Efficient Photocatalytic Hydrogen Evolution on Band Structure Tuned

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Efficient Photocatalytic Hydrogen Evolution on Band Structure Tuned Polytriazine/heptazine Based Carbon Nitride Heterojunctions with Ordered Needle-like Morphology Achieved by an In Situ Molten Salt Method Ailing Jin, Yushuai Jia, Changfeng Chen, Xin Liu, Junzhe Jiang, Xiangshu Chen, and Fei Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07243 • Publication Date (Web): 14 Sep 2017 Downloaded from http://pubs.acs.org on September 18, 2017

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The Journal of Physical Chemistry

Efficient

Photocatalytic

Hydrogen

Evolution

on

Band

Structure

Tuned

Polytriazine/heptazine Based Carbon Nitride Heterojunctions with Ordered Needle-like Morphology Achieved by an In Situ Molten Salt Method

Ailing Jin,1 Yushuai Jia,1 Changfeng Chen, Xin Liu,* Junzhe Jiang, Xiangshu Chen,* and Fei Zhang

Institute of Advanced Materials (IAM), College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang, 330022, P.R. China

[*] Corresponding authors: Email: [email protected] (X. Liu), [email protected] (X. S. Chen) [1] The authors contribute equally to this work.

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Abstract: Polymeric carbon nitride (CN) is a fascinating metal-free photocatalyst for active solar energy conversion via water splitting. However, the photocatalytic activity of CN is significantly restricted by the intrinsic drawbacks of fast charge recombination because of incomplete polymerization. Herein, an in situ ionothermal molten salt strategy has been developed to construct polytriazine/heptazine based CN isotype heterojunctions from low cost and earth-abundant urea as the single-source precursor, with the purpose of great promoting the charge transfer and separation. The engineering of crystallinity and phase structure of CN has been attempted through facile tailoring of the condensation conditions in a molten salts medium. Increasing the synthetic temperature and eutectic salts/urea molar ratio lead to the formation of CN from bulk heptazine phase to crystalline polytriazine imide (PTI) phase, while CN isotype heterojunctions are in situ created at moderate synthetic temperature and salts amount. As evidenced by the measurements of UV-vis DRS and Mott-Schottky plots, the conduction band potentials can be tuned in a wide range from -1.51 to -0.96 V by controlling the synthetic temperature and salts amount, and the apparent band gap energies are reduced accordingly. The difference in band positions between PTI and heptazine phase CN enables the formation of CN heterojunctions, greatly promoting the separation of charge carriers. These metal-free CN heterojunctions demonstrate a well ordered needle-like morphology, and the optimal sample yields a remarkable hydrogen evolution rate (4813.2 μmol h-1 g-1), improved by a factor of 12 over that of bulk heptazine-based CN and a factor of 4 over that of PTI. The enhanced photocatalytic performance can be directly ascribed to the synergistic effect of the improved crystallinity with reduced structural defects, the decreased band gap energy with tunable band positions, and the efficient separation of charge carriers induced by the formation of heterostructures.

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1. Introduction The evolution of hydrogen from water using a photocatalyst and solar energy is an ideal future energy source.1-2 The rational design and synthesis of highly active, stable yet inexpensive photocatalytic materials that operate with visible light continue to be challenges in this field.3-4 Among the various materials that have been reported for active water splitting, polymeric carbon nitride (CN) has received wide attention as it is found to be an efficient, stable, and metal-free organic photocatalyst for hydrogen production upon exposure to visible light.5 Generally, CN can be facilely prepared via thermal polycondensation of nitrogen-rich organic precursors such as dicyandiamide, cyanamide, cyanuric acid, melamine, and urea, with continuous elimination of ammonia. The semiconductor properties of CN arise from the formation of an extended π-conjugated framework consisting of triazine or heptazine units and are largely dependent on the degree of their polymerization.6 However, the characterization of CN frequently reveals the presence of structural defects including unreacted amino and/or cyano groups due to incomplete polymerization, which may stem from the kinetic problems encountered during the bulk condensation process and results in CN with a low crystallinity.7 These structural defects often act as charge trap centers during photocatalytic reactions,8 greatly reducing the charge separation efficiency and thus decreasing the photocatalytic activity. To promote charge separation and improve the photocatalytic performance of CN, one strategy is to synthesize highly crystalline CN with reduced structural defects, which can, in principle, facilitate the kinetics of charge diffusion in both the bulk and on the surface.9-11 Guo et al. reported a novel microwave-assisted thermolysis route to fabricate a highly crystalline CN by using supramolecular aggregates of melamine–cyanuric acid, which shows enhanced photocatalytic hydrogen generation benefited from reduced structural imperfections and efficient separation of electron–hole pairs.9 Controllable preparation of nano-sized morphologies is regarded as another effective route to improve its activity, including mesoporous CN, CN nanorods and CN nanosheets.12-19 The third strategy involves the construction of CN-based heterojunction photocatalysts, achieved by coupling with other semiconductors20-23 or by introduction of dopants or molecules containing heteroatoms in the synthetic process.24-26 The band alignment of two components leads to space charge 3



