Fabrication and Photocatalytic Application of Aromatic Ring

Jan 27, 2018 - These diverse applications are fundamentally based on electron–hole (e––h+) pair generation and transport under photocatalyst ill...
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Article Cite This: J. Phys. Chem. C 2018, 122, 3506−3512

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Fabrication and Photocatalytic Application of Aromatic Ring Functionalized Melem Oligomers Chu-Yen Hsu and Kao-Shuo Chang* Department of Materials Science & Engineering, National Cheng Kung University, No.1, University Road, Tainan City 70101, Taiwan ABSTRACT: This paper reports the fabrication of aromatic ring functionalized melem oligomers (AFMOs) and the study of their synergistic photodegradation properties. Highlights include the following: (1) the fabrication of various carbon nitrides (CNs) through thermal polycondensation; (2) the intrinsic modulation of the molecular nature of polymeric CNs using different amounts of melamine precursors; and (3) the extrinsic functionalization of melem oligomers (MOs) with aromatic rings, which enhanced visible light absorption, reduced dark absorption, and enhanced π electron delocalization, thus inhibiting electron−hole (e−−h+) pair recombination. X-ray diffraction and UV−vis spectroscopy qualitatively indicated the fabrication of MOs after washing with 135 °C dimethyl sulfoxide. 15N solid state nuclear magnetic resonance spectroscopy revealed that the MO sample consisted mainly of trimers before and after aromatic ring functionalization. Fourier-transform infrared spectroscopy indicated the structures of polymerized heptazine rings and the effective functionalization of aromatic rings. A scavenger study demonstrated that •O2− predominated in the AFMO photodegradation mechanism of methylene blue solutions. The AFMO sample exhibited a photodegradation rate constant of approximately 9.8 × 10−3 min−1, which was nearly 8 times higher than that of melon. The superior photocatalytic properties were attributed to the substantial improvement in visible light absorption and poor e−−h+ pair recombination because of π electron delocalization, which were verified using UV−vis and photoluminescence spectra, respectively.

1. INTRODUCTION Photocatalysis has been applied to a variety of sustainable practices,1,2 including disinfection, deodorization, water and air purification, photodegradation, water splitting, self-cleaning, and the reduction of CO2 to methane. These diverse applications are fundamentally based on electron−hole (e−− h+) pair generation and transport under photocatalyst illumination. Many promising semiconductors have been reported3 after water splitting using TiO2 was demonstrated by Honda and Fujishima in 1972.4 Polymeric carbon nitrides (PCNs), composed simply of abundant carbon and nitrogen, represent another environmentally sustainable material because of their low cost and high thermal and chemical stability.2 In addition, they possess band gaps (Eg) in the visible light active range (approximately 2.7 eV) and provide the redox potential for water splitting.5 Various approaches have been developed for fabricating CNs.2,6 Among them, thermal decomposition (polycondensation) is a simple and cheap method whose precursors are nitrogen compounds such as cyanamide, dicyandiamide, melamine, thiourea, urea, or mixtures of those.7 However, different precursors yield CNs with various polymerization, Eg, surface area, and crystallinity properties. Consequently, CNs remain elusive and complicated materials to investigate despite having been first studied by Liebig in 1834.8 With the development of X-ray techniques, a planar-structure model for CNs based on a heptazine core (C6N7) was proposed by Pauling et al. in 19379 and Redemann et al. in 1940.10 If © 2018 American Chemical Society

C6N7 completely condenses and extends indefinitely, an empirical formula similar to C3N4 is achieved.10 In addition, on the basis of the C6N7 planar-structure model, the structures of melam, melem, and melon were postulated by Redemann et al.10 However, an unsubstituted heptazine nucleus was first synthesized in 1982 by Leonard et al., who also studied its physical and spectroscopic properties using X-ray crystallography.11 In 1996, Teter and Hemley employed first-principles calculations to investigate numerous CN phases, including α-, β-, cubic-, pseudocubic-, and graphitic-C3N4 (g-CN). Among these phases, g-CN was strongly favored.12 Theoretically, g-CN has a completely condensed structure in which triazine or heptazine cores are connected by a ternary bridging nitrogen atom. By applying density functional theory, Kroke et al. calculated that heptazine-based g-CN is 30 kJ mol−1 more stable than triazine-based g-CN;13 in addition, the band positions of heptazine-based structures are more suitable for water splitting.14 Thus, heptazine-based g-CN has received substantially more attention. Melem was first synthesized in 2003 by Schnick et al., and its crystalline structure (C−N−H) was determined using X-ray diffraction (XRD), solid state nuclear magnetic resonance spectroscopy (NMR), and theoretical calculations,15 leading to a detailed understanding of the structure of g-CN. Received: December 20, 2017 Revised: January 26, 2018 Published: January 27, 2018 3506

