Graphene quantum dots: From efficient preparation to safe renal excretion
NANO RESEARCH 14(3) 674-683 (2021)
Carbon nanomaterials offer excellent prospects as therapeutic agents, and among them, graphene quantum dots (GQDs) have gained considerable interest thanks to their aqueous solubility and intrinsic fluorescence, which enable their possible use in theranostic approaches, if their biocompatibility and favorable pharmacokinetic are confirmed. We prepared ultra-small GQDs using an alternative, reproducible, top-down synthesis starting from graphene oxide with a nearly 100% conversion. The materials were tested to assess their safety, demonstrating good biocompatibility and ability in passing the ultrafiltration barrier using an in vitro model. This leads to renal excretion without affecting the kidneys. Moreover, we studied the GQDs in vivo biodistribution confirming their efficient renal clearance, and we demonstrated that the internalization mechanism into podocytes is caveolae-mediated. Therefore, considering the reported characteristics, it appears possible to vehiculate compounds to kidneys by means of GQDs, overcoming problems related to lysosomal degradation.
Formation of one-dimensional quantum crystals of molecular deuterium inside carbon nanotubes
CARBON 2021, 175, 141-154. DOI:10.1016/j.carbon.2020.12.067
Crystallization under stringent cylindrical confinement leads to novel quasi-one-dimensional materials. Substances with strong cohesive interactions can eventually preserve the symmetries of their bulk phase compatible with the restricted geometry, while those with weak cohesive interactions develop qualitatively different structures. Frozen molecular deuterium (D2), a solid with a strong quantum character, is structurally held by weak dispersive forces. Here, the formation of one-dimensional D2 crystals under carbon nanotube confinement is reported. In contradiction with its weak cohesive interactions, their structures, scrutinized using neutron scattering, correspond to definite cylindrical sections of the hexagonal close-packed bulk crystal. The results are rationalized on the grounds of numerical calculations, which point towards nuclear quantum delocalization as the physical mechanism responsible for the stabilization of such outstanding structures.
Optical properties and carrier dynamics in Co-doped ZnO nanorods
NANOSCALE ADVANCES 3(1), 214-222 (2021).
The controlled modification of the electronic properties of ZnO nanorods via transition metal doping is reported. A series of ZnO nanorods were synthesized by chemical bath growth with varying Co content from 0 to 20 atomic% in the growth solution. Optoelectronic behavior was probed using cathodoluminescence, time-resolved luminescence, transient absorbance spectroscopy, and the incident photon-to-current conversion efficiency (IPCE). Analysis indicates the crucial role of surface defects in determining the electronic behavior. Significantly, Co-doping extends the light absorption of the nanorods into the visible region, increases the surface defects, and shortens the non-radiative lifetimes, while leaving the radiative lifetime constant. Furthermore, for 1 atomic% Co-doping the IPCE of the ZnO nanorods is enhanced. These results demonstrate that doping can controllably tune the functional electronic properties of ZnO nanorods for applications.
In-situ reduction by Joule heating and measurement of electrical conductivity of graphene oxide in a transmission electron microscope
2D Materials (2021) 8, pp. 031001
Graphene oxide (GO) is reduced by Joule heating using in-situ transmission electron microscopy (TEM). The approach allows the simultaneous study of GO conductivity by electrical measurements and of its composition and structural properties throughout the reduction process by TEM, electron diffraction and electron energy-loss spectroscopy. The small changes of GO properties observed at low applied electric currents are attributed to the promotion of diffusion processes. The actual reduction process starts from an applied power density of about 2 × 1014 Wm−3 and occurs in a highly uniform and localized manner. The conductivity increases more than 4 orders of magnitude reaching a value of 3 × 103 Sm−1 with a final O content of less than 1%. We discuss differences between the reduction by thermal annealing and Joule heating.
Nanoscale Charge Density and Dynamics in Graphene Oxide
ACS Materials Letters (2021) 3, pp.1826-1831.
Graphene oxide (GO) is widely used as a component in thin film optoelectronic device structures for practical reasons because its electronic and optical properties can be controlled. Progress critically depends on elucidating the nanoscale electronic structure of GO. However, direct experimental access is challenging because of its disordered and nonconductive character. Here, we quantitatively mapped the nanoscopic charge distribution and charge dynamics of an individual GO sheet by using Kelvin probe force microscopy (KPFM). Charge domains are identified, presenting important charge interactions below distances of 20 nm. Charge dynamics with very long relaxation times of at least several hours and a logarithmic decay of the time correlation function are in excellent agreement with Monte Carlo simulations, revealing an universal hopping transport mechanism best described by Efros–Shklovskii’s law.
Graphene aerogels via hydrothermal gelation of graphene oxide colloids: fine-tuning of its porous and chemical properties and catalytic applications
Advances in Colloid and Interface Science (2021) 292 pp. 102420.
