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Facile Synthesis of Oxygen-Doped g-C3N4 Mesoporous Nanosheets for Significant Enhancement of Photocatalytic Hydrogen Evolution Performance

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15 January 2024

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Abstract
In this work, oxygen-doped g-C3N4 mesoporous nanosheets (O-CNS) were synthesized via a facile recrystallization method with the assistance of H2O2. The crystal phase, chemical composition, morphological structure, optical property, electronic structure and electrochemical property of the prepared O-CNS samples were well investigated. The morphological observation combining with nitrogen adsorption-desorption results demonstrated that the prepared O-CNS samples possessed nanosheets-like morphology with porous structure. The O doping into g-C3N4 resulted in the augment of specific surface area, which could provide more active sites for photocatalytic reaction. Simultaneously, the visible-light absorption capacity of O-CNS samples was boosted owing to the regulation of O doping. The built energy level induced by the O doping could accelerate the migration rate of photoinduced carriers, and the porous structure was most likely to speed up the release of hydrogen during photocatalyic hydrogen process. Resultantly, the photocatalytic hydrogen production rate of the optimized oxygen-doped g-C3N4 nanosheets reached up to 2012.9 μmol•h-1•g-1, which was 13.4 times higher than that of bulk g-C3N4. Thus, the significantly improved photocatalytic behavior was imputed to the porous structure, the augment of active sites, and the enhancement of visible light absorption and charge separation efficiency.
Keywords: 
Subject: Chemistry and Materials Science  -   Nanotechnology

1. Introduction

Photocatalytic hydrogen production is an advanced technology that converts renewable solar energy into clean hydrogen energy. As for this advanced technology, photocatalyst is a pivotal factor affecting the efficiency of photocatalytic hydrogen evolution. Thus, it is crucial to develop an efficient, stable and low-cost photocatalyst for facilitating the rapid development of photocatalytic hydrogen evolution [1]. Graphitic carbon nitride (g-C3N4) is a new type of organic semiconductors with the band gap of about 2.7 eV, which is reckoned as a promising photocatalytic material due to its nontoxicity, element abundance, suitable bandgap structure, stable physicochemical properties and good photoelectrochemical performance [2,3,4]. However, single component g-C3N4 still has some inherent defects, for instance, small specific surface area, limited solar light utilization, and long transfer distance of photogenerated charge [5,6], which seriously limit its potential application in photocatalytic hydrogen production.
By now, researchers have explored some effective modification methods to address the aforementioned shortcomings and improve the photocatalytic hydrogen production behavior of g-C3N4, such as element doping [7], morphology modulation [8], noble metal deposition [9] and composites with other materials [10,11,12]. Amongst these modification methods, element doping has been extensively accepted and adopted to improve cata-lytic activity due to its easy operation and simple control. As is well known, the atomic radius and physicochemical properties of O element are similar to those of C and N elements, thus it is an intriguing and challenge research to adopt oxygen doping for modifying g-C3N4 with enhanced photocatalytic hydrogen evolution performance. In previous studies, Shi ‘group [13] conducted an investigation on the thermal gas shocking synthesis and photocatalytic performance of 2D ultrathin oxygen doped g-C3N4 photocatalysts, and found that the electronic structure of oxygen-doped g-C3N4 photocatalysts was optimized and the carrier separation rate was significantly elevated. Jia and his coworkers [14] prepared S and O co-doped g-C3N4 nanosheets through a two-step annealing process with melamine as the g-C3N4 precursor for efficient hydrogen generation. Tang’ group [15] developed a defect-engineering approach to prepared O dopant and N defect in g-C3N4 framework. Saka and his coworkers [16] prepared oxygen-doped g-C3N4 through a pyrolysis route combining with the treatment of HNO3 solution. All these achievements on oxygen-doped g-C3N4 are deeply impressive; however, it is especially desirable to explore an in situ and green strategy for achieving oxygen-doped g-C3N4 with improved photocatalytic hydrogen evolution behavior.
In this work, oxygen-doped g-C3N4 porous nanosheets with large specific surface area were synthesized via a simple recrystallization method with H2O2 as the dopant source. The structure, morphology, chemical composition, optical absorption, specific surface area and carrier separation of the photocatalysts were comprehensively characterized. The photocatalytic activity and stability of the photocatalysts were evaluated by hydrogen production under the visible light illumination. In addition, the possible mechanism of the enhanced photocatalytic hydrogen evolution performance was proposed in this work. Up to now, there are limited reports on the investigation of photocatalytic hydrogen production over oxygen-doped g-C3N4 porous nanosheets.

