Ultrasound-Assisted Fabrication of Hierarchical Rodlike Graphitic

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Ultrasound-assisted Fabrication of Hierarchical Rod-like Graphitic Carbon Nitride with Fewer Defects and Enhanced Visible-light Photocatalytic Activity Zhijun Huang, Feng-Wen Yan, and Guo-Qing Yuan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03305 • Publication Date (Web): 21 Jan 2018 Downloaded from http://pubs.acs.org on January 22, 2018

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ACS Sustainable Chemistry & Engineering

Ultrasound-assisted Fabrication of Hierarchical Rodlike Graphitic Carbon Nitride with Fewer Defects and Enhanced Visible-light Photocatalytic Activity

Zhijun Huang,* Feng-Wen Yan,* and Guo-qing Yuan Beijing National Laboratory of Molecular Sciences, Key Laboratory of Green Printing, Institute of Chemistry, Chinese Academy of Sciences, Zhongguancun North First Street 2, 100190, Beijing, PR China.

ABSTRACT: The catalytic performance of graphitic carbon nitride (g-C3N4) is significantly affected by its microstructure and intrinsic defects. Here, rod-like g-C3N4 (RCN) with hierarchical structure and fewer defects is fabricated through an ultrasound-assisted molecular rearrangement strategy. Rod-like supramolecular precursors are formed by rearrangement of melem through ultrasound in water. Heating the precursors at 550 oC leads to less defective hierarchical g-C3N4 composed of nanosheets. No expensive or hazardous reagent is involved in this process. The decrease in defects has been confirmed by powder XRD, FT-IR and XPS. The special hierarchical structure provides RCN with significantly increased surface area and enhanced efficiency of light harvesting. The decrease in defects broadens the visible-light responsive range, suppresses the recombination of electron–hole pairs, and enhances the electric

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conductivity. The visible-light photocatalytic activities of RCN for RhB degradation and H2 evolution increase 16.7 and 8.7 times respectively, compared to that of bulk g-C3N4 prepared by direct heating melamine. KEYWORDS: Photocatalysis, Graphitic carbon nitride, Hierarchical structures, Defects, Molecular rearrangement

INTRODUCTION As a metal-free photocatalyst, graphitic carbon nitride (g-C3N4) has attracted global attention due to its potential applications in catalyzing water splitting and pollutant degradation.1,2 With a band gap of 2.7 eV and an absorption edge of 450 nm, it is active under visible light irradiation.3 gC3N4 can be facilely synthesized by direct heating nitrogen-rich precursors.4,5 However, the obtained samples are bulk structures and exhibit relatively low surface areas. Synthesis of nanosized morphologies, such as nanosheets, is one of the most effective routes to increase their surface areas.6 However, nanomaterials may aggregate seriously during reactions due to the high surface energy, which is unfavorable to the photocatalytic performance. Hierarchical structures composed of nanosized building blocks can effectively avoid the agglomeration.7 Thus, the fabrication of hierarchical g-C3N4 is therefore an ideal strategy to solve the uncontrolled aggregation problems. Nanocasting is one of the most important and commonly used methods to synthesize hierarchical g-C3N4. Several typical hierarchical nanostructures composed of different nanosized building blocks were fabricated by this method.8,9 Generally, nanocasting needs the coating of the hard templates with precursors and the removal of the templates by etching. The templates are generally costly and the removal of the templates by etching is not environmentally friendly.


