Treatment In Ammonia Research Articles

Enhancing electrochemical capacitor performance of N-doped tannin-derived carbons by hydrothermal treatment in ammonia

This study demonstrates that ammonia concentration when doping carbons using hydrothermal treatment has crucial impact on textural properties. The latter translates into an improvement of the electrochemical performance of carbon materials, when used into electrochemical capacitors, especially at high-rate performance. Nitrogen-doped carbons were synthesized by subjecting tannin to hydrothermal carbonization in ammonia solutions of varying concentrations. After carbonization and CO2 activation, the as-produced activated carbons (ACs) were tested as electrodes for electrochemical capacitors in symmetric configuration using 1 M H2SO4 as aqueous electrolyte. Interestingly, ammonia concentration during the hydrothermal synthesis step did not significantly affect the final N content (ca. 3 and 4 at. %) nor the nitrogen and oxygen functionalities on the surface. However, the use of ammonia had crucial impact on the textural properties developed by CO2 activation, and therefore on the electrochemical performance of the ACs. The best-performing N-doped AC showed specific electrode capacitance values, based on carbon material, of 212 F g−1 at 0.5 A g−1 and outstanding capacitance retention of ca. 71 % at 40 A g−1. It also showed high cycling stability, with capacitance retention of ca. 96 % after 30,000 cycles. Furthermore, this AC outperformed similar reported materials, achieving a specific energy of 4.6 W h kg−1 at 12.1 kW kg−1.

Growth of Phosphazene Film Onto Undoped n-InP

An anodic electrochemical treatment in liquid ammonia (-55°C, Patm) provides the formation of an ultrathin film onto p-InP (1017 atoms.cm-3) and n-InP (1018 atoms.cm-3). This overlayer exhibits a polyphosphazene like structure (H2N–P=NH) which presents a very specific spectral signature when analyzed by X-ray photoelectron spectroscopy (XPS[1]). Based on the atomic surface ratios measured, the film growth would require a nucleation step which would be followed by a phosphazene coalescence phenomenon in the two dimensions of the surface[2]. After this anodic treatment no evolution of the surface under air exposure is observed (> 1 year) [3]. This passivation layer, electrochemically obtained, appears promising for InP incorporation into (opto)electronic devices.Despite the reproducibility of the results, questions still arise, such as the anodic charge which widely varies according to the electrochemical process used. An anodic charge around 7 mC.cm-2 is required by cyclic voltametric1 whereas only 0,5 mC.cm-2 is enough by galvanometry[4] and a charge higher than 3 mC.cm-2 is observed by potentiometry[5]. Some disparities are also observed on XPS atomic surface ratios of the phosphazene like structure. For instance, the N to P133eV atomic ratio is expected to be 2 (see formula above). However, the value obtained on some of the passivated samples is slightly higher.In order to figure out these variations, the n-type is reinforced but with low doping carriers (1015atoms.cm-3) and under low photon flux. A very different behavior from n-InP (1018 atoms.cm-3) is observed by both cyclic voltametry and galvanometry. Under illumination, holes involved in the anodic process are directly proportional to the photon flux which explains the relevance of n type. n-InP (1015 atoms.cm-3) provides a photoanodic current at lower potential due to its larger space layer where an effective separation of photogenerated electron–hole pairs occurs. A larger range of potential is then available to explore the anodic treatment preventing holes by tunnel effect. By cyclic voltametry under low photon flux, onto n-InP (1015 atoms.cm-3) a constant photoanodic current is observed whereas an anodic wave was observed onto n-InP with higher doping level (1018 atoms.cm-3). As the cycles progress, the rising front of the photocurrent moves from -0.5 V to 1.2 V vs SRE while the intensity of the photocurrent remains constant.In the same illumination conditions, the galvanometry onto n-InP (1015 atoms.cm-3) required a current magnified by 200 (».500 mC.cm-2) compared to n-InP (1018 atoms.cm-3).Onto undopped n-InP the current efficiency gives then a poor performance in comparison with n-InP at higher doping level.Whatever the process used, the phosphazen film is detected by XPS. However, by galvanometry, the N to P133eV XPS atomic ratio is close to 3. A fundamental question is raised about the holes engaged during the anodic process by galvanometry. [1] D. Aureau, M. Fregnaux, A. M. Gonçalves, A. Etcheberry. Surface and Interface Analysis (2018) 1-5. [2] A.-M. Gonçalves C. Njel, D. Aureau, A. Etcheberry Thin Solid Films 538 (2013) 21–24 [3] A.-M. Goncalves, N. Mezailles, C. Mathieu, P. Le Floch, A. Etcheberry. ´Chem. Mat., 22, (2010) 3114-3120 [4] A.-M. Goncalves, C. Njel, D. Aureau, A. Etcheberry. Appl. Surf. Sci.391 (2017) 44–48 [5] C. Njel, A.-M. Goncalves, A. Etcheberry, Electrochim. Acta 139 (2014) 152–156

