Reconstructing Supramolecular Aggregates to Nitrogen Deficient g

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Reconstructing Supramolecular Aggregates to Nitrogen Deficient gC3N4 Bunchy Tubes with Enhanced Photocatalysis for H2 Production Guifang Ge, Xinwen Guo, Chunshan Song, and Zhongkui Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04227 • Publication Date (Web): 18 May 2018 Downloaded from http://pubs.acs.org on May 18, 2018

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ACS Applied Materials & Interfaces

Reconstructing Supramolecular Aggregates to Nitrogen Deficient gC3N4 Bunchy Tubes with Enhanced Photocatalysis for H2 Production Guifang Ge, † Xinwen Guo,† Chunshan Song,†,‡ Zhongkui Zhao*,† †

State Key Laboratory of Fine Chemicals, PSU-DUT Joint Center for Energy Research, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China ‡

EMS Energy Institute, PSU-DUT Joint Center for Energy Research and Department of Energy & Mineral Engineering, Pennsylvania State University, University Park, Pennsylvania 16802, United States ABSTRACT: Developing a facile method to overcome the intrinsic shortcomings of g-C3N4 photocatalyst concerning its insufficient visible-light absorption and dissatisfactory separation efficiency of charge carriers is of great significance but remains a challenge. In this work, we report, for the first time, a sapiential strategy for preparing highly-efficient nitrogen deficient g-C3N4 featuring bunchy microtubes (R-TCN) via a KOH-assisted hydrothermal treatment of rodlike melaminecyanuric acid supramolecular aggregates (RMCA) followed by heating the reconstructed RMCA, in which KOH serves as an all-rounder for breaking hydrogen bonds, accelerating hydrolysis of melamine and nitrogen defects forming. This approach endows R-TCN with unique bunchy-microtube morphology, enriched nitrogen defects, textural properties, and electronic structure, which results in narrower bandgap, higher electric conductivity, more active sites, more negative conductive band, significantly increased visible light harvesting capability and the improved separation efficiency of charge carriers. As a consequence, R-TCN shows 2.44 and 39 times higher hydrogen evolution rate (8.19 μmol h-1) than the pristine TCN from RMCA and bulk g-C3N4 from melamine. This new discovery may open a new avenue to fabricate highly efficient g-C3N4 catalysts. KEYWORDS: reconstruction, supramolecular aggregates, bunchy microtubes, graphitic carbon nitride, photocatalysis

where hydrogen bonding is used to form supramolecular aggregates with reversibility and specificity.27-31 In the last few years, there are many reports concerning the fabrication of g-C3N4 nano/micro-structures by thermal polycondensation of melamine-cyanuric acid supramolecular (MCA) aggregates.32-37 More interestingly, a smart strategy was presented for synthesizing tubular carbon nitride (TCN) photocatalysts through a facile hydrothermal (HT) technique by solely using low-cost melamine (M) as precursor,38,39 in which M was in situ transformed into cyanuric acid (CA). Nowadays, the MCA preorganization has been considered as one of the most practical strategy for fabricating g-C3N4 nano/microstructures with broad applications in energy conversion and storage, especially for visible-light photocatalysis.32-46 However, the further improvement in photocatalytic behavior including activity and stability is required. The design and modulation of microstructure and bandgap engineering of g-C3N4 were an efficient strategy to improve its photocatalysis.3,4,26 It was previously reported that the selective breaking of H-bonds existing in the intralayer framework of layered carbon nitride resulted in a dramatically improved g-C3N4 photocatalyst for hydrogen evolution resulting from the change of energy bands and structures.47 It inspired us to envisage

I. INTRODUCTION Depletion of fossil fuels and the consequent environmental concerns have become global problems. Clean hydrogen fuel from photocatalytic water splitting is an ideal carrier for converting and storing solar energy and a promising solution to solve the above issues.1-4 The design of efficient, stable, environmentally-friendly and cheap photocatalyst is the key to the practical application of photocatalytic technology.5-7 g-C3N4 has drawn growing interest in the photocatalysis area since the 20th century owing to its inexpensive precursors, chemical stability, molecular tunability, widespread availability and nontoxic nature.3,8-19 However, the application of bulk g-C3N4 is limited by its poor absorption, fast charge recombination, and low specific surface area.20,21 Developing a facile method to overcome the intrinsic shortcomings of g-C3N4 photocatalyst concerning its insufficient visible-light absorption and dissatisfactory separation efficiency of charge carriers is of great significance but remains a challenge. Several different proposals have been adopted,22-26 such as nano-structure tailoring, morphology controlling and heteroatom doping to improve its photocatalytic performance. Supramolecular preorganization is a promising selftemplate method that requires no additional templates, 1