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accumulation/depletion at the heterojunction interfaces which promotes the separation of photogenerated electrons and holes, subsequently reducing charge recombination.2, 27 Ge et al. reported a CdS quantum dots coupled CN photocatalyst with a hydrogen production rate 9 times that of pure CN owing to the high separation efficiency of charge carriers driven by heterojunctions.28 Zhang et al. synthesized CN-CNS isotype heterojunctions with enhanced hydrogen evolution activity by coating the precursor of CN onto the surface of presynthesized CNS followed by further thermal treatment.29 Dong et al. created a novel CN/CN heterojunction with improved visible light activity for the removal of NO in air by simultaneously thermal condensation of binary precursors of urea and thiourea.30 However, most of these synthetic strategies rely on the addition of extra materials during CN condensation or through a multi-step combination process, and the photocatalytic efficiency present still needs to be further enhanced compared to most inorganic semiconductors. Therefore, it is essential to develop more facile and efficient strategies for the rational design and synthesis of CN photocatalysts, especially from a single-source precursor, to further increase their crystallinity and photocatalytic performances. It is known that band structure is one of the most important intrinsic physicochemical properties of photocatalysts which is directly associated with the light harvesting, charge transfer and the photoreduction and photooxidation ability.24, 31-32 A tunable band structure of CN is beneficial for the forming of heterojunctions with other components as the energy level matching of two semiconductors is significant for the heterojunctions formation. The polymeric nature of CN provides an opportunity for the modification of its band gap and band positions by manipulating the polymerization conditions. One typical example is the tailoring of the polymeric subunits by copolymerization with barbituric acid. The resultant copolymerization products exhibit a remarkable red shift of optical absorption with increasing barbituric acid content.25 Wang and co-workers found that a sulfur mediated synthesis offers an effective approach to adjust the optical and electronic properties as well as the photoreduction and photooxidation of CN, which possesses more positive band positions than that of traditionally derived counterpart.33 Recently, a crystalline CN was synthesized from the condensation of dicyandiamide in a molten salt of LiCl–KCl.34 Further characterization of this material demonstrated that the use 4


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of a molten salt as a high-temperature solvent significantly altered the structure of typical heptazine-based CN polymer to a triazine-based CN (polytriazine imide), PTI.35 After that, many PTI phase crystalline CN were synthesized, where dicyandiamide or melamine were generally used as the precursors.36-38 Unlike kinetically limited solid state reactions, ionothermal synthesis can offer a more appropriate solvent for the CN precursors to increase the crystallinity and condensation degree, which is closely related to the property of band structure. Therefore, based on these advantages, we can envisage that the molten salt method may serve as a feasible strategy to construct CN isotype heterojunctions between different CN phases with both promoted charge separation and tunable band structure, via active control of the condensation conditions in the salt melts medium. In addition, due to the highly polar character of this special solvent, the molten salt solution at high temperature synthesis not only can improve the crystallinity but also provides a novel route to modulate the structures of CN at the nanoscale.39-40 However, to the best of our knowledge, little attention has been given to the ionothermal synthesis of CN isotype heterojunctions with a well-defined morphology and investigating the influences of salt melt conditions on the phase composition and band structure of CN. In the present work, using low cost and earth-abundant urea as the single-source precursor for the first time, metal-free PTI/heptazine based CN heterojunctions with ordered needle-like morphology have been successfully created through an in situ ionothermal molten salt method. The CN isotype heterojunctions exhibit an extraordinary hydrogen evolution rate (4813.2 μmol h-1 g-1), which leads to a factor of 12 over that of bulk heptazine-based CN and a factor of 4 over that of PTI, arising from improved crystallinity and efficient charge separation. Experimental results also show that engineering of phase structure and crystallinity of CN can be achieved by facilely regulating the condensation conditions such as synthetic temperature and eutectic salts/urea molar ratio, resulting in CN with tunable band structures. The difference in band positions between PTI and heptazine phase CN enables the formation of CN isotype heterojunctions at moderate synthetic temperature and moderate salts amount. This study highlights the important influences of molten salt conditions on the phase, band structure and photocatalysis of CN, offers a new approach of constructing band-structure-tunable heterojunctions for sustainable utilization of solar energy, and can be extended to design other 5



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well-defined polymeric heterostructures from inexpensive organic precursors for enhanced performance.

2. Experimental Section 2.1. Synthesis of Photocatalysts Urea, KCl and LiCl·H2O (analytical grade) were purchased from Sinopharm Chemical Reagent Co., Ltd. and were used without further purification. All other reagents used in this research were analytically pure and used as received. Bulk carbon nitride was prepared via thermal polycondensation of urea. Typically, urea (10 g) in an alumina crucible with a cover was heated in air at a ramping rate of 5 °C min-1 to 450 －550 °C in a muffle furnace for 5 h. The yellow product was collected after cooling to room temperature naturally and then ground into powder in an agate mortar. The sample was denoted as bulk-CN-x, where x represents the synthetic temperature, and used for further characterization and performance measurements. The CN isotype heterojunctions were synthesized using an ionothermal molten salt method. A eutectic mixture of KCl and LiCl with the molar ratio of 41:59 has a melting point about 352 °C and was employed as the high temperature solvent for CN condensation reactions. 10 g of urea with different amount of the eutectic mixture KCl/LiCl·H2O (3.1 g/3.6 g, 4.14 g/4.8 g, 6.2 g/7.2 g, 12.4 g/14.4 g and 18.5 g/21.5 g) was finely ground in an agate mortar under the irradiation of infrared lamp. Then the mixture was loaded into an alumina crucible and dried at 120 °C for 10 h in air. After complete evaporation of water, the mixture with a cover was then subjected to a heat treatment at 450－550 °C for 5 h in a muffle furnace at a ramping rate of 5 °C min-1. After it was cooled to room temperature naturally, the product was boiled and washed with deionized water thoroughly to remove residual salts. The resulting yellow powder was collected by pumping filtration, following by drying at 60 °C for 12 h. The obtained sample was denoted as ms-CN-x-y, where x and y refer to the synthetic temperature and the molar ratio of KCl to urea (y=0.25, 0.33, 0.5, 1 and 1.5 for 3.1 g/3.6 g, 4.14 g/4.8 g, 6.2 g/7.2 g, 12.4 g/14.4 g and 18.5 g/21.5 g of KCl/LiCl·H2O adding amount), respectively. 2.2. Characterization X-ray diffraction (XRD) patterns were obtained on a Rigaku RINT-2200 diffractometer 6


