Polymeric Carbon Nitride with Localized Aluminum Coordination Sites

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Polymeric carbon nitride with localized aluminum coordination sites as a durable and efficient photocatalyst for visible light utilization Chi Hun Choi, Lihua Lin, Suji Gim, Shinbi Lee, Hyungjun Kim, Xinchen Wang, and Wonyong Choi ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03512 • Publication Date (Web): 06 Apr 2018 Downloaded from http://pubs.acs.org on April 6, 2018

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ACS Catalysis

Polymeric carbon nitride with localized aluminum coordination sites as a durable and efficient photocatalyst for visible light utilization Chi Hun Choi 1, Lihua Lin 2, Suji Gim 3, Shinbi Lee1, Hyungjun Kim 3, Xinchen Wang 2, and Wonyong Choi 1,*

1

Department of Chemical Engineering/Division of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea 2

State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou, Fujian 350002, People’s Republic of China 3

Graduate School of Energy, Environment, Water, and Sustainability (EEWS), Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea *

Correspondence: [email protected] (W.C.)

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Abstract The development of efficient and yet economic photocatalysts that can utilize solar light is crucial for the sustainable future. We report a simple approach to introduce bi-dentate type of metal coordination site in polymeric carbon nitride (PCN) by an in-situ keto-enol cyclization route of acetylacetone and urea to introduce metal chelating pyrimidine derivative into the molecular framework of PCN. The resulting new metal coordination sites provide both Nand O-complexing ligands unlike the unmodified PCN that has N-ligands only. As a proofof-concept experiment, we introduced Al3+ ions into these coordination sites, which induced significant enhancements in visible light photocatalytic activities for organic pollutant degradation and H2 evolution as compared to those of bulk PCN and the conventional “nitrogen pot” metal-coordinated PCN. The optimized Al loading was as low as 0.32 wt%. The photocatalytic activities sensitively depended on the Al incorporation, and the Alincorporated sample demonstrated an excellent stability in water with showing little sign of metal leaching. While the aluminum ions complexed in the nitrogen pot little influenced the photocatalytic activity, Al3+ ions complexed by both N and O ligands in the new coordination sites significantly enhanced the photocatalytic activity of PCN. This study demonstrates a facile and scalable synthesis of PCN with alternative metal coordination sites for efficient solar energy conversion. KEYWORDS: carbon nitride, metal coordination, organic semiconductor, photocatalyst, solar energy conversion, H2 production

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Introduction Polymeric carbon nitride (PCN) has attracted significant interests owing to its versatile properties as a stable, visible light active, and environmentally benign organic semiconductor photocatalyst.1 Because of these advantages, PCN emerged as the ideal platform to convert sunlight for energy and environmental applications such as water splitting,2-5 environmental remediation,6 CO2 activation,7 and other redox chemical reactions.8 Unfortunately, pristine PCN suffers from the drawbacks including low surface area,9 low conductivity,10 and fast charge recombination,11 which inherently reduce the (photo)catalytic efficiency. Researchers employed many different strategies to increase the photocatalytic efficiency of PCN: formation of heterojunction,12 structural modification,13-14 cocatalyst inclusion,15 copolymerization,16-17 non-metal doping,18-22 and metal doping.23-27 Among these, the metal doping strategy was investigated extensively because the metal functionalization approach is ubiquitous in natural photosynthesis. The metal functionalization can effectively tune the spectroscopic charactersitics of the organometallic complexes. The most notable examples are the porphyrin and phthalocyanine derivatives where metal coordination through nitrogen atoms can formulate light-responsive organometallic compounds (e.g., chlorophyll a).23 Similar to the natural phenomenon, the introduction of metal species into the PCN structure also allowed bandgap tuning, higher visible light absorption, enhanced photoelectrochemical activities, and created another catalytic active center in the PCN network.23-27 Metal ions interact with the electron-rich “nitrogen pot” where six lone pairs from nitrogen atoms provide an ideal location for metal coordination similar to crown ether or porphyrin derivatives.28-29 Wang et al. investigated the effects of the transition metal (Fe3+ and Zn2+) inclusion into the PCN framework. They observed the metal incorporation modified electronic properties, lowered bandgap, enhanced visible light absorption, and more 3


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efficiently activated H2O2 to degrade organic dyes under visible light irradiation.23 Ding et al. showed that other 3d transition metals including Cu2+, Mn3+, Ni3+, and Co3+ also enhanced the activities of PCN similar to the previous work.25 Gao et al. further demonstrated that even alkali and alkaline earth metal ions intercalation and coordination could also increase the photocatalytic activities of PCN for hydrogen evolution.26 Even though the previous metal incorporation studies exhibited significant advancements,23-27 weak ligation and intercalation of the metallic species make the metal-incorporated PCN inherently unstable in aqueous environment and susceptible to demetallation. For example, an iron-doped PCN sample experienced a notable reduction in the iron content after the photocatalytic reaction.24 In addition, the reported works typically required a significant amount of metal precursor loading, which increases the overall manufacturing cost.23-27 Recently, Chen et al. attempted a novel copolymerization method for metal inclusion using reactive silver tricyanomethanide (AgTCM) complexes with dicyandiamide.27 The ionic interaction between the silver cation and TCM anion successfully increased the electron density of the photocatalyst and the stability of metal coordination, which resulted in higher photocatalytic activities for hydrogen evolution and alkyne hydrogenation. However, this method requires a cumbersome process of comonomer synthesis, and to our knowledge, there has not been a facile, scalable, and yet sturdy method to coordinate metal ions while altering the photophysical charactersitics such as the band energy levels and enhanced visible light absoprtion of PCN and increasing the stability and reactivity of PCN. As a new method for incorporating metal species into the PCN framework, we designed this proof-of-concept experiment by in-situ generation of chelating ligands complexed within the PCN framework. The simple keto-enol based cyclization chemistry between urea, one of the precursors of PCN, and acetylacetone (pentane-2,4-dione) results in the incorporation of 4


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2-hydroxy-4,6-dimethylpyrimidine (HDMP) within the PCN matrix (Scheme 1). The addition of either metal acetylacetonate complex or acetylacetone (aa) with any metal precursors induced a similar synthetic process; this observation suggested that the hydroxyl group and nitrogen lone pair of HDMP could serve as a bi-dentate, localized metal chelating agent to formulate a new metal coordination site (having both O- and N-ligand) upon deprotonation.3032

Among various metals, we chose aluminum as the model metal to investigate the new

metal coordination effect; other transition, alkali, and alkaline earth metal ions may have their own catalytic properties or known factors that influence the photocatalytic activities.23-27 Aluminum ions, to our knowledge, do not have any reported effects of modifying the photocatalytic performances. Because we wanted to rule out other catalytic properties of different metal ions, aluminum was proper for investigating the effect of the new metal coordination site only. We tested organic compound degradation and H2 evolution as the model reactions for this Al-incorporated PCN photocatalyst. The economical and facile synthesis method of highly active Al-incorporated PCN opens up many new possibilities that a variety of metal species can be employed to make the photochemical activities of carbon nitride more suitable for practical applications.

