School of Chemistry - Theses
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ItemThe Carbonylation of Organic Compounds by Visible Light Photoredox Catalysis( 2020)Palladium-catalysed alkoxy- and aminocarbonylation of aryl (pseudo)halides provides efficient access to aromatic esters and amides. The broad application of this approach has been restricted by functional group tolerance, high reaction temperatures and moderate catalyst efficiency. Free-radical carbonylation is a complementary approach not confined by the same inherent limitations of palladium-catalysed carbonylative cross-coupling methodology. The development of free-radical carbonylation has been hindered by the ability to selectively generate the carbon-centred radical species and the high pressures of carbon monoxide required to drive the carbonylation step. This thesis describes the development of visible light photoredox-catalysed alkoxy- and aminocarbonylation of aryl (pseudo)halides. Visible light photoredox-catalysis is a potent method to generate carbon-centred radicals selectively under mild reaction conditions. Aryl radicals can be trapped by carbon monoxide to afford carbonyl compounds. Continuous flow chemistry is utilised throughout, employing tube-in-tube semipermeable membrane reactor technology, to enable precise control over reactions conditions and safe use of carbon monoxide. Chapter 1 introduces carbonylation and elaborates on carbonylative cross-coupling of aryl (pseudo)halides. It further introduces continuous flow processing in synthetic chemistry (flow chemistry) and details the application of flow chemistry to carbonylative cross-coupling and photochemical reactions. Chapter 2 established a continuous flow platform for high pressure gas-liquid photochemistry. The flow system consisted of a pumping module, a reagent delivery module, a Teflon AF-2400 tube-in-tube reactor for saturation of the reaction stream with carbon monoxide, a photoreactor and pressure regulation devices. The photoredox-catalysed alkoxycarbonylation of aryl diazonium salts was selected to evaluate the performance of the flow system. It was determined that excellent yields of the benzoate ester could be achieved at significantly lower partial pressures of carbon monoxide and processing time than in batch. Chapter 3 details the development of a free-radical annulative addition/alkoxycarbonylation cascade reaction. The developed methodology was applied to the synthesis of a diverse library of novel 3-acetate functionalized 2,3-dihydrobenzofurans from widely accessible allyl aryl diazonium ethers. Application of the previously established continuous flow system enabled dilute reaction conditions to effectively control the propagation of competitive intermolecular radical addition side reactions without compromising on reaction throughput or space-time yield. Chapter 4 describes the development of photoredox-catalysed aminocarbonylation of aryl halides. The developed methodology was applied to the synthesis of both electron rich and electron deficient benzamides at room temperature. Spectroscopic and theoretical computational studies were conducted to elucidate the reaction mechanism. A novel tandem photoredox catalytic manifold was proposed that features the transformation of Ir(dtbbpy)(ppy)2]PF6 in the presence of DIPEA to generate a distinct highly reducing Ir-complex capable of engaging energy demanding aryl halides. Chapter 5 provides a summary of the work described in this thesis. Supplementary data is included in the appendix.
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ItemUltrasonic synthesis of advanced photocatalytic materials for use in continuous flow-through reactors( 2017)Water purification is a problem around the world. Every year, authorities introduce new standards on the amount of pollutants that could be released into the environment, and new compounds are continuously being added to the list of toxic waste. Some of these substances can be defined as persistent organic pollutants (POPs), and their degradation is generally difficult to achieve. In these cases, advanced oxidation processes are required for the decomposition of these hazardous molecules. In this thesis, the advanced oxidation process used is photocatalysis, which can be used to decompose almost any kind of organic pollutant. To enhance the photocatalytic activity of photocatalysts, researchers have focused their attention on reducing the size of the particles, reaching just a few nanometers. Unfortunately, no sufficient efforts were made in continuous flow studies, which are necessary if future industrial implementations are desired. In these systems, the photocatalyst can be used in two ways: dispersed or coated onto a surface. The former offers high activity, but its recovery at the end of the reaction can be difficult. Instead, the latter permits almost no effort in the catalyst retrieval, but its efficiency is significantly low. For these reasons, this thesis aims to investigate the conversion of nanosized catalysts into micron sized powders without a loss in activity. In this manner, the catalyst could be used as a dispersion, enhancing the degradation and reducing the costs involved in the filtration procedures. In order to accomplish this aim, microspheres were used as a