Ionic liquids (ILs) are nowadays a large and widely explored class of ionic compounds that melt below 100 °C. Due to their attractive properties, ILs are now of growing interests in a variety of inorganic materials preparation. However, most studies have put much focus on the description of new synthetic strategies. The chemical reactivity of ILs in the reactions is often neglected. In this dissertation, a series of metal chalcogenides were synthesized using the decompositions of ILs. The role or chemical reactivity of ILs in the reactions was demonstrated in detail. The hierarchical desert-rose-like SrTiO3 particles have been successfully prepared based on an ethylene glycol (EG) mediated one-pot IL-assisted solvothermal synthetic route. The used basic ionic liquid tetrabutylammonium hydroxide (TBAH) serves as an alkaline source and can also replace EG as the sole solvent to synthesize polyhedral SrTiO3, showing “all-in-one” solvent and reactant. A series of metal sulfides, such as Sb2S3, Bi2S3, PbS, CuS, Ag2S, ZnS, and CdS have been obtained from a choline chloride/thioacetamide based deep eutectic solvent (DES, an IL analog solvent) by a simple and general synthetic method. The reaction mainly proceeds in two steps: i) the dispersion of metal salts in the DES and the formation of a metal-DES complex, and ii) the decomposition of the metal-DES complex and formation of the final products. In addition, the chemical reactivity of phosphonium based ILs with selenium and tellurium at above 220 °C was systematically investigated by a series of dissolution experiments, tracking the solute selenium and tellurium species by nuclear magnetic resonance (NMR). NMR results clearly indicate some common decomposition mechanisms for quaternary phosphonium ILs at a relatively high temperature in the presence of selenium or tellurium. The decomposition of the quaternary phosphonium cations should proceed by an elimination of one alkyl substituent via an SN2 reaction, forming the respective trialkylphosphane selenides or tellurides in the presence of selenium or tellurium, which is then responsible for the genuine dissolution of selenium or tellurium. However, in the case of tellurium, the dissolution behavior is much more complicated compared to that of selenium. The coupling of P and Te which indicates a P–Te bond formation is only observed in the NMR spectra when a sufficient amount of tellurium (e.g. Te : IL = 1 : 1) is provided. The existence of a parallel-competitive IL decomposition route besides the SN2 reaction is regarded as the side reaction for the dissolution of tellurium. This may at least partially explain the relatively lower solubility of tellurium in phosphonium based ILs compared to that of selenium.

The work presented in this thesis reports the use of a series of novel thiobiuret metal complexes [M(SON(CNiPr2)2)n] (M = Cu, Ni, Fe, Zn, Cd or In; n = 2 or 3) for the first time as single source precursors for the colloidal synthesis of metal sulfide nanoparticles. Other single source precursor(s) were also used for the synthesis of CdSe, CdS, CdSe/CdS core/shell, CdSeS alloys and Cu2-xS nanoparticles in microfluidic reactors. Thermolysis experiments of [Cu(SON(CNiPr2)2)2] using only oleylamine produced Cu7S4 nanoparticles as a mixture of monoclinic and orthorhombic phases. Pure orthorhombic Cu7S4 nanoparticles were obtained when a solution of precursor in octadecene was injected into hot oleylamine whereas, Cu1.94S nanoparticles were obtained when a solution of the precursor in oleylamine was injected into hot dodecanethiol. The thermolysis of [Ni(SON(CNiPr2)2)2] gave Ni3S4 in all cases except when precursor solution in oleylamine was injected into hot octadecene which produced NiS nanoparticles. The thermolysis of [Fe(SON(CNiPr2)2)3] in oleylamine/oleylamine produced Fe7S8 nanoparticles but other combinations, in most cases, gave amorphous material. Thermolysis of [Zn(SON(CNiPr2)2)2] in oleylamine produced spherical ZnS nanoparticles. Particles with size smaller than 4.3 nm had a cubic phase, whereas the particles with size larger than 4.3 nm had a hexagonal crystal structure as suggested by the selected area electron diffraction. Powder X-Ray diffraction showed that the CdS nanoparticles obtained from the thermolysis of [Cd(SON(CNiPr2)2)2] in oleylamine were cubic under all reaction conditions except when dodecanethiol was used as an injection solvent which produced hexagonal CdS. β-In2S3 were synthesized from the thermolysis of [In(SON(CNiPr2)2)3]. Transmission electron microscopy showed that the copper, nickel and iron sulfide nanoparticles had various morphologies such as spherical, hexagonal disks, trigonal disks, rods or wires; depending on the reaction temperature, concentration of the precursor, the growth time and the solvent/capping agent combination. The zinc and cadmium sulfide nanoparticles were mostly spherical whereas the indium sulfide nanoparticles were produced in the form of ultra-thin (< 1.0 nm) nanorods or nanowires. ZnxCd1-xS and CuInS2 nanoparticles were synthesised from the 1,1,5,5-tetra-iso-propyl-4-thiobiureto complexes of Zn, Cd and Cu, In, respectively. Powder X-Ray diffraction showed that the obtained ZnxCd1-xS nanoparticles are cubic under all reaction conditions. The ZnxCd1-xS nanoparticles had an average diameter between 3.5 to 6.4 nm as shown by transmission electron microscopy. The optical properties of the ZnxCd1-xS nanoparticles were highly dependent on the ZnS to CdS precursor ratio and the solvents/capping agents. Chalcopyrite (tetragonal), wurtzite (hexagonal) or a mixture of both CuInS2 nanoparticles were obtained depending on the reaction conditions. TEM showed that the CuInS2 nanoparticles could be synthesised with different morphologies (spherical, hexagonal, trigonal or cone). Luminescent CuInS2 nanoparticles were obtained only in the absence of oleylamine. [Cd(S2CNMenHex)2], [Cd(Se2P(iPr)2)2] and [Cu(SON(CNiPr2)2)2] were used as single source precursor(s) for the synthesis of CdS, CdSe, CdSe/CdS core/shell, CdSeS alloys and Cu2-xS in microfludic reactor. The CdS nanoparticles were in size range of 5.0 to 8.0 nm whereas the CdSe nanoparticles were ultra small (ca. 2 nm) with blue luminescence. The CdSe/CdS core/shell and the CdSeS alloys were bluish green or green luminescent depending on their size. The copper sulfide nanoparticles were found to be monoclinic Cu7S4 or monoclinic Cu7S4 with minor impurities of rhombohedral Cu9S5 depending on the reaction conditions.

