Tailoring of crystalline structure of carbon nitride for superior photocatalytic hydrogen evolution

Journal of Colloid and Interface Science 556 (2019) 324–334

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Regular Article

Tailoring of crystalline structure of carbon nitride for superior photocatalytic hydrogen evolution Shuquan Huang a, Yuanguo Xu a,⇑, Feiyue Ge a, Dong Tian c, Xingwang Zhu b, Meng Xie b, Hui Xu a, Huaming Li a,⇑ a b c

School of Chemistry and Chemical Engineering, Institute for Energy Research, Jiangsu University, Zhenjiang 212013, PR China School of Pharmacy, Jiangsu University, Zhenjiang 212013, PR China State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming 650093, PR China

g r a p h i c a l a b s t r a c t A long-range atomic order carbon nitride layer plane is more favorable for the diffusion of electrons and holes to surface sites for photocatalytic reactions.

a r t i c l e

i n f o

Article history: Received 7 June 2019 Revised 31 July 2019 Accepted 18 August 2019 Available online 19 August 2019 Keywords: Porous g-C3N4 Crystalline structure Hydrogen evolution Photocatalytic

a b s t r a c t Light absorption and carrier transfer, are two sequential and complementary steps related to photocatalysis performance, whereas the collective integration of these two aspects into graphitic carbon nitride (gC3N4) photocatalyst through polycondensation optimization have seldom been achieved. Herein, we report on tailoring the crystalline structure of g-C3N4 by avoiding the formation of incompletely reacted N-rich intermediates and selective breaking the hydrogen bonds between the layers of g-C3N4 simultaneously. The obtained layer plane ordered porous carbon nitride (LOP-CN) material shows efficient photocatalytic H2 generation performance. The highest H2 evolution rate achieved is 53.8 lmol under k  400 nm light irradiation, which is 7.4 times higher than that of g-C3N4 prepared by convention thermal polycondensation. The substantially boosted photocatalytic activity is mainly ascribed to the efficient charge separation on long-range atomic order layer plane and the extended visible light harvesting ability. This work highlights the importance of crystalline structure tailoring in improving charge separation and light absorption of g-C3N4 photocatalyst for boosting its photocatalytic H2 evolution activity. Ó 2019 Elsevier Inc. All rights reserved.

1. Introduction ⇑ Corresponding authors. E-mail addresses: [email protected] (Y. Xu), [email protected] (H. Li). https://doi.org/10.1016/j.jcis.2019.08.069 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.

Using the solar energy as renewable-energy resource to address global challenges related to energy and environment issues is

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attractive. Photocatalysis water splitting provides a promising path for using of the sustainable solar energy [1,2]. To date, a large library of semiconductor photocatalysts have been elaborately developed for converting solar energy into hydrogen, which include sulfides, oxides, oxynitrides and some organic semiconductors [3]. Although some of the aforementioned photocatalysts displayed exceptional activities and stabilities, the containing of non-earth abundant elements or toxic elements hindered their large-scale photocatalytic applications [4]. It is desirable to develop inexpensive, highly efficient and durable semiconductor photocatalysts. Graphitic carbon nitride (g-C3N4), a new class of melt free organic semiconductor, is easy to obtain and does not require complicated modifications to tune the physical or chemical properties, has frequently considered as a promising material for energy and environmental photocatalysis reactions recently [5]. Specially, the term ‘‘g-C3N4” used here is only to keep consistency with the previous scientific literatures. In fact, the materials obtained through traditional thermal-induced pyrolysis of carbon- and nitrogen-containing N-rich molecules (e.g. cyanamide, dicyandiamide, melamine, urea and so on) above 525 °C is not ‘g-C3N4’ but a polymeric carbon nitride (PCN) with a zigzag manner which has been known as Liebig’s melon [6]. Melon is a NH-bridged tri-striazine 1D polymer, the hydrogen-bond between these polymer strands make them present a quasi 2D arrays like structure (Fig. 1a) [7]. However, due to the kinetic problems during the polycondensation of these N-rich molecules, complete condensation of the melon is still a challenge. The incomplete condensation of the melon primarily results in the existence of some incompletely reacted intermediates which might be obstacles for the diffusing of electrons across the plane [8]. As a result, the photo-generated electrons enabled for the photocatalytic reaction are limited and

