In situ one-step hydrothermal synthesis of oxygen-containing groups-modified g-C3N4 for the improved photocatalytic H2-evolution performance

Accepted Manuscript Title: In situ one-step hydrothermal synthesis of oxygen-containing groups-modified g-C3 N4 for the improved photocatalytic H2 -evolution performance Authors: Xinhe Wu, Fengyun Chen, Xuefei Wang, Huogen Yu PII: DOI: Reference:

S0169-4332(17)32374-7 http://dx.doi.org/doi:10.1016/j.apsusc.2017.08.050 APSUSC 36887

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Received date: Revised date: Accepted date:

9-7-2017 5-8-2017 7-8-2017

Please cite this article as: Xinhe Wu, Fengyun Chen, Xuefei Wang, Huogen Yu, In situ one-step hydrothermal synthesis of oxygen-containing groups-modified gC3N4 for the improved photocatalytic H2-evolution performance, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.08.050 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

In situ one-step hydrothermal synthesis of oxygen-containing groups-modified g-C3N4 for the improved photocatalytic H2-evolution performance

Xinhe Wu b, Fengyun Chen b,c, Xuefei Wang b, Huogen Yu a,b*

a

State Key Laboratory of Silicate Materials for Architectures, Wuhan University of

Technology, Wuhan 430070, PR China b

School of Chemistry, Chemical Engineering and Life Sciences, Wuhan University of

Technology, Wuhan 430070, PR China c

Hubei Institute of Quality Supervision and Inspection, Wuhan 430061, PR China

Tel: 86-27-87756662; Fax: 86-27-87879468 *

E-mail: [email protected]

Graphical abstract

Highlights 

An in situ one-step hydrothermal method was developed to prepare OG/g-C3N4.



Hydrothermal treatment greatly increase the specific surface area of bulk g-C3N4.



Hydrothermal treatment induce the formation of -OH and C=O on g-C3N4 surface.



The OG/g-C3N4 shows an obviously improved H2-evolution performance.

Abstract Surface modification of g-C3N4 is one of the most effective strategies to boost its photocatalytic H2-evolution performance via promoting the interfacial catalytic reactions. In this study, an in situ one-step hydrothermal method was developed to prepare the oxygen-containing groups-modified g-C3N4 (OG/g-C3N4) by a facile and green hydrothermal treatment of bulk g-C3N4 in pure water without any additives. It was found that the hydrothermal treatment (180 oC) not only could greatly increase the specific surface area (from 2.3 to 69.8 m2 g-1), but also caused the formation of oxygen-containing groups (-OH and C=O) on the OG/g-C3N4 surface, via the interlayer delamination and intralayer depolymerization of bulk g-C3N4. Photocatalytic experimental results indicated that after hydrothermal treatment, the resultant OG/g-C3N4 samples showed an obviously improved H2-evolution performance. Especially, when the hydrothermal time was 6 h, the resultant OG/g-C3N4(6h) exhibited the highest photocatalytic activity, which was clearly higher than that of the bulk g-C3N4 by a factor of ca. 7. In addition to the higher specific

surface area, the enhanced H2-evolution rate of OG/g-C3N4 photocatalysts can be mainly attributed to the formation of oxygen-containing groups, which possibly works as the effective H2-evolution active sites. Considering the facie and green synthesis method, the present work may provide a new insight for the development of highly efficient photocatalytic materials.

Keywords: g-C3N4; Modification; Hydrothermal treatment; Oxygen functionalities; Increased surface area; Hydrogen evolution photocatalyst

1. Introduction Photocatalytic H2 evolution from water splitting and photocatalytic environmental purification over semiconductor photocatalysts provides a promising way to utilize renewable resources and to resolve the serious energy crisis [1-6]. Since the first discovery of photocatalytic hydrogen evolution via water splitting from TiO2 by Fujishima et al. [7], TiO2 has been widely investigated as a photocatalyst for hydrogen production owing to its excellent photocatalytic activity and stability [8-12]. However, its effective photocatalytic reactions require near-ultraviolet (UV) irradiation (about 4% of the solar spectrum), which severely restrains its practical applications [13, 14]. Therefore, it is quite necessary to develop visible-light-driven photocatalysts for hydrogen production under sunlight irradiation [15]. Recently, various visible-light driven photocatalysts, including multicomponent oxides [16], oxynitrides [17], sulfides [18-21] and polymers [22], have been widely developed for