No description available.

Recentemente foi descoberto que diversos calcogenetos de metais de transição podem ter o estado charge density waves (CDW) suprimido a partir de pressão hidrostática e dopagens, e, por conseguinte, o estado supercondutor emerge. Nesse contexto, este trabalho apresenta um estudo sistemático de propriedades físicas de amostras poli e monocristalinas de dois compostos do sistema Nb-Te, NbTe2 e NbTe4. Com relação ao composto NbTe2, os resultados aqui apresentados demonstram que esse composto é mais um exemplo de material que exibe ambos os estados a pressão atmosférica e sem dopagens. No que tange as propriedades do NbTe4, este trabalho demonstra que amostras deficientes em telúrio tem a anomalia na curva de resistividade elétrica relacionada a formação do estado CDW amplificada e, a deficiência em telúrio é também capaz de fazer emergir supercondutividade em 5.5 K. Este trabalho também sugere algumas mudanças no diagrama de equilíbrio de fases Nb-Te publicado na base de dados da sociedade americana de metalurgia (ASM). As fases Nb5Te4 e Nb3Te4 foram identificadas como sendo fases de altas temperaturas que são formadas a partir de reações eutetóides. Além do mais, nossos resultados demonstram que a região entre as fases NbTe2 e NbTe4 consiste, na verdade, de uma região bifásica. Durante a realização deste trabalho, outro composto foi investigado, o NiTe2. Nesse composto, nossos resultados demonstram que a intercalação de Ti faz emergir um estado supercondutor em 4.5 K e cuja temperatura de transição parece insensível a pressão hidrostática. Cálculos de estrutura de bandas sugerem fortemente que o composto NiTe2 intercalado com Ti pode ser mais um exemplo de supercondutor com aspectos topológicos em sua superfície de Fermi. / Recently was demonstrated that it is possible to suppress the charge density waves (CDW) ground states while, simultaneously, a superconductor state emerges in several transition metal chalcogenides (TMC), by means of hydrostatic pressure or chemical doping. Within this context, this work presents a systematic study on physical properties of two Nb chalcogenides, NbTe2 and NbTe4. Our results demonstrate that NbTe2 is another example of a TMC which exhibit both stabilities at atmospheric pressure and without doping. Regarding the physical properties of NbTe4, we have demonstrated that Te deficiency increases significantly the anomaly in the electrical resistivity as function of temperature behavior related with the CDW formation. At the same time, Te deficiency can also cause a SC state to emerge at 5.5 K. This work also presents a review of the binary phase diagram, Nb-Te, and some changes are proposed. Nb5Te4 and Nb3Te4 were identified as high temperature phases originated from eutectoid reactions. Furthermore, our results demonstrated that the region between the phases NbTe2 and NbTe4 are, in fact, a two-phase region, differently from what is proposed in the actual version of the phase diagram. Also, during this work, another chalcogenide was investigated, NiTe2. Our results demonstrate that Ti can be intercalted between the Van der Waals gaps of the structure and consequently a superconductor state emerges at 4.5 K. The critical temperature is found to be insensitive to hydrostatic pressure. Band structure strongly suggests that NiTe2 could be another example of a superconductor with topological aspects in its Fermi surface.