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using Cu Kα radiation source at 20 mA and 40 kV. Fourier transform infrared (FTIR) spectra were recorded by Thermo Fisher Scientific Nicolet 6700. The solid-state 13C CP-MAS nuclear magnetic resonance (NMR) spectra were performed on a Varian 400 MHz spectrometer with a sample spinning rate of 10 kHz using a 5 mm zirconia rotor. The microstructures and morphologies of the samples were investigated by the high resolution transmission electron microscopy (HRTEM, JEOL JEM-2100, 200 kV) and the field emission scanning electron microscopy (FESEM, Hitachi SU-8020). Energy-dispersive X-ray spectroscopy (EDS) affiliated to TEM was used to confirm the chemical composition of the sample. The Brunauer–Emmett–Teller (BET) specific surface area of the samples was measured by nitrogen sorption experiments on a BELSORP apparatus at 77 K. The UV-vis diffuse reflectance spectra (DRS) were performed on a JASCO V-750 instrument and BaSO4 was used as the reflectance standard. X-ray photoelectron spectroscopy (XPS) was carried out on a VG-ESCALAB 250Xi instrument with an Al Kα excitation source. The photoluminescence (PL) spectra were measured at room temperature by an Edinburgh Instruments FLS980 Fluorescence Spectrometer with an excitation wavelength of 375 nm. Elemental analysis of CHN was measured by a CHNS/O analyzer (EuroVector EA3000, Italy). The photocurrent performance and Mott–Schottky plots were measured in a three-electrode system on a CHI 660A electrochemical station (Shanghai Chenhua Instruments, China). The electrolyte was a 0.5 M Na2SO4 aqueous solution (pH=7). The Pt plate and saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. The sample was deposited on the fluorine-doped tin oxide conducting glass to serve as the working electrode. The photocurrent response spectroscopy was carried out at a constant potential of +0.5 V to the working photoanode. A xenon lamp (100 mW cm-2) with global AM 1.5 G was used as light source. Mott-Schottky measurements were performed at a frequency of 1 kHz in dark. 2.3. Photocatalytic Hydrogen Production The photocatalytic hydrogen evolution reaction (HER) was carried out in a closed gas circulation system with a top-irradiation-type reactor. 50 mg of the catalyst powder was dispersed in an aqueous solution (100 mL) containing triethanolamine (TEOA, 10 vol%) as the sacrificial electron donor. A co-catalyst of 1 wt% Pt was loaded onto the synthesized 7



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catalysts by an in situ photodeposition method with H2PtCl6. The system was evacuated for 30 min to remove the air prior to light irradiation using a 300 W Xenon lamp with a 420 nm cut off filter. The evolved gases were analyzed by an online gas chromatography (GC-9790, FuLi, China) equipped with a thermal conductive detector, using argon as the carrier gas. 2.4. Photocatalytic Degradation of Methylene Blue (MB) Photocatalytic degradation of MB under visible light irradiation (λ > 420 nm, 300 W Xenon lamp) was also chosen as a probe reaction to test the photocatalytic activities of the as-prepared samples. In a typical experiment, 50 mg of the sample was placed into 100 mL of the MB solution with a concentration of 150 mg L-1. Before irradiation, the suspension was magnetically stirred in dark for 60 min to achieve adsorption-desorption equilibrium between the sample and MB. At a given time interval, 5 mL of suspension was collected and centrifuged to remove the catalyst particles for analysis. The concentration of MB was determined by recording the light absorbance at the characteristic band of 664 nm in UV-vis spectroscopy.

3. Results and Discussion 3.1. Phase, Structure and Morphology Analysis The phase composition and crystallinity of the carbon nitride samples were investigated by XRD. Figure 1c shows the XRD patterns of bulk-CN-x photocatalysts synthesized via thermal polycondensation of urea at different temperature. All three samples exhibit two characteristic peaks at 13.0o and 27.4o, corresponding to (001) and (002) diffraction planes of the heptazine-based CN (or called tri-s-triazine-based CN), which are assigned to the periodic in-plane heptazine stacking and the interlayer structural aromatic packing, respectively.5 It is observed that the full width at half-maximum (FWHM) of (002) peak for the samples becomes narrow as the synthetic temperature increases from 450 to 550 °C, indicating the improved crystallinity of bulk carbon nitride. While a significant change is found in the XRD patterns of ms-CN-x-1 obtained from ionothermal condensation of urea with the same amount of salts mixture (urea 10 g, KCl 12.4 g, LiCl·H2O 14.4 g) at different temperature (Figure 1b). ms-CN-450-1 shows typical peaks at 13.0o and 27.4o, ascribed to the structure of heptazine-based CN. However, a series of sharp peaks appear in the XRD pattern of 8


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ms-CN-550-1 at 12.0o, 20.8o, 24.1o, 24.8o, 26.6 o, 29.3o and 32.2o, which can be indexed to the (100), (110), (200), (111), (002), (102) and (210) planes of the triazine-based PTI phase, respectively.34-35 It is obvious that with elevating of the synthetic temperature the phase structure of the samples change from heptazine-based CN (ms-CN-450-1) to PTI (ms-CN-550-1) with higher crystallinity, which may be due to the higher thermal stability of the PTI compared with the heptazine phase at high temperature under the molten salts synthetic conditions.35 In comparison, the pattern of ms-CN-500-1 synthesized at moderate temperature are assumed to consist of diffraction peaks of both PTI and heptazine phase CN. The peaks at 12.0o, 20.8o, 24.1o, 26.6o, 29.3o and 32.2o can be assigned to the (100), (110), (200), (002), (102) and (210) planes of the PTI, while the peak at 27.4o corresponds to the (002) plane of the heptazine phase CN.