2. Materials and Methods

2.1. Materials

All the starting materials, containing urea ((NH2)2CO), hydrogen peroxide solution (H2O2), chloroplatinic acid (H2PtCl6), triethanolamine (TEOA), and sodium sulfate (Na2SO4) were of analytical grade, and directly used without further purification.

2.2. Preparation of g-C3N4 (u-CNB) and O-Doped g-C3N4 (O-CNS) Samples

2.2.1. Preparation of g-C3N4 (u-CNB)

The bulk g-C3N4 photocatalyst was prepared by thermal condensation polymerization. 10 g of urea was placed in the semi-closed corundum crucible and calcined in a muffle furnace at 550 °C for 2 h with the rate of 2 °C∙min-1. After being spontaneously cooled to ambient temperature, the obtained light yellow product was taken out, and fully ground into powder for further use.

2.2.2. Preparation of O-Doped g-C3N4 (O-CNS)

In a typical process, 10 g of (NH2)2CO was completely dissolved in different volumes of commercial H2O2 (30 vol%) solution under ultrasound, and then dried at 80 °C for 15 h to recrystallize (NH2)2CO. Subsequently, the recrystallized (NH2)2CO was placed in a quartz tube and heated at 550 °C for 2 h with the rate of 2 °C∙min-1 under Ar environment. Finally, the calcined products were ground and collected, and alternately washed with dilute nitric acid solution (1mol·L-1), deionized water and ethanol to obtained O-CNS samples. The volumes of H2O2 solution were 57.5 mL, 60 mL, 62.5 mL, 65 mL, 67.5 mL and 70 mL, so, the obtained O-CNS samples were respectively expressed as 57.5 O-CNS, 60 O-CNS, 62.5 O-CNS, 65 O-CNS, 67.5 O-CNS and 70 O-CNS, correspondingly.

2.3. Characterization

The X-ray diffraction (XRD) patterns of the as-prepared photocatalysts were conducted on a Bruker D8 advance diffractometer with Cu Ka radiation. X-ray photoelectron spectroscopy (XPS) was recorded on an ESCALAB 250 Xi spectrometer. The scanning electron microscopy (SEM) observation was performed on a Hitachi S-4800 electron microscope with the accelerating voltage of 5 kV. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) were conducted to investigate the porous structure of the obtained O-CNS sample on a FEI Talos F200S transmission electron microscope with the voltage of 200 kV. Brunauer-Emmett-Teller (BET) specific surface areas were recorded on a Tristar II 3020 surface area analyzer. UV-visible diffuse reflectance spectra (DRS) were collected on a TU-1901 UV-Vis spectrometer. Electron paramagnetic resonance (EPR) spectra were measured on a Bruker MEX-nano spectrometer.

2.4. Photoelectrochemical Measurements

The photoelectrochemical properties of the obtained samples were tested in Na2SO4 electrolyte solution (0.5M, pH=6.8) through a three-electrode system, which is equipped with a counter electrode (Pt sheet), a reference electrode (Ag/AgCl solution), and a working electrode (FTO conductive glass with spin coated photocatalysts) [17]. A Xenon lamp (300W) equipped with a 420 nm cut-off quartz optical filter was taken as the visible light source in the photoelectrochemical measurements. The details for the measurements were already reported in our previous work [18].