Recentemente foi descoberto que diversos calcogenetos de metais de transição podem ter o estado charge density waves (CDW) suprimido a partir de pressão hidrostática e dopagens, e, por conseguinte, o estado supercondutor emerge. Nesse contexto, este trabalho apresenta um estudo sistemático de propriedades físicas de amostras poli e monocristalinas de dois compostos do sistema Nb-Te, NbTe2 e NbTe4. Com relação ao composto NbTe2, os resultados aqui apresentados demonstram que esse composto é mais um exemplo de material que exibe ambos os estados a pressão atmosférica e sem dopagens. No que tange as propriedades do NbTe4, este trabalho demonstra que amostras deficientes em telúrio tem a anomalia na curva de resistividade elétrica relacionada a formação do estado CDW amplificada e, a deficiência em telúrio é também capaz de fazer emergir supercondutividade em 5.5 K. Este trabalho também sugere algumas mudanças no diagrama de equilíbrio de fases Nb-Te publicado na base de dados da sociedade americana de metalurgia (ASM). As fases Nb5Te4 e Nb3Te4 foram identificadas como sendo fases de altas temperaturas que são formadas a partir de reações eutetóides. Além do mais, nossos resultados demonstram que a região entre as fases NbTe2 e NbTe4 consiste, na verdade, de uma região bifásica. Durante a realização deste trabalho, outro composto foi investigado, o NiTe2. Nesse composto, nossos resultados demonstram que a intercalação de Ti faz emergir um estado supercondutor em 4.5 K e cuja temperatura de transição parece insensível a pressão hidrostática. Cálculos de estrutura de bandas sugerem fortemente que o composto NiTe2 intercalado com Ti pode ser mais um exemplo de supercondutor com aspectos topológicos em sua superfície de Fermi. / Recently was demonstrated that it is possible to suppress the charge density waves (CDW) ground states while, simultaneously, a superconductor state emerges in several transition metal chalcogenides (TMC), by means of hydrostatic pressure or chemical doping. Within this context, this work presents a systematic study on physical properties of two Nb chalcogenides, NbTe2 and NbTe4. Our results demonstrate that NbTe2 is another example of a TMC which exhibit both stabilities at atmospheric pressure and without doping. Regarding the physical properties of NbTe4, we have demonstrated that Te deficiency increases significantly the anomaly in the electrical resistivity as function of temperature behavior related with the CDW formation. At the same time, Te deficiency can also cause a SC state to emerge at 5.5 K. This work also presents a review of the binary phase diagram, Nb-Te, and some changes are proposed. Nb5Te4 and Nb3Te4 were identified as high temperature phases originated from eutectoid reactions. Furthermore, our results demonstrated that the region between the phases NbTe2 and NbTe4 are, in fact, a two-phase region, differently from what is proposed in the actual version of the phase diagram. Also, during this work, another chalcogenide was investigated, NiTe2. Our results demonstrate that Ti can be intercalted between the Van der Waals gaps of the structure and consequently a superconductor state emerges at 4.5 K. The critical temperature is found to be insensitive to hydrostatic pressure. Band structure strongly suggests that NiTe2 could be another example of a superconductor with topological aspects in its Fermi surface.

Ionic liquids (ILs) are nowadays a large and widely explored class of ionic compounds that melt below 100 °C. Due to their attractive properties, ILs are now of growing interests in a variety of inorganic materials preparation. However, most studies have put much focus on the description of new synthetic strategies. The chemical reactivity of ILs in the reactions is often neglected. In this dissertation, a series of metal chalcogenides were synthesized using the decompositions of ILs. The role or chemical reactivity of ILs in the reactions was demonstrated in detail. The hierarchical desert-rose-like SrTiO3 particles have been successfully prepared based on an ethylene glycol (EG) mediated one-pot IL-assisted solvothermal synthetic route. The used basic ionic liquid tetrabutylammonium hydroxide (TBAH) serves as an alkaline source and can also replace EG as the sole solvent to synthesize polyhedral SrTiO3, showing “all-in-one” solvent and reactant. A series of metal sulfides, such as Sb2S3, Bi2S3, PbS, CuS, Ag2S, ZnS, and CdS have been obtained from a choline chloride/thioacetamide based deep eutectic solvent (DES, an IL analog solvent) by a simple and general synthetic method. The reaction mainly proceeds in two steps: i) the dispersion of metal salts in the DES and the formation of a metal-DES complex, and ii) the decomposition of the metal-DES complex and formation of the final products. In addition, the chemical reactivity of phosphonium based ILs with selenium and tellurium at above 220 °C was systematically investigated by a series of dissolution experiments, tracking the solute selenium and tellurium species by nuclear magnetic resonance (NMR). NMR results clearly indicate some common decomposition mechanisms for quaternary phosphonium ILs at a relatively high temperature in the presence of selenium or tellurium. The decomposition of the quaternary phosphonium cations should proceed by an elimination of one alkyl substituent via an SN2 reaction, forming the respective trialkylphosphane selenides or tellurides in the presence of selenium or tellurium, which is then responsible for the genuine dissolution of selenium or tellurium. However, in the case of tellurium, the dissolution behavior is much more complicated compared to that of selenium. The coupling of P and Te which indicates a P–Te bond formation is only observed in the NMR spectra when a sufficient amount of tellurium (e.g. Te : IL = 1 : 1) is provided. The existence of a parallel-competitive IL decomposition route besides the SN2 reaction is regarded as the side reaction for the dissolution of tellurium. This may at least partially explain the relatively lower solubility of tellurium in phosphonium based ILs compared to that of selenium.