Figure 1. XRD patterns of (a) bulk-CN-500, ms-CN-550-1, and ms-CN-500-y synthesized at 500 °C with different molar ratios of molten salts to urea, (b) ms-CN-x-1 synthesized at different temperature, and (c) bulk-CN-x samples synthesized at different temperature. The peaks marked with  and * represent XRD signals of PTI and heptazine-based carbon nitride, respectively. (d) FTIR spectra of the as-prepared bulk-CN-500, ms-CN-500-1 and ms-CN-550-1. 9



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Furthermore, the influences of molten salts amount on the phase composition and crystallinity of the carbon nitride samples have been explored. The XRD patterns of ms-CN-500-y synthesized at 500 °C with different molar ratios of molten salts to urea are shown in Figure 1a, together with that of bulk-CN-500 and ms-CN-550-1 for comparison. Under the same synthetic temperature, the FWHM of (002) peak for the heptazine-based CN photocatalysts becomes gradually narrower from bulk-CN-500 to ms-CN-500-0.5, indicating that the crystallinity of CN increases with increasing of the molar ratios of molten salts/urea. However, further increasing the amount of salts mixture leads to phase composition changes of CN. Both ms-CN-500-1 and ms-CN-500-1.5 samples are composed of the hybrid of PTI and heptazine-based CN, and the content of PTI in ms-CN-500-1.5 is obviously higher compared with that in ms-CN-500-1 because of a higher adding amount of molten salts. Consequently, the XRD results clearly demonstrate that the phase structure and crystallinity of CN can be readily modulated via active control of the temperature and the eutectic salts/urea molar ratio during the ionothermal synthesis. We find that high temperature or high molar ratio of molten salts/urea favor the formation of PTI phase CN with improved crystallinity and reduced density of surface defects (ms-CN-550-1), while a hybrid of PTI and heptazine phase CN forms under moderate temperature and moderate salts amount (ms-CN-500-1, ms-CN-500-1.5). FTIR measurements were performed to analyze the chemical structure of the as-prepared CN samples (Figure 1d). Three bands can be clearly observed in the FTIR spectra of bulk-CN-500, ms-CN-500-1 and ms-CN-550-1. The strong band around 813 cm-1 corresponds to the out-of-plane ring bending of the triazine unit. The broad peak between 3000 and 3600 cm-1 is related to the N-H stretching vibration of the terminal amine group or the amine group that bridges s-triazine units. The typical stretching modes of aromatic CN heterocycles locate in the region of 1100-1700 cm-1.41-42 The spectrum of ms-CN-550-1 indicates the typical molecular structure of PTI phase. Compared to bulk-CN-500, several new peaks have emerged at 638, 989, 1281, 1377, 1454 and 2175 cm-1. The peaks at 1281, 1377 and 1454 cm-1 are attributed to the characteristic stretching modes of the triazine units.34, 38, 43 The weak band at 2175 cm-1 belongs to the stretching mode of a cyano group (C≡N).43 Although the peaks at 638 and 989 cm-1 in ms-CN-550-1 and the peak at 892 cm-1 in bulk-CN-500 are 10


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difficult to be assigned to any specific species, they indicate the significant difference in chemical structure between bulk-CN-500 and ms-CN-550-1. Moreover, it is noteworthy that the FTIR spectrum of ms-CN-500-1 is made up of peaks of both bulk-CN-500 and ms-CN-550-1, where the peaks at 892, 1239 and 1327 cm-1 are from bulk-CN-500 and the peaks at 638, 989, 1281,1377, 1454 and 2175 cm-1 originate from ms-CN-550-1. Additionally, the vibration band at 892 cm-1 in ms-CN-500-1 is observed to shift to a higher wavenumber compared to bulk-CN-500, indicative of the formation of chemical bonding between heptazine phase CN and PTI instead of a simple physical contact.

Figure 2. Solid-state 13C CP-MAS NMR spectra of (a) ms-CN-500-1, (b) bulk-CN-500 and (c) ms-CN-550-1. The spinning sidebands are marked by asterisks. The heterojunction structure of ms-CN-500-1 was further investigated by solid-state

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CP-MAS NMR, and the results are shown in Figure 2. The NMR spectrum of bulk-CN-500 shows two resolved resonances at 156.7 and 164.5 ppm, corresponding to the C(i) atoms in the CN3 of heptazine units and the C(e) atoms in terminal CN2(NH2) of heptazine units or CN2–NH–C of triazine units, respectively.8 In addition, only one peak is observed at 164.5 ppm in the spectrum of ms-CN-550-1, clearly indicating the presence of PTI units. It is noted that the spectrum of ms-CN-500-1 also shows two similar peaks at 156.7 and 164.5 ppm as bulk-CN-500 but with the latter peak obviously stronger than that of bulk-CN-500, which demonstrates the coexistence of PTI and heptazine units and the formation of PTI/heptazine 11



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based CN heterojunctions.