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Template-free assembly is another effective strategy to fabricate hierarchical g-C3N4.10 Hexagonal tubular g-C3N4 with a layered micro-nanostructure was fabricated through heating precursor obtained by self-assembly under phosphorous acid-assisted hydrothermal conditions.11 Openly-structured g-C3N4 microspheres were obtained from g-C3N4 sulfuric acid solution through crystallization strategy.12 Both the above template-free methods have drawbacks related to the tedious synthetic procedures and the use of hazardous reagents. Thus, it is still a challenge to develop facile, low-cost, and sustainable routes for the fabrication of hierarchical g-C3N4. Defects play an important role in influencing the catalytic performance of g-C3N4.13 Some defects may benefit to the photocatalytic efficiency by broadening the visible light absorption, enhancing the charge separation and serving as the active sites. Thus, extensive efforts have been devoted to introduce defects to g-C3N4.14,15 However, other defects could serve as the recombination centres for electron-hole pairs, which reduce the free carrier concentration and deteriorate the photocatalytic performance.16 Pristine g-C3N4 is generally synthesized by condensation of solid precursors at high temperature. The solid state reaction significantly restricts the mobility of the reagents. An incomplete condensation of the precursors and intermediates cannot be avoided. As a result, a large amount of defects, such as amino and cyano groups, exist in the as-prepared g-C3N4. These intrinsic defects are coated by the compact conjugated polymers and difficult to be eliminated. Few studies have been concerned about this issue. Conducting the condensation at higher temperature may reduce the defects to some extent. But this could induce the pyrolysis of g-C3N4, not only reducing the product yield, but also generating additional defects.17 Copolymerization and heating ammonium thiocyanate under ammonia atmosphere are effective approaches to prepare less defective g-C3N4.18,19 However, the use of expensive precursors and toxic gas limits their applications. Hence, it is essential to


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develop more efficient and environmentally friendly method to synthesize g-C3N4 with fewer defects. Scheme 1. Schematic illustration for fabrication of RCN.

Herein, hierarchical rod-like g-C3N4 (RCN) with fewer defects is fabricated by a facile and sustainable method as shown in Scheme 1. Except for melamine and water, no other reagent is involved in this process. The resultant RCN exhibits well developed porosity and high BET surface area (SBET) (88.6 m2 g-1). Furthermore, compared to g-C3N4 obtained by direct heating melamine, this sample possesses fewer defects and improved optical and electronic properties. As expected, the hierarchical rod-like g-C3N4 with fewer defects shows enhanced photocatalytic activity for pollutant degradation and water splitting under visible light irradiation. EXPERIMENTAL SECTION Synthesis of Catalysts. All the chemicals were reagent grade and used without further purification. For the synthesis of melem, melamine (50 g) in a covered crucible was heated at 430 oC for 4 h with a ramping rate of 5 oC min-1 in static air. After cooled to room temperature, the obtained melem was ground to fine powder. For the synthesis of rod-like melem hydrate, 10 g melem was dispersed in 50 mL water and then sonicated for 10 min. The obtained samples were collected by centrifugation, washed by ethanol and then dried at 60 oC over night.


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RCN was prepared by heating 2 g melem hydrate at 550 oC for 4 h with a ramping rate of 5 oC min-1 in static air. For comparison, BCN and UCN were also prepared under the same conditions but use melamine and urea as the precursors, respectively. Catalyst Characterization. Scanning electron microscopy (SEM) images were obtained on a Hitachi S-4800 instrument. Transmission electron microscopy (TEM) images were obtained on a JEOL JSM-2100 instrument. Nitrogen adsorption–desorption experiment was performed at 77.3 K by a Quanta-chrome Autosorb Automated Gas Sorption System (Quantachrome Corporation). Before experiments, the samples were dried at 250 oC under vacuum for 6 h. Powder X-ray diffraction (XRD) experiments were carried out on a Rigaku Rotaex diffractometer equipped with Cu Kα radiation source (40 kV, 200 mA; λ=1.54056 Å). Fourier transform infrared (FT-IR) spectra were obtained on a Bruker Tensor 27 by using KBr pellets. Elemental analysis (EA) experiments were performed on a Flash EA 1112. X-ray photoelectron spectroscopy (XPS) dates were collected on a Thermo Scientic ESCALab 250Xi using 200 W monochromated Al Kα radiation. UV-visible absorption spectra were obtained using a Shimadzu UV-2600 UV-visible spectrometer equipped with an integrating sphere assembly. BaSO4 was used as reference. Photoluminescence (PL) spectra were recorded on a Hitachi F-4500 fluorescence spectrometer with excitation at 330 nm. Time-resolved fluorescence experiments were performed on an Edinburgh FLES920. Electron paramagnetic resonance (EPR) measurements were performed on a Bruker E500 spectrometer at room temperature. Electrochemical and photoelectric properties were tested on a potentiostat Autolab PGSTAT-30 using a standard three-electrode cell with a working electrode, a platinum wire counter electrode, and a standard calomel electrode (SCE) reference electrode. Na2SO4 (0.5 M) was used as the electrolyte solution. The working electrode was prepared on indium-tin oxide (ITO) glass. A slurry was obtained by dispersing 10 mg of