H2 generation. Among them, graphitic carbon nitride (g-C3N4), has attracted much attentions on account of its low-cost, simple synthesis and good physicochemical properties [23-27]. However, the present reported g-C3N4 photocatalysts usually exhibit a low photocatalytic H2-evolution performance due to its rapid recombination of photogenerated electrons and holes. To overcome this shortcoming, numerous efforts have been developed, including nanostructure modulation (e.g., nanosheet [28], mesoporous structures [29] and hollow structures [30]), loading cocatalysts (e.g., Pd/g-C3N4 [31], Ag/g-C3N4 [32] and NiO/g-C3N4 [33]) and coupling with other semiconductors (e.g., Ag2CO3/g-C3N4 [34], TiO2/g-C3N4 [35, 36] and CdS/g-C3N4 [37]). However, the photocatalytic performance of the g-C3N4 photocatalyst is still very low, which restricts practical applications in a large-scale hydrogen production. Therefore, it is highly required to explore effective methods to enhance the photocatalytic H2-evolution performance of g-C3N4. Generally, g-C3N4 is synthesized by high-temperature calcination methods, which usually results in a heavy agglomeration and a low specific surface area. Therefore, improving the specific surface area of g-C3N4 is one of the significant strategies to enhance its photocatalytic performance. In this case, a second high-temperature calcination of g-C3N4 was widely reported to increase its specific surface area [38]. However, the second high-temperature calcination usually causes a great loss of the final g-C3N4, producing a very low productivity. Recently, it was reported that hydrothermal method was a facile and effective strategy to greatly improve the specific surface area of g-C3N4 [39-42]. For instance, Sano et al. [40]

developed an alkaline hydrothermal treatment method to enhance the specific surface area (up to 65 m2 g-1) of g-C3N4. Wei et al. [41] prepared g-C3N4 with high specific surface area (up to 43.4 m2 g-1) by a hydrothermal treatment in HNO3 solution. In addition, Hao et al. [42] also prepared the g-C3N4 photocatalyst via a deionized-water hydrothermal method to enlarge the specific surface area (up to 142.55 m2 g-1). The above results clearly demonstrated that the resulted g-C3N4 showed a noteworthy improvement of photocatalytic performance and the enhanced photocatalytic mechanism of the g-C3N4 is mainly attributed to the improved specific surface area. In fact, the hydrothermal treatment not only increases the specific surface area of g-C3N4, but also can cause the obvious change of its microstructures including the surface groups, structures and properties. Unfortunately, the surface microstructures of g-C3N4 after hydrothermal treatment have seldom been reported. Considering that the surface microstructures have a significant effect on the H+ adsorption from solution and the following interfacial H2-evolution reaction for water splitting, it is highly required and interesting to investigate the effect of surface microstructures of g-C3N4 on the photocatalytic H2-evolution performance by the facile and green hydrothermal strategy. In this article, a facile and green one-step hydrothermal modification of g-C3N4 was applied not only to improve its specific surface area, but also to induce the in situ formation of oxygen-containing groups-modified g-C3N4 (OG/g-C3N4) photocatalysts. The oxygen-containing groups (-OH, -C=O) can be gradually produced on the g-C3N4 surface via the interlayer delamination and intralayer depolymerization of bulk

g-C3N4 during hydrothermal treatment at 180 oC, and with increasing hydrothermal time more oxygen-containing groups (-OH, -C=O) are formed. The photocatalytic activities of the resultant OG/g-C3N4 are evaluated by photocatalytic hydrogen production in a lactic acid solution under visible-light irradiation. It is interesting to find that the OG/g-C3N4 photocatalysts exhibit a significant enhancement in the photocatalytic hydrogen production compared with untreated g-C3N4. A possible photocatalytic mechanism was proposed to account for the improved photocatalytic H2-evolution performance. This work opens up an appealing and facile strategy for the development of high-efficiency g-C3N4 photocatalytic materials, which may have potential applications for hydrogen production from water splitting.

2. Experimental 2.1. Preparation of g-C3N4 photocatalyst Graphite carbon nitride (g-C3N4) was simply prepared via a thermal polycondensation reaction, as reported by our previous works [43-45]. In detail, 5 g of melamine powder (> 99.9%, Alfa Aesar) was placed in a clean alumina crucible with a cover and then calcined in a muffle furnace at 550 oC for 4 h with a ramping rate of 5 oC min-1 in air atmosphere. After cooling down to room temperature, the resultant yellow bulk was ground into fine powder. 2.2. Preparation of oxygen-containing groups-modified g-C3N4 photocatalyst The oxygen-containing groups-modified g-C3N4 (OG/g-C3N4) photocatalysts were fabricated via one-step hydrothermal method, as shown in Fig. 1. Concretely, 0.5

