This dissertation describes research on the synthesis and characterization of extended heptazine–based, graphite–like carbon nitride materials (CNx), as well as molecular heptazine (C6N7) derivatives. Spurred on by recent triazine to heptazine conversion studies, a structural examination was performed on an amorphous nitrogen–rich carbon nitride material formed via the rapid and exothermic self-propagating decomposition of a triazine (C3N3) precursor, trichloromelamine (TCM). The thermally stable and insoluble CNxHy product was determined to be composed of heptazine repeat units. This conclusion was supported by 13C solid state NMR and isolation of molecular heptazine anions after base hydrolysis (structural deconstruction) of the CNxHy material. Modifications to the decomposition of TCM were explored. Introduction of a solid template (NaCl or silica) led to morphological changes in the TCM–CNx product, observed by scanning electron microscopy. It was found that the sodium salts, NaBr and NaN3, led to chloride exchange with TCM. The use of mixtures of NH4Cl and NaN3 also showed changes in the morphology of the material, while leading to slight changes in the IR spectra. A series of reactions between NaBH4 and TCM yield novel thermally stable boron carbon nitride (BCN) materials. Reactions between TCM and Li2C2 or aromatic organic solids led to CNx materials with increased carbon contents. Crystalline metal–heptazine precipitates were generated by cation exchange reaction with the base hydrolysis product of TCM–CNx, potassium cyamelurate. A structure solution was attempted for the crystalline copper cyamelurate salt, KCu[C6N7O3]·4H2O. Neutral molecular heptazines were also synthesized; these species included 2,5,8–tribromo–s–heptazine (TBH), 2,5,8–triphenyl–s–heptazine (TPH), 2,5,8–tris(diisopropylamino)–s–heptazine (TAmH), and 2–bis(trimethylsilyl)amido–5,8–dichloroheptazine (DCAH). These materials were sublimable and showed interesting optical absorption and emission properties. A polymeric heptazine material was synthesized by thermal decomposition of DCAH. Several attempts were made to synthesize polymeric materials from heptazine precursors. Extended solids with C6N8 and C9N7 stoichiometry were made through solid state metathesis reactions between trichloroheptazine and either lithium nitride or lithium carbide. Powder X–ray diffraction indicated that salt formation was occurring during these reactions and products had the desired stoichiometry by elemental analysis. It was generally observed that CNx materials containing excess carbon displayed increased thermal stability when compared to pure CNx.

Crystalline carbon nitride has been a hot topic for the last ten years because of reports claiming it could work as a photocatalyst for cheap water splitting, a catalyst for difficult reactions inorganic chemistry and the use as a potential two-dimensional semiconductor.The carbon nitride of interest in this project is poly(triazineimide) (PTI), which has a layered structure similar to graphite. Oneof the goals was to examine the synthesis parameters to try tounderstand what makes these crystallites grow. The material was primarily analyzed using scanning electron microscopy and powder x-ray diffraction. The other goal of this project was to examine the physical properties of dissolved PTI. It is currently not understood how PTI behaves in various solvents. The effect on how the freezing point depression varies in different solvents was, therefore, tested.No strong correlations of how the morphology of the produced PTIdiffered with different synthesis parameters. Freezing point measurements suggest that a solution of PTI follows Raoult's law and can be described as a true solution.