template material. The studies on continuous flow systems and their comparison to batch systems, carried out in this thesis, could be useful for future industrial implementations. For the generation of microspheres, ultrasonic emulsification technique was utilized, and the fundamental principles of ultrasound, along with those of microspheres, photocatalysis, and continuous flow reactors are discussed in Chapter 1. In Chapter 2, a structured literature review examining ultrasonic emulsification, microencapsulation, photocatalysts, and studies on continuous flow reactors, is discussed. In this thesis, chitosan (a natural amino-polysaccharide used in a wide range of applications) was chosen as a shell material for the generation of the microspheres, while nano sized TiO2 and ZnO were used as model nanosized photocatalysts. In Chapter 3, materials, analytical methods, and e experimental details used in this thesis are discussed. Three continuous flow reactors are presented, along with a new type of ultra-bright LEDs used as a light source for the photocatalytic degradation of rhodamine B, metanil yellow and methylene blue. Chapter 4 is the first chapter of result and discussion section. The role of counter ions on controlling the properties of ultrasonically generated chitosan microspheres, produced via oilin- water emulsion technique, was investigated. Various acids were used to dissolve chitosan, and it was found that the conjugate bases of the acid used (which acted as counter ions to neutralize the positive charges of ammonium ions present in the chitosan backbone) played a significant role in controlling the size, size distribution, and stability of the chitosan microspheres. In Chapter 5, the development of micron sized photocatalysts was studied. Chitosan microspheres were used for the conversion of nano sized TiO2 and ZnO (25-50 nm) into micron sized particles, possessing a size of about 10 μm. The micron sized photocatalysts possessed a photocatalytic efficiency similar to that of the nano sized powders, which was investigated in both aqueous and gas phases. In addition, the mechanism on the formation of the micron sized structures was proposed. In Chapter 6, the comparison of the photocatalytic activity of batch and continuous flow systems was investigated, using the micron sized catalyst (TiO2) previously studied. It was found that the continuous flow system is able to increase the amount of decomposable dye of up to 110% compared to that reached by the batch system. In addition, the catalyst used was found to be suitable for such continuous flow studies, with no loss in activity over a period of 42 hours. In Chapter 7, the use of ultra-bright LEDs on continuous flow systems, and the ability to apply the theory of such systems on photocatalytic reactions, were studied. It was found that the consideration of the kinetics of the photocatalytic reaction being pseudo-first order is not entirely correct, and that the new type of light source is suitable for photocatalytic degradations. In Chapter 8, some concluding remarks have been provided.
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ItemMesoporous Ti-based nanomaterials for photocatalysis and energy storage( 2017)Inspired by the discovery of the photocatalytic phenomenon in splitting water, enormous efforts have been devoted to the research of TiO2 materials. This has led to various applications ranging from photovoltaics and photocatalysis to batteries and sensors, which can be roughly divided into ‘energy’ and ‘environmental’ categories. In general, the effectiveness in the practical applications depend not only on the intrinsic properties of the TiO2 material, but also on modification to the material, including composition, morphology, and the compositional modification. As a photocatalyst, TiO2 is a wide band gap semiconductor (3.0-3.2 eV) that can be used to decompose organic compounds under ultraviolet light irradiation. An efficient strategy to extend the light response to the visible range and thus improve photocatalytic activity is by designing a heterojunction semiconductor. In this thesis pristine anatase TiO2 microspheres were used to prepare mesoporous TiO2/g-C3N4 microspheres via a nano-coating procedure followed by calcination, where the porous TiO2 acts as the active supporting scaffold and g-C3N4 as the visible light sensitizer. The composite microspheres were 8.5 folds more active in degrading phenol under visible light irradiation than mesoporous g-C3N4. Furthermore, starting with mesoporous TiO2 hollow microspheres, mesoporous brookite/anatase TiO2/g-C3N4 hollow microspheres were prepared via a facile nanocoating procedure that showed mixed phases of brookite (48 %), anatase (44 %), and rutile (8 %), incorporated with a g-C3N4 coating layer. The mesoporous hollow microspheres exhibited a unique hollow shell morphology of packed TiO2/g-C3N4 nanosheets, and a remarkable 5-fold increase in degrading phenol under visible light irradiation compared to mesoporous g-C3N4. Besides visible light photocatalysis, TiO2 can be used as an anode material for lithium-ion batteries, as it shows good gravimetric performance (336 mAh g-1) and excellent cyclability. To overcome the poor rate behaviour, slow lithium-ion diffusion, and high irreversible capacity