g of g-C3N4 powder was dispersed into 70 mL of deionized water and stirred at room temperature for 2 h. The obtained suspension was then transferred into a 100 mL of Teflon-sealed autoclave and maintained at 180 oC for a certain time. After cooling down to room temperature, the products were centrifuged, washed with deionized water and ethanol, and then dried at 60 oC for 12 h. To investigate the effect of hydrothermal time on the microstructures and photocatalytic performance of g-C3N4, the hydrothermal time is controlled to be 1, 2, 4, 6, 12 and 24 h, respectively, and the resultant OG/g-C3N4 can be denoted as OG/g-C3N4(Xh), where the X refers to the hydrothermal time. 2.3. Characterization X-ray diffraction (XRD) patterns were done on a D/MX-IIIA X-ray diffractometer (Rigaku, Japan). The morphology observation was performed on a JEM-7500F field emission scanning electronic microscopy (FESEM, Hitachi, Japan) and JEM-2100F transmission electron microscopy (TEM, JEOL, Japan). UV-visible absorption spectra were obtained using a UV-vis spectrophotometer (UV-2450, SHIMADZU, Japan). BaSO4 was used as a reflectance standard in a UV-vis diffuse reflectance experiment. Nitrogen adsorption-desorption isotherms were analyzed by a Micromeritics (ASAP 2020, USA) nitrogen adsorption apparatus. X-ray photoelectron spectroscopy (XPS) measurements were performed on a KRATOA XSAM800 XPS system with Mg K source. Fourier transform infrared (FTIR) spectra were acquired using a Nexus FT-IR spectrophotometer (Thermo Nicolet, America). The thermogravimetry (TG-DSC) is characterized by the STA-449F3 instrument (Netzsch,

Germany). Electrochemical impedance spectroscopy was performed on an electrochemical workstation (CHI660E).

2.4. Photocatalytic H2-production activity The photocatalytic hydrogen-production experiments were conducted in a 100 mL three-necked Pyrex flask at ambient temperature and atmospheric pressure, and the outlets of the flask were sealed with a silicone rubber septum. Four 420-nm LED lamps with a light intensity of 90 mW cm-2 were used as the visible-light sources, which was measured by a FZ-A visible-light radiometer (made in photoelectric instrument factory of Beijing Normal University, China). In a typical photocatalytic experiment, 50 mg of the prepared g-C3N4 photocatalyst was dispersed in a 100 mL of flat three-necked flask equipped with 80 mL of lactic acid solution (10 vol%) by stirring and sonication, and the system was then bubbled with nitrogen for 30 min to remove the dissolved oxygen. In the process of irradiation, continuous stirring was applied to keep the photocatalyst particles in suspension state. Gas (0.4 mL) was intermittently sampled through a septum, and hydrogen was analyzed by a gas chromatograph (Shimadzu GC-2014C, Japan, with nitrogen as a carrier gas) equipped with a 5 Å molecular sieve column and a thermal conductivity detector.

3. Results and discussion 3.1. The synthetic strategy of the OG/g-C3N4 photocatalysts The OG/g-C3N4 photocatalysts can be easily prepared via the hydrothermal treatment of bulk g-C3N4 at 180 oC in a pure water system, and the formation

mechanism is well illustrated in Fig. 1. First, bulk g-C3N4 was prepared through a high-temperature calcination method by using melamine as the precursor. Then, the bulk g-C3N4 can transform into the OG/g-C3N4 photocatalysts via an interlayer delamination and intralayer depolymerization progresses (Fig. 1A). During initial hydrothermal treatment, the H2O molecules can be incorporated into the interlayers of layered g-C3N4, causing the gradual delamination of bulk g-C3N4 and the formation of multilayer or few-layer g-C3N4 nanosheets (Fig. 1A). With increasing hydrothermal time, the multilayer or few-layer g-C3N4 can further be destroyed via the intralayer depolymerization, resulting in the formation of small and thin g-C3N4 nanosheets. It should be noted that compared with the bulk g-C3N4, the resultant g-C3N4 nanosheets show an obviously different surface microstructures (Fig. 1B). In addition to the terminal amino (-NH2), many oxygen-containing groups such as –OH and -C=O can be produced on the surface of g-C3N4 nanosheets (Fig. 1B), and their formation mechanism can be well explained in Fig. 1C. For the g-C3N4 nanosheet, the tri-s-triazine units (N-(C3H3N3)3) is the basis structure. During hydrothermal treatment, the H2O molecules can attack the N-(C3H3N3)3 in the g-C3N4 nanosheet to produce the (C3H3N3)3-OH, (C3H3N3)3=O and (C3H3N3)3-NH2. In fact, the (C3H3N3)3-NH2 can further be transformed into (C3H3N3)3-OH and (C3H3N3)3=O (Fig. 1C), resulting in the great production of oxygen-containing groups on the g-C3N4 nanosheets and the formation of OG/g-C3N4 photocatalysts (Fig. 1). In fact, the facile hydrothermal treatment of bulk g-C3N4 in pure water not only causes the great increase of specific surface area, but also induces the formation of