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samples in water. The slurry was spread onto the cleaned fluorine-doped tin oxide (FTO) glass. After being dried at room temperature, the working electrode was further heated at 200 oC for 12 h to improve adhesion. The uncoated areas on the electrode were isolated with epoxy resin. Total organic carbon (TOC) was analyzed using an Elementar Vario TOC analyzer. Photocatalytic Activity Test. Degradation of RhB were carried out in a quartz glass reactor. Typically, the catalysts (50 mg) were stirred in 100 mL of RhB aqueous solution (10 mg L−1) in the dark for 30 min. After the adsorption–desorption equilibrium was achieved, the light was turned on to start the photocatalytic reaction. The light was provided by a 300 W Xe lamp with a cut-off filter (λ>420 nm). The temperature was maintained at 25 oC by the flow of cooling water. At 10 min intervals, about 3 mL of the suspension was collected and centrifuged at 10000 rpm for 10 min to remove the catalyst. The upper clear liquid was analyzed by UV-vis absorption spectra. The maximum absorption was recorded at 553 nm and used for evaluating the concentration of RhB. After the reaction, the catalyst was recovered by centrifugation, washed with water and used for the next catalytic run. Photocatalytic hydrogen evolution experiments were carried out in a quartz reactor. The catalysts (100 mg) were stirred in an aqueous solution (100 mL) containing triethanolamine (10 vol%). Pt (3 wt%) was loaded on the surface of the catalyst through in situ photodeposition using H2PtCl6 as the precursor. Prior to irradiation, the suspension was evacuated several times to remove air. The light was provided by a 300 W Xe lamp with a cut-off filter (λ>420 nm). The temperature was maintained at 25 oC by the flow of cooling water. The evolved hydrogen was analyzed by a gas chromatography (GC-2014, Shimadzu) equipped with a thermal conductive detector (TCD). After the reaction, the catalyst was recovered by centrifugation, washed with water and used for the next catalytic run.


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RESULTS AND DISCUSSION

Figure 1. (a) SEM image of melem. (b,c) SEM and (d) TEM images of melem hydrate. Catalyst Characterization. The morphologies of melem and melem hydrate were observed by SEM and TEM. Figure 1a shows that melem prepared by heating melamine at 430 oC consists of irregular particles with several micrometers in size. After ultrasound in water for 10 min, the morphology of the melem changed obviously. As shown in Figure 1b, a large amount of rods were generated. The content of the rods exceeds 80% of the sample. These rods are 2−10 µm in length and 0.2−1.0 µm in diameter. The high-magnification SEM image (Figure 1c) reveals that there are numerous cracks on the surface of the rods. A developed porosity can be seen clearly from TEM image in Figure 1d. These pores in the rods are beneficial to generating hierarchical structures. The crystal structures of melem and melem hydrate were investigated by powder XRD. Ultrasonic treatment of melem in water for 10 min, melem interacts with water through hydrogen bonding, and melem hydrate is formed. During this process, molecules rearrangement occurs,


