Microscopic Revelation of Charge Trapping Sites in Polymeric Carbon

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Energy, Environmental, and Catalysis Applications

Microscopic Revelation of Charge Trapping Sites in Polymeric Carbon Nitrides for Enhanced Photocatalytic Activity by Correlating with Chemical and Electronic Structures Dipak Bapurao Nimbalkar, Monika Stas, Sheng-Shu Hou, Shyue-Chu Ke, and Jih-Jen Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02494 • Publication Date (Web): 07 May 2019 Downloaded from http://pubs.acs.org on May 7, 2019

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ACS Applied Materials & Interfaces

Microscopic Revelation of Charge Trapping Sites in Polymeric Carbon Nitrides for Enhanced Photocatalytic Activity by Correlating with Chemical and Electronic Structures Dipak B. Nimbalkar†‡, Monika Stas⁋, Sheng-Shu Hou†, Shyue-Chu Ke‡*, and Jih-Jen Wu†*

†Department

of Chemical Engineering, National Cheng Kung University, Tainan, 70101,

Taiwan.

‡Department

of Physics, National Dong Hwa University, Hualien, 97401, Taiwan.

⁋Department

of Physical Chemistry and Molecular Modeling, Opole University, Opole,

45052, Poland.

ACS Paragon Plus Environment 1

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

KEYWORDS

polymeric

carbon

nitride,

visible-light-driven

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photocatalyst,

EPR

spectroscopy, SSNMR spectroscopy, DFT calculation

ABSTRACT The influences of chemical and electronic structures on the photophysical properties of polymeric carbon nitrides (PCN) photocatalysts, which govern the microscopic mechanisms of the superior photocatalytic activity under visible-light irradiation, have been resolved in this work. Time-resolved photoluminescence and in-situ electron paramagnetic resonance measurements indicate that the photoexcited electrons in the fractured PCNs swiftly transfer to the C2p localized states where the trapped photoelectrons exhibit longer lifetime compared to those in the ordinary PCNs. Moreover, the structure deviation at the carbon (Cb) atoms around the bridging sites of heptazine ring units, where trapped photoelectrons are localized, has been determined


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in the fractured PCNs based on the

13C

solid-state nuclear magnetic resonance spectra

and the density functional theory calculations. Accordingly, the formation of fractured PCNs by breaking the in-plane hydrogen bonds at a high temperature is a promising strategy for the enhancement of photocatalytic activity.



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INTRODUCTION Polymeric carbon nitride (PCN) exhibits a two-dimensional (2D) layered structure mediating various unique properties.1-5 For instance, PCN has a moderate band gap of 2.7-2.8 eV with a visible-light absorption edge at 450-460 nm.1-5 It also possesses a suitable conduction band edge position which is more negative than the potentials of various reduction reactions, including the reductions of H2O, CO2, and O2.1, 2, 6, 7 As a metal-free semiconductor, PCN has recently attracted considerable attention for the application to the visible-light-driven photocatalyst for artificial photosynthesis and environmental remediation. PCN with the basic tectonic unit of tri-s-triazine (heptazine) can be synthesized by simply thermal polycondensation of abundant carbon nitrogenrich precursors.2, 7 Moreover, it has been evidenced that PCN is a robust material under ambient conditions, i.e., thermally stable up to ~600 oC and chemically stable both in acid and base solutions.1, 4, 7, 8 However, the bare PCN photocatalyst has the intrinsic drawbacks of high recombination rate of photocarriers and poor mass diffusion,2, 7 which impede the practical application in the solar energy conversion.


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It is well-known that the structural, optical, electronic properties of the PCN depend on the degree of condensation of the products.2, 9, 10 The photocatalytic activity of PCN is strongly influenced by the change in synthesis conditions, such as temperature, pressure, atmosphere, and reaction medium.2,

11

While a remarkable impact on the

photophysical performance has been demonstrated by a small modification in the chemical structure of PCN,12,

13

the understanding of the fundamental photophysical

factors controlling the photocatalytic activity of PCN is still deficient. The nature of active sites on the PCN structure as well as the essential criteria to boost the photocatalytic activity of the bare polymeric PCN photocatalyst remain unclear.2,

13-16

Charge carrier

trapping has been reported to play a crucial role in determining the photocatalytic activity of PCN photocatalysts.17,