oxygen-containing groups such as –OH and -C=O on the g-C3N4 surface, which can be well demonstrated by the following various characterizations. In addition, it was found that with increasing hydrothermal time, the final amounts of OG/g-C3N4 photocatalysts were gradually decreased. When the hydrothermal time was increased to 48 h, no solid OG/g-C3N4 photocatalyst could be found after hydrothermal treatment and only a very clear solution was found. In this case, when the Nessler’s regent was added into the above clear solution, some yellow precipitate was formed immediately, clearly demonstrating the formation of NH3 or NH4+ by the hydrothermal treatment of bulk g-C3N4 (Fig. 1C). The above results further suggest that the bulk g-C3N4 can be gradually and effectively delaminated and depolymerized during hydrothermal treatment. Therefore, the present OG/g-C3N4 photocatalysts can be easily and well produced only by controlling the hydrothermal time.

3.2. Morphology and microstructures of the OG/g-C3N4 photocatalysts The excellent delamination and depolymerization of bulk g-C3N4 during hydrothermal treatment can be well demonstrated by the XRD, FESEM, TEM and nitrogen adsorption-desorption isotherm. Fig. 2 shows the XRD patterns of g-C3N4 before and after different hydrothermal time. It is clear that the pure g-C3N4 exhibits obvious diffraction peaks at 27.4o and 13.1o, which can be well attributed to the interlayer (002) and intralayer (100) planes, respectively [46]. With increasing hydrothermal time, the (002) diffraction peak of OG/g-C3N4 shifts gradually from 27.4o to 27.6o, indicating the obvious shrink of interlayer distance from 0.325 to 0.323

nm owing to the introduction of O atom with a strong higher electronegativity than the N atom [47]. On the contrary, the (100) diffraction peak shifts from 13.1o to 12.2o and then decreases to 10.6o, which is attributed to the partial destroy of the periodic tri-s-triazine units of the g-C3N4 via the introduction of oxygen-containing groups (shown below). In fact, for the OG/g-C3N4(24h) sample, the new and stronger diffraction peaks at 10.6o is in good agreement with the reported melem structures [48]. Considering the excellent delamination and depolymerization of g-C3N4, the melem structures with oxygen-containing groups on the g-C3N4 surface can be easily produced during hydrothermal reaction (Fig. 1). In fact, with further increasing hydrothermal time to 48 h, the melem structures can further be dissociated, resulting in the complete destroy of the g-C3N4 structures and no suspension can be found (observation in our experiment). The FESEM, TEM images and nitrogen adsorption-desorption isotherm can further demonstrate the effective delamination and depolymerization of bulk g-C3N4, as shown in Fig. 3 and Fig. 4, respectively. For the pure g-C3N4, only bulk g-C3N4 particles with a size of several micrometers are observed (Fig. 3A and B) due to a high-temperature preparation progress, similar to the widely reported results [44, 45]. The corresponding nitrogen adsorption-desorption isotherm of pure g-C3N4 (Fig. 4a and Table 1) shows a very low specific surface area (2.3 m2 g-1) and pore volume (0.01 cm3 g-1). With increasing hydrothermal time, the resultant OG/g-C3N4 clearly shows a smaller size (ca. 100-300 nm) and sheet-like morphology (Fig. 3C-F). Based on the Fig. 4 and Table 1, the specific surface area of OG/g-C3N4 has a great