Results and Discussion Characterization of PCN and Al-functionalized PCN. The copolymerization between acetylacetone (aa) and urea to form new metal coordination centers in PCN matrix is illustrated with a proposed mechanism in Scheme 1. The synthesis scheme is comprised of the pyrolysis of urea into tri-s-triazine rings and the cyclization of acetylacetone with urea to form HDMP. The resulting tri-s-triazine ring and HDMP then undergo bimolecular condensation. The HDMP agent forms a chelating bond 5


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with the nearby metal ion (aluminum for this case) through the hydroxyl group to formulate a new metal coordination center. This simple and unique approach eliminates any pre-synthesis steps or requirements for complex synthesis procedures that were employed in previous studies.23-27 This merit also enables wide applicability for commercialization and mass production as a practical photocatalyst. If the above description of the synthesized photocatalysts is valid, the 13C solid-state nuclear magnetic resonance (SSNMR) should be almost identical when acetylacetone, Al(aa)3, or HDMP is incorporated as a dopant in the PCN matrix; this statement also applies for the aluminum-incorporated samples in that their 27Al SSNMR spectra should show similar results. The direct evidence for HDMP incorporation into the PCN matrix is crucial to support the proposed process of synthesis. The typical synthesis steps were comprised of pyrolyzing 3 g of urea to form bulk PCN and 3 g of urea with different Al(aa)3 amounts (0.005 g, 0.01 g, and 0.03 g) to synthesize Al-incorporated PCN samples in a muffle furnace. The ICP-MS analysis was performed for the Al-incorporated samples to determine the aluminum content. The Al-doped samples had 0.32 wt%, 0.45 wt%, and 1.28 wt% of aluminum for the lowest to the highest dopant amount, respectively. We first analyzed the carbon species via

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C cross-polarization/magic angle spinning solid-state nuclear magnetic

resonance (13C CP/MAS SSNMR). The PCN spectrum showed the two distinct carbon peaks at 157 ppm and 164 ppm corresponding to C1 (CN3) and C2 (CN2(NHx)) carbons, respectively (Figure 1a).33-34 The HDMP spectrum (Figure 1b) exhibited peaks at 178, 165, 160, 158, and 107 ppm. The exact assignment of these carbons was not possible because pyrimidine derivatives undergo hydrogen bonding to form self-assembled structures. Since such intermolecular arrangements and interactions show distinctive SSNMR spectra, the only peak that we could assign was 107 ppm, corresponding to C3.35-36 For the physical mixture of 6


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PCN and HDMP (Figure 1c, see Experimental and Methods for details), it was observed that HDMP peaks at 165 ppm, 160 ppm, and 158 ppm overlap with the main PCN peaks at 157 ppm and 164 ppm. This implies that the analysis of SSNMR spectra cannot provide a clear evidence that the HDMP unit is incorporated into the PCN network. For all the Alincorporated samples (Figure 1d-f), the two dominant peaks were also observed at 157 ppm and 164 ppm, similar to the PCN spectrum. With increasing the dopant concentration, the valley region intensity also increased, which might imply that the HDMP moieties are embedded into the Al-incorporated PCN matrix although it should not be taken as an evidence. More direct evidence for HDMP incorporation can be observed from Alaa(1.28%)_PCN (Figure 1f) exhibiting a new peak at 94 ppm, which might correspond to the carbon para to the HDMP hydroxyl group with Cγ (CC3) configuration (Figure 1i). In pristine HDMP 13C SSNMR spectrum, this carbon peak evolves at 107 ppm. The downshift in ppm range suggested that this carbon experiences a more electron-rich chemical environment. This observation agrees with our DFT calculation model and the proposed synthesis process (Scheme 1) that the linkage between HDMP and tri-s-trizaine unit is formed at this carbon. The electron-rich tri-s-triazine ring may donate extra electron density to this carbon to alter the chemical environment for this carbon. To observe the SSNMR spectral change more clearly, we prepared additional control PCN samples with 30 times higher dopant cocentration with respect to the Al(aa)3 used to synthesize Al-aa(0.32%)_PCN; the dopants used were HDMP or Al(aa)3. The spectral similarity of these two control samples (Figure 1g and 1h) demonstrate the successful incorporation of HDMP into the PCN matrix. The carbon peak at 94 ppm was more clearly observed in these samples, which supports the proposed synthesis mechanism that tri-s-trizaine unit and HDMP are connected via this carbon (Cγ). 7


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The optimized DFT calculation models (Figure 2g and 2h) also support this Al-N interaction since the most thermodynamically stable complex structure was obtained when both oxygen and nitrogen of HDMP participated in the metal coordination. The similarities in all the doped samples spectra indicate that HDMP incorporation is successful regardless of the kind of comonomers employed. This simple synthesis procedure to create a bi-dentate ligand (HDMP having both O- and N-coordination) is crucial for achieving highly-reacitive and yet stable organometallic complex within the PCN network. The N-based coordination are excellent in donating the electron density to the metal center to increase the electron density, which inherently increases the reactivity of the organometallic complex. However, they exhibit rather poor stability in the ambient aqueous environment and are susceptible to oxidation.37 On the other hand, the oxygen ligand also donates the electron density to the metal center (like nitrogen ligand) but it exhibits much higher resistance towards the oxidation probably due to its strong electronegativity.32, 38-39 The HDMP-based cooridnation provides both nitrogen and oxygen ligands as the efficient and strong coordination center for the metal ions, which should strengthen the stability of the metal complex and reduce the dissolution of the metal ions when photocatalysts are immersed in the aqueous environment. We also checked whether the formation of the same coordination site can be made by adding acetylacetone as a separate reagent along with other aluminum precursors such as AlCl3, Al(OCH(CH3)2)3, and Al[OC(CH3)3]3 (see Experimental and Methods for details). The 27

Al solid state nuclear magentic resonance spectra (Figure 2a-f) support the speculation that

the aluminum is in the octahedral site because the ppm range for the aluminum is near zero.40 The close similarity between Al(aa)3-doped samples and those synthesized using the separate reagents of acetylacetone and other aluminum precursors supports the successful

