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2018


Adhesion enhancement of DLC hard coatings by HiPIMS metal ion etching pretreatment


Santiago, JA; Fernandez-Martinez, I; Wennberg, A; Molina-Aldareguia, JM; Castillo-Rodriguez, M; Rojas, TC; Sanchez-Lopez, JC; Gonzalez, MU; Garcia-Martin, JM; Li, H; Bellido-Gonzalez, V; Monclus, MA; Gonzalez-Arrabal, R
Surface & Coatings Technology, 349 (2018) 787-796

ABSTRACT

Poor adhesion is a recurrent problem for the wider use of diamond-like carbon (DLC) coatings in industrial applications. In this work, we investigate the effectiveness of high-power impulse magnetron sputtering (HiPIMS) metal ion etching to improve the adhesion of DLC coatings on high speed steel substrates. The influence of HiPIMS pretreatment parameters, the metal ion selection for the process and the addition of bonding layers on the adhesion properties were studied. Daimler-Benz and nanoscratch test methods were used to evaluate the adhesion. The elemental composition, morphology and microstructure of the samples were evaluated by EELS, SEM, AFM and HRTEM. In general, samples pretreated with HiPIMS metal ion etching withstand larger critical loads than those pretreated by conventional Ar + glow discharge and bonding layers. The pretreatment is proven to be very effective at removing surface contaminants and providing a gradual interface. The selection of Cr over Ti contributes to a significant improvement on the adhesion due to the reduction of the oxygen level at the interface thus ensuring an optimal coating-substrate contact and a more compliant structure, which prevents the delamination failure.


Septiembre, 2018 | DOI: 10.1016/j.surfcoat.2018.04.090

An approach to the heating dynamics of residues from greenhouse-crop plant biomass originated by tomatoes (Solanum lycopersicum, L.)


Garzon, E; Morales, L; Ortiz-Rodriguez, IM; Sanchez-Soto, PJ
Environmental Science and Pollution Research, 25 (2018) 25880-25887

ABSTRACT

The most representative of greenhouse-crop plant biomass residues of tomatoes (Solanum lycopersicum L.) were selected for this study by using X-ray fluorescence spectrometry (XRF) and X-ray powder diffraction (XRD). The heating dynamics in air in the 600-1150 degrees C range of these residues for the production of renewable energy and the resultant ashes have been investigated. A total of 11 elements were determined by XRF in the biomass ashes and some minor elements. The content of alkaline elements and chlorides decreased as increasing heating temperature and disappeared at 1150 degrees C. Alkaline salts, NaCl and KCl, were volatilized by heating since 800 degrees C. The total contents of S and P in the biomass ashes were associated to CaSO4, and a complex phosphate identified by XRD. CaCO3 present at 600 degrees C was decomposed to CaO with disappearance at 1000 degrees C. By heating, new silicates were formed by solid-state reactions in the biomass residue. The minor elements have been found in a relative proportion lower than 0.9wt.% and they characterized the obtained ashes, with potential use as micronutrients.


Septiembre, 2018 | DOI: 10.1007/s11356-018-2577-y

Sensing and biosensing with screen printed electrodes modified with nanostructured nickel oxide thin films prepared by magnetron sputtering at oblique angles


Salazar, P; Garcia-Garcia, FJ; Gonzalez-Elipe, AR
Electrochemistry Communications, 94 (2018) 5-8

ABSTRACT

This work reports about the sensing and biosensing applications of a novel screen printed electrode (SPE) modified by nanostructured nickel oxide thin films obtained by reactive magnetron sputtering under an oblique angle configuration. Using these films as electrodes we demonstrate their ability to detect hydrogen peroxide under neutral pH conditions. Furthermore, as a proof-of-concept, NiO-modified SPEs have been developed and their cholesterol biosensing properties determined by cyclic voltammetry and chronoamperometry.


