Gergely F. Samu

Time- and energy-efficient solution combustion synthesis of binary metal tungstate nanoparticles with enhanced photocatalytic activity.

In the search for stable and efficient photocatalysts beyond TiO2 , the tungsten-based oxide semiconductors silver tungstate (Ag2 WO4 ), copper tungstate (CuWO4 ), and zinc tungstate (ZnWO4 ) were prepared using solution combustion synthesis (SCS). The tungsten precursors influence on the product was of particular relevance to this study, and the most significant effects are highlighted. Each samples photocatalytic activity towards methyl orange degradation was studied and benchmarked against their respective commercial oxide sample obtained by solid-state ceramic synthesis. Based on the results herein, we conclude that SCS is a time- and energy-efficient method to synthesize crystalline binary tungstate nanomaterials even without additional excessive heat treatment. As many of these photocatalysts possess excellent photocatalytic activity, the discussed synthetic strategy may open sustainable materials chemistry avenues to solar energy conversion and environmental remediation.

New insights into the relationship between structure and photocatalytic properties of TiO2 catalysts

This work systematically investigated the relationship between structure, morphology, photoelectrochemical (PEC) and photocatalytic (PC) properties of TiO2 catalysts. A series of TiO2 catalysts with various phase compositions (anatase-, brookite- and finally rutile-rich samples) and morphologies (1D morphology, rhomboid nanoparticles (NPs) and flower-like assemblies of nanorods) were prepared by an acidic hydrothermal treatment of hydrogen titanate nanofibres (H-TNFs). The structures of the samples, such as crystal phase composition and their spatial distribution, were extensively characterised, and the samples were tested for photocatalytic degradation of ethanol. A strong correlation is found between PEC and PC properties. PEC measurements revealed that the brookite-rich samples generated high but unstable photocurrents. The anatase and rutile-rich samples showed good stability, but for the rutile-rich samples low photocurrents were detected due to the poor conductivity of this polymorph. In contrast, the sample containing 93.2% anatase and 6.8% brookite with elongated morphology not only showed the ability to generate high photocurrents but also maintained a stable photoresponse upon an extended period of time, because of its well-balanced bi-crystalline structure and elongated morphology. Therefore, the abilities to generate high photocurrents and to maintain a stable photoresponse are equally important and probably a prerequisite for a good photocatalyst.

Electrochemical synthesis and characterization of poly(3-hexylthiophene)/single-walled carbon nanotube array hybrid materials

AbstractIn this study, we demonstrate that by directly employing single-walled carbon nanotube arrays (SWCNT-arrays)—grown on conductive substrates—as working electrodes, selective and uniform electrodeposition of a conducting polymer, namely poly(3-hexylthiophene), can be achieved on the surface of the nanotubes. The overall kinetic pattern of the electrodeposition was studied by separating the deposition charge from the one related to the redox transformation of the polymer film deposited during the precedent cycles. Both the structure and the electrochemical properties of the hybrid materials were studied as a function of the electrodeposition cycles, thus the amount of the formed polymer. The hybrids were characterized by electron microscopic (SEM, TEM) and vibrational spectroscopic (Raman spectroscopy) means. The obtained results were compared and contrasted with those gathered on macroscopic-sized multi-walled carbon nanotube array-based composites in our group recently. Overall, we conclude that electrochemical polymerization is an attractive tool to synthesize conducting polymer/SWCNT hybrid materials with controlled composition and morphology. Graphical abstractHighy organized nanostructures of conduction polymer/SWCNT array hybrids were obtained via electrodeposition

Modulation of Charge Recombination in CsPbBr3 Perovskite Films with Electrochemical Bias

The charging of a mesoscopic TiO2 layer in a metal halide perovskite solar cell can influence the overall power conversion efficiency. By employing CsPbBr3 films deposited on a mesoscopic TiO2 film, we have succeeded in probing the influence of electrochemical bias on the charge carrier recombination process. The transient absorption spectroscopy experiments conducted at different applied potentials indicate a decrease in the charge carrier lifetimes of CsPbBr3 as we increase the potential from −0.6 to +0.6 V vs Ag/AgCl. The charge carrier lifetime increased upon reversing the applied bias, thus indicating the reversibility of the photoresponse to charging effects. The ultrafast spectroelectrochemical experiments described here offer a convenient approach to probe the charging effects in perovskite solar cells.