Recently, 3D graphene aerogel has garnered a high interest aiming at benefiting of the excellent properties of graphene in devices for energy storage or environmental remediation. Hydrothermal gelation of GO dispersion is a straightforward method that offers many opportunities for tuning its properties and for processing it to devices. By adjusting hydrothermal gelation and drying conditions, it is possible to tune the density (from ~3 mg cm−3 to ~2 g cm−3), pore volume, pores size (micro to macropores), pore distribution, surface chemical polarity (hydrophobic or hydrophilic), and electrical conductivity (from ~0.5 S m−1 to S cm−1). Besides other well explored applications in energy storage or environmental remediation, graphene aerogels have excellent prospects as support for catalysis since they combine the advantages of graphene sheets (high surface area, high electrical conductivity, surface chemistry tunability, high adsorption capacity…) while circumventing their drawbacks such as difficult separation from reaction media or tendency to stacking. Compared to other 3D porous carbon materials used as catalyst support, graphene aerogels have unique porous structure. The pore walls are the thinnest to be expected for a carbon material (the thickness of monolayer graphene is 0.335 nm), hence leading to the highest exposed surface area per weight and even per volume for compacted aerogels. This has the potential to maximize the catalytic site density per reactor mass and volume while minimizing the pressure drop for continuous reactions in flow. Herein, different strategies to control the porous texture, chemical and physical properties are revised along with their processability and scalability for the implementation into different morphologies and devices. Finally, the application of graphene aerogels in the catalysis field are overviewed, giving a perspective about future directions needing further research.
Extraordinary Protective Efficacy of Graphene Oxide over the Stone-Based Cultural Heritage.
Advanced Materials Interfaces (2021) 8, pp. 2101012.
The deterioration of monumental stones is a common event occurring to our historical and cultural heritage, threatening the preservation of our social identity and its legacy to future generations. The prolonged effect of inclement weather is very harmful for particular stone materials such as limestone and dolomite, to which a truly effective and durable protective agent has not yet been found. Herein, an unprecedented performance of graphene oxide (GO) for the shielding of carbonated ornamental stones against environmental aggressions, namely intense rainfall and extreme temperatures, is demonstrated. A simple additive-free aqueous suspension of GO serves as the perfect coating, exceedingly preventing from any kind of wear or erosion after different climatic simulations. Additionally, the stones’ original aesthetics is not altered (a critical issue in monuments), together with an improvement in their mechanical properties, reasserting the unique potential of GO for conserving exposed stone-based monuments in an affordable, accessible, and revolutionary way.
Hybrids of reduced graphene oxide aerogel and CNT for electrochemical O2 reduction
Catalysts (2021) 11(11), pp. 1404
Carbon nanotubes (CNTs), graphene aerogels (GAs), and their hybrid (CNT-GA) prepared by hydrothermal treatment were tested in the electrocatalytic oxygen reduction reaction (ORR). The importance of porous structure derived from the combination of mesoporosity coming from CNTs with macroporosity stemming from GAs was evidenced because the hybrid carbon material exhibited synergistic performance in terms of kinetic current and onset potential. Different electrocatalysts were prepared based on these hybrids doped with nitrogen using different precursors and also supporting Fe nanoparticles. N-doped carbon hybrids showed higher electrocatalytic activity than their undoped counterparts. Nevertheless, both doped and undoped materials provided a mixed two and four electron reduction. On the other hand, the addition of a Fe precursor and phenanthroline to the CNT-GA allowed preparing an N-doped hybrid containing Fe nanoparticles which favored the 4-electron oxygen reduction to water, thus being an excellent candidate as a structured cathode in fuel cells.
Controlling the surface chemistry of graphene oxide: Key towards efficient ZnO-GO photocatalysts
Graphene oxide (GO) and related materials are widely reported to enhance the photocatalytic activity of zinc oxide. However, the origin of the observed performance improvements remains elusive and studies contributing to a deeper understanding of this critical issue are largely missing. In this work, we have prepared a set of benchmark ZnO-GO hybrid materials in order to systematically put under closer scrutiny the influence of the surface chemistry of GO on the photocatalytic degradation of methylene blue. The set of ZnO-GO hybrids has been synthesized in an ultrasonication process involving ZnO nanoparticles obtained in a microwave synthesis process and GO with three distinct oxidation degrees, employed in three different loading fractions. Structural and physical-chemical characterization by XRD, FTIR, Raman, UV–vis, photoluminescence and spectroscopy and XPS, consistently demonstrate the importance of the surface chemistry of GO for establishing photo-induced charge-transfer interface interactions with ZnO, facilitating the enhancement of the catalytic activity of the ZnO-GO catalyst. Optimized interface interactions thus enabled the design of a ZnO-GO catalyst exhibiting a conversion rate of 80% obtained in a time of 70 min and at a catalyst concentration of only 0.045 mg/mL.
Towards high-efficient microsupercapacitors based on reduced graphene oxide with optimized reduction degree
ENERGY STORAGE MATERIALS 2020, 25, 740-749
Reduced graphene oxide aerogels synthetized using different times of hydrothermal treatment have been tested as raw material to prepare electrochemical supercapacitors. The gravimetric electrochemical capacitance measured using 1M Na2SO4 as electrolyte was maximized for aerogels that underwent 45 min of hydrothermal treatment. The aerogels synthetized for longer durations of hydrothermal treatment exhibited higher electrical conductivity but the gravimetric capacitance drops dramatically due to an increasing resistance to diffusion of the electrolyte ions. The impeded diffusion is boosted by the intensified crosslinking between graphene sheets, which narrows the pores between them in the prepared electrode. The rGO aerogel attained for 45 min of hydrothermal treatment provided a high gravimetric capacitance of 400 F g-1 and 100 F g-1 at 50 A g-1 for three-electrode and two electrodes configuration, respectively, as well as good cyclic stability, competing with other similar carbon materials. Activation pretreatments or adding a second component (glucose, dopamine, Mn, Fe, CNT) did not provide significant change of capacitance respect to pristine rGO aerogel.