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incorporation of metal chealting coordination site through HDMP (13C CP/MAS SSNMR of these samples are provided in Figure S1). Subsequently, we conducted X-ray photoelectron spectroscopy (XPS) to analyze the surface composition of the catalysts (Figure 3a-i). The survey spectra for the analyzed samples are shown in Figure S2a-c. The Al 2p band was observed only with the Alincorporated samples as expected (Figure 3g-i). We primarily focused on carbon 1s deconvolution (Figure 3a-c) and oxygen 1s deconvolution (Figure 3d-f) to observe HDMP constituents within the polymeric network. In all spectra, we detected three significant C 1s binding energy signals of 284.7 eV, 286.6 eV, and 288.1 eV, which were attributed to the adventitious carbon contaminants and sp2 hybridized carbons, C-O or C-OH of hydroxylated carbon species, and sp2 carbon in tri-s-triazine rings (N=C-N), respectively.41-42 As we incorporated different concentrations of dopant, a new peak emerged at 285.9 eV along with an increase in intensity for 286.6 eV for C 1s signals. The new peak at 285.9 eV is assigned to the C-N carbon present in HDMP.42 The increment in C-O intensity should be induced by the hydroxyl carbon in HDMP. For oxygen, we observed two major peaks for O 1s binding energy at 531.2 eV and 532.6 eV, which were assigned to N-C-O species and surface hydroxyl groups. It should be noted that some oxygenated species were present even in the pure PCN sample since all the samples were pyrolyzed under the ambient condition (see Experimental and Methods for detail). The incorporation of the oxygen-containing functionalities are inevitable to some extent under this preparation condition. Upon introduction of the dopant, the peak at 531.2 eV increases in intensity; this result corresponds to the N-C-O species present in HDMP, similar to the carbon 1s peak at 285.9 eV and 286.6 eV. We also observed a new peak at around 534.2 eV, a peak for adsorbed oxygen

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molecules.43 The XPS results along with 13C CP/MAS SSNMR confirm that sp2 carbons, C-N bonds, N-C-O, and C-O from HDMP all exist within the carbon nitride matrix. On the other hand, the XPS N 1s deconvolution spectra displayed no discernible difference between the bulk and modified samples due to the similarities in the chemical environment for the nitrogen species (Figure S3a-c). The nitrogen deconvolution consisted of three major peaks at ~398.4 eV, 399.5 eV, and 400.8 eV. These peaks were assigned to C=NC of tri-s-triazine, N-(C)3 for nitrogen bridge in PCN, and C-NHx of incomplete polymerization or terminal amines, respectively.44 The transmission electron microscopy (TEM) images with electron energy loss spectroscopy (EELS) mapping showed a sheet-like morphology for all the synthesized samples (Figure S4a-h). To see where the aluminum was present in the doped PCN samples, EELS elemental mapping images of carbon, nitrogen, and aluminum were compared. Even elemental distributions with showing no high local intensity for C, N, and Al indicate that this particular synthesis method ensures the dispersion of metal atoms.27 Finally, powder X-ray diffraction analysis demonstrated how dopant concentration affects the crystallinity properties of PCN (see Figure 4). In XRD, the PCN spectrum showed two dominant peaks corresponding to the in-plane repeated unit (100) and interlamellar stacking (002) at 13.1° and 27.3° (2θ), respectively.45 The (100) peak began to dwindle with increasing the content of Al(aa)3. We also observed that the (002) peak intensity decreased and broadened with Al-doping with no significant peak shift. This implies that the planar structure of carbon nitride is distorted around the Al center (Figure 2h) but the interlayer distance was not changed.27 The absence of XRD peak shift seems to be ascribed to two major factors: the very low dopant concentration of 0.32 wt% and the atomically-dispersed aluminum ions within the polymeric carbon nitride matrix (see Figure S4). Because of the 10


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sparse dispersion of the local distortion Al-centers, the bulk polymer structure seems not to be heavily affected without causing a noticeable shift for (100) and (002) peaks. Optical properties of Al-functionalized PCN. We conducted UV-visible diffuse reflectance spectra analysis to see the increase in visible light absorption with increasing the dopant concentrations (Figure 5a). The PCN absorption edge ended around 460 nm, barely absorbing the violet-blue region of the visible light. The adsorption edge red-shifted upon doping, a similar trend observed in the previously reported metal-doped samples.23-27 The apparent color change from yellowish-white to darker yellow of doped samples suggests that the bandgap is altered in the metal-modified PCN specimens. We generated Tauc plots (Figure 5b-e) by extrapolating the slope of the linear region of the absorption spectra to the x-axis where the intersection of the slope and the axis represents the optical bandgap.46 The PCN exhibited 2.77 eV of the bandgap which agrees with the literature values whereas the doped samples showed a steady decrease in the bandgap of 2.71 eV, 2.53 eV, and 2.18 eV for Alaa(0.32%)_PCN, Al-aa(0.45%)_PCN, and Al-aa(1.28%)_PCN, respectively. It is also noted that Urbach’s tail with ~2.2 eV bandgap evolved with the Alaa(0.32%)_PCN and Al-aa(0.45%)_PCN. Urbach’s tail represents the localized midgap state (sub-bandgap) that allows that material to absorb lower energy photons than the actual optical bandgap.22 Previous works also suggested that sub-bandgap states provide the localized energy levels for the photogenerated carriers, which can effectively suppress the recombination.47-48 The midgap formation usually occurs when there is a structural disturbance through thermal treatment or doping, correlating with the lowered crystallinity upon introducing dopants in XRD analysis.48-49 The Urbach’s tail disappeared for the Alaa(1.28%)_PCN sample. This observation indicates that the discernible difference between the midgap and the actual bandgap was no longer found for heavily doped samples. The 11


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introduction of alternative metal coordination sites can effectively tune the overall band structure to promote more visible light absorption, which should influence the photocatalytic activity. The steady-state photoluminescence (PL) (Figure 6) also provides useful information regarding the dopant-induced sub-bandgap defects. Upon the excitation with 350 nm, the PCN sample exhibits a strong emission centered around 465 nm. When Al(aa)3 is introduced into PCN, the PL not only red-shifts but also decreases in intensity for all doped samples. It is noted that the peak maximum exhibits a gradual red-shift from 465 nm for pure PCN to 498 nm for Al-aa(0.32%)_PCN, 504 nm for Al-aa(0.45%)_PCN, and 525 nm for Alaa(1.28%)_PCN. Such behaviour was similarly observed in the previous studies that investigated the copolymerization of PCN with aromatic moieties such as barbituric acid and 2-aminobenzonitrile, which was ascribed to the extended π-conjugation.17-18,

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In

particular, a recent study which incorporated benzene ring units in the PCN structure observed a very similar PL behaviour:17 PL decreased and red-shifted with increasing the benzene dopant concentration. Since HDMP, an aromatic moiety, was introduced in this work, the red-shift can be also ascribed to the π-extension in the presence of HDMP moieties within the PCN matrix. The PL intensity decrease observed in the Al-doped samples implies either (1) that the structural distortion around the Al coordination center induces the generation of non-radiative recombination sites or (2) that the exciton dissociation is facilitated by the Al dopants. The former effect should reduce the overall photocatalytic activity whereas the latter effect should increase it. The maximal effect of Al-doping on the photocatalytic activity might be found at an optimal dopant level where the latter effect prevails.

