Wren 2018

Abstract: Platinum is the most used electrocatalyst in electrochemical energy conversion devices such as fuel cells and electrolyzes. In my talk I will highlight our recent work on understanding the surface chemistry of platinum in an aqueous electrolyte, by combining single-crystal electrochemistry, density functional theory calculations, ultra-high-vacuum modeling, in situ spectroscopy and in situ electrochemical scanning tunneling microscopy. I will challenge some existing explanations and interpretations of platinum electrochemistry, and show the sometimes surprising surface disordering of platinum that happens at both positive (anodic) and negative (cathodic) conditions, including the formation of platinum nanoparticles.

Abstract: Recently, the synthesis of efficient materials for electrocatalysis and photoelectrocatalysis has been the focus of intense research for the most different applications. In this context, titanium dioxide-based materials deserve to be highlighted because of their properties, such as low cost and nontoxicity, possibility of obtaining different morphologies and crystalline phases, and also because of the high energy of its valence band. Among the morphologies, TiO2 nanotube arrays (TNT), exhibits excellent stability, large internal surface area, and excellent electron percolation pathways for vectorial charge transfer1. These properties also make TNT a potential support material for metallic nanoparticles, such as platinum. However, the poor conductivity is one of its main drawbacks, along with its high band gap energy, requiring UV irradiation to create electron/hole pairs. Semiconductor-electrolyte interfaces present rectifying properties, thus, oxidation reactions usually do not occur on n-type semiconductors, such as TiO2, in absence of optic excitation2. In this way, in this work we evaluated the potentiality of self-doped TiO2NTs (SD-TNT) as a semi-metallic material support2 for decoration with platinum nanoparticles, aiming applications in (photo)electrocatalysis...

Abstract: PV converters based on Si have reached a status where future technological improvements need a drastic change in paradigm and pursued research ansatz. One research direction would be to manufacture multijunction solar cells which are completely based on thin film multilayer structures preferentially based on non-critical materials and manufactured by cheap thin film deposition technologies. In these cases the functionalities of the desired devices are dependent on proper engineered interfaces.  We will present at first well-known but in many cases neglected scientific challenges for surface/interface properties which need to be deduced and adjusted for thin film solar cells. The experiments presented are based on traditional surface science approaches mostly applying photoemission also with synchrotron radiation.

Abstract:  Current research on new architectures and materials for low-cost photovoltaic solar cells and the boom of nanotechnology provides new exciting fields for scientists, technologists and engineers. This talk focuses on new concepts in photovoltaics from the point of view of their fundamental mechanisms and the experimental techniques used nowadays to study them. Especial attention will be paid to materials based on metal halide perovskites, which have revolutionized modern photovoltaics due to the impressive efficiencies achieved in just a few years span. These materials have the additional advantage of being solution-processable and obtainable from relatively cheap raw materials. The efficient functioning of perovskite solar cells stems from excellent light harvesting properties in the visible range and a good charge transport properties, exemplified by exceptionally long electron and hole diffusion lengths. In this talk, experimental and theoretical methods to characterize perovskite solar cells and to extract fundamental properties that determine their efficiency will be reviewed.1–3

Abstract:  The tuning and control of mechanical properties of systems is one multidisciplinary fields of research with high potential for applications. Nature is the most important source of inspiration and the mechanical properties of seashell, ladybug legs, shark teeth, among others, have been studied in greater details [1]. Their attractive characteristics can be defined mainly by their atomic structures and geometry. Unfortunately, most of these interesting properties can not be reproduced in laboratory models. The emerge of 3D printing technology has provided a powerful tool to solve some of the bottlenecks experienced by these field. For example, 3D printing has been used to generate macro-scale structures of atomic-scale structural models [1, 2]. In this sense, one interesting system are the Schwarzites, ordered 3D porous graphene-like nanostructures with stable negatively curved sp2-hybridized carbon atoms. These structures were proposed by Mackay and Terrones in 1991. They used the concept of negative curvature in the context of periodic graphitic structures, with the same shapes as triply periodic minimal surfaces [3]. We have investigated the mechanical behavior under compressive strain of six different Schwarzites through reactive molecular dynamics (MD) simulations.

