Design of a Unique Energy-Band Structure and Morphology in a

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Design of a Unique Energy-Band Structure and Morphology in a Carbon Nitride Photocatalyst for Improved Charge Separation and Hydrogen Production Jesus Barrio, Lihua Lin, Xinchen Wang, and Menny Shalom ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02807 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 8, 2017

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ACS Sustainable Chemistry & Engineering

Design of a Unique Energy-Band Structure and Morphology in a Carbon Nitride Photocatalyst for Improved Charge Separation and Hydrogen Production Jesús Barrioa,b, Lihua Linc, Xinchen Wangc, Menny Shaloma*. a.

Department of Chemistry and Ilse Katz Institute for Nanoscale Science and Technology,

Ben-Gurion University of the Negev, Beer-Sheva 8410501, Israel. Email: [email protected] b.

Colloid Chemistry Department, Max Planck Institute for Colloids and Interfaces, 14424

Potsdam, Germany. c.

College of Chemistry, Fuzhou University, Gong Ye Road 523, Fuzhou, Fujian, Fuzhou

350002 P. R. China KEYWORDS (Carbon nitride, rational materials design, photocatalysis, supramolecular chemistry)

ACS Paragon Plus Environment

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ABSTRACT

We report the facile and environmental-friendly synthesis of an efficient carbon nitride photocatalyst for hydrogen production and dyes degradation by using a unique supramolecular assembly with an element gradient as the reactant. The element gradient is acquired through the selective removal of barbituric acid from the surface of a supramolecular assembly that comprises barbituric acid, melamine, and cyanucric acid, using hydrochloric acid as a surface modifier. The tailored design of the supramolecular aggregate results in inner and outer parts, which have carbon-rich and carbon-poor domains, respectively. Structural and optical investigations of the new assemblies reveal that the hydrogen–chlorine interaction generates a carbon gradient through the starting supramolecular assembly and to a better packing and structural alignment of the supramolecular units. Detailed X-ray photoelectron spectroscopy and photophysical studies of the final carbon nitride-like materials after calcination at 550 °C indicate that the element gradient across the starting precursor directly projects on the final carbon nitride chemical and element composition, as well as on its optical and photocatalytic properties. The spatial arrangement of the starting monomers leads to the formation of a unique energy-level structure in the final material, which is intended to improve the efficiency of charge separation under illumination and, thereby, result in a strong enhancement of photocatalytic activity toward a high hydrogen production and fast dyes degradation. This work provides new opportunities for the rational design of carbon nitride and other metal-free materials with unique


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and controllable chemical, optical, and catalytic properties for sustainable energy-related applications.

INTRODUCTION A photocatalytic water-splitting reaction, in which sunlight is directly converted into hydrogen, is one of the most promising and economically feasible future technologies for producing clean and renewable energy

1-3

. Such a photocatalytic production of hydrogen requires low-cost,

robust, and highly efficient semiconductors, which should have good charge separation properties and that can transfer charge rapidly at the semiconductor/liquid interface, display long-term stability, possess good light-harvesting properties, and have a suitable energy band position for the desired reaction

4-10

. One of the ways to manipulate charge separation under

illumination is by creating electronic junctions that facilitate charge flow in a given direction and suppress undesired recombination processes

11-14

. Despite significant progress in this field, the

design of semiconductors with good electronic and catalytic properties is still considered a major challenge, and novel approaches for the production of such semiconductors are much sought after. Over the past few years, graphitic carbon nitride (g-CN) has attracted widespread attention due to its outstanding electronic properties, which have been exploited in various applications, including in photo- and electro-catalysis catalysis

23, 24

, CO2 reduction

25

15-19

for pollutants degradation

, water splitting

26-32

20-22

, light-emitting diodes

33

heterogeneous

, solar cells

34

,


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different organic reactions35, 36 and sensing applications

37, 38

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. g-CN comprises only carbon and

nitrogen and can be synthesized by a direct calcination of C- and N-rich monomers, such as urea, cyanamide, dicyanamide, or melamine

24, 39

. However, the solid-state reaction at high

temperatures usually yields unordered materials with large grain boundaries, leading to low photocatalytic activity. Much progress in the synthesis of g-CN has been recently made by utilizing hard and soft templating methods molecules