2.5. Photocatalytic Hydrogen Evolution Experiments

Photocatalytic hydrogen production experiments were conducted in a closed glass gas circulation system (Labsolar III AG), which was connected to an Agilent 7890B gas chromatograph for online testing. A PLS-SXE-300W xenon lamp with a 420 nm cutoff filter was used as the visible light source. The vertical distance from the xenon lamp light to the surface of the mixed solution is 10 cm and the inner diameter of the reactor is 8 cm. 50 mg of photocatalyst was uniformly dispersed in the 100 mL aqueous solution containing 20% triethanolamine (TEOA) and H2PtCl6 solution (1wt%), which was respectively used as sacrificial agent and co-catalyst. Every half hour, the xenon light source was turned on and the amount of hydrogen generation was detected for one time.

3. Results

3.1. Structure and Morphology

The effect of the amount of H2O2 on the crystal structure of the as-obtained samples was analyzed by XRD and the results are shown in Figure 1. It can be seen that the u-CNB sample exhibited two clear and sharp diffraction peaks near 13.2° and 27.8°, corresponding to the (100) and (002) crystal planes of the g-C3N4, respectively [19,20]. However, after incorparating O atoms into g-C3N4 crystal, the diffraction peaks at 13.2° for all O-CNS samples disappeared. Besides above, the diffraction peak around 27.8° of O-CNS samples became wider and weaker than that of u-CNB, and the intensity gradually decreased with the increase of H2O2 content, suggesting that the introduction of O atoms affected the structure of g-C3N4 crystals.
As is presented in Figure 2, the elemental composition and chemical status of the synthesized samples were characterized by XPS. From the survey spectra (Figure 2a), three stong signals for C, N, and O element appeared for the 65 O-CNS sample, indicating the exsistence of O element. The weak signal of O 1s in u-CNB was mainly originated from adsorbed oxygen in the XPS measurement. Interestingly, the intensity of the C 1s and N 1s peaks in the 65 O-CNS sample was weaker than those in u-CNB, presumably due to the doping of O element. Figure 2b-d show the high-resolution XPS spectra. Two dominant peaks for C 1s was situated around 284.6 and 288 eV (Figure 2b), and the high-resolution spectrum of N 1s could be deconvoluted into four smaller peaks with binding energy of 398, 399.6, 400.8 and 404.2 eV (Figure 2c). Figure 2d displays the high-resolution spectrum of O 1s of the 65 O-CNS sample, in which the peak at 531.8 eV was ascribed to the formation of N-C-O material [21,22,23]. Summarily, the results mentioned above provided a strong proof for the presence and chemical status of O ions in the 65 O-CNS sample. Additionally, vacancy defects could be correspondingly formed to compensate for the valence difference of N ions and O ions.
Figure 3 shows the SEM images of u-CNB and 65 O-CNS samples. From Figure 3a, the u-CNB sample possessed a block-like structure assembly with stacking layers. That is to say, the u-CNB sample obtained by direct calcination exhibited irregularly block morphology, which was perhaps insufficient for providing abundant active sites for photocatalytic hydrogen production performance. Contrarily, a typical curled nanosheet-like structure appeared for the 65 O-CNS sample, as is seen in Figure 3b. Thereby, it is reasonably inferred that the addition of H2O2 was mostly likely to play a great role in tuning the morphological structure. The feature of such morphological structure would endow the 65 O-CNS sample with larger specific surface area, which further was benefical for augmenting active sites of the hydrogen production reaction.
The morphological structure of the 65 O-CNS photocatalyst was further examined by TEM and STEM images. As is observed from Figure 4a, the 65 O-CNS sample consists of a mass of nanosheets with a relatively thin thickness of approximately 10 nm. Furthermore, it is clear from the magnified image (Figure 4b) that some irregular pores were distributed throughout thin nanosheets, resulting in the formation of porous structure, which was beneficial for acquiring large specific surface area. The formation of porous structure was presumably associated with the bubbles produced during the polycondensation process with the assistance of H2O2. Therefore, it could be drawn a conclusion that the introduction of H2O2 indeed regulated the morphological structure. The element distribution of 65 O-CNS was revealed by high angle annular dark-field (HAADF) and energy dispersive X-ray spectroscopy (EDS) elements mapping, as illustrated in Figure 4c-f. It is clear from Figure 4c-f that C, N and O elements were uniformly distributed throughout the 65 O-CNS sample, which further confirmed the presence of O element in the 65 O-CNS sample. As for Cu element, it came from copper mesh in the sample preparation. Based on the above analysis, the oxygen-doped C3N4 thin nanosheets with porous structure were successfully prepared with H2O2 as the dopant source.

