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

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

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

abstract: Photocatalytic water splitting over suspended nanoparticles represents a potential solution for achieving CO2-neutral energy generation and storage. To design efficient photocatalysts, a fundamental understanding of the material’s structure, electronic properties, defects, and how these are controlled via synthesis is essential. Both bulk and nanoscale materials characterization, in addition to various performance metrics, can be combined to elucidate functionality at multiple length scales. In this work, two promising visible light harvesting systems are studied in detail: Pt-functionalized graphitic carbon nitrides (g-CNxHys) and TiO2-supported CeO2-x composites. Electron energy-loss spectroscopy (EELS) is used to sense variations in the local concentration of amine moieties (defects believed to facilitate interfacial charge transfer) at the surface of a g-CNxHy flake. Using an aloof-beam configuration, spatial resolution is maximized while minimizing damage thus providing nanoscale vibrational fingerprints similar to infrared absorption spectra. Structural disorder in g-CNxHys is further studied using transmission electron microscopy at low electron fluence rates. In-plane structural fluctuations revealed variations in the local azimuthal orientation of the heptazine building blocks, allowing planar domain sizes to be related to the average polymer chain length. Furthermore, competing factors regulating photocatalytic performance in a series of Pt/g-CNxHys is elucidated. Increased polymer condensation in the g-CNxHy support enhances the rate of charge transfer to reactants owing to higher electronic mobility. However, active site densities are over 3x lower on the most condensed g-CNxHy which ultimately limits its H2 evolution rate (HER). Based on these findings, strategies to improve the cocatalyst configuration on intrinsically active supports are given. In TiO2/CeO2-x photocatalysts, the effect of the support particle size on the bulk/nanoscale properties and photocatalytic performance is investigated. Small anatase supports facilitate highly dispersed CeO2-x species, leading to increased visible light absorption and HERs resulting from a higher density of mixed metal oxide (MMO) interfaces with Ce3+ species. Using monochromated EELS, bandgap states associated with MMO interfaces are detected, revealing electronic transitions from 0.5 eV up to the bulk bandgap onset of anatase. Overall, the electron microscopy/spectroscopy techniques developed and applied herein sheds light onto the relevant defects and limiting processes operating within these photocatalyst systems thus suggesting rational design strategies. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2019

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. / <p>QC 20150909</p>

Graphitic carbon nitride (g-C3N4) heterojunction composites with the semiconducting metal oxides, CeO2, ZnO and TiO2 are prepared in situ by co-calcination of the precursor materials or by a solvothermal method. The structural, morphological and the optical properties of the prepared materials are studied using various microscopy and spectroscopy techniques. The synthesized composite materials, CeO2/g-C3N4, ZnO/g-C3N4 and TiO2/g-C3N4 are more efficient in the photocatalytic degradation of the water pollutants indigo carmine (IC) and atrazine than the pure metal oxide, g-C3N4, or their physical mixtures. The CeO2/g-C3N4 and ZnO/g-C3N4 composites also exhibit improved degradation efficiencies of atrazine as compared to the individual metal oxide or g-C3N4 materials. The improved photocatalytic activity of the composites are attributed to the effective electron-hole charge separation within composite heterojunction, resulting from the well matched energy levels of the metal oxide and g-C3N4. This strategy could be helpful for the synthesis of other metal oxide and g-C3N4 composites for photocatalytic applications.