improvement after hydrothermal treatment. Specially, after hydrothermal reaction for 24 h, the resultant OG/g-C3N4 shows a specific surface area of 69.8 m2 g-1 and pore volume of 0.18 cm3 g-1. According to their pore size distribution curve (the inset in Fig. 4), the pore structure in the resultant OG/g-C3N4 samples is mainly in the range of 2-100 nm, which is in favor of the rapid diffusion and transfer of the reactants and products during photocatalytic reactions. Therefore, the effective delamination and depolymerization of bulk g-C3N4 after hydrothermal treatment cause the formation of OG/g-C3N4 nanosheets with a larger specific surface area and pore volume. To demonstrate the formation of oxygen-containing groups on the OG/g-C3N4, the FTIR, XPS and TG-DSC have been confirmed. Fig. 5A shows the FTIR spectra of bulk g-C3N4 and OG/g-C3N4 samples. For pure g-C3N4, several peaks in the range of 1200-1650 cm-1 (the peaks at 1241, 1319, 1409, 1460 and 1569 cm-1) can be assigned to the typical modes of CN heterocycles [49]. Additionally, the band at 810 cm-1 is assigned to the characteristic breathing mode of tri-s-triazine units [40], while the broad band in the range of 3000-3500 cm-1 is mainly ascribed to the N-H stretching vibration modes [50]. After hydrothermal treatment, the main absorption peaks of OG/g-C3N4 are consistent with that of pure g-C3N4, indicating that the main structure (tri-s-triazine units) of g-C3N4 is well maintained. Further observation indicates that when the hydrothermal time increases to 12 or 24 h, the resultant OG/g-C3N4 clearly shows several strong and broad absorption peaks of terminal amino group (-NH2) in the range of 2700-3550 cm-1 [51, 52], indicating that the amount of -NH2 has an obvious increase. In addition, a new characteristic peak appearing at 3390 cm-1 is

formed, which can be attributed to the stretching vibration of –OH (Fig. 1) [53]. To further observe the formation of oxygen-containing groups, their characteristic peaks in the region of 400-2000 cm-1 are shown in Fig. 5B. It is found that when the hydrothermal time is less than 6 h, the resultant samples show no evident variation in the FTIR spectra due to the limited amount of various groups (-NH2, C=O, and -OH). However, for the OG/g-C3N4(12h) and OG/g-C3N4(24h), several strong and new absorption peaks can be clearly observed. In addition to the -NH2 group at 1663 cm-1 [54], the strong absorption peaks at 1735 and 1781 cm-1 can correspond to the symmetrical and asymmetric stretching vibrations of C=O group, respectively [55], and the absorption peak at 1408 cm-1 can be ascribed to the stretching vibrations of C-OH [56]. Therefore, it is very clear that the oxygen-containing groups such as -OH and -C=O have been in situ produced on the OG/g-C3N4 surface by hydrothermal treatment. XPS, a surface analytical technique, is further employed to demonstrate the formation of oxygen-containing groups on the g-C3N4 photocatalyst. Fig. 6A shows the typical XPS survey spectra for g-C3N4 before and after hydrothermal treatment. It is found that both of the samples are mainly composed of carbon, nitrogen, and a small amount of oxygen. For the g-C3N4, the oxygen element is mainly from the adsorbed H2O in air, while that of the OG/g-C3N4 mainly come from the oxygen-containing groups (-OH and C=O). According to the XPS results (Table S1), the amounts of oxygen element in the g-C3N4, OG/g-C3N4(6h) and OG/g-C3N4(12h) photocatalysts are 1.6, 3.8 and 8.5 at%, respectively. To further exhibit the chemical

states of the oxygen element, Fig. 6B shows its corresponding high-resolution spectra of g-C3N4 samples. Compared with bulk g-C3N4, in addition to the C-OH group (532.4 eV) [57], a new peak at binding energy of 531.3 eV was formed, which can be attributed to the C=O group on OG/g-C3N4 surface [58, 59]. Thermogravimetric analyzer (TG-DSC) is used to analyze the stability of OG/g-C3N4 after the addition of oxygen-containing groups and the results are shown in Fig. 7. In the range of 35-500 oC, only a very small amount of mass loss is found for pure g-C3N4, while an obviously increased mass loss (ca. 2.36%) can be found for the OG/g-C3N4 due to the presence of oxygen-containing groups (-OH and –C=O). When the temperature is higher than 600 oC, the g-C3N4 can be completely decomposed into intermediates such as C2N2 and NH3 [60]. In addition, it is found that the temperature (738.6 oC) for the maximum endothermic peak of OG/g-C3N4(6h) is slightly lower than that (746.8 oC) of pure g-C3N4, which can be due to the fact that more defects causing by oxygen-containing groups are formed in the C-N heterocyclic structure of g-C3N4, resulting in a weaker thermal stability. The light-absorption property of g-C3N4 before and after hydrothermal treatment was characterized by UV-vis spectra, as shown in Fig. 8. The absorption edge of pure g-C3N4 is ca. 460 nm, corresponding to a band gap of 2.70 eV. With increasing hydrothermal time, the OG/g-C3N4 photocatalysts show a slight blue-shift phenomenon owing to the formation of oxygen-containing groups, in good agreement with the oxidation transformation of graphite to graphene oxide [56]. The corresponding photographs of various samples are shown in the inset of Fig. 8. It is

clear that the yellow color of g-C3N4 gradually become light brown, in good agreement with their UV-vis spectra.