DOI: 10.1021/acs.jpcc.7b12539 J. Phys. Chem. C 2018, 122, 3506−3512

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The Journal of Physical Chemistry C Considerable efforts have been dedicated to synthesizing gCN. However, the hydrogen composition of the products has always been greater than 0.6 wt %, indicating the chain structure of melon.16 In 2007, Senker et al. employed a melamine heating process, and they demonstrated the structure of crystalline zigzag-chain melonin which heptazine is connected by a secondary bridging nitrogen atom and terminated by amines, with hydrogen bonding between chainsthrough NMR, XRD, transmission electron microscopy, and other characterization tools.16 The crucial evidence for this structure was the existence of a secondary bridging nitrogen atom and a relatively high hydrogen composition (>1 wt %) compared with that of completely condensed g-CN. XRD patterns of highly crystalline melon typically exhibit two characteristic peaks at approximately 27.3 and 13.0°, which are attributed to the periodic stacking of (002) layers5 (indicating graphite-like sheets17) and the in-plane (100) stacking motif of heptazine units, respectively. In addition, on the basis of Schnick’s study,18 pyrolysis of melamine yielded trace amounts of the highly crystalline minority phases of polyheptazine imide (PHI), in addition to the majority phase of melon.18 In 2013, Tyborski et al.17 further confirmed the structure of crystalline zigzag-chain melon and suggested that ideal triazine- or heptazine-based g-CN was not achievable using thermal polycondensation because of the following reasons: (1) Fundamental differences were observed between the XRD results of their product and those of the ideal triazine-based gCN, which was theorized by Teter and Hemley in 1996,12 and the heptazine-based g-CN fabricated through ionothermal synthesis by Thomas et al. in 2008.19 (2) Their XRD results satisfactorily matched the calculated XRD patterns, in which the structures of the triclinic unit cell were used with an A−Astacking motif and flat or sinusoidally buckled melon sheets. Heptazine-based melon, which has the function of a visiblelight-driven metal-free photocatalyst, was first reported in 2009 by Wang et al.5 Various approaches have since been employed to increase the efficiency of its photocatalytic activities through the following fundamental mechanisms: (1) light absorption improvements, such as surface modification, 20 use of amorphous melon,21 increase of surface area,22−24 Eg tuning,25 doping,26−28 and copolymerization;29 (2) the recombination minimization of photogenerated charges, such as a short carrier-diffusing distance30 and heterojunction use;31 and (3) the incorporation of cocatalysts32,33 to enhance interfacial charge transfer rates.34 However, compared with heptazine-based g-CN, studies of trazine-based g-CN have been less advanced and limited in number. Highly crystalline poly(triazine imide) (PTI) was not synthesized until 2011 by Schnick et al.,35 who employed an ionothermal approach with lithium chloride and potassium chloride salt melts as the solvents. In addition, Neder, Kolb, and Senker et al. further resolved the H/Li substructure of PTI in 2016.36 The potential applications of PTI nanosheets,37 smallmolecule-doped PTI,38,39 and bulk PTI40 have also been investigated. These studies have focused only on melon and PTI/PHI; melem oligomers (MOs) have received substantially less attention. Lotsch et al. attempted to fabricate a mixture of melem monomers and oligomers by controlling the thermal polymerization of melamine at 450 °C. MOs were then screened out and observed to exhibit hydrogen evolution 2 and 3 times higher than that of the as-synthesized product and melon, respectively.41 Gambarelli and Dubois et al.25