3.2. Specific Surface Area and Optical Properties

Figure 5 shows the N2 adsorption-desorption isotherms of u-CNB and different O-doped C3N4 samples. Obviously, the N2 adsorption-desorption isotherm for O-doped C3N4 samples with higher N2 volume adsorbed at P/P0 were classified as type IV isotherms with H3 hysteresis loops, manifesting the mesoporous nature of the resulting samples. However, for u-CNB sample, a lower volume N2 adsorbed at high P/P0 was perhaps ascribed to the severe agglomeration of block-like particles [24]. Table 1 shows the specific surface area and pore size distribution of u-CNB and different O-doped C3N4 samples. Among them, u-CNB had the smallest specific surface area, attributable to the blocky structure formed by severe stacking of layers, while 65 O-CNS sample possessed the largest specific surface area due to the nanosheets stacking. The augment of specific surface area for the 65 O-CNS sample was most likely to increase the active sites for catalytic reaction, which would facilitate the significant improvement of photocatalytic hydrogen production performance.
As is well known, the light absorption capacity has a notable effect on the photocatalytic hydrogen production performance. To compare the difference of the light absorption capacity between u-CBN and 65 O-CNS sample, the UV-Vis diffuse reflection (DRS) spectra of u-CNB and 65 O-CNS samples was conducted, and the results are shown in Figure 6a. Quite evidently, the intensity of light absorption for the 65 O-CNS sample was significantly strengthened from ultraviolet to visible light region in comparison with that of u-CBN. Besides above, as compared with u-CNB, the absorption edge of 65 O-CNS sample was redshifted, indicating the visible light absorption range was broadened due to the incorporation of the doping. By extrapolating the linear portion of the Tauc plots (Figure 6b), the bandgap energy (Eg) of u-CNB was determined to be approximately 2.62 eV, while the Eg of 65 O-CNS decreased to about 2.5 eV, indicating that the introduction of O atoms could regulate the electronic structure and enhance the light capture ability [25,26,27]. Thus, the improvement of visible-light absorption capacity was likely to provide probability for producing more photo- generated carriers, and further improve its catalytic performance.

3.3. Electron Paramagnetic Resonance and Electrochemical Measurements

The unpaired electrons of the prepared u-CNB and 65 O-CNS samples were tested by electron paramagnetic resonance (EPR) at ambient temperature and the result is shown in Figure 7. In the magnetic field from 333 to 353 mT, the u-CNB and 65 O-CNS samples had only one Lorentz curve centered around a g value of 2.0034, which was considered as lone pair electrons in sp2 hybrid carbon in g-C3N4 [28]. Moreover, the EPR signal intensity of the 65 O-CNS sample was much higher than that of u-CNB, suggesting that more unpaired electrons in 65 O-CNS sample were generated under illumination. The result of EPR also revealed that the recombination of photo-generated electron-hole pairs for the 65 O-CNS sample was substantially inhibited, compared with that of u-CBN.
As previous research work domonstated [25], the photocurrent response of semiconducting photocatalysts is capable of reflecting the transfer and separation of photoexcited electrons and holes. The migration rate of photo-generated electron-hole pairs in u-CNB and 65 O-CNS samples were further analyzed through electrochemical measurement. As is presented in Figure 8a, rapid and reversible photocurrent response sprouted for u-CNB and 65 O-CNS samples, indicating the stability and reproducibility. The photocurrent density of the 65 O-CNS sample was much larger than that of u-CNB, signifying that the migration rate of photogenerated carriers of the 65 O-CNS sample was much higher than that of u-CNB, demonstrating the remarkable elevation of the separaton and migration rate of photoexcited carriers. Figure 8b presents the Nyquist plots of electrochemical impedance spectra (EIS) for u-CNB and 65 O-CNS samples. According to previously reported work [29,30,31], the smaller the radius of the curve, the smaller the internal resistance. As expected, the impedance of the 65 O-CNS sample was smaller than that of u-CBN, revealing that more excited carriers could successfully migrate to the interface due to the restriction of carrier recombination. This result of EIS was also in good accordance with the results of photocurrent analysis. Based above, we can draw a conclusion that the migration rate of photogenerated carriers of the 65 O-CNS sample were remarkably accelerated and the recombination rate of photogenerated electron-hole pairs decreased significantly due to the O doping.