decay, TiO2 nanomaterials with tuned compositions and morphologies are being investigated. Here, a promising TiO2 material has been prepared that comprises a mesoporous ‘yolk-shell’ spherical morphology in which the core is anatase TiO2 and the shell is TiO2(B). The electrochemical results indicate high specific reversible capacity at moderate current (330.0 mAh g-1) and cyclability (98 % capacity retention after 500 cycles). Lithium-sulphur batteries have attracted considerable attention as they have high specific capacity (1675 mAh g-1) and the abundance of sulphur, makes them one of the more promising next-generation battery technologies. However, commercialization of LSBs has generally been hampered by low sulphur utilization and poor long-term cyclability. These issues can be addressed, in part, by producing cathodic additives to encapsulate sulphur and polysulphides during the charge/discharge process. Mesoporous Magnéli Ti4O7 microspheres were prepared via an in-situ carbothermal reduction that exhibited large pore volume (0.39 cm3 g-1) and high surface area (197.2 m2 g-1). Strong chemical bonding of the polysulphides to Ti4O7, along with effective physical trapping in the mesopores and voids of the matrix, give superior reversible capacity (1317.6 mAh g-1) and cyclability (88 % capacity retention after 400 cycles). Ti-based materials with carefully tuned compositions, porosity, and morphologies have been constructed and tested in photocatalytic and energy storage applications revealing promising potential.
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ItemHierarchically porous titania nanostructures with high crystallinity: synthesis and photocatalytic application( 2017)Water pollution is one of the most pressing issues affecting society, consequently using titanium dioxide (TiO2) as a photocatalyst for the treatment of polluted water has attracted immense attention over past decades. However, low photocatalytic performance as a result of the fast recombination of photogenerated electron-hole pairs, few active sites and poor light utilization has restrained its real application. This thesis reports the synthesis of various novel TiO2 photocatalysts with high crystallinity and tailored nanostructures obtained by sol-gel chemistry, templating, self-assembly, solvothermal treatment and calcination. Mixed-phased hierarchically porous TiO2 networks (PTN) were prepared through sol-gel chemistry and a templating technique, followed by calcination. The PTN materials possessed reduced contact areas between TiO2 nanocrystals, significantly retarding the anatase to rutile transformation and rutile crystal growth. Compared to control samples prepared without the template, hierarchical PTN materials showed enhanced photocatalytic activity towards the degradation of methylene blue (MB) under UV light illumination. The material calcined at 600 °C for 6 h contained 15.4 % rutile and had a specific surface area of 32.2 m2 g-1, giving the highest photocatalytic activity. This enhancement was attributed to optimal rutile content and increased active sites resulting from the high surface area. Micrometer-size, monodisperse amorphous TiO2 spheres with controllable sizes were fabricated through a sol-gel process. The monodispersity, spherical shape and size were tuned by varying experimental parameters including the amount of structure-directing hexadecylamine, salt species and concentration, water amount and reaction temperature. The diameter of the spheres was determined by a competitive process between the solubility of Ti oligomers and the hydrolysis rate of titanium isopropoxide, the TiO2 precursor. Spheres with diameters up to 5.39 ± 0.68 um were achieved. The amorphous TiO2 spheres were readily converted by a solvothermal treatment and calcination process to anatase TiO2 spheres with three fascinating morphologies: ‘fluffy’ core/shell, yolk/shell and hollow nanostructures. Direct evidence was found that a surface seeding and subsequent inwards hollowing through an Ostwald ripening process lead to the formation of diverse nanostructures. The hollow microsphere calcined at 650 °C displayed a higher degradation MB rate than the benchmark, commercial Degussa (Evonik) P25. The superior photocatalytic activity of the anatase hollow structures resulted from the unique hollow structure, hierarchically porous shell and high crystallinity. The amorphous TiO2 spheres were also readily converted by a solvothermal process to pure anatase TiO2 with high thermal stability. The resultant microspheres were composed of well-crystallized anatase nanocrystals with a uniform size of 24 nm and a 77 nm pore after calcination at 900 °C. The superior thermal stability was primarily attributed to increased Ti-O-Ti bond strength and narrow crystal size distribution. Microspheres calcined at 800 or 900 °C displayed higher photocatalytic performance than P25 treated at the same temperatures. The excellent performance of the microspheres was attributed to the retention of anatase phase, presence of large pores, high crystallinity and high surface area. Overall, TiO2 photocatalyst nanostructures were fabricated by sol-gel chemistry, templating, self-assembly, solvothermal and calcination processes, and exhibited UV light photocatalytic activity that surpassed P25.