Synthesis of Hexagonal Boron Nitride Coatings Using Concentrated Solutions of Alkali Metal Borates

Continuous nanocrystalline coatings of hexagonal boron nitride (h-BN) with a thickness of ~1000‒350 nm are prepared from highly concentrated aqueous solutions of lithium and sodium borates, which display good film-forming properties. The borate solutions were applied on silicon substrates by spin coating followed by drying in air and treatment in ammonia at a temperature of 1300°C. This method is deemed promising for creating protective coatings (e.g., for protecting carbon products from oxidation at elevated temperatures).

Improving the Thermal Stability of Hot-Working Dies

By cyclic gas treatment in ammonia and in air, a composite surface layer with controlled structure may be formed on die steel. Thanks to the catalytic properties of the oxide film that appears, an extended multilayer region of chemical compounds is formed at its surface. That improves the erosion and crack resistance of the die steel, with corresponding increase in the life of hot-working dies by a factor of 2–2.5.

Femtosecond Laser Fabrication of Stable Hydrophilic and Anti-Corrosive Steel Surfaces.

We report on a novel single-step method to develop steel surfaces with permanent highly hydrophilic and anti-corrosive properties, without employing any chemical coating. It is based on the femtosecond (fs) laser processing in a saturated background gas atmosphere. It is particularly shown that the fs laser microstructuring of steel in the presence of ammonia gas gives rise to pseudoperiodic arrays of microcones exhibiting highly hydrophilic properties, which are stable over time. This is in contrast to the conventional fs laser processing of steel in air, which always provides surfaces with progressively increasing hydrophobicity following irradiation. More importantly, the surfaces subjected to fs laser treatment in ammonia exhibit remarkable anti-corrosion properties, contrary to those processed in air, as well as untreated ones. The combination of two functionalities, namely hydrophilicity and corrosion resistance, together with the facile processing performed directly onto the steel surface, without the need to deposit any coating, opens the way for the laser-based production of high-performance steel components for a variety of applications, including mechanical parts, fluidic components and consumer products.

Application of Laser Radiation for Intensification of Chemical Heat Treatment

The processes of chemical heat treatment were analyzed. The areas of their application, productivity, energy intensity, advantages and disadvantages are highlighted. The surface nitrided layers with hardness up to 800–900 HV were obtained. In this case, nitriding thickness does not exceed 0.15–0.2 mm, which is insufficient, especially if the value of residual deformations exceeds the thickness of the diffusion layer. It is shown that to increase the efficiency of nitriding, it is advisable to pre-modify the microstructure of the layer of the sample by laser irradiation. Modeling of thermal processes occurring in steels under irradiation and experimental studies have determined the conditions under which highly dispersed, with a high dislocation density, disoriented in space structures are formed in the surface layers. Dislocations, located on the grain boundaries and having a complex configuration, form paths along which nitrogen atoms can move, forming nitrides with alloy elements. It was shown that in U8 tool carbon steel (AISI M2) after laser treatment and subsequent nitriding in ammonia at a temperature of 780–800 °К for 12 h, a nitrided layer is formed. The depth of the nitrided layer without prior laser treatment is 0.115 mm, on laser-treated it varies within 0.12–0.46 mm depending on the processing speed. The results of the combined nitriding of 40Cr13 stainless steels (AISI 420 Stainless Steel) and 12Cr18Ni10Ti (AISI 321 Stainless Steel), in which the thickness of the hardened layer with a hardness of 815.7–917.7 HV is 0.4–0.5 mm, with a significantly shorter (by 3–5 times) process time and its cost, are presented. The expediency of manufacturing discretely located nitrided zones on the working surfaces of products is discussed.

