Revealing the true impact of interstitial and substitutional nitrogen doping in TiO2 on photoelectrochemical applications

Application of photocatalysts that strongly absorb within the visible range is a common strategy to improve the efficiency of photoelectrochemical (PEC) systems; this may translate to high photocurrents, but it is not always the case. Here, we show that nitrogen doping enhances visible light absorption of TiO2; however, it does not necessarily result in improved PEC performance. Depending on the applied external potential, N-doping can improve, or degrade, PEC performance either under water oxidation conditions or via hole scavenging (Na2S/Na2SO3). In this work, we developed a holistic approach to evaluate the true impact of N doping in TiO2 on PEC performance. Interstitial and substitutional N doping are experimentally explored for the first time through a simple and novel PEC approach which complemented X-ray photoelectron analyses. Using this approach, we show that interstitial N doping of anatase TiO2 dominates up to 400 °C and substitutional doping up to ca. 600 °C, without rutile formation. This reveals that the bottleneck for doping higher N-concentrations in TiO2 is the direct transformation to thermodynamically favorable N-rich phases, such as TiN/Ti2N at 700 °C, inhibiting the formation of rutile phase. Transmission electron microscopy revealed that N doping proceeds mainly from the inner to the outer tube walls via nitridation and follows a preferential pathway from interstitial to substitutional doping. Direct PEC experimental evidence on visible light activation of N doped TiO2, and the location of interband states, showed acceptor levels of 1.0 eV for substitutional and 0.7 eV for interstitial doping above the TiO2 valence band maximum. In addition, due to O vacancies and Ti3+ species, donor levels below the conduction band minimum were also created. These levels act as trapping/recombination centers for charge carriers and, therefore, the gain in the visible range due to N doping does not translate to an improved PEC performance by these structural defects. Ultimately, we show that whilst there is a benefit of visible light absorption through N doping in TiO2, the PEC performance of the samples only surpasses pristine TiO2 at relatively high biasing (>0.3 V vs. Ag/AgCl)

Characterization of Siloxane-poly(methyl methacrylate) Hybrid Films Obtained on a Tinplate Substrate Modified by the Addition of Organic and Inorganic Acids

Tinplate is used to food packaging and other types of packages. The corrosion resistance of the tinplate has been study due the necessity of an alternative to high environmental impact of chromatization process. Therefore protective coatings as hybrid films base elaborations with different acids are studied to improve the barrier effect against corrosion. The objective of this work is characterize hybrid films deposited on a tinplate from a sol made up of the alkoxide precursors 3-(trimethoxysilylpropyl) methacrylate (TMSM), tetraethoxysilane (TEOS) and poly(methyl methacrylate) (PMMA) together with one of three acids (acetic, hydrochloric or nitric acid) and to verify their action against the corrosion of the substrate. The films were obtained by a dip-coating process and cured for 3 hours at 160 °C. The film hydrophobicity was determined by contact angle measurements, and the morphology was evaluated by SEM. FTIR measurements were performed to characterize the chemical structures of the films. The electrochemical behavior of the coatings was evaluated by techniques open circuit potential monitoring (OCP), potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The results demonstrate that the siloxane-PMMA films improve the protective properties of the tinplate, with the films obtained by acetic acid addition exhibiting the greatest improvement

Niobium and niobium-iron coatings on API 5LX 70 steel applied with HVOF

The present study aimed to create and characterize niobium and niobium-iron60% coatings applied to steel API 5L X70 using the hypersonic thermal spray process (HVOF). The morphologies of the coatings were analyzed using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and profilometry, while the coatings’ hardnesses was evaluated using the Vickers hardness test. The coatings’ corrosion resistance was evaluated by monitoring their open circuit potential and potentiodynamic polarization and performing electrochemical impedance spectroscopy in a 0.05 M NaCl solution. The results showed that the niobium-iron coating contained minor porosity regions, while such defects occurred over large regions of the niobium coating. In terms of corrosion resistance, the coatings obtained in this work promoted a reduction in the substrate’s corrosion rate, but the presence of discontinuities such as porosity compromised the barrier effects of these coatings