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Photocatalytic activities. The photocatalytic activities for both 4-chlorophenol (4-CP) degradation (Figure 7) and H2 evolution (Figure 8) were assessed for each sample. For the organic oxidation part, the model pollutant 4-CP was chosen because 4-CP alone is not photolyzed under visible light irradiation nor adsorbed on the PCN surface at all. Since the photocatalytic degradation of 4-CP, the mechanisms and the production of various oxidation intermediates have been extensively investigated,53-55 4-CP has been frequently employed as a standard test substrate for the photocatalytic oxidation of organic compounds.53, 56-57 To ensure the photoactivity of the synthesized catalysts, the control experiments without light or photocatalyst were performed as well (Figure 7a). For Al-incorporated samples, Alaa(0.32%)_PCN sample showed the highest activity, but the further increase of the Al dopant concentration reduced the activity (Figure 7b) although the visible light absorption increased with the Al-dopant concentration (Figure 5a). It is often reported that the best visible light photocatalyst is not the one that exhibits the highest absorption of visible light.58-60 This is because the photocatalytic activity is determined by not only the photon absorption but also the subsequent charge separation and interfacial charge transfer processes. Lattice and surface defect sites may enhance the visible light absorption but may also serve as charge recombination sites that should reduce the overall photocatalytic activity. In this case, the Alincorporation induces more visible light absorption but the majority of charge pairs seem to be rapidly recombined in the presence of higher level of Al dopants. This is also consistent with the observation that the PL intensity is progressively reduced with increasing the Aldopant concentration (see Figure 6). The high level dopants seem to provide non-radiative recombination sites, which should decrease the photocatalytic activity.17-18, 52 The best performing Al-aa(0.32%)_PCN sample was used for further tests of oxidative degradation. We also monitored the concurrent production of chloride ions along 13


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with the degradation of 4-CP (Figure 7c). The production of chloride was stoichiometrically matched with the removal of 4-CP, which indicated that 4-CP was successfully degraded with leaving no chlorinated byproducts. Interestingly, the addition of equal mole amount of acetylacetone alone exhibited some enhancement in photocatalytic degradation and the activity enhancement was more pronounced in the presence of aluminum incorporation, which demonstrated that the metal coordination is crucial for high photocatalytic activity. The total organic carbon (TOC) analysis for the 4-CP mineralization showed that TOC removal after three-hour photocatalytic reaction was 7.3% and 49.3% for PCN and Alaa(0.32%)_PCN, respectively. Such high mineralizing activity of the Al-doped sample is highly desired as a practical remediation photocatalyst although the degradation intermediates were not fully identified in this study. On the other hand, the photocatalytic activities of PCN and Al-aa(0.32%)_PCN (for 4-CP degradation) were also measured under monochromatic irradiation (with varying the wavelength by a monochromator) and shown in Figure S5. The photoactivities of Al-aa(0.32%)_PCN were consistently higher than those of PCN over the tested wavelength region (380-540 nm). It is interesting to note that the photoactivities of Alaa(0.32%)_PCN little changed in 450-540 nm region whereas the visible light absorption efficiency of Al-aa(0.32%)_PCN rapidly decreased in that wavelength range (see Figure 5a). This implies that the visible light photons absorbed by the Al-doped PCN is very efficiently utilized to induce the subsequent charge transfers. We carried out the 4-CP degradation experiment with PCN samples that were synthesized using other aluminum precursors as dopants (see Experimental and Methods for details, Figure 7d-e) to investigate why metal coordination through HDMP is crucial for the photocatalytic activity enhancement. As shown in Figure 7d, other aluminum precursors did not show any enhancement in 4-CP degradation. The aluminum could be either coordinated 14


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in the traditionally known “nitrogen pot” or turned into aluminum hydroxide. Only after adding acetylacetone along with various aluminum precursors (Figure 7e), the doped photocatalysts displayed markedly enhanced activity. Although the direct incorporation of Al(aa)3 showed the highest activity, other aluminum precursors along with acetylacetone added as a separate reagent also exhibited comparable results (the ICP-MS Al content for these samples are summarized in Table S2). While the aluminum ions complexed in the nitrogen pot have little influnce on the photocatalytic activity, HDMP-complexed Al3+ ions significantly enhance the photocatalytic activity of PCN. This should be ascribed to the unique role of Al3+ complexed by both N and O ligands in HDMP unlike Al3+ complexed by the “nitrogen pot” in the unmodified PCN matrix. The co-presence of Al-O and Al-N coordination in Al3+-HDMP moieties seems to make the aluminum center photochemically active but the exact role needs to be further investigated for better understanding. For the reductive conversion part, we conducted H2 evolution tests under visible light irradiation and compared the rates of H2 production among different photocatalysts as shown in Figure 8a. The photo-deposited Pt (3 wt%) on PCN and triethanolamine (TEOA) were used as the cocatalyst and electron donor, respectively. Since most literature data of the photocatalytic H2 production on PCN were obtained in the presence of Pt cocatalyst and TEOA, the present activity tests were carried out under the same condition to make the direct comparison facile. However, it should be noted that the use of toxic TEOA and noble metal cocatalyst is not practically viable and should be replaced with more environmentally benign electron donors such as biomass derivatives61-63 and non-noble metal cocatalysts.4, 64 The H2 activities of all different kind of Al-incorporated samples show the same trend as the case of 4-CP degradation. The doping of aluminum induced a significant enhancement in H2 production only when it is combined with the addition of acetylacetone. The estimated 15


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quantum yield (QY) for Al-aa(0.32%)_PCN was 6.2% around 420 nm, a significant improvement compared to 1.6% of PCN. The Al-doped samples also demonstrated an excellent stability in water during six cycles of repeated photocatalysis (Figure 8b). To test the possible leaching of aluminum ions from the catalyst in suspension, a colorimetric method with eriochrome cyanine R (ECR), an effective Al3+ ion complexing reagent, was used to determine the dissoluted aluminum ion concentrations (see Experimental and Methods).65 In the photoirradiated suspension of Al-aa(0.32%)_PCN, the production of dissoluted aluminum ions determined by the ECR method was insignificant. Photocatalytic Mechanism.

Understanding why these photocatalysts doped with both

metal ions and acetylacetone performs so effectively under visible light is crucial. The charge transfer characteristics of the photocatalysts samples were investigated by electrochemical impedance spectroscopy (EIS). PCN, Al-aa(0.32%)_PCN, Al-aa(0.45%)_PCN, and Alaa(1.28%)_PCN electrodes were compared for EIS analysis to see the difference in charge transfer resistance (Figure 9). The comparison between the semicircular EIS Nyquist plots of these electrodes showed that the interfacial charge transfer resistance of the doped samples was significantly reduced, which subsequently should facilitate the interfacial redox conversions, explaining the high photocatalytic reactivity for both organic degradation and hydrogen evolution.28 The fact that the interfacial charge transfer resistance of the doped samples is the lowest with the lowest dopant (Al-aa(0.32%)_PCN) is consistent with its highest photocatalytic activity. Although Al-aa(0.32%)_PCN exhibits the lowest absorption of visible light among the Al-doped samples, it shows the highest visible light photocatalytic activity because the lower interfacial charge transfer resistance should facilitate the interfacial redox processes (i.e., photocatalytic reactions).

