The photocatalytic mechanism of TiO₂ (P25) nanoparticles coated with SiO₂ (SiO₂:TiO₂) by atomic layer deposition was investigated. The deposition of SiO₂ on TiO₂ not only gives a photocatalytic improvement for the degradation of both Rhodamine B (3.6–fold) and Acid Blue 9 (3–fold). SiO₂ deposition also changes the mechanism from direct oxidation of the pollutant at the surface of TiO₂ to a predominantly OH^⁎ radical based degradation of the pollutants originating from SiO₂:TiO₂. Low SiO₂ loadings on TiO₂, where the coating is incomplete, improve the OH^⁎ radicals generation due to the higher number of acidic Si–OH groups combined with the facilitated charge separation at the TiO₂–SiO₂ interface. As a consequence of incomplete coverage, the TiO₂ surface remains accessible, which allows both the oxidation and reduction reactions at the SiO₂:TiO₂ surface. On the other hand, high loading of SiO₂ (>3 wt.% Si) results in photocatalytic suppression due to the coverage of TiO₂ surface by SiO₂. The degradation of differently charged dyes on the SiO₂:TiO₂ surface demonstrates the independence of the adsorption properties on the photocatalytic improvement. Simultaneous degradation of two dyes demonstrated the advantage of SiO₂:TiO₂ being less selective and, therefore, better suited for general water purification.

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The promotional effects on photocatalytic hydrogen production of Cu_xO clusters deposited using atomic layer deposition (ALD) on P25 TiO₂ are presented. The structural and surface chemistry study of Cu_xO/TiO₂ samples, along with first principles density functional theory simulations, reveal the strong interaction of ALD deposited Cu_xO with TiO₂, leading to the stabilization of Cu_xO clusters on the surface; it also demonstrated substantial reduction of Ti⁴⁺ to Ti³⁺ on the surface of Cu_xO/TiO₂ samples after Cu_xO ALD. The Cu_xO/TiO₂ photocatalysts showed remarkable improvement in hydrogen productivity, with 11 times greater hydrogen production for the optimum sample compared to unmodified P25. With the combination of the hydrogen production data and characterization of Cu_xO/TiO₂ photocatalysts, we inferred that ALD deposited Cu_xO clusters have a dual promotional effect: increased charge carrier separation and improved light absorption, consistent with known copper promoted TiO₂ photocatalysts and generation of a substantial amount of surface Ti³⁺ which results in self-doping of TiO₂ and improves its photo-activity for hydrogen production. The obtained data were also employed to modify the previously proposed expanding photocatalytic area and overlap model to describe the effect of cocatalyst size and weight loading on photocatalyst activity. Comparing the trend of surface Ti³⁺ content increase and the photocatalytically promoted area, calculated with our model, suggests that the depletion zone formed around the heterojunction of Cu_xO-TiO₂ is the main active area for hydrogen production, and the hydrogen productivity of the photocatalyst depends on the surface coverage by this active area. However, the overlap of these areas suppresses the activity of the photocatalyst.

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This work presents a gas-phase approach for the synthesis of Cu₂O/TiO₂ powder-based photocatalysts using atomic layer deposition (ALD). The process is carried out in a fluidized bed reactor working at atmospheric pressure using (trimethylvinylsilyl)-hexafluoroacetulacetonate copper(I) as the Cu-precursor and H₂O vapor as the oxidizer. The saturating regime of the chemical reactions and the linear growth of ALD are achieved. In combination with the unsaturated regime, the ALD approach enables the deposition of ultrasmall Cu₂O clusters with average diameters in the range of 1.3-2.0 nm, narrow particle size distributions and tunable Cu₂O loadings on P25 TiO₂ nanoparticles. The photocatalytic performance of Cu₂O/TiO₂ photocatalysts is investigated by the degradation of organic dyes, including Rhodamine B (RhB), methyl orange, and methylene blue; the results demonstrate that the surface modification of TiO₂ nanoparticles by Cu₂O nanoclusters significantly enhances the photocatalytic activity of TiO₂. This is attributed to the efficient charge transfer between Cu₂O and TiO₂ that reduces the charge recombination. The photocatalytic reaction mechanism is further investigated for the degradation of RhB, revealing the dominating role of holes, which contribute to both direct hole oxidation and indirect oxidation (i.e. via the formation of hydroxyl radicals). Our approach provides a fast, scalable and efficient process to deposit ultrasmall Cu₂O clusters in a controllable fashion for surface engineering and modification.