18

The enhancement of photocatalytic activity by

developing the structural distortion in PCN has been demonstrated as well.2,

9, 19

However, the quantitative analysis of such traps states and the systematic investigations of the structure-dependent dynamics of photogenerated charges in the traps states of PCN are still required. Thus, it is crucial to determine how the characteristics of chemical and electronic structures impact on the photophysical



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properties of PCN photocatalysts for an understanding of the relative charge dynamics mechanisms governing the photocatalytic activity of PCN. A series of PCNs with structural deviation while keeping in-plane heptazine unit intact, which were synthesized by the simple polycondensation of melamine at 500-650 oC, were selected in this work to resolve the aforementioned fundamental issues of PCNs. Rather than the ordinary PCNs obtained at lower temperatures, the CN photocatalyst prepared at 650 oC is mainly composed of fractured PCNs which exhibits the highest photocatalytic activity. Time-resolved photoluminescence (TRPL) and low-temperature in-situ electron paramagnetic resonance (EPR) spectroscopic measurements indicate that the fractured PCNs have an obvious impact on the charge dynamics. Moreover, the structure deviation close to the bridging sites of heptazine ring units by breaking the intralayer hydrogen bonds is determined in the fractured PCNs based on the

13C

solid-

state nuclear magnetic resonance (SSNMR) spectra and the density functional theory (DFT) calculations. The small-dimension characteristic of fractured PCNs not only plays a crucial role in the reduction of the photocarrier recombination rate but also has the benefit to induce an intrinsic driving force around the photoexcited state.


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EXPERIMENTAL SECTION Synthesis of Polymeric carbon nitrides (PCNs). The PCNs were synthesized using the method described in the literature.20 Briefly, 10 g Melamine was put into a crucible covered with a lid and was heated with a 3.5 oC·min−1 ramp rate to the target temperature (400, 450, 500, 550, 600, and 650 oC) for 3 h. Characterization Techniques. The X-ray diffraction (XRD) patterns of the samples were recorded using a Rigaku X-ray diffractometer operating with Cu Kα radiation at 30 kV and 100 mA (λ = 1.54056 Å). Fourier transform infrared (FTIR) spectra were recorded on a Perkin-Elmer FT-IR spectrometer, wherein, the samples were embedded in Potassium Bromide (KBr) pellets. The surface area of the samples was determined by N2 adsorption-desorption isotherms at 77 K on a Micrometric ASAP 2020 instrument. The samples were degassed (10−3 Torr and 120 °C) for 3 h prior to the analysis. The pore size distribution curves were obtained using the BJH (Barrett–Joyner–Halenda) method. X-ray photoelectron spectroscopy (XPS) characterizations were performed using a Kα system (Thermo Scientific) with monochromatic Al Kα excitation and a charge neutralizer. Diffuse reflectance spectroscopy (DRS) (UV-visible) spectra of the



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samples were recorded on a Shimadzu UV-2550 UV−vis spectrophotometer equipped with an integrating sphere assembly, where its absorption spectrum was referenced to BaSO4. The steady-state photoluminescence (PL) spectra were recorded on a Hitachi 4500 FL fluorescence spectrometer with an excitation wavelength of 325 nm. TRPL spectroscopy measurements were conducted using a pulse laser (405 nm) for excitation. The TRPL decays at 440 nm were recorded by a time-correlated single photon counting (TCSPC) spectrometer. In-situ EPR spectra were obtained at X-band using a Bruker EMX spectrometer equipped with a Bruker TE102 cavity and operated at a 100 kHz field modulation. The ‘g’ values were calibrated based on DPPH (1,1diphenyl-2-picrylhydrazyl) sample (2.0023) and have been calculated from resonant field B0 and the resonant frequency ν using the resonance condition hν=gβB0. The samples were irradiated for 10 min by visible light (λ≥420 nm) directly in the cavity of the EPR spectrometer under the same conditions. The

13C

cross polarization/magic angle

spinning (CP/MAS) SSNMR spectra were recorded on a Bruker AVIII400HD spectrometer at resonance frequencies of 400 MHz for 1H and 100 MHz for

