Atomic Layer Deposition-Based Synthesis of Photoactive TiO2 Nanoparticle Chains by Using Carbon Nanotubes as Sacrificial Templates

Highly ordered and self supported anatase TiO2 nanoparticle chains were fabricated by calcining conformally TiO2 coated multi-walled carbon nanotubes (MWCNTs). During annealing, the thin tubular TiO2 coating that was deposited onto the MWCNTs by atomic layer deposition (ALD) was transformed into chains of TiO2 nanoparticles (~12 nm diameter) with an ultrahigh surface area (137 cm2 per cm2 of substrate), while at the same time the carbon from the MWCNTs was removed. Photocatalytic tests on the degradation of acetaldehyde proved that these forests of TiO2 nanoparticle chains are highly photo active under UV light because of their well crystallized anatase phase

Plasmonic gold-embedded TiO2 thin films as photocatalytic self-cleaning coatings

Transparent photocatalytic TiO2 thin films hold great potential in the development of self-cleaning glass surfaces, but suffer from a poor visible light response that hinders the application under actual sunlight. To alleviate this problem, the photocatalytic film can be modified with plasmonic nanoparticles that interact very effectively with visible light. Since the plasmonic effect is strongly concentrated in the near surroundings of the nanoparticle surface, an approach is presented to embed the plasmonic nanostructures in the TiO2 matrix itself, rather than deposit them loosely on the surface. This way the interaction interface is maximised and the plasmonic effect can be fully exploited. In this study, pre-fabricated gold nanoparticles are made compatible with the organic medium of a TiO2 solgel coating suspension, resulting in a one-pot coating suspension. After spin coating, homogeneous, smooth, highly transparent and photoactive gold-embedded anatase thin films are obtained

Photocatalytic acetaldehyde oxidation in air using spacious TiO2 films prepared by atomic layer deposition on supported carbonaceous sacrificial templates

Supported carbon nanosheets and carbon nanotubes served as sacrificial templates for preparing spacious TiO2 photocatalytic thin films. Amorphous TiO2 was deposited conformally on the carbonaceous template material by atomic layer deposition (ALD). Upon calcination at 550{\deg}C, the carbon template was oxidatively removed and the as-deposited continuous amorphous TiO2 layers transformed into interlinked anatase nanoparticles with an overall morphology commensurate to the original template structure. The effect of type of template, number of ALD cycles and gas residence time of pollutant on the photocatalytic activity, as well as the stability of the photocatalytic performance of these thin films was investigated. The TiO2 films exhibited excellent photocatalytic activity towards photocatalytic degradation of acetaldehyde in air as a model reaction for photocatalytic indoor air pollution abatement. Optimized films outperformed a reference film of commercial PC500

Synthesis and Characterization of Photoreactive TiO2/Carbon Nanosheet Composites

We report the atomic layer deposition of titanium dioxide on carbon nanosheet templates and investigate the effects of post-deposition annealing in a helium environment using different characterization techniques. The crystallization of the titanium dioxide coating upon annealing is observed using in-situ X-ray diffraction. The (micro)-structural characterization of the films is carried out by scanning electron microscopy and advanced transmission electron microscopy techniques. Our study shows that the annealing of the atomic layer deposition processed and carbon nanosheets templated titanium dioxide layers in helium environment results in the formation of a porous, nanocrystalline and photocatalytically active titanium dioxide-carbon nanosheet composite film. Such composites are suitable for photocatalysis and dye-sensitized solar cells applications

Plasmonic near-field localization of silver core-shell nanoparticle assemblies via wet chemistry nanogap engineering

Silver nanoparticles are widely used in the field of plasmonics because of their unique optical properties. The wavelength-dependent surface plasmon resonance gives rise to a strongly enhanced electromagnetic field, especially at so-called hot spots located in the nanogap in-between metal nanoparticle assemblies. Therefore, the interparticle distance is a decisive factor in plasmonic applications, such as surface-enhanced Raman spectroscopy (SERS). In this study, the aim is to engineer this interparticle distance for silver nanospheres using a convenient wet-chemical approach and to predict and quantify the corresponding enhancement factor using both theoretical and experimental tools. This was done by building a tunable ultrathin polymer shell around the nanoparticles using the layer-by-layer method, in which the polymer shell acts as the separating interparticle spacer layer. Comparison of different theoretical approaches and corroborating the results with SERS analytical experiments using silver and silver polymer core shell nanoparticle clusters as SERS substrates was also done. Herewith, an approach is provided to estimate the extent of plasmonic near-field enhancement both theoretically as well as experimentally

<tex>TiO_{2}$</tex> gas phase photocatalysis from morphological design to plasmonic enhancement

