Dissertations / Theses on the topic 'Carbon Nitride'

Carbon nitrides and carbon nitride derivatives are promising photocatalysts. The main focus of this thesis is the synthesis and characterization of various carbon nitrides (incompletely condensed melon, carbon nitride doped cesium titanate, ultra-long calcined melon, and OH-melem). Those carbon nitrides were then tested with regard to their photocatalytic properties. In the first part of chapter 3 of this thesis, we focus on a material called ‘‘melem oligomer’’. Two different synthesis routes were applied (open system and half open system) and the composition and structure of this material was studied. Melem with two different crystalline structures and some amorphous residues were found in the product. We also tested the photocatalytic activity of melem oligomer and confirmed hydrogen production from water with a relatively low rate of 2 μmol g-1 h-1. In the second part of chapter 3, we synthesized ultra-long calcined melamine which may have a morphology similar to the ‘‘g-C3N4 nanosheets’’. We analyzed both the composition and structure and investigated the efficiency of the presumed g-C3N4 nanosheets for hydrogen production from water. Ultra-long calcined melamine showed the best photoactivity which is twice that of melon at 490 °C. This is most likely due to the interesting morphology and high surface area. In chapter 4, melem oligomer was doped with cesium titanate in situ. Different calcination times were applied and various characterization techniques were used to investigate the composition, structure and morphology of the obtained materials. The efficiency of this hybrid photocatalyst for hydrogen production did not show higher photoactivity than the pure carbon nitrides except in the case of 16 h calcination which was the optimum calcination time overall. In chapter 5, OH-melem with a composition close to 2-oxo-6,10-diamino-s-heptazine, which could be a precursor of oxygen-doped g-C3N4, was synthesized and characterized by various techniques. Crystallinity is rather low in this oxygen containing species. NMR spectra differ from melem or cyameluric acid and XPS results confirm the presence of C=O groups. Overall, different carbon nitrides and carbon nitride derivatives were synthesized and chemically investigated to gain further knowledge on their synthesis, chemical properties and their resulting application as photocatalysts.

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.

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.

It was first performed a special method of pyrolysis of melamine in order to study the possibility of coproduction of carbon nitride and its oxidized derivatives. New compound as a graphite-like carbon nitride oxide (g-C3N4)O, which is isostructural analogue of graphite oxide, and doped with oxygen ( ~ 8.1 % ), carbon nitride (O-g-C3N4) were obtained. In contrast to known methods for the preparation of samples doped with oxygen carbon nitride a new route does not provide a preliminary synthesis of g-C3N4.The synthesized carbon nitride oxide is easily stratified and dissolved in water to form a flocculent solution which may contain not only ultra-thin nanosheets from several heptazine oxidized layers, but also the individual twodimensional monolayers. These monolayers can be a precursor for making reduced heptazine monolayer (or azagraphene). The synthesized substunces were investigated by methods chemical and X-ray analyses, IR spectroscopy, temperature-programmed desorption mass spectrometry (TPDMS) obtained products.

This thesis concerns synthesis and characterization of carbon-based materials and theinvestigation of the possible use, of a selection of these materials, in biomedicalapplications. Protein adsorption and blood plasma tests were used for this purposeutilizing a surface sensitive technique called spectroscopic ellipsometry. The materials were synthesized by physical vapor deposition and characterizedregarding microstructure, mechanical properties and optical properties. The ternaries BC-N and Si-C-N as well as carbon and carbon nitrides (CNx) of different microstructureshave been examined. In the B-C-N work, the intention was to investigate the possibilityto combine the two materials CNx and BN, interesting on their own regarding highhardness and extreme elasticity, to produce a material with even better properties.Theoretical calculations were performed to elucidate the different element substitutionsand defect arrangements in the basal planes promoting curvature in the fullerene-likemicrostructure. The Si-C-N ternary was investigated with the consideration of finding away to control the surface energy for certain applications. Amorphous carbon and threemicrostructures of CNx were analyzed by spectroscopic ellipsometry in the UV-VIS-NIRand IR spectral ranges in order to get further insight into the bonding structure of thematerial. In the second part of this work focus was held on studies of macromolecularinteractions on silicon, carbon and CNx film surfaces using ellipsometry. One purposewas to find relevance (or not) for these materials in biological environments. Materials for bone replacement used today, e.g. stainless steel, cobalt-chromium alloys andtitanium alloys suffer from corrosion in body fluids, generation of wear particles inarticulating systems, infections and blood coagulation and cellular damage leading toimpaired functionality and ultimately to implant failure. Artificial heart valves made ofpyrolytic carbon are used today, with friction and wear problems. Thus, there is still aneed to improve biomaterials. The aim of the fourth paper was to investigate theinteraction between carbon-based materials and proteins. Therefore, amorphous carbon(a-C), amorphous (a), graphitic (g) and fullerene-like (FL) CNx thin films were exposedto human serum albumin and blood plasma and the amount of protein was measured insitu using spectroscopic ellipsometry. Surface located and accessible proteins after blood plasma incubations were eventually identified through incubations in antibody solutions. Antibody exposures gave indications of surface response to blood coagulation,complement activation and clotting. The a-C and FL-CNx films might according to theresults have a future in soft tissue applications due to the low immuno-activity, whereasthe g-CNx film possibly might be a candidate for bone replacement applications. "Layered" structures of fibrinogen, a fibrous but soft protein involved in manyprocesses in our body, were grown in situ and dynamically monitored by ellipsometry inorder to understand the adsorption process and molecule arrangement onto a siliconsurface. In the last paper of this thesis, the effects of ion concentration and proteinconcentration on the refractive index of water-based solutions used in in situ ellipsometrymeasurements were demonstrated and spectral refractive index data for water solutionswith different ionic strengths and protein concentrations have been provided.

The present thesis focuses on carbon based thin films prepared by high power impulse magnetron sputtering (HiPIMS) and direct current magnetron sputtering (DCMS). Carbon nitride (CNx: 0 < x < 0.20) as well as carbon fluoride (CFx: 0.16 < x < 0.35) thin films were synthesized in an industrial deposition chamber by reactive magnetron sputtering of graphite in Ne/N2, Ar/N2, Kr/N2, Ar/CF4, and Ar/C4F8 ambients. In order to increase the understanding of the deposition processes of C in the corresponding reactive gas mixture plasmas, ion mass spectroscopy was carried out. A detailed evaluation of target current and target voltage waveforms was performed when graphite was sputtered in HiPIMS mode. First principle calculations targeting the growth of CFx thin films revealed most probable film forming species as well as CFx film structure defining defects. In order to set different process parameters into relation with thin film properties, the synthesized carbon based thin films were characterized with regards to their chemical composition, chemical bonding, and microstructure. A further aspect was the thin film characterization for possible applications. For this, mainly nanoindentation and contact angle measurements were performed. Theoretical calculations and the results from the characterization of the deposition processes were successfully related to the thin film properties. The reactive graphite/N2/inert gas HiPIMS discharge yielded high ion energies as well as elevated C+ and N+ abundances. Under such conditions, amorphous CNx thin films with hardnesses of up to 40 GPa were deposited. Elastic, fullerene like CNx thin films, on the other hand, were deposited at increased substrate temperatures in HiPIMS discharges exhibiting moderate ion energies. Here, a pulse assisted chemical sputtering at the target and the substrate was found to support the formation of a fullerene-like microstructure. CFx thin films were found to have surface energies equivalent to super-hydrophobic materials for x > 0.26 while such films were polymeric in nature accounting for hardnesses below 1 GPa. Whereas, an amorphous structure for carbon-based films with fluorine contents ranging between 16 % and 23 % was observed. For those films, the hardness increased with decreasing fluorine content and ranged between 16 GPa and 4 GPa. The HiPIMS process in fluorinecontaining atmosphere was found to be a powerful tool in order to change the surface properties of carbon based thin films.

Photocatalysis on semiconductor surfaces has grown tremendously in the last four decades. One reason for this is its analogy with photosynthesis, the most important natural photochemical process. Semiconductors to some extent can mimic the key steps of this fascinating heterogeneous photocatalytic process, i.e., photochemical charge generation, charge trapping, interfacial electron exchange and subsequent reaction. Building on this premise this thesis constitutes an investigation into the photocatalytic properties and applications of semiconducting layered framework carbon nitride based materials. Similar to traditional photocatalysts, the photocatalytic activity and efficiency of carbon nitride systems developed thus far is limited mainly by the fast recombination and low mobility of photogenerated excitons. Here, by exploiting the band alignment strategy, carbon nitride isotype (type II) and carbon nitride-niobium oxide of type II semiconductor heterojunctions were successfully constructed with the aim of suppressing the exciton recombination and improving charge extraction for the successful initiation of desirable redox chemistry. These features were demonstrated by employing the materials in heterogeneous photocatalysis for water splitting, organic pollutant decomposition and photochemical organic synthesis. Carbon nitride isotype heterojunctions constructed by controlled thermal condensation are shown to exhibit lower recombination of excitons relative to the pristine carbon nitride. As a consequence photocurrent generation and visible light driven H2 production activity was enhanced. This increase is attributed to the surface passivation and improved electron mobility of built-in electric field which arises from the topology-induced band offset of favoured type II heterojunction configuration. Building on the insights into the heterojunction-activity dependence, new type II graphitic carbon nitride (C3N4), Nb2O5 (C3N4-Nb2O5), heterojunctions synthesised via a hydrothermal method were exploited for their photodegradation ability of the organic pollutants. The synergic effect of carbon nitride and Nb2O5 coupling leads to the substantial photocatalytic activity improvement which can be attributed to the formation of an intimate interface and gradual attenuation of energy-wasteful charge recombination processes in C3N4-Nb2O5 heterojunctions materials. While water splitting and pollutant decomposition using semiconductors has received the bulk of attention, the possibilities concerning chemical synthesis are only beginning to be meaningfully exploited. We, therefore, employed carbon nitride to catalyse photo organic synthesis. It was demonstrated for the first time that carbon nitride can efficiently catalyse the photoacetalization reactions of aldehydes/ketones with alcohols, forming acetals at high yields using visible light under ambient conditions. Mechanistic studies suggest that the transient charge separation at the surface of this material is sufficient to catalyse the reaction in the absence of Lewis or Brønsted acids or solvent systems. Since the photoacetalization of aldehydes occurs under conditions similar to those of alcohols oxidation, both using visible light and carbon nitride as a catalyst, the two reactions actually proceed via different mechanisms. This study also demonstrates, visible light induced heterogeneous auto-tandem catalysis, coupling the oxidation and subsequent acetalization of alcohols in a single chemical process. This green strategy can be applicable to a wide variety of organic photo-induced synthesis.

