Elisa I. Martín

Preparation of Au nanoparticles in a non-polar medium: obtaining high-efficiency nanofluids for concentrating solar power. An experimental and theoretical perspective

This paper presents the preparation of Au nanoparticles in a non-polar medium, which is a fluid composed of the eutectic mixture of biphenyl and diphenyl oxide commonly used in Concentrating Solar Power (CSP) plants. The nanofluids prepared showed enhanced thermal properties, presenting thermal conductivity values 70% higher than those of base fluids, and isobaric specific heat values up to 10% higher. In turn, an increase of up to 36% was observed in their heat transfer coefficient, which is their efficiency as a heat transfer fluid (HTF). Also, the stability of the nanofluids was analysed using UV-vis spectroscopy, and particle size and ζ potential. The nanofluids with lower concentrations agglomerate slowly, which is considered stable for this application. Thus, these nanofluids are a promising, interesting alternative to the HTF often used in CSP plants. Also, molecular dynamics calculations were performed to better understand how the Au-nanofluid behaves in the presence of a surfactant within a temperature range between 50 and 600 K. The isobaric specific heat and thermal conductivity values followed the same experimental tendency. The analysis of the radial distribution functions (RDFs) and spatial distribution functions (SDFs) showed that, as the temperature rose, an exchange took place between the surfactant and diphenyl oxide molecules in the first layer of molecules around the metal. This movement incorporated a directionality that may play a part in the enhanced thermal properties. The surfactant participates as an active component within the Au-nanofluid, contributing to efficient heat transfer processes.

Convergent study of Ru–ligand interactions through QTAIM, ELF, NBO molecular descriptors and TDDFT analysis of organometallic dyes

We report a theoretical study of a series of Ru complexes of interest in dye-sensitised solar cells, in organic light-emitting diodes, and in the war against cancer. Other metal centres, such as Cr, Co, Ni, Rh, Pd, and Pt, have been included for comparison purposes. The metal–ligand trends in organometallic chemistry for those compounds are shown synergistically by using three molecular descriptors: quantum theory of atoms in molecules (QTAIM), electron localisation function (ELF) and second-order perturbation theory analysis of the natural bond orbital (NBO). The metal–ligand bond order is addressed through both delocalisation index (DI) of QTAIM and fluctuation index (λ) of ELF. Correlation between DI and λ for Ru–N bond in those complexes is introduced for the first time. Electron transfer and stability was also assessed by the second-order perturbation theory analysis of the NBO. Electron transfer from the lone pair NBO of the ligands toward the antibonding lone pair NBO of the metal plays a relevant role in stabilising the complexes, providing useful insights into understanding the effect of the ‘expanded ligand’ principle in supramolecular chemistry. Finally, absorption wavelengths associated to the metal-to-ligand charge transfer transitions and the highest occupied molecular orbital (HOMO)--lowest unoccupied molecular orbital (LUMO) characteristics were studied by time-dependent density functional theory.

Hybrid Perovskite, CH3NH3PbI3, for Solar Applications

The effect of the incorporation of NH4+ into the CH3NH3+ sites of the tetragonal perovskite CH3NH3PbI3 is analysed. Also, how it affects the introduction of Cd2+ cations into Pb2+ sites for a perovskite with 25źat.% of NH4+ is addressed. The incorporation of NH4+ into perovskite leads to a dramatic loss of crystallinity and to the presence of other phases. Moreover, the NH4PbI3 was not found. The less formation of perovskite when NH4+ is incorporated is due to geometrical factors and not changes in the chemical state bonding of the ions. Also, the samples where perovskite is formed show similar band gap values. A slight increase is observed for samples with x=0.5 and 0.75. For the sample with x=1, a drastic increase of the band gap is obtained. Periodic-DFT calculations agree with the experimental structural tendency when NH4+ is incorporated and the density of states analysis confirmed the experimental band gap. The perovskite with 25źat.% of NH4+ was selected for studying the effect of the concentration of Cd on the structural and electronic properties. The theoretical band gap values decreased with the Cd concentration where the narrowing of Cd s-states in the conduction band plays an important role.