Figure 3. SEM and TEM images of (a) bulk-CN-500 and (b-g) ms-CN-500-1. The dotted circles in (f) indicate the crystalline domains. The morphological characterizations of the synthesized bulk-CN-500 and ms-CN-500-1 were conducted by field emission SEM. As shown in Figure 3a, the bulk-CN-500 sample derived from the traditional route exhibits a loosely aggregated lamellar morphology with a porous structure. Interestingly, the molten salt synthesis of ms-CN-500-1 results in an obviously different morphology, which consists of dense thick, needle-like rods that are assembled in a highly oriented way (Figure 3b-c). The different morphology between bulk-CN-500 and ms-CN-500-1 can be ascribed to the solvating effect of KCl and LiCl on the growth of carbon nitride. It is frequently reported that the strong polarizing nature of the ionic solvent of molten salts at high temperature facilitates the formation of well-defined nanostructures with highly crystalline character due to the improved mass transport and nucleation processes.39-40 The microstructure of ms-CN-500-1 was further analyzed by bright-field TEM, and the typical images are shown in Figure 3d-g. The obtained products are uniform, needle-shaped nanorods with a length of around 200-400 nm and a diameter of about 10-20 nm. It is expected that the charge transfer would be significantly promoted within the high-aspect-ratio nanorod structures. At higher magnification, individual crystalline domains with distinct lattice fringes can be recognized (indicated by dotted circles), although some part is amorphous (Figure 3f). To understand the composition of the crystalline domains, representative HRTEM image was taken from the needle point and displays the clear spacing of 0.34 nm, which can be assigned to the (002) crystal plane of PTI (Figure 3g). Thus, the 12


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crystalline domains in Figure 3f are likely from PTI while the amorphous domains are probably associated with heptazine-based CN with relative low crystallinity. The clear heterojunction interface is located between the crystalline PTI and amorphous heptazine-based CN. This result confirms the formation of heterojunction between PTI and heptazine phase CN in ms-CN-500-1, which is also consistent with XRD, FTIR and NMR results. The chemical composition of the ms-CN-500-1 sample was studied by EDS spectrum (Figure 4a), indicating the presence of C, N, O, K and Cu elements, among which the Cu signal was from Cu grids used as TEM support for the powder sample. In addition, EDS mapping was employed to characterize the distribution of elements and reveals a uniform distribution of C, N and K in ms-CN-500-1 photocatalyst (Figure 4b-d).

Figure 4. (a) EDS analysis and the elemental mapping images (b) C, (c) N and (d) K of ms-CN-500-1.

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Figure 5. XPS results of (a) the survey spectrum and high-resolution spectra of (b) C 1s ,(c) N 1s, (d) K 2p, (e) Cl 2p, and (f) Li 1s for ms-CN-500-1. To get more insights into the surface chemical states of ms-CN-500-1, the sample was characterized by XPS measurements and the results are shown in Figure 5. The survey spectrum confirms the presence of C, N and O as the major elements in ms-CN-500-1 (Figure 5a). The signals of K 2p and K 2s are very weak in ms-CN-500-1 due to the low content of K+ (2.91%) in the CN framework after thorough washing with boiling water. High resolution spectra were recorded in the C 1s, N 1s, K 2p, Cl 2p and Li 1s regions. The C 1s spectra in Figure 5b can be divided into three peaks at 284.6, 286.1 and 287.9 eV, which are ascribed to 14


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the sp2 C–C bonds from carbon-containing contaminations, the sp3-hybridized carbon from the defects of CN surface, and the sp2-bonded carbon in the N-containing aromatic ring (N−C=N), respectively, with the latter considered to be the major carbon species in the polymeric CN framework.44-45 The N 1 s binding energy can be deconvoluted into four peaks of 398.4, 400.2, 401.0 and 404.3 eV (Figure 5c). The strongest N 1s peak at 398.4 eV corresponds to the sp2-hybridized nitrogen (C–N=C). The N 1s peak located at 400.2 eV is attributed to the tertiary nitrogen (N–(C)3). The N 1s peak at 401.0 eV is indicative of the amino functional groups (C–N–H), consistent with the FTIR measurement. The weakest N 1s peak at 404.3 eV is due to the charging effects.46-47 The sp2-hybridized nitrogen (C–N=C, 398.4 eV), the tertiary nitrogen (N–(C)3, 400.2 eV) together with the sp2-hybridized carbon (N−C=N, 287.9 eV) confirm the presence of the heptazine heterocyclic ring units. The K 2p spectrum exhibits two characteristic peaks of K+ at 292.7 and 295.5 eV in Figure 5d, while the Cl 2p (Figure 5e) and Li 1s (Figure 5f) spectra show negligible signals at 197.6 and 55.1 eV, respectively, indicating the existence of very trace Cl and Li+ in ms-CN-500-1, which is in good agreement with the previous reports.8,

48

According to the structural models of

heptazine-based CN and PTI,8 the theoretical atomic ratio of NC3 to C−N=C is 1:6 for the heptazine-based CN, and the atomic ratio of nitrogen in aromatic ring (NC3 and C−N=C) to amino nitrogen (C–N–H) is 2:1 for the PTI. For ms-CN-500-1, the former value determined from quantitative analysis of N 1s spectrum is 1:7.5, less than that of heptazine-based CN, and the latter value is 7.1:1, larger than that of PTI, which suggests that PTI and heptazine-based structure coexist in ms-CN-500-1. The results of elemental analysis (EA) and synthetic conditions of CNs are summarized in Table 1. Note that the C/N molar ratio as well as the apparent formula of molten salts synthesized ms-CN-500-y is almost the same as that of bulk-CN-500, implying that the molten salt process has not changed the elemental composition of CN samples. Therefore, the results of XRD, FTIR, NMR, SEM, TEM, and XPS together with EA demonstrate the successful preparation of PTI/heptazine based CN isotype heterojunctions with facile in situ molten salt method using urea as the single-source precursor. The bulk carbon nitride sample (bulk-CN-550, bulk-CN-500, bulk-CN-450) consists of heptazine-based CN phase, the ms-CN-550-1 sample is composed of PTI phase, and the ms-CN-500-1 (or 15



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ms-CN-500-1.5) sample consists of PTI/heptazine based CN isotype heterojunctions doped with trace K+ under moderate synthetic temperature and moderate molten salts amount. Table 1. Summary of the Elemental Analysis Results and the Synthetic Conditions of Carbon Nitrides. Sample