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a moderate activity is exhibited by the pristine g-C3N4. In order to extract the electrons as many as possible, past attempts focused on the extrinsic physical struct alteration to minimizes the distance for photo-generated carriers, which mainly contained 2D gC3N4 manufacture [9–12] and nano-g-C3N4 construction [13–15]. Whilst these aforementioned methods have always progressed to more efficient g-C3N4-based systems, most of these low dimension structure materials are limited in light response due to the blue shift caused by quantum confinement effect [16,17]. There is a pressing need to develop unique structured g-C3N4 with broad light absorption and superior charge transfer ability for scalable photocatalytic activity. In general, the long-range atomic order structure is typically considered as a key fact to improve the charge diffusion and separation efficiency [18,19]. By contrast, a disordered structure (such as amorphous and porous semiconductor) may have many defects might hinder the diffusion of photoexcited charge carriers due to the abundant recombination and trapping centers. However, disorder structures have advantages of extending light absorption [20]. Considering that light harvesting is another indispensable element for achieving high solar energy conversion efficiency, the exploitation of long-range atomic order structural carbon nitride in layer planes but porous between layers may be a promising strategy to collectively resolve the charge diffusion length as well as to expand light absorption. For example, the successful attempts in TiO2 porous single-crystals [21], single unit cell Bi2WO6 nanosheet [22] and PDINH supramolecular [23] have achieved high visible light activity. Moreover, the orderdisorder interfaces in g-C3N4 reported by Xie’ group had verified the superiority in hot-carrier generation recently [24]. This calls for future efforts on constructing a plane order porous g-C3N4 for highly efficient photocatalysts.

Fig. 1. (a) Top view of the layer atomic structure with completely reacted melon. (b) Top view of the layer atomic structure with incompletely reacted intermediates. C, N and H atoms were denoted by gray, blue and white balls, respectively. (c) and (d) show the calculated electrostatic potentials along the x and y direction of layer with completely reacted melon, respectively. (e) and (f) present the calculated electrostatic potentials along the x and y direction of layer with incompletely reacted intermediates (Fig. S1). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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As aforementioned, the g-C3N4 material is thermal-induced condensation from some N-rich carbon-molecules, and some incompletely reacted intermediates (including amino and cyanogroups [25]) will be formed, which result in hydrogen bonds presented not only in the planes but also between the layers [26]. On the basis of the above elucidation, a high-quality layer plane carbon nitride could be obtained if the incomplete reaction pathways can be avoided in the condensation process [19]. Fig. 1c–f present the electrostatic potentials along the basal plane across the hydrogen bonds-located regions (Fig. S1). The calculated potential barriers across the x and y direction of layer with completely reacted melon are 0.83 and 1.58 eV, respectively. For the layer with incompletely reacted intermediates, the potential barriers will increase to 1.18 eV (x-axis) and 1.66 eV (y-axis), which are much larger than that of layer with completely reacted melon. Suggesting that the transports of charge carriers in the plane of layer with completely reacted melon are more efficient. In addition, the removal of intermediate hydrogen bonds could selectively weak the interaction between the layers and resulting in a porous structure. According to the calculation results of free energy profiles from dicyandiamide to melem in different synthesis atmospheres of Jun et al. [27], the deamination steps were thermodynamically favorable in O2 atmosphere. Motivated by this research result, we proposed here for the first time the pure O2 assist concept into the tailoring of crystalline structure of carbon nitride for superior photocatalytic hydrogen evolution. Our study demonstrated that the thermal-induced condensation of melem (Fig. S2) [28] at pure O2 atmosphere could lead to a layer plane order porous structured carbon nitride material. The obtained layer plane order porous carbon nitride materials exhibited much higher photocatalytic H2 generation activity compared to the conventional bulk carbon nitride, which is about 16 times higher at k  420 nm. The improved photocatalytic activities could be assigned to the increased charge carrier transfer efficiency and photo-absorption. 2. Materials and methods 2.1. Chemicals and reagent Melamine (C3H6N6, 99%) and chloroplatinic acid (H2PtCl6
6H2O, 99%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China) and used without further purification. Deionized (DI) water were used in the whole experiment. 2.2. Synthesize of pure bulk g-C3N4 6.0 g of melamine was encased in a corundum crucible with a cover. Then the corundum crucible was heated to 550 °C for 6 h under air condition in a muffle furnace. The heating rate was 5 °C/min. The obtained sample was named as G-CN. 2.3. Synthesize of melem 6.0 g of melamine was heated to 425 °C for 6 h at a rate of 5 °C/ min under Ar condition in quartz tube muffle furnace. Subsequently, the obtained white precursor was ground to powder and stored in a dryer bottle. 2.4. Synthesize of layer plane order porous g-C3N4 0.4 g of the as-prepared melem powder was laid on a cubic ceramic crucible (21 * 50 mm). And then, the cubic ceramic crucible was heated to 500, 530 and 560 °C for 4 h at a rate of 5 °C/min under pure dry oxygen condition in quartz tube muffle