Hard wear-resistant coatings have been widely applied 10 cutting tools to increase their durability and improve tribological properties. Physical vapour deposited TiAIN-based coatings. used in dry cutting performances. Have shown excellent hot hardness and oxidation resistance. The main handicap of these coatings is their brittleness. Another type of coatings arc amorphous. rather than crystalline. for example those based on fullerene-like carbon nitride (CNx). Such coatings possess high elasticity. but relatively low hardness. In this study. bi-Iayer coatings. designed as 1I CNx-based outer layer and a TiAlN-based inner layer. were deposited on either a silicon substrate or a high speed steel M2 substrate. The CNx outer layer incorporated either Ti/AI or Cr to improve its hardness. Cr was also added to the TiAIN based inner layer in some of the coatings. Detailed microstructural characterization and nanoindentation. to assess contact damage. \\CTC carried out on these coatings. The results showed that the microstructure of these coatings plays a critical part in the contact response. The addition ofCr into the CNx layer improves the toughness of the CNx layer. It acts as an inhibitor to the propagation of shear cracks initiated from the inner TiAIN layer. The incorporation of Cr into the TiAIN-based inner layer results in refinement of grain size and solid solution hardening. subsequently, this hinders inter-columnar sliding. Which results in the plastic deformation occurring lit a higher load. The application of a Cr interlayer was found to enhance the adhesion strength between the coating and the substrate. It was found the thicker. and harder. coatings on the ductile steel substrate generated inclined and lateral cracking. In contrast. thinner coatings initiated inter-columnar sliding and shear cracking followed by substrate plastic now. In turn. significant edge cracking (circumferential cracking) occurred in the CNx outer layer along the periphery of indent. It was also noted that nano-indentation testing using both Berkovich and spherical indenters produced different mechanical response and deformation microstructures in the coatings deposited on silicon substrates.

In the industry of metal cutting tools the conditions are extreme; the temperature can vary thousand degrees rapidly and the pressure can be tremendously high. To survive this kind of stress the cutting tool must be both hard and tough. In order to obtain these properties different coatings are used on a base of cemented carbide, WC-Co. Common coatings are hard ceramics like titanium nitride and titanium carbon-nitride with an outer layer of aluminium oxide. In this thesis the possibility of using titanium dioxide as an interlayer between titanium carbon-nitride and aluminium oxide to control the morphology and phase of aluminium oxide is investigated. Of the different aluminium oxide phases only the alpha-Al2O3 is stable. The titanium carbon-nitride coatings are made by CVD (chemical vapour deposition); also the alumina is deposited by CVD. The titanium dioxide was deposited by atomic layer deposition (ALD) which is a sequential CVD technique that allows a lower deposition temperature and better control of the film growth than CVD. The obtained thin films were analyzed using XRD, Raman spectroscopy, ESCA and SEM. To test the adhesion of the coatings the samples were sand blasted. A thin interlayer of titanium dioxide causes the aluminium oxide to grow as alpha-Al2O3, thinner TiO2 gave better adhesion.

This thesis is concerned with the electrical properties of the disordered amorphous material, amorphous carbon and carbon nitride films. At first, hydrogenated amorphous carbon was deposited using rf plasma enhanced chemical vapour deposition with methane as a precursor. Changing the deposition parameter, such as the input power results in the negative self bias modification and thus changes in the properties of the deposited film. The hydrogen condition in the deposition chamber is important for the growth of these films. With the demonstration of an achieve electronic device, a resonance tunnel diode, was reported using pulsed laser deposition; pulsed laser deposited amorphous carbon and carbon nitride thin films were studied. To understand the electrical properties of these films, a wide range of measurements were performed. The surface morphology was examined using atomic force microscopy and scanning tunnelling microscopy. The microstructure was investigated using electron energy loss spectroscopy giving data on the sp2 fraction, density, nitrogen content and Raman spectroscopy showed the degree of sp2 clustering. The band structure was investigated using electron energy loss spectroscopy, scarnning tunnelling spectroscopy and ultraviolet photoelectron spectroscopy, giving information on the density of empty conduction band states, close to the Fermi level and the occupied valence band states. The joint density of states was also measured by ultraviolet-visible-infrared optical transmittance, spectroscopic ellipsometry and Photothermal deflection spectroscopy. Electrical characterisations were carried out using both sandwich and coplanar structures. Pulsed laser annealing of amorphous carbon films was also studied, and the change on the surface morphology, microstructural and electrical properties studied. The conduction mechanism in amorphous carbon films at high electric fields was found to be based on classical Poole-Frenkel conduction, and the dielectric constants estimated from the model were found to be consistent with optical measurements. The neutral trapping centres were postulated to be localised sp2 sites below the conduction band according the analysis of the total band structure. Low field conduction in amorphous carbon films were thought to be controlled by band tail hopping through localised sp2 sites. Laser annealing shows the increase of the number of the sp2 sites which increase the conductivity of the film. However, the sp2 clustering does not necessarily increase the conductivity of the film. The optical band gap in high stress amorphous carbon films can be smaller than the other reports, as a bandtail exists in the bandgap which contributes to the hopping and Poole- Frenkel conduction process. The influence to the nitrogen atoms incorporated to laser deposited amorphous carbon nitride films was also studied. It was found that the nitrogen gas background pressure in the deposition chamber strongly affects the properties of the films. It was demonstrated that a higher nitrogen pressure does not always give rise to higher nitrogen content in the films. Higher nitrogen pressure reduces the velocity of the incident carbon species ablated by the laser, and less dense (less stress) films were deposited. Consequently, the conductivity of the film was reduced. However, the conduction mechanism appears still to be similar to that of amorphous carbon. The analysis of the change in the band structure due to the incorporation of the nitrogen atoms supports the analysis. Thus, the entire band structure of amorphous carbon was linked to the electrical conduction mechanism at both high and low electric fields, including the effect of nitrogen atom incorporation, and pulsed laser annealing. In this thesis we report the highest field effect mobility of a-C and a-CNx films ever reported in the literature of 0.01-0.02 cm2/Vs. This mobility is obtained due to the very high electric field that can be applied to our devices.

This Thesis details the synthesis, characterisation, and appl ications of micro/mesoporous carbon materials with tunable porosity prepared via template carbonisation. The main focus is the development of carbonaceous porous materials via for energy related gas storage applications. The Thesis investigates three synthesis strategies, namely; (i) nanocasting via liquid impregnation using zeolitic imidazolate framework (ZIF-8) as template, (ii) combination of liquid impregnation and chemical vapour deposition (CVD) using ZIF -8 as template and (iii) use of zeolite 13X and Y as templates for porous N-doped (carbon nitride type) materials. All the templated carbon materials were additionally activated to enhance their porosity and evaluated for gas (hydrogen and C02) storage.

Research efforts have been focused in the development of hard and wear resistant coatings over the last few decades. These protective coatings find applications in the industry such as cutting tools, automobile and machine part etc. Various ceramic thin films like TiN, TiAlN, TiC, SiC and diamond-like carbon (DLC) are examples of the films used in above applications. However, increasing technological and industrial demands request thin films with more complicated and advanced properties. For this purpose, B-C-N ternary system which is based on carbon, boron and nitrogen which exhibit exceptional properties and attract much attention from mechanical, optical and electronic perspectives. Also, boron carbonitride (BCN) thin films contains interesting phases such as diamond, cubic BN (c-BN), hexagonal boron nitride (h-BN), B[sub4]C, β-C[sub3]N[sub4]. Attempts have been made to form a material with semiconducting properties between the semi metallic graphite and the insulating h-BN, or to combine the cubic phases of diamond and c-BN (BC[sub2]N heterodiamond) in order to merge the higher hardness of the diamond with the advantages of c-BN, in particular with its better chemical resistance to iron and oxygen at elevated temperatures. New microprocessor CMOS technologies require interlayer dielectric materials with lower dielectric constant than those used in current technologies to meet RC delay goals and to minimize cross-talk. Silicon oxide or fluorinated silicon oxide (SiOF) materials having dielectric constant in the range of 3.6 to 4 have been used for many technology nodes. In order to meet the aggressive RC delay goals, new technologies require dielectric materials with K<3. BCN shows promise as a low dielectric constant material with good mechanical strength suitable to be used in newer CMOS technologies. For optical applications, the deposition of BCN coatings on polymers is a promising method for protecting the polymer surface against wear and scratching. BCN films have high optical transparency and thus can be used as mask substrates for X-ray lithography. Most of the efforts from different researchers were focused to deposit cubic boron nitride and boron carbide films. Several methods of preparing boron carbon nitride films have been reported, such as chemical vapor deposition (CVD), plasma assisted CVD, pulsed laser ablation and ion beam deposition. Very limited studies could be found focusing on the effect of nitrogen incorporation into boron carbide structure by sputtering. In this work, the deposition and haracterization of amorphous thin films of boron carbon nitride (BCN) is reported. The BCN thin films were deposited by radio frequency (rf) magnetron sputtering system. The BCN films were deposited by sputtering from a high purity B[sub4]C target with the incorporation of nitrogen gas in the sputtering ambient. Films of different compositions were deposited by varying the ratios of argon and nitrogen gas in the sputtering ambient. Investigation of the oxidation kinetics of these materials was performed to study high temperature compatibility of the material. Surface characterization of the deposited films was performed using X-ray photoelectron spectroscopy and optical profilometry. Studies reveal that the chemical state of the films is highly sensitive to nitrogen flow ratios during sputtering. Surface analysis shows that smooth and uniform BCN films can be produced using this technique. Carbon and nitrogen content in the films seem to be sensitive to annealing temperatures. However depth profile studies reveal certain stoichiometric compositions to be stable after high temperature anneal up to 700ºC. Electrical and optical characteristics are also investigated with interesting results. The optical band gap of the films ranged from 2.0 eV - 3.1 eV and increased with N[sub2]/Ar gas flow ratio except at the highest ratio. The optical band gap showed an increasing trend when annealed at higher temperatures. The effect of deposition temperature on the optical and chemical compositions of the BCN films was also studied. The band gap increased with the deposition temperature and the films deposited at 500[degrees]C had the highest band gap. Dielectric constant was calculated from the Capacitance-Voltage curves obtained for the MOS structures with BCN as the insulating material. Aluminum was used as the top electrode and the substrate was p-type Si. Effect of N[sub2]/Ar gas flow ratio and annealing on the values of dielectric constant was studied and the dielectric constant of 2.5 was obtained for the annealed BCN films. This by far is the lowest value of dielectric constant reported for BCN film deposited by sputtering. Lastly, the future research work on the BCN films that will be carried out as a part of the dissertation is proposed.
ID: 030422822; System requirements: World Wide Web browser and PDF reader.; Mode of access: World Wide Web.; Thesis (Ph.D.)--University of Central Florida, 2011.; Includes bibliographical references (p. 112-130).
Ph.D.
Doctorate
Electrical Engineering and Computer Science
Engineering and Computer Science

The connection between ritual and the interior is interrogated through a theoretical framework integrating Van Gennep’s Rites of Passage Theory and Turner’s Theory of Liminality. A multi-faceted methodological framework is developed from the interrogation of the disciplinary edges of multiple methodologies, addressing the experiential, cultural and subjective dimensions of ritual. This new way of exploring the interior demonstrates how knowledge can be acquired from the body’s immersion in unfolding ritual situations, revealing elements of ritual and interior in relation to one another and the generation of new theories on the interior.