3.3. Photocatalytic Hydrogen Production Performance

The photocatalytic performance of different O-doped samples was evaluated by splitting water into hydrogen under visible light irradiation, and the results are shown in Figure 9. It can be seen from Figure 9a that the u-CNB sample had a very poor photocatalytic hydrogen production activity with a hydrogen production rate of 149.9 μmol∙h-1∙g-1. Satisfactorily, the hydrogen production performance of O-CNS samples was greatly improved with the increase of H2O2 amount. When the amount of H2O2 reached 65 mL, the obtained sample had the optimal hydrogen production rate of 2012.9 μmol∙h-1∙g-1, which was 13.4 times than that of u-CNB. With further increasing the amount of H2O2, the hydrogen production rate of the obtained samples did not increase constantly, but gradually decreased (Figure 9b). The reason for this trend can be summarized as follows. Firstly, the destruction of pore structures induced by excess H2O2 probably made an important contribution to the decrease of hydrogen evolution rate,which could also be evidenced by the decrease of surface area (Figure 5) with the increase of H2O2 volumes from 65mL to 70mL [32]. Secondly, the excess of H2O2 could result in the formation of more defects, which would act as recombination centers, leading to the increase of the recombination rate of photoinduced e--h+ pairs. Accordingly, the hydrogen production rate of O doped CNS photocatalysts decreased when the loading amount of H2O2 exceeded the critical value of 65 mL. More importantly, the hydrogen production rate of the O-doped g-C3N4 was higher than that of most metal-free g-C3N4 photocatalysts under similar conditions (Table 2) [13,33,34,35,36,37]. In addition, the cycling experiments over the 65 O-CNS sample were conducted. As is found from Figure 9c, the photocatalytic hydrogen production rate of the 65 O-CNS sample exhibited a slight decline after three cycle tests. Additionally, the XRD pattern in Figure 9d showed that the crystal structure of 65 O-CNS exhibited no obvious change after cyclic experiments of photocatalytic hydrogen production. The above results indicated that the 65 O-CNS sample was of high stability as a photocatalyst for photocatalytic hydrogen production.

