Visible light response semiconductor nanomaterials for heterogeneous photocatalysis in liquid phase

The development of sustainable and green technologies powered by renewable energy sources is highly desired to address the growing global energy need and water scarcity problems. Heterogeneous photocatalysis emerged in the past decades as promising solar-powered technology for environmental remediation applications such as wastewater treatment. The photoactivity of the materials is believed to be governed by complex mechanisms, still it was shown that it may be critically dependent on the following material properties (i) ability and effectiveness to absorb incident photons, (ii) charge separation efficiency, (iii) charge utilization efficiency, (iv) morphology including the size and shape of the nanostructure and its distribution and (v) the crystal structure, phase composition and crystallinity...etc Hence, most strategies aiming to improve the performance of photocatalytic materials may focus on one or more of the aforementioned aspects. Beside developing new materials or modifying existing systems, the development of sustainable, easy-to-operate systems are highly desired for developing countries such as Africa where almost half of the population are affected by water scarcity of some sort. For this motivation the immobilization of powder catalyst could be one attractive solution. In this thesis three experimental systems are presented. In the first two the effect of material properties on the photoactivity whereas in the third chapter the immobilization of powder catalyst was investigated. The first experimental project aimed to study the effect of synthesis parameters of WO3 nanostructures on its morphology, phase composition, optical properties and ultimately on the photoactivity. Understanding the role of process parameters to gain control over the material properties is still a challenge but is of great interest in photocatalysis. Here, a hydrothermal synthesis method was employed to synthesize WO3 nanostructures with various morphologies, crystal phases and optical properties. The effect of the solution pH, the polymeric surface modulator and the added EtOH was investigated on the material properties and on the photocatalytic activities. It was found that the crystal structure and the morphology of WO3 was influenced by the solution pH in the first place. It was proposed that stabilization effects between the crystal phase and the morphology could also influence the crystallization process beside supersaturation. It was revealed that despite the highest surface area of W-2.01-P20E, reduced oxidation state did not promote high photo-response. Instead the photoactivity of WO3 was seen as the compromise of the material properties including the optical, structural properties and the oxidation state. In the second experimental project the effect of Ag co-catalysis was studied on TiO2- Cu2O heterostructure formation. Coupling a wide band gap (TiO2) and a narrow band gap (Cu2O) semiconductor could benefit from extended light absorption properties and additionally from enhanced charged separation. In this study a facile wet chemical synthesis method was coupled with a UV treatment step to fabricate TiO2-Ag-CuxO ternary hybrid nano-materials. The effect of the Ag loading (1-5%) and the synthesis sequence of the Ag deposition step was evaluated on the material properties as well as on the visible photocatalytic activity. It was revealed that both the amount and the order of the Ag-deposition altered the material properties considerably. Typically TiO2/CuxO/Ag (TCA) catalysts had better visible light absorption properties but reduced affinity to adsorb methyl orange (MO) to their surface. Whereas, TiO2/Ag/CuxO (TAC) catalysts in general had better dye adsorption properties relative to TCA and had more efficient decoloration properties under visible light. TOC and HPLC-MS analysis revealed that MO and possibly its degradation products were mainly mineralized and/or adsorbed to the surface of TAC catalyst with 5% nominal Ag content in the visible process generating limited amount of byproducts in the final solution. The third experimental project focused on the immobilization of the previously prepared powder TiO2-Cu2O nanostructure. In this work a fluorine-doped tin oxide (FTO) glass sheet was used as a substrate and the doctor-blade coating technique has been employed to make TiO2-Cu2O thin films. Although this technique has a widespread use in the fabrication of solar cells to the best of our knowledge this is the first report on supported TiO2-Cu2O photocatalytic systems prepared by this method. To optimize the performance of the TiO2- Cu2O thin film under visible light irradiation, the chemical composition of the doctor-blading paste and the temperature of the final thermal treatment step was studied. It was found that both the paste composition and the heat treatment step played an important role in the material properties. When the film contained ethyl cellulose the minimum temperature to remove organic additives was 350 ◦C. Whereas for the films containing only alpha terpineol 300 ◦C was sufficient. It was revealed that the higher temperature treatment resulted in more oxidized films which were also shown in their deeper colour. The most effective film under visible light irradiation was TC-0-300 which contained no cellulose and was treated at the lowest temperature

Doped TiO2 Nanowires for Applications in Dye Sensitized Solar Cells and Sacrifical Hydrogen Production