Abstract: There has been growing interest in the photoelectrochemical conversion of carbon dioxide (a potent greenhouse gas and a contributor to global climate change) to useful carbon-based fuels or chemicals. The reaction products are of potential importance to energy technology, food research, medical applications and fabrication of plastic materials. Given the fact that the CO2 molecule is very stable, its electroreduction processes are characterized by large over-potentials. It is often postulated that, during electroreduction, the rate limiting step is the protonation of the adsorbed CO product to form the CHO adsorbate. In this respect, the proton availability and its mobility at the photo(electro)chemical interface has to be addressed. On the other hand, competition between such parallel processes as hydrogen evolution and carbon dioxide reduction has also to be considered. By proper combination of certain metal (Au, Ag, Cu) nanoparticles with nonstoichiometric mixed-metal (WO3+ZrO2) selective reduction of CO2 to alcohols is in acid media feasible.

Recently, we have concentrated on the development of hybrid materials by utilizing combination of metal oxide semiconductors thus capable of effective photoelectrochemical reduction of carbon dioxide. For example, the combination  of conducting polymers, or titanium (IV) oxide, and copper (I) oxide has been considered before and after sunlight illumination. Application of the hybrid system composed of both above-mentioned oxides resulted in high current densities originating from photoelectrochemical reduction of carbon dioxide mostly to methanol (CH3OH) as demonstrated upon identification of final products. Among important issue is intentional stabilization, activation, and functionalization of the mixed-metal-oxide-based photoelectrochemcal interface toward better long-term performance and selectivity production of small organic molecules (C1-C4) and other chemicals. In this respect, ultra-thin films of conducting polymers (simple or polyoxometallate-derivatized) and supramolecular complexes (with  nitrogen containing ligands and certain transition metal sites), sub-monolayers of metals (Cu, Au), networks of noble metal (Au, Ag) nanoparticles or layers of robust bacterial biofilms have been considered. The photo-biocathode with Cu-containing enzyme has induced the reduction of not only oxygen but carbon dioxide as well, under illuminations with photon energies higher than silicon band gap. In the presentation, special attention will be paid to mechanistic aspects of electroreduction of carbon dioxide, fabrication and characterization of highly selective and durable semiconductor photoelectrode materials and to importance of the reaction conditions.

Abstract: For an efficient technological development, the availability of suitable materials is an unavoidable challenge. To make solar technology competitive with fossil fuels, sustainable and cost-effective solar absorber materials are needed. In this regards, we are particularly interested in multi-cation complex alloy materials as they offer flexible pathways to tune their electronic and optical properties. Theoretical prediction of new materials and their experimental realization remains an important challenge in this time. However, once the material is predicted, the primary challenge is to determine whether the material (pristine or doped) can be synthesized as pure-phase in thermodynamic equilibrium conditions. We will present a density functional theory (DFT) studies on our newly predicted solar absorber materials: CuBiW2O8 (CBTO)  and CuSnW2O8 (CTTO) . We’ll discuss their electronic and optical properties, and analyze their stabilities. Chemical potential landscape analyses revealed that both could be synthesized at flexible experimental growth conditions. Like other Cu-based compounds, the formations of Cu vacancies are probable and create shallow hole states. On the other hand, Cu2S is earth abundant, non-toxic and an important semiconductor with many applications. Previously it sought wide attention to scientific community as a promising photovoltaic material since Cu2S based thin film solar cells demonstrated nearly 10% conversion efficiency. The major limitation of this materials is excessive Cu vacancy formation tendency. We have predicted a new phase of Cu2S: acanthite . Our recently published theoretical study on the acanthite phase of Cu2S showed that Cu vacancy formation tendency can be reduced with Ag alloying