3.3. Photocatalytic performance and mechanism of OG/g-C3N4 photocatalysts Photocatalytic H2-production performances of OG/g-C3N4 photocatalysts were evaluated under visible light irradiation using 10 vol% of lactic acid aqueous solutions. Pt (1 wt%) was used as a cocatalyst to reduce the overpotential for hydrogen evolution, and the results are shown in Fig. 9 and Table 1. It can be seen that the pure g-C3N4 exhibits an obvious H2 production activity (5.4 mol h-1). With increasing hydrothermal time, the H2-evolution rate of OG/g-C3N4 has an significant increase, and the OG/g-C3N4(6h) exhibits the highest photocatalytic performance with a H2-evolution rate of 37.6 mol h-1, which is about 7 times higher than the naked g-C3N4. Further increasing hydrothermal time would cause the decreased photocatalytic H2-evolution activity of OG/g-C3N4. In fact, in addition to the hydrothermal time, the hydrothermal temperature also has an important effect on the photocatalytic performance of OG/g-C3N4 photocatalysts. It was found that with increasing hydrothermal time from 110 to 180 oC, the resulting OG/g-C3N4 photocatalysts exhibit a gradually increased photocatalytic H2-evolution performance (Fig. S1). To further investigate the stability of photocatalytic performance, a recycling test of the OG/g-C3N4(6h) was performed and the corresponding results were displayed in Fig. 9B. It is found that the OG/g-C3N4 sample has a good repeating H2-evolution performance.

The above results clearly suggest that the photocatalytic H2-evolution performance of bulk g-C3N4 can be greatly improved by the present hydrothermal treatment technology. Considering the facile and green hydrothermal strategy, it is very essential to further consider the potential photocatalytic mechanism of the resultant OG/g-C3N4 photocatalyst. Apparently, according to the N2 adsorption-desorption isotherms (Fig. 4 and Table 1), the improved specific surface areas of OG/g-C3N4 photocatalysts are one of the most important factors for the high photocatalytic H2-evolution performance. However, when the hydrothermal time is increased from 6 to 24 h, the resultant OG/g-C3N4 shows a gradual increased specific surface area, while the corresponding H2-evolution performance exhibits a decreased activity. Therefore, it is very clear that there are some other important factors that controlling the final H2-evolution activity. According to the FTIR, XPS, TG-DSC and UV-vis spectra results, it is believed that the oxygen-containing groups in the OG/g-C3N4 should be another important factor for the significantly improved photocatalytic performance and a possible photocatalytic mechanism of OG/g-C3N4 is proposed in Fig. 10. On one hand, the oxygen-containing groups such as-OH and C=O on the g-C3N4 surface can not only boost the dispersion and hydrophilicity of g-C3N4 in the aqueous solution, but also play a significant role in improving the separation efficiency of photogenerated electrons and holes [61], which are in favor of the following photocatalytic oxidation (lactic acid oxidation) and reduction (H+ reduction) reactions. On the other hand, the oxygen in the oxygen-containing groups can effectively adsorb the H+ from aqueous solution and work as the interfacial active

sites for the H2-evolution reaction. The corresponding electrochemical impedance spectroscopy spectra (EIS) reveals that the OG/g-C3N4(6h) exhibits the smallest arc radius (Fig. S2), clearly suggesting that it has the lowest interfacial charge transfer resistance and the best charge-separation efficiency. Therefore, the OG/g-C3N4 clearly exhibits an improved photocatalytic performance compared with the naked g-C3N4. To further demonstrate the significant role of oxygen-containing groups for the high H2-evolution activity of g-C3N4, the above OG/g-C3N4(6h) was calcined at 450 o

C for 4 h with the aim of removing the oxygen-containing groups. The resultant

sample is referred to be OG/g-C3N4(450oC), and its XRD and FTIR results are shown in Fig. 11. It is found that the prepared OG/g-C3N4(450oC) show a similar XRD and FTIR spectra as the naked g-C3N4 in Fig. 2 and 5, clearly suggesting the successful removal of oxygen-containing groups. The photocatalytic experimental result (Fig. 9A) indicates that after the removal of oxygen-containing groups, the OG/g-C3N4(450oC) exhibits a remarkably lower H2-evolution performance (6.0 mol h-1) than the OG/g-C3N4(6h), although they show comparable specific surface areas (43.5 m2 g-1, Table 1). Therefore, the above results strongly demonstrate that the greatly improved H2-evolution activity of OG/g-C3N4 can be mainly attributed to the formation of oxygen-containing groups during hydrothermal treatment.