This thesis explores the synthesis of metal oxide 1-D nanowires using a sol-gel method in supercritical carbon dioxide (sc-CO2), as an environmental friendly enabling solvent. Porous nanowires were synthesized and their performance was tested in dye sensitized solar cell and sacrifical hydrogen production. Titanium isopropoxide (TIP) was used as a precursor for titania synthesis while copper, bismuth and indium were examined as dopants, respectively. The sol-gel reactions were catalyzed by acetic acid in CO2 at a temperature of 60 °C and pressure of 5000 psi. It was observed that acetic acid/monomer ratio \u3e 4 produced nanowires while a lower ratio led to the formation of various morphologies, depending on reaction conditions. The synthesized undoped and doped nanowires were characterized by electron microscopy (SEM and TEM), N2 physisorption, FTIR, XRD, XPS, thermal analysis. These results showed high aspect ratio nanowires as observed by SEM (15-25) with surface areas ranging from 40 to 126 m2/g. These surface areas are comparable and sometimes exceeded the surface area of Degussa P25 (i.e. 50 m2/g). Copper doped nanowires were examined as sacrificial photocatalytic catalysts in ‎Chapter 3. It was found that the modified sol-gel synthesis approach in supercritical CO2 produced primarily a single oxidation state (Cu (I)) of active Cu2O/TiO2, which was confirmed by XPS and XANES analyses. Wt % of copper and initial concentration of sacrificial agent were optimized to enhance hydrogen production with the results compared to undoped titanium nanowires and Degussa P25. The Cu2O/TiO2 nanowires showed improved hydrogen production at 1 % Cu (I) loading which produced about 10 times more hydrogen than Degussa P25 and 47 times more than undoped nanowires, respectively. ‎Chapter 4 discusses the synthesis and application of Indium (In) doped titania for DSSCs and sacrificial hydrogen production. Indium has high conductivity, transparency for visible light, and good electron mobility, making it an attractive dopant for such applications. In ‎Chapter 5, sacrificial hydrogen production by bismuth titanate nanowires using formaldehyde as the sacrificial agent is examined. Bismuth titanate was previously shown theoretically to have all the requirements to be an effective photocatalyst to produce hydrogen when doped with transition metals. However, little experimental work has been reported on the performance of bismuth titanate nanowires. Bismuth titanate nanowires were synthesized using a sol-gel methodology in supercritical CO2 with different levels of bismuth loading (1, 1.4, and 2 mol % bismuth). These nanowires were investigated for their sacrificial photocatalytic hydrogen production, which was compared to that of undoped titanium nanowires and P25. ‎Chapter 6 deals with the experimental results of bismuth titanate, which is known as an active visible light photocatalyst, with most of its applications focused on remediation of water and wastewater. However, very limited applications of bismuth titanate in DSSCs have been reported in the literature. The effect of Bi loading on TiO2 nanowires for DSSCs was investigated by testing the J-V curves of the solar cells. The influence of Bi on the internal processes of electron transport was also evaluated by electrochemical impedance spectroscopy. In order to integrate these nanowires into polymeric systems for easy processability and application, the synthesis of polysulfone polymers was examined in ‎Chapter 7. These polymers were designed to contain carboxylic functional groups to enable coordination with titanium and titanium doped photocatalyst. The goal was to produce an organic-inorganic membrane for photocatalytic membrane reactor that can reduce fouling by degradation of pollutants. The membrane was also examined to be cast as nanotubes using an anodic aluminum oxide (AAO) template method

METAL OXIDE HETEROSTRUCTURES FOR EFFICIENT PHOTOCATALYSTS

Photocatalytic processes over semiconducting oxide surfaces have attracted worldwide attention as potentially efficient, environmentally friendly and low cost methods for water/air purification as well as for renewable hydrogen production. However, some limitations to achieve high photocatalytic efficiencies have been found due to the fast recombination of the charge carriers. Development of heterostucture photocatalysts by depositing metals on the surface of semiconductors or by coupling two semiconductors with suitable band edge position can reduce recombination phenomena by vectorial transfer of charge carriers. To draw new prospects in this domain, three different kinds of heterostructures such as n-type/n-type semiconductor (SnO2/ZnO), metal/n-type semiconductor (RuO2/TiO2 and RuO2/ZnO) and p-type/n-type semiconductor (NiO/TiO2) heterojunction nanomaterials were successfully prepared by solution process. Their composition, texture, structure and morphology were thoroughly characterized by FTIR, X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM) and N2 sorption measurements. On the other hand, a suitable combination of UV–visible diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS) data provided the energy band diagram for each system. The as-prepared heterojunction photocatalysts showed higher photocatalytic efficiency than P25 TiO2 for the degradation of organic dyes (i.e. methylene blue and methyl orange) and the production of hydrogen. Particularly, heterostructure RuO2/TiO2 and NiO/TiO2 nanocomposites with optimum loading of RuO2 (5 wt %) and NiO (1 wt %), respectively, yielded the highest photocatalytic activities for the production of hydrogen. These enhanced performances were rationalized in terms of suitable band alignment as evidenced by XPS/UPS measurements along with their good textural and structural properties. This concept of semiconducting heterojunction nanocatalysts with high photocatlytic activity should find industrial application in the future to remove undesirable organics from the environment and to produce renewable hydrogen

