phitic carbon nitride based on rational interpretation of chemical a

Subscriber access provided by UNIV OF MISSOURI ST LOUIS

Characterizing electronic structure near energy gap of graphitic carbon nitride based on rational interpretation of chemical analysis Kouki Akaike, Kenichi Aoyama, Shunsuke Dekubo, Akira Onishi, and Kaname Kanai Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b05316 • Publication Date (Web): 15 Mar 2018 Downloaded from http://pubs.acs.org on March 15, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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

Chemistry of Materials

Characterizing electronic structure near energy gap of graphitic carbon nitride based on rational interpretation of chemical analysis Kouki Akaike*‡, Kenichi Aoyama‡, Shunsuke Dekubo, Akira Onishi, Kaname Kanai Department of Physics, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda-city, Chiba 278-8510, Japan ABSTRACT: Graphitic carbon nitride (g-CN) has attracted enormous interests in an application to a visible-light-driven photocatalyst, particularly for hydrogen evolution via water splitting. Despite intensive photocatalytic works to achieve higher hydrogen-evolution rate, the chemical and electronic structures that are essential for the water photolysis reactions have not comprehensibly understood. To reveal the fundamental properties, we utilized well-oriented g-CN films for reliable analyses with several types of electron spectroscopy. Comparing Xray photoelectron spectra of the g-CN film with those of a g-CN monomer, melem, provided a definite peak assignment of the spectra, from which we identified g-CN as melon. The analysis with ultraviolet photoelectron and inverse photoemission spectroscopy (UPS and IPES) for the melon film clarified energy distributions of the occupied and unoccupied electronic states near the energy gap of melon, respectively. Band structure calculations of a melon crystal revealed orbital characters of the electronic states. The calculations also implied that the energy dispersion of only the lowest unoccupied molecular orbital is present along melon chains. The energy structure of melon, determined by the UPS and IPES spectra, was demonstrated to be preferable for water splitting. The results shown in this study will facilitate designs of superior polymeric photocatalysts.

1. INTRODUCTION Utilizing nearly inexhaustible energy from the sun is an environmentally friendly technique to produce the energy that sustains the global society. This approach would help us gradually estrange from the life that relies fully on relentless consumption of fossil fuels. Photocatalytic hydrogen evolution thus becomes a target of intensive researches since the electrochemical photolysis of water was reported in 19721. Molecular hydrogen is a clean energy resource, because only water is a by-product of the exothermic reaction between hydrogen and oxygen molecules. Thus, the energy generation using hydrogen molecules sets our expectations that the emission amount of greenhouse-effect gases, such as carbon dioxide produced by the combustion of petroleum-based fuels, can be reduced by using fuel cell2,3. Among a number of proposed photocatalysts for this technology4–10, graphitic carbon nitride (g-CN) is a rising material since 200911,12, which comprises only light elements (carbon and nitrogen) and is featured by a layered graphitic-like structure13,14. The polymer exhibits excellent chemical11 and thermal13 stabilities, and potential molecular tunabilities15,16 as well as the photoabsorption in visible-light range. Until now, great efforts have been made to promote photocatalytic reactions by the increase in surface areas of g-CN12,17, reducing the energy gap for a better photoabsorption by heteroatom doping18–22 and copolymerization with other building blocks23–25, and facilitating free carrier generation at the interfaces with noble and transition metals26–28, and with inorganic semiconductors29–34.

The synthesis of g-CN is a facile solid-state pyrolysis of inexpensive nitrogen-rich organic molecules, like triamino-s-triazine (melamine) (Scheme 1). Upon heating the precursor, it is readily converted into 2,5,8-triamino-tri-s-triazine (melem) by deammonation, which is reportedly more thermodynamically stable than the derivatives consisting of s-triazine13,35. Further polycondensation leads to a linear melem polymer, melon, and finally a heptazine-based polymer with the two-dimensional covalent network (g-C3N4) is believed to be formed after full de-ammonation36. However, as several researchers have already pointed out13,16, most of the synthesized g-CN contains significant hydrogen atoms. Together with recently reported X-ray diffraction (XRD) and neutron diffraction study14 and definite identification by Lotsch et al. in 200713, g-CN generally obtained by thermal polycondensation is melon rather than g-C3N4 (Scheme 1). Nevertheless, we can still see the erroneous identification of g-CN as “g-C3N4”. In the recent photocatalytic works, crystal structure and chemical traces of gC3N4 seem to be justified by the routine assignment of XRD, infra-

Sc heme 1. Processes of thermal polycondensation of melamine. Gray, blue and white balls in the molecules represent carbon, nitrogen and hydrogen atoms, respectively.

ACS Paragon 1Plus Environment

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

Page 2 of 12

red (IR) absorption spectroscopy as well as X-ray photoelectron spectroscopy (XPS) without addressing the validity of the established interpretation. To end up the contradictory identification derived from conventional analyses, concise but definite characterizations of g-CN are necessary.

melem shows a reasonable XRD profile [Figure 1(a)], which is in good agreement with that measured for crystalline melem in ref. 16. Elemental analysis determined its atomic ratio to be C:32.99, H:2.53, N:64.39, which is again in good agreement with the stoichiometry calculated from the molecular structure of melem.

Such a situation would obstruct theoretical and experimental investigations of the electronic structure near the energy gap of g-CN, which actually influences photocatalytic reactions. This is because calculations of the band structure and density-of-states (DOS) require reliable chemical and structural models of a target. Moreover, to exemplify the calculated occupied and unoccupied DOS, analyses with ultraviolet photoelectron spectroscopy (UPS) and inverse photoemission spectroscopy (IPES) are essential. However, a conductive specimen is a must to reliably determine the energies of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) that are responsible for photocatalytic reactions. Since as-synthesized g-CN through the pyrolysis is powdery with poor electric conductivity, more suitable specimen for reliably acquiring photoelectron spectra is necessary from the technical point of view.

Melem thin films for UPS/IPES measurements were prepared by vacuum deposition on a polycrystalline Au substrate. The substrate was fabricated by evaporation onto a doped Si substrate and then was exposed to air, followed by UV/ozone treatment for 15 min. The film for XPS measurements was vacuum-deposited on a highly doped Si substrate with naturally oxidized layers. Thicknesses of both films were 3 nm, which were monitored with a quartz microbalance. Prior to the use, the Si substrate was ultrasonicated in water, acetone, and isopropanol for 10 min. each, followed by UV/ozone treatment for 15 min.

In this article, we utilized well-oriented g-CN films for the analysis with photoelectron spectroscopy. The chemical composition of the film is the same as the powdery bulk sample. First, we present that g-CN was identified as melon, employing a prototypical technique commonly used in photocatalytic researches, XPS, in comparison to the results for a monomer of g-CN, melem. The analysis with UPS and IPES for the melon film determined the energies of the HOMO and LUMO with respect to vacuum level, and exemplified that the energy structure of melon is thermodynamically favorable for water splitting under visible-light illumination. Theoretical band calculations based on the crystal structure of melon recently determined by Fina et al.14 rationalized the measured DOS with UPS and IPES, and further implied preferable electron transport along melon chains owing to the energy dispersion of the LUMO.