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Graphene oxide (GO) has immense potential for widespread use in diverse in vitro and in vivo biomedical applications owing to its thermal and chemical resistance, excellent electrical properties and solubility, and high surface-to-volume ratio. However, development of GO-based biological nanocomposites and biosensors has been hampered by its poor intrinsic biocompatibility and difficult covalent biofunctionalization across its lattice. Many studies exploit the strategy of chemically modifying GO by noncovalent and reversible attachment of (bio)molecules or sole covalent biofunctionalization of residual moieties at the lattice edges, resulting in a low coating coverage and a largely bioincompatible composite. Here, we address these problems and present a facile yet powerful method for the covalent biofunctionalization of GO using colamine (CA) and the poly(ethylene glycol) cross-linker that results in a vast improvement in the biomolecular coating density and heterogeneity across the entire GO lattice. We further demonstrate that our biofunctionalized GO with CA as the cross-linker provides superior nonspecific biomolecule adhesion suppression with increased biomarker detection sensitivity in a DNA-biosensing assay compared to the (3-aminopropyl)triethoxysilane cross-linker. Our optimized biofunctionalization method will aid the development of GO-based in situ applications including biosensors, tissue nanocomposites, and drug carriers.

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In-situ resource utilization (ISRU) is increasingly acknowledged as an essential requirement for the construction of sustainable extra-terrestrial colonies. Even with decreasing launch costs, the ultimate goal of establishing colonies must be the usage of resources found at the destination of interest. Typical approaches towards ISRU are often constrained by the mass and energy requirements of transporting processing machineries, such as rovers and massive reactors, and the vast amount of consumables needed. Application of self-reproducing bacteria for the extraction of resources is a promising approach to reduce these pitfalls. In this work, the bacterium Shewanella oneidensis was used to reduce three different types of Lunar and Martian regolith simulants, allowing for the magnetic extraction of iron-rich materials. The combination of bacterial treatment and magnetic extraction resulted in a 5.8-times higher quantity of iron and 43.6% higher iron concentration compared to solely magnetic extraction. The materials were 3D printed into cylinders and the mechanical properties were tested, resulting in a 400% improvement in compressive strength in the bacterially treated samples. This work demonstrates a proof of concept for the on-demand production of construction and replacement parts in space exploration.

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Gold nanoparticles have been extensively studied for their applications in catalysis. For Au nanoparticles to be catalytically active, controlling the particle size is crucial. Here we present a low temperature (105 °C) thermal atomic layer deposition approach for depositing gold nanoparticles on TiO₂ with controlled size and loading using trimethylphosphino-trimethylgold(iii) and two co-reactants (ozone and water) in a fluidized bed reactor. We show that the exposure time of the precursors is a variable that can be used to decouple the Au particle size from the loading. Longer exposures of ozone narrow the particle size distribution, while longer exposures of water broaden it. By studying the photocatalytic activity of Au/TiO₂ nanocomposites, we show how the ability to control particle size and loading independently can be used not only to enhance performance but also to investigate structure-property relationships. This study provides insights into the mechanism underlying the formation and evolution of Au nanoparticles prepared for the first time via vapor phase atomic layer deposition. Employing a vapor deposition technique for the synthesis of Au/TiO₂ nanocomposites eliminates the shortcomings of conventional liquid-based processes opening up the possibility of highly controlled synthesis of materials at large scale.