Septiembre, 2018 | DOI: 10.1016/j.elecom.2018.07.020

Improving the activity of gold nanoparticles for the water-gas shift reaction using TiO2-Y2O3: an example of catalyst design


Plata, JJ; Romero-Sarria, F; Suarez, JA; Marquez, AM; Laguna, OH; Odriozola, JA; Sanz, JF
Physical Chemistry Chemical Physics, 20 (2018) 22076-22083

ABSTRACT

In the last ten years, there has been an acceleration in the pace at which new catalysts for the water-gas shift reaction are designed and synthesized. Pt-based catalysts remain the best solution when only activity is considered. However, cost, operation temperature, and deactivation phenomena are important variables when these catalysts are scaled in industry. Here, a new catalyst, Au/TiO2-Y2O3, is presented as an alternative to the less selective Pt/oxide systems. Experimental and theoretical techniques are combined to design, synthesize, characterize and analyze the performance of this system. The mixed oxide demonstrates a synergistic effect, improving the activity of the catalyst not only at large-to-medium temperatures but also at low temperatures. This effect is related to the homogeneous dispersion of the vacancies that act both as nucleation centers for smaller and more active gold nanoparticles and as dissociation sites for water molecules. The calculated reaction path points to carboxyl formation as the rate-limiting step with an activation energy of 6.9 kcal mol(-1), which is in quantitative agreement with experimental measurements and, to the best of our knowledge, it is the lowest activation energy reported for the water-gas shift reaction. This discovery demonstrates the importance of combining experimental and theoretical techniques to model and understand catalytic processes and opens the door to new improvements to reduce the operating temperature and the deactivation of the catalyst.


Septiembre, 2018 | DOI: 10.1039/c8cp03706j

A direct in situ observation of water-enhanced proton conductivity of Eu-doped ZrO2: Effect on WGS reaction


Garcia-Moncada, N; Bobadilla, LF; Poyato, R; Lopez-Cartes, C; Romero-Sarria, F; Centeno, MA; Odriozola, JA
Applied Catalysis B-Environmental, 231 (2018) 343-356

ABSTRACT

Eu-doped ZrO2 solid solutions have been synthesized in order to prepare proton conductors as water-enhancer additives for the WGS reaction. Elemental characterization has been carried out revealing homogeneous dopant distribution resulting in fluorite-type solid solutions for Eu2O3 contents up to similar to 9 mol.%. Representative samples of the Eu-doped ZrO2 series have been analysed by Impedance Spectroscopy (IS) in inert, oxygen and wet conditions. The solid solution with 5 mol.% of Eu2O3 has presented the highest conductivity values for all tested conditions indicating an optimal amount of dopant. Moreover, the presence of vapour pressure results in an increment of the conductivity at temperatures lower than 300 degrees C, meanwhile at higher temperatures the conductivity is the same than that in inert conditions. To elucidate these results, in situ DRIFTS studies were carried out. These experiments evidenced the existence of water dissociation at oxygen vacancies (band at 3724 cm(-1)) as well as the presence of physisorbed water at temperatures up to similar to 300 degrees C where the band at 5248 cm(-1) characteristic of these species disappeared. These results points to a layer model where the physisorbed water interacts with surface hydroxyls generated by dissociated water that improves the proton conductivity through Grotthuss' mechanism in the RT-300 degrees C temperature range. These samples were successfully tested in WGS reaction as additive to a typical Pt-based catalyst. The presence of the mixed oxide reveals an increase of the catalyst' activity assisted by the proton conductor, since improves the water activation step.


Septiembre, 2018 | DOI: 10.1016/j.apcatb.2018.03.001

Performance improvement in olive stone's combustion from a previous carbonization transformation


Gomez-Martin, A; Chacartegui, R; Ramirez-Rico, J; Martinez-Fernandez, J
Fuel, 228 (2018) 254-262