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that controlling H-bonds splitting of MCA should enhance photocatalysis of g-C3N4 prepared by the extensively-investigated MCA supramolecular preorganization method. It is well known that alkali can not only accelerate the H-bonding breaking but also promote the consecutive transformation of melamine to cyanurodiamide (CDA), ammelide (AM), CA, and even to CO2 and NH3 via hydrolysis (Figure S1). Therefore, we hypothesized that the H-bonds mediated MCA supramolecular aggregates might be reconstructed by alkali assisted HT process. As a consequence, the microstructures and energy band structures of g-C3N4 derived from the reconstructed MCA (R-MCA) composed of diverse precursors could be effectively modulated. However, to the best of our knowledge, there is no report on the reconstruction of MCA to fabricate g-C3N4. Moreover, it was illustrated that the introduction of nitrogen defects into g-C3N4 framework can promote its visible light photocatalytic performance owing to its ability in tuning band structures and serving as trapping sites for photogenerated charge carriers to retard their recombination.13,48-52 Among the diverse methods,53-57 the alkali assisted polymerization of carbon nitride precursors has been considered as one of the most efficient and simple strategy to introduce nitrogen defects. Therefore, we envision that the nitrogen-deficient g-C3N4 micro/nano-structures can be fabricated through thermal polymerization of R-MCA precursor, in which the alkali serves as an all-rounder for promoting H-bonds breaking, adjusting donor-acceptor pairs in the reconstruction process of MCA and enhancing the formation of nitrogen defects in g-C3N4 framework. In this communication, we report, for the first time, a facile alkali-assisted reconstruction strategy for synthesizing nitrogen deficient g-C3N4 bunchy microtubes (R-TCN, looser gray powder than TCN, Figure S2) using the previously reported rodlike MCA aggregates (RMCA) via an alkali assisted HT method followed by the residual alkali (no extra alkali is required) assisted nitrogen defects introduction into g-C3N4 framework in the thermal polycondensation of reconstructed RMCA (RRMCA), in which KOH serves as an all-rounder for breaking hydrogen bonds, accelerating hydrolysis of melamine and nitrogen defects forming. The aforementioned new strategy endows R-TCN with unique bunchy microtubes morphology, enriched nitrogen defects, narrowing band gap energy (Eg), increased electric conductivity, more active sites by larger specific surface area, more negative conductive band. These lead to significantly improved visible light harvesting capability and separation efficiency of photoinduced charge carriers. As a result, R-TCN shows 2.44 times higher hydrogen evolution rate (HER, 8.19 μmol g-1) than pristine TCN prepared by thermal condensation of RMCA. The alkali-assisted reconstruction of MCA aggregates and introduction of nitrogen defects have been extended to synthesizing the other nitrogen deficient bundles of slim carbon nitride (R-HSCN) and bundles of carbon fibers (R-

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TCNT) with promoted photocatalytic HER by using spherical MCA aggregates (SMCA) formed by equal molar M and CA in DMSO and thin fibrous MCA ones (FMCA) in HT process at 180 oC as precursors, respectively. The new method developed in the present work may lay a new avenue to fabricate highly efficient carbon nitride catalysts for diverse transformations including but not limited to photocatalysis.