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The nitrogen isotherms and BET analysis were conducted for all the synthesized samples to see how the doping with metal with acetylacetone changes the surface area of the material. As shown in Figure S6a, the BET surface area analysis exhibited that the surface area increases from 52.4 m2/g for pure PCN to 76.2 m2/g for Al-aa(0.32%)_PCN and 124.6 m2/g for Al-aa(1.28%)_PCN. The surface area of acetylacetone-doped PCN (denoted as aa_PCN) was 46.6 m2/g, which shows that the surface area was not enhanced by the acetylacetone incorporation. In comparison, we also measured the surface area of the Aldoped samples that were prepared without acetylacetone (see Figure S6b-d). It is interesting to note that the surface area of Al-iPrO_PCN, Al-tBuO_PCN, and Al-Cl_PCN (113.8, 68.4, and 119 m2/g, respectively) is higher (or comparable) than Al-aa(0.32%)_PCN (76.2 m2/g) although they exhibit much lower photocatalytic activity than Al-aa(0.32%)_PCN. On the other hand, upon introduction of acetylacetone along with these aluminum precursors, the synthesized catalysts exhibited a small increase compared to the bulk PCN in the BET surface area (66.5 m2/g for Al-iPrO+aa_PCN, 63.2 m2/g for Al-tBuO+aa_PCN, and 67.1 m2/g for Al-Cl+aa_PCN) despite their higher photocatalytic activities. The photocatalytic activity generally shows a poor correlation with the catalyst surface area especially for non-adsorbing substrates like 4-CP.57 The photocatalytic activities of Al-doped PCN samples depend on the presence of Al coordination centers along with other properties influenced by the introduction of new metal coordination centers but the catalyst surface area effect seems to be insignificant. The generation of photooxidants on the photocatalysts under visible light irradiation is critical for the oxidative degradation. The scavenger test was performed to see which oxidant species (h+, •OH, and O2-) is mainly responsible for the organic degradation. As shown in Figure S7, the addition of EDTA (hole scavenger)66 and TBA (hydroxyl radical 17


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scavenger)67 had little effect on the degradation of 4-CP for both PCN and Alaa(0.32%)_PCN. However, when p-benzoquinone (p-BQ), a superoxide scavenger,68 was added, the photocatalytic activity was completely suppressed for both PCN and Alaa(0.32%)_PCN. This result implies that the main oxidation power of the photocatalysts is related with the role of superoxide radical.69 Therefore, the main effect of Al-doping in PCN photocatalysts seems to facilitate the conduction band (CB) electron transfer to O2 with generating more superoxide radicals. The lower charge transfer resistance of the Al-doped PCN supports this conclusion as well. However, it should be noted that the photo-oxidative power of PCN without involving hydroxyl radicals are rather limited. The flat band positions were estimated by Mott-Schottky analysis at different frequencies for PCN (Figure 10a) and Al-aa(0.32%)_PCN (Figure 10b). The resultant spectra show positive slopes for both catalyst samples, a typical sign of n-type semiconductor characteristic.70 The intersection between the extrapolated line and the x-axis determines the flat band potential (EFB). The measured EFB of PCN and Al-aa(0.32%)_PCN to be -1.33 V (vs. Ag/AgCl at pH 6) and -1.02 V (vs. Ag/AgCl at pH 6), respectively.71 Figure 10c shows the DFT results on the calculated band edge positions (i.e., HOMO and LUMO levels) of cluster models of PCN and Al-aa(0.32%)_PCN (DFT optimized structures of these cluster models are shown in Figure 2). Three different DFT methods consistently predicted the general trend of downshifted band edge positions of both LUMO and HOMO levels after the metal-functionalization. This trend should be similarly reflected in the positioning of CB and valence band (VB) in the bulk-sized PCN, which is consistent with the flat band potential determined by Mott-Schottky analysis. This result also shows that the HOMO-LUMO gap is slightly lowered by Al-doping, which is consistent with the redshift in the absorption onset upon incorporating Al (Figure 5a). It should be mentioned that a 18


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quantitative comparison between the experimental data and DFT calculation is not adequate because of the incompleteness of xc-functional and model system employed in the calculation. Figure 11 illustrates the band position alignments of PCN and Al-aa(0.32%)_PCN, which were estimated from the experimental and DFT results. The band alignment showed that the dioxygen reduction to generate superoxide radical, E°(O2/O2-) = -0.33 VSHE, is energetically allowed even with the downward shift of the CB edge after Al incorporation, which is consistent with the scavenger test that showed the major active species is the superoxide radical. For hydrogen evolution, the CB position of the Al-incorporated PCN has an enough driving force to enable H2 formation (E°(H+/H2) = -0.177 VSHE at pH 3).

Conclusion In summary, this study demonstrated a new and simple method of stable metal incorporation in PCN matrix to enhance the photocatalytic activities markedly for both organic compound degradation and H2 production. Using aluminum acetylacetonate as a simple precursor for metal incorporation into the PCN framework eliminated the complex process of co-monomer synthesis and fabrication steps. The Al-incorporated PCN exhibited sufficient durability in an aqueous environment without showing any sign of Al leaching during the repeated cycles of photocatalysis. Another advantage of this direct coordination strategy using acetylacetone was that only a very small amount of metal precursor was needed to optimize the photocatalytic activity and alter chemical/physical characteristics of the synthesized catalysts. Whereas 1~10 wt% of the metal precursor was typically required to boost the photocatalytic activity in the previous studies,23-27 the optimal Al content we found in this work was as low as 0.32 wt%. The photocatalytic activities of the Al-doped PCN depended heavily on the presence of the new metal coordination centers for both the 19


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degradation of 4-CP and H2 production. A systematic characterization of Al-doped PCN samples revealed that the metal incorporation significantly changed the optical, electrochemical, and photocatalytic properties. The new metal coordination through acetylacetone provides a novel and simple way to enhance the photocatalytic activities of bare carbon nitride markedly for a variety of applications utilizing solar visible light. This unique PCN material that contains metal ions incorporated with acetylacetone calls for further investigations to test different metal species other than Al. This study proposed a new strategy that can be applied to the development of highly efficient carbon nitride-based photocatalysts for various solar conversion with employing a simple synthetic approach.