The research undertaken in this project has involved the synthesis of the carbon nitride material C6N9H3.HCl under high pressures and temperatures in the piston cylinder apparatus. The synthesis conditions of the material have been perfected and the relatively well crystallised products have been characterised in several ways to try to gain a better understanding of the structure of the material. Characterisation techniques used include; elemental analysis, X-ray diffraction, infrared spectroscopy, scanning and transmission electron microscopy, X-ray photoelectron spectroscopy, visible, ultraviolet and Fourier Transform Raman spectroscopy. The behaviour of the C6N9H3.HCl material has also been studied further under very high pressures. Diamond anvil cell techniques and synchrotron X-ray radiation have been used to investigate the materials structural behaviour up to pressures in excess of 60 GPa. The decompression of the material has also been investigated. Investigation of the behaviour at high pressures using FTIR was also attempted. The ambient pressure characterisation, high pressure behaviour and decompression of the material have been compared in detail to computational calculations performed on the model structure proposed for the material. Interesting advances in the understanding of the ambient pressure structure and of the high pressure behaviour of the material have been made and the significant structural changes in the material at very high pressures have been explained by the comparison of the experimental and computational results. Due to the highly fluorescent nature of the material under visible Raman lasers it was not possible to obtain Raman spectra of the material in the laboratory at UCL. Therefore the material has been studied using UV and FT Raman techniques using equipment in laboratories in Lyon and Grenoble in France and in Nottingham, UK, in an attempt to obtain vibrational spectra without exciting electronic transitions and to gain more information about the structure.

In this study, the chemical bonding in hard and elastic amorphous carbon nitride (a-CNx) films is investigated with x-ray photoelectron spectroscopy (XPS) and 15N, 13C, and 1H nuclear magnetic resonance (NMR) spectroscopy. The films were deposited by DC Magnetron sputtering in a pure nitrogen discharge on Si(001) substrates at 300--400??C. Nanoindentation measurements reveal an elastic modulus of ∼50 GPa and a hardness of ∼5 GPa, thus confirming our films are highly elastic but resist plastic deformation.;Our 13C NMR study demonstrates the absence of sp 3-bonded carbon in this material. Collectively, our N(1s) XPS, 13C NMR, and 15N NMR data suggest a film-bonding model that has an aromatic carbon structure with sp2-hybridized nitrogen incorporated in heterocyclic rings. We demonstrate that the nitrogen bonding is predominantly in configurations similar to those in pyridine and pyrrole. In addition, the data indicate that the a-CNx films prepared for this study have low hydrogen content, but are hydrophilic. Specifically, results from 15N and 13C cross polarization (CP) and 1H magic angle spinning (MAS) NMR experiments suggest that nitrogen sites are susceptible to protonation from water absorbed during sample preparation for the NMR experiments. The sensitivity of the surface of a-CNx to water absorption may impact tribological applications for this material.;In accord with our XPS and NMR spectroscopic studies on a-CN x films, we propose a film-structure model consisting of buckled graphitic planes that are cross-linked together by sp2 hybridized carbons. The curvature and cross-linking is attributed to a type of compound defect, which is formed by placing a pentagon next to single-atom vacancy in a graphite layer. Our proposed film structure is called the pentagon-with-vacancy-defect (5VD) model. Using Hartree-Fock calculations, we show that the 5VD, film-structure model is compatible with our XPS, NMR, and nanoindentation measurements and with previous transmission electron microscopy (TEM) and computational work.

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.

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

There is an every increasing need to develop more durable and higher performing coatings for use in a range of products including tools, devices and bio-implants. Nano-structured coatings either in the form of a nanocomposite or a multilayer is of considerable interest since they often exhibit outstanding properties. The objective of this thesis was to use advanced plasma synthesis methods to produce novel nano-structured coatings with enhanced properties. Coatings consisting of combinations of aluminum (Al), aluminum nitride (AlN) and amorphous carbon (a-C) were investigated. Cathodic vacuum arc deposition and unbalanced magnetron sputtering were used to prepare the coatings. By varying the deposition conditions such as substrate bias and temperature, coatings with a variety of microstructures were formed. A comprehensive range of analytical methods have been employed to investigate the stoichiometry and microstructure of the coatings. These include Transmission Electron Microscopy (TEM), Scanning Transmission Electron Microscopy (STEM), Electron Energy Loss Spectroscopy, Auger Electron Spectroscopy, X-ray diffraction and Raman spectroscopy. In addition to the investigation of microstructure, the physical properties of the coatings were measured. Residual stress has been recognized as an important property in the study of thin film coatings since it can greatly affect the quality of the coatings. For this reason, residual stress has been extensively studied here. Hardness measurements were performed using a nano indentation system, which is sensitive to the mechanical properties of thin films. This thesis undertook the most comprehensive investigation of the Al/AlN multilayer system. A major finding was the identification of the conditions under which layers or nanocomposite form in this system. A model was developed based on energetics and diffusion limited aggregation that is consistent with the experimental data. Multilayers of a-C and Al were also found to form nanocomposites. No hardness enhancement as a function of layer thickness or feature size was observed in either the Al/AlN or a-C/a-C systems. It was found that the most important factor which determines hardness is the intrinsic stress, with films of high compressive stress exhibiting the highest hardness. Nano-structured multilayers of alternating high and low density a-C were investigated. For a-C multilayers prepared using two levels of DC bias, evidence of ion beam induced damage was observed at the interfaces of both the low and high density layers. In addition, the structure of the high density (ta-C, known as tetrahedral amorphous carbon) layers was found to be largely unchanged by annealing. These results extend our understanding of how a-C form from energetic ion beams and confirms the thermal stability of ta-C in a multilayer. This thesis also presented the first attempt to synthesis a-C multilayered films with a continuously varying DC bias in sinusoidal pattern. The resulting films were shown to have a structurally graded interface between layers and verified that ion energy and stress are the most important factors which determine the structure of a-C films.

Artificial photosynthesis utilises solar-light for clean fuel H2 production and is emerging as a potential solution for renewable energy generation. Photocatalytic systems that combine a light harvester and catalysts in one-pot reactor are promising strategies towards this direction. Yet, most of the reported systems function by consuming excess amount of expensive sacrificial reagents, preventing commercial development. In this thesis, carbon nitrides (CNx) have been selected as non-toxic, stable and low-cost photocatalysts. CNx are first introduced as efficient light harvesters, to couple alcohol oxidation with proton reduction, in the presence of a Ni-based molecular catalyst. This system operated in a single compartment while the oxidation and reduction products were collected in the solution and gaseous phases, respectively, demonstrating a closed redox system. In the presence of an organic substrate and absence of a proton reduction catalyst, photoexcited CNx was found to accumulate long-lived "trapped-electrons", which enables decoupling oxidation and reduction reactions temporarily and spatially. This allows solar H2 generation in the dark, following light exposure, replication light and dark cycle of natural photosynthesis in an artificial set-up. The stability of the designed system was found to be limited by the Ni-based molecular catalyst, and the spectroscopic studies revealed electron transfer from CNx to catalyst as the kinetic bottleneck. Graphene based conductive scaffolds were introduced to the CNx-Ni system, to accelerate the rate of electron transfer from CNx to the Ni catalyst. Time-resolved spectroscopic techniques revealed that introducing these conductive binders enabled better electronic communication between CNx and Ni, resulting in significantly enhanced photocatalytic activity. To improve the solar-light utilisation and the photocatalytic performance of bulk CNx, a straightforward ultra-sonication approach was introduced. This pre-treatment was found to break aggregates of bulk CNx, and the resulting activated CNx had significantly improved activity. The activated CNx showed record activities per gram of the material used, for H2 evolution with a molecular Ni catalyst. The use of abundant waste sources instead of organic substrates was investigated in the presence of activated CNx. The system demonstrated to photoreform purified and raw lignocellulose samples into H2 in the presence of various H2 evolution catalysts over a wide range of pH.

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.

