Morphology Control and Photocatalysis Enhancement by the One-Pot

Letter pubs.acs.org/Langmuir

Morphology Control and Photocatalysis Enhancement by the OnePot Synthesis of Carbon Nitride from Preorganized HydrogenBonded Supramolecular Precursors Yohei Ishida,†,‡ Laurent Chabanne,‡ Markus Antonietti,‡ and Menny Shalom*,‡ †

Department of Applied Chemistry, Graduate Course of Urban Environmental Sciences, Tokyo Metropolitan University, Minami-ohsawa 1-1, Hachiohji, Tokyo 192-0397, Japan ‡ Department of Colloid Chemistry, Max Planck Institute of Colloids and Interfaces, 14424 Potsdam, Germany S Supporting Information *

ABSTRACT: We present an efficient synthesis of a modified carbon nitride photocatalyst by using supramolecular complexes of cyanuric acid, melamine, and 2,4-diamino-6-phenyl-1,3,5-triazine as precursors. We combined a self-templating approach for morphology control with the modification of photophysical properties by altering the chemical structure of the material. The resulting carbon nitrides exhibit high surface areas, defined morphologies, and a strong enhancement of light absorption in the visible-light region. A detailed analysis shows that the ratio changes of the three raw monomers resulted in different carbon nitride morphologies, absorption, and emission properties, along with the incorporation of different numbers of phenyl groups in the resulting carbon nitride structures. The modified carbon nitrides exhibit superior activity in the photodegradation of rhodamine B, up to 16 times that of bulk carbon nitride. The pyrolysis of rationally chosen supramolecular hydrogen-bonded precursors constitutes a synthetic pathway for the simple one-pot preparation of efficient, metal-free carbon nitride photocatalysts.

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INTRODUCTION Carbon nitride materials have attracted much attention for the past several years in the field of carbon-based materials because of their outstanding catalytic and photocatalytic properties.1−4 This cheap, easily available, metal-free organic semiconductor demonstrates high performance in many energy-related field such as the oxidation of hydrocarbons, water splitting,5−8 and the reduction of oxygen in fuel cells.9,10 Commonly, bulk graphitic carbon nitride (g-C3N4), the most stable allotrope under ambient conditions, is prepared by condensation from suitable molecular precursors such as urea, cyanamide, dicyandiamide (DCDA), melamine, and so forth.5,11,12 In general, the photocatalytic activity of semiconductor materials depends on their surface area, light absorption, and ability to have efficient charge separation under illumination.13 Therefore, to manipulate the C3N4 electronic and catalytic performance, modifications of carbon nitride syntheses have been studied intensively. One limitation of the solid-state condensation of simple monomers is the formation of disorganized 2D agglomerate structures with small grain sizes, resulting in small surface areas and low catalytic performance.5 The most common pathways to increasing the surface area of C3N4 are hard-templating approaches using silica nanoparticles or mesoporous silica to create porous C3N4.14 However, this type of templating is not © 2014 American Chemical Society

time- and cost-efficient because of the use of sacrificial material and sometimes hazardous chemicals for the removal of the template. Recently, the Thomas group2 and we15 reported a well-ordered hollow carbon nitride photocatalyst that was prepared by using the cyanuric acid−melamine (CM) supramolecular complexes16,17 as a starting material. The condensation of the CM complex at 550 °C results in an efficient carbon nitride photocatalyst resulting from the improvement of its electronic and optical properties alongside a larger surface area (∼70 m2 g−1) than that of a bulk carbon nitride prepared from DCDA (∼10 m2 g−1). In addition, self-templating18 of the CM complex is simpler, safer, and cheaper than previous approaches that involve silica nanostructures.14 To increase the photoactivity of prepared C3N4 further, it is crucial to enhance its light absorption toward the visible region and improve the charge separation efficiency under illumination. The typical pathways for modifying the photophysical properties of C3N4 are the introduction of new organic moieties into its structure, for example, by copolymerization with monomers such as barbituric acid19 and the integration of heteroatoms within the carbon nitride structure.20 Because Received: October 27, 2013 Revised: January 2, 2014 Published: January 2, 2014 447

