A Solvothermal Synthesis of TiO2 Nanoparticles in a Non-Polar Medium to Prepare Highly Stable Nanofluids with Improved Thermal Properties.

Nanofluids are systems with several interesting heat transfer applications, but it can be a challenge to obtain highly stable suspensions. One way to overcome this challenge is to create the appropriate conditions to disperse the nanomaterial in the fluid. However, when the heat transfer fluid used is a non-polar organic oil, there are complications due to the low polarity of this solvent. Therefore, this study introduces a method to synthesize TiO2 nanoparticles inside a non-polar fluid typically used in heat transfer applications. Nanoparticles produced were characterized for their structural and chemical properties using techniques such as X-ray Diffraction (XRD), Raman spectroscopy, Transmission Electron Microscopy (TEM), Fourier Transform Infrared (FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). The nanofluid showed a high stability, which was analyzed by means of UV-vis spectroscopy and by measuring its particle size and ζ potential. So, this nanofluid will have many possible applications. In this work, the use as heat transfer fluid was tested. In this sense, nanofluid also presented enhanced isobaric specific heat and thermal conductivity values with regard to the base fluid, which led to the heat transfer coefficient increasing by 14.4%. Thus, the nanofluid prepared could be a promising alternative to typical HTFs thanks to its improved thermal properties and high stability resulting from the synthesis procedure.

Revealing at the molecular level the role of the surfactant in the enhancement of the thermal properties of the gold nanofluid system used for concentrating solar power.

A molecular dynamics study based on gold nanofluids performed with and without the presence of tetraoctylammonium halide as a surfactant in a base fluid is presented. The base fluid consisting of a mixture of biphenyl and diphenyl oxide is used in concentrating solar power (CSP) plants. The radial distribution functions (RDFs) and spatial distribution functions (SDFs) were analysed with the temperature. Theoretical results indicate that the surfactant acts as a kind of net around the nanoparticle that plays an active role in enhancing the thermal properties of the gold nanofluid system. A greater lability of the base fluid-surfactant interactions than the base fluid-gold nanoparticle interactions is observed. At lower temperatures, there is an inner layer around the gold nanoparticle with two surfactant molecules close to the metal. At a higher temperature a ratio of gold nanoparticles : diphenyl oxide molecules of 1 : 4 is maintained in the inner layer for the systems with and without the presence of a surfactant. At the highest temperatures, the presence of the surfactant in a second shell impedes the approximation of the fifth diphenyl oxide molecule. Thus, the surfactant affects the macroscopic properties of the gold nanofluid system at the molecular level.

MoS2/Cu/TiO2 nanoparticles: synthesis, characterization and effect on photocatalytic decomposition of methylene blue in water under visible light.

Photodegradation processes are of great interest in a range of applications, one of which is the photodecomposition of pollutants. For this reason, analysing nanoparticles that improve the efficiency of these processes under solar radiation are very necessary. Thus, in this study, TiO2 was doped with Mo and Cu using low-temperature hydrolysis as the method of synthesis. Pure TiO2 and x%MoS2/Cu/TiO2 nanoparticles were prepared, where x is the theoretical quantity of MoS2 added (0.0%, 1.0%, 5.5%, 10.0%), setting the nominal quantity of Cu at 0.5 wt.%. The samples obtained were characterized by X-ray diffraction, Raman spectroscopy, X-ray electron spectroscopy and UV-Vis spectroscopy in diffuse reflectance mode. The results suggest that the TiO2 structure was doped with the Mo6+ and Cu2+ ions in the position of the Ti4+. The x%MoS2/Cu/TiO2 samples presented lower band gap energy values and greater optical absorption in the visible region than the pure TiO2 sample. Lastly, the photocatalytic activity of the samples was assessed by means of the photodegradation of methylene blue under visible light. The results show that when the quantity of Mo in the co-doped samples increased (x%MoS2/Cu/TiO2) there were significant increases of up to 93% in the photocatalytic activity.

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.