furnace. The obtained samples were named as 500 LOP-CN, 530 LOP-CN, and 560 LOP-CN, respectively. 2.5. Characterization Bruker D8 diffractometer (Cu-Ka as the radiation, k = 1.5418 Å) was used to create the X-ray diffraction (XRD) pattern, on which the operational program was 2h = 10–60° (5°/min). Fourier transformed infrared (FT-IR) spectra of the samples were produced in a 1 cm2 KBr flake on the Thermo Scientific Nicolet iS-50 instrument. A Thermo Fisher DXR with a 532 nm Nd:YAG excitation source (Thermo Fisher Technology (China) Co., Ltd.) instrument was used to draw the Raman spectra. Atomic force microscope (AFM) images and height profiles were acquired through a Nanosurf Flex-Axiom microscope. Scanning electron microscope (SEM) and Transmission electron microscope (TEM) images of the samples were obtained on field emission microscope (JEOL JWSM7800F) and transmission electron microscope (JEOL JEM-2100F), respectively. The specific surface area and pore size distribution of the samples were calculated from the N2-sorption isotherms collected on a Tristar 3020 (Mike Murray (Shanghai) Instrument Co., Ltd.) high-performance micropore analyzer via brunaueremmett-teller (BET) method. X-ray photoelectron spectroscopy (XPS) spectra were collected by using the MKII X-ray photoelectron spectrometer on Mg Ka radiation. All the obtained XPS spectra have been calibrated to the C 1s peak at 284.6 eV. UV–visible diffuse reflectance spectrum (DRS) were recorded on a UV3600Plus UV–vis spectrophotometer (Shimadzu Corporation, Japan), in which the samples were compacted on the surface of a BaSO4 tablet. Static and time-resolved decay fluorescence spectra were obtained on a Varian Cary Eclipse spectrometer. 2.6. Photocatalytic reactions The photocatalytic hydrogen evolution reactions were performed in a top-irradiation Pyrex vessel which was connected with a glass closed gas circulation system. Typically, 30 mg photocatalysts contained 3% Pt were dispersed in 100 mL DI water which contained 10% triethanolamine (TEOA) through ultrasonic treatment. Then the suspension was put in the Pyrex vessel and sealed on the circulation system. Before the photocatalytic reaction, the whole system was evacuated three times to remove all the detectable air gas. The reaction temperature was kept at 10 using a water cycle system. A 300 W Xe lamp was used as the light source. Incident lights were altered by using a cut-off filter. The produced H2 were analyzed through a gas chromatography (GC-7900) apparatus which was equipped with a thermal conductive detector (TCD) and a 5A molecular sieve column, Ar was used as the carrier gas. 3. Results and discussions Due to the sublimation of melamine around 300 , one-step thermal-induced condensation of melamine in a semi-closed vessel is necessary. This semi-enclosed chamber is poor of oxygen even if in air condition at high temperature. Therefore, to bring the precursor into contact with O2 sufficiently, melamine was heated at 425 °C for 6 h to produce the melem firstly. Then, these obtained melem were further heated on a fully opened cubic ceramic crucible in pure dry O2 atmosphere to produce the LOP g-CN, a synthesis schematic has been displayed in Fig. 2. The XRD patterns in Fig. 3c show that all the as-prepared samples display two peaks around 13.0° and 27.7°, which are assigned to the (1 0 0) and (0 0 2) planes, arising from the hydrogen bonds dominated longrange atomic order intralayer periodic aromatic segments and

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Fig. 2. Schematic illustration for the preparation of the LOP g-CN samples.

