Magdalena Skompska

Epitaxial growth of hexagonal boron nitride (hBN) on sapphire is of fundamental importance for the development of van der Waals heterostructures, yet the mechanism of crystallographic alignment across a weakly bonded interface remains unresolved. Here, we demonstrate that graphitic carbon nitride (g‐C ₃ N ₄ ), unintentionally formed during Metal–Organic Vapor Phase Epitaxy (MOVPE) from triethylboron and ammonia precursors, plays a pivotal role in transferring lattice orientation from the Al ₂ O ₃ substrate to the overgrown hBN film. High‐resolution transmission electron microscopy, electron energy loss spectroscopy, X‐ray diffraction, photoluminescence, and X‐ray photoelectron spectroscopy consistently reveal a nanometer‐thick interfacial region composed of AlN:O and g‐C ₃ N ₄ . Despite its partial amorphization upon post‐growth cooling, the layered structure of g‐C ₃ N ₄ provides the necessary structural flexibility to accommodate lattice mismatch and transmit crystallographic information across the interface. This finding establishes g‐C ₃ N ₄ as an effective buffer enabling epitaxial growth of two‐dimensional hBN on a three‐dimensional substrate. These insights suggest that the presence of carbon at the growth interface, often regarded as detrimental, can in fact be critical for achieving wafer‐scale, high‐quality epitaxial hBN, opening new directions for interface engineering in 2D material synthesis.

Photocatalytic processes have been studied as promising solutions to mitigate the impact of pollutants on aquatic environments. Here, the enhancement of photocatalytic performance and stability of titanate nanostructures (TNS), a well-established photocatalyst, were investigated through Sr modification. Structural characterization confirmed Sr in-corporation in the crystalline structure, mainly in the interlayers. The sample Sr(5%)TNS, synthesized with 5% (wt.), exhibited fine lamellar morphology, different from the elongated nanowires of pristine TNS. The photocatalytic performance of the Sr-modified sample was studied for the removal of a model pollutant, caffeine, under UV-Vis and visible irradiation. A clear enhancement in the caffeine removal rate was observed using Sr(5%)TNS as a photocatalyst, when compared with the pristine material. Further improvement in the photocatalytic performance was obtained by combining Sr(5%)TNS with graphitic-like carbon nitride (g-C₃N₄) as a novel composite film. This proved to be a promising strategy for enhancing both the visible-light photocatalytic efficiency and the stability of the films, while also facilitating their reuse. Various configurations of the hybrid system were tested, and the best results for caffeine degradation and catalyst robustness were achieved with a 4:1 ratio of Sr(5%)TNS to g-C₃N₄. Mechanisms for charge transfer in irradiated Sr(5%)TNS particles, and in Sr(5%)TNS/g-C₃N₄ composite films are proposed and discussed.

The hybrid system BiVO₄/g-C₃N₄ is a prospective photocatalyst because of the favorable mutual alignment of the energy bands of both semiconductors. However, the path of the photocatalytic process is still unclear because of contradictory information in the literature on whether the mechanism of charge carrier separation at the BiVO₄/g-C₃N₄ interface is band-to-band or Z-scheme. In this work, we clarified this issue by comparative photocatalytic studies with the use of systems without a mediator and with different kinds of mediators including Au nanoparticles, fullerene derivatives, and the Fe³⁺/Fe²⁺ redox couple. Additionally, the charge transfer dynamics at the BiVO₄/g-C₃N₄ and BiVO₄/mediator/g-C₃N₄ interfaces were investigated by time-resolved photoluminescence (TRPL) measurements, while the influence of the mediator on the surface recombination of the charge carriers was verified by intensity-modulated photocurrent spectroscopy (IMPS). We proved that the charge carrier separation at the BiVO₄/g-C₃N₄ interface occurs according to the mechanism typical for a heterojunction of type II, while the incorporation of the mediator between BiVO₄ and g-C₃N₄ leads to the Z-scheme mechanism. Moreover, a very strong synergetic effect on caffeine (CAF) degradation rate was found for the system BiVO₄/Au/g-C₃N₄ in the presence of Fe³⁺ ions in the CAF solution.

Exfoliated g-C₃N₄ is a well-known semiconductor utilized in heterogenous photocatalysis and water splitting. An improvement in light harvesting and separation of photogenerated charge carriers may be obtained by polymer doping with sulfur. In this work, we incorporate sulfur into the polymer chain by chemical polymerization of trithiocyanuric acid (C₃N₃S₃H₃) to obtain C₃N₃S₃. The XRD measurements and TEM images indicated that C₃N₃S₃, in contrast to g-C₃N₄, does not exist in the form of a graphitic structure and is not exfoliated into thin lamellas. However, both polymers have similar optical properties and positions of the conduction and valence bands. The comparative studies of electrochemical oxygen reduction and hydrogen evolution indicated that the overpotentials for the two processes were smaller for C₃N₃S₃ than for g-C₃N₄. The RDE experiments in the oxygen-saturated solutions of 0.1 M NaOH have shown that O₂ is electrochemically reduced via the serial pathway with two electrons involved in the first step. The spectroscopic experiments using NBT demonstrated that both polymers reveal high activity in the photocatalytic reduction of oxygen to superoxide anion radical by the photogenerated electrons.

High-quality perovskite film with large grains and therefore reduced grain boundaries plays a significant role in improving the power conversion efficiency (PCE) and ensuring good long-term stability of the perovskite solar cells. In this work, we found that adding camphorsulfonic acid (CSA), a Lewis base, to the perovskite solution results in the crystallization of larger perovskite grains. By varying the concentration of CSA, we found that the optimal concentration of the additive is 1 mg/mL, which leads to an 20% increase in PCE of the cells compared to the reference CSA-free cell. Interestingly, we observed that the PCE of cells with an excess of CSA was initially poor, but may increase significantly over time, possibly due to CSA migration to the hole-transporting layer, leading to an improvement in its conductivity.

In the present work, we report the successful synthesis and characterization of six (two new) fullerene mono- and di-pyrene derivatives based on C₆₀ and C₇₀ fullerenes. The synthesized compounds were characterized by spectral methods (ESI-MS, ¹H-NMR, ¹³C-NMR, UV-Vis, FT-IR, photoluminescence and photocurrent spectroscopy). The energy of HOMO and LUMO levels and the band gaps were determined from cyclic voltammetry and compared with the theoretical values calculated according to the DFT/B3LYP/6-31G(d) and DFT/PBE/6-311G(d,p) approach for fully optimized molecular structures at the DFT/B3LYP/6-31G(d) level. Efficiency of solar cells made of PTB7: C₆₀ and C₇₀ fullerene pyrene derivatives were analyzed based on the determined energy levels of the HOMO and LUMO orbitals of the derivatives as well as the extensive spectral results of fullerene derivatives and their mixtures with PTB7. As a result, we found that the electronic and spectral properties, on which the efficiency of a photovoltaic cell is believed to depend, slightly changes with the number and type of pyrene substituents on the fullerene core. The efficiency of constructed solar cells largely depends on the homogeneity of the photovoltaic layer, which, in turn, is a derivative of the solubility of fullerene derivatives in the solvent used to apply these layers by spincoating.