4. Conclusions In summary, a facile hydrothermal method has been developed to prepare the highly efficient g-C3N4 photocatalysts. It is found that the hydrothermal treatment

(180 oC) not only can greatly increase the specific surface area (from 2.3 to 69.8 m2 g-1), but also causes the formation of oxygen-containing groups (-OH and C=O) on the OG/g-C3N4 surface, via the interlayer delamination and intralayer depolymerization of bulk g-C3N4. All the resultant OG/g-C3N4 photocatalysts clearly show an improved photocatalytic H2-evolution activity, and the OG/g-C3N4(6h) exhibits the highest photocatalytic performance, which is clearly higher than the bulk g-C3N4 by a factor of 7. The enhanced H2-evolution rate of OG/g-C3N4 photocatalysts can be mainly attributed to the formation of oxygen-containing groups (-OH and C=O), which can effectively adsorb the H+ from aqueous solution and work as the interfacial active sites for the H2-evolution reaction.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (51472192, 21477094, and 51672203). This work was also financially supported by the Fundamental Research Funds for the Central Universities (WUT 2017IB002).

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Figure captions Fig. 1. (A) Graphical illustration for the synthesis of OG/g-C3N4 photocatalysts by a hydrothermal method; step 1 representing an interlayer delamination progress; step 2 representing an intralayer depolymerization progress; (B) the surface microstructures of resultant OG/g-C3N4 photocatalyst; (C) the formation mechanism of oxygen-containing groups during hydrothermal reaction. Fig. 2. XRD patterns of various samples: (a) g-C3N4, (b) OG/g-C3N4(2h), (c) OG/g-C3N4(6h), (d) OG/g-C3N4(12h) and (e) OG/g-C3N4(24h). Fig. 3. FESEM images of (A) g-C3N4, (C) OG/g-C3N4(6h), and (D) OG/g-C3N4(12h); TEM images of (B) g-C3N4 and (E,F) OG/g-C3N4(12h). Fig. 4. Nitrogen adsorption-desorption isotherms and their corresponding pore size distributions (inset) for (a) g-C3N4 and (b) OG/g-C3N4(6h). Fig. 5. (A) FTIR spectra and (B) their corresponding high-magnification spectra in the region of 400-2000 cm-1 for various samples: (a) g-C3N4, (b) OG/g-C3N4(2h), (c) OG/g-C3N4(6h), (d) OG/g-C3N4(12h) and (e) OG/g-C3N4(24h). Fig. 6. (A) XPS survey spectra and (B) O 1s high-resolution XPS spectra of (a) g-C3N4 and (b) OG/g-C3N4(12h). Fig. 7. TG and DSC curves for the (a) g-C3N4 and (b) OG/g-C3N4(6h). Fig. 8. UV-vis diffused reflectance spectra and their corresponding photographs of various samples: (a) g-C3N4, (b) OG/g-C3N4(2h), (c) OG/g-C3N4(6h) and (d) OG/g-C3N4(12h). Fig. 9. (A) The photocatalytic H2-production activities of various samples: (a) g-C3N4,

(b) OG/g-C3N4(1h), (c) OG/g-C3N4(2h), (d) OG/g-C3N4(4h), (e) OG/g-C3N4(6h), (f) OG/g-C3N4(12h), (g) OG/g-C3N4(24h) and (h) OG/g-C3N4(450oC); (B) Photocatalytic cycling test of typical OG/g-C3N4(6h) photocatalyst. Fig. 10. Photocatalytic mechanism of OG/g-C3N4 photocatalyst: (A) oxygen-containing groups on the OG/g-C3N4 surface adsorb hydrogen ions from solution; (B) the oxygen-containing groups work as the active sites to promote H2 evolution. Fig. 11. (A) XRD patterns and (B) FTIR spectra of the OG/g-C3N4(6h) photocatalyst before (a) and after (b) calcination at 450oC.

Fig. 1. (A) Graphical illustration for the synthesis of OG/g-C3N4 photocatalysts by a hydrothermal method; step 1 representing an interlayer delamination progress; step 2 representing an intralayer depolymerization progress; (B) the surface microstructures of resultant OG/g-C3N4 photocatalyst; (C) the formation mechanism of oxygen-containing groups during hydrothermal reaction.

d c

14

002

b 100

Relative intensity (a.u.)

e

21 28 2 Theta (degree)

a 35

Fig. 2. XRD patterns of various samples: (a) g-C3N4, (b) OG/g-C3N4(2h), (c) OG/g-C3N4(6h), (d) OG/g-C3N4(12h) and (e) OG/g-C3N4(24h).