Lithium-ion batteries (LIBs) are widely applied in a variety of applications such as mobile phones, laptop chargers, and electric vehicles. Thanks to a high energy density of about 120 to 220 Wh kg-1, LIBs are used for a long time, however, the present technology is unable to satisfy the increasing energy storage requirements. Therefore, increasing the energy density of LIBs to improve the performance is very important. Because of that the specific capacity and operation voltage of the anode and cathode materials determine the energy density, improving these two parameters is the key point. This can be achieved in two ways, one being the optimization of the electrode materials of existing LIBs, both cathode and anode, the other is the development of new battery systems to replace LIBs, potassium-ion batteries (KIBs) and sodium-ion batteries (NIBs) are examples of such new systems. In any case, the selection of the electrode materials is crucial. With a rapid development of two-dimensional (2D) materials, leading directly to an increase interest in exploring 2D materials in order to serve as possible electrode materials, based on their unique 2D structures, large conductivity, and most importantly, wide specific surface area. Among them lays graphene-like carbon-nitride materials with lightweight properties. These materials have collected spotlights in multiple fields that are concerned with energy harvesting and storage. The metallic monolayer C6N3 is a very recently discovered member in this family, which is chemically, mechanically, dynamically, and thermodynamically stable through the first-principal calculations. In this work, we investigate the monolayer C6N3 performance as a potential and promising foundation for the anode material of LIBs/NIBs/KIBs. According to our theoretical investigation, the metallic monolayer C6N3 should be an effective anode material for the LIBs/NIBs/KIBs, which combines high specific capacity and low average open-circuit voltage.

The need for green and renewable fuels has led to the investigation of ways to exploit renewable resources. Solar among all the renewables is the most powerful and its conversion into usable energy would help in solving the energy problem our society is facing. Photocatalytic water splitting for hydrogen production is an example of solar energy storage into chemical bonds. The hydrogen produced in this way can then be employed as carbon free fuel creating the “Hydrogen Cycle”. This work investigates the structure and the activity of graphitic carbon nitride (g-C₃N₄), an organic semiconductor that proved a suitable photocatalyst for hydrogen production from water. Synthesised by thermal polycondensation of melamine it is a graphitic like material with a band gap of 2.7 eV which makes it a visible light active catalyst. In a first instance the effect of the synthesis conditions on its structure and morphology are investigated to find the optimum parameters. The temperature of condensation is varied from 450°C up to 650°C and the length from 2.5 h to 15 h. The structural changes are monitored via X-ray diffraction (XRD) and elemental analysis while the effect on the morphology and the band gap of g-C₃N₄ are investigated by mean of scanning electron microscopy and UV-Vis absorption. Subsequently, a study of the crystal structure of the catalyst is carried out. Using structures proposed in the literature, X-ray diffraction and neutron scattering simulations are used to narrow down the number of possible 3D structures. After structural characterisation, the activity of g-C₃N₄ for photocatalytic hydrogen evolution is evaluated. It is confirmed that loading 1 wt.% Pt on its surface significantly increases the hydrogen evolution rate. The attention then focuses on the loading procedures, the reduction pre treatments of the co-catalyst and the reasons of the different performances when different procedures are employed. The catalytic system is characterised by mean of X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and XRD. By investigating the composition and the morphology of the platinum nanoparticles under different conditions, the main factors responsible for the changes in activity of g-C₃N₄ for hydrogen evolution are identified. Additionally, the role of the co catalyst and its interaction with g-C₃N₄ is also elucidated. Finally, taking forward the knowledge acquired on the Pt-g-C₃N₄ system, the effect on the hydrogen evolution rate of alloying platinum with a second metal (Cu, Ag, Ni and Co) is studied. The nanoparticles are characterised by XRD and TEM. A screening of the loading procedures and bimetallic systems is performed to identify the most promising for photocatalytic hydrogen evolution with the aim of bringing them towards further investigation.

Thesis advisor: Dunwei Wang / Water splitting has been recognized as a promising solution to challenges associated with the intermittent nature of solar energy for over four decades. A great deal of research has been done to develop high efficient and cost-effective catalysts for this process. Among which tantalum nitride (Ta₃N₅) has been considered as a promising candidate to serve as a good catalyst for solar water splitting based on its suitable band structure, chemical stability and high theoretical efficiency. However, this semiconductor is suffered from its special self-oxidation problem under photoelectrochemical water splitting conditions. Several key unique properties of graphitic carbon nitride (g-C₃N₄) render it an ideal choice for the protection of Ta₃N₅. In this work, Ta₃N₅/g-C₃N₄ hybrid photoanode was successfully synthesized. After addition of co-catalyst, the solar water splitting performance of this hybrid photoanode was enhanced. And this protection method could also act as a potential general protection strategy for other unstable semiconductors. / Thesis (MS) — Boston College, 2018. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