II. EXPERIMENTAL SECTION Preparation of Bulk Carbon Nitride (GCN). Bulk g-C3N4 was prepared by a standard polymerization.3 Briefly, 2 g melamine was loaded in a quartz boat and calcined at 500 °C for 4 h with a heating rate of 2.5 °C min-1. Finally, the stiff product was ground to homogeneous powder. Preparation of Tubular Carbon Nitride (TCN). According to the previous report,38 0.8 g melamine and 0.96 g phosphorous acid were dispersed in 80 mL deionized water and then stirred for 1 h. Then the mixture was transferred into a Teflon-lined autoclave and heated at 180 °C for 10 h. The mixture was filtered, washed with enough DI water to completely remove all phosphorus, and then the solids were dried at 60 °C to obtain rodlike melamine-cyanuric acid supramolecular aggregates (RMCA). The resulting RMCA aggregates were calcined at 500 °C for 4 h under nitrogen atmosphere with a ramp rate of 2.5 °C min-1 to get pristine tubular carbon nitride (TCN). Preparation of Carbon Nitride Bunchy Tubes through KOH Assisted Reconstruction Process (R-TCN). The asprepared rodlike melamine-cyanuric acid supramolecular aggregates (RMCA) from above work were dispersed into KOH aqueous solutions with different amounts of KOH (0.004-0.1 g) with strongly stirring, then transferred into a Teflon-lined autoclave at 180 °C for 8 h (R-RMCA-x, and x is denoted as the adding amount of KOH). Subsequently, the solid was filtered and washed with water. Resulting white powder was dried at 60 °C and calcined at 500 °C for 4 h with the heating rate of 2.5 °C min-1 under nitrogen flow. Resulting powder was washed with water to remove partial alkali that exists in the aggregates, and the carbon nitride bunchy tubes were obtained (R-TCN). For comparison, HT-TCN was prepared by using the similar procedure as above except for the hydrothermal process in absence of KOH. The precursor of HT-TCN (supramolecular aggregates prior to calcination) was designed as HT-RMCA. The water washing free carbon nitride bunchy tube (WF-R-TCN) was prepared by the similar procedure to R-TCN except for the water washing process was eliminated before the calcination process in N2 atmosphere. The as-prepared R-TCN supramolecular assembly was washed by using a large amount of deionized water to completely remove alkali on the RTCN assembly before calcination, and then calcined at 500 °C for 4 h with the heating rate of 2.5 °C min-1 under nitrogen flow to obtain the alkali-free R-TCN bunchy tube (CRA-R-TCN). Preparation of Hollow Spherical Carbon Nitride (HSCN). According to the previous report,45 0.50 g melamine was 2


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dissolved into 20 mL DMSO by ultrasonic process to get solution A. 0.51 g cyanuric acid was dissolved into 20 mL DMSO by ultrasonic process to get solution B. The solution B was poured into solution A to form a mixture. The mixture was continuously stirred for 30 min, the spherical melamine-cyanuric acid supramolecular aggregates (SMCA) were recovered by filtering, washing and the subsequential drying at 60 oC with ethanol to remove DMSO. The hollow spherical carbon nitride (HSCN) was obtained by calcining SMCA at 500 °C for 4 h under nitrogen atmosphere with a ramp rate of 2.5 °C min-1. Preparation of the Reconstructed HSCN through KOH Assisted Hydrothermal Process (R-HSCN). The asprepared spherical melamine-cyanuric acid supramolecular aggregates (SMCA) were dispersed into KOH aqueous solution with 0.02 g of KOH with strongly stirring, and then transferred into a Teflon-lined autoclave at 180 °C for 8 h. Subsequently, the solid was filtered and washed with water. Resulting white powder was dried at 60 °C to obtain the reconstructed SMCA (RSMCA) with a flower-like morphology composed of fibrous supramolecular aggregates. R-SMCA was calcined at 500 °C for 4 h with the heating rate of 2.5 °C min-1 under nitrogen flow. Finally, the bundles of thin carbon nitride tubes (R-HSCN) were obtained. For reference, the SMCA aggregates were treated by hydrothermal process in the absence of alkali to obtain the treated SMCA (HTSMCA). Preparation of Thin Carbon Nitride Tubes (TCNT). According to the previous report,33 0.40 g melamine was dispersed into 40 mL H2O by ultrasonic process to get dispersion A. 0.405 g cyanuric acid was dispersed into 40 mL H2O by ultrasonic process to get dispersion B. The dispersion B was gradually poured into dispersion A to form a mixture. The mixture was continuously stirred for 30 min. The resulting mixture was transferred into Telfon-lined autoclave to perform a hydrothermal process at 180 oC for 4 h. The micro-fibrous melamine-cyanuric acid supramolecular aggregates (FMCA) were obtained by a filtering, washing and drying process at 60 oC. The thin carbon nitride tubes (TCNT) was obtained by the calcination of FMCA at 500 °C for 4 h under nitrogen atmosphere with a ramp rate of 2.5 °C min-1. Preparation of the Reconstructed TCNT through KOH Assisted Hydrothermal Process (R-TCNT). The asprepared spherical melamine-cyanuric acid supramolecular aggregates (FMCA) were dispersed into KOH aqueous solution with 0.02 g of KOH with strongly stirring, and then transferred into a Teflon-lined autoclave at 180 °C for 8 h. Subsequently, the solid was filtered and washed with water. Resulting white powder was dried at 60 °C to obtain the reconstructed FMCA (RFMCA) with bundles of fibrous melamine-cyanuric acid supramolecular aggregates. R-FMCA was calcined at 500 °C for 4 h with the heating rate of 2.5 °C min-1 under nitrogen flow. Finally, the bundles of carbon nitride fibers (R-TCNT) were obtained after the complete removal of K