synthesized melem monomers, dimers, and trimers from cyameluric chloride-based compounds at a very low temperature (−95 °C), at which dimers and trimers were also observed to exhibit a substantial red shift in their absorbance. Thus, the objective of the current study was to fabricate various MOs and their derivatives in order to study their synergistic photodegradation properties. Two approaches were employed to demonstrate their effectiveness in enhancing photocatalytic properties: (1) intrinsically modulating the molecular nature of PCN using different amounts of melamine precursors and (2) extrinsically functionalizing MOs with aromatic rings to enhance absorption in a visible light range, reduce dark absorption, and improve π electron delocalization to inhibit e−−h+ pair recombination. Our results suggested that aromatic ring functionalized melem oligomers (AFMOs) were successfully fabricated. They hold great promise as efficient and environmentally friendly photocatalysts for photodecomposing methylene blue (MB) organic dye.

2. EXPERIMENT CNs were fabricated using melamine precursors in the current study. Melem was synthesized by heating melamine powder (approximately 0.28 mmol) at 450 °C for 5 h in a sealed quartz ampule under an Ar atmosphere.32 After natural cooling to room temperature, the ampule was intentionally broken to collect white melem powder at the bottom. Two polymerization levels of MOs were prepared by using 1 or 5 g of melamine powder.41 Each was placed into a lidcovered aluminum oxide crucible (not airtight), which was placed in a quartz tube furnace, and then purged using argon at room temperature to sustain an inert environment inside the tube. The sample was subsequently heated to 450 °C at 7.5 °C/ min and held for 12 h under a steady argon flow. The obtained polycondensed powder was then grounded, placed in dimethyl sulfoxide (DMSO) at various temperatures (r.t., 90 °C, and 135 °C), and stirred for 5 days. The resulting suspension was centrifuged at 10000g. The precipitates were washed many times with acetone and deionized water and finally dried at 60 °C for 12 h. This completed the preparation of the MOs. AFMO was prepared using a procedure modified from the recipe proposed by Teng et al. in 2016.42 MOs (0.5 g) were added to 50 mL of ethanol, stirred for 6 h, and partially dried, allowing the sample to turn into a slurry mixture. The mixture was placed into a covered aluminum oxide crucible (not airtight) and annealed at 450 °C for 3 h in a tube furnace using a steady argon flow. Melon and aromatic ring functionalized melon (AFM) were also prepared for comparison. The preparation was almost the same as that for the MOs and AFMO, except that heating was performed at 550 °C for 3 h. Various characterization tools were employed to study properties of the CNs. XRD was used to study phases and crystallinity, and UV−vis spectroscopy was used to study absorption and Eg. 15N solid state NMR was performed to investigate the polymerization states of the CNs. The functional groups attached after aromatic ring functionalization and the heptazine rings of MOs were determined through Fouriertransform infrared spectroscopy (FTIR). Photoluminance (PL) spectroscopy was employed to investigate the recombination behavior of the photoinduced e−−h+ pairs of the materials. The photocatalytic properties of the CNs were studied by measuring the rates at which they photodecomposed MB solutions (5 ppm) under a solar simulator (265-W Xe lamp; approximately 3507

DOI: 10.1021/acs.jpcc.7b12539 J. Phys. Chem. C 2018, 122, 3506−3512

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The Journal of Physical Chemistry C 884 W/m2), which was equipped with a cutoff filter for the 425 nm wavelength and below. Prior to each measurement, sample solutions were maintained in the dark for 30 min to achieve an absorption−desorption equilibrium. The data were then collected every 30 min during the reaction.

were effectively removed. In addition, the crystallinity of these samples after the DMSO wash had deteriorated, particularly for the sample after the 135 °C DMSO wash. The explanation is that hot DMSO enabled the dissolution of melem monomers and the breaking of hydrogen bonds among MO units, leading to low crystallinity. The latter also implied that abundant surface terminations and defects (e.g., non-hydrogen-bonded amines) were created, which potentially act as photocatalytic active sites.43 Thus, the 135 °C DMSO wash was beneficial for fabricating CNs as photocatalysts. Figure 1b compares the UV−vis absorption spectra of the 1 g, 5 g, and melon samples. The melon sample exhibited an absorption edge at approximately 470 nm. However, blue shifts (approximately 450 nm) were observed for the 1 and 5 g samples. The 5 g sample was shifted even more than the 1 g sample; the span of the absorption edge from melon also indicated low polymerization in the 5 g sample. Thus, in addition to crystallinity and optical properties, XRD and UV− vis results also qualitatively demonstrated the polymerization levels of the 1 and 5 g samples. 15 N solid state NMR was employed to quantitatively study the polymerization level of the 5 g sample after the 135 °C DMSO wash (Figure 2a). The peaks at approximately 157, 121,

3. RESULTS AND DISCUSSION XRD was used to study the phases and crystallinities of the CNs, including the MO prepared using 1 g (black curve) and 5 g (blue curve) of melamine powder precursors (Figure 1a).