This thesis describes the synthesis, characterization and photocatalytic applications of carbon nitride (C3N4) and titanium dioxide (TiO2) materials. C3N4 was prepared from the thermal decomposition of a trichloromelamine (TCM) precursor. Several different reactor designs and decomposition temperatures were used to produce chemically and thermally stable orange powders. These methods included a low temperature glass Schlenk reactor, a high mass scale stainless steel reactor, and decomposition at higher temperatures by the immersion of a Schlenk tube into a furnace. These products share many of the same structural and chemical properties when produced by these different methods compared to products from more common alternate precursors in the literature, determined by infrared spectroscopy (IR), nuclear magnetic resonance spectroscopy (NMR), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and elemental analysis. C3N4 is capable of utilizing light for photocatalysis due to its moderate band gap (Eg), measured to be between 2.2 and 2.5 eV. This enables C3N4 to be used in the photocatalytic degradation of organic dyes and the production of hydrogen via the water-splitting reaction. C3N4 degraded methylene blue dye to less than 10% of its initial concentration in less than an hour of UV light illumination and 60% under filtered visible light in 150 minutes. It also degraded methyl orange dye to below 20% in 70 minutes under UV light and below 60% in 150 minutes under visible light. Using precious metal co-catalysts (Pt, Pd, and Ag) photo-reduced onto the surface of C3N4, hydrogen was produced from a 10% aqueous solution of triethanolamine at rates as high as 260 μmol h-1 g-1. C3N4 was also modified by mixing the precursor with different salts (NaCl, KBr, KI, KSCN, and NH4SCN) as hard templates. Many of these salts reacted with TCM by exchanging the anion with the chlorine in TCM. The products were mostly prepared using the high temperature Schlenk tube reactor, and resulted in yellow, orange, or tan-brown products with Eg values between 2.2 and 2.7 eV. Each of these products had subtle differences in the IR spectra and elemental composition. The morphology of these C3N4 products appeared to be more porous than unmodified C3N4, and the surface area for some increased by a factor of 4. These products demonstrated increased activity for photocatalytic hydrogen evolution, with the product from TCM-KI reaching a peak rate as high as 1,300 µmol h-1 g-1. C3N4 was coated onto metal oxide supports (SiO2, Al2O3, TiO2, and WO3) with the goal of utilizing enhanced surface area of the support or synergy between two different semiconductors. These products typically required higher temperature synthesis conditions in order to fully form. The compositions of the SiO2 and Al2O3 products were richer in nitrogen and hydrogen compared to unmodified C3N4. The higher temperature reactions with C3N4 and WO3 resulted in the formation of the HxWO3 phase, and an alternate approach of coating WO3 on C3N4 was used. The degradation of methyl orange showed a significant increase in adsorption of dye for the composites with SiO2 and Al2O3, which was not seen with any of the individual components. The composite between C3N4 and TiO2 showed improved activity for hydrogen evolution compared to unmodified C3N4. The surface of TiO2 was modified by the reductive photodeposition of several first row transition metals (Mn, Fe, Co, Ni, and Cu). This process resulted in the slight color change of the white powder to shades of light yellow, blue or grey. Bulk elemental analysis showed that these products contained between 0.04-0.6 at% of the added metal, which was lower than the targeted deposit amount. The Cu modified TiO2 had the largest enhancement of photocatalytic hydrogen evolution activity with a rate of 8,500 µmol h-1 g-1, a factor of 17 higher than unmodified TiO2.