by washing process with a large amount of water, confirmed by ICP test. Samples Characterizations. X-ray diffraction (XRD) patterns of the samples were obtained on Rigaku Smart Lab diffractometer, using a nickel-filtered Cu Kα X-ray source at a scanning rate of 5° over the range between 5° and 80°. Transition electron microscopy (TEM) was carried out on Tecnai F30 electron microscope (FEI Co. Ltd.) at 300 kV. Scanning electron microscopy (SEM) was performed on JSM-5600LV (JEOL). The specific surface areas were calculated via the BET method, and the pore size distributions were calculated from an adsorption branch of the isotherm. Fourier transform infrared (FTIR) spectra were recorded on a Bruker EQUINOX55 infrared spectrometer with the samples dispersed in KBr pellets. X-ray photoelectron spectroscopy (XPS) were conducted on an ESCALAB 250 XPS system, monochromatized Al Kα X-ray radiation (15 kV, 150 W, 500 mm, pass energy = 50 eV) were used as X-ray source. The UV-vis spectra (DRS) were collected on a UV–Vis spectrometer (JASCO V-550). Photoluminescence spectra (PL) were measured using a fluorescence spectrometer (Hitachi F-7000). The N/C atomic ratio was determined with an elemental analyzer (Vario EL, Germany). Photocatalytic Hydrogen Production. Photocatalytic hydrogen production tests were carried out in a topilluminated vessel connected to a closed gas mixed and the irradiation area of the reactor was 21.2 cm2 at 15 oC.54 10 mg catalyst was suspended in 100 ml 10 vol% of TEOA aqueous solution at ambient pressure in N2 atmosphere. 3 wt% Pt was used as co-catalyst by in-situ photodeposition method. The light source was a 300W Xenon lamp (PLSSXE300/300UV, Perfectlight, Beijing) with a 420 nm cutoff filter (λ > 420 nm) with 130 mW cm-2 light intensity (tested at the middle position of reaction aqueous solution). The evolved H2 was determined by a gas chromatograph (GC9790, FuLi). The apparent quantum efficiency (AQE) was performed with 50 mg catalyst using the same system at different wavelengths, calculated by the following equation: AQE = (2 × number of evolved H2 molecules/number of incident photons) × 100%. Photoelechemical Measurements. The photocurrent responses were carried out with an electrochemical station (CHI 660E, Shanghai Chenhua, China) with a conventional three-electrode cell. Ag/AgCl electrode was used as a reference electrode, and a platinum wire was used as counter electrode. The as-prepared samples were coated on ITO glass (2.0 ×2.0 cm2) was used as working electrodes. Na2SO4 (0.5 mol L-1) was electrolyte.

III. RESULTS AND DISCUSSION Figure 1 shows the morphology of aggregates and g-C3N4 by SEM and TEM. From Figure 1a and 1c, the hexagonal structured RMCA with a width about 80 μm and a length up 350 μm converted into a bundle of slender rods (RRMCA) by the KOH assisted HT process. R-CNT featuring