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leading to a hydrogen-bonded rosette-like supramolecular structure as shown in Scheme 1.20 The broad reflection of melem hydrate at 26–28o (d spacing = 0.32–0.34 nm) can be attributed to the layered arrangement of the supramolecular structure (Figure 2). The π−π interactions between the layers may induce the stacking of the planar sheets in a perpendicular direction, resulting in the formation of rods. Two sharp diffraction peaks at 6.1o and 12.2o, corresponding to (110) and (220) peaks of melem hydrate respectively, are derived from the channels perpendicular to the plane. For comparison, melem hydrate was also prepared by stirring melem in water for 1 h. Some rods were found in the obtained sample. However, the content of the rods decreased largely. A large amount of irregular particles can be observed. This suggests that the layered melem hydrate may be exfoliated from the particles by ultrasound and self-assembles to rods.

Figure 2. Powder XRD patterns of melem and melem hydrate. Heating melem hydrate at 550 oC for 4 h, RCN is obtained. A large number of pores with tens of nanometers in size appear in the surface of RCN (Figure 3a). The rod-like morphology is well maintained after heating. The accompanied mass loss results in hollow structure consisting of numerous nanosheets (Figure 3b). Some hollow hemispherical structures are generated due to the partial interconnection and overlapping of the nanosheets. TEM image in Figure 3c clearly shows a hierarchical rod-like structure composed of nanosheets with random orientations. From


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the high-resolution TEM image (Figure 3d) of the detailed local structure, several interconnected nanosheets can be observed clearly. These nanosheets are rolled up to reduce the surface energy, as revealed by the darker edge.

Figure 3. (a,b) SEM and (c,d) TEM images of RCN.

Figure 4. Nitrogen adsorption–desorption isotherms of BCN and RCN. The textural structure of RCN was analyzed by nitrogen adsorption–desorption measurements and compared with that of BCN. The isotherms in Figure 4 show that both BCN and RCN


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exhibit type IV isotherm with H3 hysteresis loop, indicating the existence of mesopores.3 These mesopores are derived from the interconnection and overlapping of the curved nanosheets as demonstrated by SEM and TEM images. The SBET of RCN is determined to be 88.6 m2 g-1, almost 8 times higher than that of BCN (11.3 m2 g-1). Due to the large voids in RCN, the total pore volume is up to 0.37 cm3 g-1. RCN with large SBET and pore volume could provide more catalytic active sites and facilitate the mass diffusion during photocatalytic reactions. Increasing the surface area of g-C3N4 by introducing porosity can be achieved using template methods.21,22 However, complicated post-treatments and the use of hazardous agents (HF or NH4HF2) limit their large-scale applications. RCN with high surface area and developed porosity in this work is facilely prepared by ultrasound and heating. Except for water and melamine, no other agent is involved in this sustainable process. Powder XRD was employed to investigate the crystallinity (Figure 5a). BCN prepared directly from melamine demonstrates two characteristic reflection peaks of g-C3N4. The one at 12.9o is ascribed to the in-planar repeating tri-s-triazine units (100), and the other at 27.4o corresponds to the interlayer-stacking structures (002).23 The I002/I100 ratio is determined to be 13.9. Compared to BCN, RCN shows a slightly weakened (002) diffraction peak. This can be explained by the fact that RCN is composed of nanosheets. The intensity of (100) peak for RCN increased significantly, which leads to a largely decreased I002/I100 ratio (4.5). This indicates that the periodicity of the tri-s-triazine units in RCN is improved greatly. In other words, RCN with higher crystallinity and fewer defects is obtained. As aforementioned, melem hydrate supramolecular structure can be formed by reaction of melem with water. The π−π interactions between supramolecular may induce the stacking of the planar sheets in a perpendicular direction, resulting in the formation of rods with ordered channel structure. Temperature-dependent powder


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XRD experiment performed by Sophia and co-worker20 confirmed that the symmetric channel structure does not collapse when water is removed at high temperature due to the strong intermolecular hydrogen-bonding interconnecting. As a result, g-C3N4 with higher crystallinity and fewer defects could be obtained after heating at 550 oC.