Micro-Raman Spectroscopy for the Determination of Local Temperature Increases in TiO2 Thin Films due to the Effect of Radiation.

This study applied a classic method involving Raman spectroscopy and the use of Stokes and anti-Stokes peaks to measure the temperature of TiO2 thin films found in dye-sensitized solar cells (DSSCs). In addition, three mathematical formulae were used and analyzed to estimate the increase in temperature generated solely by the effect of the radiation. The tests and calculations performed showed an increase in the temperature of the TiO2 film. That is, the films were heated by the radiation they were exposed to. A temperature increase of up to 30 K was detected for the sample with a single layer of TiO2, and over 40 K for the sample with three layers for the highest radiation powers used, and greater increases in temperature were observed in the thicker films.

A Study of Overheating of Thermostatically Controlled TiO2 Thin Films by Using Raman Spectroscopy.

In this study a classic Raman spectroscopy method is applied and the intensity ratio of Stokes and anti-Stokes peaks is used to measure the temperature of thermostatically controlled TiO2 thin films. In addition, three mathematical formulae are used and analyzed to estimate the temperature of the TiO2 thin films. Overheating of the samples above the thermostatically controlled temperature was observed while recording the Raman spectra, with a temperature increase of up to 30 K being detected. DFT-periodic calculations showed that the anatase (101) surface had a smaller band gap than bulk anatase. Thus, it can absorb the laser radiation with a wavelength of 532 nm that is used in the experimental setup. Part of the absorbed photon energy transfers into phonon energy, heating up the anatase phase, thus leading to the heating of the samples. Moreover, overheating of the samples indicates that the experimental method used in this study can lead to deviations in their real absolute temperature values.

Revealing the role of Pb(2+) in the stability of organic-inorganic hybrid perovskite CH3NH3Pb1-xCdxI3: an experimental and theoretical study.

This paper presents the synthesis of organic-inorganic hybrid perovskite CH3NH3Pb1-xCdxI3. The effect of incorporating Cd(2+) or Pb(2+) on the stability of the perovskite structure was analysed from a theoretical and experimental viewpoint. The XRD results showed that the tetragonal perovskite structure was formed for x values of up to 0.5, which seems to indicate that the presence of a considerable amount of Pb(2+) is necessary to stabilise the structure. In turn, UV-Vis spectroscopy showed how the presence of Cd(2+) led to a reduction in the optical band gap of the perovskite structure of up to 9% for CH3NH3Pb0.5Cd0.5I3 with regard to the MAPbI3 structure. Moreover, periodic-DFT calculations were performed to understand the effect of the increased concentration of Cd on the structural and electronic properties of MAPbI3 perovskites. The analysis of both the ELF and the non-covalent interaction (NCI) index show the important role played by the Pb(2+) ions in stabilizing this kind of hybrid perovskite structures. Finally, the DOS analysis confirmed the experimental results obtained using UV-Vis spectroscopy. The theoretical band gap values decreased as the concentration of Cd increased.

New insights into organic-inorganic hybrid perovskite CH3NH3PbI3 nanoparticles. An experimental and theoretical study of doping in Pb²âº sites with Sn²âº, Sr²âº, Cd²âº and Ca²âº.