Fig. 3. (a) Top and (b) side view of the atomic structure of melon, C, N and H atoms were denoted by gray, blue and green balls, respectively. (c) XRD patterns of G-CN and LOP-CN obtained from different temperatures. (d) EPR signals of G-CN sheet and LOP-CN sheets. (e) FT-IR and (f) Raman spectra of G-CN and LOP-CN obtained from different temperatures. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

the van der Waals forces controlled layer stacking along the c-axis, respectively (Fig. 3a and b) [6,26,29]. Interestingly, compare to the G-CN, a distinctive peak evolution can be observed over these LOPCN materials with changing the synthesis temperature. With the temperature increasing from 500 to 560 °C, the peak intensity of (1 0 0) planes is enhanced at first and then decreased sharply. On the other hand, the (0 0 2) planes are continuously decreased and become broad. The enhanced peak intensity at 13.0° indicates a long-range atomic order intralayer structure, while the weaken and broadened peak on 27.7° suggests a disturbance of the interlayer stacking [20,30]. This result verifies that the crystalline structure of carbon nitride could be precisely tailored by this pure O2 assist thermal-induced condensation of melem strategy. In order to offer a convincing proof for the order intralayer of LOP-CN, room-temperature electron paramagnetic resonance (EPR) signals of G-CN sheet and LOP-CN sheet which were prepared by liquidexfoliation of their mother materials were performed (Fig. S3 displayed the AFM results of the exfoliated G-CN and 530 LOP-CN). As shown in Fig. 3d, the EPR signal of 500 and 530 LOP-CN sheet

is weaker than that of G-CN sheet, indicating the decreased defect density in the layer [31]. The 530 LOP-CN sheet has the weakest EPR intensity, which suggests the high quality of layer of 530 LOP-CN sample. On the other hand, the EPR signal of 560 LOP-CN sheet is enhanced, the increment in the EPR intensity might be caused by the breaking of hydrogen bonds of the layer of 560 LOP-CN under high temperature [26]. The EPR results are marched well with the XRD analysis. FT-IR spectra of the G-CN and different LOP-CN samples are showed in Fig. 3e, a sharp peak is found at 809 cm 1, which can be assigned to the bending vibrations of heptazine rings. The set of peaks between 1600 and 1200 cm 1 region are caused by the vibrations of the CAN stretching in the nitrogen heterocycles rings [32]. In addition, these peaks are not changed with increasing the synthesis temperature, suggesting the basic block of carbon nitride are maintained. Broad bands between 3000 and 3500 cm 1 are belonging to modes of adsorbed water molecule and residual nitrogen precursor species stretching vibrations [9,33]. An obvious decrease in the peak intensity of -NH2 species stretching vibrations

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occurs in these LOP-CN samples when the temperature gets high, indicating less residual nitrogen precursor species exist in the LOP-CN samples. We could not eliminate the -NH2 species completely and obtain a fully condensed purely carbon nitride in this study. Raman spectrum of the G-CN displays a typical CAN stretching vibrations in the 1200–1700 cm 1 region (Fig. 3f) [8]. Sharp peaks can be observed at 703, 752 and 975 cm 1. The 703 and 752 cm 1 peaks are caused by a doubly degenerate mode of the in-plane bending vibrations of the CAN@C. The second peak located at 975 cm 1 is belong to the symmetric N-breathing vibrations of heptazine [34]. A substantial broadened over these peaks is observed in these LOP-CN samples, which is similar to the XRD analysis, again confirm the porous structure. The porous structure was verified by the TEM images and the derived pore size distribution plots from nitrogen sorption isotherms. As observed in Fig. 4, the G-CN displays a typical layer slate-like morphology with a smooth surface (Fig. 4a). Compare to the G-CN, different porous that through the whole particle of LOP-CN samples are found. When the thermal polymerization temperature is 500 °C (Fig. 4b), the obtained material shows a spongy structure with irregular mesopores. Further increase the thermal polymerization temperature to 530 °C, slit holes with a uniform orientation and are nearly parallel to each other can be observed (Fig. 4c). This phenomenon can be explained by eliminating of the hydrogen bonds between layers might led to the generation of the slit holes along the intralayer. The same orientation of the slit holes is corresponding with the arrangements of melon strands on the intralayer. With the polymerization temperature increasing to 560 °C, the collapse of porous occurred (Fig. 4d) due to the breaking of hydrogen bonds on both intralayer and interlayer of carbon nitride at high temperature [20]. The evolution pore struc-