ABSTRACT

Under the framework of circular economy, agricultural wastes are an interesting carbon-based feedstock for thermal energy and power generation. Their use could extend the availability of biomass-based fuel and, at the same time, would reduce negative environmental effects. However, depending on the residues' characteristics, their direct combustion in boilers presents some challenges which could be overcome with a carbonization pretreatment. In this paper, the main mechanisms of thermochemical transformation of an abundant agricultural waste, olive stone, into biochar products via slow carbonization are analyzed, with emphasis on the effect of peak carbonization temperature. Thermogravimetric and differential scanning calorimetry analysis are used to evaluate the performance of the resulting biochars compared to raw olive stone in combustion processes and to assess the correlation between the peak carbonization temperature and compositional and fuel properties. Results show that with a prior treatment up to an optimum temperature of 800 degrees C the energy density is increased up to three times compared to the raw material. These findings suggest that carbonization of olive stones reduces the barriers to their direct use in current biomass boiler technology.


Septiembre, 2018 | DOI: 10.1016/j.fuel.2018.04.127

Iron-Catalyzed Graphitic Carbon Materials from Biomass Resources as Anodes for Lithium-Ion Batteries


Gomez-Martin, A; Martinez-Fernandez, J; Ruttert, M; Heckmann, A; Winter, M; Placke, T; Ramirez-Rico, J
Chemsuschem, 11 (2018) 2776-2787

ABSTRACT

Graphitized carbon materials from biomass resources were successfully synthesized with an iron catalyst, and their electrochemical performance as anode materials for lithium-ion batteries (LIBs) was investigated. Peak pyrolysis temperatures between 850 and 2000 degrees C were covered to study the effect of crystallinity and microstructural parameters on the anodic behavior, with a focus on the first-cycle Coulombic efficiency, reversible specific capacity, and rate performance. In terms of capacity, results at the highest temperatures are comparable to those of commercially used synthetic graphite derived from a petroleum coke precursor at higher temperatures, and up to twice as much as that of uncatalyzed biomass-derived carbons. The opportunity to graphitize low-cost biomass resources at moderate temperatures through this one-step environmentally friendly process, and the positive effects on the specific capacity, make it interesting to develop more sustainable graphite-based anodes for LIBs.


Agosto, 2018 | DOI: 10.1002/cssc.201800831

Synthesis and characterization of SiC/Si3N4 composites from rice husks


Real, C; Cordoba, JM; Alcala, MD
Ceramics International, 44 (2018) 14645-14651

ABSTRACT

SiC-Si3N4 composites have been obtained by carbothermal reduction of rice husk under a nitrogen-argon atmosphere at 1450 degrees C, which is a lower temperature than those used by other authors. On the other hand, tailoring the argon/nitrogen ratio led to the obtained of SiC-Si3N4 composites across the whole range of compositions. Phosphoric acid treatment permited the synthesis of the composite without a pyrolysis step. The final products were characterized by X-ray diffractometry, IR spectroscopy and scanning electron microscopy.


Agosto, 2018 | DOI: 10.1016/j.ceramint.2018.05.090

Solar pilot plant scale hydrogen generation by irradiation of Cu/TiO2 composites in presence of sacrificial electron donors


Maldonado, MI; Lopez-Martin, A; Colon, G; Peral, J; Martinez-Costa, JI; Malato, S
Applied Catalysis B-Environmental, 229 (2018) 15-23

ABSTRACT

A Cu/TiO2 photocatalyst has been synthesised by reducing a Cu precursor with NaBH4 onto the surface of a sulphate pretreated TiO2 obtained by a sol-gel procedure. The catalyst, that shows a clearly defined anatase phase with high crystallinity and relatively high surface area, and contains Cu2O and CuO deposits on its surface, has been used to produce hydrogen in a solar driven pilot plant scale photocatalytic reactor. Different electron donor aqueous solutions (methanol, glycerol, and a real municipal wastewater treatment plant influent) have been tested showing similar or even higher energy efficiency than those obtained using more expensive noble metal based photocatalytic systems. The glycerol solutions have provided the best reactive environments for hydrogen generation.