2. EXPERIMENTAL SECTION Materials. At first, we performed thermogravimetry (TG) – differential thermal analysis (DTA) of melamine powder (Wako Pure Chemical Industries, Ltd. 99.0%) under He atmosphere to determine the synthesis temperatures of melem and melon. The data is shown in Figure S1 in Supporting Information. Temperature was increased at the heating rate of 10°C min–1. The weight steeply decreased around 300°C, which was attributed to the melem formation by referring a previous report35. The synthesis of melem was carried out in a quartz tube that was placed in a commercial tube furnace (Koyo Thermo Systems Co., Ltd., KTF035N1) under nitrogen atmosphere (purity: 99.99995%)37. Melamine of 1 g was placed in the bottom of a test tube (Nichiden Rika-Glass Co., Ltd., P-18SM), and then the tube was capped with the aluminum foil with a pinhole at the center of it. Therefore, the solid-state polycondensation proceeded under the semi-closed system. The heating rate was fixed at 10°C min–1. After naturally cooled the reactor down to room temperature, the powder slightly tinged with beige at the bottom of test tube was collected. To obtain pure melem, we optimized annealing conditions of melamine and found that 310°C for 3 h was optimal. To further purify the product, it was loaded into an alumina crucible and degassed under high vacuum (the pressure was lower than 10–5 Pa). The purified

Melon thin films were fabricated using the aforementioned setup. Conductive substrates [commercial Al and Au (Nilaco, purity 99%), highly oriented pyrolytic graphite (HOPG) (Grade ZYA, mosaic spread 0.4 ± 0.1, NEWMET KOCH, SPI)] were placed at 9.5 cm from the aluminum cap of the test tube, where at the same time 0.5 g melamine was loaded at the bottom. Melamine was heated up to 450°C at the rate of 1°C min–1 and that temperature was kept for 5 h. After that, the reactor was cooled down to room temperature at about the rate of 2°C min–1. As for the HOPG substrate, a fresh surface was obtained by cleaving it with a scotch tape before the use. Attention: These heating processes involve generation of ammonia gas as a by-product. We thus connected the ammonia trap filled with water to the quartz tube, but the water of the trap flowed back to the reactor on the course of cooling the furnace down. Hence, before the cooling process, we closed the three-way stopcock that is connected to the trap by plastic tubes. Analytical methods. TG-DTA and elemental analysis were performed using the equipment in Tokyo University of Science [TG-DTA2010SA (Bruker AXS) and 2400II CHNS (Parkin Elmer), respectively]. XRD for powder and thin films specimens were carried out using respective diffractometers [Ultima IV and SmartLab (Rigaku), respectively] with Cu Kα as a radiation source. The structural models with lattice planes were generated by VESTA38. XPS spectra of melem and g-CN films were recorded using PHI 5000 VersaProbe (Ulvac-Phi) with monochromatic Al Kα as an excitation source. The core levels of powdery g-CN were measured using AXIS Nova (KRATOS ANALYTICAL) with monochromatic Al Kα. The photoemission peak assigned to C-C bonding was used to calibrate the energy axis of the powder XPS spectra. The XPS spectra present in this study were analyzed with Voigt functions with a software, XPSPEAK41 (by Raymund W.M. Kwok). For determining the chemical composition of melem and melon, the values of inelastic mean-free-paths of the photoelectrons from C 1s and N 1s were obtained from QUASES-IMFP-TPP2M Ver3.0 by S. Tougaard39. UPS measurements were performed under ultra-high vacuum (base pressure < 5 × 10–8 Pa) using a homemade apparatus with an electron analyzer (SES200, VG Scienta) and helium discharge lamp. He I α resonance line (21.22 eV) was used as an excitation source to acquire the UPS spectra. The energies of vacuum levels were deduced using the secondary electron cutoff of UPS spectra at normal emission with the sample biased by –5 V. IPES spectra were also recorded in situ on the same specimen using a commercial


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


apparatus (PSP Vacuum Technology). A band-pass type photon detector for photons of hν = 9.3 eV consists of a channeltron coated with NaCl and placed behind a SrF2 window. During IPES measurements, negative charging and/or damages of organic films by the electron beam were a concern, but we observed no attenuation or broadening of the spectra during the measurements. The substrate temperature during the vacuum deposition of melem and acquisition of photoelectron spectra was kept at room temperature. The energy resolution of the UPS (IPES) deduced from the Fermi edge of the vacuum-deposited Au film was 0.1 eV (0.4 eV). Theoretical calculations. Density of states (DOS) of melem and melem-dimers, and of melon were calculated using the Materials Studio module, in DMol3 and CASTEP, respectively (Dassault Systèmes BIOVIA)40. The molecular structure of melem was created in the Materials Studio and automatically tidied up to keep them plausible. As for modeling two melem-dimers and crystalline melon, the crystal structure, determined by XRD and simulations in ref. 14, was assumed for the calculations of energy levels. Density functional theory (DFT) calculations were performed using the B3LYP exchange-correlation function, and dnd (DMol3) and plane wave (CASTEP) basis sets. The simulated DOS were obtained by convoluting delta functions at respective molecular orbitals with Gaussian functions with FWHM of 0.5 eV for Figure 3, and Figures S5 and S7 in Supporting Information.

3. RESULTS Characterization of melem and g-CN with XRD and XPS. First of all, it is essential to mention fundamental properties

of powdery melem and g-CN before stating properties of the g-CN films. The XRD profile of the powdery melem is shown at the bottom of Figure 1(a). The intense diffractions were observed at 2θ of 12–14° and 25–28°, which is in good agreement with a reported XRD profile for pure melem16,35. The powdery g-CN, on the other hand, exhibits a relatively dull profile with characteristic Bragg peaks at 12.8° and 27.7° [top curve in Figure 1(a)]. The overall feature is in excellent agreement with the XRD profile of melon reported in previous literature13,14,16,41. The polymeric carbon nitride showing the XRD was identified as melon by Lotsch et al. based on their multiple analyses13. The powdery g-CN we synthesized is thus melon. According to the structural model of melon reported in ref. 14, the diffractions at 12.8° and 27.7° were presumably assigned to (210) [separation between parallel melem chains, Figure 1(c)] and (002) [separation between graphitic sheets, Figure 1(d)] planes, respectively14. g-CN films we fabricated by vapor deposition polymerization37,42 appear to have a similar chemical composition to the powdery bulk melon. The C 1s and N 1s XPS spectra of the film [bottom curves of Figures 2(a) and (b), respectively] are similar to those for a powdery specimen (compared in Figure S2, Supporting Information). Both spectra are in good agreement with the XPS spectra of pure melon16. The obtained films were thus used as a model specimen of melon for detailed characterizations of chemical and electronic structures. We will show later that, even without the comparison with the results in previous literature, the XPS spectra of the g-CN film can be interpreted as the formation of melon [see Figure 2(a) and (b)].

Fig ure 1. XRD profiles of powdery melem and g-CN, and of g-CN films. (a) XRD patterns of powdery melem and g-CN. (b) Out-of-plane XRD profiles of g-CN films deposited on Al, Au, and HOPG substrates. As for the profile of HOPG, the specimen was a graphite layer adhesive to a scotch tape. For all film specimens, only the (002) diffraction of melon was detected (indicated by an arrow), indicating the graphitic sheets of melon are oriented in parallel to the substrate surface. (c) (210) plane and (d) (002) plane of melon are illustrated. Note: since the XPS spectra of the g-CN film are similar to those of powdery melon (see Figure S2), labels of ‘melon’ are used instead of ‘g-CN’ in both (a) and (b).



