Hydroxide | EPFL Graph Search

Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, a ligand, a nucleophile, and a catalyst. The hydroxide ion forms salts, some of which dissociate in aqueous solution, liberating solvated hydroxide ions. Sodium hydroxide is a multi-million-ton per annum commodity chemical. The corresponding electrically neutral compound HO• is the hydroxyl radical. The corresponding covalently bound group –OH of atoms is the hydroxy group. Both the hydroxide ion and hydroxy group are nucleophiles and can act as catalysts in organic chemistry. Many inorganic substances which bear the word hydroxide in their names are not ionic compounds of the hydroxide ion, but covalent compounds which contain hydroxy groups. Hydroxide ion The hydroxide ion is naturally produced from wate

Impact of cation redox chemistry on continuous hydrothermal synthesis of 2D-Ni(Co/Fe) hydroxides

Andreas Stephan Lesch, Massimo Rosa

Continuous hydrothermal flow synthesis (CHFS) is a facile, upscalable and cost-efficient synthetic method enabling the nanostructuring of advanced functional materials in steady conditions, i.e. not in batch synthesis. In this paper, we use CHFS to crystallize NiCo- and NiFe-hydroxides in water solution with 2D nanofeatures. By tuning the synthetic parameters, we disclose the key role of the cation redox chemistry in the transition between two competitive phases: from 2D-nanoplatelets of brucite to layered double hydroxides (LDH). For controlling the precipitation of different Ni, Fe, Co-hydroxide phases, we propose the combined use of an oxidizing (H2O2) and a complexing (NH3) agent. At temperatures as low as 80 degrees C, the presence of H2O2 and a low concentration of NH3 favour the Ni2+/Co3+ over Ni2+/Co2+ oxidation states, shifting the product structure from brucite phase (temperatures > 80 degrees C) to LDH. Conversely, for the NiFe-hydroxides the transition from LDH (temperatures = 80 degrees C) to brucite phase (temperatures > 80 degrees C) is controlled by the reaction temperature only. Due to the high stability of Fe3+, the synthesis of NiFe products by CHFS does not require oxidizing and complexing agents, resulting in a robust process for large-scale production.

ROYAL SOC CHEMISTRY2019

Preparation, characterization and photocatalytic activity of commercial TiO2 powders co-doped by N and S