Highly anisotropic titanium nitride nanowire arrays for low-loss hyperbolic metamaterials fabricated via dynamic oblique deposition

Hyperbolic metamaterials (HMMs) with highly anisotropic metal nanowires exhibit unique optical properties arising from their extraordinary optical anisotropy. Although metal nanowires are often fabricated by embedding noble metals such as silver in an anodic alumina membrane dielectric host, the low melting point of noble metals limits their utilization in high-temperature applications, and there are fabrication issues such as overfilling or discontinuous islands within the host pores. Thus, metal nanowires with a high melting point for HMMs and alternative fabrication techniques are desired. In this study, we fabricated a highly anisotropic nanowire array (NWA) using titanium nitride, which has a high melting point, via dynamic oblique deposition of titanium and subsequent thermal treatment in ammonia. Spectra of ellipsometric parameters were well-fitted by a Fresnel reflection theoretical model considering the optical anisotropy, in which the effective permittivity was described using effective medium theory and the Drude–Lorentz model. The out-of-plane component of the effective permittivity was negative at λ > 850 nm, whereas the in-plane component was positive, indicating that the fabricated NWA behaves as a HMM. The figure of merit in the near-infrared range was higher than that of conventional multilayer TiN HMMs. The NWA presented here is a promising candidate for HMMs in high-temperature applications due to the high melting point of titanium nitride.

Significant enhancement of the oxygen reduction activity of self-heteroatom doped coal derived carbon through oxidative pretreatment

Functional low-cost catalysts derived from earth abundant materials are vitally important for enabling mass implementation of renewable energy conversion systems, such as fuel cells and metal-air batteries. Herein, we report the activation of earth-abundant coal to realize very active and stable catalysts for the oxygen reduction reaction (ORR) through oxidative pretreatment followed by thermal treatment under ammonia forming self-doped heteroatoms and defects. The coal thermally pretreated in air at 350 °C for 30 min followed by treatment in ammonia for 2 h presented very good electrocatalytic ORR activity with onset potential of 0.978 V, half-wave potential of 0.871 V, Tafel slope of 57 mV dec−1, and predominant 4-electron reduction of O2 to OH−. The influence of the preoxidation step prior to ammonia treatment on the properties of the coal-derived carbon and hence the ORR has been specifically studied. The optimized catalyst also displayed stable performance during the ORR in 0.1 M NaOH for a test period of 35 h and exhibited excellent tolerance to methanol. The preoxidation step facilitates elimination of aliphatic and amorphous carbon, condensation of aromatic structures and the formation of initial pore structures, and subsequent removal of unstable heteroatoms from aromatic carbon rings to form surface and edge defects as potential active sites for promoting the ORR. Using pristine coal as raw material to produce functional ORR catalysts will be of great importance for real applications.

Cost-effective preparation of metal-free electrocatalysts by phosphoric acid activation of lignocellulosic materials for oxygen reduction reaction

Activated carbons have been prepared by phosphoric acid activation of lignocellulosic precursors, an industrial method, followed by heat treatment in ammonia. Thus, a cost-effective, scalable and metal-free electrocatalyst was developed for use in oxygen reduction reaction (ORR) at fuel cell cathodes. The physicochemical properties of the activated carbons have been analyzed by elemental analysis, X-ray photoelectron spectroscopy, X-ray diffraction, and nitrogen adsorption. The ORR electrocatalytic performances of the activated carbons have been investigated by cyclic voltammetry and linear sweep voltammetry in an alkaline electrolyte. The results showed that phosphorus-containing groups are key to endowing phosphoric-acid-activated carbons with comparable electrocatalytic activity to that of commercial Pt/C. This was because these phosphorus-containing groups facilitated the formation of both nitrogen-containing groups and defects in the microstructure. Besides, heat treatment of phosphoric-acid-activated carbons in ammonia produced a highly developed mesopore structure and thus kinetically facilitated the ORR.