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In this work, we report molecular layer deposition (MLD) of ultrathin poly(ethylene terephthalate) (PET) films on gram-scale batches of ultrafine particles for the first time. TiO2 P25 nanoparticles (NPs) are coated up to 50 cycles in an atmospheric-pressure fluidized-bed reactor at 150 °C using terephthaloyl chloride and ethylene glycol as precursors. Ex-situ diffuse reflectance infrared Fourier transform spectroscopy, thermogravimetric analysis, and transmission electron microscopy show the linear growth at 0.05 nm/cycle of uniform and conformal PET films, which are unattainable with conventional wet-phase approaches. The sub-nanoscale and nanoscale PET films not only suppress the photocatalytic activity of TiO2 NPs by hindering the access of water and reactant molecules to the TiO2 surface but also improve the dispersibility of TiO2 NPs in both organic and aqueous media. Still, the bulk optical properties, electronic structure, and surface area of TiO2 are essentially unaffected by the MLD process. This study demonstrates the industrial relevance of MLD to simultaneously suppress the photoactivity and enhance the dispersibility of commercial TiO2 P25 nanopowders, which is crucial for their use for example as UV-screening agents in sunscreens and as white pigments in paints. Moreover, by rapidly modifying the surface properties of particles in a controlled manner at the sub-nanometer scale, particle MLD can serve many other applications ranging from nanofluids to emulsions to polymer nanocomposites.

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Photocatalysts for water purification typically lack efficiency for practical applications. Here we present a multi-component (Pt:SiO₂:TiO₂ (P25)) material that was designed using knowledge of reaction mechanisms of mono-modified catalysts (SiO₂:TiO₂, and Pt:TiO₂ ) combined with the potential of atomic layer deposition (ALD). The deposition of ultrathin SiO₂ layers on TiO₂ nanoparticles, applying ALD in a fluidized bed reactor, demonstrated in earlier studies their beneficial effects for the photocatalytic degradation of organic pollutants due to more acidic surface Si–OH groups which benefit the generation of hydroxyl radicals. Furthermore, our investigation on the role of Pt on TiO₂ (P25), as an improved photocatalyst, demonstrated that suppression of charge recombination by oxygen adsorbed on the Pt particles, reacting with the separated electrons to superoxide radicals, acts as an important factor for the catalytic improvement. Combining both materials into the resulting Pt:SiO₂:TiO₂ (P25) nanopowder exceeded the dye degradation performance of both the individual SiO₂:TiO₂ (P25) (1.5 fold) and Pt:TiO₂ (P25) (4-fold) catalysts by 6-fold as compared to TiO₂ (P25). This approach thus shows that by understanding the individual materials’ behavior and using ALD as an appropriate deposition technique enabling control on the nano-scale, new materials can be designed and developed, further improving the photocatalytic activity. Our research demonstrates that ALD is an attractive technology to synthesize multicomponent catalysts in a precise and scalable way.

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This work presents the evaluation of the photocatalytic activity of P25 TiO₂ particles, coated with SiO₂, using atomic layer deposition (ALD) for the photocatalytic removal of methylene blue, oxidation of methanol and inactivation of Escherichia coli bacteria in water and its comparative evaluation with bare P25 TiO₂. Two different reactor configurations were used, a slurry reactor with the catalyst in suspension, and a structured reactor with the catalyst immobilized in macroporous foams, that enables the long-term operation of the process in continuous mode, without the necessity of separation of the particles. The results show that the incorporation of SiO₂ decreases the efficiency of the photocatalytic oxidation of methanol, whereas a significant improvement in the removal of methylene blue is achieved, and no significant changes are observed in the photocatalytic inactivation of bacteria. Adsorption tests showed that the improvements, observed in the removal of methylene blue by the incorporation of SiO₂, was mainly due to an increase in its adsorption. The improvement in the adsorption step as part of the global photocatalytic process led to a significant increase in its removal efficiency. Similar conclusions were reached for bacterial inactivation where the loss of photocatalytic efficiency, suggested by the methanol oxidation tests, was counteracted with a better adherence of bacteria to the catalyst that improved its elimination. With respect to the use of macroporous foams as support, a reduction in the photocatalytic efficiency is observed, as expected from the decrease in the available surface area. Nevertheless, this lower efficiency can be counteracted by the operational improvement derived from the easy catalyst reuse.