Julian Andrés Rengifo Herrera

Heterogeneous photocatalysis over TiO2 using the sunlight seems to be a promising technology for waste-water treatment and drinking water production. However, the overall efficiency of TiO2 under natural sunlight is limited to the UV-driven activity (λ < 400 nm), accounting only to ~ 4% of the incoming solar energy on the Earth's surface. The increase of the TiO2 absorption towards wavelengths more abundant on the planet's surface has, thus become recently an interesting challenge. In this framework, the main objective of this work was to study the modification of TiO2 by non metallic doping (N and S) in order to increase its light absorption in the visible region (45% of the light hitting the terrestrial surface). Commercial TiO2 powders were doped by a simple preparation method consisting of manual grinding them with a N or S precursor (thiourea) and then annealing at 400 and 500 °C. At both temperatures N, S co-doped commercial TiO2 powders with visible response were obtained. However, the annealing step produced a reduction on the specific surface area and dehydroxylation causing a detrimental effect on the photocatalytic UV-driven activity principally on photocatalyst annealed at 500 °C. E. coli bacteria inactivation, phenol and di-chloroacetate (DCA) oxidation was achieved when the co-doped photocatalyst annealed at 400 °C was illuminated upon UV light the •OH radical being the main oxidative species, that was detected by Electronic Spin Resonance (ESR) spin-trapping with DMPO. In contrast, upon visible light irradiation, N, S co-doped TiO2 powders showed a diminished oxidation power since phenol and DCA were not oxidized. On the other hand, it was possible to inactivate E. coli cells demonstrating that under these conditions, the photocatalytic mechanism was different. In order to elucidate this mechanism, characterization by Diffuse Reflectance Time Resolved Spectroscopy (DRTRS), Low Temperature-ESR and ESR-spin trapping measurements were done. By DRTRS measurement, it was found that under visible light excitation, electrons would be promoted from N, S localized states within the band gap to the conduction band. These electrons were probably trapped on shallow traps such as oxygen vacancies (Vo) allowing them to reach lifetimes of ms. LT-ESR data revealed that N, S co-doping probably could benefit the formation of Vo. ESR spin trapping with TMP-OH as a singlet oxygen quencher, revealed the formation of singlet oxygen as the main oxidative species instead of the •OH radical. Thus, it was suggested that a photo-promoted electron, instead of the localized hole on N, S states, would be the carrier charge playing the main role in photocatalytic reactions on N, S co-doped commercial TiO2 powders. When the electron is trapped on Vo, it could react with molecular oxygen previously adsorbed producing superoxide radical (•O2-). Finally, the oxidation of •O2- by localized holes seems to be thermodynamically favored leading the singlet oxygen (1O2) production. It is well known that singlet oxygen is a Reactive Oxygen Species with a lower oxidative power than the hydroxyl radical. Singlet oxygen is not able to oxidize organic substances such as phenol and DCA but this species is very toxic to microorganism producing lipid peroxidation reaction on biological membranes. It was also concluded that N, S co-doped TKP 102 annealed at 400 °C did not present an enhancement on their photocatalytic activity towards phenol oxidation and E. coli inactivation when simulated solar light was used. Undoped Degussa P-25 was the commercial powder with highest photocatalytic activity. Evidences are reported about the classical photocatalytic process where •OH radicals are produced mainly by oxidation of water or hydroxyl ions with photo-induced valence band holes prevails upon simulated solar light exposition. Localized states induced by N or S-doping and responsible of visible light absorption did not play an important role on the photocatalytic activity of these novel materials under the experimental conditions used.

EPFL2009

Stability and physical properties of alkylsilane and alkylphosphonate self-assembled monolayer (SAM) films on metal oxide substrates characterized at the micro- and nanoscale