Nanometric nickel exsolution in the hexagonal perovskite Ba8Ta6NiO24: Survey of the structural, magnetic and catalytic features

Nickel nanoparticles that decorate the surface of a parent oxide are well known to be developed by some cubic-type perovskites from the exsolution of their intrinsic Ni2+ species in reducing conditions with subsequent catalytic properties. In this work, we show similar phenomenon in a hexagonal-type perovskite. The reducing treatment in ammonia of Ba8Ta6NiO24, a hexagonal perovskite-type oxide, leads to the exsolution of nickel nano-sized particles at the surface of the precursor leaving oxygen vacancies and partially depleted but rearranged Ni/Ta sites. Spectroscopic methods, X-ray diffraction, TEM and TGA were employed to investigate the Ni exsolution while TGA was used to investigate the thermal behavior and its reversibility. 50% of the initial nickel content transferred into Ni0 nanoparticles as determined from M(H) magnetization plots and TGA, in agreement with the concomitant creation of oxygen vacancies in the preserved Ni2+ coordination (50%). The cationic vacancies (Vc) involve face-sharing octahedral dimers in which Ni/Ta/Vc rearranged through the shared face window, but preserved a non-centrosymmetric cationic partition. Ni exsoluted oxides often show a catalytic activity; we present here an original example of a hexagonal perovskite with catalytic activity toward syngas production by dry reforming of methane with carbon dioxide.

A Generalizable Top-Down Nanostructuring Method of Bulk Oxides: Sequential Oxygen-Nitrogen Exchange Reaction.

A thermal reaction route that induces grain fracture instead of grain growth is devised and developed as a top-down approach to prepare nanostructured oxides from bulk solids. This novel synthesis approach, referred to as the sequential oxygen-nitrogen exchange (SONE) reaction, exploits the reversible anion exchange between oxygen and nitrogen in oxides that is driven by a simple two-step thermal treatment in ammonia and air. Internal stress developed by significant structural rearrangement via the formation of (oxy)nitride and the creation of oxygen vacancies and their subsequent combination into nanopores transforms bulk solid oxides into nanostructured oxides. The SONE reaction can be applicable to most transition metal oxides, and when utilized in a lithium-ion battery, the produced nanostructured materials are superior to their bulk counterparts and even comparable to those produced by conventional bottom-up approaches. Given its simplicity and scalability, this synthesis method could open a new avenue to the development of high-performance nanostructured electrode materials that can meet the industrial demand of cost-effectiveness for mass production.

Designing and facilely synthesizing a series of cobalt nitride (Co4N) nanocatalysts as non-enzymatic glucose sensors: A comparative study toward the influences of material structures on electrocatalytic activities

Designing high-efficiency electrocatalysts for glucose concentration detection plays a pivotal role in developing various non-enzymatic glucose detection devices. Herein, we have successfully designed and synthesized various cobalt nitrides (Co4N) by using different weak bases (i.e. hexamethylenetetramine (HMT), urea, and ammonium hydroxide (AH)) through nitridation treatment in ammonia (NH3) atmosphere. Physical characterization results demonstrate that Co4N-NSs (nanosheets) with vast meso/macropores and large BET surface are successfully constructed once adding carbon paper and HMT into precursors. As the synergistic effect of metallic character of Co4N phase, excellent electroconductibility of pyrolytic carbon, and large surface area, Co4N-NSs surfaces can form more Co4+ active sites in electrochemical reaction processes. Meanwhile, the abundant meso/macroporous structures constructed in Co4N-NSs further promoted its mass transfer ability. Benefitting from the above mentioned advantages, Co4N-NSs therefore exhibit more excellent glucose oxidation ability than another three control samples (i.e. Co4N-HMT, Co4N-Urea and Co4N-AH). When used for glucose detection, the optimal Co4N-NSs display excellent detection parameters as well, such as: a wide linear range of 0.6–10.0mM, a large sensitivity of 1137.2uAcm−2mM−1 glucose, a low detection limit of 0.1µM, a small response time of 1.7s, good reproducibility and stability, and the excellent anti-interference to other electroactive molecules and Cl−. Upon utilized for measuring glucose concentrations in human blood serum samples, the detection results on Co4N-NSs are accurate and satisfying as well. This work opens a new possibility for boosting electrochemical catalysis abilities of Co4N samples by the structure design.