This paper presents the synthesis of the organic-inorganic hybrid perovskite, CH3NH3PbI3, doped in the Pb(2+) position with Sn(2+), Sr(2+), Cd(2+) and Ca(2+). The incorporation of the dopants into the crystalline structure was analysed, observing how the characteristics of the dopant affected properties such as the crystalline phase, emission and optical properties. XRD showed how doping with Sn(2+), Sr(2+) and Cd(2+) did not modify the normal tetragonal phase. When doping with Ca(2+), the cubic phase was obtained. Moreover, DR-UV-Vis spectroscopy showed how the band gap decreased with the dopants, the values following the trend Sr(2+) < Cd(2+) < Ca(2+) < CH3NH3PbI3 ≈ Sn(2+). The biggest decrease was generated by Sr(2+), which reduced the CH3NH3PbI3 value by 4.5%. In turn, cathodoluminescence (CL) measurements confirmed the band gap obtained. Periodic-DFT calculations were performed to understand the experimental structures. The DOS analysis confirmed the experimental results obtained using UV-Vis spectroscopy, with the values calculated following the trend Sn(2+) ≈ Pb(2+) > Cd(2+) > Sr(2+) for the tetragonal structure and Pb(2+) > Ca(2+) for the cubic phase. The electron localization function (ELF) analysis showed similar electron localizations for undoped and Sn(2+)-doped tetragonal structures, which were different from those doped with Sr(2+) and Cd(2+). Furthermore, when Cd(2+) was incorporated, the Cd-I interaction was strengthened. For Ca(2+) doping, the Ca-I interaction had a greater ionic nature than Cd-I. Finally, an analysis based on the non-covalent interaction (NCI) index is presented to determine the weak-type interactions of the CH3NH3 groups with the dopant and I atoms. To our knowledge, this kind of analysis with these hybrid systems has not been performed previously.

Tm-doped TiO2 and Tm2Ti2O7 pyrochlore nanoparticles: enhancing the photocatalytic activity of rutile with a pyrochlore phase.

Tm-doped TiO2 nanoparticles were synthesized using a water-controlled hydrolysis reaction. Analysis was performed in order to determine the influence of the dopant concentration and annealing temperature on the phase, crystallinity, and electronic and optical properties of the resulting material. Various characterization techniques were utilized such as X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy and UV-vis spectroscopy. For the samples annealed at 773 and 973 K, anatase phase TiO2 was obtained, predominantly internally doped with Tm(3+). ICP-AES showed that a doping concentration of up to 5.8 atom % was obtained without reducing the crystallinity of the samples. The presence of Tm(3+) was confirmed by X-ray photoelectron spectroscopy and UV-vis spectroscopy: the incorporation of Tm(3+) was confirmed by the generation of new absorption bands that could be assigned to Tm(3+) transitions. Furthermore, when the samples were annealed at 1173 K, a pyrochlore phase (Tm2Ti2O7) mixed with TiO2 was obtained with a predominant rutile phase. The photodegradation of methylene blue showed that this pyrochlore phase enhanced the photocatalytic activity of the rutile phase.

Introducing "UCA-FUKUI" software: reactivity-index calculations.

A new software (UCA-FUKUI) has been developed to facilitate the theoretical study of chemical reactivity. This program can calculate global parameters like hardness, softness, philicities, and Fukui condensed functions, and also local parameters from the condensed functions. To facilitate access to the program we have developed a very easy-to-use interface. We have tested the performance of the software by calculating the global and local reactivity indexes of a group of representative molecules. Finite difference and frontier molecular orbital methods were compared and their correlation tested. Finally, we have extended the analysis to a set of ligands of importance in coordination chemistry, and the results are compared with the exact calculation. As a general trend, our study shows the existence of a high correlation between global parameters, but a weaker correlation between local parameters.

Thermo-selective Tm(x)Ti(1-x)O(2-x/2) nanoparticles: from Tm-doped anatase TiO2 to a rutile/pyrochlore Tm2Ti2O7 mixture. An experimental and theoretical study with a photocatalytic application.

This is an experimental and theoretical study of thulium doped TiO2 nanoparticles. From an experimental perspective, a method was used to synthesize thulium-doped TiO2 nanoparticles in which Tm(3+) replaces Ti(4+) in the lattice, which to our knowledge has neither been reported nor studied theoretically so far. Different proportions of anatase and rutile phases were obtained at different annealing temperatures, and XRD and Raman spectroscopy also revealed the presence of a pyrochlore phase (Tm2Ti2O7) at 1173 K. Thus, the structure of the Tm-doped nanoparticles was thermally-controlled. Furthermore, XPS showed the presence of Tm(3+) in the samples synthesized, which produces oxygen vacancies to maintain the local neutrality in the lattice. The presence of Tm(3+) in the samples led to changes in the UV-Vis absorption spectra, so they showed photoluminescence properties and new states in the band gap, which produce a new lower energy electronic transition than the main TiO2 one. Periodic DFT calculations were performed to understand the experimentally produced structures. The production of oxygen vacancies was analysed and the changes generated in the structure were fully detailed. The DOS and PDOS analyses confirmed the experimental results obtained using UV-Vis spectroscopy, and showed that the new electronic states in the band gap are due to interactions of the f state of Tm and the p state of O. Likewise, the charge study and the ELF analysis indicate that when Tm is introduced into the TiO2 structure, the Ti-O bond around the oxygen vacancy is strengthened. Finally, an example of a photocatalytic application was developed to show the high efficiency of the samples due to the heterojunction in the interfaces of the phases in the samples, which improved the charge separation and the good charge carrier mobility due to the presence of the pyrochlore phase, as was also shown theoretically.