Surface cleanliness of the melon film was checked with a survey XPS spectrum [Figure S3, Supporting Information]. The result suggested minor contributions of elements other than expected carbon and nitrogen. Therefore, surface electronic structures measured in this study can be not influenced by e.g. water adsorption on the melon surface, although melon is prone to water adsorption13 likely due to hydrogen bonding with heptazine units37. We found that the melon film exhibits a preferential orientation of graphitic sheets. Out-of-plane XRD profiles for the films deposited on Au and Al substrates are shown in Figure 1(b). Notably, a marked Bragg peak was observed at 27.7–27.8°, which corresponds to a d-spacing of 3.2 Å. In comparison to the powder XRD of melon [Figure 1(a)], the observed diffraction can be assigned to (002) plane (probably π-stacking of melon). Besides, the absence of the other diffractions [for example, from (210) plane] along the surface normal suggests that the graphitic sheets are preferably oriented in parallel to the substrate surface, in accordance with the carbon nitride film prepared from guanidine carbonate37. The same orientation of the graphitic sheets was found for the film fabricated on a HOPG substrate. The measurement of out-ofplane XRD for the film on a HOPG substrate did not directly allow us to extract the information of the film itself, because of the significant diffractions originated from the substrate [Figure S4, Supporting Information]. However, by peeling off the melon film with a scotch tape, we could finally detect the diffraction from (002) plane of melon [indicated by an arrow in Figure 1(b)], because a thickness of the graphite layers that is adhesive to the tape becomes thinner. Only the Bragg peak from the π-stacking of melon was observed at 27.8°, again indicating the parallel orientation of the graphitic sheets. Such a preferential orientation observed for all melon films can be suitable for surface scientific analyses, because the graphitic sheet, where photocatalytic reactions take place, is exposed to the irradiation of excitation light or electron-beam used for acquiring photoelectron spectra. Hereafter, detailed chemical analyses with XPS are presented for providing a rational interpretation of core level spectra of carbon

Page 4 of 12

nitrides. The comparable XPS analysis for the melon film and a vacuum-deposited melem film clarified the chemical environment of carbon and nitrogen atoms in melon. The C 1s XPS spectrum for a vacuum-deposited melem film has a dominant peak at 288.7 eV [Figure 2(a)]. The N 1s spectrum of the melem film shown in Figure 2(b) is asymmetry with a shoulder structure at the binding energy between 401 and 402 eV. Integrated area intensities of the C 1s and N 1s spectra yielded C/N ratio of 0.60, in perfect agreement with the value determined by elemental analysis (0.60) and calculated stoichiometry of melem (0.60). Consequently, the dominant signal in the C 1s spectrum is safely assigned to sp2-carbons of melem. According to routine fitting processes, the N 1s spectrum could be deconvoluted with three Voigt functions labeled as I (399.2 eV), II (400.1 eV) and III (401.6 eV) [Figure 2(b)]. The ratios of the area intensity relative to the total spectral intensity were found to be 66%, 24% and 10% for the peaks I, II, and III, respectively. These percentages well correspond to the atomic concentrations of the nitrogen atoms in melem [NI: 60%, NII: 30%, NIII: 10%, see the upper inset of Figure 2(b)]. Moreover, the measured binding energies of the peaks I and III are in line with those for sp2- and central nitrogen atoms of 3,5,11,13-tetracycl[3,3,3]azine43 (a melem analogue, see Table 1 for the details of the peak assignment for this molecule). As a consequence, we found that the peaks I-III are attributed to sp2-nitrogen on heptazine ring (C=N-C), amino groups (C-NH2), and a central nitrogen singly bonded to three carbons N-(C)3, respectively. The above assignment is summarized in Table 1. Note that our assignment of the N 1s XPS spectrum for the melem film is in good agreement with that for heptazineoligomers15, but contradicts with that for 2-oxo-6,10-diamino-sheptazine (OH-melem)44. Partial density of states (PDOS) for an isolated melem calculated by DFT further support the above interpretation of the N 1s spectrum [Figure S5(a), Supporting Information]. Comparing the C 1s XPS spectrum of the melon film with that of the melem film indicated a similarity of the chemical environment of carbon atoms in the monomer and polymer. As shown at the

Fig ure 2. XPS spectra of melem and melon films. The spectra in C 1s (a) and N 1s regions (b) are shown. Black dots are the measured spectra after subtracting Shirley backgrounds. Peaks labeled as ‘S’ in (a) and (b) at higher binding energy might be attributed to shake-up satellites. Labels for respective peak components resolved by standard peak fitting with Voigt function correspond to atomic labels marked in molecular structures illustrated at the left side of the all spectra.


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

Chemistry of Materials Tab le 1. Summary of C 1s and N 1s core level binding energies for melem and melem derivatives. Values of binding energy for respective peak components are given in electronvolt. Assignment labeled by asterisks is taken from the respective references.

bottom of Figure 2(a), the intense peak was observed at 288.5 eV. Because the measured binding energy is close to that for the C 1s peak of melem (288.7 eV), the dominant C 1s signal should be attributed to sp2-carbons of heptazine rings. Note that a smaller signal was detected at 285.0 eV, and, at the same time, a very weak component was resolved at 286.2 eV. Although origin of the latter peak is still elusive (therefore we presumably assigned this peak to a contamination, see Table 2), the former has been assigned to adventurous carbon unintentionally incorporated into carbon nitrides45–47. Because we used HOPG as the substrate, in principle, the photoelectron emission from the substrate is possibly detectable if the thickness of the film is enough thin in comparison to a probing depth of present XPS measurements in C 1s region (photoelectron inelastic mean-free-path is 3-4 nm at photoelectron kinetic energy of around 1200 eV)39. To check the origin of this peak, we compared the C 1s XPS spectrum of the melon film on HOPG with that of the film deposited onto Al [Figure S6(a), Supporting Information]. The C-C contribution was detected as well in the film on Al, but its relative intensity to the main peak is smaller than the case of the melon film on HOPG. Therefore, the stronger peak at 285.0 eV in Figure 2(a) can be likely due to the photoelectron emission from the HOPG substrate, because the preparation condition was identical for the films on both the substrates. We now turn to the analysis of the N 1s spectrum for the melon film. The definite assignment of the N 1s spectrum for melem leads to a better interpretation of the spectrum for melon compared to previous reports. The N 1s XPS spectrum of the melon film shown at the bottom of Figure 2(b) is asymmetry with a shoulder at high binding energy side, similar to that of melem film [top curve in Figure 2(b)]. However, a closer look at the normalized N 1s spectra of the melon and melem highlights a slight difference in the spectral shape between them. As shown in Figure S6(b) in Supporting Information, the intensity around ~400 eV in the melon spectrum, which corresponds to a contribution from amino groups