The past decades, photocatalysis has emerged as a powerful technology for pollution abatement and energy applications, but its use in gaseous environment is less obvious than in water borne applications. Therefore this thesis specifically aims at understanding and improving TiO2-based photocatalysis in all its facets towards gas phase processes. In first instance a suitable photoreactor is developed and validated. A simple suspension-based coating strategy enables to immobilize powderous photocatalytic materials onto glass bead supports that are packed around a light source in a cylindrical glass reactor tube. The presented design offers several advantages such as good catalyst immobilization, efficientlight utilization, intimate contact with gaseous pollutants and a catalyst weight gain by a factor of 25 compared to self-supporting pellets. This glass bead photoreactor is used in a comparative study on TiO2-basedphotocatalytic materials, in which both technological and economical parameters are considered. By performing a cost effectiveness analysis, PC500 (Cristal Global) is determined to be a very active and cost efficient photocatalyst in the degradation of gaseous acetaldehyde and even outperforms the typical benchmark Aeroxide P25 catalyst (Evonik). In the next part of the work, fundamental insight is gathered in the driving factors for gas phase photocatalytic reactions by investigating the different nature of the P25 and PC500 catalysts. The high surface area andsophisticated yet accessible pore system of PC500 seem to dominate overthe superior electronic efficiency of P25. These findings are supportedby photocatalytic gas phase experiments, photocurrent measurements, N2-sorption data, X-ray diffraction patterns, UV-VIS spectroscopy, thermo-gravimetric analysis and acetaldehyde adsorption measurements, amongst others. The dominating effect of surface area on gas phase photocatalytic activity is exploited in the development of well-immobilized, spacious TiO2 thin films. These films are prepared by depositing a thin, conformal layer of TiO2 onto sacrificial carbonaceous templates by means of atomic layer deposition. The carbonaceous templates are either carbon nanosheets or multi-walled carbon nanotubes, both grown on silicon wafers. After application of a calcination step, the sacrificial template is removed, TiO2 is crystallized into the anatase phase and the as-deposited continuous TiO2 layer has transformed into an interconnected network of nanoparticles. Electron microscopy images show that the overall appearance of the films is entirely commensurate to the original template morphologies. The employed strategy allows to fabricate spacious thin films with surface area enhancement factors of up to 260 with regard to a dense, flat TiO2 film. Thus obtained thin films exhibit superior photocatalytic activity compared to a reference film consisting of the photoactive PC500 catalyst. For the testing of these thin films another type of photoreactor was developed. The use of the constructed single pass flow through, slit-shaped, flat bed photoreactor is visualized by computational fluid dynamic simulations. As an intermission the use of surface photovoltage measurements is discussed in relation to photocatalytic activity. The research question is simply whether surface photovoltage measurements can be used as a quick screening tool for assessing photocatalytic performance. The answer is less straightforward. Based on several practical case studies, the trend between photovoltage and photoactivity isinvestigated using a custom-made photovoltage set-up. For TiO2-based materials with variable anatase/rutile ratios, catalytic activity is determined to be proportional to photovoltage readings. In contrast, in the case of variable amounts of silver nanoparticles deposited on the TiO2 surface that act as electron traps under UV illumination, an inverse relation is observed. Furthermore, a significant effect of the catalyst humidity is detected, but the most important conclusion to be drawn from thisstudy is that surface photovoltage measurements can only probe electronic properties. The vast impact of morphological parameters on photocatalytic activity is not accounted for by this technique. Therefore care should be taken when interpreting surface photovoltage data in relation to photocatalysis. Finally, successful attempts have been madeto extend the TiO2 photoactivity window towards the visible light region of the spectrum. This is achieved by exploiting surface plasmon resonance effects of gold-silver alloy nanoparticles. In the first part of theresearch a theoretical, predictive model is established that enables topredict the plasmon resonance wavelength of such alloy nanoparticles, based on the combined effect of particle size and alloy composition. The model is developed using theoretical simulations of extinction spectra based on Mie theory and dielectric data from literature. The proposed model indicates that mainly the alloy composition determines the resonance wavelength, while particle size is of minor importance. In the second part of the investigation, gold-silver alloy nanoparticles are deposited on the TiO2 surface. By merely altering the alloy composition of the deposited nanoparticles, plasmonic photocatalysts can be prepared that display surface plasmon resonance in a 70 nm broad window in the visible light wavelength range that is roughly centered at the maximum intensity wavelength of solar radiation. Thus obtained plasmonic photocatalysts are tested towards their self-cleaning performance in the degradation of a solid layer of stearic acid located at the catalyst-air interface. The highest quantum efficiency is obtained when the resonance wavelength of theplasmonic catalyst exactly matches that of the incident light. This is demonstrated for the case of Au0.3Ag0.7 nanoparticles on TiO2 under 490 nm illumination, provided by a custom-made LED array. In conclusion, in this work TiO2 gas phase photocatalysis is investigated in its broadest sense. Different aspects are discussed ranging from reactordevelopment, over techno-economic analysis, morphological catalyst design, characterization, structure-activity relation, interpretation of photovoltage measurements, to visible light activity using plasmonics. We are hopeful that this altogether leads to better understanding and novel insight into this fascinating research domain.nrpages: 213status: publishe