Protic Ionic Liquids Used as Metal-Forming Green Lubricants for Aluminum: Effect of Anion Chain Length

Among the applications for protic ionic liquids (PILs), lubrication is one of the newest and the most promising. In this work, ammonium-based protic ionic liquids were tested as lubricant fluids for aluminum-steel contacts. PILs were synthesized with 2-hydroxyethylamine (2HEA) and a carboxylic acid (formic and pentanoic), aiming to understand the effect of two different anion chain lengths on the lubricant behavior. The synthesized PILs were characterized by RMN, FTIR and TGA. Wear tests, conducted using a ball-on-plate configuration, showed that the increase of the anion carbon chain length in the PIL structure reduced significantly the coefficient of friction value. Besides, after the wear tests, the PILs structural integrity was not affected. In the same way, bending under tension (BUT) tests evidenced that the performance for stamping conditions of the PIL with the longest anion carbon chain was similar to that of the commercial lubricant. Since, both formed a uniform tribofilm, developed the same lubrication regime and the drawing forces values were close and constant. Hence, the ionic liquid obtained with 2HEA and pentanoic acid (2HEAPe) is as suitable as the commercial lubricant for metal forming processes

Corrosion process in NaCl/Na2SO4 solutions of AISI 316L stainless steel treated by ionic nitriding

This work is a corrosion study of an AISI 316L austenitic stainless steel after different plasma ion nitriding conditions. The properties of the coated AISI 316L were measured. The corrosion resistance was evaluated by cyclic voltammetry and salt spray testing. Sample surfaces were characterised by SEM and the phase determination with XRD analysis. Nanohardness was determined by using a Vickers indenter. The nitrided samples treated at 400°C, in a gaseous mixture of 5%N2 and 95%H2 for 4 and 5 h, had the best corrosion resistance. For the salt spray test, where the potential range is near the equilibrium potential, the nitrided material presented a reduced corrosion resistance. Through the control of the process parameters it is possible to improve the stainless steel surface hardness, combined with a good corrosion resistance. The highest value of temperature and N2 proportion increased the surface roughness and hardness, though it caused a decrease in corrosion resistance

Corrosion resistance of hybrid films applied on tin plate: precursor solution acidified with nitric acid (ph=3)

Siloxane – poly (methylmethacrylate)-based materials are systems formed by a silicon network, to which chains of poly (methylmethacrylate) are linked by covalent bonds or by physical interactions. Their stability and adherence allow their application on substrates like tin plate in order to increase the corrosion resistance. The aim of this work is to coat tin plate with a hybrid film obtained from a sol consisting of alkoxide precursors: 3 - (trimethoxysilylpropyl) methacrylate (TMSM) and poly (methyl methacrylate) PMMA. Effect of tetraethoxysilane (TEOS) addition was evaluated. Morphology was evaluated by SEM and contact angle. Electrochemical behavior was evaluated by open circuit potential (OCP), potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Results showed that siloxane-PMMA film obtained with a higher addition of TEOS had higher thickness. However, intense densification caused by TEOS addition promoted crack formation, thereby compromising the corrosion resistance

Ionic liquids (il) corrosion on a285 carbon steel

In this work, corrosion caused by different ionic liquids on Grade A – A285 carbon steel plates at room temperature (25°C) is studied. Ionic liquids were prepared via acid – base neutralization. The ionic liquid hydrophilic or hydrophobic character was determined by the wettability technique. Corrosion behavior for each system was analyzed by electrochemical measurements (open circuit potential monitoring and potentiodynamic polarization). Substrate morphology was analyzed by scanning electron microscope and energy dispersive X-ray spectroscopy before and after the electrochemical measurements. The results obtained showed that Grade A – A285 carbon steel in contact with the ionic liquids presented low corrosion rates, being the highest one the corresponding for the 2- hydroxyethylamominum lactate system and the lowest corrosion rate for 2-hydroxyethylammonium acetate system