<p dir="ltr">With climate change creating the need for renewable energy to replace fossil fuels, solar energy technologies are primed to dominate the energy sector. And while photovoltaics have improved significantly in recent decades, continued evolution of this technology requires research into new fabrication techniques and new materials. The solution processing of metal chalcogenide semiconductors offers an opportunity to fabricate photovoltaics in a low-cost and high-throughput way. However, for this methodology to make a commercial impact a variety of challenges around the fundamental chemistry and materials science need to be addressed. Furthermore, while solution processing has been applied heavily to the Cu(In,Ga)(S,Se)₂ family of materials, these techniques can also open doors for emerging materials like Cu₂ZnSnSe₄, Ag₂ZnSnSe₄, and the chalcogenide perovskites.</p><p dir="ltr">In solution processed Cu(In,Ga)(S,Se)₂ devices, researcher have generally started with a Cu(In,Ga)S₂ film that is then selenized to form the final Cu(In,Ga)(S,Se)₂ material. However, this process has been connected to the formation of a problematic “fine-grain” layer. To solve this issue, the molecular precursors from amine-thiol chemistry were modified to produce soluble molecules with metal selenium bonding. This enabled direct solution deposition of CuInSe₂ films that could be processed without forming a fine grain layer.</p><p dir="ltr">Reactive dissolution chemistry (or “alkahest” chemistry) is useful for solution processing because it can enable the direct use of metal or metal chalcogenide precursors, bypassing the potential impurities from metal salt precursors. However, the commonly used amine-thiol reactive solvent system is better suited to making metal sulfides than metal selenides because the thiol acts as a sulfur source. To address this limitation, a new alkahest based on alkylammonium polyselenide solutions was developed which could reactively dissolve a wide range of metals, metal chalcogenides, and metal oxides. This generalizable chemistry enabled the synthesis of a wide range of binary and multinary metal chalcogenides including Cu(In,Ga)Se₂, Cu₂ZnSnSe₄, and Ag₂ZnSnSe₄.</p><p dir="ltr">Emerging metal chalcogenide semiconductors composed of earth-abundant and non-toxic elements that can exhibit strong optoelectronic properties and high stability are a target of significant interest. Chalcogenide perovskites like BaZrS₃ and BaHfS₃ are an intriguing option to satisfy these requirements but have rarely been studied because of synthesis difficulties, historically being made by solid-state reactions or the sulfurization of oxides around 1000 °C. Here a solution-based approach that only requires moderate temperatures of 550-575 °C was developed utilizing a hybrid ink containing soluble metal thiolates and nanoparticulate metal hydrides.</p><p dir="ltr">The hybrid ink was an important proof of concept that chalcogenide perovskites could be synthesized at these moderate temperatures. However, it relies on complex and difficult to handle precursors. A simpler route would be to use air-stable precursors to make an oxide perovskite and subsequently sulfurize the material. However, this route has historically used excessively high temperatures. Therefore, a new sulfurization step was conceived based on thermodynamic arguments that includes both sulfur and hafnium sulfide as an oxygen sink. This redesigned sulfurization enabled the conversion of BaZrO₃ into BaZrS₃ at temperatures around 575 °C.</p><p dir="ltr">Finally, an energy systems and economic analysis was performed to consider how photovoltaics might be incorporated into agricultural lands. This work showed that when compared with traditional photovoltaics or a PV Aglectric concept, using corn for ethanol is an inefficient way to generate both food and energy from a given unit of land.</p>

Magister Scientiae - MSc (Chemistry) / This work involves the synthesis of copper telluride-polypropylenimine tetra(5-(2-thienyl) salicylaldimine) (CuTe@PPI) and copper antimonide-polypropylenimine tetra(5-(2-thienyl) salicylaldimine) (CuSb@PPI) core-shell nanoparticles (NPs), using two-pots and one-pot synthesis methods, respectively. Their morphology was studied by X-ray diffraction spectroscopy (XRD), high resolution transmission electron microscopy (HRTEM) and high resolution scanning electron microscopy (HRSEM); while their structures were characterized by Fourier transform infrared spectroscopy (FTIR) and elemental analysis. Photophysical properties of the core-shell NPs were determined from ultraviolet-visible absorption spectroscopy (UV-Vis) and photoluminescence spectroscopy (PL). For core-shell NPs produced via two-pots method only CuTe@PPI exhibited ? ? ?* and n ? ?* which indicate that CuSb@PPI produced via two-pots method was unsuccessfully synthesized. The ? ? ?* and n ? ?* transitions indicate the presence of polypropylenimine tetra(5-(2-thienyl) salicylaldimine) (PPI) on the surface of CuTe NPs and CuSb NPs. FTIR confirmed coordination of PPI on the surface of CuTe NPs and CuSb NPs by showing a shift in wavenumber of C=N group bands from PPI. HR-TEM showed that the CuTe@PPI synthesized via one-pot method have a wide particles sizes distribution with an average particles size of 13.60 nm while for CuTe@PPI synthesized via two-pots it was impossible to determine the particles size due to aggregation. CuSb@PPI synthesized via twopots method and one-pot method has a wide particles sizes distribution with an average size of 7.98 nm and 11.61 nm respectively. The average particles sizes determined by HR-SEM were found to be 35.24 nm (CuTe@PPI two-pots method), 33.90 nm (CuTe@PPI one-pot method), 18.30 nm (CuSb@PPI two-pots method), and 16.18 nm (CuSb@PPI one pot method). / 2021-08-31