55-58

40-45

, introducing additional heteroatoms

46-54

and

that can affect the electronic structure and catalytic activity, and using a

supramolecular preorganization of the g-CN monomers (melamine, cyanuric acid, and others) prior to their calcination at high temperatures

59-61

. The supramolecular route relies on a non-

covalent interaction between the g-CN monomers, which can lead to versatile morphologies and structures 62-67. Moreover, the sequence of the monomers in the starting supramolecular assembly can directly project onto the electronic and photocatalytic properties of the final g-CN product by creating beneficial electronic structures, such as heterojunctions 68, 69 and defect/surface states 70, thus allowing a rational design of the photo-physical and catalytic properties of the final g-CN product by appropriately selecting the starting monomers. The supramolecular approach enables the large-scale synthesis of highly ordered materials, such as hollow spheres and tubes, rods, and needles, without any further templating-based techniques, hazarded chemicals (i.e. acid/base), organic solvents or metal salts. The structure of the supramolecular assembly is determined by the interaction between the monomers and by their solubility in a given solvent (including the solvent–monomer interactions)

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. The final g-CN product can inherit the sequence of the

monomers in the starting supramolecular assembly, thereby allowing the creation of unique electronic and photo-physical properties of the final g-CN product by a smart design of the monomers.


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Here, we demonstrate the synthesis of a photoactive g-CN with a unique energy bands structure, which leads to a good catalytic activity. We synthesized the modified g-CN by utilizing a novel supramolecular assembly, in which the g-CN precursor comprises an atomic gradient of carbon and nitrogen across the assembly. We obtained this assembly by manipulating the interactions between hydrochloric acid, melamine, cyanuric acid, and barbituric acid in water. X-ray photoelectron spectroscopy (XPS) depth profile studies unravel that the formation of an atomic gradient occurs due to a specific replacement of barbituric acid solely by a Cl- anion in the pristine supramolecular assembly. A thermal gravimetric analysis (TGA), coupled with Fourier transform infrared spectroscopy (FTIR), indicates that the strong interaction between melamine and Cl- dramatically suppresses melamine sublimation and degradation during the calcination process. We demonstrate that changes in the sequence of the monomers and in the atomic gradient in the starting materials is projected on the final energy levels structure of the final gCN product. This novel structure enhances g-CN photo-activity for dye degradation and hydrogen production due to a better charge separation of photo-excited charges. EXPERIMENTAL SECTION Synthesis of catalysts. All supramolecular assemblies were prepared by using a 1:1:0.1 (molar ratio) of cyanuric acid:melamine:barbituric acid (CMB), as described previously 68, in 50 ml of acidified solutions with various volumes of HCl 37%: 0 ml ('CMB'), 1 ml (2% v/v; CMB– HCl2%), 2.5 ml (5% v/v; CMB–HCl5%), or 5 ml (10% v/v; CMB–HCl10%), corresponding to chlorine:CMB ratios of 0.6, 1.5, and 3, respectively. Then, the complexes were mixed for 4 h with an automatic shaker and centrifuged for 5 min at 6000 rpm. The resulting powders were dried overnight at 60 °C in a vacuum oven and then calcined at 550 °C for 4 h (heating rate: 2.3 °C/min) under an inert nitrogen atmosphere.