Photoelectrochemistry by Design: Tailoring the Nanoscale Structure of Pt/NiO Composites Leads to Enhanced Photoelectrochemical Hydrogen Evolution Performance

Photoelectrochemical hydrogen evolution is a promising avenue to store the energy of sunlight in the form of chemical bonds. The recent rapid development of new synthetic approaches enables the nanoscale engineering of semiconductor photoelectrodes, thus tailoring their physicochemical properties toward efficient H2 formation. In this work, we carried out the parallel optimization of the morphological features of the semiconductor light absorber (NiO) and the cocatalyst (Pt). While nanoporous NiO films were obtained by electrochemical anodization, the monodisperse Pt nanoparticles were synthesized using wet chemical methods. The Pt/NiO nanocomposites were characterized by XRD, XPS, SEM, ED, TEM, cyclic voltammetry, photovoltammetry, EIS, etc. The relative enhancement of the photocurrent was demonstrated as a function of the nanoparticle size and loading. For mass-specific surface activity the smallest nanoparticles (2.0 and 4.8 nm) showed the best performance. After deconvoluting the trivial geometrical effects (stemming from the variation of Pt particle size and thus the electroactive surface area), however, the intermediate particle sizes (4.8 and 7.2 nm) were found to be optimal. Under optimized conditions, a 20-fold increase in the photocurrent (and thus the H2 evolution rates) was observed for the nanostructured Pt/NiO composite, compared to the benchmark nanoparticulate NiO film.

Electrochemistry and Spectroelectrochemistry of Lead Halide Perovskite Films: Materials Science Aspects and Boundary Conditions

The unique optoelectronic properties of lead halide perovskites have triggered a new wave of excitement in materials chemistry during the past five years. Electrochemistry, spectroelectrochemistry, and photoelectrochemistry could be viable tools both for analyzing the optoelectronic features of these materials and for assembling them into hybrid architectures (e.g., solar cells). At the same time, the instability of these materials limits the pool of solvents and electrolytes that can be employed in such experiments. The focus of our study is to establish a stability window for electrochemical tests for all-inorganic CsPbBr3 and hybrid organic–inorganic MAPbI3 perovskites. In addition, we aimed to understand the reduction and oxidation events that occur and to assess the damage done during these processes at extreme electrochemical conditions. In this vein, we demonstrated the chemical, structural, and morphological changes of the films in both reductive and oxidative environments. Taking all these results together as a whole, we propose a set of boundary conditions and protocols for how electrochemical experiments with lead halide perovskites should be carried out and interpreted. The presented results will contribute to the understanding of the electrochemical response of these materials and lead to a standardization of results in the literature so that comparisons can more easily be made.

Tailoring the interfaces in conducting polymer composites by controlled polymerization

Abstract This chapter summarizes the various strategies for the controlled synthesis of conducting polymer/inorganic nanostructure interfaces. We aimed to give a brief overview on the wealth of possibilities, also highlighting the particular importance of each method. The presented approaches span through chemical, electrochemical, and photo-polymerization of the conducting polymer (CP), but certain examples for the in situ formation of the inorganic component was also demonstrated. One common feature in all cases was the precise control over the properties of the organic/inorganic interface. Such control may include the manipulation of the composition, morphology, and optoelectronic properties of the hybrid assembly. It is also important that various comparative studies evidenced the superior performance of such prepared hybrid materials compared to their counterparts obtained via simple mechanical mixing. We really hope that the materials science community will increasingly exploit these smart methods in the future, to obtain nanohybrid materials with advanced properties.