tures also can be verified by the derived pore size distribution plots from nitrogen sorption isotherms (Fig. S4). As observed, G-CN has no pore structure. Little pores appear in 500 LOP-CN sample. When the temperature rises to 530 °C, the pore volume increases quickly. Same with the result of TEM, the collapse of some pore occurs when the temperature increases to 560 °C. As the pore morphology changing, the BET specific surface areas of the samples also change largely. The BET specific surface area of G-CN, 500 LOP-CN, 530 LOP-CN and 560 LOP-CN calculated from the N2 adsorptiondesorption isotherms are 6.08, 18.57, 31.17 and 14.58 m2 g 1, respectively (Fig. S5). The 530 LOP-CN possesses the highest BET surface area, which shows 6 times higher than G-CN. A larger surface area could provide more potential reaction sites for Pt deposition thereby offering more H2 evolution sites. Combining all the structural characterization above, it evidently demonstrates the successful formation of layer plane ordered porous carbon nitride material. XPS was undertaken to investigate the information of the surface compositions and chemical state of these materials. The survey spectra are provided in Fig. S6, in which only the peaks of C, N and O had been seen in all the profiles. High-resolution spectra of C 1s, N 1s and O 1s are presented in Fig. 5. The C 1s spectra (Fig. 5a) could be divided into three distinct peaks: C@N bond (284.6 eV) of the framework of g-C3N4, surface CAO species (286.2 eV) and graphitic carbon (288.1 eV). Notably, the peak intensities of the surface CAO species increases with the increase of polymerization temperature, suggesting trace amount of carboxylate groups can be involved into the surface of g-C3N4 by heating in dry oxygen [35]. High-resolution spectra of N 1s (Fig. 5b) have been fitted into three peaks at 398.6 eV, 399.2 eV and 400.7 eV, which are belonging to sp2-hybridized nitrogen C@NAC,

Fig. 4. TEM images of the as-prepared samples: (a) G-CN. (b) 500 LOP-CN. (c) 530 LOP-CN. (d) 560 LOP-CN.

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Fig. 5. High-resolution XPS spectra of (a) C 1s. (b) N 1s. (c) O 1s.

inner N atoms bonded N-(C)3 and the terminal -NHx groups, respectively [36]. Almost no change is observed in the N 1s spectra, indicating little influence to the framework of g-C3N4 by this dry O2 assist thermal-induced condense process. For the high-resolution spectra of O 1s (Fig. 5c), the bonds belong to surface oxides C@O, CAOH, adsorbed water and CAOAC are fitted out. Differ from the spectra of C 1s and N 1s, the O species in these samples change a lot. First, the peak of surface oxides C@O became negligible in these LOP-CN samples. Second, the ratio of the adsorbed water compositions keeps increasing till the temperature up to 530 °C and then decreasing at 560 °C. For the form one, the surface oxides C@O species can be observed on G-CN but not LOP-CN, suggesting fewer defects in LOP-CN and the framework of tri s triazine block is very stable. While for the latter one, the increased adsorbed water might due to the evolution of the porous structure of the samples. This phenomenon can be verified by the change of hydrophilicities of the samples. As shown in Fig. S7, the water contact angle of G-CN, 500 LOP-CN, 530 LOP-CN and 560 LOPCN is 25.2°, 17.8°, 4.7° and 5.7°, respectively. The decreased water contact angles suggestion the pore structure improved the hydrophilicity [37,38]. UV–vis DRS in Fig. 6a displays an obvious red shift of the absorption edge in these LOP-CN compare to that of G-CN, indicating the light harvesting ability have been improved in these LOPCN samples. The apparent visuals of G-CN and 560 LOP-CN samples are presented inset the figure, one can see that the colour changes step-by-step from pale yellow to deep yellow. It is noteworthy that the absorption of 530 LOP-CN is much higher than those of other samples. The sharp absorption edge of 530 LOP-CN sample could be ascribed to the order layer plane, which has been frequently observed on the crystalline carbon nitrides [4,8,31,39]. Corresponding Tauc’s plots of (ahv)2 vs. (hv) of the samples are displayed in Fig. 6b. As observed, the band gap energy (Eg) of G-CN, 500 LOP-CN, 530 LOP-CN and 560 LOP-CN is 2.78 eV, 2.69 eV, 2.65 eV and 2.61 eV, respectively. In order to further understand the influence of porous structure on the conduction band (CB) and valence band (VB) of the photocatalysts, Mott–Schottky plots and X-ray photoelectron spectroscopy valence band (XPS-VB) were conducted. As shown in Fig. S8, the flat-band position of G-CN, 500