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Photocatalysts, contrary to conventional catalysts, utilize light to drive a reaction. By absorption of photons, electrons are excited and reach higher states to initiate a redox reaction. This principle can be broadly used to produce chemicals such as hydrogen via the reduction of water or transform carbon dioxide into solar fuels, i.e., methane, methanol, and formaldehyde. Besides chemical production, the reductive/oxidative potential of the excited electrons/holes can be utilized to degrade molecules. This is especially useful for self-cleaning materials or for cleaning waste water streams from molecules such as dyes, pesticides, or pharmaceuticals, as water pollution levels increasingly harm human health, especially in developing countries. Despite the great potential and vast research activities over the last decades, the development of cheap and efficient photocatalysts and especially the evaluation of the mechanism of photocatalytic degradation pathways are still lacking, which hampers the translation from lab to application. TiO2 (P25) is a cheap and non-toxic photoactive material, yet too inefficient for water cleaning. This thesis focuses on the surface modification of TiO2 (P25), minimizing various drawbacks of TiO2 (P25) and on the evaluation of the photocatalytic mechanisms. Atomic layer deposition (ALD) allows us to precisely control the deposition of many different materials at the atomic level, an advantage that could improve the photocatalyst design and optimize the activity.@en

The role of Pt on photocatalytic substrates such as TiO₂ (P25) for the decomposition of organic pollutants is still controversial in the scientific community. The well-observed behavior of an optimum catalytic activity as a function of the Pt loading is usually explained by the shift from charge separation to charge recombination behavior of Pt clusters. However, experiments supporting this explanation are still lacking to give a concise understanding of the effect of Pt on the photocatalytic activity. Here, we present an experimental study that tries to discriminate the different effects influencing the photocatalytic activity. Using atomic layer deposition in a fluidized bed reactor, we prepared TiO₂ (P25) samples with Pt loadings ranging from 0.04 wt % to around 3 wt %. In order to reveal the mechanism behind the photocatalytic behavior of Pt on P25, we investigated the different aspects (i.e., surface area, reactant adsorption, light absorption, charge transfer, and reaction pathway) of heterogeneous photocatalysis individually. In contrast to the often proposed prolonged lifetime of charge carriers in Pt-loaded TiO₂, we found that after collecting the excited electrons, Pt acts more as a recombination center independent of the amount of Pt deposited. Only when dissolved O₂ is present in the solution, charge recombination is suppressed by the subsequential consumption of electrons at the surface of the Pt clusters with the dissolved O₂ benefited by the improved O₂ adsorption on the Pt surface.

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We employed atomic layer deposition (ALD) to deposit ultrathin SiO₂ layers on P25 TiO₂ nanoparticles to fabricate TiO₂/SiO₂ core/shell nanostructures. The ALD process was carried out in a fluidized bed reactor working at atmospheric pressure using SiCl₄ and H₂O as precursors, enabling the deposition of SiO₂ at 100 °C with the ability to control the thickness at the sub-nanometer level. By controlling the thickness of the SiO₂ in a very narrow range, i.e., below 2 nm, the photocatalytic activity of TiO₂ can be tuned. In particular, an enhancement was obtained for the SiO₂ layers with a thickness below 1.4 nm, in which the layer with a thickness of about 0.7 nm exhibited the highest photocatalytic activity; for SiO₂ layers thicker than 1.4 nm, the photocatalytic activity was strongly suppressed. The photocatalytic activity enhancement and the degradation mechanism of RhB by the TiO₂/SiO₂ photocatalysts were investigated by combining X-ray photoelectron spectroscopy, UV–Vis absorption spectroscopy, photoluminescence spectroscopy and the aid of charge carrier and radical scavengers. Our findings have revealed an improvement of photogenerated charge separation due to the SiO₂ coating and the dominating role of hydroxyl radicals in the degradation of RhB.

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Graphene's maximized surface-to-volume ratio, high conductance, mechanical strength, and flexibility make it a promising nanomaterial. However, large-scale graphene production is typically cost-intensive. This manuscript describes a microbial reduction approach for producing graphene that utilizes the bacterium Shewanella oneidensis in combination with modern nanotechnology to enable a low-cost, large-scale production method. The bacterial reduction approach presented in this paper increases the conductance of single graphene oxide flakes as well as bulk graphene oxide sheets by 2.1 to 2.7 orders of magnitude respectively while simultaneously retaining a high surface-area-to-thickness ratio. Shewanella-mediated reduction was employed in conjunction with electron-beam lithography to reduce one surface of individual graphene oxide flakes. This methodology yielded conducting flakes with differing functionalization on the top and bottom faces. Therefore, microbial reduction of graphene oxide enables the development and up-scaling of new types of graphene-based materials and devices with a variety of applications including nano-composites, conductive inks, and biosensors, while avoiding usage of hazardous, environmentally-unfriendly chemicals.

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