Figure 1. (a) XRD results of various CN derivatives. DMSO wash was applied to the 5 g melamine sample at various temperatures (25, 90, and 135 °C). (b) UV−vis absorption spectra of the 1 and 5 g melamine and melon samples.

Melon (purple curve) and melem (red curve) were also plotted for reference; the two samples exhibited characteristic peaks consistent with those described in the literature.6 Compared with the 1 g sample, the 5 g sample exhibited more observable characteristic peaks,41 indicating its more melem-like properties (e.g., short chains). The sample also exhibited better crystallinity, as indicated by a smaller full width at halfmaximum of the peaks. The crystallinity variation between the two samples was caused by the amount of byproduct NH3 gas that was generated during polycondensation, which was originally reported in the formation of crystalline melon by considering the equilibrium of chemical reactions.20 When a crucible of the same volume was used, large quantities of melamine precursors (5 g) yielded a considerable amount of NH3 gas (6C3H6N6 → C18H12N28 + 8NH3), which led to slow condensation and favored the formation of melem and lowpolymerization MOs. In addition, ample reaction time enabled low-polymerization MOs to rearrange to develop hydrogen bonding among MO units, leading to enhanced crystallinity. By contrast, small quantities of melamine precursors (1 g) yielded little NH3 gas and a fast condensation reaction, leading to a melon-like structure (long twisted chains) and poor crystallinity. Although the 5 g sample potentially consisted of lowpolymerization MOs and possessed abundant active sites on its surface area, it may also have contained a wide Eg melem monomer, which is undesirable for visible-light-driven photocatalysis. Thus, DMSO washing at 25 °C (dark green curve), 90 °C (light green curve), and 135 °C (orange curve) was applied to remove any melem monomers from the sample, on the basis of Schnick’s study15 (Figure 1a). As seen in the figure, most of the characteristic peaks disappeared after DMSO washing compared with the 5 g sample that was not washed with DMSO (blue curve), indicating that the melem monomers

Figure 2. (a) 15N solid state NMR result of the 5 g melamine sample after the 135 °C DMSO wash. The inset shows nitrogen locations in a heptazine ring. (b) Average polymerization chain length (n) of melon and the 1 and 5 g melamine samples after the 135 °C DMSO wash.

101, and 80 ppm were attributed to the peripheral nitrogen, center nitrogen, secondary bridging amine (NH), and terminal amine (NH2), respectively, in the heptazine ring (inset of Figure 2a).41 The observed secondary bridging amine peak directly indicated that the heptazine rings were polymerized to form PCN. To deduce the average polymerization chain length (n), a calculation reported by Lau et al.41 was applied. The ratio of integrated peak areas of NH2 to NH of the sample was estimated at approximately 2.52. Thus, the corresponding n was calculated as approximately 3, indicating that the sample comprised MOs and consisted mainly of trimers. The same calculation strategies were applied to melon and the 1 g sample after the 135 °C DMSO wash, yielding n values of approximately 40 and 5, respectively (Figure 2b). The quantitative results showed that the 5 g sample contained more primary amine than secondary bridging amine, and it had less polymerization than the 1 g sample. Thus, NMR analysis strongly supported the XRD and UV−vis results. To potentially improve the photocatalytic properties of MO, the 5 g sample was subjected to surface modification through aromatic ring functionalization. UV−vis spectroscopy was used 3508

DOI: 10.1021/acs.jpcc.7b12539 J. Phys. Chem. C 2018, 122, 3506−3512

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The Journal of Physical Chemistry C to study the absorption behaviors of various samples (Figure 3a). Melon (blue curve) and MO (orange curve) were