13C

using a


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double-resonance magic-angle spinning (MAS) probe with 4 mm o.d. rotors. The MAS spinning speed was set at 12 kHz. Computational Details. Geometry optimizations were performed by DFT through the generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) functional21 combined with the 6-31G (d,p) basis set in gas phase. The calculations were performed with zero-point energy (ZPE) correction. The full geometry optimization was done without any restriction. All optimized structures contain no imaginary frequency. The electronic band gaps were calculated using time-dependent (TD)– DFT.22 The calculations of 13C nuclear isotropic shielding for optimized geometries were performed using B3-LYP level of theory (Becke’s three parameter hybrids function combined with the Lee-Yang-Parr correlation function)23 combined with Jensen’s pcS-2 basis set.24 The calculated magnetic parameters are corrected by the constant scale factor. Full natural bond orbital (NBO) population analyses were also done. All calculations were carried out using the Gaussian 16 program package.25 Photocatalytic Activity Measurements. Photocatalytic activity of the PCN sample was investigated by measuring the degradation of RhB dye solution under ambient



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conditions. 20 mg of PCN catalyst was added in 100 ml of RhB dye solution (10 mg/L aqueous solution). The initial concentration of RhB was denoted as C0. The solution was stirred for 0.5 h in the dark to get the adsorption-desorption equilibrium. A white light LED lamp (visible light: output 65 W) was used as the simulated visible light source. Magnetic stirring of the suspension was continued throughout the reaction. At a defined 1 h time interval, 3 ml aliquots of the solution were collected and measured the concentration of RhB. RESULTS AND DISCUSSION Structural characterizations of the products prepared by polycondensation of melamine at different temperatures were first conducted using XRD and FTIR spectroscopy. Figure S1a shows the XRD patterns of those products. It reveals that the characteristic peaks of melamine and melem derivatives2, 18 appear in the XRD patterns of the products prepared at 400 oC and 450 oC. A weak diffraction peak at 2 = 13.1o and a strong peak at 2 = 27.5o are acquired from the products prepared at temperatures  500 oC, which are pertaining to (210) and (002) planes of PCNs, respectively.26-28 It indicates that the completely structural phase change from melamine


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to PCN occurs at 500 oC. For simplicity, the samples prepared at temperature  450 oC are named as M-T while those produced at temperature  500 oC are named as PCN-T, hereafter. As summarized in Figure 1a, the interlayer d spacing of the PCN-T sample slightly decreases with the preparation temperature, indicating that a denser stacking of the conjugated aromatic system is formed due to the week Van der Waals interaction.2, 20, 29-32

Moreover, the decrease in the intensity ratio of (210) to (002), as shown in Figure

1a, reflecting the disordering of the intralayer structural packing motif, is also observed in the PCN-T samples as increasing the preparation temperature.2, 27 The FTIR spectra of the products are shown in Figure S1c. Rather than the broad bands obtained in the spectra of M-T samples, the sharp peaks appear between 1150 and 1700 cm−1 in the spectra of the PCN-T samples due to the distinctive stretching mode of the heptazine heterocyclic ring (C6N7) units.2, 20, 33 Moreover, the peak at 808 cm−1 is corresponding to the breathing vibration of triazine ring units and the peak at 888 cm-1 is assigned as the deformation mode of N-H in amino groups.34 These strong peaks are the characteristic variations of PCNs.29, 34, 35 The broad bands between at 2900–3300 cm−1 in the spectra



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of the PCN-T samples are the stretching modes of N-H, which have been assigned to the terminal amino groups.20, 29, 35 The compositions and chemical states of PCN-T samples were studied by XPS analyses. Only C and N elements exist in the PCN-T samples have been confirmed by the survey spectra shown in Figure S2a. Figure S2b shows the binding energies of C 1s and N 1s core electrons in the PCN-T samples remain almost the same, suggesting identical chemical states for carbon and nitrogen atoms.9, 35 The dominant C 1s peak at 288.1 eV is pertaining to N=C-N coordination of sp2 hybridized heptazine units.9,

35, 36

The three peaks centered at 398.7, 400.2 and 401.3 eV obtained by deconvolution of the N 1s spectra of the PCN-T samples correspond to the C-N=C, N-(C)3 and C-N-H bonding of heptazine units, respectively.2, 9, 36 The area ratios of these peaks are also displayed in Figure S2b, showing that there is no systematic change in the ratio of CN=C, N-(C)3 and C-N-H bondings of the PCN-T samples with preparation temperature based on the XPS measurements. Figure 1b shows the DRS of the PCN-T samples. The absorption bands with absorption edges at ~460 nm in these spectra are attributed to the intrinsic