As a potential solution to the global energy and environmental pollution, design and synthesis of artificial photocatalysts with high activities have attracted increasing scientific interests worldwide. In recent years, the graphitic carbon nitride (g-C3N4) has shown new possible applications in the photocatalytic field due to its unique properties. However, the photocatalytic efficiency of the pristine g-C3N4 is greatly limited by the high recombination rate of the photo-induced electron-hole pairs. In this thesis, the aim is to design and fabricate efficient g-C3N4 based photocatalysts with enhanced photocatalytic activities under a visible light irradiation. In order to achieve this goal, two strategies have been employed in the present thesis. First, the as-obtained g-C3N4 was used as the host material to construct staggered-aligned composite photocatalysts by selecting semiconductors with suitable band positions. By this method, three kinds of g-C3N4-based composite photocatalysts such as g-C3N4/ZnS nanocage, g-C3N4/m-Ag2Mo2O7 and g-C3N4/MIL-88A were successfully fabricated. Second, the microstructure of the g-C3N4 was modified by the H2O2-treatment at an elevated temperature and ambient pressure. In this study, the g-C3N4 was prepared by a simple pyrolysis of urea. As for all the as-synthesized phtocatalysts, the structures, morphologies and the optical properties were carefully characterized by the following techniques: XRD, SEM, TEM, FT-IR and DRS. Also, the band edge positions of m-Ag2Mo2O7 and MIL-88A were studied by the Mott-Schottky methods. Thereafter, the photocatalytic activities were evaluated by using a solution of rhodamine B (RhB) as a target pollutant for the photodegradation experiments performed under a visible light irradiation. The results showed that all the aforementioned g-C3N4-based photocatalysts exhibited enhanced photocatalytic activities in comparison with the pristine g-C3N4. For the case of the g-C3N4-based composite photocatalysts, the enhancement factor over the pristine g-C3N4 can achieve values ranging from 2.6 to 3.4. As for the H2O2-treated g-C3N4, the degradation rate constant can be 4.6 times higher than that of the pristine g-C3N4. To understand the key factors in new materials design, we also devote a lot of efforts to elucidate the basic mechanisms during the photocatalytic degradation of organic pollutant. Based on the results of the active species trapping (AST) experiments, the main active species in each photocatalytic system were determined. In the g-C3N4/m-Ag2Mo2O7 and the g-C3N4/MIL-88A system, three kinds of active species of ·O2-, h+ and ·OH were found to be involved in the photocatalytic reaction. Among them, the ·O2- and h+ were the main active species. In the g-C3N4/ZnS and H2O2-treated g-C3N4 photocatalytic systems, the main active species was determined as the ·O2-. The reaction pathways of these active species were also demonstrated by comparing the band edge positions with the potentials of the redox couple. In addition, the relationship between the active species and the photocatalytic behaviors of N-de-ethylation and conjugated structure cleavage were studied. Finally, possible mechanisms to explain the enhanced photocatalytic activities were proposed for each photocatalytic system. The results in this thesis clearly confirm that the photocatalytic activity of the g-C3N4 based photocatalyst can efficiently be enhanced by constructions of staggered-aligned composites and by modification of the microstructure of the g-C3N4. The enhanced photocatalytic performance can mainly be ascribed to the efficient separation of the photo-induced electron-hole pairs and the increase of the active sites for the photocatalytic reaction.

QC 20150909

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

In this work, we have investigated temperature dependent electrical transport properties of carbon based two-dimensional (2D) nanomaterials. Various techniques were employed to synthesize the samples. For instance, high quality large area graphene and boron, nitrogen doped graphene (BNC) were grown using thermal catalytic chemical vapor deposition (CVD) method. Liquid phase exfoliation technique was utilized to exfoliate graphene and graphitic carbon nitride samples in isopropyl alcohol. Chemical reduction technique was used to reduce graphene oxide (rGO) by utilizing ascorbic acid (a green chemical) as a reducing agent. Detailed structural and morphology characterization of these samples was performed using state of the art microscopy as well as spectroscopic techniques (for example; Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), UV-Visible spectroscopy as well as Raman Spectroscopy). The low temperature (5 K< T <400 K) electrical transport properties of these materials show substantial difference from sample to sample studied. For instance, CVD grown graphene film has displayed metallic behavior over a wide range of temperature (5 K < T <300 K). At higher temperatures, resistivity followed linearly with the temperature (ρ(T) ~T). A power law dependence (ρ(T) ~ T4) observed at lower temperatures. Where as liquid phase exfoliated graphene and graphitic carbon nitride samples displayed nonmetallic nature: increasing resistance with decrease in temperature over a wide range (8 K < T < 270 K) of temperature. Electrical transport behavior in these samples was governed by two different Arrhenius behaviors in the studied temperature range. In the case of rGO and BNC layers, electrical conduction show two different transport mechanisms in two different temperature regimes. At higher temperatures, Arrhenius-like temperature dependence of resistance was observed indicating a band gap dominating transport behavior. At lower temperatures, Mott's two dimensional-Variable Range Hopping (2D-VRH) behavior was observed.

The nature and role of defects is of primary importance to understand the physical properties of C and BN single walled nanotubes. Transmission electron microscopy (TEM) is a well known powerful tool to study the structure of defects in materials. However, in the case of SWNTs, the electron irradiation of the TEM may knock out atoms. This effect may alter the native structure of the tube, and has also been proposed as a potential tool for nanoengineering of nanotubular structures. Here we develop a theoretical description of the irradiation mechanism. First, the anisotropy of the emission energy threshold is obtained via density functional based calculations. Then, we numerically derive the total Mott cross section for different emission sites of carbon and boron nitride nanotubes with different chiralities. Using a dedicated STEM microscope with experimental conditions optimised on the basis of derived cross-sections, we are able to control the generation of defects in nanotubular systems. Either point or line defects can be obtained with a spatial resolution of a few nanometers. The structure, energetics and electronics of point and line defects in BN systems have been investigated. Stability of mono- and di- vacancy defects in hexagonal boron nitride layers is investigated, and their activation energies and reaction paths for diffusion have been derived using the nudged elastic band method (NEB) combined with density functional based techniques. We demonstrate that the appearance of extended linear defects under electron irradiation is more favorable than a random distribution of point defects and this is due to the existence of preferential sites for atom emission in the presence of pre-existing defects, rather than thermal vacancy nucleation and migration.

This thesis presents results of a computational investigation focusing on two classes of materials: • Those of interest as anodes in rechargeable Li ion batteries, particularly TiO2 polymorphs and a novel Ti oxysulfide; and • A graphitic carbon nitride, where the high pressure structural and electronic transformations, and intercalation properties are studied. The first part of the work considers the Li intercalation in TiO2 Brookite. Calculations identify stable sites and low energy configurations during the intercalation process up to full occupancy. It is demonstrated that Li intercalation is homogeneous but consists of three domains corresponding to different ordering patterns. It is shown that Li intercalation is strongly limited by the diffusion of Li ions, especially at high Li content, resulting in the absence of intercalation at normal conditions, and intercalation to high concentrations at elevated temperature or in the nanophase. Later, Li intercalation in the TiO2-B structure is investigated and the most favourable energy sites for Li ions in the structure have been identified including the energy favourable migration pathways. These have been studied by calculating the diffusion barriers in all possible directions within the substrate material. Calculations indicate that the TiO2-B structure has a lower density than other titanates, and the calculated cell voltage is in the range of 0.8 to 1.45 V for low Li content, with the Z direction showing the highest mobility for Li ions; the TiO2-B structure therefore yields a high Li mobility. Calculations show that brookite is suitable to host Li ions, and yields relatively constant voltage values which are suitable for use in actual Li batteries (Brookite anode value calculated as 1.7 V); TiO2-B generates a voltage value which is lower than the brookite voltage which means it is a promising anode in rechargeable Li batteries. In the second part of the thesis a graphitic carbon nitride of composition C6N9H3.HCl is studied. In the first part of the work, the interlayer bonding mechanism that takes place between consecutive layers at high pressure is analysed. Calculations performed also allowed identification of symmetrical hydrogen bonding, a feature which is rarely observed, and the formation of carbon-chlorine bonds (both observations at elevated pressures). In the next stage of the study on Carbon Nitride, intercalation behaviour within the graphitic structure is examined (hence connecting this part of the work to the work performed on the TiO2 substrates). Results indicate that the ability of the examined carbon nitride to act as an anode for rechargeable Li batteries is low because the intercalation energy is not constant as a function of the Li content, in addition to strong structural deformations occurring during intercalation. The combination of factors affecting the cyclabilty of intercalation/deintercalation is expected to make the substrate a poor rechargeable battery. The final section of this thesis is dedicated to the study of the Li intercalation properties of a layered oxysulfide structure of composition Y4Ti4O10S4. Calculations indicate that the material appears to be unsuitable for use as an electrode for Li ion rechargeable batteries due to unsuitable Li intercalation energies, and large structural changes occurring within the substrate during the cycling process.

Un procédé dénommé SMAT (Surface Mechanical Attrition Treatment) a été développé pour la fabrication des structures nanocristallines à la surface des matériaux cristallins. Les propriétés mécaniques ainsi que les propriétés de diffusion avec une réactivité exceptionnelle sont nettement améliorées pour l’échantillon SMATé. Par conséquent, les métaux SMATés sont considérés pour la fabrication de nanomatériaux de carbone. Des échantillons en acier inoxydable (AISI 316L), Co, Ni et Ti SMATés ont été soumis au procédé CVD simple pour la synthèse des nanomatériaux de carbone. Les nanomatériaux de carbone sont obtenus en surface de ces échantillons. Ces produits sont caractérisés par MET, MEB, DRX et spectrométrie RAMAN, indiquant une présence de CNFs sur les échantillons SMATés en acier inoxydable 316 L, Co, Ti ainsi qu’une présence de CNFs et MWNTs sur Ni SMATé. Le mécanisme de la croissance de CNFs sur les métaux de transition traités par le SMAT a été illustré schématiquement. Les effets de différents paramètres sont ainsi discutés. La seconde partie de la thèse concerne un nouveau développement de la machine SMAT. Pour former une couche nanostructurée plus épaisse en surface de matériaux, un nouveau système de SMAT a été développé. Les échantillons en l’acier inoxydable 316 ont été traités sous traction et à haute température en utilisant le nouveau système de SMAT. Puis, ces échantillons ont été nitrurés par un procédé de nitruration assisté par plasma. Les échantillons ont été caractérisés par microscopie optique, DRX, nanoindentation, et la machine de micro-dureté
Since the development of the new technique SMAT (Surface Mechanical Attrition Treatment), great success has been achieved. The mechanical properties and the diffusion properties of materials treated by SMAT are greatly improved. Carbon nanomaterials such as carbon nanofibers (CNFs) and carbon nanotubes (CNTs) have attracted special attention due to their unique properties and potential application. Since the diffusion properties of materials have been improved after the SMAT process, a SMAT process followed by a CVD process, i. E. Hybrid SMAT, is tailored for synthesizing carbon nanomaterials in-situ on the surface of bulk metallic materials. 316L stainless steel, pure Co, pure Ni and pure Ti plate were subjected to hybrid SMAT process to synthesize carbon nanomaterials. The effects of main parameters are discussed. The products were investigated by SEM, TEM, XRD and RAMAN characterizations. Growth mechanism was proposed. The second part of work concerns the development of SMAT machine and the formation of nitride nanomaterials on bulk metallic materials. A new SMAT system that can provide various treating conditions was developed to form a thicker nanostructured surface layer. 316 stainless steel samples were subjected to the new system, treating under traction and under thermal stress respectively. The treated samples were investigated by optical micros-copy, XRD and nanoindentation. Treated samples were submitted to the nitriding process to form nitride nanomaterials. The nitride samples were investigated by optical microscopy and microhardness tester