The Role of Surfactants in the Stability of NiO Nanofluids: An Experimental and DFT Study

This study shows an analysis of the stability of nanofluids based on a eutectic mixture of diphenyl oxide and biphenyl, which is used as a heat transfer fluid (HTF) in concentrating solar energy, and NiO nanoparticles. Two surfactants are used to analyse the stability of the nanofluids: benzalkonium chloride (BAC) and 1-octadecanethiol (ODT). From an experimental perspective, the stability is analysed by means of UV/Vis spectroscopy, particle size measurements through the dynamic light-scattering technique, and ζ-potential measurements. The results show that the stability of the nanofluids improves with the use of BAC. DFT calculations are performed to understand the role played by the surfactants. The interaction of the surfactants with both the fluid and the NiO (100) surface is studied. Quantum theory of atoms in molecules (QTAIM) analysis shows that hydrogen bridge interactions favour the stability of the fluid-surfactant mixture. The more stabilising NiO-surfactant interaction involves the Ni-H interaction of the -SH and -CH3 groups of ODT and BAC. Also, nanofluids with BAC are favoured over those with ODT, which is in agreement with experimental results. The structural and electronic effects of incorporating the surfactant onto the NiO (100) surface are shown by using electron localisation function analysis, the non-covalent interaction index and projected density of states.

Modeling the interactions of phthalocyanines in water: From the Cu(II)-tetrasulphonate to the metal-free phthalocyanine

A quantum and statistical study on the effects of the ions Cu(2+) and SO(3)(-) in the solvent structure around the metal-free phthalocyanine (H(2)Pc) is presented. We developed an ab initio interaction potential for the system CuPc-H(2)O based on quantum chemical calculations and studied its transferability to the H(2)Pc-H(2)O and [CuPc(SO(3))(4)](4-)-H(2)O interactions. The use of the molecular dynamics technique allows the determination of energetic and structural properties of CuPc, H(2)Pc, and [CuPc(SO(3))(4)](4-) in water and the understanding of the keys for the different behaviors of the three phthalocyanine (Pc) derivatives in water. The inclusion of the Cu(2+) cation in the Pc structure reinforces the appearance of two axial water molecules and second-shell water molecules in the solvent structure, whereas the presence of SO(3)(-) anions implies a well defined hydration shell of about eight water molecules around them making the macrocycle soluble in water. Debye-Waller factors for axial water molecules have been obtained in order to examine the potential sensitivity of the extended x-ray absorption fine structure technique to detect the axial water molecules.

Theoretical study on the hydrophobic and hydrophilic hydration on large solutes: The case of phthalocyanines in water

A theoretical study on the hydration phenomena of three representative Phthalocyanines (Pcs): the metal-free, H2Pc, and the metal-containing, Cu-phthalocyanine, CuPc, and its soluble sulphonated derivative, [CuPc(SO3)4](4-), is presented. Structural and dynamic properties of molecular dynamics trajectories of these Pcs in solution were evaluated. The hydration shells of the Pcs were defined by means of spheroids adapted to the solute shape. Structural analysis of the axial region compared to the peripheral region indicates that there are no significant changes among the different macrocycles, but that of [CuPc(SO3)4](4-), where the polyoxoanion presence induces a typically hydrophilic hydration structure. The analyzed water dynamic properties cover mean residence times, translational and orientational diffusion coefficients, and hydrogen bond network. These properties allow a thorough discussion about the simultaneous existence of hydrophobic and hydrophilic hydration in these macrocycles, and indicate the trend of water structure to well define shells in the environment of hydrophobic solutes. The comparison between the structural and dynamical analysis of the hydration of the amphipathic [CuPc(SO3)4](4-) and the non-soluble Cu-Pc shows a very weak coupling among the hydrophilic and hydrophobic fragments of the macrocycle. Quantitative results are employed to revisit the iceberg model proposed by Frank and Evans, leading to conclude that structure and dynamics support a non-strict interpretation of the iceberg view, although the qualitative trends pointed out by the model are supported.

MoS2 nanosheets vs. nanowires: preparation and a theoretical study of highly stable and efficient nanofluids for concentrating solar power

The nano-colloidal suspension of nanomaterials in a base fluid, typically named a nanofluid, is a promising system that shows interesting properties, such as those related to heat transfer processes. Obtaining nanofluids with high stability is a priority challenge for this kind of system. So, a rationalization of the preparation of nanofluids is clearly needed. Thus, this study presents a methodology based on liquid phase exfoliation that makes it possible to prepare stable nanofluids and control the morphology of the nanostructures, which is defined by the surfactant used. Two stable nanofluids were prepared based on MoS2 nanosheets and MoS2 nanowires and a typical heat transfer fluid (HTF) used in high temperature applications. Periodic-Density Functional Theory (periodic-DFT) calculations were performed to rationalize why different nanostructures were obtained according to the surfactant used. Finally, enhancements in thermal properties were found, being up to 57% for thermal conductivity and up to 7.5% for isobaric specific heat. Therefore, these nanofluids are a promising alternative to the typical HTF used, which is a eutectic mixture of biphenyl and diphenyl oxide. Also, to our knowledge, controlling the nanostructures obtained and the rationalization of the methodology for the preparation of stable nanofluids is reported for the first time. This leads to highly stable nanofluids with improved thermal properties, promising for application in concentrating solar power.