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 transition of the conjugated aromatic system.20,

37, 38

Compared to other three

PCN-T samples, the significantly blue-shifted  transition is acquired from PCN650. The blue shift of  transition, which has been recognized to be attributed to the quantum size confinement effect8,

9, 31, 35, 39, 40

occurring as increasing the degree of

thermal decomposition of PCNs, indicates the fractured PCN structure existing in PCN650. The formation of fractured PCNs at 650 oC was also confirmed by the increased specific surface area and pore size of PCN-650 compared to other three PCN samples, as shown in Figures 1c and S3. The absorption band in the wavelength range of 460625 nm, which is assigned to n transition by lone pairs of edge N atoms in the heptazine ring units,9,

37

appears in the spectra of the PCN-T samples prepared at

temperatures  550 oC. The intensity of n transition is enhanced first at 600 oC and saturates as further increasing the temperature to 650 oC. It has been reported that n transition is spatially forbidden for the perfectly symmetric in-plane unit.9, 15, 37, 41 The presence of the absorption band of n transition indicates the formation of an asymmetric planar structure in the PCN-550, PCN-600, and PCN-



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Figure 1. (a) Interlayer d spacings and the intensity ratios of (210)/(002) diffraction peaks obtained from XRD patterns of PCN-T samples. (b) DRS and Tauc plots (inset) of M-T and PCN-T samples, (c) Surface areas and rate constants of visible-light-driven RhB degradation, and (d) TRPL decay curves at 440 nm of PCN-T samples.

650 samples. It is consistent with the XRD results (Figure 1a) that the enriched disordering of the intralayer structural packing motif in the PCN-T samples prepared at high temperatures. Accordingly, the optical absorption characteristics of PCN-650, i.e.,


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blue-shifted  transition and significant n transition; reveal its unique in-planar asymmetric structure. Photodegradation of RhB under visible light irradiation was conducted to evaluate the photocatalytic activities of the PCN-T samples as shown in Figure S4. The RhB concentrations which remain the initial value during 5 h irradiation without adding any photocatalyst is illustrated in this Figure S4. After 5 h irradiation, 31% and 73% of RhB were degraded using PCN-500 and PCN-550 as photocatalysts, respectively. The RhB was degraded 100% after 5 h in the presence of PCN-600 whereas the same condition can be reached in 3 h with the addition of PCN-650 in the RhB dye solution under visible-light irradiation. The apparent first-order reaction rate constants of RhB photodegradation in the presences of these PCN catalysts were estimated using the kinetic data acquired within 3 h due to the fast photodegradation conducted by PCN650. The reaction rate constants and specific surface areas (Figure S3) of the PCN-T photocatalysts are illustrated in Figure 1c for comparison. Apparently, the reaction rate constants exhibit a positive correlation with the specific surface areas. Both specific surface area and activity rate constant of PCN-600 are 2.7 times larger than those of



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PCN-550. However, PCN-500 shows much lower reaction rate constant due to its inferior light absorption ability at   420 nm. With 50% less specific surface area, the reaction rate constant of PCN-500 is one-third that of PCN-550 catalyst. On the other hand, the reaction rate constant of PCN-650 increases 2.2-fold compared to PCN-600 although the surface area only enlarges 1.45 times. Accordingly, in addition to the light absorption ability and the specific surface area, there are other factors influence the photocatalytic activities of these PCN-T photocatalysts. The steady-state PL measurements were conducted to examine the photocharge recombination in the PCN-T samples. With an excitation wavelength of 325 nm, as shown in Figure S5a, the PL bands centered at ~460 nm are acquired from these PCNT samples, which are ascribed to  band-to-band) recombination of electrons and holes.15, 20 The emission intensities of the PCN-T samples show the order: PCN-500 > PCN-550 > PCN-600  PCN-650, suggesting that charge recombination is suppressed in the PCN-T samples prepared at higher temperatures. As shown in Figure S5b, the shoulders of the emission bands extending to longer wavelengths are clearly presented in the normalized PL spectra of the PCN-T samples prepared at higher temperatures. It