Afin de résoudre les problèmes environnementaux et énergétiques modernes, ces dernières années ont vu le développement de catalyseurs photocataytiques capables d’utiliser la lumière solaire. En effet, les possibles applications des semiconducteurs présentant des propriétés photocatalytiques dans les domaines de la production d’hydrogène ou la dégradation de polluants organiques ont généré un grand intérêt de la part de la communauté scientifique. Actuellement, les photocatalyseurs à base de dioxyde de titane (TiO₂) et de nitrure de carbone graphitique (g-C₃N₄) sont considérés comme les matériaux les plus étudiés pour leurs faibles coûts et leurs propriétés physico-chimiques exceptionnelles. Cependant, la performance photocatalytique de ces matériaux reste encore limitée, à cause de la recombinaison rapide des porteurs de charge et et d'une absorption limitée de la lumière. En générale, malgré des caractéristiques exceptionnelles, ces matériaux ne contribuent pas significativement à la séparation de charge et l’absorption de la lumière lorsqu’ils sont produits par des méthodes conventionnelles. L'objectif de cette thèse est de développer de nouvelles voies pour la production de matériaux efficaces basés sur TiO₂ et g-C₃N₄). Nous avons d'abord préparé de la triazine (CxNy) qui fonctionne comme un co-catalyseur d'oxydation ce qui facilite la séparation des paires «électron-trou» dans le système du photocatalyseur creux de type Pt-TiO₂-CxNy. La présence simultanée de Pt et de CxNy, qui servent comme co-catalyseurs de réduction et d'oxydation, respectivement, a permis une amélioration remarquable des performances photocatalytiques du TiO₂. De plus, nous avons développé une nouvelle approche, en utilisant un procédé de combustion de sphère de carbone assisté par l’air, pour préparer du C/Pt/TiO₂ . Ce matériau possède de nombreuses propriétés uniques qui contribuent de manière significative à augmenter la séparation « électron-trou », et en conséquence, à améliorer la performance photocatalytique. Dans le but de développer un matériau qui soit capable de fonctionner sous les rayons du soleil et dans l'obscurité, nous avons développé un photocatalyseur creux à double enveloppes : le Pt-WO₃/TiO₂-Au. Ce matériau a montré non seulement une forte absorption de la lumière solaire, mais aussi une séparation des charges élevée et une haute capacité de stockage d'électrons. Par conséquent, ce type de photocatalyseurs a montré une dégradation efficace des polluants organiques, à la fois sous la lumière visible (λ ≥ 420 nm) et dans l'obscurité. En ce qui concerne le g-C₃N₄, nous avons exploité la relation entre les lacunes d’azote et les propriétés plasmoniques des nanoparticules d’or (Au). Ce type de photocatalyseur du Au/g-C₃N₄ a été préparé en présence d’alcali suivi par une post calcination. En effet, les lacunes d’azote ainsi produites permettent le renforcement des interactions entre l’or et le g-C₃N₄ et des propriétés plasmoniques de l’or. Ces caractéristiques exceptionnelles renforcent l'utilisation efficace de l’énergie solaire ainsi que la séparation des paires « électron-trou », ce qui contribuent à la performance photocatalytique pour la production d'hydrogène du photocatalyseur. Afin d’améliorer la capacité d’absorption de la lumière visible de g-C₃N₄, une nouvelle voie de synthèse dénommée « poly-alcaline » a été développée. La possibilité d’ajouter du polyéthylèneimine (PEI) et de l’hydroxyde de potassium (KOH) pour générer de nombreux centres lacunaires en azote ainsi que des groupes hydroxyles dans la structure du matériau, a été explorée afin d’optimiser l’efficacité du matériau. De telles modifications ont démontré leurs capacités à réduire la bande interdite et à provoquer plus facilement la séparation de charges améliorant ainsi les propriétés photocatalytiques du photocatalyseur vis-à-vis de la production d’hydrogène. Cette méthode ouvre donc une nouvelle voie d’avenir pour préparer des photocatalyseurs nanocomposites efficaces possédant à la fois, une forte d’absorption de la lumière et une bonne séparation de charges.
Afin de résoudre les problèmes environnementaux et énergétiques modernes, ces dernières années ont vu le développement de catalyseurs photocataytiques capables d’utiliser la lumière solaire. En effet, les possibles applications des semiconducteurs présentant des propriétés photocatalytiques dans les domaines de la production d’hydrogène ou la dégradation de polluants organiques ont généré un grand intérêt de la part de la communauté scientifique. Actuellement, les photocatalyseurs à base de dioxyde de titane (TiO₂) et de nitrure de carbone graphitique (g-C₃N₄) sont considérés comme les matériaux les plus étudiés pour leurs faibles coûts et leurs propriétés physico-chimiques exceptionnelles. Cependant, la performance photocatalytique de ces matériaux reste encore limitée, à cause de la recombinaison rapide des porteurs de charge et et d'une absorption limitée de la lumière. En générale, malgré des caractéristiques exceptionnelles, ces matériaux ne contribuent pas significativement à la séparation de charge et l’absorption de la lumière lorsqu’ils sont produits par des méthodes conventionnelles. L'objectif de cette thèse est de développer de nouvelles voies pour la production de matériaux efficaces basés sur TiO₂ et g-C₃N₄). Nous avons d'abord préparé de la triazine (CxNy) qui fonctionne comme un co-catalyseur d'oxydation ce qui facilite la séparation des paires «électron-trou» dans le système du photocatalyseur creux de type Pt-TiO₂-CxNy. La présence simultanée de Pt et de CxNy, qui servent comme co-catalyseurs de réduction et d'oxydation, respectivement, a permis une amélioration remarquable des performances photocatalytiques du TiO₂. De plus, nous avons développé une nouvelle approche, en utilisant un procédé de combustion de sphère de carbone assisté par l’air, pour préparer du C/Pt/TiO₂ . Ce matériau possède de nombreuses propriétés uniques qui contribuent de manière significative à augmenter la séparation « électron-trou », et en conséquence, à améliorer la performance photocatalytique. Dans le but de développer un matériau qui soit capable de fonctionner sous les rayons du soleil et dans l'obscurité, nous avons développé un photocatalyseur creux à double enveloppes : le Pt-WO₃/TiO₂-Au. Ce matériau a montré non seulement une forte absorption de la lumière solaire, mais aussi une séparation des charges élevée et une haute capacité de stockage d'électrons. Par conséquent, ce type de photocatalyseurs a montré une dégradation efficace des polluants organiques, à la fois sous la lumière visible (λ ≥ 420 nm) et dans l'obscurité. En ce qui concerne le g-C₃N₄, nous avons exploité la relation entre les lacunes d’azote et les propriétés plasmoniques des nanoparticules d’or (Au). Ce type de photocatalyseur du Au/g-C₃N₄ a été préparé en présence d’alcali suivi par une post calcination. En effet, les lacunes d’azote ainsi produites permettent le renforcement des interactions entre l’or et le g-C₃N₄ et des propriétés plasmoniques de l’or. Ces caractéristiques exceptionnelles renforcent l'utilisation efficace de l’énergie solaire ainsi que la séparation des paires « électron-trou », ce qui contribuent à la performance photocatalytique pour la production d'hydrogène du photocatalyseur. Afin d’améliorer la capacité d’absorption de la lumière visible de g-C₃N₄, une nouvelle voie de synthèse dénommée « poly-alcaline » a été développée. La possibilité d’ajouter du polyéthylèneimine (PEI) et de l’hydroxyde de potassium (KOH) pour générer de nombreux centres lacunaires en azote ainsi que des groupes hydroxyles dans la structure du matériau, a été explorée afin d’optimiser l’efficacité du matériau. De telles modifications ont démontré leurs capacités à réduire la bande interdite et à provoquer plus facilement la séparation de charges améliorant ainsi les propriétés photocatalytiques du photocatalyseur vis-à-vis de la production d’hydrogène. Cette méthode ouvre donc une nouvelle voie d’avenir pour préparer des photocatalyseurs nanocomposites efficaces possédant à la fois, une forte d’absorption de la lumière et une bonne séparation de charges.
The utilization of solar light-driven photocatalysts has emerged as a potential approach to deal with the serious current energy and environmental issues. Over the past decades, semiconductor-based photocatalysis has attracted an increasing attention for diverse applications including hydrogen production and the decomposition of organic pollutants. Currently, titanium dioxide (TiO₂) and graphitic carbon nitride (g-C₃N₄)-based photocatalysts have been considered as the most investigated materials because of their low cost, outstanding physical and chemical properties. However, their photocatalytic performances are still moderate owing to the fast charge carrier recombination and limited light absorption. The main target of the research presented in this thesis is to develop novel routes to prepare efficient materials based on TiO₂ and g-C₃N₄. These materials possess prominent features, which contribute to address the fast charge separation and light absorption problems. We firstly have prepared triazine (CxNy) acting as an oxidation co-catalyst, which efficiently facilitates electron-hole separation in a Pt-TiO₂-CxNy hollow photocatalyst system. The co-existence of Pt and CxNy functioning as the reduction and oxidation co-catalysts, respectively, has remarkably enhanced the photocatalytic performance of TiO₂. Next, we have also developed a new approach employing the air- assisted carbon sphere combustion process in preparing C/Pt/TiO₂. This material possesses many salient properties that significantly boost the electron-hole separation leading to enhanced photocatalytic performance. In an attempt to design a material that can operate under sunlight and in darkness, we have introduced Pt-WO₃/TiO₂-Au double shell hollow photocatalyst. The material has shown not only strong solar light absorption but also efficient charge separation and electron storage capacity. As a result, this type of photocatalyst exhibits a high activity performance for the degradation of organic pollutants both under visible light (λ ≥ 420 nm) and in the dark. Regarding to g-C₃N₄, we have explored the relationship between nitrogen vacancies and the plasmonic properties of Au nanoparticles employing alkali associated with the post-calcination method to prepare Au/g-C₃N₄. In fact, the produced nitrogen vacancies in the structure of g-C₃N₄ essentially enhance the interaction at Au/g-C₃N₄ interface and the plasmonic properties of Au nanoparticles. These outstanding features contribute to enhance the utilization of solar light and electron-hole separation that prompt the photocatalytic performance towards hydrogen production. Finally, we have employed a novel poly-alkali route to prepare a strong visible light absorption photocatalyst-based g-C₃N₄. The co-existence of PEI and KOH, which induces numerous nitrogen vacancies and incorporated hydroxyl groups in the structure of the resulted material, has been explored for the first time. These modifications have been proved to narrow the bandgap and facilitate the charge separation leading to enhance the solar light-driven hydrogen production. This method also opens up a new approach to prepare efficient nanocomposite photocatalysts possessing both strong light absorption and good charge separation.
The utilization of solar light-driven photocatalysts has emerged as a potential approach to deal with the serious current energy and environmental issues. Over the past decades, semiconductor-based photocatalysis has attracted an increasing attention for diverse applications including hydrogen production and the decomposition of organic pollutants. Currently, titanium dioxide (TiO₂) and graphitic carbon nitride (g-C₃N₄)-based photocatalysts have been considered as the most investigated materials because of their low cost, outstanding physical and chemical properties. However, their photocatalytic performances are still moderate owing to the fast charge carrier recombination and limited light absorption. The main target of the research presented in this thesis is to develop novel routes to prepare efficient materials based on TiO₂ and g-C₃N₄. These materials possess prominent features, which contribute to address the fast charge separation and light absorption problems. We firstly have prepared triazine (CxNy) acting as an oxidation co-catalyst, which efficiently facilitates electron-hole separation in a Pt-TiO₂-CxNy hollow photocatalyst system. The co-existence of Pt and CxNy functioning as the reduction and oxidation co-catalysts, respectively, has remarkably enhanced the photocatalytic performance of TiO₂. Next, we have also developed a new approach employing the air- assisted carbon sphere combustion process in preparing C/Pt/TiO₂. This material possesses many salient properties that significantly boost the electron-hole separation leading to enhanced photocatalytic performance. In an attempt to design a material that can operate under sunlight and in darkness, we have introduced Pt-WO₃/TiO₂-Au double shell hollow photocatalyst. The material has shown not only strong solar light absorption but also efficient charge separation and electron storage capacity. As a result, this type of photocatalyst exhibits a high activity performance for the degradation of organic pollutants both under visible light (λ ≥ 420 nm) and in the dark. Regarding to g-C₃N₄, we have explored the relationship between nitrogen vacancies and the plasmonic properties of Au nanoparticles employing alkali associated with the post-calcination method to prepare Au/g-C₃N₄. In fact, the produced nitrogen vacancies in the structure of g-C₃N₄ essentially enhance the interaction at Au/g-C₃N₄ interface and the plasmonic properties of Au nanoparticles. These outstanding features contribute to enhance the utilization of solar light and electron-hole separation that prompt the photocatalytic performance towards hydrogen production. Finally, we have employed a novel poly-alkali route to prepare a strong visible light absorption photocatalyst-based g-C₃N₄. The co-existence of PEI and KOH, which induces numerous nitrogen vacancies and incorporated hydroxyl groups in the structure of the resulted material, has been explored for the first time. These modifications have been proved to narrow the bandgap and facilitate the charge separation leading to enhance the solar light-driven hydrogen production. This method also opens up a new approach to prepare efficient nanocomposite photocatalysts possessing both strong light absorption and good charge separation.