LOP-CN, 530 LOP-CN and 560 LOP-CN are estimated to be at 1.25 V, 1.31 V, 1.41 and 1.43 V vs. Ag/AgCl electrode at pH = 6.8, which correspond to 0.65 V, 0.71 V, 0.81 V and 0.87 V vs. normal hydrogen electrode (NHE) at pH = 0, respectively. The flat-band position is approximately equal to that of the Fermi level and the XPS-VB of all the samples are conducted to be 2.2 eV (Fig. 6c). Accordingly, the VB position of G-CN, 500 LOP-CN, 530 LOP-CN and 560 LOP-CN are determined at 1.55 V, 1.49 V, 1.39 V and 1.33 V, respectively. Combine with the results of Eg determined by DRS, the CB position of G-CN, 500 LOP-CN, 530 LOP-CN and 560 LOP-CN can be calculated at 1.21 V, 1.25 V, 1.26 V and 1.28 V, respectively. A diagram of the band structure of the as-prepared samples is shown in Fig. 6d. As observed, the band energy is upshifted with the increased treatment temperature, suggesting a stronger redox ability can be realized by this O2 assist thermal-induced condensation. Fig. 7 gives the photocatalytic H2 evolution performance of the as-prepared G-CN and LOP-CN catalysts. From Fig. 7a, it is clear that both G-CN and LOP-CN samples show a steady hydrogen production rate (HER) under visible light irradiation. The HER are gradually enhanced with the temperature increase to 530 °C. Further, increasing the temperature to 560 °C results in a rapid decrease in photocatalytic performance. The reduced photocatalytic activity beyond 530 °C is probably due to the disturbance of the plane atomic arrangements and collapse of the porous as observed in the XRD patterns (Fig. 3c) and TEM images (Fig. 4d), which significantly decrease the carrier separation efficiency and the activity sites for the desired redox reactions (detailed mechanism study will be introduced in the next section). The HER of 530 LOP-CN is 53.7 lmol h 1 under k  400 nm, which offers 7.4 times higher than that of G-CN (7.2 lmol h 1) (Fig. 7c). In addition, this increment fact could be raised to 16 under k  420 nm (Fig. 7d). Wavelength-dependent apparent quantum yield (AQY) of H2 evolution corresponds the optical absorption spectrum of 530 LOP-CN (Fig. 7b), indicating that the H2 production is mainly driven by photoinduced electrons in photocatalyst. The AQY is 6.2% under k = 400 ± 5 nm and maintained at 3.3% under k = 420 ± 5 nm irradiation. The stability of 530 LOP-CN sample was evaluated by cycle tests. As shown in Fig. S9, no obvious

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Fig. 6. (a) UV–vis DRS spectra of G-CN and LOP-CN obtained from different temperatures. (b) The Tauc’s plots of (ahv)2 vs. (hv) transformed from DRS. (c) XPS-VB of the as-prepared samples. (d) Schematic energy level diagrams of the as-prepared samples.

Fig. 7. (a) Photocatalytic H2 evolution performance of G-CN and LOP-CN with different synthesis temperatures under visible light irradiation. (b) Wavelength dependence hydrogen evolution quantum yield of 530 LOP-CN. The comparison of HER between G-CN and LOP-CN samples under light irradiation of (c) k  400 nm and (d) k  420 nm. The error bars were carried out by estimating the standard deviation from triplicate experiments (n = 3).