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ItemFabrication of PVDF–TiO2 electrospun membranes incorporating with carbon nitride for solar fuel production and organic pollutant photodecomposition( 2016)Semiconductor–mediated photocatalysis for the decomposition of pollutants and production of industrially important species, i.e., methane by photoreduction of CO2 (g) is an emerging technology. However, problems, including low quantum efficiency, visible light inactivity and the difficulty to deploy and recover the photocatalyst, have to be mitigated. In order to enhance the photocatalytic activity of titanium dioxide, the sensitization of TiO2 with visible light active carbon nitrides (CNx) was proposed. Nonetheless, as an important step in the fabrication of a photocatalytic device, the integration of photocatalytic nanoparticles into a solid matrix, such as an electrospun fibrous membrane, forms a research objective in this thesis. A low temperature synthesis route to fabricate TiO2 nanoparticles with different crystal phase compositions was developed. The Ti–precursor concentration (9–45 mM) and the presence of Cl– during hydrothermal treatment influenced the TiO2 crystal phase composition. Overall, anatase–rich TiO2 samples showed higher photocatalytic decomposition activity than rutile–rich samples. However, all samples and a commercial TiO2 reference produced only trace amounts of methane during CO2 photoreduction. A polyvinylidene fluoride (PVDF)–TiO2 nanocomposite was fabricated by electrospinning followed by a low temperature hydrothermal treatment to induce the in situ growth of TiO2 nanoparticles on the electrospun PVDF nanofibres. The crystal phase composition of TiO2 was tuned by manipulating the concentration of the Ti–precursor (0.030–0.125 M) and acidity (pH <0–6.5) in the hydrothermal solution. The surface accessibility, crystal phase composition and the presence of Ti3+ within the nanocomposite significantly influenced the photocatalytic activity for CO2 reduction and organic pollutant decomposition. The maximum production of methane was 19.8 µmol per gram of photocatalyst per hour (quantum efficiency for the photomethanation reaction, Q. E.CH4 : 0.44 %) under UV irradiation. The visible light absorption of the PVDF–TiO2 nanocomposite was enhanced by the addition of CNx. A facile, low temperature wet–chemical synthesis was developed for CNx. The synthesized CNx possessed C=O functional groups that resulted in a negatively charged surface across pH 3–9, and led to an enhanced adsorption capacity and organic pollutant photodegradation under visible light irradiation. CNx also showed a relatively high capacity for heavy metal ion adsorption. Unfortunately, the CNx particles were too large for successful incorporation into the PVDF–TiO2 nanofibres. As an alternative, graphitic–CNx quantum dots (g–CNQDs) were synthesized by microwave heating, and were introduced into the PVDF–TiO2 nanofibres during electrospinning. The g–CNQDs were evenly distributed along the nanofibres, and significantly extended the photoresponse of the nanocomposite into the visible range. Methane production from CO2 photoreduction increased with the amount of g–CNQDs incorporated into the nanocomposite, with a maximum production of 39.8 µmol of methane per gram of photocatalyst per hour (Q.E.CH4 = 0.58%) under simulated sunlight irradiation.