Carbon nitrides are a unique family of nitrogen-rich carbon materials with multiple beneficial properties for effective alkali metal ion transport/storage. Graphitic carbon nitride (g-C3N4) is considered the most viable member of the carbon nitride family because of its high nitrogen content, wide structure with several nitrogen-defect pore sites, ease of synthesis, affordability, and scalability. Also, g-C3N4 delivers a lithium ion battery (LIBs) theoretical capacity of 524 mAh/g unlike graphite which records only 327 mAh/g. However, due to the ineffective intercalation/deintercalation reaction of Li+ with C3N4 it suffers low capacity, poor conductivity and structural deformation when applied as an anode material for battery application. Due to this problem, the application of g-C3N4 for LIBs has slowed down, and the prospects of g-C3N4 for emerging battery systems such as potassium ion batteries (KIBs) have not been explored. In this thesis, we present unique strategies to resolve the problems of irreversible Li+ intercalation, poor conductivity, and structural destruction, and explore g-C3N4-based composites for KIB system. In the first study, one-dimensional carbon nitride nanofibers were designed and proved to be a more effective and better performing anode material for LIBs than bulk g-C3N4. This work was accomplished by combining theoretical computing and experimental techniques, Density functional theory calculation showed that the edges of the 1D-g- C3N4 nanofibers exhibited a suitable Li adsorption energy for stress-free adsorption and desorption of adsorbed Li-atoms. Moreover, our synthesized 1D-g-C3N4 nanofiber possessed edges and pores, as well as higher pyridinic nitrogen content unlike the bulk g-C3N4. The 1D-g-C3N4 nanofiber delivered a superior specific capacity of 181.7 mAh/g, a specific capacity of 138.6 mAh/g after 5000 cycles when cycled at 10C along with excellent stability and power density. This performance remains the highest amongst reported C3N4 anode materials in literature. Carbon nitride/graphene (C3N4/graphene) heterostructure is commonly reported for lithium ion batteries and this heterostructure design occurs in different configurations of 1D/2D or 2D/2D. However, a clear theoretical understanding of how the configuration of such heterostructure affects battery performance is not established. By using a first principle theory approach we studied the 1D/2D and 2D/2D C3N4/graphene heterostructures with a focus on their conductivity, charge transfer, bond structure and rearrangement/breakage and theoretical reversible capacity. In all our study, the DFT results showed that 1D/2D C3N4/graphene delivers superior charge transfer, electronic conductivity, theoretical capacity, and structural integrity compared to 2D/2D configuration. This work expanded upon the relationship between the heterostructure configuration and the electrochemical performance, this work will encourage the design of effective heterostructures for rechargeable batteries. Motivated by the result of the 1D/2D C3N4/graphene heterostructure for LIBs, we employed it for potassium ion battery application. When the fabricated 1D-g-C3N4 nanofiber was employed in potassium ion batteries, the high nitrogen content facilitated K+ adsorption; however, the K-atom diffusion barrier was too high for effective adsorption/desorption. Therefore, we combined the 1D-g-C3N4 nanofiber with 2D reduced graphene oxide (rGO) to design a 1D/2D C3N4/rGO composite for stable and effective potassium storage. In this work, we also combined the use of Density Functional Theory calculations and experimental battery testing along with high powered characterization techniques to study the storage mechanism of the composite electrode material for potassium ion battery. The 1D/2D composite benefitted from the larger surface area and conductivity of 2D reduced graphene oxide and the nitrogen rich active sites of the 1D-g-C3N4 nanofiber. Additionally, DFT calculations showed that the graphene structure from 2D rGO possessed lower K-atom diffusion barrier and superior conductivity which provided shorter ionic transport distances and boosted electronic conductivity in the composite. Thanks to the synergistic interaction between the 1D-g-C3N4 nanofiber and 2D rGO, the electrode delivered a remarkable specific capacity of 464.9 mAh/g after 200 cycles at 1 A/g and 228.6 mAh/g after 1000 cycles at 10 A/g, which is one of the best potassium ion battery anode material performance reported so far. Another approach to exploring the benefits of the 1D-g-C3N4 nanofiber is to use it as a source of N-doped carbon. Metal oxides such as cobalt oxide (Co3O4) have been widely applied as anode materials in rechargeable LIBs but the small d-spacing limits their application for large-sized metal ion batteries such as potassium ion batteries. Moreover, through DFT calculations we proved that the poor performance of Co3O4 for KIBs is due to poor conductivity, high diffusion barrier, and weak potassium interaction. Thanks to the concept of interfacial engineering, we fabricated a hierarchical composite of Co3O4@N-doped carbon in which the N-doped carbon is derived from 1D-g-C3N4. The material design approach for the composite involved coating the surface of Co3O4 with N-doped carbon such K+ can be effectively transported through the that at the interface both materials via multiple ionic pathways. Furthermore, the structural design of the composite enabled increased Co3O4 spacing for effective K+ diffusion, improved conductivity, and protection of the core structure from damage. Based on the entire composite, a capacity of 448.7 mAh/g was delivered after 40 cycles, and 213 mAh/g was retained after 740 cycles when cycled at 500 mA/g. This work combined the principle of material boundary engineering with theoretical computation to design a composite anode material whose performance exceeded that of most metal-oxide-based KIB anodes reported in literature. In summary, the strategies presented in this thesis show that the morphology and electronic properties of g-C3N4 can be manipulated to resolve the problems of irreversible intercalation/deintercalation, poor conductivity, and structural deformation. Moreover, the application of g-C3N4 has been extended to potassium ion batteries and we are the first research group to demonstrate this in literature. Also, the electrochemical performances recorded from experimental battery testing and theoretical computation (DFT simulation) shows that g-C3N4 and g-C3N4-based materials are promising advanced anode materials for LIBs and KIBs. These strategies can be extended to other members of the carbon nitride family such as CN, C2N, C3N etc. for different metal-ion batteries.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
School of Environment and Sc
Science, Environment, Engineering and Technology
Full Text

Le nitrure de carbone graphitique (gCN) est un semi-conducteur organique ayant dernièrement attiré l'attention par sa capacité à photocatalyser la séparation de l'eau. Il a récemment été montré que le gCN était un polymère basé sur le cycle heptazine C6N7, mais son arrangement tridimensionnel reste encore très peu connu. En effet, sa faible solubilité empêche l'utilisation des techniques de caractérisation classiques, et le terme gCN recouvre en réalité une large gamme de composés différents, selon les conditions de synthèse utilisées (choix du précurseur, température…). L'obtention de modèles moléculaires, de structures maîtrisées et bien définies, serait donc d'une grande aide dans la compréhension du lien structure-propriétés. Ceci est le but des travaux présentés dans ce manuscrit. La réactivité du chlorure de cyaméluryle, un précurseur monomérique, a été étudiée, et un protocole de substitution sélective quantitative par les amines secondaire aliphatique a été déterminé. L'utilisation de synthèses par déprotonation ou par activation thermique ont permis l'obtention de deux dimères et d'un trimère linéaire solubles. Les oligomères synthétisés ont été caractérisés par de nombreuses techniques (diffraction des rayons X, RMN, IR, absorption UV-vis, fluorescence, électrochimie), et les valeurs obtenues ont été corroborées à celle obtenues par DFT. De façon générale, une diminution des énergies des transitions électronique est observée quand la taille de chaîne augmente, et l'application de méthodes d'extrapolation suggère que les oligomères linéaires sont des bon modèle moléculaire du gCN
Graphitic carbon nitride (gCN) is an organic semi-conductor which has lately attracted a lot of attention when its photocatalytic properties were highlighted for water splitting. It has been recently shown to be based on the heptazine core, but its three-dimensional structure remains elusive. This is first due to its poor solubility which prevents the use of classical characterization techniques, and second to the fact that changes in synthesis experimental conditions (precursors, temperature…) yield different materials. The synthesis of tailored and well-defined molecular models would therefore certainly be of great interest to better understand the structure-properties relationship of this material. This is the aim of the work presented in this manuscript. The reactivity of cyameluryl chloride, a monomeric precursor, has been studied, and a protocol for a quantitative selective substitution by aliphatic secondary amines has been determined. The use of deprotonation by a strong base or thermal treatment yielded two dimers and one linear trimer. The oligomers have been characterized by several technique (X-ray diffraction, NMR, IR, UV-vis absorption, emission, electrochemistry), and the obtained data were in close agreement to the ones observed in DFT. As a rule of thumb, a decrease of the electronic transition energies is observed for an increasing chain length. The application of extrapolation methods to the experimental data suggests that oligomers are relevant molecular models for gCN

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.