Fig. 3. FESEM images of (A) g-C3N4, (C) OG/g-C3N4(6h), and (D) OG/g-C3N4(12h); TEM images of (B) g-C3N4 and (E,F) OG/g-C3N4(12h).

-1 Quantity adsorbed (cm3g )

125

75

Pore volume

100

a b

50

1

10

100

Pore diameter (nm)

25 0 0.0

0.2 0.4 0.6 0.8 Relative pressure (p/p0)

1.0

Fig. 4. Nitrogen adsorption-desorption isotherms and their corresponding pore size distributions (inset) for (a) g-C3N4 and (b) OG/g-C3N4(6h).

Relative intensity (a.u.)

A e d c b a

810

e d c b a

1408

Relative intensity (a.u.)

B

1781 1735 1631

3600 3000 2400 1800 1200 600 Wavenumber (cm-1)

1800 1500 1200 900 600 Wavenumber (cm-1) Fig. 5. (A) FTIR spectra and (B) their corresponding high-magnification spectra in the region of 400-2000 cm-1 for various samples: (a) g-C3N4, (b) OG/g-C3N4(2h), (c) OG/g-C3N4(6h), (d) OG/g-C3N4(12h) and (e) OG/g-C3N4(24h).

Relative intensity (a.u.) 528

O 1s

b

a

0

B

N 1s

C 1s

Relative intensity (a.u.)

A

200 400 600 Binding energy (eV)

531.3 C=O

532.4 C-OH

O 1s

b

a 531

534

537

Binding energy (eV)

Fig. 6. (A) XPS survey spectra and (B) O 1s high-resolution XPS spectra of (a) g-C3N4 and (b) OG/g-C3N4(12h).

-2.36%

100 -89.79%

DSC (mW/mg)

4.5 3.0 1.5 0.0 -1.5

80 60 40

a b

-3.0

o

738.6 C

20

o

746.8 C

150

TG (%)

6.0

300 450 600 o Temperature ( C)

0 750

Fig. 7. TG and DSC curves for the (a) g-C3N4 and (b) OG/g-C3N4(6h).

Fig. 8. UV-vis diffused reflectance spectra and their corresponding photographs of various samples: (a) g-C3N4, (b) OG/g-C3N4(2h), (c) OG/g-C3N4(6h) and (d) OG/g-C3N4(12h).

Rate of H2 evolution (mol h-1 )

A

40 30 20 10 0 a

b

H2 production (mol)

B 150

c

bubble

d

e

bubble

f

g

h

bubble

120 90 60 30 0

0

2

4 6 8 10 Irradiation time (h)

12

Fig. 9. (A) The photocatalytic H2-production activities of various samples: (a) g-C3N4, (b) OG/g-C3N4(1h), (c) OG/g-C3N4(2h), (d) OG/g-C3N4(4h), (e) OG/g-C3N4(6h), (f) OG/g-C3N4(12h), (g) OG/g-C3N4(24h) and (h) OG/g-C3N4(450oC); (B) Photocatalytic cycling test of typical OG/g-C3N4(6h) photocatalyst.

Fig. 10. Photocatalytic mechanism of OG/g-C3N4 photocatalyst: (A) oxygen-containing groups on the OG/g-C3N4 surface adsorb hydrogen ions from solution; (B) the oxygen-containing groups work as the active sites to promote H2 evolution.

Relative intensity (a.u.)

A b

a

14

21 28 2 Theta (degree)

35

Relative intensity (a.u.)

B b a

3500

2800 2100 1400 Wavenumber (cm-1)

700

Fig. 11. (A) XRD patterns and (B) FTIR spectra of the OG/g-C3N4(6h) photocatalyst before (a) and after (b) calcination at 450oC.

Table 1 Effects of hydrothermal time on the physical properties and photocatalytic H2-evolution performance of the g-C3N4.

Sample

SBET (m2 g-1)

Pore volume (cm3 g-1)

Average pore size (nm)

Activity (mol h-1 )

g-C3N4

2.3

0.01

21.2

5.4

OG/g-C3N4(2h)

14.7

0.04

15.8

5.9

OG/g-C3N4(4h)

16.9

0.05

13.6

17.3

OG/g-C3N4(6h)

47.2

0.13

13.2

37.6

OG/g-C3N4(12h)

52.4

0.14

8.2

29.7

OG/g-C3N4(24h)

69.8

0.18

8.0

11.6

OG/g-C3N4(450oC)

43.5

0.13

15.8

6.0