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ItemStructured photocatalysts for solar energy storage with improved boundary layer mass transfer( 2016)As solar energy contributes more of our energy needs, energy storage becomes critical. There are two main strategies for solar energy storage; in batteries or as chemical energy. These two strategies fit differently into future renewable energy infrastructures. Batteries suit small, distributed photovoltaic installations such as rooftop solar panels. They are less suited to industrial scale installations not attached to the grid. Just as green plants store energy whilst the sun shines, and use it overnight, we need systems that efficiently store the solar energy as chemical energy in forms that can readily be converted back to electrical energy as required. Methods for capturing and storing solar energy using photocatalytic as against photovoltaic cells include water-splitting, conversion of carbon dioxide to methane, methanol or hydrocarbons, and direct recharging of redox flow batteries. All of these have the same requirements as leaves – light capture, the need to flow liquids through the cells with a minimum of pumping energy, and molecules that store the energy in a useful way. The capture and storage of solar energy is an active field. Focus in this work was on methods of energy storage other than conventional batteries, and on work using variations of the Z-Scheme approach to light capture as it allows a greater proportion of the available spectrum to be captured. For water splitting, closely coupled schemes, where hydrogen and oxygen are generated simultaneously, were considered too dangerous to scale up and so were largely ignored. The Z-Scheme can be split into an oxidation reactor and a reduction reactor, with a recirculating shuttle molecule. In water splitting these would generate oxygen and hydrogen gas respectively. This allows separate photocatalysts to be used for the two reactions. In this thesis, activated tungsten oxide was chosen as the photo-oxidation catalyst due to its high selectivity for converting ferric ions to oxygen and ferrous ions, and anatase was selected as the basis of the photoreduction catalyst because of its low cost, availability, and the large amount of work carried out controlling surface area, porosity and chemistry of anatase photocatalysts. The photocatalysts were screened as slurries of dispersed beads and particles. This allowed rapid and simple photocatalyst preparation, but did not allow continuous reactor operation due to the need to separate the catalyst from the reactant solution. Catalysts were characterised by nitrogen adsorption for surface area and porosity, scanning and transmission electron microscopy for particle size, crystal size and morphology. Spectrometry was used to look at the absorption and scattering of light by the catalysts, and to monitor progress of reaction colorimetrically. Colorimetric actinometry was used to calibrate the light source, a mercury-xenon arc lamp. Methylene blue decolourization was used to follow the reduction reaction, and ferrous and ferric bipyridyl complexes were used to follow the oxidation reaction. Results from slurry reactors proved difficult to analyse. Calculations were carried out that showed that slurries did not behave as simply as assumed in most of the literature. In particular there was evidence of mass transfer limitation from the bulk liquid to the photocatalyst surface, even in well-stirred reactors, and variability in light capture and light loss by transmission and back scattering introduced significant errors. A 65 mm diameter heterogeneous reactor was designed and built to study continuous reaction. The initial design used thin film photocatalysts mounted on fused quartz, produced by evaporative casting. Data were logged continuously using an in-line cuvette connected to a CCD spectrophotometer. This reactor system was effective, with high reproducibility. The main source of errors was bubbles passing through the cuvette. Using this reactor, reaction rates were shown to be limited by boundary layer diffusion between the flowing liquid and the catalyst, not diffusion within the catalyst layer as had been expected. The reaction appeared to follow Langmuir-Hinshelwood kinetics closely, where the diffusion limitation depends in part on the binding of the substrate to the photocatalyst surface. So diffusion had a much greater impact on the bleaching of the weakly binding methylene blue over anatase than on the strongly binding ferric ions over tungsten oxide. To overcome boundary layer diffusion it is necessary to develop catalysts structured to optimise both light capture and mass transfer. To retain the advantages of thin film casting, engineering approaches to reducing boundary layer diffusion were developed. Designs tested included the “Leaf” reactor and a static mixer. Both were produced using 3D printing. Little or no improvement over the standard thin film reactors was observed, probably because the resolution of the printer was not fine enough (a target of less than 180 μm was estimated from thin film results, with the effective printer resolution being greater than 1,500 μm). A more successful approach was the development of structured catalysts based on supporting the active components on a felted quartz mat. Unlike glass fibre supported photocatalysts reported in the literature, the quartz mat fibres were laid down in a more random and open structure that did not bias liquid flow. Supported photocatalysts were prepared using evaporative casting, and growing thin films onto the fibres using sol-gel or aqueous methods. High reaction rates per unit area were obtained using structured catalysts, although some diffusion limitation remained at practical flowrates. The results obtained clearly demonstrate that the next generation of photocatalytic cells must be designed and engineered to further reduce or eliminate diffusion limitation while maintaining low flow resistance and hence energy lost to pumping. This should combine finite element modelling, improved methods of coating substrates with photocatalysts, and optimisation of catalyst activity by controlling surface area, porosity and light capture to ensure efficient use of as much of the solar spectrum as possible. The most cost effective reactions for energy storage, transport and recovery have yet to be identified.