3.4. Photocatalytic Mechanism

In order to further investigate the enhancement mechanism of photocatalytic activity of oxygen-doped C3N4, the Mott-Schottky (M-S) plots of u-CNB and 65 O-CNS samples were depicted in Figure 10. The M-S plots of u-CNB and 65 O-CNS exhibited positive slopes, indicating the typical characteristics of n-type semiconductors. By intercepting on the horizontal axis in the M-S plots, the flat potentials of u-CNB and 65 O-CNS were calculated to be -0.58 eV and -0.69 eV (vs. NHE), respectively. The 65 O-CNS sample showed a more negative flat band potential than u-CNB, indicating a larger amount of electron accumulation and faster separation rate of photo-generated carriers in the 65 O-CNS photocatalyst. For n-type semiconductor, the potential of flat band is about 0.2 eV more positive than that of conduction band (CB) [38]. Therefore, the conduction band of u-CNB and 65 O-CNS was estimated to be -0.78 eV and -0.89 eV (vs. NHE), respectively. Combining with the UV-vis DRS results above, the valence band (VB) of the u-CNB and 65 O-CNS sample were calculated to be 1.84 eV and 1.61 eV(vs. NHE), respectively. On the basis of above demonstration, a proposed mechanism for the enhancement of photocatalytic behavior is schematically portrayed in Figure 11. Upon the exposure to visible light irradiation, the electrons (e-) in O-CNS are excited, and migrate from the VB to CB, leaving holes in the VB (h+). Then, the excited electrons will transfer from the CB of O-CNS to the surface of Pt particles. The accumulating electrons on the surface of Pt particles combine with hydrogen ions to produce H2. At the same time, TEOA is oxidized to form TEOA+ by photogenerated holes duiring the oxidation reaction process. The possible reaction equations involved in photocatalytic hydrogen evolution are as follows.
O-CNS + hv → h+ + e-
H2O + h+ → H+ + ·OH
TEOA + h+ → TEOA+
e- + H+ → H2
Summarily, the O doping induced the formation of porous structure, the decrease of band gap, the regulation of energy level, and the improvement of the separation and migration rate of photogenerated carriers. The porous structure could presumably speed up the release of hydrogen during photocatalyic hydrogen process. The regulation of energy band endowed O-CNS with higher reduction capacity. The built energy levels formed by vacancy defects facilitated the efficient separation and transfer of photoexcited electrons. Therefore, the 65 O-CNS sample exhibited highest photocatalytic hydrogen production rate due to the synergistic effect of the improved visible light absorption capacity, the augmented active sites, and the enhanced photogenerated carrier separation rate.

5. Conclusions

In the present work, oxygen-doped g-C3N4 porous nanosheets were achieved through an in situ, easily controllable and environmentally friendly approach, using H2O2 as oxygen source. The modification of oxygen doping resulted in the formation of porous structure, vacancy defects and boosted visble-light absorption capacity. The vacancy defects shaped into novel energy level, which effectively restricted the recombination of photoinduced carriers. Simultaneously, the amount of O doping in g-C3N4 significantly affected the photocatalytic hydrogen evolution performance. Benefiting from the optimum amount of H2O2, the 65 O-CNS sample possessed the highest photocatalytic hydrogen production performance. The notable improvement of photocatalytic hydrogen evolution performance over the 65 O-CNS sample was mainly attributed to the porous structure, the increase of active sites, the boosted visible-light absorption capacity and the enhancement of photogenerated charge carriers’ separation efficiency. This work will provide a new pathway for pursuing novel strategy for the preparation of element doped C3N4 photocatalysts with exceptional photocatalytic hydrogen evolution performance.

Author Contributions

Tiekun Jia conceived and designed the experiments; Zhao Deng carried out the synthetic experiment and photo-catalytic performance of the as-prepared samples; Jingjing Li measured the as-prepared samples; Tiekun Jia wrote the paper; Dongsheng Yu analyzed the data; Joong Hee Lee provided precise instruction.