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and R-TCNT featuring bundles (ca. 15 μm) of carbon nitride fibers (ca. 1.7 μm diameter) were prepared (Figure S4c,S4d and Figure S5c), respectively, which definitely differed from those of HSCN (hollow sphere, ca. 3 μm diameter) and TCNT (ca. 1-3 μm diameter) that derived from pristine SMCA and FMCA, respectively (Figure S4a, S4b and Figure S5d). The control experiments were performed to explore the possible reconstruction process. The as-prepared RMCA precursor was subjected to alkali-assisted HT treatment under different KOH dosage (R-RMCA-x, x=0.004, 0.01, 0.02, 0.1 g KOH) and HT time (tHT=1, 8, 12 h). From Figures S6 and S7, H-bonds of RMCA can be broken at a low KOH dosage, and R-RMCA-0.004 featuring irregular rod with ca. 10 μm of width was formed. Interestingly, using more KOH, R-RMCA-0.01 features a rod shape with ca. 18 μm of width, which is much larger than that of R-RMCA-0.004 although high KOH concentration can enhance H-bonds breaking. From Table S1, the increasing atomic ratio of N/C (nN/C) can be seen. This implies that the hydrolysis of peeled melamine from RMCA complex by breaking H-bonds takes place along with the H-bonds splitting process (Figure S1). Increasing KOH dosage to 0.02 g, the bundles (ca. 20 μm) of microrods (ca. 2 μm) can be synthesized (Figure S6d), along with nN/C rising up to 1.550 (Table S1), ascribed to the competitive processes incluing H-bonds splitting, hydrolysis of melamine, and also the reassembly of the melamine and converted CDA, AM and CA by hydrolysis. However, the further increasing KOH dosage up to 0.1 g leads to the formation of single R-RMCA rod with ca. 2 μm of width, and a decrease in nN/C can be observed (Table S1), ascribed to the hydrolysis of CA to CO2 and NH3 which is significantly accelerated by high KOH concentration. Figure S7 presents the evolution of bunchy microtubes of R-RMCA-0.02. The rod with rough surface (ca. 22 μm) was formed for 1 h, and bunchy microrods were formed for 8 h. The bundles were broken while tHT was prolonged to 12 h. The increasing nN/C value can be observed if tHT is not more than 8 h, and it decreases along with prolonging hydrothermal time (Table S2). On the basis on the aforementioned observation, a tentative reconstruction process of RMCA to produce bunchy microrods was proposed (Scheme 1). Firstly, the thick hexagonal prism microrods are split into slim rods through alkali assisted H-bonds breaking. After, the further H-bonds splitting, the hydrolysis of melamine, and reassembly of melamine and CA with the as-formed CDA and AM competitively happen, besides the hydrolysis of CA to CO2 and NH3. As a consequence, the supramolecular bundles of microrods with an increasing atomic ratio of nitrogen to carbon are formed. The R-TCN-x carbon nitride micro/nano-structures were prepared through thermal polycondensation of RRMCA-x precursors. The R-TCN-0.02 catalyst featuring bundles of microtubes shows the highest photocatalytic activity for hydrogen evolution. Interestingly, R-TCN-0.02 shows much higher activity than R-TCN-0.1 featuring a single microtube. From Figure S9, even with similar light

Figure 1. SEM images of RMCA (a), TCN (b), R-RMCA (c), RTCN (d), and HT-RMCA (e). TEM images of R-TCN (f).

unique bunchy g-C3N4 tubes was prepared through heating R-RMCA at 500 oC for 4 h under N2 atmosphere (Figure 1d and 1f), which is definitely different from TCN derived from RMCA with thick and long tube characteristics (Figure 1b). In order to clarify the promoting role of alkali, the similar HT process was performed on RMCA except for the absence of alkali to obtain HT-RMCA. From Figure 1e, there is no visible difference between HT-RMCA and RMCA except for HTRMCA featuring rough surface. However, remarkable reconstruction of SMCA can be observed by alkali-free HT process (Figure S3b). The treated SMCA under alkali-free conditions (HT-SMCA) features much different morphology from the pristine SMCA (Figure S3a and S3b). The obvious change in HT-SMCA but almost no change in HT-RMCA can be ascribed to the much larger size of RMCA (ca. 80 μm width) than that of SMCA (ca. 2 μm). In combination of feature of HT-RMCA, HT-SMCA, and RRMCA, it can be concluded that the HT process without alkali assistance can also break H-bonds of MCA aggregates, but alkali can promote H-bonds breaking and reconstruction of MCA supramolecular aggregates. The reconstruction method have been extended to synthesize flower-like aggregates (R-SMCA) from the spherical MCA (SMCA) and bundles of fibrous aggregates (R-FMCA) from thin MCA fibers (FMCA) (Figure S3c, S5a, S5b). Through thermal polymerization of R-SMCA and RFMCA, the R-HSCN featuring bundles (ca. 6.5 μm diameter) of slim carbon nitride nanotubes (ca. 260 nm) 4