for melem [peak II of the top spectrum in Figure 2(b) and Table 1], decreases, whereas the intensity around 401.5 eV slightly increases. This can be interpreted as follows. Upon polycondensation of melem, a fraction of the amino groups in melem may be used to form covalent bonds between neighboring melem units. Hence, we speculated that the nitrogen bridging two heptazine units [(C)2NH] would give a new contribution at higher binding energy sides of the original peak for melem [401.5 eV in Figure S6(b)]. To substantiate the interpretation on the spectral change, we calculated PDOS of the two melem-dimers that is the minimum model of crystalline melon. The theoretical results suggest that four types of nitrogen can be distinguished [see Figure S5(b), Supporting Information]: sp2-nitrogen on heptazine ring (green, the rightmost peak in the energy range), C-NH2 (pink), (C)2-NH (purple), and central N-(C)3 of heptazine ring (blue, the leftmost peak). Note that C-NH2 highlighted in yellow should have the same energy with C-NH2 (pink) in actual melon-crystal, because of the equivalence of relative atomic positions in (002) plane. By taking this result into account, in contrast to the most of previous literature in which the major fraction of the N 1s spectrum of synthesized carbon nitrides is resolved into two16,47,48 or three components22,36,49–51 (Table 2), the N 1s spectrum of the melon film could be deconvoluted with four Voigt peaks A-D [Figure 2(b)] that locate at 398.9 eV, 399.5 eV, 401.5 eV and 400.6 eV, respectively. In reference to the calculated PDOS for a melem dimer as well as to the peak assignment of N 1s spectrum for melem (Table 1), the peaks A and C were assigned to sp2-nitrogens and central N-(C)3 of heptazine ring, respectively. As for peak B, the measured binding energy is in excellent agreement with that of nitrogen in neutral amine52. According to the simulated PDOS for the melem dimers [Figure S5(b)], the peaks B and D can be attributed to nitrogen atoms of C-NH2 (amino group) and (C)2-NH (nitrogen bridging two melem units), respectively. To further support this assignment, the ratios of area intensities relative to total spectral intensity were calculated. The values were found to be 66.2%, 14.3%, 9.3%, and



Page 6 of 12

Tab le 2. Summary of C 1s and N 1s core level binding energies for graphitic carbon nitride. The values are given in electronvolt. Product names and assignment labeled by asterisks are taken from the respective references.

10.2% for peaks A-D, respectively. These values are in good agreement with the stoichiometry of NA-ND in melon (NA: 67%, NB: 11%, NC: 11%, ND: 11%). Besides, a C/N ratio deduced from the XPS spectra was 0.70 (C-C contribution detected at 285.0 eV was not included), again in line with the calculated value of melon (0.67). We emphasize that the deduced C/N ratio is much smaller than that for perfect g-C3N4 (0.75). These results indicate that the product pyrolysis of melamine is not g-C3N4 but melon. Our peak assignment of the XPS spectra is summarized in Table 2 together with that reported in previous literature. Occupied and unoccupied states of melem and melon. With the aforementioned identification of the synthesized g-CN, we could eventually characterize the occupied and unoccupied electronic states of the carbon nitrides, employing UPS and IPES, respectively. For photocatalytic water splitting, the valence and conduction bands of a photocatalyst must locate below and above

the standard potentials of water oxidation and hydrogen reduction, respectively. UPS and IPES measurements can directly determine the values of the frontier orbitals with respect to vacuum level, which are useful material parameters to assess whether the energy structure of a photocatalyst is preferable for the anticipated reactions. The bottom black curves in Figure 3(a) present the UPS and IPES spectra of the melem film. Calculated DOS (blue) for an isolated melem shown below them qualitatively rationalized the measured energy distributions in both occupied and unoccupied regions. The distribution of molecular orbitals for melem (vertical bars shown under the calculated DOS curve) indicates that the shoulder structure of the first peak in the UPS spectrum at the low binding energy side corresponds to the HOMO [denoted as ‘H’ in Figure 3(a)], and that the LUMO resides in the first peak of the IPES spectrum [‘L’ in Figure 3(a)]. PDOS for melem extracted from DFT calculations [Figure S7(a), Supporting Information] suggests


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


Fig ure 3. Experimentally determined occupied and unoccupied states of melem and melon. (a) UPS and IPES spectra of melem (black) and melon (red). The detected bands were assigned according to PDOS in occupied and unoccupied states shown in Figure S7. Blue curves below the measured spectra are calculated DOS with DFT. Vertical bars under the simulated DOS indicate the locations of molecular orbitals. ‘H’ and ‘L’ under the bars of melem near the band gap denote the locations of the HOMO and LUMO of melem, respectively. Their orbital characters are shown in (b) and (c), respectively. Atomic labels in the peak assignment shown above the measured spectra are the same as the labels used in Figure 2. (d) Energy diagrams of melem and melon with respect to vacuum level. The standard potentials of proton reduction and water oxidation with respect to vacuum level are also shown in gray to compare them with the energy structure of melon characterized in ultrahigh vacuum condition. The values of the potentials were taken from ref. [55].

that the 2p orbitals of sp2-nitrogen on the heptazine ring dominate the occupied states near the Fermi level (EF) with less contributions from 2p orbitals of carbon atoms and nitrogen atoms of amino groups. On the other hand, the orbitals of sp2-nitrogen and carbon atoms on the ring constitute the unoccupied states. This may explain the orbital characters of the HOMO and LUMO for melem shown in Figures 3(b) and (c), respectively. The second band in the IPES spectrum also includes a large contribution from 2s orbitals of nitrogen atoms in amino groups. The HOMO and LUMO spectral onsets, determined by the cross points between a baseline and a tangent at the inflection points of the first peaks in the UPS and IPES spectra, respectively, and energy of the vacuum level relative to EF (EVL = 5.2 eV for melem film) yielded the ionization energy (I) of 7.0 eV and electron affinity (A) of 3.0 eV for melem [see the energy diagram in Figure 3(d)]. The energy gap was thus determined to be 4.0 eV. The spectral shapes of UPS and IPES spectra for the melon film are similar to those of melem, except for a moderate decrease in an energy gap upon the polycondensation of melem. Top red curves in Figure 3(a) show the UPS and IPES spectra of the melon film on a HOPG substrate. Again, the calculated DOS (shown by blue curves under the measured spectra) agrees with the measured spectra. Here, it should again be noted that the C 1s XPS spectrum of melon film contains small contributions from carbon contaminations [see Figure 2(a) and Table 2]. However, the N 1s core level analysis suggests that their influence on the chemical bonds in heptazine backbone and terminal groups of melon is negligible. Besides, because theoretical DOS qualitatively reproduces the measured spectra, these small amounts of carbon contaminations do not significantly alter significantly the surface electronic structure of melon near EF.

The theoretical investigations on the occupied and unoccupied electronic states of melon clarified analogous feature to melem in PDOS of melon [Figure S7(b), Supporting Information]. 2porbitals of sp2-nitrogen on heptazine rings mainly constitute the occupied states near the energy gap, whereas the orbitals of both sp2-carbon and nitrogen contribute to the unoccupied states near [Figure S7(b), Supporting Information]. The shape of the first peak of the UPS spectrum with a shoulder structure at lower binding energy is quite similar to the valence band spectra previously measured with XPS16,19,53. As done for melem, the energy onsets of the HOMO and LUMO and EVL (EVL = 4.2 eV for melon film) yielded I of 6.8 eV and A of 3.4 eV for melon [Figure 3(d)]. It was noted from the comparison of the energy diagrams that I decreases by 0.2 eV, whereas A increases by 0.4 eV upon the polymerization of melem. The energy gap (transport gap) of melon was found to be 3.4 eV, which is 0.7 eV larger than the optical gap reported for powdery melon11. The exciton binding energy, given by the energy difference between optical and transport gaps, was found to be 0.7 eV, from which the exciton dissociation into free charges at the melon surface appears unfavorable to occur at room temperature. Band calculations. Finally, band calculations provided further insights of the occupied and unoccupied electronic states of melon. Figure 4(a) shows the dispersion relation of melon, which was calculated by assuming the structural parameters determined in ref. 14. The energy dispersion of the HOMO is as large as 0.22 eV, present only along π-stacking [Γ-Y, A-B, and D-E directions, see Figure 4(c)], whereas the dispersion of the LUMO reaches the total of 0.38 eV along both the π-stacking and melon chains [A-Y and E-C directions, see Figure 4(b)]. Notably, the energy dispersion of only the LUMO is present along melon chains, in good agreement with previous theoretical results11. Moreover, along Γ-Z



Page 8 of 12

Fig ure 4. Band structure of melon near the band gap. (a) Dispersion relations of the HOMO (bottom curve) and LUMO (top curve) are illustrated. Note that only LUMO exhibits the energy dispersion along polymer chains (A-Y and E-C directions). (b) Molecular alignment in ab plane [see also Figures 1(c) and (d)] and corresponding reciprocal lattice. (c) Reciprocal lattice is illustrated with melon sheets π-stacked along c-axis.

direction (along a-axis), band dispersions of both the HOMO and LUMO are negligible, probably due to the weak interaction between melon chains (hydrogen bonding)13,48, leading to little overlaps of the molecular orbitals among them.