TiO2 Gas Phase Photocatalysis from Morphological Design to Plasmonic Enhancement

The past decades, photocatalysis has emerged as a powerful technology for pollution abatement and energy applications, but its use in gaseous environment is less obvious than in water borne applications. Therefore this thesis specifically aims at understanding and improving TiO2-based photocatalysis in all its facets towards gas phase processes. In first instance a suitable photoreactor is developed and validated. A simple suspension-based coating strategy enables to immobilize powderous photocatalytic materials onto glass bead supports that are packed around a light source in a cylindrical glass reactor tube. The presented design offers several advantages such as good catalyst immobilization, efficientlight utilization, intimate contact with gaseous pollutants and a catalyst weight gain by a factor of 25 compared to self-supporting pellets. This glass bead photoreactor is used in a comparative study on TiO2-basedphotocatalytic materials, in which both technological and economical parameters are considered. By performing a cost effectiveness analysis, PC500 (Cristal Global) is determined to be a very active and cost efficient photocatalyst in the degradation of gaseous acetaldehyde and even outperforms the typical benchmark Aeroxide P25 catalyst (Evonik). In the next part of the work, fundamental insight is gathered in the driving factors for gas phase photocatalytic reactions by investigating the different nature of the P25 and PC500 catalysts. The high surface area andsophisticated yet accessible pore system of PC500 seem to dominate overthe superior electronic efficiency of P25. These findings are supportedby photocatalytic gas phase experiments, photocurrent measurements, N2-sorption data, X-ray diffraction patterns, UV-VIS spectroscopy, thermo-gravimetric analysis and acetaldehyde adsorption measurements, amongst others. The dominating effect of surface area on gas phase photocatalytic activity is exploited in the development of well-immobilized, spacious TiO2 thin films. These films are prepared by depositing a thin, conformal layer of TiO2 onto sacrificial carbonaceous templates by means of atomic layer deposition. The carbonaceous templates are either carbon nanosheets or multi-walled carbon nanotubes, both grown on silicon wafers. After application of a calcination step, the sacrificial template is removed, TiO2 is crystallized into the anatase phase and the as-deposited continuous TiO2 layer has transformed into an interconnected network of nanoparticles. Electron microscopy images show that the overall appearance of the films is entirely commensurate to the original template morphologies. The employed strategy allows to fabricate spacious thin films with surface area enhancement factors of up to 260 with regard to a dense, flat TiO2 film. Thus obtained thin films exhibit superior photocatalytic activity compared to a reference film consisting of the photoactive PC500 catalyst. For the testing of these thin films another type of photoreactor was developed. The use of the constructed single pass flow through, slit-shaped, flat bed photoreactor is visualized by computational fluid dynamic simulations. As an intermission the use of surface photovoltage measurements is discussed in relation to photocatalytic activity. The research question is simply whether surface photovoltage measurements can be used as a quick screening tool for assessing photocatalytic performance. The answer is less straightforward. Based on several practical case studies, the trend between photovoltage and photoactivity isinvestigated using a custom-made photovoltage set-up. For TiO2-based materials with variable anatase/rutile ratios, catalytic activity is determined to be proportional to photovoltage readings. In contrast, in the case of variable amounts of silver nanoparticles deposited on the TiO2 surface that act as electron traps under UV illumination, an inverse relation is observed. Furthermore, a significant effect of the catalyst humidity is detected, but the most important conclusion to be drawn from thisstudy is that surface photovoltage measurements can only probe electronic properties. The vast impact of morphological parameters on photocatalytic activity is not accounted for by this technique. Therefore care should be taken when interpreting surface photovoltage data in relation to photocatalysis. Finally, successful attempts have been madeto extend the TiO2 photoactivity window towards the visible light region of the spectrum. This is achieved by exploiting surface plasmon resonance effects of gold-silver alloy nanoparticles. In the first part of theresearch a theoretical, predictive model is established that enables topredict the plasmon resonance wavelength of such alloy nanoparticles, based on the combined effect of particle size and alloy composition. The model is developed using theoretical simulations of extinction spectra based on Mie theory and dielectric data from literature. The proposed model indicates that mainly the alloy composition determines the resonance wavelength, while particle size is of minor importance. In the second part of the investigation, gold-silver alloy nanoparticles are deposited on the TiO2 surface. By merely altering the alloy composition of the deposited nanoparticles, plasmonic photocatalysts can be prepared that display surface plasmon resonance in a 70 nm broad window in the visible light wavelength range that is roughly centered at the maximum intensity wavelength of solar radiation. Thus obtained plasmonic photocatalysts are tested towards their self-cleaning performance in the degradation of a solid layer of stearic acid located at the catalyst-air interface. The highest quantum efficiency is obtained when the resonance wavelength of theplasmonic catalyst exactly matches that of the incident light. This is demonstrated for the case of Au0.3Ag0.7 nanoparticles on TiO2 under 490 nm illumination, provided by a custom-made LED array. In conclusion, in this work TiO2 gas phase photocatalysis is investigated in its broadest sense. Different aspects are discussed ranging from reactordevelopment, over techno-economic analysis, morphological catalyst design, characterization, structure-activity relation, interpretation of photovoltage measurements, to visible light activity using plasmonics. We are hopeful that this altogether leads to better understanding and novel insight into this fascinating research domain.nrpages: 213status: publishe