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decrease in the HER indicates the good photo-durability of the 530 LOP-CN sample. Moreover, the FT-IR spectra and XRD pattern of the 530 LOP-CN after cycle test were recorded, which further confirm the good stability of 530 LOP-CN (Fig. S10). Since the photocatalytic reactions were carried out in the presence of Pt cocatalyst, the Pt nanoparticles on these obtain samples also have been investigated for exploring the rationale of the enhanced photocatalytic performance. Fig. 8 gives the TEM images and the corresponding Pt particles size distribution histogram of the used photocatalysts. As can be seen, the average Pt size of Pt/ G-CN, Pt/500 LOP-CN, Pt/530 LOP-CN and Pt/560 LOP-CN estimated from the Gaussian-fitting curve are 29.8, 20.7, 17.8 and 9.77 nm, respectively. Clearly, the average Pt size of Pt/LOP-CN are smaller than that of Pt/G-CN. On the one hand, the decreased Pt size on Pt/500 LOP-CN and Pt/530 LOP-CN should be attributed to the increased surface area (Fig. S5). On the other hand, the smallest Pt size of Pt/560 LOP-CN (which possesses a relative lower surface area than that of 530 LOP-CN) might be caused by the loss of terminal amino functional groups at high temperature [26], since the amine groups of carbon nitride are commonly considered as the preferred site for in situ reduction of Pt0 from PtCl26 [40]. These results suggest that appropriately increase the synthesized temperature could not only optimize the structure but also prevent the loss of deposition sites of Pt co-catalysts. At the meantime, the chemical state of these Pt cocatalysts also had been investigated via XPS. As shown in Fig. S11, the deconvolution of the overlapped high-resolution XPS spectra of Pt 4f peaks could produce four binding energies centered around 72.2, 73.3, 74.5 and

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76.6 eV. The peak of about 72.2 and 74.5 eV are associated with the metallic Pt0 4f7/2 and Pt0 4f5/2, respectively. The rest of two peaks at about 74.5 and 76.6 eV can be assigned to the Pt2+ 4f7/2 and Pt2+ 4f5/2, respectively. So, it suggests that the species of Pt in the spent Pt/G-CN and Pt/LOP-CN are consist of metallic Pt0 and oxide PtO, the present of PtO can be attributed to partial oxidation of Pt0 during the photodeposition process [41]. Next, to better understand the relationships of structure and activity, photoluminescence (PL) spectra were conducted. From Fig. 9a, it can be seen that the steady state PL spectra of G-CN is much higher than that of those LOP-CN samples. The intensities of these spectra are in an order of G-CN > 500 LOP-CN > 560 LOPCN > 530 LOP-CN. The decrease in the PL intensity indicates that the electron-hole separation efficiencies have been improved. Therefore, the lowest PL intensity of 530 LOP-CN implies its outstanding carrier separation rate. Note that the order of carrier separation rate cannot match the photocatalytic performance very well, which should be attributed to the multistep process of a photocatalytic reaction [42]. The PL can only reflect part of the charge carrier transfer step instead of the photocatalytic performance [16]. Regardless, the carrier separation efficiency is enhanced. Since only the 530 LOP-CN exhibits high quality layer plane in the prepared four samples, we then chose the G-CN and 530 LOP-CN to further study the charge transfer properties related to the high-quality layer plane. Fig. 9b displays the time-resolved fluorescence spectra of G-CN and 530 LOP-CN, it is found that the carrier’s lifetime has been prolonged on 530 LOP-CN. By fitting their decay kinetics, the calculated average lifetimes of G-CN and

Fig. 8. TEM images of the spent Pt/G-CN and Pt/LOP-CN photocatalysts. Inset figures in each image are the size distribution histogram with Gaussian-fitting curve of the Pt particles: (a) Pt/G-CN; (b) Pt/500 LOP-CN; (c) Pt/530 LOP-CN; (d) Pt/560 LOP-CN.

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Fig. 9. (a) Steady state PL spectra of G-CN and LOP G-CN samples. (b) Time-resolved fluorescence spectra of G-CN and 530 LOP G-CN, inset figure is the lifetimes of G-CN and 530 LOP G-CN calculated from fitting decay kinetics. (c) Photocurrent density of G-CN and 530 LOP G-CN under visible light (k  420 nm) with and without addition of 1 mM methylviologen dichloride (MVCl2). (d) EIS Nyquist plots of G-CN and 530 LOP-CN. (e) Steady state PL spectra of the spent Pt/G-CN and Pt/530 LOP G-CN photocatalysts. (f) Time-resolved fluorescence spectra the spent Pt/G-CN and Pt/530 LOP G-CN photocatalysts.