The ever increasing demand for energy due to over consumption of non-renewable fossil fuels has emphasized the need for alternate, sustainable and efficient energy conversion and storage systems. In this direction, electrochemical energy conversion and storage systems involving various fundamental electrochemical redox processes such as hydrogen evolution (HER), oxygen reduction (ORR), oxygen evolution (OER), hydrogen oxidation (HOR) reactions and others become highly important. Electrocatalysts are often used to accelerate the kinetics of these reactions. Platinum (Pt), ruthenium oxide and iridium oxide (RuO2 and IrO2) are known to be the state of the art catalysts for several of these reactions due to favouarable density of states (DOS) near the Fermi level, binding energy with the reactant species, chemical inertness etc. Apart from HER, OER and ORR, chlorine evolution reaction (Cl-ER) is another industrially important reaction associated with water purification, disinfection, bleaching, chemical weapons and pharmaceuticals. Dimensionally stable anodes (RuO2/IrO2 mixed with TiO2 on Ti) are the most commonly used catalysts for this process. Issues related to surface poisoning, corrosion and cost of the catalysts, in addition to selectivity and specificity towards a particular reaction are various aspects to be addressed. For example, Pt is not very specific for ORR in presence of methanol in addition to high cost and corrosion in certain media. On the other hand, DSA can efficiently catalyze both OER and Cl-ER, and hence there is overlap of the two processes in the potential range available. There is an on going search for efficient, cost-effective, stable catalysts that possess high specificity for a particular redox reaction. Towards this goal, the present study explores certain layered (phospho)chalcogenides for catalyzing HER, ORR, OER and Cl-ER. The present thesis is structured in two parts, where the first part explores the multi-functional catalytic aspects of new classes of compounds based on layered transition metal mixed chalcogenides (MoS2(1-x)Se2x) and ternary phosphochalcogenides (FePS3, FePSe3 and MoPS). In addition, lithium insertion and desinsertion has been studied with the aim of using the layered materials for rechargeable batteries. The second part of the thesis explores organic electrode materials with active carbonyl groups such as rufigallol, polydihydroxyanthrachene succinic anhydride (PDASA) as battery electrodes. Additionally, covalently functionalized transition metal phthalocyanines with reduced graphene oxide are studied as counter electrodes in dye sensitized solar cells (DSSCs). MoS2(1-x)Se2x (x = 0 to 1) compositions are solid solutions of MoS2 and MoSe2 in different ratios. They crystallize in hexagonal structure with space group P63/mmc (D6h4) having Mo in trigonal prismatic coordination like the pristine counterparts. X-Ray diffraction studies reveal that Vegard’s law (figure 1a) is followed and hence complete miscibility of MoS2 and MoSe2 is established. MoS2(1-x)Se2x (x = 0 to 1) are layered in nature and the layers are held together by long range, weak van der Waal’s forces. This gives us the flexibility of exfoliation to produce corresponding few-layer materials (figure 1b). Figure 1. (a) Variation of lattice parameter corresponding to (002) reflection of MoS2(1-x)Se2x with different x values. (b) Scanning electron micrograph of few-layer MoS2(1-x)Se2x (x = 0.5). The electrocatalytic activity of the few-layer sulphoselenides have been studied towards HER in aqueous 0.5 M H2SO4 and towards Cl-ER in 3 M aqueous NaCl (pH = 3) solution. The mixed chalcogenides exhibit very good activities for both HER and Cl-ER as compared to the activity of their pristine counter parts (i.e. MoS2 and MoSe2) (figures 2a and 2b). Electrocatalytic activity on different compositions reveal that MoS1.0Se1.0 exhibits the maximum activity. Additionally, it has been observed that MoS1.0Se1.0 shows high specificity for Cl-ER with negligible interference of OER. Figure 2. Voltammetric data for (a) hydrogen evolution reaction (in 0.5 M aqueous H2SO4) and (b) chlorine evolution reaction (in 3 M aqueous NaCl solution, pH = 3) on MoS2(1-x)Se2x (x = 0, 0.5, 1). Figure 3. (a) XRD pattern of MoS2(1-x)Se2x (x = 0.5) electrode after a cycle of Li insersion and deinsersion (red) along with as-synthesized material (black) (b) Cycling behaviour of rGO supported (black) and pristine (red) MoS2(1-x)Se2x (x = 0.5) as electrode in rechargeable lithium-ion battery. The equiatomic MoS1.0Se1.0 has also been studied as an anode material for rechargeable lithium batteries. The cyclic voltammogram and characterization after charge-discharge cycle (figure 3a) indicate intercalation of Li with in the layers followed by conversion type formation of Li-S and Li-Se type compounds. The pristine material shows continuous capacity fading while the composites of sulphoselenides functionalized with conducting carbon supports such as rGO, MWCNT, super P carbon, toray carbon show marked improvement in capacity as well as cycling behavior. The rGO functionalized MoS1.0Se1.0 reveals ~1000 mAh/g of stable specific