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Letter

changes in the number, and possibly strength, of hydrogen bonds. For example, the C1−M0−Mp1 complex formed a structure consisting of large, smooth pillarlike aggregates (Figure 1b) whereas C1−M0.5−Mp0.5 formed a less homogeneous morphology with smaller, ill-defined aggregates along with a few pillarlike aggregates, which are smaller and less defined than observed for C1−M0−Mp1. The highly crystalline structure of C−M−Mp complexes is reflected in the X-ray diffraction (XRD) patterns (Figure 1c). The appearance of new in-plane diffraction peaks for the C−M−Mp complex at 11.4, 18.8, and 22.8° (indexed as (100), (110), and (200), respectively) with respect to the raw materials confirmed the formation of a long-range-ordered complex. Further evidence for the formation of the C−M−Mp complex is given by recording the FT-IR spectra as shown in Figure 1d. It is well known that hydrogen bonds between amino and keto groups result in several differences in their FT-IR spectra.21 The formation of hydrogen bonds within the C1−M0−Mp1 complex results in a shift of the CO stretching vibration from 1693 to 1704 cm−1 whereas the triazine ring stretching vibration was shifted from 823 to 787 cm−1. Moreover, the stretching vibrations of the phenyl group in Mp were shifted from 1533 and 1625 to 1541 and 1653 cm−1, respectively, as a result of the new arrangement of the molecule within the C−M−Mp complex.22 To prepare photoactive modified carbon nitrides (CNs), C− M−Mp complexes were pyrolyzed at 550 °C for 4 h under a nitrogen atmosphere at a heating rate of 2.3 K min−1. (The detailed synthesis procedures are outlined in the SI.) SEM images of the resulting CNs show various morphologies that are strongly dependent on the molar ratio of raw materials. The pillarlike structure of the C1−M0−Mp1 complex before heating (Figure 1b) formed a fiber-type morphology of CN (Figure 2a), and the less-defined aggregates of the C1−M0.5−Mp0.5

simple morphology control of carbon nitride was successfully demonstrated in previous reports,2,15 we investigated the possibility of improving the photophysical properties of the material while maintaining control of its morphology and a high surface area. The enhancement of light harvesting results in a higher concentration of charge carriers that can participate in the photocatalysis process. Consequently, an increase in the photocatalysis efficiency should be obtained. In this letter, a modified carbon nitride photocatalyst is simply prepared by the pyrolysis of the supramolecular complex of cyanuric acid (C), melamine (M), and 2,4-diamino-6-phenyl1,3,5-triazine (Mp) with different ratios as a precursor. The chemical structure, morphology, and photophysical properties of the resulting carbon nitrides are fully characterized by XRD, FT-IR, SEM, elemental analysis, and absorption and emission spectroscopy. These materials exhibit ordered morphologies with a strong absorption enhancement in the visible-light region with respect to that of the original CM complex. The photocatalytic activity was investigated by measuring the degradation of rhodamine B dye under visible light illumination in the presence of the resulting carbon nitrides.

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RESULTS AND DISCUSSION Supramolecular C1−M1 − x−Mpx complexes were prepared by mixing different molar ratios (x = 0, 0.05, 0.2, 0.5, 0.8, 0.95, and 1) of C, M, and Mp in water (Figure 1a). Although the three

Figure 1. (a) Hydrogen-bonded supramolecular C−M−Mp complex and the proposed carbon nitride structure after calcination. (b) SEM images of the C1−M0.5−Mp0.5 and C1−M0−Mp1 complexes. (c) XRD patterns. (d) FT-IR spectra of the C1−M0−Mp1 complex and raw materials.

raw materials are not completely soluble in water, partial solubility allows the formation of an ordered C−M−Mp complex after 4 h. Scanning electron microscopy (SEM) images of C1−M0−Mp1 and C1−M0.5−Mp0.5 complexes prior to pyrolysis are shown in Figures 1b and S1. The crystal morphologies of C1−M1 − x−Mpx complexes are highly dependent on the molar ratio of M and Mp as a result of the

Figure 2. (a) SEM images of structured C−M−Mp CNs with different molar ratios after condensation at 550 °C for 4 h. (b) FT-IR spectra and (c) XRD patterns of C−M−Mp CNs after condensation at 550 °C for 4 h. 448