Experimental and Methods Materials. The following chemicals were purchased and used without further purification: urea (Sigma-Aldrich, ≥98%), aluminum acetylacetonate (Al(aa)3, Sigma-Aldrich, 99%), aluminum isopropoxide (Al[OCH(CH3)2]3, Sigma-Aldrich, ≥99.99%), aluminum tri-tertbutoxide (Al[OC(CH3)3]3, Sigma-Aldrich), acetylacetone (aa, Sigma-Aldrich, ≥99%), hexachloroplatinic acid hexahydrate (H2PtCl6⋅6H2O, Sinopharm Chemical Reagent Co., Ltd), triethanolamine (TEOA, Sinopharm Chemical Reagent Co., Ltd), Na2SO4 (Na2SO4, CicaReagent, 99%), ethanol (Samchun, 99.5%), methanol (J.T. Baker, ≥99.9%), poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (PEG-PPG-PEG, SigmaAldrich, Mn average ~2800), anhydrous terpineol (Fluka, 97.0%), tert-butanol (TBA, SigmaAldrich,

≥99.0%),

p-benzoquinone

(p-BQ,

Sigma-Aldrich,

99.5%),

ethylenediaminetetraacetic acid disodium salt dihydrate (EDTA, Sigma-Aldrich, 99.0101.0%),

2-hydroxy-4,6-dimethylpyrimidine (HDMP, Alfa Aesar, 97%), perchloric acid

(HClO4, Sigma-Aldrich, 70%), phosphoric acid (H3PO4, Sigma-Aldrich, ≥85 wt% in H2O), 20


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and acetonitrile (J.T. Baker, ≥99.9%). Aluminum chloride hydrate (AlCl3⋅xH2O, SigmaAldrich, 99.999%) was dried in an oven at 80 °C for at least 72 hours before usage. Ultrapure (14 MΩ⋅cm) deionized water was prepared by a Barnstead purification system.

Synthesis of bulk polymeric carbon nitride.

Bulk polymeric carbon nitride (denoted as

PCN) was synthesized via thermal pyrolysis of urea similar to the experimental procedure described by the reported work.47 A typical batch run contained 3 g of urea in a lidded alumina crucible. Then, the crucible was heated in a muffle furnace at a ramping rate of 5 °C per minute to 550 °C. The temperature was maintained for two hours before cooling to room temperature. The resultant biscuit-like powder was washed with deionized water for several times and dried in an oven at 80 °C for 24 h for future usage.

Synthesis of aluminum acetylacetonate functionalized PCN. Aluminum-functionalized PCN was prepared via similar steps to the method described in the previous section. 3 g of urea and aluminum acetylacetonate (Al(aa)3) (0.005g, 0.01g, and 0.03g) were put into an alumina crucible after grinding them for an hour to reach complete homogeneity. Then, the crucible was heated in a muffle furnace at a ramping rate of 5 °C per minute to 550 °C. The temperature was maintained for two hours and decreased to room temperature. After cooling, the final product was washed with deionized water several times and dried in an oven at 80 °C for 24 h for future usage. After analyzing the metal content through ICP-MS analysis, the prepared catalysts were denoted as Al-aa(0.32%)_PCN, Al-aa(0.45%)_PCN, and Alaa(1.28%)_PCN from the lowest to highest dopant concentration, respectively. For SSNMR analysis, the sample denoted as Al-aa30x_PCN was synthesized by using 30 times more

21


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dopant than that for Al-aa(0.32%)_PCN. The heating procedure was identical to the aforementioned procedure.

Synthesis of PCN using other aluminum precursors. The equal mole amount of aluminum precursor (AlCl3, Al(OCH(CH3)2)3, or Al[OC(CH3)3]3) (with respect to that of Al(aa)3 used for the synthesis of Al-aa(0.32%)_PCN) was ground with 3 g of urea for an hour to reach homogeneity. The resultant powder was heated to 550 °C with the ramping rate of 5 °C per minute. After maintaining the temperature at 550 °C for two hours, the resultant catalysts were cooled to room temperature. The final products were washed with deionized water several times and put into an oven at 80 °C for 24 h before usage. The catalysts were denoted as Al-Cl_PCN, Al-iPrO_PCN, and Al-tBuO_PCN according to the precursor used, AlCl3, Al(OCH(CH3)2)3, and Al[OC(CH3)3]3, respectively. Instead of using Al(aa)3 as a single precursor, different Al-incorporated samples were also prepared by using a mixture of aluminum precursor and acetylacetone (aa) as separate reagents. The equal mole amounts of aluminum precursor (AlCl3, Al(OCH(CH3)2)3, or Al[OC(CH3)3]3) and acetylacetone (with respect to that of Al(aa)3 used for the synthesis of Al-aa(0.32%)_PCN) were ground with 3 g of urea for an hour to reach homogeneity. The subsequent synthesis process was the same as the above. The catalysts prepared were denoted as Al-Cl+aa_PCN, Al-iPrO+aa_PCN, and Al-tBuO+aa_PCN according to the precursor used, AlCl3, Al(OCH(CH3)2)3, and Al[OC(CH3)3]3, respectively. Synthesis of 2-hydroxy-4,6-dimethylpyrimidine (HDMP) incorporated PCN. The incorporation of HDMP within the PCN matrix was achieved in two methods. When acetylacetone was used, 3 g of urea and 4.7 µL of acetylacetone were put into an alumina crucible. The molar amount of acetylacetone was equal to that used to synthesize Al22


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aa(0.32%)_PCN; this is the three times in mole amount with respect to the Al(aa)3 used to make Al-aa(0.32%)_PCN because aluminum acetylacetonate has three mole equivalent of acetylacetone. The crucible was heated to 550 °C with the ramping rate of 5 °C in a muffle furnace. The temperature was set at 550 °C for two hours before cooling to room temperature. This sample was denoted as aa_PCN. Another method directly introduced HDMP molecule with urea. The sample denoted as HDMP_PCN contained 30 times molar amount of HDMP with respect to Al(aa)3 used to make Al-aa(0.32%)_PCN. The resultant mixture was heated to 550 °C with the ramping rate of 5 °C after grinding them for an hour. The temperature was also maintained at 550 °C for two hours before cooling. The resultant powder was ground and washed with deionized water several times and dried in an oven at 80 °C for 24 hours before usage. As a control sample, a physical mixture of 0.1 g of PCN and 0.057 g of HDMP was also prepared and denoted as HMDP+PCN Characterization methods for bulk and functionalized PCN. The morphology and atom composition of PCN and Al-modified PCN samples were characterized using high-resolution transmission electron microscopy (HR-TEM; JEOL JEM-2100F) and electron energy loss spectroscopy (EELS) analysis using a Cs-corrected line at National Institute for Nanomaterials Technology (Pohang, Korea). X-ray diffraction (XRD) patterns were measured with PANalytical X’Pert PRO with Cu Kα radiation. Diffuse reflectance spectra were recorded on Shimadzu UV-2600 spectrophotometer; utilizing Kubelka-Munk equations, FR = =

1 − 2

FR ∙ hν/ = hν − the data were fitted to show the visible light absorption and optical bandgap (Eg).72 X-ray photoelectron spectroscopy (XPS) was analyzed to see the surface elemental composition 23


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with monochromatic Al Kα radiation (Theta Probe AR-XPS System, 1486.6 eV, Busan Center, KBSI, Korea). Brunauer-Emmett-Teller (BET) surface areas were analyzed by nitrogen gas physisorption experiment (AutoporeIV, UPA-150, ASAP2010, Jeonju Center, KBSI, Korea). Inductive Coupled Plasma Mass Spectrometry (ICP-MS) was conducted with Quadrupole mass filter with Argon plasma (X2, I-CAPQ, Attom, Neptune, Ochang Center, KBSI, Korea).