4. DISCUSSION Here we at first discuss the validity of the revised assignment of the N 1s core level spectra for melon. As already seen in Table 2, most previous literature that mentioned their products as “g-C3N4” reported that the second peak from the lower binding energy (~400 eV) was attributed to N-(C)3, a central nitrogen of the heptazine ring. That is the obvious difference from our conclusions. We found that N-(C)3 component appears at the highest binding energy in the N 1s range except for shake-up satellites (around 401 eV, see Table 2) in comparison to the melem spectra. If one assumes that three components constitute the N 1s XPS spectrum of melon and the second peak is attributed to N-(C)3 as most literature supposed (see Table 2), the atomic ratio of NA to NC in melon [Figure 2(b)] would not coincide with the calculated value (6), which contradicts with the fact that IR absorption spectroscopy on the melon film confirmed the presence of heptazine ring42. The fact that melon consists of linear heptazine-polymers does not appear to make a difference in chemical shifts of N 1s spectra between melem and melon. We, however, noticed that the peaks attributed to NA, NB, and NC of melon shift toward lower binding energy in comparison to the corresponding nitrogen atoms of melem (see Table 2). It is noteworthy that the magnitude of the energy shift is functional group-dependent (NI of melem to NA of melon: 0.3 eV, NII to NB: 0.6 eV, NIII to NC: 0.1 eV). In general, binding energy of a core level is influenced by not only chemical environment but also the Fermi level position of a specimen. Since the work functions for both the molecular films used for the XPS analysis were determined to be 4.2 eV by UPS, they do not account for the chemical shifts. Thus, the ionization energies of N 1s levels of respective nitrogen in melon film should be different from those in melem film, likely owing to changes in local electrostatic potentials generated by neighboring non-covalent atoms. The smallest change in binding energy for N-(C)3 (0.1 eV) is understandable: Since the corresponding nitrogen is confined in a heptazine ring, the change in the local potential, possibly caused by polymerization

of melem and assembly of melon chains, can be small. On the other hand, local electrostatic-environment around nitrogen of exterior heptazine-rings and amino-groups can be easily altered by adjacent melon-chains. The difference in the chemical shifts of N 1s between heptazine rings and amino groups is qualitatively supported by the PDOS analysis shown in Figure S5(b) of Supporting Information. Energy separation between the sp2-nitrogen (green) and amino-group (pink) components for the melem dimers is smaller than that for an isolated melem [Figure S5(a), Supporting Information]. This result might imply that the relative energy of nitrogen in melem is altered by the periodic alignment of melon chains. Another point of the revised assignment is that C-NH2 and (C)2NH contributions were resolved. The reason why the contribution of the bridging nitrogen in melon locates at higher binding energy than that of C-NH2 is likely explained by the slight difference in electron negativity (χ) between C and H. Because carbon has a larger χ than hydrogen54, nitrogen adjacent to two carbons may thus be more positive, resulting in the higher binding energy of (C)2-NH component than C-NH2 [Figure S5(b), Supporting Information]. One might concern if our interpretation of the XPS spectra, in particular the N 1s spectrum, is valid to a more general powdery specimen. Figure 5 illustrates the analysis results of the XPS spectrum of the bulk specimen using four Voigt functions. Notably, the simulated spectra (red curves) of not only the C 1s but also N 1s spectra are in good agreement with the measured spectra (black dots). The C/N ratio was calculated to 0.67, in excellent agreement with stoichiometry of melon. Besides, the ratios of area intensities relative to total spectral intensity for the components A-D were found to be 65%, 14%, 9%, and 12%, respectively. These values are again in good agreement with the stoichiometry of NA-ND in melon. These results exemplified that the revised interpretation on the chemical analysis with XPS spectra is valid independently of sample form. We further point out that N 1s XPS data reported in some literature do not appear correctly analyzed. For instance, the height of the background did not match with raw data47, and N-(C)3 contribution was missing in the assignment16. To correctly identify the synthesized g-CN, we would suggest that one has to evaluate the area intensity ratio of respective nitrogen components resolved by


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


Fig ure 5. Spectral deconvolution of the powder melon XPS in C 1s (a) and N 1s (b) regions. In (a), three Voigt components as well as shake-ups were assumed as the analysis for the film specimen [Figure 2(a)]. The binding energy of a peak attributable to unknown contamination (286.6 eV) was not fixed to simulate the measured spectrum. As for (b), the relative energies of each atomic contribution were fixed to be identical with those used for the film specimen. Initially, the relative area intensities of respective components were set to be the same as Figure 2(b). Then, by using a common full-width-at-half maximum and Gaussian/Lorentzian mixing ratio, all parameters except the relative peak energy were optimized to reproduce the spectra. The simulated spectra [red-dashed and -solid lines in (a) and (b), respectively (see also Figure 2)] that assume the revised peak assignment well reproduced the measured N 1s spectra. The labels of the resolved contributions used here are the same as those in Figure 2.

peak fitting, in order to know a detailed chemical structure in the products with XPS. The energy diagram illustrated in Figure 3(d) shows that the LUMO is 0.2 eV more stabilized in energy than the HOMO upon the thermal polycondensation of melem. Taking into account the results of the band calculations [Figure 4(a)], the larger shift of the LUMO can be a consequence of the broader development of the unoccupied states due to the energy dispersion along melon chains. This can be rationalized as follows. As shown in Figure 3(b), the LUMO of melem comprises the contribution from 2p-orbital of NII [nitrogen atom of –NH2, see the inset in Figure 2(b)]. The polycondensation subsequently forms the covalent bonds between melems bridged by (C)2-NH groups. As a consequence, the LUMO of melem forms the band in a melon chain. On the other hand, the HOMO of melem does not have the contribution from the amino groups [Figure 3(c)]. Because the molecular orbital is confined on the heptazine ring, the polymerization does not lead to the orbital splitting of the HOMO in a melon chain. The predicted larger dispersion of the LUMO may suggest superior electron transport along melon chains to hole, which would promote separation of free charges and suppress their recombination.