Figure 4. FTIR spectroscopy of MO before (blue curve) and after (orange) aromatic ring functionalization. The inset presents a schematic of the hypothetical structure of AFMO. Figure 3. (a) UV−vis spectra of melon, AFM, MO, and AFMO. The corresponding colors are also shown. (b) Tauc plots of melon and MO samples.

compared. The two samples that were subjected to aromatic ring functionalization (AFMO and AFM) exhibited no distinctive absorption edges and substantially stronger absorptions in the visible light wavelength range compared with those of MO and melon. The tailing effect resulted from the formation of abundant surface terminations and defects,43 as indicated in Figures 1a and 6. The enhanced absorption implied a beneficial aromatic ring functionalization strategy for photocatalysis. Indeed, aromatic ring functionalization could be intuitively distinguished according to the color variation of the samples, in which light yellow (lower two images of Figure 3a) turned into brown (upper two images of Figure 3a) after aromatic ring functionalization. The optical Eg of melon and MO were analyzed using Tauc plots (Figure 3b), in which (αhν)2 was plotted against hν because of the direct transitions of the materials (α is the absorption coefficient, h represents Planck’s constant, and ν denotes the irradiation frequency). The Eg of MO and melon were calculated as approximately 2.9 and 2.8 eV, respectively. FTIR spectroscopy was utilized to study the functional groups of MOs before (blue curve) and after (orange curve) aromatic ring functionalization (Figure 4). The observed characteristic bands at approximately 810, 1248, and 3000− 3300 cm−1 for the two samples were attributed to the breathing mode of the heptazine ring, the secondary bridging amine, and the terminal amino groups, respectively. The two curves essentially exhibited a similar trend10 and indicated the structure of polymerized heptazine rings. However, the band at approximately 1579 cm−1, which was observed only in the AFMO sample, indicated the effective functionalization of aromatic rings. The schematic of a hypothetical structure of AFMO is presented in the inset. In addition, 15N solid NMR was also employed to ensure that aromatic ring functionalization did not substantially affect the polymerization in MOs (Figure 5). The ratio of integrated peak areas of NH2 to NH of the sample was estimated at 2.1, which corresponded to an n of approximately 3.6. The result suggested that, on average, the AFMO sample sustained its structure and consisted of trimers. The slight increase in n compared with that of the sample without functionalization (n

Figure 5. (a) 15N solid NMR results of AFMO. (b) Average ratios of NH2 to NH and the chain length (n) of AFMO.

= 3) was attributed to the heat treatment during functionalization, which triggered an increase in polymerization reactions. Photodegradation of MB was conducted to study the photocatalytic properties of CNs (Figure 6) because MB exhibited a low sensitization effect and it was still considered an appropriate dye in evaluating the intrinsic visible light photocatalytic property.44 To ensure that an absorption−

Figure 6. Photocatalytic activities of CNs. Absorption−desorption equilibrium of AFMO in the dark is presented as an example. •OH and •O2− scavenger results were also studied. The inset presents the photodegradation rate constant (k) of the CNs. 3509