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Scheme 1. The plausible reconstructing process of rodlike melamine-cyanuric acid supramolecular aggregates (RMCA) to produce bunchy carbon nitride microtubes. absorption edge, R-TCN-0.02 exhibits a higher visible light absorption than R-TCN-0.1, ascribed to light scattering by the holes and slits within bundles of g-C3N4 microtubes that is confirmed by pore size distribution curves (Figure S10). The alkali-assisted reconstruction strategy can be extended to the other MCA supramolecular aggregates for preparing highly efficient g-C3N4 photocatalyst with unique morphologies (Figure S4, S5 and S11). From Figure S12, the developed R-TCN shows 2.5 times higher specific surface area than TCN. The residual alkali on RMCA precursor is proposed to assist the introduction of nitrogen defects containing cyano groups and nitrogen vacancies into g-C3N4 framework.55 From Figure 2a, in comparison with pristine TCN, R-TCN features weaker and broader XRD peaks appearing at around 13.1o and 27.5o corresponding to the in-plane structural packing motif and interlayer stacking reflection of aromatic segments, respectively.58 This implies the structural deformation of tri-triazine units and a reduced interplanar order.59 From Figure 2b, with the increase in residual alkali (from pristine TCN to CRAR-TCN, then to R-TCN and WF-R-TCN), a progressive decrease in the intensity of N-H stretching peaks appearing at the zone of 3000-3300 cm-1 and a consecutive increase in the stretching vibration of cyano groups (–C≡ N)60 can be observed, suggesting the decreasing N-H groups and the introduction of cyano groups during the synthesis of R-TCN. Elemental analysis shows that the nN/C value of R-TCN is smaller than pristine TCN (from Tables S2 and S3, increasing nitrogen content in whole framework by reconstruction of RMCA but a decrease in the part of framework by alkali-assisted nitrogen defects introduction), indicating the presence of nitrogen defects. From Figure 2c, the increasing intensity of peaks located at 284.6 eV suggests the loss of nitrogen to form nitrogen vacancies in g-C3N4 framework. The increase in XPS peak intensity at 286.4 eV corresponding to C-NHx (x=1,2) on the edge of tri-s-triazine implies the introduction of cyano groups,61 which possess similar C1s binding energies to

Figure 2. a) XRD patterns, b) FT-IR spectra, c) C1s and d) N1s XPS spectra of calcined carbon nitrides TCN and R-TCN. e) UV/Vis light absorption spectra and band gap energies (inset) and f) photoluminescence spectra of the as-calcined GCN, TCN and R-TCN carbon nitride structures.

C-NHx (x=1,2). From Figure 2d, three main peaks observed at 398.5, 400.1, and 400.9 eV can be assigned to sp2-hybridized aromatic N atoms (C-N=C), tertiary nitrogen N-(C)3, and amino groups (C-N-H), respectively.54 Compared to TCN, the N-(C)3 peak of RTCN slightly shifts to lower binding energy, suggesting the generation of cyano groups.62 Further, the slightly lower N/C atomic ratio of 1.04 for R-TCN can be attributed to the C-N=C vacancies in the framework (Table S4). This is consistent with the observation from FT-IR (Figure 2b). The optoelectronic properties of pristine TCN and the developed R-TCN bunchy tubes were measured by UV/Vis light absorption and photoluminescence spectra. UV/Vis light absorption spectra and calculated band gaps for GCN, TCN and R-TCN are shown in Figure 2e; the reconstruction of hexagonal tubes increases the light absorption over the full wavelength range, and the optical gap of R-TCN mildly blue-shifted approximately 14 nm. This probably resulted from reconstruction of thick tube increasing the refraction/ reflection of light. The band gap from the Tauc plots of R-TCN is 2.47 eV, while that of TCN is 2.56 eV. It is well known that the photoluminescence spectrum can be used to characterize the migration, transfer, and separation efficiency of photogenerated charge carriers. The GCN reveals a strong photoluminescence quenching (Figure 2f). The PL 5



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confirmed by the decreased hemicycle radius measured by electrochemical impedance spectroscopy (EIS) and the transient photocurrent response. In Figure 3c, in comparison with the pristine TCN, a marked decrease Nyquist plots arc radius for R-TCN is observed, which suggests that the electronic resistance of R-TCN is smaller than TCN. On the other hand, a significantly strengthened photocurrent over R-TCN was generated, which is 3 times higher than that of the pristine TCN (Figure 3d), illustrating that the separation efficiency of the photoinduced charge carries is obviously enhanced.65 Therefore, we can deduce that the promoted photocatalytic performance can originate from the unique bunchy-microtube morphology, enriched nitrogen defects, narrower bandgap, higher electric conductivity, more active sites by larger specific surface area, more negative conductive band, the significantly improved visible light harvesting capability and separation efficiency of charge carriers, and the promoted visible light absorbance and charge separation and transfer may play a dominant role.