Figure 5. (a) Powder XRD patterns and (b) FT-IR spectra of BCN and RCN. The chemical structures were investigated by FT-IR spectra (Figure 5b). RCN shows several bands in 1200–1650 cm-1 region, which are ascribed to the typical stretching modes of C–N heterocycles. The band at 813 cm-1 is ascribed to the breathing vibration of the tri-s-triazine units.24 BCN shows a similar but slightly different FT-IR spectrum. The signals at 3426 and 3256 cm-1, corresponding to the stretching vibrations of N–H in amino groups, are observed for


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BCN.25 These amino groups are formed by incomplete polymerization. RCN with fewer defects exhibits weaker absorption in the N–H stretching region. The presence of nitrogen-containing defects was also approved by EA measurements. As summarized in Table 1, the C/N atomic ratio of BCN is 0.653, much lower than the theoretical value (0.75). The extra nitrogen atoms are attributed to the nitrogen-containing defects. With fewer defect, RCN displays a higher C/N atomic ratio (0.685). Heating g-C3N4 at 550 oC in air, surface nitrogen atoms may lose due to the thermal oxidation etching.26 Thus, the surface C/N atomic ratios of RCN and BCN determined by XPS are increased largely to 1.518 and 0.954, respectively. Table 1. EA results of BCN and RCN. N

C

H

Oa

C/Nb

C/Nb,c

(wt%)

(wt%)

(wt%)

(wt%)

RCN

60.86

35.74

1.43

1.97

0.685

1.518

BCN

61.31

34.37

1.87

2.45

0.653

0.954

a

Calculated from the C/H/N EA results. bAtomic ratio. cDetermined by XPS. XPS measurement is an effective method to investigate defect. N1s and C1s XPS spectra of

the samples presented in Figure 6 further confirm the decrease in defects. N1s XPS spectra of BCN and RCN were fitted into three peaks. The peaks at 399.3 and 398.6 eV belong to N atoms in tri-s-triazine units (C=N–C) and N–(C)3, respectively. Both the two N species are involved in the core structure of g-C3N4. The peak at 400.8 eV corresponds to C–N–H, which is derived from the amino defects.27 The content of the N atoms in amino defects for BCN is 23.6%. This value decreases obviously to 14.5% for RCN, indicating that RCN bears fewer defects. Similar result is also obtained by the analysis of C1s XPS spectra. Three peaks were fitted from the C1s spectra. The peak at 284.9 eV is attributed to pure carbon, such as graphite or amorphous carbon (C–C).28 The peaks at 288.1 and 288.6 eV correspond to sp2-hybridized carbon in N=C–N and


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=C–NH, respectively.29 The defect content can be semi-quantitatively analyzed by the area ratio of =C–NH to N=C–N. BCN exhibits a higher ratio (0.76) than that of RCN (0.63), indicating that RCN possesses fewer amino groups.

Figure 6. (a) N1s and (b) C1s XPS spectra of BCN and RCN. The optical properties of the samples were investigated by UV-visible diffuse reflection spectra (Figure 7). BCN exhibits moderate absorption intensity. The band gap of BCN determined by (αhv)1/2 versus photon-energy plots is 2.71 eV, corresponding to an absorption edge of 458 nm. The absorption intensity of RCN improved significantly. It is believed that the special hierarchical micro-nanostructure allows the multiple reflections and scattering of incident light, which results in the enhancement of light harvesting.9,30 The absorption edge of RCN is


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red-shifted slightly to a longer wavelength and the band gap decreases to 2.69 eV. Therefore, more visible light can be collected by RCN. As mentioned above, RCN is composed of nanosheets. It should be known that, compared to bulk g-C3N4, g-C3N4 nanosheets generally exhibit a blue-shifted absorption due to the quantum confinement effect.31,32 We attribute the broadened visible-light responsive range of RCN to the increased crystallinity. The higher crystallinity may induce an improved π-electron delocalization in the conjugated polymer.28

Figure 7. UV-visible diffuse reflection spectra of BCN and RCN.