I and A are also useful to judge if desired photocatalytic reaction(s) potentially occur. As seen from Figure 3(d), the electronic structure of melon was found to be energetically favorable for water splitting, because the HOMO and LUMO of melon locate deeper and shallower than the standard potentials of water oxidation (5.67 eV with respect to vacuum level)55 and proton reduction (4.44 eV)55, respectively. However, we concerned that the direct comparison of I and A determined by UPS and IPES in ultrahigh vacuum with the standard potentials is valid, because solvation effects and image charge effects of a conductive electrode likely influence the energy levels determined by cyclic voltammetry (CV)56. The following equation was proposed to correlate solid-state I determined

by UPS with oxidation potentials of spherical organic semiconductors vs. ferrocene/ferrocenium (Fc/Fc+) reference solute56,

EHOMO = –(1.4 ± 0.1) ! (qVCV ) – (4.6 ± 0.08) eV

(1).

Assuming this equation is valid for both the HOMO and LUMO, we roughly calculated oxidation and reduction potentials of melon as 1.6 V and –0.9 V vs. Fc/Fc+, respectively. To scale these values with respect to the standard hydrogen electrode (SHE), 0.2 V has to be added56. The corrected oxidation and reduction potentials were 1.8 V and –0.7 V, respectively, which are close to the values determined by CV57. Comparing these estimated values with the standard potentials of proton reduction (–0.41 V vs. SHE at pH=7) and water oxidation (0.82 V vs. SHE at pH=7) again suggests that driving force for water splitting with melon is sufficient. Although the energy structure of melon appears favorable for water splitting, we found the large exciton binding energy (0.7 eV), which implies the potentially huge attraction between hole and electron in generated excitons. However, transient absorption spectroscopy on “g-C3N4” obtained from the calcination of urea under ambient conditions revealed that the excitons of >65% are split into free hole and electron without the use of cocatalyst and sacrificial agent58. Further investigations will be necessary to address this discrepancy, but, as most organic semiconductors used in solar cell applications59, melon needs an interface for facilitating exciton dissociation and selective collection of generated free charges.

5. CONCLUSIONS In summary, we utilized the well-oriented melon films prepared by vapor deposition polymerization for characterizing the chemical and electronic structures of the rising photocatalyst for hydrogen evolution via water splitting. The N 1s XPS spectra of the melon films were reappraised with the aids of the XPS analysis of the melem film and the calculated PDOS for melem-dimers. In stark contrast to previous literature, the N 1s spectra were resolved



by four Voigt functions that include the contributions from both – NH– and –NH2. We demonstrated the formation of melon, not gC3N4 by the pyrolysis of melamine. The use of the melon films enabled us to directly characterize the electronic structure near the energy gap of melon with UPS and IPES. The band calculations implied that electron transport might be preferred along melon chains. This would be beneficial for hydrogen evolution. The energy structure of melon was also found to be favorable for photocatalytic water splitting. Comparison of the optical gap with the transport gap determined by UPS and IPES suggests the relatively large exciton binding energy of melon, which appears counterintuitive to the report that excitons of over 65% are dissociated into free charges58. We finally emphasize that utilizing the melon film for characterizations with photoelectron spectroscopy will open the way to provide the opportunity of addressing energetics and chemistry at the interface, for instance, between melon and cocatalyst such as platinum. Moreover, in principle, the terminal amino-groups of the melon film can be substituted with the cyanamide moiety, which is relevant to photocatalytic reaction sites16. Once we obtain the modified melon film, it is possible to evaluate not only chemical composition but also the energy structure, including unoccupied states, of newly synthesized melon derivatives. In particular, the rational interpretation of the N 1s XPS spectrum of melon by providing the evidence of the peak assignment will be helpful to identify newly synthesized carbon nitrides that are even in a powdery form. With these expected potentials of our approach, a study on the interface between melon and platinum is now ongoing in our group to elucidate electronic structures and photocarrier energetics. We hope that these fundamental studies, including the present work, will ultimately contribute to establish ground rules of designing polymeric and hetero-structured photocatalysts that exhibit superior hydrogen evolution rate.

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

(11)

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. TG-DTA of melamine, supplementary XRD and XPS results, PDOS of melem and melem-dimers.

(12)

AUTHOR INFORMATION Corresponding Author (13)

*E-mail: [email protected]

Author Contributions K. Akaike and K. Aoyama equally contributed to this work.

ACKNOWLEDGMENT We thank Prof. M. Tadokoro and Prof. K. Tokiwa of Tokyo University of Science for assistance of TG-DTA and XRD measurements, respectively. We also appreciate Dr. K. Tajima and Dr. K. Nakano of RIKEN for the use of XPS equipment. K. Akaike and K.K. acknowledge JSPS for financial supports (15H06636 and 16K05956, respectively).

(14)

(15)

REFERENCES (1)

Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238 (5358), 37–38.

(16)

Page 10 of 12

Mehta, V.; Cooper, J. S. Review and Analysis of PEM Fuel Cell Design and Manufacturing. J. Power Sources 2003, 114 (1), 32–53. Farrauto, R.; Hwang, S.; Shore, L.; Ruettinger, W.; Lampert, J.; Giroux, T.; Liu, Y.; Ilinich, O. New Material Needs for Hydrocarbon Fuel Processing: Generating Hydrogen for the PEM Fuel Cell. Annu. Rev. Mater. Res. 2003, 33 (1), 1–27. Dempsey, J. L.; Brunschwig, B. S.; Winkler, J. R.; Gray, H. B. Hydrogen Evolution Catalyzed by Cobaloximes. Acc. Chem. Res. 2009, 42 (12), 1995–2004. Hinnemann, B.; Moses, P. G.; Bonde, J.; Jørgensen, K. P.; Nielsen, J. H.; Horch, S.; Chorkendorff, I.; Nørskov, J. K. Biomimetic Hydrogen Evolution: MoS2 Nanoparticles as Catalyst for Hydrogen Evolution. J. Am. Chem. Soc. 2005, 127 (15), 5308–5309. Hou, Y.; Laursen, A. B.; Zhang, J.; Zhang, G.; Zhu, Y.; Wang, X.; Dahl, S.; Chorkendorff, I. Layered Nanojunctions for Hydrogen-Evolution Catalysis. Angew. Chem. Int. Ed. 2013, 52 (13), 3621–3625. Sprick, R. S.; Jiang, J.-X.; Bonillo, B.; Ren, S.; Ratvijitvech, T.; Guiglion, P.; Zwijnenburg, M. A.; Adams, D. J.; Cooper, A. I. Tunable Organic Photocatalysts for Visible-Light-Driven Hydrogen Evolution. J. Am. Chem. Soc. 2015, 137 (9), 3265–3270. Chen, X.; Shen, S.; Guo, L.; Mao, S. S. SemiconductorBased Photocatalytic Hydrogen Generation. Chem. Rev. 2010, 110 (11), 6503–6570. Zong, X.; Yan, H.; Wu, G.; Ma, G.; Wen, F.; Wang, L.; Li, C. Enhancement of Photocatalytic H2 Evolution on CdS by Loading MoS2 as Cocatalyst under Visible Light Irradiation. J. Am. Chem. Soc. 2008, 130 (23), 7176– 7177. Li, Y.; Wang, H.; Xie, L.; Liang, Y.; Hong, G.; Dai, H. MoS 2 Nanoparticles Grown on Graphene: An Advanced Catalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2011, 133 (19), 7296–7299. Wang, X.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J. M.; Domen, K.; Antonietti, M. A MetalFree Polymeric Photocatalyst for Hydrogen Production from Water under Visible Light. Nat. Mater. 2009, 8 (1), 76–80. Wang, X.; Maeda, K.; Chen, X.; Takanabe, K.; Domen, K.; Hou, Y.; Fu, X.; Antonietti, M. Polymer Semiconductors for Artificial Photosynthesis: Hydrogen Evolution by Mesoporous Graphitic Carbon Nitride with Visible Light. J. Am. Chem. Soc. 2009, 131 (5), 1680–1681. Lotsch, B. V.; Döblinger, M.; Sehnert, J.; Seyfarth, L.; Senker, J.; Oeckler, O.; Schnick, W. Unmasking Melon by a Complementary Approach Employing Electron Diffraction, Solid-State NMR Spectroscopy, and Theoretical Calculations—Structural Characterization of a Carbon Nitride Polymer. Chem. Eur. J. 2007, 13 (17), 4969–4980. Fina, F.; Callear, S. K.; Carins, G. M.; Irvine, J. T. S. Structural Investigation of Graphitic Carbon Nitride via XRD and Neutron Diffraction. Chem. Mater. 2015, 27 (7), 2612–2618. Zambon, A.; Mouesca, J.-M.; Gheorghiu, C.; Bayle, P. A.; Pécaut, J.; Claeys-Bruno, M.; Gambarelli, S.; Dubois, L. S-Heptazine Oligomers: Promising Structural Models for Graphitic Carbon Nitride. Chem. Sci. 2016, 7 (2), 945–950. Lau, V. W.; Moudrakovski, I.; Botari, T.; Weinberger, S.; Mesch, M. B.; Duppel, V.; Senker, J.; Blum, V.; Lotsch, B. V. Rational Design of Carbon Nitride Photocatalysts by Identification of Cyanamide Defects as Catalytically Relevant Sites. Nat. Commun. 2016, 7, 12165.