Figure 3. a) Electronic band structures of GCN, TCN and RTCN. b) Photocatalytic activity and stability of GCN, TCN, and R-TCN for H2 evolution under visible light irradiation (λ> 420 nm, 10 mg catalyst). c) Electrochemical impedance spectra (EIS) of GCN, TCN, and R-TCN. d) Transient photocurrent response of GCN, TCN, and R-TCN under visible light irradiation.

IV. CONCLUSIONS In summary, the present work developed a sapiential strategy for synthesizing novel nitrogen-deficient g-C3N4 micro/nano-structures by alkali-assisted reconstruction of melamine cyanuric acid supramolecular aggregates, in which KOH serves as an all-rounder for promoting Hbonds breaking, modulating the hydrolysis process of the released melamine from MCA by splitting H-bonds, and increasing nitrogen defects. As a consequence, the fabricated novel micro/nano-structured g-C3N4 photocatalysts shows superior photocatalytic activity in hydrogen evolution under visible light irradiation the pristine g-C3N4 photocatalysts, ascribed to the narrowing band gap energy, promoting electric conductivity, more active sites by larger specific surface area, more negative conductive band, unique bunchy microtubes morphology, enriched nitrogen defects, and the resulting significantly improved visible light harvesting capability and separation efficiency of photoinduced charge carriers. This discovery in the present work may open a new horizon for fabricating highly efficient g-C3N4 based catalysts for diverse transformations including but not limited to photocatalysis.

intensity for R-TCN decreased drastically in comparison with those of GCN and TCN, which demonstrates the recombination of photogenerated charge carriers was substantially suppressed.63 The valence band (VB) edge potential of R-TCN and TCN were measured by the XPS valence band (Figure S13). The energy band positions are presented in Figure 3a. The VB energy levels of R-TCN, TCN and GCN were determined to be 1.42, 1.57 and 1.69 eV, respectively. From Figure 3a, in comparison with that of TCN, the VB of R-TCN negatively shifts by 0.15 eV, indicating the altered electronic structure owing to the KOH assisted hydrothermal reconstruction process. The conduction band (CB) edge of R-TCN, TCN and GCN were calculated to be -1.05, -0.99 and -0.95 eV, respectively. The negative shift in the CB of the R-TCN leads to a larger thermodynamic driving force for photocatalytic reactions.64 The photocatalytic performance of GCN, TCN and RTCN were examined in H2 production reaction with visible light. As shown in Figure 3b, the hydrogen evolution rate (8.19 μmol h-1) with R-TCN is higher than that of TCN (3.35 μmol h-1), which is 39 times higher than that on bulk g-C3N4 (0.21 μmol h-1). Moreover, hydrogen evolution without noticeable deactivation was observed after four cycles. The apparent quantum efficiency (AQE) of R-TCN under 420 nm is 1.9% (Figure S14), ca. 1.24 times higher than that reported for P-TCN (1.5% of AQY) if it was tested in our instrument. R-TCN shows 2.5 times larger specific surface area (53 m2 g-1) than TCN (21 m2 g-1). Thus it could provide more surface active sites.38 Furthermore, the promoted light absorption of R-TCN by the light scattering owing to the holes and slit within the bundles of g-C3N4 microtubes can also favor the improvement in photocatalytic activity under visible light irradiation. The improved charge transport can be clearly

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.xxxx. Route for hydrogen-bonds breaking and hydrolysis for reconstruction, optical photographs of TCN and R-TCN, SEM of pristine and reconstructed diverse carbon nitride nano/micro-structures, elemental analysis, the other photocatalytic performance, DRS, pore size distribution, N2 adsorption-desorption isotherms, XPS for VB, AQY of R-TCN (PDF)

AUTHOR INFORMATION Corresponding Author 6


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*[email protected]

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ORCID Xinwen Guo: 0000-0002-6597-4979. Chunshan Song: 0000-0003-2344-9911. Zhongkui Zhao: 0000-0001-6529-5020.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially support by the National Natural Science Foundation of China (U1610104, 21676046) and the Chinese Ministry of Education via the Program for New Century Excellent Talents in Universities (NCET-12-0079).

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Graphical Abstract


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Reconstructing Supramolecular Aggregates to Nitrogen Deficient g

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