Enamul Hoque

Self-assembled monolayer (SAM) films have attracted immense attention for both fundamental and applied research. A SAM is composed of a large number of molecules with a head group that chemisorbs onto a substrate, a tail group that interacts with the outer surface of the film, and a spacer (backbone) chain group that connects the head and tail groups resulting in a coating. Interactions between spacer groups of different molecules, such as van der Waals forces and/or hydrogen bonding, hasten SAM film formation and contribute to its stability. In this dissertation, SAM and thin films have been formed onto copper and aluminum oxide surfaces by reaction with 1H,1H,2H,2H-perfluorodecyldimethylchlorosilane (PFMS), 1H,1H,2H,2H-perfluorodecyltrichlorosilane (PFTS), 1H,1H,2H,2H-perfluorodecylphosphonic acid (PFDP), octylphosphonic acid (OP), decylphosphonic acid (DP), and octadecylphosphonic acid (ODP). The properties and stability of the films were investigated employing complementary surface analysis techniques: X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), friction force microscopy (FFM), a derivative of AFM, contact angle measurements (CAMs), and Fourier transform infrared reflection/absorption spectroscopy (FT-IRRAS). The perfluoroalkylsilane SAM on Cu is found to be extremely hydrophobic typically having sessile drop static contact angles of more than 130° for pure water and a surface energy of 14 mJ/m2 (mN/m). FFM showed a significant reduction in the adhesive force and friction coefficient of PFMS modified Cu (PFMS/Cu) compared to unmodified Cu. Treatment by exposure to harsh conditions showed that PFMS/Cu SAM can withstand boiling nitric acid (pH=1.8), boiling water, and warm sodium hydroxide (pH=12, 60 °C) solutions for at least 30 minutes. Furthermore, no SAM degradation was observed when PFMS/Cu was exposed to warm nitric acid solution for up to 70 min at 60 °C or 50 min at 80 °C. XPS and FT-IRRAS data reveal a coordination of the PFMS silicon (Si) atom with a cuprate (CuO) molecule present on the oxidized copper substrate. The data give good evidence that the stability of the SAM film on the PFMS modified oxidized Cu surface is largely due to the formation of a siloxy-copper (-Si-O-Cu-) bond via a condensation reaction between silanol (-Si-OH) and copper hydroxide (CuOH). For a PFTS modified Cu surface (PFTS/Cu), the sessile drop static contact angle of pure water has been measured to be more than 125° and the surface energy to be typically less than 16 mJ/m2. Stability tests show that the PFTS/Cu film can survive in boiling pure water for one hour, boiling nitric acid (pH 1.5 or 1.8) for 30 minutes, sodium hydroxide solution (pH 12, 70 °C) for 30 minutes, and autoclave conditions (steam at 134 °C and 3 atmospheres) for 15 minutes. The more commonly used self-assembled monolayer (SAM) modifications of Cu surfaces, e.g. thiol compounds, are significantly less stable than PFTS/Cu. Extremely hydrophobic (low surface energy) and stable PFMS/Cu SAMs and PFTS/Cu films could be useful as corrosion inhibitors in micro/nanoelectronic devices and/or as promoters for anti-wetting, low adhesion surfaces or drop-wise condensation on heat exchange surfaces. XPS analysis confirmed the presence of perfluorinated and non-perfluorinated alkylphosphonate molecules on the PFDP, DP, and ODP SAMs deposited at the aluminum oxide coated silicon (Al/Si) surfaces. The sessile drop static contact angle of pure water on PFDP SAMs was typically more than 130° and on DP and ODP typically more than 125° indicating that the phosphonic acid SAMs reacted with Al samples were very hydrophobic. The surface roughness for PFDP/Al, DP/Al, ODP/Al, and bare Al was approximately 35 nm, as determined by AFM. The surface energy for PFDP/Al was determined to be approximately 11 mJ/m2 by the Zisman plot method compared to 21 mJ/m2 and 20 mJ/m2 for DP/Al and ODP/Al, respectively. PFDP/Al gave the lowest adhesion and friction force while bare Al gave the highest. The adhesion and friction forces for ODP/Al and DP/Al SAMs were in between. ODP, DP, and OP SAMs have been studied in detail on relatively flat aluminum oxide surfaces. The rms surface roughness for ODP/Al, DP/Al, OP/Al, and bare Al was less than 15 nm, as determined by AFM. The sessile drop static contact angle of pure water on ODP/Al and DP/Al was typically more than 115° and on OP/Al typically less than 105°. The surface energy for ODP/Al and DP/Al was determined to be approximately 21 mJ/m2 and 22 mJ/m2, respectively, compared to 26 mJ/m2 for OP/Al. ODP/Al and OP/Al were studied by FFM to better understand their micro-/nano-tribological properties. ODP/Al gave the lowest coefficient of friction values while bare Al gave the highest. The adhesion forces for ODP/Al and OP/Al were comparable. The chemical stability of ODP/Al, PFDP/Al, DP/Al, OP/Al, and PFMS/Al SAMs has been inspected by exposure to warm nitric acid (pH 1.8, 30 min, 60-95 °C). The XPS data and stability against harsh chemical conditions indicate that a type of bond forms between a phosphonic acid (PA) or silane molecule and the oxidized Al surface. Stability tests using warm nitric acid (pH 1.8, 30 min, 60-95 °C) show ODP/Al SAMs to be most stable followed by PFDP/Al, DP/Al, PFMS/Al, and OP/Al SAMs. For PFTS/Al, stability tests demonstrate that modified aluminum is able to survive exposure to warm nitric acid (pH = 1.8, 60 °C, 30 min) indicating some degree of robustness. Hydrophobic, low adhesion, and robust aluminum surfaces have useful applications for micro/nano-electromechanical systems (MEMS/NEMS), such as digital micro-mirror devices (DMDs). These studies are expected to aid in the design and selection of proper lubricants for MEMS/NEMS.

EPFL2007