COORDENAÇÃO DE APERFEIÇOAMENTO DO PESSOAL DE ENSINO SUPERIOR
O nitreto de gálio (GaN) é um dos mais interessantes e promissores materiais para aplicação em dispositivos óptico- eletrônicos. GaN pode ser usado para a fabricação de diodos e lasers azuis. O desenvolvimento deste tipo de material está relacionado com três campos principais: 1) deposição de camadas de GaN cristalino; 2) produção de nano- filamentos a partir de reações confinadas no interior de nanotubos de carbono; 3) síntese de GaN em pó por diferentes métodos químicos. Recentemente, novas técnicas de deposição adotaram a sublimação de pós de GaN como fonte de gálio para a produção de nanofilamentos de GaN, filmes finos ou cristais. Estes métodos de sublimação mostram a necessidade do emprego de pós de GaN. No presente trabalho, é apresentada uma nova rota para a produção de pós de GaN a partir da reação gás-sólido entre Ga2O3 e NH3(g) utilizando o carbono como agente redutor no interior de um novo tipo de reator, disposto verticalmente. A partir desta rota obteve-se pós de GaN com conversões aproximadamente de 100% e com estrutura cristalina hexagonal. A quantidade de GaN obtida variou de acordo com os parâmetros experimentais adotados. Através de uma análise estatística foi possível determinar a influência da temperatura, razão molar de carbono/Ga2O3 e do tempo experimental sobre a taxa de produção de GaN.
It is well known that gallium nitride (GaN) is one of the most interesting and promising materials for optoelectronic devices. GaN can be used for manufacturing blue light- emitting diodes and lasers. Development of this material is concerned with three main areas 1) deposition of GaN crystalline layers onto different substrates; 2) manufacturing of GaN nanorods from chemical reactions in the confined spaces provided by carbon nanotubes; 3) synthesis of GaN powders by different chemical methods. Recently, new deposition techniques have adopted sublimation of GaN powders as gallium source to produce GaN nanorods, thin films or bulk crystals. These sublimation methods rely on the supply of GaN powders. This thesis presents a new route to produce GaN powder from gas-solid chemical reaction between Ga2O3 and NH3 using carbon as reducing agent in a new reactor design. The GaN powder obtained from this route possesses a hexagonal crystal structure and was found to correspond to almost 100% conversion of Ga2O3. The amount of GaN present in the powders varied with experimental parameters. A statistical analysis showed the influence of temperature, carbon/Ga2O3 ratio and experimental time on the production of GaN powder.

In the first section of the thesis graphitic carbon nitride was for the first time synthesised using the high-temperature condensation of dicyandiamide (DCDA) – a simple molecular precursor – in a eutectic salt melt of lithium chloride and potassium chloride. The extent of condensation, namely next to complete conversion of all reactive end groups, was verified by elemental microanalysis and vibrational spectroscopy. TEM- and SEM-measurements gave detailed insight into the well-defined morphology of these organic crystals, which are not based on 0D or 1D constituents like known molecular or short-chain polymeric crystals but on the packing motif of extended 2D frameworks. The proposed crystal structure of this g-C3N4 species was derived in analogy to graphite by means of extensive powder XRD studies, indexing and refinement. It is based on sheets of hexagonally arranged s-heptazine (C6N7) units that are held together by covalent bonds between C and N atoms. These sheets stack in a graphitic, staggered fashion adopting an AB-motif, as corroborated by powder X-ray diffractometry and high-resolution transmission electron microscopy. This study was contrasted with one of many popular – yet unsuccessful – approaches in the last 30 years of scientific literature to perform the condensation of an extended carbon nitride species through synthesis in the bulk. The second section expands the repertoire of available salt melts introducing the lithium bromide and potassium bromide eutectic as an excellent medium to obtain a new phase of graphitic carbon nitride. The combination of SEM, TEM, PXRD and electron diffraction reveals that the new graphitic carbon nitride phase stacks in an ABA’ motif forming unprecedentedly large crystals. This section seizes the notion of the preceding chapter, that condensation in a eutectic salt melt is the key to obtain a high degree of conversion mainly through a solvatory effect. At the close of this chapter ionothermal synthesis is seen established as a powerful tool to overcome the inherent kinetic problems of solid state reactions such as incomplete polymerisation and condensation in the bulk especially when the temperature requirement of the reaction in question falls into the proverbial “no man’s land” of classical solvents, i.e. above 250 to 300 °C. The following section puts the claim to the test, that the crystalline carbon nitrides obtained from a salt melt are indeed graphitic. A typical property of graphite – namely the accessibility of its interplanar space for guest molecules – is transferred to the graphitic carbon nitride system. Metallic potassium and graphitic carbon nitride are converted to give the potassium intercalation compound, K(C6N8)3 designated according to its stoichiometry and proposed crystal structure. Reaction of the intercalate with aqueous solvents triggers the exfoliation of the graphitic carbon nitride material and – for the first time – enables the access of singular (or multiple) carbon nitride sheets analogous to graphene as seen in the formation of sheets, bundles and scrolls of carbon nitride in TEM imaging. The thus exfoliated sheets form a stable, strongly fluorescent solution in aqueous media, which shows no sign in UV/Vis spectroscopy that the aromaticity of individual sheets was subject to degradation. The final section expands on the mechanism underlying the formation of graphitic carbon nitride by literally expanding the distance between the covalently linked heptazine units which constitute these materials. A close examination of all proposed reaction mechanisms to-date in the light of exhaustive DSC/MS experiments highlights the possibility that the heptazine unit can be formed from smaller molecules, even if some of the designated leaving groups (such as ammonia) are substituted by an element, R, which later on remains linked to the nascent heptazine. Furthermore, it is suggested that the key functional groups in the process are the triazine- (Tz) and the carbonitrile- (CN) group. On the basis of these assumptions, molecular precursors are tailored which encompass all necessary functional groups to form a central heptazine unit of threefold, planar symmetry and then still retain outward functionalities for self-propagated condensation in all three directions. Two model systems based on a para-aryl (ArCNTz) and para-biphenyl (BiPhCNTz) precursors are devised via a facile synthetic procedure and then condensed in an ionothermal process to yield the heptazine based frameworks, HBF-1 and HBF-2. Due to the structural motifs of their molecular precursors, individual sheets of HBF-1 and HBF-2 span cavities of 14.2 Å and 23.0 Å respectively which makes both materials attractive as potential organic zeolites. Crystallographic analysis confirms the formation of ABA’ layered, graphitic systems, and the extent of condensation is confirmed as next-to-perfect by elemental analysis and vibrational spectroscopy.
Die vorliegende Arbeit befasst sich mit der Synthese und Charakterisierung neuer Allotropen und Nanostrukturen von Karbonitriden und berührt einige ihrer möglichen Anwendungen. Alle gezeigten, ausgedehnten, kovalent verbundenen Karbonitridgerüste wurden in einem ionothermalen Syntheseprozess – einer Hochtemperaturbehandlung in einem eutektischen Salzgemisch als ungewöhnlichem Lösungsmittel – aus einfachen Präkursormolkülen erzeugt. Der Kondensationsmechanismus folgt einer temperaturinduzierten Deaminierung und Bildung einer ausgedehnten, aromatischen Einheit; des dreifach substituierten Heptazines. Die Dissertation folgt vier übergreifenden Themen, beginnend mit der Einleitung in Karbonitridsysteme und der Suche nach einem Material, welches einzig aus Kohlenstoff und Stickstoff aufgebaut ist – einer Suche, die 1834 mit den Beobachtungen Justus von Liebigs „über einige Stickstoffverbindungen“ begann. Der erste Abschnitt zeigt die erfolgreiche Synthese von graphitischem Karbonitrid (g-C3N4); einer Spezies, welche auf Schichten hexagonal angeordneter s-Heptazineinheiten beruht, die durch kovalente Bindungen zwischen C- und N-Atomen zusammengehalten werden, und welche in einer graphitischen, verschobenen Art und Weise gestapelt sind. Der zweite Abschnitt berührt die Vielfalt von Salzschmelzensystemen, die für die Ionothermalsynthese geeignet sind und zeigt auf, dass die bloße Veränderung der Salzschmelze eine andere Kristallphase des graphitischen Karbonitrides ergibt – das g-C3N4-mod2. Im dritten Abschnitt wird vom Graphit bekannte Interkallationschemie auf das g-C3N4 angewendet, um eine Kalliuminterkallationsverbindung des graphitischen Karbonitirdes zu erhalten (K(C6N8)3). Diese Verbindung kann in Analogie zum graphitischen System leicht exfoliiert werden, um Bündel von Karbonitridnanoschichten zu erhalten, und weist darüberhinaus interessante optische Eigenschaften auf. Der vierte und letzte Abschnitt handelt von der Einführung von Aryl- und Biphenylbrücken in das Karbonitridmaterial durch rationale Synthese der Präkursormoleküle. Diese ergeben die heptazinbasierten Frameworks, HBF-1 und HBF-2 – zwei kovalente, organische Gerüste.