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CHARACTERIZATION X-ray diffraction patterns were measured on a Bruker D8 Advance instrument, using Cu Kα radiation. Nitrogen sorption measurements were taken with N2 at 77 K after the samples had been degassed at 150 °C in vacuum for 20 h, using a Quantachrome Quadrasorb SI porosimeter. The apparent surface area of the final g-CN products was calculated by applying the BrunauerEmmett-Teller (BET) model to the isotherm data points of the adsorption branch. Elemental analysis was accomplished as a combustion analysis using a Vario Micro device. SEM images were captured on a LEO 1550-Gemini instrument. TEM images were recorded on a Tecnai 12 TWIN instrument. AFM images were recorded on Dimension 3100 SPM with Nanoscope 4 controller. EDS was measured using an OXDORD instrument in a FEI VERIOS 460L. The FTIR spectra used for compound characterization were recorded on a Varian1000 Spectrometer. Optical absorbance spectra were measured with a Varian spectrophotometer equipped with an integrating sphere. The emission spectra were recorded on an LS-50B PerkinElmer instrument. Fluorescence lifetimes were recorded by using Fluorolog TCSPC HORIBA Scientific instrument. The excitation and detection wavelengths are 370 nm and 470 nm, respectively. The XPS data were collected by using an X-ray photoelectron spectrometer ESCALAB 250 ultrahigh vacuum (1×10-9 bar) apparatus with an AlKα X-ray source and a monochromator. The X-ray beam size was 500 µm and survey spectra was recorded with a pass energy (PE) of 150 eV and high energy resolution spectra were recorded with a PE of 20 eV. To correct for charging effects, all spectra were calibrated relative to a carbon C 1s peak, positioned at 284.8 eV. The depth profile of the sample was obtained by combining a sequence of Ar ion gun etch cycles interleaved with XPS measurements from the current surface. The sputtering rate was approximately 0.07 nm/sec. The XPS results were processed by using the AVANTGE software.


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Mass-loss and spectroscopic data of the functional groups were obtained in a Thermogravimetric Analyzer Q500 coupled with an FTIR Microscope Nicolet iN10. Hydrogen evolution measurements with 3% Pt as a co-catalyst and triethanolamine (TEOA) as a hole-scavenger were performed using a side-irradiated closed steel reactor equipped with a Teflon inlet, a thermocouple, a pressure sensor, a magnetic stir bar, and a thermostat, connected to a Schlenk line

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. During the experiment, the build-up of pressure was monitored as a function of

irradiation duration. A white LED array was used as the irradiation source. All catalytic experiments were conducted under an argon atmosphere. The double-walled and thermostatically controlled reaction vessel was connected to a digital pressure sensor (Type-P30, DP = -0.1%, WIKA Alexander Wiegand SE & Co. KG) to monitor the pressure increase due to hydrogen evolution; 50 mg of the sample was placed inside the reactor and the reactor was then evacuated and refilled with argon several times. H2O and TEOA were pre-treated before use. H2O was first degassed for 1 h in a vacuum in an ultrasonic bath and then purged with argon for 1 h. TEOA was purged for 1 h with argon. The solvent mixture (38 ml), composed of water and TEOA in a 9:1 (v/v) ratio, and 39.4 µl of a H2PtCl6 solution, 8% in water (corresponding to a theoretical value of 3 wt% Pt loading onto the catalyst, total concentration of 0.207 mM), was added and the temperature was maintained at 25 °C with a thermostat. After being stirred for 10 min to reach thermal equilibrium, the reaction mixture was started by switching on a 50 W (the illumination intensity on the photocatalyst surface was 10000 µmol s-1m-2) white LED array (Bridgelux BXRA-50C5300; λ > 410 nm). The amount of evolved gas was continuously monitored by a time-dependent pressure increase. The hydrogen evolution rate was calculated ac-cording to the ideal gas law:


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Equation 1: ࢔ሶ =

࢔ ࢚

= ૚૙૞

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∆࢖ࢂ ࡾࢀ࢚

where n ̇ is the hydrogen evolution rate (micromoles per hour), n is the number hydrogen moles (micromoles), t is the illumination time (hours), ∆p is the pressure increase (bars) during irradiation time t, V is the volume headspace above the reaction solution, R is the universal gas constant (8.314 J mol-1K-1), and T is the temperature (298 K). To confirm that the evolved gas was hydrogen, the headspace of the reactor was analyzed by mass spectrometry (Pfeiffer Vacuum ThermoStar GSD 301 T gas analyzing system, using argon as the carrier gas) after the test. The apparent quantum yield (AQY) for H2 evolution was determined by LEDs equipped with 420 nm band-pass filter. The irradiation area was 9 cm2 and the irradiation intensity was measured by averaging 10 points in the irradiation area. The average intensity of 420 nm monochromatic light was 9.1 mW cm-2 (Newport 2936-R). The AQY was calculated as follow: AQY=Ne/Np×100% = 2M/Np×100%, where Ne is the amount of reaction electrons, Np is the incident photons, M is the amount of H2 molecules. The photocatalytic activity was further evaluated by the degradation of Rhodamine B (RhB) and p-nitrophenol (p-NP) under white light irradiation. In a typical photocatalytic degradation experiment, RhB or p-NP solution (20 ml, 20 mgL-1) and carbon nitride (20 mg) were mixed in a glass vial in darkness with continuous stirring until the adsorption-desorption equilibrium between the dye and the catalyst was obtained. After turning on the light, aliquots were withdrawn from the suspension at given time intervals. The concentration of remaining RhB, p-NP in solution was spectrophotometrically monitored by optical absorption values (at 554 nm of UV-vis absorption spectra for RhB and 317 nm of UVvis absorption spectra for p-NP) on an ultraviolet-visible spectrophotometer during the photodegradation process.


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RESULTS AND DISCUSSION The FT-IR spectroscopy and XRD analysis provide evidence for the formation of different supramolecular assemblies (Figure 1). The stretching vibration of the C = O groups of cyanuric and barbituric acid are shifted from 1724 cm-1 to 1703 cm-1 due to the hydrogen-C = O interaction. At a high HCl concentration (10%), a new stretching vibration emerges that can be attributed to N-H symmetric and antisymmetric stretching vibrations of NH3 (3550 cm-1)

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,

reflecting the structural degradation of the supramolecular assembly. The XRD analysis reveals that HCl strongly affects the structure of the supramolecular assembly; the XRD pattern of the non-halogenated assembly shows well-resolved peaks at 10.7o, 19.8o, 21.75o, and 27.8o, which can be indexed as (100), (110), (200) corresponding to the in-plane pattern, and (002) of the inplanar packing. The insertion of HCl dramatically changes the structure – only a sharp peak is observed at ~28°, corresponding to the d-spacing of 0.328 nm – reflecting the rearrangement of the supramolecular units toward one direction and the excellent three-dimensional packing. As compared with the minimal distance in carbons/graphite (0.336 nm), the close distance between the supramolecular layers indicates the tightly packed structure and the strong charge interaction between the units of the crystal. The high intensity of this peak suggests that this complex probably entails a long range order, which is directed by the halogen-hydrogen interaction.


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Figure 1. FT-IR spectra (a) and XRD (b) patterns of CMB and CMB– HCl assemblies. Solid state UV-vis absorption spectroscopy similarly indicates the tightly packed structure of the CMB–HCl assembly (Figure 2). The absorption properties of the CMB–HCl assembly (at 2% and 5% HCl) differ from those of CMB alone, especially in the range of 250–500 nm, wherein three peaks (at 262, 337, and 417 nm) are strongly enhanced and a red-shift in the absorption is observed, reflecting the strong π-π interaction and the improved electron delocalization within the CMB–HCl assemblies

74

. The TEM images indicate better layered structure for the CMB–

HCl5% compared to the pristine CMB, further supporting the improved π-π interaction (Figure S1†). At a higher HCl concentration (10%), the π-π interaction strength declines, probably due to the elimination of the supramolecular structure resulting from the strong protonation of the supramolecular units.


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Figure 2. UV-vis diffuse reflectance spectra of CMB supramolecular assemblies with or without HCl, before calcination. Another proof for the structural and morphological changes resulting from the addition of HCl to the pristine CMB assembly is provided by SEM and TEM images (Figure 3 and Figure S1†); the pristine CMB assembly exhibits a rod-like morphology, whereas the addition of HCl results in the formation of small supramolecular particles, which are oriented uniformly. The TEM analysis of the CMB–HCl5% compared to CMB further confirms that the acid treatment results in the improvement of the layered structure and to the formation of smaller supramolecular particles. We note that we chose to focus on CMB–HCl5% due to its superior photo-activity, as demonstrated below.