DOI: 10.1021/acs.jpcc.7b12539 J. Phys. Chem. C 2018, 122, 3506−3512

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The Journal of Physical Chemistry C desorption equilibrium was achieved prior to measurement, all photodecompositions were performed in the dark for the first 30 min; our results indicated that the equilibrium was established within that period. AFMO (blue dotted curve) is presented as an example. The self-photodegradation of MB (blank, without photocatalysts) was also measured for comparison (gray curve). In this measurement, approximately 10% of the MB was degraded, as estimated after a dark absorption correction. Melon (purple curve), AFM (red curve), MO (green curve), and AFMO (blue curve) photodegraded approximately 10, 24, 53, and 60%, respectively, of the MB in 90 min, which were also estimated after a dark absorption correction. The following three features were observed: (1) Both MO-based materials exhibited substantially greater photocatalytic activity than melon-based materials because more active sites were sustained with lower polymerization in MO-based materials. (2) AFM and AFMO exhibited enhanced photocatalytic properties compared with their corresponding materials without aromatic ring functionalization because this process enabled a wide range of visible light absorption, as illustrated in Figure 3. (3) The amount of dark absorption for AFMO was substantially lower than that of MO, indicating another purpose of aromatic ring functionalization for MO. However, aromatic ring functionalization did not exert a substantial influence on dark absorption for melon (similar dark absorption behaviors were observed for melon and AFM) because of the high crystallinity and hard exfoliation properties of melon. To determine the crucial radicals responsible for the photodegradation mechanism of AFMO, tert-butanol (hydroxyl radical (•OH) scavenger) and sodium disulfate (superoxide radical (•O2−) scavenger) were studied. As observed in the figure, both scavengers substantially inhibited the photodegradation properties of AFMO; the •O2− scavenger (blue dashed curve) suppressed photodecomposition even more than the •OH scavenger (blue dot dashed curve), indicating the •O2− predominates in the photodegradation properties of AFMO. The photodegradation rate constant (k) for each sample was also quantitatively deduced by plotting −ln(C/C0) against irradiation time (inset of Figure 6; C0 and C denote the original and residual concentrations of MB, respectively) to study their photodegradation kinetic behavior. A first-order linear relationship was observed for each sample; the corresponding k values (×10−3 min−1, slope for each curve) were estimated at 9.8, 8.4, 2.9, and 1.3 for AFMO, MO, AFM, and melon, respectively. The AFMO sample exhibited a k value approximately 7.5 times higher than that of melon, suggesting the effectiveness of lowpolymerization MOs and aromatic ring functionalization for photocatalysis. Our results strongly indicated that AFMO holds great promise as a photocatalyst. PL was used to study the recombination behavior of MO (green curve) and AFMO (blue curve) (Figure 7), because PL emission signals are indicative of the recombination feasibility of photoinduced e−−h+ pairs; strong recombination leads to high PL signals. Figure 7a presents the results using a 325 nm excitation light source. AFMO exhibited substantially lower intensity than MO, indicating its poorer e − −h + pair recombination. The explanation is that π electrons were delocalized from the heptazine ring to the attached aromatic rings, inhibiting the direct e−−h+ pair recombination.42 This observation strongly supports the hypothetical structure of AFMO (Figure 4), and its superior photocatalysis. By contrast, when a 532 nm excitation light source was employed, AFMO

Figure 7. PL spectra of MO and AFMO using light sources of different excitations: (a) 325 nm; (b) 532 nm.

exhibited a substantially stronger PL emission signal, which was attributed to strong absorption in the visible light range (the tailing effect) (Figure 3a). However, a considerably lower intensity was observed for the MO because the light source energy was insufficiently strong to trigger e−−h+ pairs in MO.

4. CONCLUSIONS CNs including melem monomers, MOs, AFMO, melon, and AFM were fabricated using melamine precursors through thermal polycondensation for this study. DMSO washing at 135 °C was determined to effectively remove melem monomers in the as-fabricated samples. 15N solid state NMR results suggested that the heptazine rings of the sample that was washed in DMSO at 135 °C were polymerized to form MOs, which consisted primarily of trimers. UV−vis spectroscopy illustrated that the aromatic ring functionalized samples (i.e., AFMO and AFM) exhibited substantially enhanced absorption in the visible light range (the tailing effect), although no distinctive absorption edges were observed. Aromatic ring functionalization was also deduced from the color variation of the samples, light yellow before and brown afterward. FTIR spectroscopy further supported the existence of heptazine rings and the effective functionalization of aromatic rings for the MO samples. Scavenger experiments indicated that •O2− was predominant in the AFMO samples, which exhibited a photodegradation capability almost 8 times higher than that of melon. The superior photocatalytic properties were determined on the basis of its poor e−−h+ pair recombination because of π electron delocalization, which was indicated by the substantially weak PL intensity under 325 nm irradiation. The tailing effect of absorption in the visible light range was also verified using a 532 nm excitation light source in the PL measurement. Our results indicated that the AFMO sample holds great promise as a photocatalyst.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +886-6-2757575, ext. 62922. ORCID

Kao-Shuo Chang: 0000-0002-2876-9255 Notes

The authors declare no competing financial interest. 3510

DOI: 10.1021/acs.jpcc.7b12539 J. Phys. Chem. C 2018, 122, 3506−3512

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The Journal of Physical Chemistry C

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ACKNOWLEDGMENTS This study was partially supported by the Ministry of Science and Technology (MOST), Taiwan, under grants MOST 1052221-E-006-028 and MOST 105-2923-E-006-005-MY2.

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