Dr. Peter William de Oliveira

Abstract Free standing, binder free, and conductive additive free mesoporous titanium dioxide/carbon hybrid electrodes were prepared from co‐assembly of a poly(isoprene)‐block‐poly(styrene)‐block‐poly(ethylene oxide) block copolymer and a titanium alkoxide. By tailoring an optimized morphology, we prepared macroscopic mechanically stable 300 μm thick monoliths that were directly employed as lithium‐ion battery electrodes. High areal mass loading of up to 26.4 mg cm−2 and a high bulk density of 0.88 g cm−3 were obtained. This resulted in a highly increased volumetric capacity of 155 mAh cm−3, compared to cast thin film electrodes. Further, the areal capacity of 4.5 mAh cm−2 represented a 9‐fold increase compared to conventionally cast electrodes. These attractive performance metrics are related to the superior electrolyte transport and shortened diffusion lengths provided by the interconnected mesoporous nature of the monolith material, assuring superior rate handling, even at high cycling rates.

Abstract Pure and Nb-doped TiO2 photocatalysts with highly ordered alternating gyroid architecture and well-controllable mesopore size of 15 nm via co-assembly of a poly(isoprene)-block-poly(styrene)-block-poly(ethylene oxide) block copolymer are synthesized. A combined effort by electron microscopy, X-ray scattering, photoluminescence, X-ray photoelectron spectroscopy, Raman spectroscopy, and density functional theory simulations reveals that the addition of small amounts of Nb results in the substitution of Ti4+ with isolated Nb5+ species that introduces inter-bandgap states, while at high concentrations, Nb prefers to cluster forming shallow trap states within the conduction band minimum of TiO2. The gyroidal photocatalysts are remarkably active toward hydrogen evolution under UV and visible light due to the open 3D network, where large mesopores ensure efficient pore diffusion and high photon harvesting. The gyroids yield unprecedented high evolution rates beyond 1000 µmol h−1 (per 10 mg catalyst), outperforming even the benchmark P25-TiO2 more than fivefold. Under UV light, the Nb-doping reduces the activity due to the introduction of charge recombination centers, while the activity in the visible triple upon incorporation is owed to a more efficient absorption due to inter-bandgap states. This unique pore architecture may further offer hitherto undiscovered optical benefits to photocatalysis, related to chiral and metamaterial-like behavior, which will stimulate further studies focusing on novel light–matter interactions.

Anti-reflection and photocatalytic properties are desirable for improving the optical properties of electronic devices. We describe a method of fabrication a single-layer, anti-reflective (AR) thin film with an additional photocatalytic property. The layer is deposited on glass substrates by means of a low-cost dip-coating method using a SiO2-TiO2 solution. A comparative study was undertaken to investigate the effects of TiO2 concentrations on the photocatalytic properties of the film and to determine the optimal balance between transmittance and photocatalysis. The average transmittance increases from T = 90.51% to T = 95.46 ± 0.07% for the wavelengths between 380 and 1200 nm. The structural characterization indicated the formation of thin, porous SiO2-TiO2 films with a roughness of less than 7.5 nm. The quality of the samples was evaluated by a complete test program of the mechanical, chemical and accelerated weathering stability. This results open up new possibilities for cost-effective AR coatings for the glass and solar cell industries.

Coating on large surfaces is a critical issue in both academic studies and industrial production. This work proposes a novel method of coating a large flat substrate (50 × 100 cm2) via a wet chemical process using a very small amount (20 ml) of coating solution. The sol material consisted of surface-modified silicon dioxide (SiO2) nanoparticles (10–30 nm), which have the optimal antireflective (AR) function in the visible spectral range for thin films with a thickness ranging from 110 to 120 nm. Ellipsometry results demonstrate a homogeneous thickness of the AR coating on glass (109.4 ± 2.7 nm). A deviation of less than 3% over a large coated surface was observed. Crack-free coatings with homogeneous morphology on the surface of the coatings were observed using scanning electron microscopy. The AR effect was confirmed with UV–vis measurements, with an average transmittance of 91.1% and 94.7%, respectively, in visible wavelengths for the one-sided and double-sided AR coatings (in comparison to 88% for uncoated glass).

Copper indium gallium diselenide (CIGS) nanoparticles are one the promising materials for solar cell technology applications. In this study, CIGS nanostructures were synthesized using a solvothermal method. By adding different ammonium salts, the synthesis time could be reduced by up to 2 h compared to the routine solvothermal method. The effects of ammonium halides on the synthesis process and on particle growth were investigated. Structural properties and size information of the synthesized nanoparticles were obtained by X-ray diffraction and transmission electron microscopy. The CIGS particles were found to be tetragonal with average sizes ranging from 20 to 100 nm. The optical band gap of these structures was found 1.38 and 1.46 eV. Shortening the synthesis time of dispersed CIGS nanoparticles could be interesting for the development of cost-effective or non-vacuum technologies.