Synthetic

nKCl/nurea

temperature (°C )

C

N

H

Atomic

Apparent

(wt. %)

(wt. %)

(wt. %)

C/N ratio

formula

bulk-CN-500

500

0:1

32.018

56.154

7.358

0.67

C3N4.5H8.3

ms-CN-500-0.25

500

1:4

27.840

48.048

6.409

0.68

C3N4.4H8.3

ms-CN-500-0.33

500

1:3

27.375

46.499

7.172

0.69

C3N4.4H9.4

ms-CN-500-0.5

500

1:2

26.970

45.221

6.751

0.70

C3N4.3H9.0

ms-CN-500-1

500

1:1

28.443

47.708

7.030

0.70

C3N4.3H8.9

ms-CN-500-1.5

500

1.5 : 1

27.217

45.481

7.241

0.70

C3N4.3H9.6

ms-CN-550-1

550

1:1

27.265

46.010

2.838

0.69

C3N4.3H3.7

3.2. Visible Light Photocatalytic Activity of CN Heterojunctions Visible-light-induced photocatalytic H2 generation was then carried out on the produced CN samples loading 1 wt% Pt in the presence of TEOA as the sacrificial electron donor. As shown in Figure 6e-f, for the bulk carbon nitride synthesized via traditional thermal polycondensation method, the photocatalytic H2 generation activity increases with increased condensation temperature. The highest HER rate of 1001.0 μmol h-1 g-1 is achieved with the bulk-CN-550 sample due to the improved crystallinity, which is more than two times larger than that of bulk-CN-450 (430.8 μmol h-1 g-1) and bulk-CN-500 (410.2 μmol h-1 g-1). The photocatalytic activity in hydrogen production over molten salt synthesized samples is dependent on the synthetic temperature and the adding amount of eutectic salts. Under the same adding amount of molten salts in urea (urea 10 g, KCl 12.4 g, LiCl·H2O 14.4 g), the H2 production rate of the ms-CN-x-1 samples first increases and then decreases as the synthetic temperature increases from 450 to 550 °C (Figure 6c-d). Notably, ms-CN-500-1 composed of PTI/heptazine based CN isotype heterojunctions shows a HER rate up to 4813.2 μmol h-1 g-1; this value is nearly 8 times higher than that of ms-CN-450-1 with a bulk heptazine-based 16


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structure and about 4 times larger than that of ms-CN-550-1 with a PTI structure. This result clearly demonstrates the advantage of constructing PTI/heptazine CN heterojunctions on photocatalytic H2 generation. The promoted charge separation and migration on the heterojunction interfaces may be one of the main factors that enhance the photocatalytic activity and will be discussed in detail in the following section.

Figure 6. (a,c,e) Time course of photocatalytic hydrogen production under visible light (λ > 420 nm) and (b,d,f) comparison of HER rates of bulk carbon nitride and molten salt synthesized ms-CN-x-y. The H2 production of ms-CN-500-y synthesized at 500 °C with different molar ratios of salts/urea are shown in Figure 6a-b. A steady H2 evolution as a function of irradiation time is observed for the ms-CN-500-y samples. Obviously, increasing the salts amount first gives rise 17



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to the significantly increased HER rates, and further increasing leads to the HER rate decline. Among all the samples, the ms-CN-500-1 photocatalyst exhibits the highest activity under the same reaction conditions, improved by a factor of 12 over the HER rate of bulk-CN-500 with a heptazine-based structure (410.2 μmol h-1 g-1) and a factor of 4 over that of ms-CN-550-1 with a PTI structure (1282.4 μmol h-1 g-1), which is the highest reported HER rate of CN synthesized from a single-source precursor.6,

48-52

In particular, the ms-CN-500-1.5 with

heterojunction structure shows lower activity than that of the ms-CN-500-1, while the initial H2 generation amount of ms-CN-500-1 and ms-CN-500-1.5 are essentially the same during the first 1.5 hours. As the reaction time increases, the H2 generation amount is comparatively reduced for the ms-CN-500-1.5 sample, probably due to the excess content of PTI that may have an adverse effect on the photocatalytic activity enhancement. The above results indicate that the photocatalytic H2 generation activity of carbon nitirde can be effectively improved by tuning of phase structure and crystallinity via ionothermal molten salt synthesis of PTI/heptazine based CN heterojunctions.

Figure 7. (a) Photocatalytic degradation of MB, (b) first-order kinetic plots, and (c) corresponding rate constant k values of bulk-CN-500, ms-CN-500-1 and ms-CN-550-1 under visible light irradiation (λ > 420 nm). The MB dye was used as probing molecule to investigate the photocatalytic degradation property of bulk-CN-500, ms-CN-500-1 and ms-CN-550-1. Remarkably, as illustrated in Figure 7a, ms-CN-500-1 exhibits the best photocatalytic performance and approximately 60% of MB can be eliminated from the high-concentration dye solution (150 mg L-1). The photodegradation reaction follows the pseudo-first-order kinetic equation53 (Figure 7b-c), and the corresponding rate constant is calculated to be 6.37 E-3 min-1 for ms-CN-500-1, which is much higher than that of bulk-CN-500 (2.31 E-5 min-1) and ms-CN-550-1 (4.17 E-4 min-1), revealing the superior photocatalytic degradation activity besides photocatalytic H2 generation 18


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for heterostructured ms-CN-500-1.