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Figure 3. SEM images of CMB and CMB-HCl supramolecular assemblies. We used XPS analysis to elucidate the structural changes, bond strengths, and element composition of the pristine CMB and CMB–HCl5% assemblies (Figure 4). The Cl 2p3, 2p1 XPS patterns reveal the presence of two different chemical states in the halogenated material – at 197.5 eV (Cl2p3), 199.1 eV (Cl2p1) and 201.7 eV (Cl2p3), 203.4 eV (Cl2p1)– corresponding to NH–Cl, and CO–Cl bond energies, respectively. These values indicate that the Cl anions can react with melamine, cyanuric acid, and barbituric acid units and have a prominent role in the alignment of the supramolecular monomers within the CMB–HCl assembly (Scheme S1†). The protonation of the amine groups is reflected in the new supramolecular assembly, as the N1s values for C-N-C and C-N-H co-ordination are shifted to a higher binding energy, namely, from 398.9 eV and 400.3 eV to 399.1 eV and 400.6 eV, respectively. The peak at ~ 406 eV is due to the charging effects. The C1s spectra displays three main binding energies for the CMB and CMB–HCl5%: C-C (284.8 eV and 284.9 eV, respectively) originating from the barbituric acid; C-(N)3 coordination from the melamine units (288.1 eV for both); and O = C-(N)2 specie that


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belongs to the cyanuric and barbituric acids (289.6 eV and 289.7 eV, respectively). In the CMB powder, a small contribution corresponding to adventitious C-O-C at 286.1 eV was found. The integration of the signals reveals a significant 5% decrease in the weight percentage of the C-C specie and a 4% increase of the C-(N)3 coordination bond (4% more) for the CMB–HCl5% assembly as compared with the pristine CMB assembly, probably resulting from the different interactions of the monomer in HCl. This finding implies that HCl can selectively remove barbituric acid from the full assembly.


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Figure 4. XPS spectra of C1s (a) and N1s (b) of the pristine CMB assembly and of C1s, (c), Cl2p3-2p1 (d), and N1s (e) of the CMB–HCl5% assembly. To test our postulation regarding the selective barbituric acid removal by the HCl addition, and to unravel the content of carbon and nitrogen species through the supramolecular assembly, we conducted a depth profile analysis of the CMB (Figure S2†) and CMB–HCl5% assembly for C1s (Figure 5 and Figure S3†) by sputtering the samples with Ar at 0.07 nm/s and elucidating the composition at t = 20 s (1.4 nm), 40 s (2.8 nm), 60 s (4.2 nm), and 80 s (5.6 nm). This analysis reveals that the acid modification increases carbon along the supramolecular assembly, indicating the formation of a gradient of elements. The integration of the signals shows that the weight percentage of the C-C species increases with depth while the C-(N)3 coordination decreases, demonstrating that the Cl- selectively removes the barbituric acid from the surface of the supramolecular assembly, thus forming an inner part that is more “carbon-rich” than the “nitrogen-rich” surface, as confirmed by XPS depth prolife (Figure 5). However, the XPS depth profile of the CMB assembly doesn’t show any clear trend of the different carbon species ratio (Figure S2†).


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Figure 5. Diagram of the variation with depth of the C1s weight percentage for C-C (black), C-N (red), and C-O (blue) species in the CMB–HCl5% assembly. The total carbon atomic weight is ~37%. Elemental analysis further proves the change in carbon amounts and the presence of HCl in the supramolecular structure of the starting materials. The total carbon amount and the C/N ratio confirm the gentle removal of barbituric acid from the structure, probably due to better solubility of barbituric acid in HCl compared to the other monomers (Table S1†), while the total weight clearly indicates the presence of Cl- ions. The amount of hydrogen slightly increases with the addition of HCl, indicating the protonation of the structure (Figure S4† and Table S2†). Photoactive carbon nitride materials were acquired by the calcination of the supramolecular assemblies at 550 °C for 4 h under a nitrogen atmosphere with a heating ramp of 2.3 K/min. The condensation of the supramolecular assemblies into carbon nitride materials can be well followed by TGA coupled with FTIR. This TGA-FTIR analysis (Figure S5†) reveals that the degradation