碩士
國立臺北科技大學
機電整合研究所
89
This study investigates the feasibility of DLC and C-N film as the FED candidate materials. Thin films deposited by R. F. Sputtering method, home-made nano-diamonds target, and inlet the C-N reaction gases. Results show that the film structure appeared column type, accompanied the vacuum pressure increasing the column structure deteriorating, and the film became flat, smooth, and dense finally. The DLC thin film of column structure conducted by hydrogen plasma etching, the column became thin, and the threshold electric field also became lower. Besides, the result showed that the threshold electric field decreased in contained nitrogen carbon film and the carbon nitride film, which suggested that the sub-energy band formed when nitrogen doped into films. The lowest threshold electric field 12.81 V/μm, the biggest current density 1.35 mA/cm2 in this study were obtained when the films were growing above the silicon pyramid array and Au as the interlayer. In conclusion, the results suggested that the field emission characteristics are boosted when adding the metal interlayer, inletting the nitrogen gas and performing the silicon pyramid array.

Colloidal processing methods were developed in order to disperse highly concentrated 1.0, 2.0, and 6.0 vol% single-walled carbon nantoube (SWNT)-Si 3N4 aqueous composite suspensions. Interparticle pair potentials were developed between individual Si3N4 particles and SWNT bundles by coating them with cationic surfactant molecules of cetyltrimethylammonium bromide (CTAB). Zeta potential, viscosity, and sedimentation measurements were conducted on SWNTs and Si3N4 particle suspensions in order to optimize the pH and amount of adsorbed CTAB. The composite suspension viscosity was pH sensitive and adjusted accordingly before consolidation into three-dimensional solid parts using a rapid prototyping fabrication method called robocasting. High-density composites were produced using spark plasma sintering and structurally intact SWNTs were directly observed in the final sintered microstructure using scanning electron microscopy and Raman spectroscopy. When processed with SWNTs the highly insulative ceramic became electrically conductive and resulted in increased grindability for the otherwise hard to machine ceramic. The high hardness, fracture toughness and density of Si 3N4 was maintained for the composite due to the detailed development of colloidal processing and sintering methods used during fabrication. In addition, the thermal conductivity of the ceramic was reduced with the incorporation of well-dispersed SWNTs. Indentation load studies on the composites revealed sub-surface chipping and deformation around the indent before radial crack development indicating a degree of damage tolerance over the monolith. Along the wake of the crack SWNTs were also observed bridging the crack therefore showing their potential to act as toughening agents in brittle ceramics.

碩士
國立交通大學
應用化學系所
93
The thesis are divided into two parts. In the first part, we used 2,4,6-trichloro-1,3,5-triazine as the precursor to react with sodium which have been achieved by decomposing sodium hydrate at 623 K. At a low reaction temperature of 623 K, carbon nitride nanosphere、porous carbon nitride material and byproduct sodium chloride, would be produced. In the second part, we used porous anodic alumina oxide as the template. At 623 K, 2,4,6-trichloro-1,3,5-triazine reacted with the reactive template, which formed from thermally decomposing NaH on AAO, generated carbon nitride nanotube and byproduct sodium chloride inside the template channels. Well-ordered carbon nitride nanotube bundles with a high content of nitrogen were isolated after the sodium chloride and template were removed by deionized water and 48% HF, respectively. The diameter of the carbon nitride nanotubes was 300 nm, the length was 60 μm, and the wall thickness was 50 nm.

碩士
國立中興大學
環境工程學系所
106
A visible-light-driven graphitic carbon nitride/reduced graphene oxide/α-Sulfur composite (CNRGOS8) was synthesized as efficient photocatalysts for environmental applications. The photocatalytic reactivity of the fabricated CNRGOS8 was determined by the degradation of Rhodamine B (RhB) and tetracycline (TC). The effects of pH, mixed ratio of catalysts, dosage of photocatalyst were optimized, and the reaction kinetics and reaction pathway were studied. The results indicated the optimized pH values for RhB and TC degradation was 3 and 7, respectively. In addition, no deterioration of the efficiency was found for CNRGOS8 (70:5:25) after 5 cycles of operation. Such a result was indicative of a prolonged lifetime of the CNRGOS8 (70:5:25). With probing by the scavengers, 2-Propanol (·OH capture reagent), benzoquinone (O2•－ capture reagent) and sodium oxalate (h+ capture reagent), the major reactive species were identified as superoxide radicals and hydroxyl radicals. The abundant natural organic matter such as humic acid (HA) oftentimes coexists with the pollutants in the aquatic environment, which may affect the efficiency and alter reaction pathways of photodegradation of the pollutants. An enhanced electron transfer and reactive oxide species production were found for CNRGOS8 (70:5:25) in the presence of humic acid. However, the overall removal efficiency of the pollutants was suppressed due to the competition between CNRGOS8 (70:5:25) and the coexisting HA for the active sites.

碩士
國立交通大學
應用化學系
91
The reaction between hexachlorocyclopentadiene (C5Cl6) and Na, which have been achieved by decomposing NaH thermally, can afford diversified morphologies of nano-sized carbon at 623 K, including urchin-like spheres, hollow spheres with cubic core as well as tubulate structures. The formation of cubic-cored hollow spheres and tubulate structure might be caused by NaCl by-product. The formation of diversified morphologies may be attributed to heterogeneous contact of the two reactants. Besides, we successfully developed a new strategy for formation of an reactive template by thermally decomposing NaH on AAO, which can react with carbonhalides to produce carbon tubes. First of all, reacting the reactive template with C5Cl6 at 623 K generates carbon nanotubes inside the template channels. Well-ordered carbon nanotube bundles ( diameter of 300 nm, length of 60 μm, wall thickness of 20 nm ) were isolated after the template was removed by 48％ HF. Lowering the reaction temperate to 473 K led to the formation of carbon nanotubes with porous wall. The pore size distribution data, caculated from nitrogen desorption branch of isotherms by BJH ( Barret-Joyner-Halenda ) method, showed that the mesoporous walls were uniform with narrow pore size distribution centered at 4 nm. Moreover, well-ordered CNx nanotubes with a high content of nitrogen ( x ∼17％ ) were carried out by reacting pentachloropyridine ( C5NCl5 ) with the reactive template at 623 K. The diameter of the CNx nanotubes was 300 nm, the length was 60μm, and the wall thickness was 20 nm.

碩士
逢甲大學
化學工程學系
106
In this study, molybdenum oxides with various oxidation states were decorated on two-dimensional graphitic carbon nitride (gCN) to enhance the photocatlytic activity of CO2 reduction under visible light irradiation and to correlate the characteristics of photocatalysts to the efficiency of CO2 conversion. Part 1: Preparation and Properties of Molybdenum Trioxide / Graphitic Carbon Nitride Composite for Photocatalysis Molybdemum trioxide/graphitic carbon nitride was prepared by calcination and hydrothermal method. Ammonium molybdate tetrahydrate was the precursor of molybdenum and melamine was for gCN. Molybdenum trioxide (MoO3) was obtained by hydrothermal method (210 °C) and graphitic carbon nitride was by calcination method (450 °C) at Air or Ar. XRD result confirmed the crystal phase of MoO3 and characteristic peaks of gCN. SEM images confirmed the morphology of MoO3 and gCN. TEM images presented the distribution of MoO3. A significant red shift, compared to pure MoO3, revealed from UV-VIS spectra of samples with the presence of gCN. After calcination under Ar, some MoO3 reduced to MoO2 as evidenced from ESCA results of MoO3-gCN-Ar and thus increased photocatalytic activity. Photocatalytic reduction of CO2 showed MoO3-gCN-Ar (8 W, 254 nm) could successfully convert CO2 into CO, and the yield of CO was 0.067 mol/gcat. Part 2: Preparation and Properties of Molybdenum Oxide Quantum Dots / Graphitic Carbon Nitride Composite for Photocatalysis In order to further improve the photocatalytic activity of CO2, molybdenum oxide quantum dots/graphitic carbon nitride composites (MoOx-QDs-gCN) were prepared in the second part of this study. Molybdenum oxide quantum dots were prepared by hydrothermal method (80 °C) using molybdenum powder as the precursor. The melamine was used as a precursor to prepare graphitic carbon nitride (gCN) by calcining at 500 °C and 550 °C under air atmosphere. The obtained molybdenum oxide quantum dots were mixed with gCN and then calcined at 300 °C. XRD, ESCA and EDX results confirmed that the catalyst contained molybdenum oxide with various oxidation states on gCN. TEM images showed after calcination graphitic carbon nitride still mentain their characteristic structure. A significant red shift of the absorption edge of MoOx-QDs-gCN, compared to gCN, was observed from UV-VIS spectra. Carbon dioxide photocatalytic reduction results showed MoOx-0.3gCN (8 W, 254 nm) had the best conversion yield of CO and the yield of CO was 0.418 mol/gcat. Part 3：In-Situ Preparation and Properties of Molybdenum Oxide / Graphitic Carbon Nitride Composite for Photocatalysis In the third part of this study, in-situ preparation of molybdenum oxide (MoOx) on gCN was attemped to improve the interaction of gCN and MoOx. Thermal condensation method was applied to fabricate graphitic carbon nitride(gCN). Different amounts of molybdenum disulfide (MoS2) were dissolved with hydrogen peroxide solution, followed by the addition of gCN. Strong oxidation of hydrogen peroxide with molybdenum disulfide was ulilized to replace the sulfur atoms of molybdenum disulfide to oxygen atoms. The presence of nitrogen active sites on gCN surface has electronic affinity with molybdenum ions. After the deposition of molybdenum oxide particals on gCN, the remained sulfur ions were removed by neutralization by alkali. After centrifugated and washed, we could obtain the composite photocatalysts and named XMS-0.1CN, where X indicates the volume of MoS2. The results of XPS-Mo3d confirmed that the photocatalyst 30MS-0.1CN had the highest ratio of Mo4+ (compared with the other photocatalyst). The appearance of Mo4+ could improve the charge transport capacity and promote photocatalytic activity. The UV-Vis and Tauc Plot analysis results were shown that the in-situ synthesized photocatalyst had a red shift of the absorption edge compared with pure gCN and the band-gap was narrowed by 0.1 to 0.2 eV. The results of PL spactrum analysis showed that the photoluminescence intensity of in-situ prepared photocatalyst was lower than that from that emitted from pure gCN. This result indicated that molybdenum oxide after in-situ growth could effectively reduce the recombination efficiency of photogenerated electron-holes. In the photocatalytic reduction of carbon dioxide, carbon monoxide was the only successful conversion product. 30MS-0.1CN had the highest conversion yield, 3.937 mol/gcat, as the highest Mo4+ ratio was estimated by ESCA. This study showed that the in-situ growth of the photocatalyst was indeed effective at improving photocatalytic activity compared to pure gCN.