Encapsulated iron-based oxygen reduction electrocatalysts by high pressure pyrolysis

Non-precious metal catalysts (NPMCs) are candidate materials to replace platinum for proton exchange membrane fuel cells (PEMFCs). Herein we reported a type of iron-based NPMCs prepared by high pressure pyrolysis for the oxygen reduction reaction (ORR) in acidic media. The catalysts are in form of carbon microspheres in a sub-microscale consisting of iron-containing nanoparticles encapsulated by graphitic layers. By tailoring temperatures and duration of pyrolysis, the best ORR catalyst was achieved at 700 °C and 75 min, which exhibits an onset potential of 0.85 V at 0.1 mA cm−2 and a half-wave potential of 0.72 V in acid media. After 10,000 potential cycles, only 25 mV shift of half-wave potential is observed showing excellent stability. An analogue material prepared from nitrogen-free precursors shows significant electrochemical activity though it is much lower than that from the nitrogen containing precursors and can be improved by post treatment in ammonia. These results indicate the contribution to the catalysis from surface nitrogen functionalities and encapsulated metal-containing nanoparticles.

Novel highly active and selective Fe-N-C oxygen reduction electrocatalysts derived from in-situ polymerization pyrolysis

Among several candidates considered to replace expensive platinum based catalysts for oxidation reduction reaction (ORR) in fuel cells, iron-nitrogen-carbon (Fe-N-C) composites attract the most interest. In this work, we design a novel method to prepare Fe-N-C catalysts for ORR in both acid and alkaline media. It involves initial polymerization of different amino-pyridines to form a C-N-C backbone, followed by their high-temperature pyrolysis on silica template, HF etching and second heat treatment in ammonia. Polymerization of 2-amino-6-methylpyridine and 2,6-diaminopyridine was done by in-situ and ex-situ templating methods. The catalysts synthesized by this method possess half-wave potentials of ~ 0.88V versus RHE in O2 saturated 1M KOH electrolyte, and the best catalyst based on 2-amino-6-methylpyridine was seen to achieve an E1/2 greater than 0.7V in the O2 saturated 0.5M sulfuric acid electrolyte. Importantly, the catalysts prepared by this novel method exhibit remarkably low peroxide yield in both acid and base media (< 4% and < 1.3%, respectively) as revealed by rotating ring disk electrode (RRDE) studies.

Electrochemical capacitance of titanium nitride modified lithium titanate nanotube array

Titanium nitride modified lithium titanate nanotube array (TiN-Li4Ti5O12 NTA) was designed as the electrode material of lithium-ion supercapacitor for electrochemical energy storage. Li4Ti5O12 NTA was prepared through lithiation treatment of anodic TiO2 NTA and then annealing treatment in argon. TiN-Li4Ti5O12 NTA was formed through coating TiO2 sol on the surface of Li4Ti5O12 NTA and then full nitridation treatment in ammonia. Partially nitridized Li4Ti5O12 (N-Li4Ti5O12) NTA was also prepared through a controlled nitridation treatment of Li4Ti5O12 NTA in ammonia. TiN-Li4Ti5O12 NTA with full TiN coating layer exhibited higher conductivity than N-Li4Ti5O12 NTA with partially incorporated TiN. The capacitance accordingly increased from 79.58 F g−1 for N-Li4Ti5O12 to 143.83 F g−1 for TiN-Li4Ti5O12 at 0.5 A g−1 in 0.5 M Li2SO4. The capacitance retention ratio also increased from 72.95% to 82.41% after 1000 cycles at 3.0 A g−1. The all-solid-state supercapacitor was constructed using TiN-Li4Ti5O12 NTA electrode and polyvinyl alcohol gel electrolyte containing Li2SO4, showing high capacitance of 40.45 F g−1, energy density of 32.36 Wh kg−1 and power density of 0.6 kW kg−1 at the high output voltage of 2.4 V and current density of 0.5 A g−1. Therefore, TiN-Li4Ti5O12 NTA could act as a feasible electrode material of lithium-ion supercapacitor.