Solid-State NMR.

13

C solid state nuclear magnetic resonance spectra were obtained with

cross-polarization magic angle spinning (13C CP/MAS SSNMR; Bruker Avance III HD 400, Brucker, Germany, 100.66 MHz; Zirconia Rotor, 8-10 kHz, Western Seoul Center, KBSI, Korea).

27

Al solid state nuclear magnetic resonance spectra were obtained with magic angle

spinning (27Al MAS SSNMR;

unity

INOVA, Agilent Technologies, U.S.A, Zirconia Rotor, 22

kHz, Western Seoul Center, KBSI, Korea).

Photoluminescence.

Steady-state photoluminescence spectrum

(Microtime-200,

Daegu

Center, KBSI, Korea) was measured for the samples by guiding emission signal using an optical

fiber

to

the

external

spectrometer

(F-7000,

Hitachi).

The

steady-state

photoluminescence was measured for the synthesized catalysts from 365 nm to 800 nm with time-correlated single-photon counting method. The excitation wavelength was 350 nm (HORIBA, FluoroMax-4). Photocatalytic activity measurements for the degradation of organic compound. The prepared catalysts were suspended in ultrapure deionized water in a 50 mL Pyrex vial. The model pollutant used was 4-chlorophenol (4-CP). The pH was adjusted to 3.0 using HClO4. A typical batch run contained 0.5 g/L of catalyst and 100 µM of 4-CP in 30 mL solution. The 24


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reactor was allowed to be equilibrated under the dark and ambient air condition and magnetic stirring for 30 minutes to establish the organic substrate adsorption equilibrium and dioxygen dissolution in the catalyst suspension (air saturation). The irradiation from a 300-W Xenon arc lamp (Oriel) was filtered by a 420-nm long pass filter and a 10-cm IR water filter and then focused onto the reactor. The photocatalytic activity was monitored by measuring the concentration of 4-CP with a high-performance liquid chromatograph (HPLC, Agilent Technologies 1260 infinity) with the ZORBAX column (Agilent Technologies, ZORBAX 300SB-C18, Stablebond Analytical, 4.6 × 150 mm, 5-micro). The concurrent generation of chloride ion was analyzed by an ion chromatograph (IC, Dionex DX-120). The sample aliquot (1 mL) was taken each hour for both HPLC and IC analysis. The photocatalyst was removed by a syringe filter (Millex®-LCR Low Protein Binding Hydrophilic LCR (PTFE) membrane, 0.45 µm pores). After three hours of irradiation, the remaining solution was filtered and analyzed with a total organic carbon instrument (TOC, Shimadzu TOC-VCSH) to estimate the degree of mineralization. When the irradiation wavelength-dependent photocatalytic activity was measured (see Figure S5), the Xe-lamp light was filtered and adjusted at a specific wavelength by a monochromator (Newport, Oriel 77250). Since the monochromatic light intensity was much lower than the polychromatic irradiation, the photocatalytic reactions were carried out in a small quartz cuvette (3 mL). The other procedure was similar to the aforementioned experimental process.

Photocatalytic measurements for H2 evolution. Photocatalytic hydrogen production was carried out in a Pyrex top-irradiation reaction vessel connected to a closed gas circulation system. 50 mg catalyst powder was dispersed in an aqueous solution (100 mL) containing 10 mL of 10 vol% TEOA as a sacrificial agent. The Pt cocatalyst of 3 wt% was deposited on the 25


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catalyst surface using an in situ photodeposition method with using H2PtCl6 as a precursor. The reaction solution was evacuated several times to remove the dissolved air completely before irradiation under 300-W Xe lamp. The wavelengths of the incident light were controlled by using a set of long-pass cut-off filters. The temperature of the reaction solution was maintained at room temperature using cooling water during the reaction. The evolved gas was analyzed by a gas chromatograph equipped with a thermal conductivity detector (TCD) and a 5-Å molecular sieve column. Argon was used as a carrier gas. The apparent quantum yield (AQY) for the H2 evolution was determined by replacing the Xe lamp with a 420-nm LED. The 420 nm bandpass filter was also used to narrow the bandwidth of the light source. The irradiation (at 420 nm) area and average intensities were 9 cm2 and 11.9 mW⋅cm-2 (ILT 950 spectroradiometer), respectively. The AQY was calculated as follow: AQY =

2 =

where Ne is the number of reacted electrons, Np is the total number of incident photons, and M is the number of produced H2 molecules. Aluminum Dissolution Test.

The possible dissolution of aluminum ions from the Al-

doped CN photocatalyst was tested by a colorimetric method that was described elsewhere.65 To briefly summarize, the reagent solution was prepared by dissolving eriochrome cyanine R (ECR) in deionized water (100 mg/L) through vigorous sonication. The Al3+-ECR compexes induce a strong absorption peak at 533 nm (6.5x104 M-1cm-1) and the dissolved aluminum concentration was calibrated by using standard solutions of 10-50 µM Al3+ ions prepared by dissolving aluminum chloride in deionized water. The catalyst (PCN and Alaa(0.32%)_PCN) was dispersed in deionized water (0.5 g/L) and the catalyst suspension was irradiated for 3 h under magnetic stirring. The irradiated PCN catalyst was removed by a 26


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syringe filter, and the filtrate solution was mixed with ECR solution, then the UV-visible absorption spectra were recorded (Agilent 8453) to determine the dissoluted aluminum concentrations. (Photo)Electrochemical Measurements. The PCN and Al-modified PCN electrodes were prepared by a simple Dr. Blade method. 0.5 g of catalyst was ground for two hours with 0.2 mL of PEG-PPG-PEG and 3.0 mL of terpineol. The resultant mixture was annexed onto the clean FTO glass with the electrode area of 2 cm × 2 cm. The resulting electrode was annealed in a muffle furnace at 450 °C for an hour with a ramping rate of 5 °C per minute under the ambient atmosphere. After cooling to room temperature, the prepared working electrode, an Ag/AgCl/NaCl 3M (+0.209 V vs. SHE) reference electrode (RE-5B, BASi), and a Pt wire counter electrode were immersed in an aqueous Na2SO4 solution (0.1 M, pH ~6) in a standard three-electrode cell with a quartz window. The electrolyte solution was pre-purged with ultrapure Ar gas for 30 min before immersion and continuously purged throughout the experiment. Electrochemical impedance spectroscopic analysis (EIS) was performed at a potential of +0.20 V (vs. Ag/AgCl/NaCl 3M) with an AC voltage of 50 mV RMS under visible light irradiation (λ > 420 nm); the frequency ranged from 1 MHz to 0.1 Hz. MottSchottky plot was generated by sweeping voltage range from -0.8 V to 0.8 V at a frequency of 1, 3, and 5 kHz in the dark.