ACS Paragon 10 Plus Environment

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

(17)

(18)

(19)

(20)

(21)

(22)

(23)

(24)

(25)

(26)

(27)

(28)

(29)

(30)

(31)

Chemistry of Materials Goettmann, F.; Fischer, A.; Antonietti, M.; Thomas, A. Chemical Synthesis of Mesoporous Carbon Nitrides Using Hard Templates and Their Use as a Metal-Free Catalyst for Friedel–Crafts Reaction of Benzene. Angew. Chem. Int. Ed. 2006, 45 (27), 4467–4471. Zhang, G.; Zhang, M.; Ye, X.; Qiu, X.; Lin, S.; Wang, X. Iodine Modified Carbon Nitride Semiconductors as Visible Light Photocatalysts for Hydrogen Evolution. Adv. Mater. 2014, 26 (5), 805–809. Liu, G.; Niu, P.; Sun, C.; Smith, S. C.; Chen, Z.; Lu, G. Q. (Max); Cheng, H.-M. Unique Electronic Structure Induced High Photoreactivity of Sulfur-Doped Graphitic C3N4. J. Am. Chem. Soc. 2010, 132 (33), 11642–11648. Yue, B.; Li, Q.; Iwai, H.; Kako, T.; Ye, J. Hydrogen Production Using Zinc-Doped Carbon Nitride Catalyst Irradiated with Visible Light. Sci. Technol. Adv. Mater. 2011, 12 (3), 034401. Fang, J.; Fan, H.; Li, M.; Long, C. Nitrogen Self-Doped Graphitic Carbon Nitride as Efficient Visible Light Photocatalyst for Hydrogen Evolution. J. Mater. Chem. A 2015, 3 (26), 13819–13826. Li, J.; Shen, B.; Hong, Z.; Lin, B.; Gao, B.; Chen, Y. A Facile Approach to Synthesize Novel Oxygen-Doped gC3N4 with Superior Visible-Light Photoreactivity. Chem. Commun. 2012, 48 (98), 12017. Kofuji, Y.; Isobe, Y.; Shiraishi, Y.; Sakamoto, H.; Tanaka, S.; Ichikawa, S.; Hirai, T. Carbon Nitride– Aromatic Diimide–Graphene Nanohybrids: Metal-Free Photocatalysts for Solar-to-Hydrogen Peroxide Energy Conversion with 0.2% Efficiency. J. Am. Chem. Soc. 2016, 138 (31), 10019–10025. Zhang, J.; Zhang, G.; Chen, X.; Lin, S.; Möhlmann, L.; Dołęga, G.; Lipner, G.; Antonietti, M.; Blechert, S.; Wang, X. Co-Monomer Control of Carbon Nitride Semiconductors to Optimize Hydrogen Evolution with Visible Light. Angew. Chem. Int. Ed. 2012, 51 (13), 3183–3187. Zhang, J.; Chen, X.; Takanabe, K.; Maeda, K.; Domen, K.; Epping, J. D.; Fu, X.; Antonietti, M.; Wang, X. Synthesis of a Carbon Nitride Structure for Visible-Light Catalysis by Copolymerization. Angew. Chem. Int. Ed. 2010, 49 (2), 441–444. Zhang, G.; Lan, Z.-A.; Wang, X. Surface Engineering of Graphitic Carbon Nitride Polymers with Cocatalysts for Photocatalytic Overall Water Splitting. Chem. Sci. 2017. Indra, A.; Menezes, P. W.; Kailasam, K.; Hollmann, D.; Schröder, M.; Thomas, A.; Brückner, A.; Driess, M. Nickel as a Co-Catalyst for Photocatalytic Hydrogen Evolution on Graphitic-Carbon Nitride (sg-CN): What Is the Nature of the Active Species? Chem. Commun. 2016, 52 (1), 104–107. Yue, X.; Yi, S.; Wang, R.; Zhang, Z.; Qiu, S. Cadmium Sulfide and Nickel Synergetic Co-Catalysts Supported on Graphitic Carbon Nitride for Visible-Light-Driven Photocatalytic Hydrogen Evolution. Sci. Rep. 2016, 6 (1). Pan, C.; Xu, J.; Wang, Y.; Li, D.; Zhu, Y. Dramatic Activity of C3N4/BiPO4 Photocatalyst with Core/Shell Structure Formed by Self-Assembly. Adv. Funct. Mater. 2012, 22 (7), 1518–1524. Zhu, Y.-P.; Li, M.; Liu, Y.-L.; Ren, T.-Z.; Yuan, Z.-Y. Carbon-Doped ZnO Hybridized Homogeneously with Graphitic Carbon Nitride Nanocomposites for Photocatalysis. J. Phys. Chem. C 2014, 118 (20), 10963–10971. Chai, B.; Peng, T.; Mao, J.; Li, K.; Zan, L. Graphitic Carbon Nitride (g-C3N4)–Pt-TiO2 Nanocomposite as an Efficient Photocatalyst for Hydrogen Production under Visible Light Irradiation. Phys. Chem. Chem. Phys. 2012, 14 (48), 16745.