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of the monomers at high temperatures is strongly quenched. In situ FTIR measurements reveal that the HCl modification mainly inhibits melamine degradation and sublimation at elevated temperatures, probably due to the strong melamine–Cl interaction. Elemental analysis of the final materials (Table S3†) shows that the carbon amounts and the C/N ratio decrease with increasing amounts of HCl in the starting precursor (CMB: 0.63, as compared with 0.61 for CMB– HCl10%–C3N4). This finding indicates that the starting composition directly projects onto the element ratios in the final materials. Hydrogen values are around 2%, which means that most of the amino groups reacted during condensation. In addition, the total weight indicates the absence of oxygen, implying that most of the oxygen from the C = O groups in the cyanuric and barbituric acid was eliminated. FT-IR spectroscopy and XRD analysis of the final materials (Figure 6a and b) further prove the formation of carbon nitride materials after calcination. The analysis demonstrates typical stretching modes of CN heterocycles from 1200 to 1600 cm-1 and the breathing mode of triazine units at 800 cm-1. The dense packing and the strong intramolecular interaction in the starting CMB–HCl2% and CMB–HCl5% assemblies lead to stronger and narrower inter-planar stacking peak around 27.2° [indexed as (002)], as compared with the pristine CMB-C3N4 and the CMBHCl10%–C3N4 materials. We used SEM to investigate the morphology of the new carbon nitride materials. The SEM images show a direct correlation between the initial supramolecular assembly and the final materials after calcination at 550 °C (Figure 6c). The rod-like morphology of the pristine CMB is converted to a roll-like morphology in CMB–C3N4, while the box-like CMB–HCl2% and CMB– HCl5% particles are converted to hollow particles. At a higher (10%) HCl concentration in the starting CMB, continuous wires are obtained in the CMB–HCl10%–C3N4, probably due to the


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partial degradation of the starting supramolecular assembly at high HCl amount. The retainment of all the initial structures reflects the solid-state reaction without a melting step as an intermediate. The main structural arrangements remained stable owing to the strong preorganization of the structure of the starting monomers, as described above. Furthermore, the composition and homogeneity of the samples were tested by EDX (Figure S6†), which confirmed the presence of C and N.

Figure 6. (a) FT-IR spectra, (b) XRD spectra, and (c) SEM images of CMB–HCl–C3N4 materials. TEM and AFM measurements were conducted in order to analyze the layered structure of the materials (Figure 7, Figure S7† and Figure S8†). The TEM images of CMB-C3N4 (Figure 7 and


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Figure S7a†) shows a porous roll-like morphology, while the CMB 5%HCl C3N4 (Figure S7b†) displays thinner sheets structure with a better crystallinity. In accordance with the TEM analysis, AFM images (Figure 7 and Figure S8†) reveal that the CMB 5%HCl C3N4 exhibits a uniform layered structure with a thickness of ~3 nm (corresponding to ~9 layers) while the CMB-C3N4 is less homogeneous and thicker due to the folding of the layers ensuing from the roll-like morphology.


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Figure 7. TEM images, AFM images and thickness plot for CMB–C3N4 (a1-a3) and CMB– HCl5%–C3N4 (b1-b3). More images and thickness measurements are provided in Figure S7† and Figure S8†. To further elucidate the changes in electronic properties, we preformed XPS measurements for the most photoactive CMB–HCl5%–C3N4 and CMB–C3N4 samples (Figure 8). For CMB-C3N4, the C1s binding energies show main carbon specie at 288.0 eV corresponding to a C-N-C


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coordination, small residues of a O = C-(N)2 bond from cyanuric and barbituric acid at 289.3, and a C-C bond at 285.1 eV, originating from barbituric acid. In the N1s spectrum, several binding energies are observed; the main one shows the C-N-C groups (398.5 eV), a tertiary nitrogen N-(C)3 at 399.9 eV, and an additional weak signal at 401.1 eV corresponding to the remaining amino groups (C-N-H). For CMB-HCl5%–C3N4, the weight percentage of the C-C bond declines, underpinning that the design of elements in the supramolecular precursor is preserved in the final C3N4 due to the removal of barbituric acid by HCl, showing as well the contributions corresponding to C-O adventitious (286.5 eV), C-N-C coordination (288.3 eV) and O = C-(N)2 bond from cyanuric and barbituric (289.5 eV). Significantly, the N1s spectrum indicates that the HCl treatment leads to the enhancement of protonated amino groups in the final material (30% in atomic weight vs 17% in the pristine CMB-C3N4). A depth profile analysis (Figure 8e and f) demonstrates that the carbon amount and weight percentage of the C-C population increase across the final carbon nitride material, indicating that the final carbon nitride materials inherit the starting molecular organization in the supramolecular assembly.