Acknowledgments

The authors express grateful thanks to and the Department of Science and Technology of Henan Province, China (Henan Science and Technology Research Program, 222102520005 and 232102521021), and the Education Department of Henan Province for the fund support (21A430026).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. XRD patters of u-CNB and O-CNS samples.
Figure 1. XRD patters of u-CNB and O-CNS samples.
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Figure 2. (a) Survey spectra, (b) C 1s spectra, and (c) N 1s spectra of u-CNB and 65 O-CNS, (d) O 1s spectra of 65 O-CNS.
Figure 2. (a) Survey spectra, (b) C 1s spectra, and (c) N 1s spectra of u-CNB and 65 O-CNS, (d) O 1s spectra of 65 O-CNS.
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Figure 3. SEM images of (a) u-CNB and (b) 65 O-CNS.
Figure 3. SEM images of (a) u-CNB and (b) 65 O-CNS.
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Figure 4. (a) TEM image, (b) HAADF-STEM image, (c, d, e) elemental mapping images of C, N, and O elements, (f) energy diffraction spectrum of 65 O-CNS.
Figure 4. (a) TEM image, (b) HAADF-STEM image, (c, d, e) elemental mapping images of C, N, and O elements, (f) energy diffraction spectrum of 65 O-CNS.
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Figure 5. N2 adsorption-desorption isotherms of the as-obtained samples.
Figure 5. N2 adsorption-desorption isotherms of the as-obtained samples.
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Figure 6. (a) UV-Vis DRS spectra and (b) the plot of (αhv)1/2 vs photon energy (hv) for bandgap energies of u-CNB and 65 O-CNS.
Figure 6. (a) UV-Vis DRS spectra and (b) the plot of (αhv)1/2 vs photon energy (hv) for bandgap energies of u-CNB and 65 O-CNS.
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Figure 7. EPR spectra of u-CNB and 65 O-CNS.
Figure 7. EPR spectra of u-CNB and 65 O-CNS.
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Figure 8. (a) Photocurrent curves and (b) electrochemical impedance spectra of u-CNB and 65 O-CNS electrodes.
Figure 8. (a) Photocurrent curves and (b) electrochemical impedance spectra of u-CNB and 65 O-CNS electrodes.
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Figure 9. (a) Photocatalytic hydrogen production curves, (b) hydrogen production rate for u-CNB and O-CNS, (c) photocatalytic durability of 65 O-CNS and (d) XRD pattern of 65 O-CNS after cyclic runs.
Figure 9. (a) Photocatalytic hydrogen production curves, (b) hydrogen production rate for u-CNB and O-CNS, (c) photocatalytic durability of 65 O-CNS and (d) XRD pattern of 65 O-CNS after cyclic runs.
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Figure 10. Mott-Schottky plots of u-CNB and 65 O-CNS.
Figure 10. Mott-Schottky plots of u-CNB and 65 O-CNS.
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Figure 11. The mechanism of the photocatalytic hydrogen production of u-CNB and O-CNS under visible light irradiation.
Figure 11. The mechanism of the photocatalytic hydrogen production of u-CNB and O-CNS under visible light irradiation.
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Table 1. The specific surface area and pore size distribution value of the samples.
Table 1. The specific surface area and pore size distribution value of the samples.
Samples BET (m2·g-1) Average Pore
Width (nm)		 Pore Volume
(cm3·g-1)
u-CNB 13.9 9.125 0.0567
57.5 O-CNS 33.8 27.256 0.327
60 O-CNS 32.4 34.321 0.465
62.5 O-CNS 34.5 40.539 0.544
65 O-CNS 48.2 43.246 0.583
67.5 O-CNS 31.3 46.784 0.614
70 O-CNS 28.8 49.653 0.638
Table 2. Comparison of hydrogen evolution of element-doped g-C3N4 photocatalysts.
Table 2. Comparison of hydrogen evolution of element-doped g-C3N4 photocatalysts.
Samples Light Source Reactant Solution Hydrogen Evolution
(μmol h-1 g-1)		 Reference
O-doped g-C3N4 nanosheets 300W Xe Lamp (λ > 420 nm) 80 mL water+20 mL TEOA, 1% H2PtCl6 2012.9 This work
Pt/g-C3N4 nanotube 300W Xe Lamp (λ > 420 nm) TEOA aqueous solution (100 mL, 10 vol%) 5304 Ref [33]
S-doped g-C3N4 300W Xe Lamp (λ > 420 nm) 50 mL aqueous TEOA solutions (10 vol%), 2% Pt 161.32 Ref [34]
Oxygen-doped g-C3N4 sheets 300 W Xenon lamp (λ > 420 nm) 100 mL aqueous solution containing 10 vol% TEOA, 3% Pt 2200 Ref [15]
N-Defective and S-Doped g-C3N4 300 W Xe lamp (λ > 420 nm) 10 mL TEOA+ 90 mL deionized water, 3% Pt 5651.5 Ref [35]
Carbon defective g-C3N4 300 W Xe lamp (λ > 420 nm) 100 mL of 10 vol% TEOA aqueous solution, 1% Pt 1534 Ref [36]
K-doped g-C3N4 300 W Xe lamp (λ > 400 nm) 100 mL containing 10 vol % TEOA, 3% Pt 1337.2 Ref [37]
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