Antonio Esau Del Rio Castillo

Graphene–Perovskite Solar Cells Exceed 18 % Efficiency: A Stability Study

Interface engineering is performed by the addition of graphene and related 2 D materials (GRMs) into perovskite solar cells (PSCs), leading to improvements in the power conversion efficiency (PCE). By doping the mesoporous TiO2 layer with graphene flakes (mTiO2 +G), produced by liquid-phase exfoliation of pristine graphite, and by inserting graphene oxide (GO) as an interlayer between the perovskite and hole-transport layers, using a two-step deposition procedure in air, we achieved a PCE of 18.2 %. The obtained PCE value mainly results from improved charge-carrier injection/collection with respect to conventional PSCs. Although the addition of GRMs does not influence the shelf life, it is beneficial for the stability of PSCs under several aging conditions. In particular, mTiO2 +G PSCs retain more than 88 % of the initial PCE after 16 h of prolonged 1 sun illumination at the maximum power point. Moreover, when subjected to prolonged heating at 60 °C, the GO-based structures show enhanced stability with respect to mTiO2 +G PSCs, as a result of thermally induced modification at the mTiO2 +G/perovskite interface. The exploitation of GRMs in the form of dispersions and inks opens the way for scalable large-area production, advancing the possible commercialization of PSCs.

Binder-free graphene as an advanced anode for lithium batteries

We report the fabrication of binder-free anodes for lithium-ion batteries (LIBs) based on graphene nanoflakes on-demand designed and produced by liquid phase exfoliation of graphite. A solvent exchange process is exploited to first remove the N-methyl-2-pyrrolidone used for the exfoliation of graphite and then to re-disperse the exfoliated single-(SLG) and few-layer (FLG) graphene flakes, at a high concentration (∼5 g L−1), in an environmentally friendly solvent, i.e., ethanol. Anodes are realized by drop-casting the SLG- and FLG-based ink in ethanol under ambient conditions on a Cu foil without any binder or conductive agents, typically used for the fabrication of conventional LIBs. We tested our SLG- and FLG-based anodes in a half-cell configuration, achieving a reversible specific capacity of ∼500 mA h g−1 after 100 cycles at a current density of 0.1 A g−1, with coulombic efficiency >99.5%. We also tested the SLG- and FLG-based anode in a full-cell configuration, exploiting commercial LiNi0.5Mn1.5O4 as a cathode. The battery operates around 4.7 V with a flat-plateau voltage profile and a reversible specific capacity of ∼100 mA h g−1. The proposed electrode fabrication process is fast, low cost and industrially scalable opening the way to the optimization of energy and power densities, lifetime and safety of LIBs, while minimizing their cost and environmental impact.

Few-layer MoS2 flakes as a hole-selective layer for solution-processed hybrid organic hydrogen-evolving photocathodes

High-efficiency organic photocathodes, based on a regioregular poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester (rr-P3HT:PCBM) bulk heterojunction sandwiched between charge-selective layers, are emerging as efficient and low-cost devices for solar hydrogen production by water splitting. Nevertheless, stability issues of the materials used as charge-selective layers are hampering the realization of long-lasting photoelectrodes, pointing out the need to investigate novel and stable materials. Here, we propose MoS2 flakes, produced by Li-aided exfoliation of the bulk counterpart, as an efficient atomic-thick hole-selective layer for rr-P3HT:PCBM-based photocathodes. We carried out p-type chemical doping to tune on-demand the MoS2 Fermi level in order to match the highest occupied molecular orbital level of the rr-P3HT, thus easing the hole collection at the electrode. The as-prepared p-doped MoS2-based photocathodes reached a photocurrent of 1.21 mA cm−2 at 0 V vs. RHE, a positive onset potential of 0.56 V vs. RHE and a power-saved figure of merit of 0.43%, showing a 6.1-fold increase with respect to pristine MoS2-based photocathodes, under simulated 1 sun illumination. Operational activity of the photocathodes over time and under 1 sun illumination revealed a progressive stabilization of the photocurrents at 0.49 mA cm−2 at 0 V vs. RHE. These results pave the way towards the exploitation of layered crystals as efficiency-boosters for scalable hybrid organic H2-evolving photoelectrochemical cells.