Photocatalytic solar fuel generation is the most green, sustainable and viable approach to deal with both the ever-growing energy crisis and environmental issues, simultaneously. The work presented in this thesis is focused on the development of new organic carbonaceous semiconductors, typically, carbon quantum dots (CQDs) and graphitic carbon nitride (g-C3N4), and porphyrin small molecules and their hybrids with graphitic carbon nitride, meanwhile, their application in the field of photocatalytic solar fuel generation. In the chapter 1, a general review about background and mechanism of photocatalytic solar fuel generation are introduced first. Next, the features and developments of porphyrin and graphitic carbon nitride for the photocatalytic redox reaction are discussed. In chapter 2, the synthesis, characterization methods and photocatalytic experiment in details are described. In chapter 3, gram-scale CQDs are facilely synthesized by simple thermal treatment of citric acid monohydrate, and microporous 1D nanorods of g-C3N4 are prepared through template-free chemical approach. The photocatalytic properties of 1D protonated g-C3N4 (HCN) modified with different amount of CQDs were evaluated by the rate of H2- evolution under visible light irritation. The results demonstrate that g-C3N4/CQDs with the optimal CQDs amount of 0.25 wt.% afford the highest H2-production rate of 382 μmol h-1 g-1 with apparent quantum yield (AQY) of 1.9% which was about 3-fold of pure g- C3N4. The composite g-C3N4/CQDs show a remarkable stability against the photocorrosion within a continuous experiment period over 12h. The enhanced photocatalytic H2-production activity could be attribute to the intimate interface between CQDs and g-C3N4, which not only significantly improves the visible-light absorption, but also prolongs the lifetime of charge carriers and suppresses the recombination of photogenerated electron-hole pairs. This work showed that low-cost and metal-free CQDs could be an efficient photosensitizer to promote photocatalytic hydrogen generation. In chapter 4, we reported a new array of push-pull isomeric naphthalimide- porphyrins (ZnT(p-NI)PP) to investigate the effect of naphthalimide and molecular constitution on light driven hydrogen evolution from water splitting. These compounds were synthesized by integration of four naphthalimide moieties on meso-substituion of porphyrin macrocycle through para phenyl linker. Porphyrins were characterized by UV- Vis, Fluorescence and DFT calculations and compared with those of zinc tertapheylporphyrin (ZnTPP). When these porphyrins were employed as photocatalyst for the photocatalytic hydrogen production (PHP) with platinum co-catalyst, they delivered high hydrogen efficiency compared to that of ZnTPP. Particularly, ZnT(p-NI)PP exhibited 203 times higher hydrogen efficiency than the ZnTPP. This could be ascribed to the efficient exciton dissociation into holes and electrons at the photoexcited state of ZnT(p-NI)PP and then electrons were transferred to the proton through platinum. These results indicate that introduction of naphthalimide at meso-position of porphyrin through para phenyl linker is a versatile strategy to improve the photocatalytic hydrogen evolution of porphyrin based materials. In addition, the other two isomeric naphthalimide conjugated porphyrins through meta-and ortho-phenyl linker, ZnT(m-NI)PP and ZnT(o-NI)PP are also developed for photocatalytic H2 production. The para-linked isomer, ZnT(p-NI)PP delivered a much higher H2 production rate of 973 μmol h−1g -1 compared to that of ZnT(m-NI)PP (597 μmol h−1g −1) and ZnT(o-NI)PP (54 μmol h−1g −1), respectively. This could be attributed to the efficient intramolecular energy transfer from the naphthalimide to the porphyrin ring. In chapter 5, a series of NP/g-C3N4 hybrids of graphitic carbon nitride (g-C3N4) with naphthalimide-porphyrin (ZnT(p-NI)PP, labelled as NP) have been developed for photocatalytic hydrogen production. Planar naphthalimide-porphyrins are adsorbed onto flexible two-dimensional g-C3N4 through π-π stacking, which are characterized by scanning electronic microscopy and X-ray photoelectron spectroscopy. Except for its function as photosensitizer, NP in the hybrids could efficient inhibit the charge recombination by electron transfer for the lower lowest unoccupied molecular orbital of NP than g-C3N4, whereas facilitate energy transfer from g-C3N4 donor to NP acceptor for efficient overlap of emission spectrum of NP and absorption of g-C3N4. As a result, the hybrid containing weigh ratio of 2% NP (2%NP/g-C3N4) exhibits an enhanced photocatalytic hydrogen production rate (HPR) of 2297 μmol g−1 h −1, while pristine g- C3N4 shows a HPR of 698 μmol g−1 h −1. The 2%NP/g-C3N4 shows the best performance when compared with the reported hybrids of g-C3N4 with Zn(II) -tetrakis(4- carboxylphenyl) porphyrin (ZnTCPP/g-C3N4) and Zn(II)-tetrakis(4- hydroxyphenyl)porphyrin (ZnTHPP/g-C3N4) in photocatalytic hydrogen production under the same conditions. In the chapter 6, the future work on photocatalytic CO2 reduction, perspectives and conclusions are included

No description available.