LOP-CN are 3.25 and 4.239 ns (inset Fig. 9b), respectively. Suggesting that the carrier transfer efficiency have been enhanced [43–45]. In addition, the surface charge transfer efficiency was further measured by comparison of the transient photocurrent response increments in methylviologen dichloride (MVCl2) solution as well as an electrochemical impedance spectra (EIS) test [46,47]. Because the MV2+ is a kind of electron scavenger, the addition of MV2+ into the electrolyte would greatly increase the photocurrent density (detail mechanism is conducted in Supporting information). As shown in Fig. 9c, the increment of G-CN and 530 LOP-CN is 2.74 and 3.79 times, respectively. Undoubtedly, the 530 LOP-CN possesses a higher photo-excited charge carrier transfer efficiency than G-CN, which should be ascribed to the fast carrier transfer property of plane long-range order structure [18]. Fig. 9d displays the EIS Nyquist plots of G-CN and 530 LOP-CN, which indicates the

530 LOP-CN material possesses a small electron resistance [48–51]. Furthermore, to further verify the high charge carrier separation and transfer rate on 530 LOP-CN sample, the comparison of PL and time-resolved fluorescence spectra of the spent Pt/G-CN and Pt/530 LOP-CN photocatalysts were conducted. As shown in Fig. 9e, the PL intensity of Pt/530 LOP-CN is still weaker than that of Pt/G-CN, suggesting the introduction of Pt cocatalyst would not obstruct the enhancement of charge carrier separation on 530 LOPCN. Interestingly, the corresponding time-resolved fluorescence spectra of the spent Pt/G-CN and Pt/530 LOP-CN photocatalysts (Fig. 9f) show almost no changes upon that of G-CN and LOP-CN materials (Fig. 9b). This result is unusual since in most photocatalytic systems, the decoration of cocatalysts strongly accelerated the decay [52,53], implying the similar interactions with the Pt cocatalyst over the G-CN and 530 LOP-CN materials [7].

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Lastly, considering that the surface area may pay a direct factor for enhancing the photocatalytic activities because a larger surface area may potentially provide more reaction sites for Pt deposing, it is necessary to exclude the contributions of the increased surface area to the photocatalytic performance. In our case, for the best 530 LOP-CN sample, the specific surface area increase factor is about 5 from 6.08 to 31.17 m2 g 1, which cannot match the great HER boosting by a factor of 16 from 0.31 to 5.1 mmol h 1 at k  420 nm. Therefore, the ordered plane atomic arrangements which improved the carrier separation efficiency might pay primary responsibility to the photocatalytic activity enhancement. In conclusion, the significant photocatalytic H2 evolution activity enhancement of the LOP-CN could be attributed to the following three factors: Firstly, the long-range atomic order intralayer structure inhibit the recombination of photo-excited electrons and holes by accelerating charge separation and fast migration. Secondly, the porous structure formed by slit holes along the intralayer offers more abundant metal–semiconductor interfaces for Pt deposition, thereby improving the H2 evolution reactions. Thirdly, the layer plane ordered porous structure synergistically modulate light absorption and carrier transfer two basic steps of photocatalysis.

[2]

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[4] [5]

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[7]

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4. Conclusions [11]

In summary, by judiciously thermal-induced condensation of melem in pure O2 atmosphere, two photocatalytic activity favorable features were created. One is the remarkable charge separation efficiency as a result of long-range plane order atomic arrangement which benefit the carrier’s migration by improving the diffusion lengths of electrons in-plane. The other is the abundant slit holes formed caused by eliminating the hydrogen bonds between layers which not only offers more reaction sites for Pt deposition but also extends the visible light absorption. As a consequence of these favorable features in improving two basic steps of photocatalysis: light absorption and charge transfer, the asprepared layer plane ordered porous carbon nitride exhibited a significantly enhanced photocatalytic H2 evolution performance under visible light. This work mainly highlights the tailoring of crystalline structure of carbon nitride could be achieved by a simple pure O2 assist thermal-induced condensation of melem strategy. Compared with the recent phase tailoring means on carbon nitride materials, the advantage of this O2 assist thermal-induced phase adjusting method is simpler and greener. Furthermore, benefiting from the high-quality of the layer plane and high purity, the LOP-CN sheets support broad potential applications including solar energy conversion, sensors, and energy storage. Acknowledgements This work is financially supported by the National Natural Science Foundation of China (No. 21777063, 21676128, 21407065). Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX18_2266). The High Performance Computing Platform of Jiangsu University.

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Appendix A. Supplementary material

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Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.08.069.

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