Agosto, 2018 | DOI: 10.1016/j.apcatb.2018.02.005

Effect of milling mechanism on the CO2 capture performance of limestone in the Calcium Looping process


Benitez-Guerrero, M; Valverde, JM; Perejon, A; Sanchez-Jimenez, PE; Perez-Maqueda, LA
Chemical Engineering Journal, 346 (2018) 549-556

ABSTRACT

This work analyzes the relevant influence of milling on the CO2 capture performance of CaO derived from natural limestone. Diverse types of milling mechanisms produce contrasting effects on the microstructure of the CaO formed after calcination of the milled limestone samples, which affects crucially the kinetics of carbonation at conditions for CO2 capture. The capture capacity of CaO derived from limestone samples milled using either shear or impact based mills is impaired compared to as-received limestone. After calcination of the milled samples, the resulting CaO porosity is increased while crystallinity is enhanced, which hinders carbonation. Conversely, if the material is simultaneously subjected to intense impact and shear stresses, CaO porosity is promoted whereas CaO cristanillity is reduced, which enhances carbonation in both the reaction and solid-state diffusion controlled regimes.


Agosto, 2018 | DOI: 10.1016/j.cej.2018.03.146

On the determination of thermal degradation effects and detection techniques for thermoplastic composites obtained by automatic lamination


Martin, MI; Rodriguez-Lence, F; Guemes, A; Fernandez-Lopez, A; Perez-Maqueda, LA; Perejon, A
Composites part A-Applied science and manufacturing, 111 (2018) 23-32

ABSTRACT

Automatic lay-up and in-situ consolidation with thermoplastic composite materials is a technology under research for its expected use in the profitable manufacturing of structural aeronautical parts. This study is devoted to analysing the possible effects of thermal degradation produced by this manufacturing technique. 
Rheological measurements showed that there is negligible degradation in PEEK for the temperatures reached during the process. Thermogravimetric analysis under linear heating and constant rate conditions show that thermal degradation is a complex process with a number of overlapping steps. A general kinetic equation that describes the degradation of the material with temperature has been proposed and validated. Attenuated total reflectance Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy confirmed that there is no remarkable degradation. The use of a combination of in-situ and ex-situ experimental techniques, including kinetic modelling, not only provides reliable information about degradation but also allows setting optimal processing conditions.


Agosto, 2018 | DOI: 10.1016/j.compositesa.2018.05.006

Electrical conduction mechanisms in graphene nanoplatelet/yttria tetragonal zirconia composites


Poyato, R; Osuna, J; Morales-Rodriguez, A; Gallardo-Lopez, A
Ceramics International, 44 (2018) 14610-14616

ABSTRACT

Yttria tetragonal zirconia polycrystalline (3YTZP) ceramic composites with 5, 10 and 20 vol% graphene nano-platelets (GNPs) were prepared by spark plasma sintering (SPS) and their electrical conductivity as a function of temperature was characterized. The composites exhibit anisotropic microstructures so the electrical conductivity studies were carried out in two directions: perpendicular (sigma(perpendicular to)) and parallel (sigma(parallel to)) to the SPS pressing axis. The composites with 5 and 10 GNP vol% showed high electrical anisotropy, whereas the composite with 20 GNP vol % exhibited nearly isotropic electrical behavior. sigma(perpendicular to) shows metallic-type behavior in the composites with 10 and 20 vol% GNP revealing that charge transport takes place through defect-free GNPs. For the composite with 5 vol % GNP the observed semiconductor-type behavior was explained by a two dimensional variable range hopping mechanism. sigma(parallel to)shows metallic-type conductivity in the composite with 20 GNP vol% and positive d sigma(parallel to)/dT slope in the composites with 5 and 10 GNP vol%.