Electronic and structural properties of highly aluminum ion doped TiO(2) nanoparticles: a combined experimental and theoretical study.

This study presents the experimental and theoretical study of highly internally Al-doped TiO2 nanoparticles. Two synthesis methods were used and detailed characterization was performed. There were differences in the doping and the crystallinity, but the nanoparticles synthesized with the different methods share common features. Anatase to rutile transformation occurred at higher temperatures with Al doping. X-ray photoelectron spectroscopy showed the generation of oxygen vacancies, which is an interesting feature in photocatalysis. In turn, the band-gap energy and the valence band did not change appreciably. Periodic density functional calculations were performed to model the experimentally doped structures, the formation of the oxygen vacancies, and the band gap. Calculation of the density of states confirmed the experimental band-gap energies. The theoretical results confirmed the presence of Ti(4+) and Al(3+) . The charge density study and electron localization function analysis indicated that the inclusion of Al in the anatase structure resulted in a strengthening of the TiO bonds around the vacancy.

Experimental and theoretical study of the electronic properties of Cu-doped anatase TiO2.

A good correlation was obtained between the electronic properties of Cu-doped anatase TiO2 by virtue of both physical chemistry characterization and theoretical calculations. Pure and Cu-doped TiO2 were synthesized. The composition, structural and electronic properties, and the band gap energy were obtained using several techniques. The method of synthesis used produces Cu-doped anatase TiO2, and XRD, XPS and Raman spectroscopy indicate that Cu atoms are incorporated in the structure by substitution of Ti atoms, generating a distortion of the structure and oxygen vacancies. In turn, the band gap energy of the synthesized samples decrease drastically with the Cu doping. Moreover, periodic density functional theory (DFT-periodic) calculations were carried out both to model the experimentally observed doped structures and to understand theoretically the experimental structures obtained, the formation of oxygen vacancies and the values of the band gap energy. From the analysis of density of states (DOS), projected density of states (PDOS) and the electron localization function (ELF) a decrease in the band gap is predicted upon increasing the Cu doping. Thus, the inclusion of Cu in the anatase structure implies a covalent character in the Cu-O interaction, which involves the appearance of new states in the valence band maximum with a narrowing in the band gap.

A methodology for improving laser beam induced current images of dye sensitized solar cells.

Using the laser beam induced current (LBIC) technique for the study of solar cells and photovoltaic devices, it is possible to obtain images representing the different degrees of quantum efficiency observed on the surface of these elements. Dye sensitized solar cells (DSSCs) or photoelectrochemical solar cells, in contrast to those based on solid-solid interfaces, show a slow response to irradiance variations--up to tens of seconds. This is basically due to both viscous matter transport processes and load transfer. This response is inappreciable when the device is functioning continuously but when a LBIC scan is performed, in which the laser moves quickly from one point to another, the slow response produces a memory effect and the signal generated at one given point depends on the conversion efficiency coefficients of the previously excited positions, resulting in diffuse images and a lack of sharpness. This work presents a methodology to correct high-resolution LBIC mappings of DSSCs using an algorithm based on the kinetics of the discharge process of the irradiated zone. The validity of the proposed method has been evaluated by carrying out experiments where the algorithm has been applied to LBIC mappings.