(32)

(33)

(34)

(35)

(36)

(37)

(38) (39)

(40) (41)

(42)

(43)

(44) (45)

(46)

(47)

Gu, L.; Wang, J.; Zou, Z.; Han, X. Graphitic-C3N4Hybridized TiO2 Nanosheets with Reactive {001} Facets to Enhance the UV- and Visible-Light Photocatalytic Activity. J. Hazard. Mater. 2014, 268, 216–223. Wang, Y.; Shi, R.; Lin, J.; Zhu, Y. Enhancement of Photocurrent and Photocatalytic Activity of ZnO Hybridized with Graphite-like C3N4. Energy Environ. Sci. 2011, 4 (8), 2922. Cao, S.-W.; Liu, X.-F.; Yuan, Y.-P.; Zhang, Z.-Y.; Liao, Y.-S.; Fang, J.; Loo, S. C. J.; Sum, T. C.; Xue, C. Solarto-Fuels Conversion over In2O3/g-C3N4 Hybrid Photocatalysts. Appl. Catal. B Environ. 2014, 147, 940–946. Jürgens, B.; Irran, E.; Senker, J.; Kroll, P.; Müller, H.; Schnick, W. Melem (2,5,8-Triamino-Tri- s -Triazine), an Important Intermediate during Condensation of Melamine Rings to Graphitic Carbon Nitride: Synthesis, Structure Determination by X-Ray Powder Diffractometry, Solid-State NMR, and Theoretical Studies. J. Am. Chem. Soc. 2003, 125 (34), 10288–10300. Thomas, A.; Fischer, A.; Goettmann, F.; Antonietti, M.; Müller, J.-O.; Schlögl, R.; Carlsson, J. M. Graphitic Carbon Nitride Materials: Variation of Structure and Morphology and Their Use as Metal-Free Catalysts. J. Mater. Chem. 2008, 18, 4893. Arazoe, H.; Miyajima, D.; Akaike, K.; Araoka, F.; Sato, E.; Hikima, T.; Kawamoto, M.; Aida, T. An Autonomous Actuator Driven by Fluctuations in Ambient Humidity. Nat. Mater. 2016, 15 (10), 1084–1089. Momma, K.; Izumi, F. VESTA 3 for Three-Dimensional Visualization of Crystal, Volumetric and Morphology Data. J. Appl. Crystallogr. 2011, 44 (6), 1272–1276. Tanuma, S.; Powell, C. J.; Penn, D. R. Calculations of electron inelastic mean free paths. V. Data for 14 organic compounds over the 50–2000 eV range. Surf. Interf. Anal. 1994, 21, 165. Dassault Systèmes BIOVIA, Materials Studio, Release 2016, San Diego: Dassault Systèmes, 2016. Wang, J.; Hao, D.; Ye, J.; Umezawa, N. Determination of Crystal Structure of Graphitic Carbon Nitride: Ab Initio Evolutionary Search and Experimental Validation. Chem. Mater. 2017, 29 (7), 2694–2707. Bian, J.; Li, J.; Kalytchuk, S.; Wang, Y.; Li, Q.; Lau, T. C.; Niehaus, T. A.; Rogach, A. L.; Zhang, R.-Q. Efficient Emission Facilitated by Multiple Energy Level Transitions in Uniform Graphitic Carbon Nitride Films Deposited by Thermal Vapor Condensation. ChemPhysChem 2015, 16 (5), 954–959. Boutique, J. P.; Verbist, J. J.; Fripiat, J. G.; Delhalle, J.; Pfister-Guillouzo, G.; Ashwell, G. J. 3, 5, 11, 13Tetraazacycl [3.3.3] Azine: Theoretical (Ab Initio) and Experimental (X-Ray and Ultraviolet Photoelectron Spectroscopy) Studies of the Electronic Structure. J. Am. Chem. Soc. 1984, 106 (16), 4374–4378. Wang, H. Investigations into carbon nitrides and carbon nitride derivatives. PhD Thesis, Ludwig-MaximiliansUniversität München, 2013. Lin, L.; Ou, H.; Zhang, Y.; Wang, X. Tri- s -TriazineBased Crystalline Graphitic Carbon Nitrides for Highly Efficient Hydrogen Evolution Photocatalysis. ACS Catal. 2016, 6 (6), 3921–3931. Dai, H.; Gao, X.; Liu, E.; Yang, Y.; Hou, W.; Kang, L.; Fan, J.; Hu, X. Synthesis and Characterization of Graphitic Carbon Nitride Sub-Microspheres Using Microwave Method under Mild Condition. Diam. Relat. Mater. 2013, 38, 109–117. Li, X.; Zhang, J.; Shen, L.; Ma, Y.; Lei, W.; Cui, Q.; Zou, G. Preparation and Characterization of Graphitic Carbon Nitride through Pyrolysis of Melamine. Appl. Phys. A 2009, 94 (2), 387–392.



(48)

(49)

(50)

(51)

(52)

Niu, P.; Yang, Y.; Yu, J. C.; Liu, G.; Cheng, H.-M. Switching the Selectivity of the Photoreduction Reaction of Carbon Dioxide by Controlling the Band Structure of a g-C3N4 Photocatalyst. Chem. Commun. 2014, 50 (74), 10837. Yu, J.; Wang, K.; Xiao, W.; Cheng, B. Photocatalytic Reduction of CO2 into Hydrocarbon Solar Fuels over gC3N4–Pt Nanocomposite Photocatalysts. Phys. Chem. Chem. Phys. 2014, 16 (23), 11492. Liu, J.; Liu, Y.; Liu, N.; Han, Y.; Zhang, X.; Huang, H.; Lifshitz, Y.; Lee, S.-T.; Zhong, J.; Kang, Z. Metal-Free Efficient Photocatalyst for Stable Visible Water Splitting via a Two-Electron Pathway. Science 2015, 347 (6225), 970–974. Miller, T. S.; Jorge, A. B.; Suter, T. M.; Sella, A.; Corà, F.; McMillan, P. F. Carbon Nitrides: Synthesis and Characterization of a New Class of Functional Materials. Phys. Chem. Chem. Phys. 2017, 19 (24), 15613–15638. Raymundo-Pinero, E.; Cazorla-Amorós, D.; LinaresSolano, A.; Find, J.; Wild, U.; Schlögl, R. Structural Characterization of N-Containing Activated Carbon Fibers Prepared from a Low Softening Point Petroleum Pitch and a Melamine Resin. Carbon 2002, 40 (4), 597– 608.

(53)

(54) (55) (56)

(57)

(58)

(59)

Page 12 of 12

Niu, P.; Yin, L.-C.; Yang, Y.-Q.; Liu, G.; Cheng, H.-M. Increasing the Visible Light Absorption of Graphitic Carbon Nitride (Melon) Photocatalysts by Homogeneous Self-Modification with Nitrogen Vacancies. Adv. Mater. 2014, 26 (47), 8046–8052. Cotton, F. A.; Wilkinson, G.; Gaus, P. L. Basic Inorganic Chemistry 3rd Edition; Wiley: New York, 1995. Trasatti, S. The absolute electrode potential: an explanatory note (Recommendations 1986). Pure Appl. Chem. 1986, 58, 955. Dandrade, B.; Datta, S.; Forrest, S.; Djurovich, P.; Polikarpov, E.; Thompson, M. Relationship between the Ionization and Oxidation Potentials of Molecular Organic Semiconductors. Org. Electron. 2005, 6 (1), 11–20. Wu, P.; Wang, J.; Zhao, J.; Guo, L.; Osterloh, F. E. Structure Defects in g-C3N4 Limit Visible Light Driven Hydrogen Evolution and Photovoltage. J. Mater. Chem. A 2014, 2 (47), 20338–20344. Corp, K. L.; Schlenker, C. W. Ultrafast Spectroscopy Reveals Electron-Transfer Cascade That Improves Hydrogen Evolution with Carbon Nitride Photocatalysts. J. Am. Chem. Soc. 2017, 139 (23), 7904–7912. Kippelen, B.; Brédas, J.-L. Organic Photovoltaics. Energy Environ. Sci. 2009, 2 (3), 251.

Insert Table of Contents artwork here


phitic carbon nitride based on rational interpretation of chemical a

Recommend Documents