Multifunctional Oxide-Based Materials: From Synthesis to Application

The book deals with novel aspects and perspectives in metal oxide and hybrid material fabrication

Magnetic field directed self-assembly of gold Pickering emulsion for preparing patterned film.

Patterning plays a vital in role in sensor-based devices like surface-enhanced Raman spectroscopy (SERS), surface-enhanced infrared absorption (SEIRA), radio frequency (RF) antennas and many others. The linear array spacing and width of gold strips has been shown to increase the local intensity through near-field coupling with diffracted electromagnetic waves. This rise in local charge boosts vibrational energies of molecules in close-surface contact or proximity, resulting in increased IR absorption. The strip-like or any other types of patterns are efficiently achieved through top-down nanofabrication processes like atomic-force-deposition, nanoimprinting, UV-Lithography etc., which involve high capital cost, complex processing and occasionally low throughput. This research was therefore undertaken with the aim of reducing the process complexities and improving scalability, by applying a magnetic and spin coating directed self-assembly (MSCDS) to prepare optically sensitive dipole-dipole chain-like ordered arrays of the gold nanoparticle Pickering ferrofluid in polyvinyl alcohol (PVA) emulsion, in the form of a thin film on glass and silicon substrate. Previously-conducted MSCDS processes lacked the control over the dimensions of the prepared patterns. Here, the static magnetic field approach was taken to modify the MSCDS process to overcome the limitation of pattern dimension control, providing tuneability for optical applications. Quantitative image analysis of the patterned thin film allowed for the measurement of pattern geometrical dimension (chain length-CL, chain gap-CG and chain thickness-CT), which was then correlated with processing parameters such as magnetic field configurations (single, compound and concentric), spinning speeds and viscosities of Pickering emulsion. Upon optimization, spectroscopical characterisation was performed on prepared patterned thin film to demonstrate the capability of the modified MSDS process in enhancing the molecular detection at low concentrations. The UV-vis spectra of the patterns demonstrated the impact of CT and CG on the degree of gold-iron oxide nanoscale interactions leading to tuneability of absorption bands between 390-700nm. The coupling of the increased optical sensitivity through enhanced charge transfer dynamics with the mid-infra-red range grating order (CT+CG) resulted in an amplification in vibrational band excitation of molecular bonds. For example, SEIRA measurements of thin film patterns showed a vibrational signal enhancement in asymmetric vibration of -CH2 (2920cm-1) bonds of PVA by 40%, as CT increased by 178% from 1.2μm at probing 45 degree grazing angle. Furthermore, the magneto-optical SERS phenomenon - involving local polarization of gold nanoparticles through the neighbouring magnetised iron oxide nanoparticle in the presence of external magnetic field - was exploited to reveal the varying degree of enhancement in peaks related to Rhodamine 6G (R6G) coated on thin film nanostructure, which was dependent on magnetized CT/CG morphology; especially the C-C-C ring (671 cm-1), for which the Raman peak increased by 12,000% when magnetized by a 43mT field. In summary, the modified MSCDC process is cheap with an expandable throughput rate ( > 0.1 m2/h) and flexible designs, offering both nanoscale and microscale tuneability of pattern dimensions. Even with higher defectivity (~14%) in comparison to the nanoimprinting method, this method can potentially be used to create repetitive array-like structure. Furthermore, the use of iron oxide reduces the cost without sacrificing the optical performance and thus contributes to the optical tuneability of the thin film nanostructure, thereby making the entire product a potential absorbing antenna and microfluidics thin film for biomolecule detection

From nanoparticle networks to metal-organic frameworks: synthesis, structural engineering and applications