Pure g-C3N4 and MoS2 modified g-C3N4 materials were synthesized using a facile heating method and a low-temperature hydrothermal method, respectively. The obtained samples were characterized by XRD pattern and N2 adsorption-desorption technique at 77K. The adsorption and photocatalytic performance of all obtained samples were investigated by discoloration of direct black 38 dye in the dark and under visible light irradiation. The results showed that all obtained samples exhibited good discoloration efficiency of direct black 38 dye. The two factors including pH values and Mo loading effected mainly on elimination efficiency of direct black 38 dye. MoS2 modified g-C3N4 materials possessed the more enhanced adsorption and photocatalytic performance in comparison to pure g-C3N4 at pH value of 3.5, with adsorbent dosage of 0.1 g/L. Furthermore, it was found that the adsorption process and photo-catalysis simultaneously occurred under visible light irradiation and followed up a pseudo-second-order kinetic reaction of Langmuir - Hinshelwood model. / g-C3N4 và g-C3N4 biến tính bởi MoS2 đã được tổng hợp theo phương pháp nung đơn giản và phương pháp thủy nhiệt ở nhiệt độ thấp tương ứng. Các mẫu tổng hợp đã được đánh giá đặc trưng bởi các phương pháp hiện đại như giản đồ nhiễu xạ tia X, phương pháp hấp phụ-khử hấp phụ N2 ở 77K. Khả năng hấp phụ và quang hóa xúc tác của các vật liệu tổng hợp đã được nghiên cứu bởi quá trình phân hủy màu thuốc nhuộm direct black 38 trong điều kiện bóng tối và chiếu sáng bởi ảnh sáng nhìn thấy của đèn chiếu sáng sợi đốt wolfram (220V-100W). Các kết quả nghiên cứu chỉ ra rằng các mẫu tổng hợp đều có hiệu suất xử lý màu cao đối với thuốc nhuộm direct black 38. Hai yếu tố gồm pH dung dịch và hàm lượng MoS2 ảnh hưởng chính đến hiệu suất xử lý màu direct black 38. g-C3N4 biến tính bởi MoS2 luôn thể hiện hiệu suất hấp phụ và quang hóa cao hơn so với g-C3N4 tinh khiết. Hơn nữa, khi được chiếu sáng bởi ánh sáng nhìn thấy thì quá trình hấp phụ và quá trình quang hóa thuốc nhuộm direct black 38 trên các vật liệu tổng hợp đã xảy ra đồng thời và mô hình Langmuir - Hinshelwood động học bậc 2 đã được đề xuất cho quá trình này.

The path towards mitigation of anthropogenic greenhouse gas emissions lies in the transition from conventional to sustainable energy resources. The Hydrogen Economy, a cyclic economy based on hydrogen as a fuel, is suggested as a tool in the necessary energy transition. Photocatalysis makes use of sunlight to promote thermodynamically non-favoured reactions such as water splitting, allowing for sustainable hydrogen production. Harvesting thermal energy along with photonic energy is an interesting concept to decrease the activation energy of water splitting (i.e. ΔG = + 237.2 kJ∙mol−1). This work aims to confront this hypothesis in a gas phase photo-thermal reactor designed specifically for this study. The photocatalyst chosen is graphitic carbon nitride (g-C3N4), an organic semiconductor possessing a narrow band gap (i.e. 2.7 eV) as well as a band structure which theoretically permits water splitting. The photocatalytic performance of Pt/g-C3N4 for hydrogen evolution was tuned by altering its synthetic temperature. Electron paramagnetic resonance was used to gain insight on the evolution of the photocatalyst activity with synthesis temperature. Then, gold nanoparticles were deposited on g-C3N4 surface. Localized surface plasmon resonance properties of gold nanoparticles are reported in the literature to be influenced by temperature. Therefore Au/g-C3N4 appeared as a promising candidate for photo-thermal water splitting. X-ray spectroscopy unveiled interesting observations on the gold oxidation state. Moreover, under specific reduction conditions, gold nanoparticles with a wide variety of shapes characterized by sharp edges were formed. Finally, the development of the photo-thermal reactor is presented. The design process and the implementation of this innovative reactor are discussed. The reactor was successfully utilized to probe photoreactions. Then, the highly energy-demanding photocatalytic water splitting was proven not to be activated by temperature in the photo-thermal apparatus.