One-dimensional zinc oxide (ZnO) nanostructures of different sizes were synthesized on silicon wafer substrates by controlling the substrate condition using chemical etching and a vapor-liquid-solid (VLS) process. In this work, a thin layer of gold was deposited on the silicon substrate and used as catalyst. Gold is one of the most frequently-used catalysts in the chemical vapor-deposition method. By annealing the gold-coated thin films at different temperatures, the layer was transformed into gold islands, due to the dewetting effect. To investigate the effect of the dewetting process on ZnO nanostructures, samples with various thicknesses are annealed at different temperatures. The results are compared with the uncoated and chemically-etched silicon wafers. Structural and morphological properties of the samples were analyzed using X-ray diffraction and scanning-electron microscopy. Formation of nanowires and nanorods was observed, and their sizes were dependent on the sizes of the gold islands. Photoluminescence spectra of the samples at room temperature were measured and visible emissions were observed from the synthesized nanostructures at room temperature.

Glass coatings are of great interest for biomedical implant application due to their excellent properties. Nowadays they are used in different fields including drug delivery, for bone tissue regeneration or as implant. Nevertheless they can only be applied using high temperatures. Therefore their usage in the field of cardiovascular implant application is still restricted. Accordingly new developments in this field have been carried out to overcome this problem and to coat cardiovascular implants. Here, novel glass-like coatings have been developed and applied using sol-gel technique at moderate temperatures. The biocompatibility and selectivity have been analyzed using human endothelial cells. The obtained results clarify that the developed compositions can either promote or suppress endothelial cell growth only by altering the sintering atmosphere. A later application as thin layer on cardiovascular implants like stents is conceivable.

Different AlN nanostructures were synthesized by using chemical vapor condensation method. The raw materials were a mixture of Al and NH4Cl powder with different weight ratios as well as Cu and Mn powders, which were used as dopants. The effects of the source materials, dopants, and annealing on optical and structural properties of AlN nanostructures were investigated. Scanning electron microscopy results showed different nanostructures, including nanowires and nanoparticles with different sizes and morphologies. The photoluminescence spectroscopy of the samples was performed at room temperature. Photoluminescence spectra of the samples showed intense peaks in visible regions from undoped, Mn-doped, and Cu-doped samples. Due to the light emission from the samples at different wavelengths, these nanostructures can be used in optoelectronic devices.

In this work, we prepared thin films of aluminum oxide (Al2O3) with different thicknesses, using a wet chemical process. The Al2O3 nanoparticles with an average size of 40 nm were dispersed in water and deposited on soda glass substrates. The morphology of the resulting thin films was characterized by means of scanning electron microscopy. The optical properties of the thin films were studied by measuring reflectance and transmittance. A theoretical description of the reflection and transmission mechanism of the films was developed by measuring the thickness and spectral behavior of the refractive index. Numerical evaluations were used for modeling the optical spectra of the thin films of alumina. By fitting numerical curves to the experimental data, the extinction coefficient and refractive index were obtained. The dielectric constant and optical properties of the colloidal solution of the particles were also studied.

Transparent conducting aluminum doped zinc oxide Al:ZnO (AZO) layers have been deposited by spin coating on glass substrates using two different sols, 1-propanolic solution of Zinc and Aluminum salts (conventional sol) and a suspension of already crystalline AZO nanoparticles redispersed in 1-propanol. The coatings have been sintered in air at 600 °C for 15 min. and then post annealed in a reducing atmosphere at 400 °C for 90 min. The influence of the aluminum content in the coating sol (sol-gel layer) and in the redispersed nanoparticles (nanoparticulare suspension layer) on the optical properties and electrical resistivity have been investigated. A single step spin coated thin layer is obtained, so that multilayers coating have been used to lower the obtained sheet resistance. The visible transmission of both types of layers is high (T > 80%). The influence of the sintering temperature and the optimum doping concentration are investigated. Seven layers synthesized with Al/Zn = 1 mol.% and submitted to reducing treatment in forming gas (N2:H2 = 92:8) exhibited a sheet resistance R = 0.42 k (ρ = 7.9 × 10–3 Ω · cm) with an average transmittance of 80% at 550 nm for layer deposited from conventional sol and 36 k (ρ = 2.5 × 10–1 Ω · cm) for nanoparticles suspension layer.