3.3. The Mechanism of Photocatalytic Activity Enhancement

Figure 8. (a) UV-vis diffuse reflectance spectra and (b) plots of (αhν)1/2 vs. photon energy (hν) for bulk-CN-500, ms-CN-500-1 and ms-CN-550-1. The light harvesting, energy band structure, surface area and separation efficiency of charge carriers may be the main factors influencing the performance of photocatalysts. Therefore, to probe the origin of the high photocatalytic activity of ms-CN-500-1, the samples were analyzed by UV-vis DRS, Mott–Schottky plots, nitrogen adsorption-desorption isotherms, PL spectra and photocurrent experiments. The optical absorption spectra of ms-CN-500-1, bulk-CN-500 and ms-CN-550-1 are presented in Figure 8a. All the samples exhibit typical semiconductor absorptions in the visible region, which is in good agreement with previous reports.38, 54 The as-obtained ms-CN-500-1 shows the widest band edge at around 465 nm in comparison to bulk-CN-500 at 418 nm and ms-CN-550-1 at 450 nm, which is beneficial for harvesting visible light and enhancing the photocatalytic activity. In particular, the optical properties of ms-CN-500-y are greatly dependent on the conditions of ionothermal synthesis, as indicated by the UV-vis DRS for ms-CN-500-y prepared using different amounts of molten salts (Figure S1). Interestingly, the absorption edge is gradually moved towards longer wavelengths, indicating a decreased apparent band gap with increasing solvent amount. The modified optical properties can also be observed from the color change of the ms-CN-500-y samples (Figure S2). The color of bulk-CN-500 is pale yellow, whereas the color changes to bright yellow, and then brownish yellow for the samples from ms-CN-500-0.25 to 19



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ms-CN-500-1.5. As shown in Figure 8b, the apparent band gap energies (Eg) estimated from the intercept of the tangents to the plots of (αhν)1/2 vs. photon energy are 2.46, 2.67 and 2.65 eV for ms-CN-500-1, bulk-CN-500 and ms-CN-550-1, respectively.55-57

Figure 9. Mott–Schottky plots of (a) bulk-CN-500 and (b) ms-CN-500-1 and ms-CN-550-1. The energetic positions of the valence and conduction band edge of the as-prepared photocatalysts were measured by Mott-Schottky plots. Figure 9 displays the data of bulk-CN-500, ms-CN-500-1 and ms-CN-550-1, which show positive slopes in the linear region of the Mott-Schottky plots, indicative of the nature of n-type semiconductors. Because the flat band potential (EFB) of the n-type semiconductors is close to the bottom edge of the conduction band (CB), which can be calculated by the X intercept when 1/C2 is zero,58 the CB potentials of bulk-CN-500, ms-CN-500-1 and ms-CN-550-1 are calculated to be approximately -1.51 (Figure 9a), −1.00 and −0.96 V (Figure 9b) (vs. SCE, pH = 7), respectively. Combined with the apparent band gap energies obtained from Figure 8b, the valence band (VB) potentials of bulk-CN-500, ms-CN-500-1 and ms-CN-550-1 are determined to be 1.16, 1.46 and 1.69 V (vs. SCE, pH = 7), respectively.

20


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Figure 10. Determined apparent band gap energies and band positions of carbon nitrides. Band gap energies are in eV. It has been reported that the band gap and energy band position are important physicochemical properties of semiconductors that determine photocatalytic activity,59-60 and the results for the as-prepared carbon nitride samples are supplied in Figure 10. It is noticeable that the synthetic temperature and molten salts amount have strong impacts on the apparent band gap energies and the band positions of CB and VB, which are directly associated with the photoreduction and photooxidation ability. As the solvent amount increases, the ms-CN-500-y samples show gradually narrowed band gap and consequently widened absorption band edge compared to bulk-CN-500 (from 2.67 to 2.34 eV), while the CB potentials become more and more positive (from -1.51 to -0.96 V) with ms-CN-550-1 (PTI) possessing the most positive CB position. As a result, the band structures of CN synthesized in KCl/LiCl molten salts system can be successfully tuned by controlling the synthetic temperature and the molar ratios of eutectic salts to urea, which is a direct consequence of the improved crystallinity and phase composition changes of CN as determined by XRD (Figure 1a-b). The molecular engineering in a salt melts medium can alter the condensation of CN, greatly modulating its packing structure and hence adjusting the band structures. Such tunable CB and VB potentials are in favor of tuning the photoreduction and photooxidation ability of CN, thus giving rise to improved performance profiles in photocatalytic water splitting and environmental

pollutant

degradation.

Moreover,

one

can

find

that

bulk-CN-500

(heptazine-based CN) is of the largest band gap and most negative CB potential while 21



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ms-CN-550-1 (PTI) of the most positive CB. The remarkable difference in band positions between PTI and heptazine phase enables the formation of CN isotype heterojunction with well-matched band structure. The specific surface area of bulk-CN-500, ms-CN-500-1 and ms-CN-550-1 were measured by nitrogen adsorption-desorption isotherms. As shown in Figure 11, the typical type III isotherms are observed for all three samples, indicating the presence of porous structures probably caused by the liberation of gases during thermal condensation of urea. The BET specific surface area (SBET) of ms-CN-500-1 is determined to be 35.8 m2 g-1, close to that of bulk-CN-500 (34.4 m2 g-1) and slightly larger than that of ms-CN-550-1 (25.7 m2 g-1). Considering the fact that the HER rate and photodegradation rate constant of ms-CN-500-1 are much higher than that of bulk-CN-500, although both of them have very similar SBET. This indicates that the surface area does not play a primary role in enhancing the photocatalytic performance of CN, because in most solid-liquid phase photocatalysis processes it is the band structure and charge separation rather than mass transfer that essentially determines the reaction rate.61

Figure 11. N2 sorption isotherms of bulk-CN-500, ms-CN-500-1 and ms-CN-550-1. The room-temperature PL emission spectra were performed to investigate the separation and recombination of photogenerated electrons and holes. As illustrated in Figure 12a, a strong and broad emission band is observed for bulk-CN-500 centered at approximately 450 nm, which is related to the radiative recombination of charge carriers with the energy of light close to the band gap of bulk-CN-500. With regard to ms-CN-500-1, the profile of PL 22


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spectrum is similar to that of bulk-CN-500 and ms-CN-550-1, while an evident red shift of the peak position (463 nm) is observed owing to the reduced band gap. Meanwhile, the PL intensity of ms-CN-500-1 shows a remarkable decrease compared with bulk-CN-500 and ms-CN-550-1, implying that the recombination of photogenerated electron-hole pairs is greatly inhibited.