Size-Tuning of WSe2 Flakes for High Efficiency Inverted Organic Solar Cells

The development of large-scale production methods of two-dimensional (2D) crystals, with on-demand control of the area and thickness, is mandatory to fulfill the potential applications of such materials for photovoltaics. Inverted bulk heterojunction (BHJ) organic solar cell (OSC), which exploits a polymer-fullerene binary blend as the active material, is one potentially important application area for 2D crystals. A large ongoing effort is indeed currently devoted to the introduction of 2D crystals in the binary blend to improve the charge transport properties. While it is expected that the nanoscale domains size of the different components of the blend will significantly impact the performance of the OSC, to date, there is no evidence of quantitative information on the interplay between 2D crystals and fullerene domains size. Here, we demonstrate that by matching the size of WSe2 few-layer 2D crystals, produced by liquid-phase exfoliation, with that of the PC71BM fullerene domain in BHJ OSCs, we obtain power conversion efficiencies (PCEs) of ∼9.3%, reaching a 15% improvement with respect to standard binary devices (PCE = 8.10%), i.e., without the addition of WSe2 flakes. This is the highest ever reported PCE for 2D material-based OSCs, obtained thanks to the enhanced exciton generation and exciton dissociation at the WSe2-fullerene interface and also electron extraction to the back metal contact as a consequence of a balanced charge carriers mobility. These results push forward the implementation of transition-metal dichalcogenides to boost the performance of BHJ OSCs.

Thermal Stability and Anisotropic Sublimation of Two-Dimensional Colloidal Bi2Te3 and Bi2Se3 Nanocrystals

The structural and compositional stabilities of two-dimensional (2D) Bi2Te3 and Bi2Se3 nanocrystals, produced by both colloidal synthesis and by liquid phase exfoliation, were studied by in situ transmission electron microscopy (TEM) during annealing at temperatures between 350 and 500 °C. The sublimation process induced by annealing is structurally and chemically anisotropic and takes place through the preferential dismantling of the prismatic {011̅0} type planes, and through the preferential sublimation of Te (or Se). The observed anisotropic sublimation is independent of the method of nanocrystal’s synthesis, their morphology, or the presence of surfactant molecules on the nanocrystals surface. A thickness-dependent depression in the sublimation point has been observed with nanocrystals thinner than about 15 nm. The Bi2Se3 nanocrystals were found to sublimate below 280 °C, while the Bi2Te3 ones sublimated at temperatures between 350 and 450 °C, depending on their thickness, under the vacuum conditions in the TEM column. Density functional theory calculations confirm that the sublimation of the prismatic {011̅0} facets is more energetically favorable. Within the level of modeling employed, the sublimation occurs at a rate about 700 times faster than the sublimation of the {0001} planes at the annealing temperatures used in this work. This supports the distinctly anisotropic mechanisms of both sublimation and growth of Bi2Te3 and Bi2Se3 nanocrystals, known to preferentially adopt a 2D morphology. The anisotropic sublimation behavior is in agreement with the intrinsic anisotropy in the surface free energy brought about by the crystal structure of Bi2Te3 or Bi2Se3.

Exfoliation of Few-Layer Black Phosphorus in Low-Boiling-Point Solvents and Its Application in Li-Ion Batteries

The liquid-phase exfoliation (LPE) of black phosphorus (BP) is a strategic route for the large-scale production of phosphorene and few-layer BP (FL-BP) flakes. The exploitation of this exfoliated material in cutting-edge technologies, e.g., in flexible electronics and energy storage, is however limited by the fact that the LPE of BP is usually carried out at a high boiling point and in toxic solvents. In fact, the solvent residual is detrimental to device performance in real applications; thus, complete solvent removal is critical. Here, we tackle these issues by exfoliating BP in different low-boiling-point solvents. Among these solvents, we find that acetone also provides a high concentration of exfoliated BP, leading to the production of FL-BP flakes with an average lateral size and thickness of ∼30 and ∼7 nm, respectively. The use of acetone to produce less defective few-layer BP flakes (FL-BPacetone) from bulk crystals is a straightforward process which enables the rapid preparation of homogeneous th...

Solution-Processed Hybrid Graphene Flake/2H-MoS2 Quantum Dot Heterostructures for Efficient Electrochemical Hydrogen Evolution

We designed solution-processed, flexible hybrid graphene flake/2H-MoS2 quantum dot (QD) heterostructures, showing enhanced electrocatalytic activity for the hydrogen evolution reaction (HER) with respect to their native individual components. The 2H-MoS2 QDs are produced through a scalable, environmentally friendly, one-step solvothermal approach from two-dimensional (2D) 2H-MoS2 flakes obtained by liquid phase exfoliation (LPE) of their bulk counterpart in 2-Propanol. This QDs synthesis avoids the use of high boiling point and/or toxic solvents. Graphene flakes are produced by LPE of graphite in N-Methyl-2-pyrrolidone. The electrochemical properties of 2H-MoS2 QDs and their HER-favorable chemical and electronic coupling with graphene enable to reach current density of 10 mA/cm² at an overpotential of 136 mV, surpassing the performances of graphene flake/2H-MoS2 (1T-MoS2) flake heterostructures. Our approach provides a shortcut, viable and cost-effective method for enhancing the 2D materials electrocataly...