discharge capacity for 500 cycles (figure 3b). In the next two chapters, new class of transition metal-based layered materials FePS3 and FePSe3, containing both P and chalcogen (S and Se) is indroduced for electrocatalysis. FePS3 crystallizes in monoclinic symmetry with an indirect band gap of ~1.55 eV while FePSe3 possesses rhombohedral crystal structure with comparatively low band gap (~1.3 eV) as shown in figure 4a. The FePS3 and FePSe3 have been exfoliated as has been done for MoS1.0Se1.0 (liquid exfoliation method) using acetone as the solvent. Stable colloids with few-layer nanosheets having lamellar morphology and lateral sizes of ~100 to 200 nm are obtained. Electrical characterization indicates that they are semiconducting and the conductivity of the Se analogue is ~50 times higher than that of the S analogue (figure 4b). Figure 4. (a) Catholuminescence of FePX3 ( X = S and Se) reveals the band gap of the material. Band gap of the S analogue is 1.52 eV and that of the Se analogue is 1.33 eV (b) Resistivity of FePX3 ( X = S and Se) as a function of temperature. The tri-functional electrocatalytic activities on rGO-few layer FePX3 (X = S and Se) have been evaluated for HER over a wide pH range (0.5 M H2SO4, 0.5 M KOH, phosphate Figure 5. Catalytic activity of rGO-few-layer FePX3 (X = S, Se) towards HER in (a) aqueous 0.5 M H2SO4 and (b) 3.5 wt % NaCl solutions. (c) ORR activity of the catalysts in oxygen saturated 0.5 M KOH (d) OER behaviour on the catalysts in 0.5 M KOH at a rotation speed of 1600 rpm. buffer, pH 7 and 3.5 % NaCl), ORR and OER in alkaline media (0.5 M KOH). The studies clearly reveal that both rGO-FePS3 and rGO-FePSe3 exhibit excellent HER activity in acidic media (figure 5a) with high stability. The HER studies in 3.5 wt % aqueous NaCl solution (figure 5b) suggests that the catalysts are effective in evolving hydrogen from sea-water environment. Studies on ORR activity (figure 5c) indicate that the rGO composites of both S and Se analogues follow 4-electron pathways to produce water as the final product. They are also found to be highly methanol tolerant. In the case of OER (figure 5d), XPS characterization of the electrodes after the voltammetric studies reveals the presence of very thin layer of Fe2O3 (not detectable by XRD). All the three reactions (HER, ORR and OER) catalyzed by the Se analogue are better than the S analogue (figure 5). This could be due to the low band gap and high conductivity of FePSe3 as compared to FePS3. The over potential to achieve 10 mAcm-2 current density is ~108 mV for rGO-few-layer FePS3 catalyst where in the case of rGO-few layer FePSe3, it is ~97 mV (table 1). Table 1. Catalytic activities of rGO-few layer FePS3 and rGO-few layer FePSe3 towards HER, ORR and OER. Reaction studied rGO-FePS3 rGO-FePSe3 HER (η @ 10mAcm-2) ~108 mV ~97 mV ORR (peak potential) ~0.81 V ~0.87 V OER (η @ 10mAcm-2) ~470 mV ~430 mV It is likely that there is a strong interaction between FePX3 (metal d-orbital) and rGO, as observed from the downward shift of Fe 2p peak in high resolution XPS studies. This interaction may extend the density of states of metal d-orbitals thereby improving the catalytic activities. The next chapter deals with molybdenum-based phosphosulphide compound (MoPS). Molybdenum-based phosphide catalysts have been explored recently as excellent catalysts for various electrochemical reactions such as HER. It is expected that the catalyst containing both S and P will show positive effects on catalytic activities due to the synergy between S and P. In the present study, P incorporated MoS2 is studied towards HER. The XRD pattern of the as-synthesized crystal suggests the presence of mixed phase of MoS2, MoP2 and MoP while the elemental mapping in microscopy indicates the ratio of Mo, P and S to be 1:1:1. The electrochemical HER in 0.5 M H2SO4 indicates that the activity is improved drastically as compared to bulk and few-layer MoS2. The next section explores the use of different organic electrode materials possessing active carbonyl groups for Li-storage studies. The advantage of the use of carbonyl-based compounds lies in the high reversible activity towards Li ion insersion and de-insersion. Rufigallol (figure 6a) exhibits very stable capacity of ~200 mAh/g (at C/20 rate) upto 500 Figure 6. (a) and (c) Schematic representation of rufigallol and poly-dihydroanthracene succinic anhydride (PDASA) respectively. (b) and (d) Cyclic behaviour of rufigallol (at C/20 rate) and PDASA (at 20 mAg-1 current rate) in Li-storage devices. (e) and (f) represent the coulombic efficiency of rufigallol (at C/20 rate) and PDASA (at 20 mAg-1 current rate) as a function of number of cycles. cycles along (figure 6b) and with very good rate capability. A triptycene-based mesoporous polymer, PDASA (figure 6c) is introduced and explored as efficient electrode material for Li-storage. PDASA exhibits very high capacity of ~1000 mAh/g at a current rate of 50 mA/g upto 1000 cycles (figure 6d). Even at very high current rates (3A/g) excellent cyclability is observed. The mechanistic details of lithium uptake and release are studied using various spectroscopic techniques. In both the cases the coulombic efficiency observed is ~80 to 90 % (figures 6e and f). Figure 7. (a) Digital photograph of the dye sensitized solar cell with rGO-Co-TAPc counter electrode. (b) Photoconversion efficiency of DSSCs with different counter electrodes as mentioned in the figure. (c) Photo conversion efficiency