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complex before heating (Figure 1b) formed a sheetlike morphology (Figure 2a). These SEM images demonstrate that the resulting morphologies of CNs can be easily manipulated by controlling the molar ratio of the precursors. The substitution of Mp molecules for M in the complex results in higher degrees of freedom and better flexibility of the complex as a result of the formation of only two hydrogen bonds per molecule of Mp (instead of three for M). In addition, the size of the phenyl group along with the loss of one hydrogen bond might allow the 2D structure to bend more freely, as opposed to the three-hydrogen-bond complex. Interestingly, regardless of the different morphologies, all seven of the as-made C−M−Mp CNs of this series have a large BET surface area of ca. 75 ± 5 m2 g−1 (Table S1 in the SI). These values are to be compared to 12.1 m2 g−1 for the bulk carbon nitride that was prepared by the condensation of melamine at 550 °C for 4 h. Clear evidence for the creation of CN structures was obtained by XRD and FT-IR analysis (Figure 2b,c). Several strong bands in the 1200−1600 cm−1 region, which correspond to the typical stretching modes of CN heterocycles, were observed.23 Additionally, a peak was found at around 800 cm−1, another characteristic feature of the heptazine unit. Interestingly, this peak was gradually shifted from 813 to 808 cm−1 when increasing the fraction of Mp. This shift is probably due to the morphology of the resulting CNs, suggesting that C1− M0−Mp1 CN has a denser packing structure than C1−M1−Mp0 CN.22 The XRD patterns of C−M−Mp CNs were similar to that of the bulk CN, supporting the formation of carbon nitride. The strong interplanar stacking peak of the aromatic systems (indexed as (002)) was gradually shifted from 27.1 to 27.6° by increasing the Mp amount within the complex. In agreement with the FT-IR results, the shift in the XRD peaks to higher angles indicates a denser packing of the layers, which presumably follows the improved crystallinity of the resulting CN (see also Figure S2). The formation of CN structures was also confirmed by elemental analysis (Table S1). The C/N molar ratios of the resulting CNs varied from 0.70 for C1−M1−Mp0 to 1.0 for C1− M0−Mp1; these C/N values clearly show that the phenyl group of Mp (which contains only carbon and hydrogen and no nitrogen) is included in the resulting CN structures. Using the value of C/N = 1.0, the chemical formula of C1−M0−Mp1 is calculated to be close to (C6N8)3−(C6H5)1, indicating that on average one phenyl group is present for three melem units. Moreover, the small amount of hydrogen (less than 2% besides phenyl components) suggests that most of the NH2 and the OH groups have been eliminated during condensation. The influence of the addition of Mp to the electronic properties of C−M−Mp CNs as organic semiconductors was studied by absorption and emission spectra. The UV−vis diffuse reflectance absorption spectra of C−M−Mp CNs are shown in Figure 3a. The absorption band edge of C−M−Mp CNs is gradually red shifted along with the enhancement of the optical density in the visible region for higher Mp concentrations. These spectral changes are likely due to the presence of phenyl groups in the resulting CN structure that increase the conjugated system. The elemental analysis data together with the absorption spectra show a direct correlation between the spectral properties and the number of phenyl group within the CN structure. Another possible reason for the spectral shift is the morphological effect of the resulting CN

Figure 3. (a) Absorption and (b) emission spectra for C−M−Mp CNs obtained after the condensation at 550 °C for 4 h. The excitation wavelength for emission spectra was set at 350 nm for all experiments. (c) Relative concentration (C/C0) of rhodamine B as a function of illumination time for C−M−Mp CNs. *Bulk CN was prepared by the condensation of melamine at 550 °C for 4 h. The samples were illuminated by white LED as the light source.