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indicates that multiple radiative transitions take place in those PCN-T samples after the  stimulation.15, 20, 42 Moreover, a small tail in the shorter wavelength ranging from 420 nm to 380 nm is also shown in the PL spectrum of PCN-650.8, 35, 43 The broader emission band of PCN-650 suggests its distinct electronic structure compared to other PCN-T samples, which is consistent with the DRS results in Figure 1b. The dynamics of charge recombination in the PCN-T samples were investigated by TRPL using a 405 nm pulse laser for excitation. The TRPL decay curves of these PCNT samples at 440 nm are shown in Figure 1d. The average PL lifetimes of PCN-500, PCN-550, PCN-600, and PCN-650 are 1370, 905, 492, and 739 ps, as respectively summarized in Table S1. Compared to PCN-500, the shortening in  emission lifetime acquired in the other three PCN-T samples ensures that there are fast nonradiative deactivation processes taking place before the  transition. The possible nonradiative-relaxation pathways in PCN-T samples are that photocharges trap to the localized states.2, 15, 44, 45 Accordingly, more efficient charge separation occurs in PCN-600, PCN-650, PCN-550, and PCN-500 in sequence. The order of the charge separation efficiencies: PCN-600 > PCN-550 > PCN-500 is consistent with the decrease



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in the emission intensity of the steady-state PL band centered at ~460 nm (Figure S5a). It confirms that the charge recombination is inhibited through efficient charge separation.13 Interestingly, PCN-650 shows a longer average PL lifetime but comparable steady-state emission intensity with PCN-600. It indicates that the concentration of steady-state photogenerated charges in PCN-650 is lesser than that in PCN-600, which again suggests the dissimilar electronic structure of PCN-650. To investigate the natures of charge location and dynamics of the PCN-T samples, EPR spectroscopic measurements were carried out at 77 K in the dark and under visible light (  420 nm) irradiation conditions. As respectively revealed in Figure 2a and Figure S6, an isotropic EPR signal at g-value ~2.004 were acquired from the PCNT samples, which is apparently pertaining to the unpaired electron on the  conjugated state of PCN aromatic rings with a dominant C2p character.9,

17, 18, 46

For quantitative

analysis of the photoexcited electrons localized in the C2p states of the PCN-T samples, the variation of double integrated EPR signal intensity was acquired in the dark and under visible light irradiation as shown in Figure 2b. In the dark, as reducing the degree of polymerization by thermal decomposition, the population of the trapped C2p state in


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the samples increases with the preparation temperature up to 600 oC and afterward decreases at 650 oC. Under visible-light irradiation, PCN-500, PCN-550, and PCN-600 possess almost identical quantities of photoexcited electrons trapped in the C2p states, whereas PCN-650 exhibits less trapped photoelectrons. It has been reported that the C2p states with the photoexcited electrons mainly perform as the active centers that drive the photocatalytic activity of the PCN photocatalysts.17,

18

According to the EPR

results shown in Figure 2b, however, PCN-650 with



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Figure 2. (a) EPR spectra of M-T and PCN-T samples recorded at 77 K in dark. (b) Double integration intensities of EPR signal intensities in dark (blue color) and under visible light (hv) irradiation (λ  420 nm) (cyan color + blue color). The original Vis-dark EPR spectra are given in Figure S6. (c) Time evolution of in-situ EPR signal intensities


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of PCN-T samples during visible light on and off as well as fitting curves with doubleexponential decay.

the highest photocatalytic activity exhibits the lower concentrations of the trapped C2p state in the dark as well as under visible light. Thus, the recombination dynamics of the C2p trapped photoelectrons in these PCN-T samples were further investigated by exploiting EPR spectroscopy at 77 K. In-situ monitoring of the EPR intensity contributed by the C2p trapped electrons was conducted with visible light on/off in a static magnetic field hold on to the central component of the EPR signal17 as denoted by an arrow in Figure 2a. The time evolutions of the normalized EPR intensities are presented in Figure 2c. During 10-min irradiation, the EPR intensity of PCN-650 reaches the saturated level faster compared to PCN-500, PCN-550, and PCN-600, as revealed in Figure 2c. These results are consistent with the variations of the double integrated EPR signal intensities of each PCN-T sample acquired under visible-light irradiation and dark displayed in Figure 2b (cyan color), showing less population of the photoexcited C2p trapped