of Pt and rGO-Co-TAPc based DSSCs as function of storage time. (d) Schematic illustration of DSSC wherein the energy level of the counter electrodes and electrolyte are shown for different M-TAPcs. In a slightly different direction, metal phthalocyanine - rGO composites (rGO-M-TAPc; M = Co, Zn, Fe) have been explored as counter electrodes in DSSC. Figure 7a depicts the digital image of a DSSC constructed using rGO-Co-TAPc as the counter electrode. It has been observed that rGO-cobalt tetraamino phthalocyanine (rGO-Co-TAPc) counter electrode exhibits ~6.6 % of solar conversion efficiency (figure 7b) and is close to that of standard DSSC (Pt counter electrode) under identical experimental conditions and are highly stable (figure 7c). Other metal phthalocyanines show less efficiency and is analysed based on the relative positions of HOMO energy levels of the materials and the energy level of the redox system (I-/I3- system) as given in figure 7d. The thesis contains eight chapters on aspects discussed above along with summary and future perspectives given at the end. It is devided into various chapters in two sections, one comprising inorganic chalcogenide-based electrocatalysts and another comprising organic electrode materials. Appendix I discusses the Na-storage behaviour of MoS1.0Se1.0 and appendix II describes the Li-storage behaviour of rGO functionalized benzoquinone and diamino anthraquinone electrode materials.

In recent years, there has been an increasing interest on renewable energy sources as substitute to fossil fuels. Among various processes of energy generation, electrochemical methods such as storage and conversion systems, electrolysis of water (production of H2 and O2), fuel cells, batteries, supercapacitors and solar cells have received great attention. The core of these energy technologies is a series of electrochemical processes, which directly depend on the nature of ‘electro catalyst’. The design and preparation of an electro catalyst is based on new concepts such as controlled surface roughness, atomic topographic profiles, defined catalytic sites, atomic rearrangements, and phase transitions during electrochemical reactions. Good electro catalysts should possess low over potential, high exchange current density, high stability, low cost and high abundance. The most fundamental reactions in the area of electrochemistry are hydrogen evolution (HER) and oxygen reduction (ORR) reactions. They are important in different energy systems such as fuel cells and batteries. Platinum has been a favoured electro catalyst due to its high activity, favourable density of states at Fermi level and chemical inertness. The low abundance, however, limits its large scale applications. Alternate materials with high catalytic activities are always required. In this particular direction, metal chalcogenides such as sulphides and selenides have attracted attention in recent years. The present thesis describes the synthesis of different phases of palladium and nickel chalcogenides and their applicability in various electrochemical reactions, both in aqueous and organic media. First part includes the synthesis of highly crystalline palladium selenide phases namely Pd17Se15, Pd7Se4 and Pd4Se by employing facile single source molecular precursor method. Pure palladium selenide phases are prepared by thrombolysis of highly processable intermediate complexes formed from metal and selenium precursors. Continuous films of different dimensions on various substrates (glass, ITO, FTO etc.) could be prepared (figure 1). This is one of the requirements for processing any new material. Thickness of the films could be altered by changing the volume of precursor complex coated on the substrate. All the phases are found to be metallic in nature with resistivity values in the range of 30 to 180 µΩ.cm. Figure 1. (a) Scanning electron micrograph and (b) photographic image of Pd17Se15 prepared on different substrates glass (1), Si (2), fluorine doped tin oxide (FTO) (3) and DSSC solar cell fabricated using FTO coated Pd17Se15 as the counter electrode (4). Other components of DSSC are given in the experimental section. All the palladium selenides phases are shown to be catalytically active towards electrochemical reactions such as HER and ORR. It is observed that the activities of the phases depend on the stoichiometric ratio of palladium to selenium. Higher the palladium content in the phase, higher is the catalytic activity observed. Therefore, the activities of the chalcogenides can be easily tuned by varying the ratio of metal to chalcogen. Tafel slopes of 50–60 mV/decade are observed for all three phases towards HER indicating that Volmer- Heyrovsky mechanism is operative. The exchange current densities are in the range of 2.3 x 10-4 A cm-2 to 6.6 x 10-6 A cm-2 (figure 2a). Figure 2. (a) Linear sweep voltammograms of Pd17Se15, Pd7Se4 and Pd4Se in 0.5 M H2SO4 (HER) and (b) 0.1 M KOH (ORR) at a scan rate of 2 mVs-1. These phases are found to be highly robust and stable under different pH conditions. Stability of the phases is confirmed by characterizing the catalysts post-HER process, using various techniques such as XPS, XRD and SEM. High activities observed for Pd4Se is explained based on electrochemically active surface area values determined from under potential deposition studies and also based on DFT calculations. Computational studies reveal the presence of different charge distribution on palladium in all the three phases