This thesis employs experimental and theoretical methods to characterise carbon nitride solids and proposes a generalstructural model for amorphous carbon nitride (a-C:N). It finds that a-C:N deposited by several methods is essentially identical, with similar bonding environments for carbon and nitrogen atoms. Using evidence from several techniques, the saturation of nitrogen in an sp2 carbon matrix is discussed. The experimental studies on a range of carbon nitride solids show no evidence for a crystalline form of carbon nitride. In addition to the experimental characterisation of a-C:N, ab initio molecular dynamics were used to investigate bonding and structure in carbon nitride. These simulations show that the most common form of nitrogen bonding was three-fold sites with a lone pair of electrons. Two-fold nitrogen sites were also found in agreement with experimental findings. An increase of nitrogen in a-C:N decreases the sp3-carbon fraction, but this is not localised on the nitrogen and the effect is most severe at high densities. A simulation of a low density/high nitrogen content network shows that the nitrogen saturation seen experimentally may be due to the formation of N2 dimers and C-N molecules which are easily driven out of the structure. The ab initio simulations also explore the nature of charged nitrogen and carbon sites in a-C:N. An analysis based on Wannier Function centres provided further information about the bonding and allowed for a detailed classification of these sites. The removal of electrons from the networks caused structural changes that could explain the two-state conductivity in ta-C:N memory devices. Finally, a theoretical study of the electron energy-loss near-edge structure (ELNES) calculated using multiple scattering theory is presented. The calculated ELNES of diamond, graphite and boron, silicon and carbon nitride structures compare well to experiment and supports the experimental finding that no crystalline carbon nitride had (or has) been produced. These ELNES calculations will however, provide a means of identifying crystalline beta-C3N4 should it be synthesised.

Carbon is a fascinating element which can be observed in a large variety of morphologies and atomic structures due to its chemical ability to form different hybridizations. The present PhD thesis proposes the synthesis of several carbon-based materials using a unique and quite simple technique: the oxy-acetylene combustion flame method. From crystalline sp3- diamond to planar sp2- graphite, from the unidirectional nanotubes, needles and rods to bidimensional petals, a large variety of carbon materials are synthesized under the atmospheric pressure. These various carbon forms have been produced using a set of different experimental parameters. Both the input gas ratio and the substrate temperature are found to play a key role in the synthesis of these new carbon materials. The high quality of the graphitic phases can be correlated to the large acetylene content in the gas and to the high temperature of the substrate. Some specific morphologies such as petal-like single graphite crystals have been synthesized. Their sizes reach up to 20 mm. These bidimensional carbon materials are of particular importance to investigate fundamental physics in ideal low-dimensional systems. Polyhedral graphite crystals, which exhibit a unidirectional morphology, have also been produced. Their crystal structure is found to be highly graphitic although they display a cylindrical/polyhedral shape. Preliminary measurements of their field emission properties reveal a huge emission current, which is higher than the emission current obtained for multi-wall carbon nanotubes. The latter have also been synthesized in large amount and high quality using our oxy-acetylene combustion flame technique. At last, using again the same experimental set-up, a crystalline carbon nitride phase has been synthesized for the first time using a specific molecule called “melamine” as an organic precursor. Several experimental techniques, such as Energy Dispersive X-ray (EDX), X-ray Photoelectron Spectroscopy (XPS), Electron Energy Loss Spectroscopy (EELS), X-ray diffraction and Raman and infra-red spectroscopies have been used to analyze both the chemical composition and the crystalline structure of this new material, revealing a graphitic-C3N4 phase.