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electrons in CN-650 compared to the other three PCN-T samples. The annihilations of C2p trapped electrons EPR signals were acquired after visible light off, as shown in Figure 2c, which demonstrates slow decay patterns due to liquid nitrogen (77 K) environment. The lifetime of the photoexcited C2p trapped electrons in the PCN-T samples are extracted from the normalized bi-exponential decay curves of the C2p trapped electron EPR signals shown in Figure 2c. As listed in Table S2, the lifetimes of the photoexcited C2p trapped electrons in the PCN-T samples show the order: PCN-650 > PCN-600 > PCN-550 > PCN-500. Interestingly, the photoexcited electrons trapped in the C2p states of the PCN-650 exhibit the longest lifetime although the total number of the photoexcited C2p trapped electrons in PCN-650 is the lowest among these PCN-T samples. As aforementioned results, PCN-650 with the highest photocatalytic activity possesses a rather distinct electronic structure than others. It suggests that the unique chemical structure of PCN-


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Figure 3.

13C

CP-MAS SSNMR spectra of PCN-T samples. Inset: assignments of

carbon atoms.

650 with a high concentration of the active sites on the molecular level of the conjugated aromatic rings is responsible for the superior photocatalytic activity. However, no significant difference in chemical structure is resolvable from FTIR and XPS analyses due to their spectroscopic confines, as respectively shown in Figures S1c and S2. Accordingly,

13C

CP-MAS SSNMR spectra of the PCN-T samples were further

recorded to probe the microscopic structural characteristics of the PCN networks. As shown in Figure 3, two main

13C

signals at 164.4 and 156.3 ppm which are attributed to



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Ce and Ci backbone skeletons denoted in the inset are obtained in the spectra of all PCN-T samples.29,

42, 47-49

Moreover, no signal appears in the typical triazine-based

region of ~168 ppm in these

13C

SSNMR spectra.48 It confirms that only heptazine

based building blocks exist in the PCN-T samples. Interestingly, a peak at 162.4 ppm is getting stronger in the

13C

SSNMR spectra of the PCN-T samples prepared at higher

temperatures. Compared to identical terminal Ce atoms of heptazine based building units, the slightly upfield shift of the peak at 162.4 ppm reflects that carbon entities are hindered or shielded. Based on the DFT calculation, this signal is pertaining to Cb carbon atoms as shown in the inset of Figure 342,

50

and the details of which will be

discussed later. In comparison with the PCN-T samples prepared at lower temperatures, the distinct signals of Ce, Cb, and Ci in the

13C

SSNMR spectrum of

PCN-650 indicate that the local chemical environments of PCN networks are changed and structural motif of PCN-650 is no longer symmetric.42, 49, 50 DFT calculations were performed on various geometric structures of carbon nitrides, including bend, zigzag, and hypothesized graphitic carbon nitride (g-C3N4) models as shown in Figure S7, to provide an insight into the local structure of heptazine skeletons.


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The details of the calculations are given in supporting information. The chemical structure of the carbon nitride composed of the strands of heptazine ring units with the zigzag arrangement is the only geometric assembly that satisfies the experimental results obtained in this work without any symmetry restriction.29 The stabilized longrange 2D planar zigzag configuration and reliable band gap energies are well corroborated by the experimentally observed intraplane (210) diffraction26,

27in

XRD

patterns (Figure S1a and b) and the absorption edges in DRS spectra (Figure 1b), respectively. From the structural point of view, the alternating arrangement of the heptazine units through NH-bridge bondings in the zigzag fashion seems to be necessary to release the strain as well as to keep the structure in a planar configuration. The orbital characters of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for melem oligomers are shown in Figure S7. It illustrates that the 2p orbitals of sp2 nitrogen on the heptazine ring dominate the HOMO, while the orbitals of sp2 nitrogen and carbon atoms contribute to LUMO.9, 26 Moreover, the HOMO and LUMO of the tetramer (4x1) oligomer, as also shown in Figure 4a,



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clearly reveal that charge density is mainly localized around the adjacent atoms of bridging sites of heptazine ring units. The

Figure 4. (a) DFT optimized zigzag structure for tetramer (4x1) oligomer and their corresponding HOMO and LUMO distributions. (b) Schematic of atomic positions in optimized zigzag 2D tetramer (4x2) melem oligomer. Gray, blue and white balls represent C, N, and H respectively.