M. Tech. (Department of Chemistry, Faculty of Applied and Computer Sciences), Vaal University of Technology. / The presence of organic and inorganic pollutants in aqueous environments is one of the major challenges confronting man. It is therefore important to develop sensitive, versatile and cheap techniques for their detection. Arsenic (III), 2-chlorophenol (2-CP) and sulfamethoxazole (SMX) are priority pollutants that pose health threats to humans and animals. This study was thus aimed at exploring two promising carbon nanomaterials as electrode modifiers for the electrochemical sensing of arsenic (III), 2-CP and SMX in water. Glassy carbon electrode (GCE) was modified with a nanocomposite of carbon dots (CDs) and graphitic carbon nitride (g-C3N4) and used as a sensor for the analytes in aqueous media. The CDs was prepared by a facile one-pot hydrothermal method using pine cone as the carbon source; g-C3N4 and g-C3N4/CDs nanocomposite were prepared via the microwave irradiation heating method. CDs, g-C3N4 and g-C3N4/CDs were dropped-dried on the surface of bare GCE. Transmission electron microscopy (TEM), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) were used to characterize the prepared materials. GCE, g-C3N4/GCE, CDs/GCE and g-C3N4/CDs/GCE electrodes were electrochemically investigated by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) using a ferrocyanide [Fe (CN) 6]3-/4- redox probe. The current and the reversibility of the redox probes were enhanced in the presence of modifiers. The electrochemical behavior of arsenic (III), 2-CP and SMX on different electrodes (GCE, CDs/GCE, g-C3N4/GCE and g-C3N4/CDs/GCE) were investigated by differential pulse voltammetry (DPV) under optimized conditions in a phosphate buffer solution (pH 7.6, 6 and 5 for 2-CP, As (III) and SMX respectively). The results demonstrated that the g-C3N4/CDs/GCE electrode significantly enhanced the oxidation peak current of all three analytes. The detection sensitivity of the analytes was greatly improved, suggesting that this new modified electrode has great potential in the determination of trace level of arsenic (III), 2-CP and SMX in water. The oxidation peak currents displayed a linear relationship to concentrations for 2-CP (0.5 - 2.5 μM, R2=0.958, n=5), arsenic (III) (2 - 10 μM R2=0.978, n=5) and SMX (0.3 - 1.3 μM R2=0.9906, n=5). The detection limits of 0.62 μM, 1.64 μM and 0.10 μM were obtained for 2-CP, arsenic and SMX, respectively. Phenol and 4-chloro-3-methyl-phenol were found to interfere with the detection of 2-CP, while, Cu2+, Zn2+, Pb2+ and Cd2+ were the only significant ions that interfered with the electrochemical detection of arsenic (III). EDTA was used as a ligand to mask the interference effects of copper, cadmium, lead and zinc on arsenic sensing. The modified electrode (g-C3N4/CDs/GCE) was used to determine arsenic, 2-CP and SMX in spiked tap and effluent water samples by the standard addition method and the results showed percentage recoveries varying from 93-118% for 2-CP, 98-100% for arsenic and 80-105% for SMX. The outcomes of this study established that the nanocomposite material represents an easy and sensitive sensing platform for the monitoring of arsenic (III), 2-CP and SMX in aqueous media.

В работе проведено исследование структурных, оптических и электронных свойств графитоподобного нитрида углерода (g-C3N4), синтезированных методом пиролиза мочевины при температурах 450–600°С. Метод функционала плотности использован для подтверждения выводов, основанных на экспериментальных данных. Был выполнен анализ литературных данных по g-C3N4, были рассмотрены структурные особенности получаемого материала и их оптическое поглощение. Также приведено описание используемых экспериментальных методов и математическое описание используемого метода моделирования – теории функционала плотности. Структура синтезированных образцов g-C3N4 определена по спектрам рентгеновской дифракции и ИК поглощения. Оптическое поглощение образцов исследованы по спектрам диффузного отражения, преобразованным в соответствии с теорией Кубелки-Мунка, оценка ширины энергетической щели для прямых и непрямых оптических переходов проводилась по методу Тауца. Была установлена линейная зависимость параметров кристаллической решетки и ширины щели от температуры синтеза. Для подтверждения экспериментальных результатов было проведено моделирование возможных структур g-C3N4 методом функционала плотности в пакете Quantum ESPRESSO. Была проведена оптимизация положений атомов для различных модельных структур, рассчитаны ИК спектры и зонные структуры электронных состояний. / In this work we investigated structural, optical and electrical properties of graphitic carbon nitride (g-C3N4) synthesized by thermal treatment of urea at 450– 600°C. Density functional theory calculations were performed to support suggestions based on experimental data. Available research data for g-C3N4 was studied, with interest in structural and absorption properties of the material. Additionally, the experimental methods and theory behind DFT-calculations were described. We determined the structure of g-C3N4 samples based on XRD and IR spectra. Optical absorption was investigated using diffuse reflectance spectra transformed with Kubelka-Munk function. Energy band gap was evaluated with Tauc plot for direct and indirect optical transitions. We show that crystal lattice parameters and band gap linearly depend on the synthesis temperature. To further prove our assumptions based on experimental data we also performed DFT-calculations for g-C3N4 in Quantum ESPRESSO software package. We calculated IR spectra and electronic band structures for models of each sample.