Agosto, 2018 | DOI: 10.1016/j.ceramint.2018.05.082

Synergy achieved in silver-TiO2 nanocomposites for the inhibition of biofouling on limestone


Becerra, J; Zaderenko, AP; Sayagues, MJ; Ortiz, R; Ortiz, P
Building and Environment, 141 (2018) 80-90

ABSTRACT

Biodeterioration of stone monuments is estimated to be as high as 20-30% of the total degradation suffered by Cultural Heritage constructions. With regard to this problem, bactericidal treatments are mainly based on cleaning. These processes, while effective in the short term, require frequent reapplications increasing potential damages to the monument. Silver nanoparticles offer many advantages over traditionally employed products, such as their prolonged biocide efficacy and their low toxicity to humans and environment. The aim of this study was to evaluate the applicability and effectiveness of seven nanocomposite treatments based on titanium dioxide and/or silver nanoparticles to prevent biodeterioration of limestone monuments. These nanocomposites were characterized by UV Visible spectrophotometry, Dynamic Light Scattering and Electron Microscopy. To assess their bactericidal activity, accelerated weathering tests were performed on limestones from the quarry of Utrera, a source widely employed in such iconic monuments as the Cathedral of Seville (Spain). Furthermore, the samples of biopatina employed in our assays stemmed from the fa ades of historical buildings from Seville. Our results show that silver and titanium dioxide nanocomposites stabilized by citrate achieve a high biocide effect while maintaining color alterations at a low level.


Agosto, 2018 | DOI: 10.1016/j.buildenv.2018.05.020

Unravelling the Role of Oxygen Vacancies in the Mechanism of the Reverse Water-Gas Shift Reaction by Operando DRIFTS and Ultraviolet-Visible Spectroscopy


Bobadilla, LF; Santos, JL; Ivanova, S; Odriozola, JA; Urakawa, A
ACS Catalysis, 8 (2018) 7455-7467

ABSTRACT

The reaction mechanism of the reverse water gas shift (RWGS) reaction was investigated using two commercial gold-based catalysts supported on Al2O3 and TiO2. The surface species formed during the reaction and reaction mechanisms were elucidated by transient and steady-state operando DRIFTS studies. It was revealed that RWGS reaction over Au/Al2O3 proceeds through the formation of formate intermediates that are reduced to CO. In the case of the Au/TiO2 catalyst, the reaction goes through a redox mechanism with the suggested formation of hydroxycarbonyl intermediates, which further decompose to CO and water. The Ti-3+ species, the surface hydroxyls, and oxygen vacancies jointly participate. The absence of carbonyl species adsorbed on gold particles during the reaction for both catalysts indicates that the reaction pathway involving dissociative adsorption of CO2 on Au particles can be discarded. To complete the study, operando ultraviolet visible spectroscopy was successfully applied to confirm the presence of Ti3+ and to understand the role of the oxygen vacancies of TiO2 support in activating CO2 and thus the subsequent RWGS reaction.


Agosto, 2018 | DOI: 10.1021/acscatal.8b02121

Structural Reversibility of LaCo1-xCuxO3 Followed by In Situ X-ray Diffraction and Absorption Spectroscopy


Pereniguez, Rosa; Ferri, Davide
Chemphyschem, 19 (2018) 1876-1885

ABSTRACT

Combinations of perovskite-type oxides with transition and precious metals exhibit a remarkable self-regenerable property that could be exploited for numerous practical applications. The objective of the present work was to study the reversibility of structural changes of perovskite-type oxides under cyclic reducing/oxidizing atmosphere by taking advantage of the reducibility of LaCoO3. LaCoO3 +/- and LaCo0.8Cu0.2O3 +/- were prepared by ultrasonic spray combustion and were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS) and temperature-programmed reduction (TPR). XRD and XAS data confirmed that copper adopted the coordination environment of cobalt at the B-site of the rhombohedral LaCoO3 under the selected synthesis conditions. The structural evolution under reducing atmosphere was studied by insitu XRD and XANES supporting the assignment of the observed structural changes to the reduction of the perovskite-type oxide from ABB'O-3 (B'=Cu) to B'(0)/ABO(3) and to B'B-0(0)/A(2)O(3). Successive redox cycles allowed the observation of a nearly complete reversibility of the perovskite phase, i.e. copper was able to revert into LaCoO3 upon oxidation. The reversible reduction/segregation of copper and incorporation at the B-site of the perovskite-type oxides could be used in chemical processes where the material can be functionalized by segregation of Cu and protected against irreversible structural changes upon re-oxidation.