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calculated band gaps of the optimized structures of the melem oligomers with zigzag arrangement are presented in Table S3. When decreasing the length of melem oligomer, the band gap of the melem oligomer increases as a result of the positively shifted LUMO energy level and almost unchanged HOMO energy levels, as shown in Figure S8. The positive shift of LUMO energy levels suggests the reduction ability of the photoelectrons in the small melem oligomers is enhanced.43 Furthermore, the calculated atomic distances and bond angles of the melem oligomers, as illustrated in Figure 4b, are calculated by DFT. The results are listed in Table S4. Compared to the 2D tetramer (4x2), CN zigzag structure exhibits a longer distance in closer N1-N2 atoms as well as a larger bond angle between C1-N3(H1)-C2 networks in the 1D-layered tetramer (4x1) as denoted in Figure 4b. This suggests that the structural deviations at the bridging sites of heptazine ring units are mainly caused by the repulsion of nitrogen lone pairs in close N1-N2 contacts.49 The simulations of the

13C

chemical shifts in the stabilized 1D and 2D zigzag CN

structures were further conducted using DFT method. As shown in Figure 5a, three distinct carbon resonances of Ce, Cb, and Ci atoms in the 1D layer tetramer (4x1)



oligomer are determined by the DFT simulation. The calculated

Page 28 of 48

13C

chemical shifts of

the Ce, Cb, and Ci atoms in a 1D tetramer are consistent with those experimentally acquired from the

13C

SSNMR spectrum of PCN-650, as illustrated in Figure 3.

Therefore, a strong agreement between DFT and SSNMR results evidently reveals that the carbon resonance at 162.4 ppm is ascribed to the bridging-site carbon (Cb) atoms. This assignment is consistent with the recent work resolved from 2D

13C-15N

SSNMR

correlation spectrum.42 The slightly upfield shift of the Cb peak compared to Ce peak may be ascribed to the more charge density distributed at the deviated structure which reduces the interaction between the proton and the adjacent carbon atom nucleus. It should be addressed that the simulated chemical-shift parameters remain unchanged if the length of the 1D melem oligomer increases. Nevertheless, when more melem oligomers forming the 2D zigzag fashion (tetramers (4x2)) as shown in Figure 5b, some Cb atoms slightly down-field shift back to ~164.4 ppm, resulting in the decrease in the yield of chemical shift at ~162.4 ppm compared to that of 1D zigzag CN structure. Accordingly, the DFT results support the SSNMR analyses that the


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Figure 5. DFT calculated 13C NMR chemical shift parameters (Ce= ~164.4, Cb= ~162.4, and Ci= ~156.3 ppm) for melem oligomers (a) 1D tetramer (4x1), and (b) 2D tetramer (4x2). The NBO charge distributions of DFT optimized oligomer structures: (c) 1D and (d) 2D tetramers. Gray, blue, and white balls represent C, N, and H, respectively.

peak at 162.4 ppm is getting stronger with fractured PCNs prepared at higher temperatures. The DFT results suggest that the chemical shift of Cb atom in the PCNs is influenced by the formation of the (N-H)•••N intralayer hydrogen bonding through the



Page 30 of 48

bridged NH of the 1D zigzag CN structure with the N atom in the heptazine ring of the adjacent 1D zigzag CN within the 2D structure, as denoted by red dotted line in Figure 5b. The NBO charge distributions on the 1D-tetramer (4x1) and 2D-tetramer (4x2) oligomers are shown in Figures 5c and 5d, respectively. In the 1D oligomer, highdensity negative charges are distributed at the adjacent atoms of bridging sites of the heptazine ring units to form the nucleophilic sites. On the other hand, the positive charges are mainly distributed at the tertiary heptazine nitrogen atoms which act as the electrophilic sites. Obviously, the negative charges distributed at the bridging sites of the oligomer are getting less once the 2D zigzag fashion is formed. Only those bridging sites on the edge of the CN sheet remain the negative-charge-rich regions. Therefore, the NBO analyses suggest that a high concentration of the nucleophilic sites is achievable by the formation of fractured PCNs. Based on the aforementioned results, the structural geometries and dimensions of the PCN-T samples, which are determined by the preparation temperatures, have direct correlations with the charge dynamics and photocatalytic activities. Compared to the