Figure 12. (a) PL emission spectra and (b) transient photocurrent responses of the as-prepared bulk-CN-500, ms-CN-500-1 and ms-CN-550-1. To further evaluate the efficient charge separation in ms-CN-500-1, the transient photocurrents were measured during repeated on/off illumination cycles at a potential of 0.5 V (Figure 12b with the dark current subtracted). All the samples present prompt and reproducible responses to the changes of the external light source. The photocurrent density of ms-CN-500-1 is much higher than that of ms-CN-550-1 and bulk-CN-500. The enhanced photocurrent generated in ms-CN-500-1 further confirms the facilitated separation of photogenerated carriers, which may originate from the effective charge transfer between PTI and heptazine CN unit due to the improved crystallinity, the orientation within the nanorod structures, and the formation of heterojunctions. Therefore, the intrinsic shortcomings of fast charge recombination in polymeric CNs have been addressed via the molten salt construction of isotype heterojunctions, and a better photocatalytic performance has been achieved.

23



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Figure 13. Schematic illustration of photogenerated electron-hole transport and separation at the PTI/heptazine based carbon nitride heterojunction interface. As noted above, the enhanced photocatalytic performance of ms-CN-500-1 is a consequence of the synergistic effect of the improved crystallinity with reduced structural defects, the decreased apparent band gap energy with tunable band structure, and the efficient separation of charge carriers induced by the formation of PTI/heptazine based CN heterojunctions through ionothermal molten salt method. The promotional charge separation mechanism of ms-CN-500-1 is illustrated in Figure 13. Since PTI and heptazine phase CN possess different band positions, the band alignment between the two components leads to the formation of Type II heterojunction. Due to their narrow band gaps, both of them can be excited by visible light to produce photogenerated electron-hole pairs. Once PTI and heptazine phase CN integrate together, the photogenerated electrons tend to migrate from heptazine phase CN to PTI because of the more positive CB potential of PTI, and meanwhile the photogenerated holes transfer from PTI to heptazine phase CN because of the more negative VB potential of heptazine CN. The potential difference is the major driving force for the efficient interfacial charge migration and separation. As a result, the redistribution of electrons on one side and holes on the other side of the heterojunction is achieved, greatly reducing the electron-hole recombination, and thus enhancing the photocatalytic activity. Besides, the improved crystallinity of CN heterojunctions decreases the charge-trap-site density, together with the oriented nanorod structures greatly facilitating charge transfer, which further improves the photocatalytic activity. So we might conclude that the engineering crystallinity and phase composition of carbon nitride through active control of synthetic 24


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temperature and eutectic salts/urea molar ratio will be an effective approach for optimizing photocatalytic properties.

4. Conclusions The novel metal-free PTI/heptazine based CN heterojunctions were constructed through an in situ ionothermal molten salt method and demonstrated with well ordered needle-like morphology. Using easily available and earth-abundant urea as the single-source precursor for the first time, the ionothermal synthesis process is facile, economic, and environmentally benign. This study attempts the engineering of crystallinity and phase composition of CN via active control of the condensation conditions in a molten salts medium, resulting in CN with tunable band structures. Increasing the synthetic temperature and eutectic salts/urea molar ratio favor the formation of CN from bulk heptazine phase to crystalline PTI phase, cause the CB shift to a more positive potential, and reduce the band gap energies accordingly, as evidenced by the measurements of UV-vis DRS and Mott-Schottky plots. The difference in band positions between PTI and heptazine phase CN enables the formation of CN isotype heterojunctions with well-matched band structure at moderate synthetic temperature and moderate salts amount, greatly promoting the separation of charge carriers. The optimized synthesis yields a ms-CN-500-1 heterojunction photocatalyst with a remarkable HER rate of 4813.2 μmol h-1 g-1, improved by a factor of 12 over that of bulk heptazine-based CN and a factor of 4 over that of PTI, which is the highest reported HER rate of CN synthesized using a single-source precursor. The enhanced photocatalytic performance of ms-CN-500-1 can be ascribed to the synergistic effect of the improved crystallinity with reduced structural defects, the decreased apparent band gap energy with tunable band positions, and the efficient separation of charge carriers induced by the formation of heterostructures. The strategy of the facile ionothermal construction of band-structure-tunable heterojunctions offers new opportunities for sustainable utilization of solar irradiation, and could be extended to design other well-defined polymeric heterostructures from inexpensive organic precursors for enhanced performance. These metal-free PTI/heptazine based CN heterojunctions can also be expected to be applied in other solar energy related areas of organic photosynthesis, CO2 photofixation, and photovoltaic devices. 25



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Supporting Information Available: UV-vis diffuse reflectance spectra and color of the ms-CN-500-y samples. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgments The research reported in this work was financially supported by the National Natural Science Foundation of China (No. 21503100), the Natural Science Foundation of Jiangxi Province (No. 20161BAB213071 and 20151BAB213010), the Project of Education Department of Jiangxi Province (No. GJJ150325), and the Sponsored Program for Cultivating Youths of Outstanding Ability in Jiangxi Normal University.

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Efficient Photocatalytic Hydrogen Evolution on Band Structure Tuned

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