Agosto, 2018 | DOI: 10.1002/cphc.201800069

Mechanochemical synthesis, structural, magnetic, optical and electrooptical properties of CuFeS2 nanoparticles


Dutkova, E; Bujnakova, Z; Kovac, J; Skorvanek, I; Sayagues, MJ; Zorkovska, A; Kovac, J; Balaz, P
Advanced Powder Technology, 29 (2018) 1820-1826

ABSTRACT

The rapid mechanochemical synthesis of nanocrystalline CuFeS2 particles prepared by high-energy milling for 60 min in a planetary mill from copper, iron and sulphur elements is reported. The CuFeS2 nanoparticles crystallize in tetragonal structure with mean crystallite size of about 38 ± 1 nm determined by XRD analysis. HRTEM study also revealed the presence of nanocrystals with the size of 5–30 nm with the tendency to form agglomerates. The Raman spectrum confirms the chalcopyrite structure. Low temperature magnetic data for CuFeS2 support the coexistence of antiferromagnetic and paramagnetic spin structure. Moreover, the hysteresis loops taken at temperatures from 5 K to 300 K revealed a presence of very small amount of ferromagnetic phase, which seems to be associated with the non-consumed elemental Fe in as-prepared nanoparticles. The optical band gap of CuFeS2nanoparticles has been detected to be 1.05 eV, larger than band gap of the bulk material. The wider gap possibly resulted from the nano-size effect. Photoresponses of CuFeS2nanoparticles were confirmed by I-V measurements under dark and light illumination. It was demonstrated that mechanochemical synthesis can be successfully employed in the one step preparation of nanocrystalline CuFeS2 with good structural, magnetic, optical and electrooptical properties.


Agosto, 2018 | DOI: 10.1016/j.apt.2018.04.018

Surface modification of Ti-6Al-4V alloys manufactured by selective laser melting: Microstructural and tribo-mechanical characterization


Torres, Y; Sarria, P; Gotor, FJ; Gutierrez, E; Peon, E; Beltran, AM; Gonzalez, JE
Surface & Coatings Technology, 348 (2018) 31-40

ABSTRACT

Medical grade of both titanium (Ti) and Ti6A14V alloy are recognized as the metallic biomaterials with the better outcomes for clinical repair of bone tissue thanks to their suitable mechanical properties and corrosion resistance. However, those Ti advantages are not enough to avoid failure risks of bone implants; between 5 and 10% of Ti implants fail due to a deficient osseointegration, within 5 years of post-implantation. Most of these failures indicate the necessity of getting a better biomechanical-biofunctional balance. Microstructural and tribo-mechanical characterizations were performed on Ti6A14V samples obtained by selective laser melting and subjected to different surface treatments (thermal stress relief, acid etching, chemical treatment and thermochemical treatment). Scanning electron microscopy and X-ray diffraction were used for detailed characterization of the elemental composition, phase analysis and surface morphology. Micro-hardness and scratch tests were employed to evaluate the tribo-mechanical properties, which were improved after consecutive surface treatments. Protuberances with spherical morphology, as a remainder of the original powder, were present on the surface. The resulting modified surfaces were constituted by rutile (major phase) and anatase (minor phase). Submicro-nano-topographies were obtained after the chemical and thermochemical treatments.