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Figure 8. (a) XPS valence band spectra and (b) band structures of BCN and RCN. The valence band (VB) of BCN and RCN were examined by VB XPS spectra (Figure. 8). The VB minimum for BCN and RCN are determined to be 2.44 and 2.0 eV, respectively. BCN gives an obvious band tail. This is associated with the presence of defects and terminal groups. The VB of BCN is 0.44 more positive than that of RCN, indicating that the electronic structure is altered significantly. In combination with the band gap dates, the conduction band (CB) maximum of BCN and RCN were calculated to be −0.27 and −0.69 eV, respectively. Apparently, The CB of RCN shifts to a more negative value. The notable difference between the band structures of BCN and RCN may be due to the different crystallinity and defect content. Low-defected g-C3N4 contains less nitrogen, which may led to up-shift of the VB. Quantum confinement effects may also affect the band structure significantly.33,34 The work function of the XPS was 4.20 eV, making 0 V versus NHE equal to -4.50 eV versus vacuum. Thus, the VB potential is calculated


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to be 2.14 V for BCN and 1.7 V for RCN (Figure. 8b). Although the band structure of RCN changed obviously, it is still satisfy the thermodynamic requirements for degradation of RhB and hydrogen evolution.

Figure 9. (a) PL spectra and (b) time-resolved fluorescence decay spectra of BCN and RCN. Optical and electronic properties improvements were further investigated by PL spectra (Figure 9a). PL emission occurs during the recombination of the electron-hole pairs, which could reduce the photocatalytic efficiency.35 Defects such as amino groups may serve as the recombination centres. BCN with a large amount of defects shows a strong PL emission peak centred at 455 nm. The emission peak of RCN is red-shifted to 470 nm, which is in agreement with the UV-visible diffuse reflection spectra. The PL intensity of RCN decreased drastically,


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indicating a lower electron–hole recombination rate.36 Apparently, the decrease in defect concentration is the reason for the suppression of this energy-wasteful process. In addition, reducing the size of the building blocks to the nanoscale level could reduce the transportation distance of the charges, which decreases the possibility of the recombination. Thus, the PL intensity decreases significantly. Time-resolved fluorescence spectra were used to investigate the lifetime of charge carriers. As shown in Figure 9b, the spectra of BCN and RCN were well fitted into double exponential decay model. Obviously, RCN displays a prolonged lifetime. The shorter lifetime is increased from 1.44 ns for BCN to 2.46 ns for RCN and the longer lifetime is increased from 9.82 ns for BCN to 13.12 ns for RCN. This prolonged lifetime means that more photo-generated charges could reach the surface of the photocatalyst and be captured by reactive substrates.

Figure 10. EPR spectra of BCN and RCN. The existence of unpaired electrons in the photocatalyst was proved by EPR spectra (Figure 10). BCN exhibits a symmetrical Lorentzian line centred at a g value of 2.0034. This could be attributed to the unpaired electrons in π-conjugated CN aromatic rings of carbon atoms.37 The Lorentzian line is greatly enhanced for RCN. Thus, high crystallinity and low defect concentration can help to increase the density state of conduction band. The transient


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photocurrent responses of BCN and RCN were recorded for four on-off cycles of intermittent irradiation under visible light. As exhibited in Figure 11, both BCN and RCN show photocurrents with good reproducibility, indicating that the electrodes exhibit steady photoelectrochemical performance under visible light irradiation. The photocurrent density of RCN is about 3.5 times higher than that of BCN. The enhanced photocurrent response of RCN is benefited from the improved conductivity and charge separation efficiency.38,39