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PCN photocatalysts prepared at low temperatures  600 oC, PCN-650 with fractured PCNs demonstrates superior photocatalytic activity under visible light irradiation. TRPL results (Figure 1d) indicate that the photoexcited electrons in fractured PCNs are able to swiftly transfer to the C2p localized states to impede their recombination with holes. According to EPR measurements (Figure 2), the concentration of the photoexcited electrons in the C2p states of fractured PCNs is less than those in PCNs, which may be attributed to the significantly blue-shifted  transition (Figure 1b), as a result of fewer photoelectrons generated under visible-light irradiation. Nevertheless, the photoelectrons in the C2p states of fractured PCNs exhibit a longer lifetime compared to those of PCNs, which mainly contributes to the superior photocatalytic activity of fractured PCNs. The results of

13C

SSNMR measurements (Figure 3) and DFT

calculations (Figures 4, 5a-b) further indicate that the trapped C2p photoelectrons are localized at the carbon atoms (Cb) around the bridging sites of heptazine ring units. By the correlations between experimental and theoretical results, the PCNs formed by the polycondensation of melamine at 500-650oC possess stabilized long-range 2D planar zigzag configuration, as shown in Figure S9. Moreover, the structure deviation of



Page 32 of 48

fractured PCNs by breaking the intralayer hydrogen bonds at high temperature, which increases the charge densities of Cb atoms, is correlated with the photoelectrons with longer lifetime trapped at C2p localized states of PCN-650. This is the crucial factor resulting in the enhancement of the photocatalytic activity of PCN-650 compared to those prepared at lower temperatures. The resolved correlation in this work is consistent with the recent reports that breaking hydrogen bonds in the CN network enhances the photocatalytic activities of disorder/amorphous CNs.51-53 The simulations of NBO charge distributions further indicate that the fractured PCNs increases the density of nucleophile sites (Figures 5c-d). These nucleophilic sites which are beneficial to the surface adsorption may also play a role in the photocatalytic activity of fractured PCNs. In addition to the high-density nucleophilic sites and the long lifetime of the trapped photoelectrons, the characteristics of the shorter photocharge transfer paths as well as the higher surface area (Figure 1c) also contribute to the superior photocatalytic activity of fractured PCNs photocatalyst. CONCLUSIONS


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In this work, PCN photocatalysts with structural deviation while keeping in-plane heptazine unit intact were synthesized by the polycondensation of melamine at 500-650 oC.

The microscopic mechanism for the enhanced photocatalytic activity of the fractured

PCNs prepared by thermal decomposition of PCNs at high temperature has been revealed by correlating with the chemical and electronic structures. The structure deviation of fractured PCNs by breaking the intralayer hydrogen bonds at high temperature increases the charge density of the carbon (Cb) atoms close to the bridging sites of heptazine ring units. It is correlated with the photoelectrons with longer lifetime trapped at C2p localized states of fractured PCNs, i.e. PCN-650, which is the vital aspect for the enhancement of the photocatalytic activity of PCN-650 compared to those prepared at lower temperatures. Moreover, the increase in the density of nucleophilic sites can be achieved by the formation of the fractured PCNs. Accordingly, the formation of fractured PCNs by breaking the intralayer hydrogen bonding chains is a promising strategy for the enhancement of photocatalytic activity.



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ASSOCIATED CONTENT

Supporting Information.

The characterization results of XRD analysis, chemical analysis, BET data, photocatalytic activity, steady state and normalized PL spectra, EPR spectra, and DFT calculations of PCNs as well as the schematics of ordinary PCN and fractured PCN were added in the supporting information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author * Email: [email protected] and [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT


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We thank Prof. Chia-Hung Lee for providing the BET data and Dr. Hsun-Wei Cho for his technical support in TRPL measurements. This project has received funding from the Ministry of Science and Technology in Taiwan under Contracts No. MOST-106-2811-E006-057, MOST-105-2221-E-006-251-MY3, MOST-105-2112-M-259-007-MY3. DFT calculations were carried out using resources provided by Wroclaw Centre for Networking and Supercomputing (http://wcss.pl), grant No. 478.

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Table of Contents (TOC)


Microscopic Revelation of Charge Trapping Sites in Polymeric Carbon

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