Graphene-Based Hole-Selective Layers for High-Efficiency, Solution-Processed, Large-Area, Flexible, Hydrogen-Evolving Organic Photocathodes

Regio-regular poly(3-hexylthiophene-2,5-diyl) (rr-P3HT), the work-horse of organic photovoltaics, has been recently exploited in bulk heterojunction (BHJ) configuration with phenyl-C61-butyric acid methyl ester (PCBM) for solution-processed hydrogen-evolving photocathodes, reaching cathodic photocurrents at 0 V vs. RHE (J0V vs RHE) of up to 8 mA cm-2. The photoelectrochemical performance of these photocathodes strongly depends on the presence of the electron (ESL) and hole (HSL) selective layer. While TiO2 and its sub-stoichiometric phases are consolidated ESL materials, the currently used HSLs (e.g., MoO3, CuI, PEDOTT:PSS, WO3) suffer electrochemical degradation under hydrogen evolution reaction (HER)-working conditions. In this work, we use solution-processed graphene derivatives as HSL to boost efficiency and durability of rr-P3HT:PCBM-based photocathodes, demonstrating record-high performance. In fact, our devices show cathodic J0V vs RHE of 6.01 mA cm-2, onset potential (Vo) of 0.6 V vs. RHE, ratiome...

Engineered MoSe2‐Based Heterostructures for Efficient Electrochemical Hydrogen Evolution Reaction

Two-dimensional transition metal-dichalcogenides are emerging as efficient and cost-effective electrocatalysts for hydrogen evolution reaction (HER). However, only the edge sites of their trigonal prismatic phase show HER-electrocatalytic properties, while the basal plane, which is absent of defective/unsaturated sites, is inactive. Here, we tackle the key challenge that is increasing the number of electrocatalytic sites by designing and engineering heterostructures composed of single-/few-layer MoSe2 flakes and carbon nanomaterials (graphene or single-wall carbon nanotubes (SWNTs)) produced by solution processing. The electrochemical coupling between the materials that comprise the heterostructure effectively enhances the HER-electrocatalytic activity of the native MoSe2 flakes. The optimization of the mass loading of MoSe2 flakes and their electrode assembly via monolithic heterostructure stacking provided a cathodic current density of 10mAcm-2 at overpotential of 100mV, a Tafel slope of 63mVdec-1 and an exchange current density (j0) of 0.203 Acm-2. In addition, electrode thermal annealing in a hydrogen environment and chemical bathing in n-butyllithium are exploited to texturize the basal planes of the MoSe2 flakes (through Se-vacancies creation) and to achieve in situ semiconducting-to-metallic phase conversion, respectively, thus they activate new HER-electrocatalytic sites. The as-engineered electrodes show a 4.8-fold enhancement of j0 and a decrease in the Tafel slope to 54mVdec-1.

Biotransformation and Biological Interaction of Graphene and Graphene Oxide during Simulated Oral Ingestion

The biotransformation and biological impact of few layer graphene (FLG) and graphene oxide (GO) are studied, following ingestion as exposure route. An in vitro digestion assay based on a standardized operating procedure (SOP) is exploited. The assay simulates the human ingestion of nanomaterials during their dynamic passage through the different environments of the gastrointestinal tract (salivary, gastric, intestinal). Physical-chemical changes of FLG and GO during digestion are assessed by Raman spectroscopy. Moreover, the effect of chronic exposure to digested nanomaterials on integrity and functionality of an in vitro model of intestinal barrier is also determined according to a second SOP. These results show a modulation of the aggregation state of FLG and GO nanoflakes after experiencing the complex environments of the different digestive compartments. In particular, chemical doping effects are observed due to FLG and GO interaction with digestive juice components. No structural changes/degradation of the nanomaterials are detected, suggesting that they are biopersistent when administered by oral route. Chronic exposure to digested graphene does not affect intestinal barrier integrity and is not associated with inflammation and cytotoxicity, though possible long-term adverse effects cannot be ruled out.