Figure 11. Transient photocurrents (λ>420 nm) of BCN and RCN. Photocatalytic Activity of RCN. Degradation of RhB under visible light irradiation is used to evaluate the catalytic activities of RCN. For comparison, the catalysts prepared from melamine (BCN) and urea (UCN) were tested under the same reaction conditions. As shown in Figure 12a, the degradation of RhB is negligible without photocatalyst. RhB can be degraded over all the tested catalysts under visible light irradiation. BCN gives moderate photocatalytic activity. The degradation rate is 23% after irradiation for 2 h. The degradation rate constant was calculated to be 0.0021 min-1. Clearly, RCN shows a much increased photocatalytic activity. Almost all of the RhB were degraded within 2 h. The degradation rate constant is 0.035 min-1, about 16.7 times higher than that of BCN. It is reasonable that, the improved catalytic activity of RCN is attributed to the high SBET (88.6 m2g-1). The high SBET not only provides more available active


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sites, but is also beneficial to the adsorption and diffusion of reactants. The optimized optical and electronic property is another reason for the improved catalytic activity. To verify this, the catalytic performance of UCN with a comparable SBET (71.4 m2g-1) to RCN was examined. UCN is composed of nanosheets. Due to the quantum confinement effect, UCN shows a narrowed visible light responsive range.40,41 RCN with optimized optical and electronic properties exhibits a broadened visible light absorption and an enhanced absorption intensity. As a result, the catalytic activity of RCN increases obviously under visible light irritation compared to that of UCN. Recycle experiment was carried out for degradation of RhB to study the stability of the catalyst. The catalytic activity shows no obvious decrease after four consecutive runs, suggesting the good stability of RCN. The used catalyst was characterized by TEM (Figure 13a). Due to the mild reaction condition, the morphology of the RCN after photocatalytic reaction demonstrates no observable change. To exclude the dye sensitized effect for RCN during the photocatalytic reaction, UV-visible diffuse reflection spectrum of the used catalyst was recorded (Figure 13b). Compared to the fresh catalyst, the used catalyst exhibits no obvious difference in the spectrum. The weak new peak at around 525 nm may be attributed to the adsorbed reaction product. The mineralization ability of RCN was also investigated by TOC analysis. The TOC of the fresh RhB aqueous solution is 5.10 ppm. This value is decreased slowly to 4.81 and 4.53 ppm after irradiation for 2 and 4 h, respectively. The TOC removal rate is much lower than the partial breakdown rate. Therefore, it can be deduced that the RhB is firstly degraded to smaller molecules with high rate during the photocatalytic reaction. The generated small molecules will further be oxidized into CO2 and H2O with much lower rate. The color change of the dye is mainly due to partial breakdown of the organic compound, rather than mineralization. This


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phenomenon has already been observed from other g-C3N4 photocatalysts both with and without co-catalysts. 42,43

Figure 12. Photocatalytic activity of the photocatalysts for (a) RhB degradation and (b) hydrogen evolution. The photocatalytic performance of RCN for splitting of water was further tested. All photocatalysts were loaded with Pt (3%) as the co-catalyst. Triethanolamine (10 vol%) was used as the scavenger. As shown in Figure 12b, BCN shows an obvious visible-light photocatalytic activity. The hydrogen evolution rate (HER) reaches 28 µmol h-1. As expected, the catalytic activity of RCN is much higher. A HER of 246 µmol h-1 is obtained, which is 8.7 times higher than that of BCN. The stability of RCN was examined by performing the reactions for four cycles. A total amount of 1.11 mmol H2 is produced steadily within 5 hours in the first run. This value decreases slightly to 1.04 mmol for the fourth run, suggesting the high stability of RCN against photocorrosion.


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Figure 13. (a) TEM image of the used RCN. (b) UV-visible diffuse reflection spectra of the fresh and used RCN. CONCLUSIONS In conclusion, ultrasound-assisted rearrangement of melem leads to the formation of rod-like precursor with ordered supramolecular structure at molecular level. Less defective RCN with a hierarchical micro-nanostructure is prepared by heating the precursor at 550 oC. The decrease in defects has been well confirmed by powder XRD, FT-IR, EA and XPS. The special hierarchical structure composed of nanosheets provides RCN with significantly increased surface area and developed porosity. The unique physical and chemical structure of RCN broadens the visiblelight responsive range, improves the light harvesting efficiency, suppresses the recombination of electron–hole pairs, and increases the electric conductivity. As a result, the visible-light photocatalytic activities of RCN for RhB degradation and H2 evolution increase 16.7 and 8.7 times respectively, compared to that of BCN. The TOC measurements revealed that the degradation is mainly attributed to the partial breakdown of the RhB molecular. This work has demonstrated a facile and sustainable strategy for fabricating less defective g-C3N4 hierarchical structure with enhanced photocatalytic performance through an ultrasound assisted molecular rearrangement strategy.