Nanoparticle networks, self-assembled from flame generated hot aerosols consisting of ceramic nanoparticles with well-controlled particle size, are promising materials for many different applications, especially for photodetectors and VOC sensors. Furthermore, the great structural flexibilities of these self-assembled nanoparticle networks including tuneable thickness and hierarchical porosity, precisely-controlled averaged particle size as well as chemical composition make them as potential platforms for templated materials synthesis via chemical conversion. On the other hand, metal-organic framework (MOF), is a growing family of microporous materials consisting of metal cations connected by organic linkers. Their unique properties, including a narrow pore size distribution (intrinsic porosity), designable topology, high accessible surface area, and chemical mutability, make MOFs promising materials for a variety of applications including gas storage, separation, catalysis, biotechnology, optics, microelectronics and energy production/storage. However, there are still several bottlenecks hindering the structural engineering of metal-organic frameworks, especially for pure crystalline MOF materials, including limited attainable thickness, scalability, poor mechanical stability (i.e. brittle nature of MOFs), hard to realize the morphological control (e.g. tuneable extrinsic hierarchical porosity) and geometric designs on pure crystalline MOF components. Thus, a facile synthetic approach for MOF structuring is highly desirable, which could afford the fabrication of three-dimensional MOF materials with possibly unlimited thickness, free-standing feature, the control over extrinsic hierarchy as well as pre-determined designs of MOFs while maintain their crystalline property and intrinsic extreme accessible surfaces. Firstly, we started with the synthesis of pure ZnO nanoparticle networks and the optimization of their particle size. Later, using the ZnO nanoparticle networks with an optimal particle size, a high-performing UV photodetector has been prepared to show a proof of concept application of such structural engineering. After achieving the first structural control over ZnO nanoparticle networks, a multi-dimensional control has been further investigated associated with its potential use for multi-functional devices including transparent conductive oxides and gas sensors. Given the successful structural control over nanoparticle networks, considering the existing bottlenecks in current MOF fabrication, this multi-dimensional structural control has been successfully replicated to MOF preparation via a means of gas phase conversion. Therefore, in this thesis, a systematic study has been presented from the synthesis and applications of nanoparticle networks to those of metal-organic frameworks in the sequence of: (i) the synthesis of three-dimensional nanoparticle networks (i.e. ZnO-based metal oxide nanoparticle networks), (ii) the realization of a precise particle size control over the synthesized nanoparticle networks (e.g. ZnO) and the use of resulted optimal structure for photodetector application, (iii) the realization of chemical composition manipulation over the synthesized nanoparticle networks (e.g. ZnO nanoparticle networks with varied Al doping concentrations) and the use of the resulted structures as proof of concept applications for both porous conductive electrodes and VOC sensor, (iv) the establishment of a synthetic pathway from nanoparticle networks to metal-organic frameworks based on the replication of the structural control over nanoparticle networks towards metal-organic frameworks, and the proof of concept application of the resulted free-standing metal-organic frameworks monolith for effective molecular sieve in batteries, and (v) the use of the established fabrication approach (i.e. from nanoparticle networks to metal-organic frameworks) for monolithic metal-organic framework patterning

Development of new devices for time-resolved Raman spectroelectrochemistry

La Tesis Doctoral desarrollada por D. David Ibáñez Martínez que lleva por título “Development of new devices for time-resolved Raman spectroelectrochemistry” ha sido realizada en el área de Química Analítica de la Universidad de Burgos y dirigida por los doctores Álvaro Colina Santamaría y Mª Aránzazu Heras Vidaurre. En este trabajo se han desarrollado nuevas celdas y dispositivos espectroelectroquímicos que permiten el estudio tanto de nuevos materiales como de mecanismos químicos de reacción complejos difícilmente abordables mediante otras técnicas de análisis convencionales. El gran avance instrumental desarrollado en esta Tesis ha permitido el estudio, mediante espectroelectroquímica Raman, de diversos sistemas de gran interés para la sociedad, tales como polímeros conductores, nanotubos de carbono, nanopartículas metálicas, bases del ADN o interfases entre dos disoluciones inmiscibles

Nanofabrication

We face many challenges in the 21st century, such as sustainably meeting the world's growing demand for energy and consumer goods. I believe that new developments in science and technology will help solve many of these problems. Nanofabrication is one of the keys to the development of novel materials, devices and systems. Precise control of nanomaterials, nanostructures, nanodevices and their performances is essential for future innovations in technology. The book "Nanofabrication" provides the latest research developments in nanofabrication of organic and inorganic materials, biomaterials and hybrid materials. I hope that "Nanofabrication" will contribute to creating a brighter future for the next generation

SYNTHESIS OF INTEGRATED NANOCATALYSTS WITH MESOPOROUS SILICA/SILICATE AND MICROPOROUS MOFS

Ph.DDOCTOR OF PHILOSOPH