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Figure 8. XPS spectra for C1s (a) and N1s (b) of CMB–C3N4 and for C1s (c) and N1s (d) of CMB–HCl5%–C3N4, and a diagram of the variation with depth for the C1s and N1s weight percentage (e) and for the C1s species (C-C and C-N-C); (f) in the CMB–HCl5%–C3N4 material. The electronic properties of the CMB–C3N4 materials were evaluated by photo-physical measurements. The carbon decline in the starting supramolecular precursor leads to a blue-shift of the absorption spectra with increasing HCl concentrations in the starting precursor due to a lower number of C-C bonds, as compared with the pristine CMB–C3N4 (Figure 9a). The slight blue-shift of the absorption spectrum further supports the selective removal of the barbituric acid


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from the starting supramolecular assembly. Photoluminescence (PL) measurements are considered to be a very sensitive tool for detecting electronic changes such as surface states and heterojunctions in semiconductors due to the formation of alternative radiative paths. It is important to note that the emission of CMB–C3N4 is already low due to the formation of an in situ heterojunction, as was shown previously

68

. Nevertheless, the acidic treatment results in

further quenching of the emission of the final materials due to an additional alteration of the electronic structure. In accordance with the latter, The PL lifetime measurements of the CMB– HCl5%–C3N4 is slightly shorter compared to the CMB–C3N4 (5.7 ns and 6.2ns). This alteration can be attributed to the formation of new energy-band structure, induced by the starting atomic composition of the supramolecular reactant and the better layered morphology which facilitates the charge transfer between the layers. The new energy-band structure quenches the radiative recombination due to beneficial charge separation properties. To evaluate the photo-activity of the CMB–HCl–C3N4-like materials with respect to the pristine CMB–C3N4, we measured the hydrogen evolution in a water/triethanolamine (TEOA) solution, with Pt as the co-catalyst, under a white-light illumination (Figure 9c). All CMB–HCl–C3N4 materials demonstrate a higher catalytic activity than that of the carbon nitride made from melamine solely (M–C3N4; 380 µmol H2/h·g vs ~10 µmol H2/h·g), and the most photoactive material, CMB–HCl5%–C3N4, produces almost 40 times more H2 than M–C3N4. The Quantum yield of CMB–HCl5%–C3N4 under blue light illumination (420 nm) was found to be 1.6 %. This value is relativity high, considering that no templating, heteroatoms doping, or other modifications are involved. In addition, the photocatalyst demonstrates good stability for three consecutive cycles (Figure S9†).


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Figure 9. (a) UV-vis diffuse reflectance spectra of the materials after calcination. (b) Emission spectra of CMB–HCl–C3N4 materials after calcination. (c) Hydrogen evolution performance of different carbon nitride materials. Further proof of the enhanced photocatalytic activity is given by the RhB dye and p-nitrophenol (p-NP) degradation as a function of time under a white light illumination (Figure 10 and Figure S10†, S11†). The CMB–HCl5%–C3N4 and CMB–HCl10%–C3N4 showed the highest photoactivity, both degraded the RhB dye much faster than using CMB–C3N4 (Figure 10b and Figure S10†). CMB–HCl5%–C3N4 photodegraded ~60% of p-NP, which is considered more stable than the organic dyes, after 90 min while CMB C3N4 photodegraded only 30% of the dye (Figure 10a and Figure S11†). We note that the degradation of p-NP was acquired without any metal as co-catalyst.