Agosto, 2018 | DOI: 10.1016/j.surfcoat.2018.05.015

The effect of vitreous phase on mullite and mullite-based ceramic composites from kaolin wastes as by-products of mining, sericite clays and kaolinite


Sanchez-Soto, PJ; Eliche-Quesada, D; Martinez-Martinez, S; Garzon-Garzon, E; Perez-Villarejo, L; Rincon, JM
Materials Letters, 223 (2018) 154-158

ABSTRACT

Mullite precursors were prepared using kaolin waste, sericite clay containing kaolinite and industrial kaolin with addition of alumina in a wet medium to synthesize mullite (72 wt% Al2O3 and 28 wt% SiO2). Uniaxial pressed bars of the powdered mullite precursors were fired in the range 1400-1600 degrees C with soaking times 30-120 min. The resultant materials were studied by XRD and SEM-EDX. Bulk densities, apparent porosities and flexural strengths in four points were determined in the fired bars at 1500, 1550 and 1600 degrees C. It was concluded that the thermal behaviour of these mullite precursors was influenced by the presence of impurities in the raw materials. These impurities originate a liquid phase forming a glassy phase which produces a progressive and enhanced densification of the mullite materials by reaction sintering at 1500-1600 degrees C. The technical properties were also influenced by the relative proportion of vitreous phase. The microstructure of characteristic mullite crystals was revealed by SEM. It was emphasized the use of kaolin waste by-products of mining and sericite clays as valuable raw materials for mullite preparation.


Julio, 2018 | DOI: 10.1016/j.matlet.2018.04.037

Photo/Electrocatalytic Properties of Nanocrystalline ZnO and La–Doped ZnO: Combined DFT Fundamental Semiconducting Properties and Experimental Study


Ahsaine, A.H.; Slassi, A.; Naciri, Y.; Chennah, A.; Jaramillo‐Páez, C.; Anfar, Z.; Zbair, M.; Benlhachemi, A.; Navío, J.A.
Chemistry Select, 3 (2018) 7778-7791

ABSTRACT

This work reports the synthesis of nanocrystalline ZnO and 5% La‐doped ZnO (La/ZnO) materials for photo/electrocatalytic degradation of Rhodamine B. The samples were characterized by X‐Ray diffraction, scanning and transmission electron microscopy, X‐Ray photoelectron spectroscopy and diffuse reflectance spectra. The effect of La doping on electronic structure was investigated using density functional theory calculations (DFT), La‐doped ZnO showed an n‐type metallic nature compared to pristine ZnO and La doping creates occupied states within the band gap edge. Under UV light, La/ZnO showed higher kinetic constant and efficiency than ZnO. A possible mechanism was elaborated on the basis of DFT and active trapping measurements. Different initial Rhodamine B concentration were studied to assess the electro‐oxidation of RhB. The electrochemical degradation of RhB over La/ZnO spindles electrode was pronounced with three time's high kinetic constant. The superior electro/photoactivity of La/ZnO was due to its unique morphology, high charge separation of the charge carriers and higher conductivity induced by La‐doping (intermediary levels). Superoxide ions and holes were the main active species for the photodegradation. Whereas, synergetic effect of hydroxyl radicals and hypochlorite ions were responsible of the high RhB electrocatalytic degradation.


Julio, 2018 | DOI: 10.1002/slct.201801729

Origin of Light-Induced Photophysical Effects in Organic Metal Halide Perovskites in the Presence of Oxygen


Anaya, M; Galisteo-Lopez, JF; Calvo, ME; Espinos, JP; Miguez, H
Journal of Physical Chemistry Letters, 9 (2018) 3891-3896

ABSTRACT

Herein we present a combined study of the evolution of both the photoluminescence (PL) and the surface chemical structure of organic metal halide perovskites as the environmental oxygen pressure rises from ultrahigh vacuum up to a few thousandths of an atmosphere. Analyzing the changes occurring at the semiconductor surface upon photoexcitation under a controlled oxygen atmosphere in an X-ray photoelectron spectroscopy (XPS) chamber, we can rationalize the rich variety of photophysical phenomena observed and provide a plausible explanation for light-induced ion migration, one of the most conspicuous and debated concomitant effects detected during photoexcitation. We find direct evidence of the formation of a superficial layer of negatively charged oxygen species capable of repelling the halide anions away from the surface and toward the bulk. The reported PL transient dynamics, the partial recovery of the initial state when photoexcitation stops, and the eventual degradation after intense exposure times can thus be rationalized.


Julio, 2018 | DOI: 10.1021/acs.jpclett.8b01830

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