Figure 5 .
Figure 5. Atomic structure of C 3 N 4 NS and C-C 3 N 4 NS.

FigureFigure 6 .
Figure6Dshows the photocatalytic rhodamine B (RhB) degradation results of the series of samples under visible-light illumination.Three sets of parallel experiments were presented to rule out the contingency of the experiment.All data errors are within reasonable tolerances, which might result from the difference in degree of centrifugation.As shown in Figure6D, after visible-light illumination, for the C 3 N 4 NS photocatalyst, only 50% RhB could be degraded in 60 min.However, all C-C 3 N 4 NS samples exhibited higher photocatalytic degradation rates.Among them, the 0.4 C-C 3 N 4 NS showed the highest degradation performance, which reached approximately 95% RhB degradation in only 40 min.

Figure 6 .
Figure 6.(A) Photocatalysis hydrogen evolution and (B) volume histogram of series of samples.(C) Photocatalysis hydrogen evolution stability test of 0.4 C-C 3 N 4 NS.(D) Photocatalysis rhodamine B (RhB) degradation performances of series of samples.

Figure 6 .
Figure 6.(A) Photocatalysis hydrogen evolution and (B) volume histogram of series of samples.(C) Photocatalysis hydrogen evolution stability test of 0.4 C-C3N4 NS. (D) Photocatalysis rhodamine B (RhB) degradation performances of series of samples.

Figure 8 .
Figure 8. (A) UV-Vis diffuse reflection and (B) the estimated band gaps of series of samples.(C) VB-XPS spectra for C3N4 NS and 0.4 C-C3N4 NS samples.

Figure 7 .
Figure 7. (A) Photoinduced current-time curves and (B) Electrochemical impedance spectroscopy (EIS) results of C 3 N 4 NS and 0.4 C-C 3 N 4 NS electrodes.(C) Photoluminescence spectra of series of samples.(D) Time-resolved fluorescence spectra of C 3 N 4 NS and 0.4 C-C 3 N 4 NS.

Figure 7 .
Figure 7. (A) Photoinduced current-time curves and (B) Electrochemical impedance spectroscopy (EIS) results of C3N4 NS and 0.4 C-C3N4 NS electrodes.(C) Photoluminescence spectra of series of samples.(D) Time-resolved fluorescence spectra of C3N4 NS and 0.4 C-C3N4 NS.

Figure 8 .
Figure 8. (A) UV-Vis diffuse reflection and (B) the estimated band gaps of series of samples.(C) VB-XPS spectra for C3N4 NS and 0.4 C-C3N4 NS samples.

Figure 8 . 12 Figure 9 .
Figure 8. (A) UV-Vis diffuse reflection and (B) the estimated band gaps of series of samples.(C) VB-XPS spectra for C 3 N 4 NS and 0.4 C-C 3 N 4 NS samples.

Figure 10 .
Figure 10.Mechanism of the photocatalysis performance improvement under visible light.

Figure 9 .
Figure 9. Band structure of the C 3 N 4 NS and 0.4 C-C 3 N 4 NS.

Figure 10 11 Figure 9 .
Figure10shows the mechanism for the improvement of the photocatalysis performance under visible light.The substitution of C atoms for N atoms in the tri-s-triazine ring maintained the 2D conjugated system of g-C 3 N 4 , while the introduction of doped carbon atoms accelerates the charge mobility, thus enhancing the photocatalytic performance.

Figure 10 .
Figure 10.Mechanism of the photocatalysis performance improvement under visible light.

Figure 10 .
Figure 10.Mechanism of the photocatalysis performance improvement under visible light.

Figure 11 .
Figure 11.Schematic illustration for the formation of C-C3N4 NS.

Figure 11 .
Figure 11.Schematic illustration for the formation of C-C 3 N 4 NS.