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AUTHOR INFORMATION Corresponding Author * Fax: +86-010-62559373. E-mail: [email protected]. * Fax: +86-010-62559373. E-mail: [email protected]. ORCID Zhijun Huang: 0000-0003-2205-6091 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21304101) REFERENCES (1) Wang, X.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J. M.; Domen, K.; Antonietti, M., A metal-free polymeric photocatalyst for hydrogen production from water under visible light. Nature Mater. 2009, 8, 76–80. (2) Zheng, Y.; Liu, J.; Liang, J.; Jaroniec, M.; Qiao, S. Z., Graphitic carbon nitride materials: controllable synthesis and applications in fuel cells and photocatalysis. Energy Environ. Sci. 2012, 5, 6717–6731. (3) Wang, Y.; Wang, X.; Antonietti, M., Polymeric graphitic carbon nitride as a heterogeneous organocatalyst: from photochemistry to multipurpose catalysis to sustainable chemistry. Angew. Chem., Int. Ed. 2012, 51, 68–89. (4) Cao, S.; Low, J.; Yu, J.; Jaroniec, M., Polymeric photocatalysts based on graphitic carbon nitride. Adv. Mater. 2015, 27, 2150-2176.


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No expensive or hazardous reagent is involved in the process for the ultrasound-assisted fabrication of hierarchical rod-like g-C3N4 with fewer defects.


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Figure 1. (a) SEM image of melem. (b,c) SEM and (d) TEM images of melem hydrate. 82x82mm (300 x 300 DPI)



Figure 2. Powder XRD patterns of melem and melem hydrate. 80x60mm (300 x 300 DPI)


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Figure 3. (a,b) SEM and (c,d) TEM images of RCN. 82x82mm (300 x 300 DPI)



Figure 4. Nitrogen adsorption–desorption isotherms of BCN and RCN. 80x58mm (300 x 300 DPI)


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Figure 5. (a) Powder XRD patterns and (b) FT-IR spectra of BCN and RCN. 80x125mm (300 x 300 DPI)



Figure 6. (a) N1s and (b) C1s XPS spectra of BCN and RCN. 80x129mm (300 x 300 DPI)


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Figure 7. UV-visible diffuse reflection spectra of BCN and RCN. 84x63mm (300 x 300 DPI)



Figure 8. (a) XPS valence band spectra and (b) band structures of BCN and RCN. 70x106mm (300 x 300 DPI)


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Figure 9. (a) PL spectra and (b) time-resolved fluorescence decay spectra of BCN and RCN. 80x127mm (300 x 300 DPI)



Figure 10. EPR spectra of BCN and RCN. 80x60mm (300 x 300 DPI)


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Figure 11. Transient photocurrents (λ>420 nm) of BCN and RCN. 80x57mm (300 x 300 DPI)



Figure 12. Photocatalytic activity of the photocatalysts for (a) RhB degradation and (b) hydrogen evolution. 75x98mm (300 x 300 DPI)


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Figure 13. (a) TEM image of the used RCN. (b) UV-visible diffuse reflection spectra of the fresh and used RCN. 82x46mm (300 x 300 DPI)



Scheme 1. Schematic illustration for fabrication of RCN. 84x43mm (300 x 300 DPI)


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Table of Contents 84x29mm (300 x 300 DPI)


Ultrasound-Assisted Fabrication of Hierarchical Rodlike Graphitic

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