structures. However, it was previously observed that a denser packing of the carbon nitride structure results in a blue-shifted absorption.15 Because the interlayer packing of C1−M0−Mp1 is tighter than that of C1−M1−Mp0, we attribute the red shift in the absorption spectrum of C−M−Mp CNs to the chemical structure of the resulting CNs, not to their morphology. The band gaps of C−M−Mp CNs are calculated to be 2.25−1.55 eV by the absorption edges as summarized in Table S1. The emission spectra of C−M−Mp CNs excited at 350 nm are shown in Figure 3b. In accordance with the absorption spectral shifts, the emission peaks were gradually red shifted from C1− M1−Mp0 to C1−M0−Mp1. The decrease in intensity with increasing Mp amounts is probably due to the creation of a new charge-transfer path resulting from the presence of phenyl groups in the CN structure. The photocatalytic effectiveness of the C−M−Mp CNs is shown in a typical model reaction, the degradation of rhodamine B (RhB) as a function of time under visible-light illumination (Figure 3c). For all of the C−M−Mp CNs, full RhB degradation was already achieved after only 40 min (with different rate constants), whereas bulk CN exhibited only poor activity (less than 30% after 40 min). It should be noted that no degradation of RhB occurred under illumination in the absence of CNs (blank in Figure 3c). For all CNs, the amount of RhB dye adsorbed to the CNs was less than 15%, and the photodegradation yield was calculated after the subtraction of this fraction. The photodegradation of RhB can occur in the following ways: (1) cleavage of the all-conjugated structure or (2) N-de-ethylation.24,25 In the first process, the position of the main absorption peak at 554 nm remains constant while only the peak intensity decreases. In the second path, the main absorption peak at 554 nm is blue shifted up to 498 nm (RhB only). Examples of the photodegradation of RhB under illumination in the presence of C1−M0−Mp1 and bulk CNs 449

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Langmuir are shown in Figure S3. The absorption spectra of the RhB dye at different times show a decrease of the main peak at 554 nm, which corresponds to the cleavage of the RhB structure. The degradation rates varied among samples, but for all C−M−Mp CNs, almost 50% RhB degradation was observed after 10 min. The most efficient RhB degradation was observed in the presence of C1−M0−Mp1 CN, and the degradation rates gradually decreased with lower Mp concentration, from C1− M0−Mp1 to C1−M1−Mp0. Moreover, the most efficient catalyst demonstrates high stability and no chemical changes were observed after 4 h of RhB photodegradation as shown in Figure S4. The photocatalytic activity of solid semiconductors in a one-photon conversion reaction such as RhB degradation usually depends on (1) the surface area of the catalyst, (2) the absorption spectra, and (3) the charge separation efficiency under illumination.13 In the present system, the introduction of a phenyl group into the CN structure has little influence on the surface area (Table S1) but results in increased light absorption and probably, as suggested by the absorption and emission spectra, more efficient charge separation. The existence of the electron-rich phenyl groups within the structure can improve the charge separation under illumination thanks to the creation of different localization sites for holes and electrons in CN. C1− M0−Mp1 CN, which was the most efficient catalyst of all, exhibited RhB degradation that was ca.16 times as efficient as that of bulk CN, thanks to the improvement in both its morphological and photophysical properties.

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REFERENCES

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CONCLUSIONS We showed a path to synthesize nanostructured carbon nitride materials by compositional modifications of supramolecular monomer complexes. The resulting C−M−Mp CNs also showed a variety of morphologies that are strongly dependent on the monomer composition in the complex, among them little disks and fiber bundles. The insertion of phenyl groups within the CN structure results in an increase in the optical density along with an impressive red shift into the visible region with higher Mp ratios. In addition, the increase in the amount of Mp results in strong quenching of the emission spectra, which indicates the formation of new charge-transfer paths. C1−M0−Mp1 CN shows ca. 16-fold improved photocatalytic activity compared to that of bulk CN. This work demonstrates that the rational, controlled substitution of monomers in the CM complex can lead, after condensation, to materials with improved photoelectronic properties, in addition to the variation of morphology and surface area resulting from the controlled formation of supramolecular complexes. ASSOCIATED CONTENT

S Supporting Information *

Experimental details, SEM, XRD, and FT-IR for the C−M−Mp complex before heating. BET surface areas and elemental analysis. Absorption spectra for RhB degradation experiments. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

Y.I. acknowledges JSPS Research Fellow PD from the Japan Society for the Promotion of Science (JSPS).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 450

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