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ItemPorous titania-based composite materials and their high-throughput photocatalytic evaluation for environmental remediation( 2016)Semiconductor-mediated photocatalysis is a promising technology for environmental remediation. Among various materials, titania is a well-known photocatalyst, yet much improvement is still required to further improve its activity. This thesis presents some approaches used to optimize the photocatalytic activity of porous titania-based materials that are physically viable for practical operations. Specifically, the effect of the addition of nitrogen during synthesis and silver nanoparticles combined with various templating methods were examined. A high-throughput testing system based on parallelization and miniaturization of methylene blue photodegradation reactions was also developed to facilitate the photocatalytic evaluation in an efficient manner. Hierarchically porous, anatase titania thin films of varied thickness were fabricated by a one-pot, soft-templating technique combined with a phase separation route. The pore structure was readily tuned by adjusting the concentration of the polymeric components added during the sol-gel synthesis. Poly(vinylpyrrolidone) (PVP) altered the three dimensional pore structure, generating macroporous networks within the films. The highest photocatalytic activity under UV irradiation normalized by the accessible surface area was obtained by porous titania thin films prepared using 1:1:0 poly(ethylene glycol):PVP:F127. The addition of F127 did increase the overall photocatalytic activity, but lowered the activity per unit area because of obstructed light penetration. In order to effectively utilize the visible light, mesoporous anatase titania with nitrogen doping was prepared by a template-free, sol-gel synthesis route. The effect of calcination conditions and the type of titania precursor were investigated, highlighting their profound influence upon the adsorption and visible light activity. The titania crystallization in the presence of nitrogen was also studied using in situ synchrotron powder diffraction. The nitrogen modified titania prepared from titanium (IV) butoxide and diethanolamine calcined at 350 °C for 10 h exhibited a high methylene blue adsorption capacity (85 mg g-1) and high photocatalytic activity under visible light. The prominent photocatalytic performance was attributed to the synergetic effect from the abundant nitrogen content (10.91 at. %), relatively high specific surface area (154.8 m2 g-1), and enhanced surface acidity (isoelectric point ≈ 2.7). To further enhance the practicality of the titania composites with nitrogen modification, the synthesis method was then extended to obtain porous monolithic structures. The goal of this study was to investigate the relationship between the photocatalytic activity and the diverse porous morphologies produced using the phase separation route and agarose gel templating. The amount of polymer used in the phase separation induced monoliths and the infiltration method in the preparation of agarose templated monoliths were shown to affect both the physicochemical and optical properties. This comparative study showed that the highest UV and visible light activity for methylene blue removal was achieved by the agarose-templated monoliths that were infiltrated at 60 °C. This was accredited to their higher surface area and higher nitrogen content compared to those of the monoliths obtained from phase separation. The addition of nitrogen and silver nanoparticles was carried out simultaneously with a hard templating technique using silica spheres packed into a three dimensional “opal” structure to further optimize the performance of titania under visible light. All of the opal templated samples in this work performed better than the commercial titania, P25. The highest photocatalytic enhancement, showing more than eight times higher activity than the non-modified titania, was achieved by the opal templated sample prepared with 1.0 mol % of silver. Although both the nitrogen and silver addition and templating enhanced the visible light activity, the most significant improvement was afforded by the utilization of the silica opal template that gave rise to high surface area (>100 m2 g-1) and promoted the surface charge interaction.
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ItemEnergy efficiency and advantages of ultrasonic synthesis of nanomaterials( 2015)The physico-chemical effects of ultrasound (US) have been used widely for synthesising various materials. The focus of this project is to evaluate the energy efficiency and advantages of ultrasonic synthetic process. Poly(methyl methacrylate) and poly(methyl methacrylate)-CaCO3 nanocomposites were synthesised by conventional and US-assisted (USK) emulsion polymerization. Although the conversions obtained were similar for both processes, nanocomposites produced by USK were smaller with a narrower particle size distribution. In another study, the photocatalytic activity of CdS nanoparticles synthesized using US were compared with those synthesized using mechanical agitation on the basis of energy input. Samples synthesized using a US horn (USH) and a high shear homogeniser (HSH) showed a lower photocatalytic activity compared to those synthesized in an US bath (USB) and using mechanical stirring (NUS). However, when the power input per unit volume (W/L) is considered, the order of effectiveness of the catalysts is USB>NUS>HSH>USH, suggesting that the mild cavitation conditions generated in the USB process are sufficient to produce an efficient photocatalyst. Overall, US assistance provides improvement in conversions/yields and the dispersive effects help obtain smaller particle sizes and narrower size distributions. However, when the increased energy requirements are taken into account it is obvious that when combining US with conventional material synthesis techniques, it is imperative to choose not only the right amount of energy input but also, the right mode of US input in order to synthesize the most efficacious nanomaterials.