DFT Calculations. Density functional theory (DFT) calculations were performed for PCN and Al-aa_PCN systems using Jaguar 8.4 program (Schrödinger, L.L.C.). To appease the band-gap underestimation problem of conventional exchange-correlation (xc) functionals based on local density approximation (LDA) or generalized gradient approximation (GGA), 27


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we cross-compared results from one meta-GGA functional of M06-L73 and results from two hybrid functionals of B3LYP and B3PW91 having 20% of Hartree-Fock exchange B3LYP. Pople’s split-valence double-zeta basis set of 6-311G** was used for DFT calculations.

Associated Content Supporting Information. Additional characterization data of PCN and Al-doped PCN samples: Table S1-2, Figure S1-S7. This supporting information is available free of charge via the Internet at http://pubs.acs.org.

Author Information Corresponding Author. [email protected] Notes The authors declare no competing financial interest.

Acknowledgements This work was financially supported by the Global Research Laboratory (GRL) Program (No. NRF-2014K1A1A2041044), “Next Generation Carbon Upcycling Project” (Project No. 2017M1A2A2042517), and KCAP (Sogang Univ.) (No. 2009-0093880), which were funded by the Korea Government (Ministry of Science and ICT) through the National Research Foundation (NRF). The authors would also like to thank Dr. Dipankar Barpuzary for assisting electrode synthesis.

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Scheme 1. Overall chemical synthesis reaction between acetylacetone, urea, and metal ions (aluminum in this case) to form localized metal coordination site through in-situ formation of HDMP in the PCN network.

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Figure 1. 13C cross-polarization/magic angle spinning solid-state nuclear magnetic resonance spectra (13C CP/MAS SSNMR) for pure PCN (a), HDMP (b), the physical mixture of HDMP + PCN (c), Al-aa(0.32%)_PCN (d), Al-aa(0.45%)_PCN (e), Al-aa(1.28%)_PCN (f). The catalysts prepared with higher dopant concentration (30 times more mol% dopant with respect to Al-aa(0.32%)_PCN) are shown in (g) for HDMP as dopant and (h) for Al(aa)3 as dopant. The unit structure (i) was shown to facilitate the carbon assignment. (the * symbol indicates the spinning side bands of the solid-state NMR spectra).

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Figure 2. The 27Al solid-state NMR (27Al SSNMR) of Al-aa(0.32%)_PCN (a), Alaa(0.45%)_PCN (b), and Al-aa(1.28%)_PCN (c). The catalysts prepared with other aluminum precursors (AlCl3, Al[OCH(CH3)2]3, and Al[OC(CH3)3]3) along with addition of acetylacetone are shown in (d-f). DFT optimized atomistic structures of cluster models of PCN (g) and Al(aa)3 PCN (h). All the structures from three different DFT methods (M06-L, B3LYP, and B3PW91) are nearly the same, and thus the results obtained using M06-L functional are representatively shown. Blue, gray, white, red and purple colors denote nitrogen, carbon, hydrogen, oxygen, and aluminum, respectively.

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Figure 3. X-ray photoelectron spectra (XPS) for C 1s, O 1s, and Al 2p bands for PCN (a,d, and g), Al-aa(0.32%)_PCN (b, e, and h), and Al-aa(1.28%)_PCN (c, f, and i). For C 1s deconvolution, the following symbols, I* (sp2 C species), II* (C-N bond species), III* (C-O bond species), and IV* (N=C(-N)2 species) are used to indicate the corresponding binding energy bands obtained from the band deconvolution. For O 1s, the following symbols, I* (NC-O species), II* (surface –OH groups), and III* (adsorbed O2) were used to indicate the corresponding binding energy.

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Figure 4. X-ray diffraction patterns (XRD) of PCN, aa_PCN, Al-aa(0.32%)_PCN, Alaa(0.45%)_PCN, and Al-aa(1.28%)_PCN photocatalysts.

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Figure 5. (a) UV-visible diffuse reflectance spectrum (DRS) of PCN, Al-aa(0.32%)_PCN, Al-aa(0.45%)_PCN, and Al-aa(1.28%)_PCN photocatalysts; (b-e) The bandgap analysis through modification of Kubelka-Munk function for the above samples. BaSO4 was used as the reference. 40


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Figure 6. Steady-state photoluminescence (PL) spectra for PCN, Al-aa(0.32%)_PCN, Alaa(0.45%)_PCN, and Al-aa(1.28%)_PCN photocatalysts. The excitation wavelength was 350 nm. The maximum photoluminescence wavelength observed are: 465 nm for PCN, 498 nm for Al-aa(0.32%)_PCN, 504 nm for Al-aa(0.45%)_PCN, and 525 nm for Al-aa(1.28%)_PCN.

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Figure 7. Time profiles of photocatalytic degradation of 4-CP with different PCN samples. (a) control experiments without either photocatalyst or light. (b) Effect of Al(aa)3 dopant concentration. (c) Comparison of the photocatalytic degradation of 4-CP with the concurrent production of chloride. (d) Effect of other aluminum precursors (AlCl3, Al[OCH(CH3)2]3, and Al[OC(CH3)3]3). (e) Effect of adding acetylacetone as a separate reagent along with other aluminum precursors. The experimental conditions were: [catalyst] = 0.5 g/L, [4-CP] = 100 µM, pH = 3, λ > 420 nm, air-equilibrated.

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Figure 8. (a) The photocatalytic hydrogen evolution experiments for the prepared catalysts. (b) The hydrogen evolution recycle tests with Al-aa(0.32%)_PCN for six cycles. The experimental conditions were: [catalyst] = 0.5 g/L, 10 mL of 10 vol% TEOA, 90 mL of deionized water, and 3 wt% Pt cocatalyst.

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Figure 9. Electrochemical impedance spectroscopic (EIS) Nyquist plots of the PCN, Alaa(0.32%)_PCN, Al-aa(0.45%)_PCN, and Al-aa(1.28%)_PCN catalysts with +0.20 V (vs. Ag/AgCl) with an AC voltage of 50 mV RMS under visible light irradiation (λ > 420 nm); the frequency ranges from 1 MHz to 0.1 Hz.

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Figure 10. Mott-Schottky flat band potential analysis for PCN (a) and Al-aa(0.32%)_PCN (b); 0.1 M of Na2SO4 (pH ~6) as the electrolyte, platinum wire for counter electrode, and Ag/AgCl (+0.209 vs. SHE) as the reference electrode at 1000 Hz, 3000 Hz, and 5000 Hz. (c) Band edge locations (HOMO and LUMO) calculated from three different DFT methods of M06-L, B3LYP, and B3PW91. The numbers in parentheses represent the HOMO-LUMO gap. (c) Schematic diagram of plausible band edge alignment of PCN and Alaa(0.32%)_PCN (downshifted by ~0.3 V for conduction band, ~0.2 V for valence band in accordance with the experimental and DFT results).

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Figure 11. The standard reduction potentials of E°(O2/O2-) and E°(H+/H2) are shown as blue and pink dashed line, respectively. All values are with respect to SHE at pH = 3.

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Polymeric Carbon Nitride with Localized Aluminum Coordination Sites

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