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Figure 10. CMB-C3N4 materials photoactivity on p-NP (a) and RhB (b). The enhanced photocatalytic activity could have been emanated from several factors: better light harvesting, which can lead to more charge carriers, better charge separation under illumination, a large surface area providing more catalytic sites and the morphology of the final materials. Nitrogen sorption measurements indicate that there is no clear correlation between the surface area and the photocatalytic activity (Figure S12†). The blue-shift in the absorption spectra of the most active catalysts clearly indicates that there is no connection between the light-harvesting properties of the carbon nitride materials and their photocatalytic activity. However, the emission quenching, alongside the carbon gradient through the material strongly suggests that the modified electronic properties together with the uniform and thin layered structure of the CMB– HCl5%–C3N4 (Figure 7, Figure S7† and Figure S8†) are responsible for the enhanced activity. In light of the data obtained in this study, we propose a mechanism in which the starting atomic gradient in the supramolecular reactants leads to the formation of an energy-band gradient in the final C3N4 materials. We note that the thickness of the CMB–HCl5%–C3N4 in the photocatalytic measurement is around 3 nm as determined from the AFM data (Figure 7 and Figure S8†). The core of the synthesized carbon nitride is composed of a semiconductor with a smaller band-gap


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than that of the outer layer, owing to the alteration of the C/N ratio (Scheme 1). In addition, the insertion of carbon into the C3N4 structure was proven by our and other groups to shift the conduction band energy of the materials toward more positive potentials, further supporting the formation of energy levels gradient due to the carbon changes in the final material (Figure S13†) 75,76

. The modified electronic structure as well as the improved layered structure and morphology

together with the better dispersibility and crystallinity of the CMB–HCl5%–C3N4 (Figure 7, Figure S7† and Figure S8†) further facilitates the charge separation of excitons under illumination, leading to a greater enhancement of carbon nitride photo- and catalytic activity toward a highly efficient hydrogen production.

Scheme 1. Graphic representation of the proposed carbon nitride photoactive material from the supramolecular assembly and the corresponding energy-band structure. We note that the round shape of the materials in the scheme is given only for simplification and it does not represent the materials morphology. CONCLUSIONS We demonstrate the synthesis of an efficient carbon nitride photocatalyst for dye degradation and hydrogen production by a rational design of the monomers used to form supramolecular assemblies with an element gradient, which serve as the reactant in a high-temperature reaction.


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This synthetic method allows to target the photophysical and catalytic properties of carbon nitride by the smart design of the starting supramolecular assemblies, without any templating methods or other chemical treatment. The utilization of a hydrogen-chlorine interaction leads to the creation of a carbon gradient through the starting supramolecular assembly, which can be directly transferred to the final carbon nitride material, underpinning the importance of the sequence of the monomers in the starting precursor. The rearrangement of the monomers in the supramolecular assembly leads to the formation of a unique energy-level structure in the final materials, which is proposed to improve the efficiency of charge separation under illumination, resulting in an enhancement of photocatalytic activity toward higher hydrogen production. Furthermore, TGA coupled with FTIR measurements reveal that melamine degradation and sublimation are dramatically quenched at high temperatures, indicating the strong hydrogenchlorine interaction. This work opens new opportunities for the facile and environmentallyfriendly synthesis of carbon nitride and other metal-free materials with specific electronic structures and high catalytic properties by a rational design of the underlying monomers. ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website. XPS, TGA, elemental analysis, TEM and EDX of the starting assemblies and the carbon nitride materials as well as the photophysical and photocatalytic properties of the carbon nitride materials are given in the supporting information file. AUTHOR INFORMATION Corresponding Author


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Department of Chemistry, Ben-Gurion University of the Negev, Beersheba 009728, Israel. Email: [email protected] Author Contributions The manuscript was written through contributions of all authors and they all have given approval to the final version of the manuscript. ACKNOWLEDGMENT The authors thank Katharina Otte from Max Planck Institute for Colloids and Interfaces for her help. We also thank Dr. Sofiya Kolusheva and Dr. Nathalia Frumin from Ben-Gurion University of the Negev for their help with TGA, FTIR and XPS measurements. REFERENCES 1

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For Table of Contents Use Only

Synopsis An efficient carbon nitride photocatalyst for hydrogen production and dyes degradation was synthesized by a rational design of the elements sequence in the staring monomers.


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Design of a Unique Energy-Band Structure and Morphology in a

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