which is likely to be another reason for varied activities. Palladium selenides are also explored as catalysts towards ORR in alkaline medium. Kinetic parameters and reaction mechanism are determined using RDE studies. All the three phases are found to be active and Pd4Se shows the highest activity, following a direct 4 electron reduction pathway (figure 2b). Other two phases follow 2 electron pathway terminating at hydrogen peroxide stage. Catalytic activity of Pd17Se15 is further improved by Nano structuring of the material and by synthesizing the material on active supports such as rGO, acetylene black and today carbon. ORR plays an important role in metal-air batteries. The palladium chalcogenides are used as electrodes in metal-air batteries. Specific energy density observed in the case of Mg-air primary batteries is higher for Pd4Se than the other two phases (figure 3a). Figure 3. (a) Discharge curves of Mg-O2 battery with different phases of palladium selenides as cathodes. Constant current density of 0.5 mA cm-2 is used for discharge. (b) Characteristic J–V curves of DSSCs with Pd17Se15, Pd7Se4 and Pt as counter electrodes. Versatility of these phases is further studied towards redox reaction in non-aqueous medium (I3-/I-). This reaction plays a crucial role in the regeneration of the dye in dye-sensitized solar cells (DSSC). Palladium selenide phases prepared on FTO plates are employed as counter electrodes in DSSC. The solar light conversion efficiencies are found to be 7.45 and 6.8% for Pd17Se15 and Pd7Se4 respectively and are comparable to that of platinum (figure 3b). The reason for high activities may be attributed to high electronic conductivity and low work function of the phases. The following chapter deals with the synthesis of palladium sulphide phases (Pd4S and Pd16S7) using both hydrothermal and single source precursor methods. Electro catalytic activities of the phases are shown towards HER and ORR and Pd4S exhibits better catalytic activities than that of Pd16S7 phase. Direct electrochemistry of cytochrome c is achieved on Pd4S with ∆E of ~64 mV (figure 4a). Electrochemical oxidation of ethanol, ethylene glycol (EG) and glycerol are also studied on the Pd4S phase and the activity is found to follow the order, glycerol > ethylene glycol > ethanol (figure 4b). Figure 4. (a) Cyclic voltammograms of Pd4S in (1) 0.1 M phosphate buffer solution (pH 7.0) and (2) in presence of 0.2 mM cytochrome c at a scan rate of 50 mVs-1 and (b) Voltammograms of Pd4S in presence of different alcohols (ethanol, EG and glycerol) in 1 M KOH solution at sweep rate of 50 mVs-1. Concentration of alcohols used is 0.1 M. The effect of dimensionality on the electro catalytic activity of nickel selenide phases forms part of the next chapter. Nickel selenide (NiSe) nanostructures possessing different morphologies of wires, spheres and hexagons are synthesized by varying the selenium precursors namely, selenourea, selenium dioxide (SeO2) and potassium selenocyanate (KSeCN), respectively using hydrothermal method. The different selenium precursors result in morphologies that are probably dictated by the by-products as well as relative rates of amorphous selenium formation and dissolution. The three different morphologies are used as catalysts for HER, ORR and glucose oxidation reactions. The wire morphology is found to be better than that of spheres and hexagons for all the reactions. Among the reactions studied, NiSe is found to be good for HER and glucose oxidation while ORR seems to terminate at the peroxide stage. In alkaline medium, nickel forms hydroxides and oxy-hydroxides and these oxyhydroxides are catalytically active towards the oxidation of glucose. Therefore, nickel selenides are employed as highly selective non-enzymatic glucose sensors and detection limit of 5 µM is observed. Electrical measurements on a single nanowire and a hexagon morphology of NiSe are carried out on devices fabricated by focused ion beam (FIB) technique (figure 5). The semiconducting nature of NiSe is revealed in the I-v measurements. The band gap of the material is found to be 1.9 eV and hence the single nanowire and hexagon are shown to act as visible light photodetector. Figure 5. SEM images of (a) single NiSe nanowire and (b) single NiSe hexagon with Pt contacts fabricated by FIB technique. Figure 6. Cyclic voltammograms of NiSe nanowires in 0.5 M aqueous NaOH in the (i) absence and (ii) the presence of 0.5 mM glucose, at a scan rate of 20 mVs-1 and (b) Galvanostatic discharge performance of Ni3Se2 with different morphologies (A, B and C represent Ni3Se2 prepared from SeO2, selenourea and KSeCN respectively). The next chapter includes the synthesis of different morphologies of Ni3Se2 using three different selenium precursors (SeO2, KSeCN and selenourea) and the study of their activities towards electrochemical reactions such as HER and glucose oxidation (figure 6a). Electrical measurements demonstrated the metallic behaviour of the material. These are also shown to be efficient electrode materials in energy storage devices such as supercapacitors with high specific capacitance of 2200 F/g (figure 6b). The studies are summarized in the last chapter with scope for further work. The appendixes show preliminary studies on electrooxidation of glycerol and propanol on Pd supported on TiN, synthesis of other selenides of Ni, Cu, Ag and Ti, and electro synthesis of metal-organic frameworks. (For figures pl refer the abstract pdf file)