Anions and cations distribution in M5+/N3- co-alloyed TiO2 nanotubular structures for photo-electrochemical water splitting

Co-alloyed (M,N) titanium dioxide nanotubes were synthesized by a simple anodization process with M-Ti alloys (M = Ta or Nb), followed by a thermal treatment in ammonia to introduce nitrogen in those nanostructure. The photo-electrochemical performances of these co-alloyed samples was compared with the ones of N doped and undoped TiO2 NTs. Different conditions of thermal treatment under ammonia were studied in order to control the amount of nitrogen introduced in the TiO2 structure and tend to achieve a balance of charges between Ta5+/Nb5+ cations and N3- anions. The structure and composition of these materials were characterized by X-Ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS) and Transmission Electron Microscopy (TEM) EDX mapping. This combination of methods confirm the successful introduction of doping atoms, and allowed us to estimate the amount of M5+ cations and N3- anions in the TiO2 NTs. TEM EDX mapping indicates a heterogeneous distribution of Nb5+ and Ta5+ cations in the nanotubes that can originate from the use of highly concentrated alloys. External quantum efficiency measurements were used to determine the photo-electrochemical activity of our samples in different spectral domains. A significant improvement of activity in the visible region of the solar spectra was observed for co-alloyed TiO2 samples when compared to undoped nanotubes but also when compared to N doped TiO2. The obtained results indicate that the parameters of the NH3 thermal treatments should be finely controlled to improve the conversion efficiency in the visible domain for the co-alloyed samples by introducing a precise amount of N3- in substitution of oxygen in the TiO2 lattice and to avoid the damaging of the nanotubular structure.

NH3 Post‐Treatment Induces High Activity of Co‐Based Electrocatalysts Supported on Carbon Nanotubes for the Oxygen Evolution Reaction

AbstractCobalt oxide nanoparticles were deposited on nitrogen‐doped carbon nanotubes (NCNTs) through impregnation by using cobalt nitrate as a precursor and subsequent drying and calcination. Co loadings were prepared in the range from 4 to 40 wt%, and hydrogen and ammonia were applied in the thermal post‐treatment of the CoOx/NCNT samples. The Co3O4 spinel structure was detected in all samples, while the thermal treatment in ammonia and hydrogen led to the formation of CoO and metallic Co in addition. Treatment in ammonia resulted in the partial reduction of Co3O4 to CoO and nitrogen doping of the oxides, leading to excellent electrocatalytic activity in the oxygen evolution reaction (OER) and stability despite of the lower Co oxidation states compared with the sample calcined in air. In contrast, the sample reduced in hydrogen showed a lower activity and stability in the OER. The high activity of the ammonia‐treated sample can be assigned to improved conductivity, favorable surface properties with surface nitrogen improving the hydrophilicity of the catalysts, and the more facile transformation to the OER‐active layered cobalt oxyhydroxide phase under anodic conditions.

Capacitive performance of molybdenum nitride/titanium nitride nanotube array for supercapacitor

Molybdenum nitride (MoNx) depositing on titanium nitride nanotube array (TiN NTA) was designed as MoNx/TiN NTA for supercapacitor electrode material. MoNx/TiN NTA was fabricated by electrodepositing molybdenum oxide onto titanium dioxide NTA and one-step nitridation treatment in ammonia. MoNx/TiN NTA involved top-surface layer of MoNx nanoparticles and underlying layer of TiN NTA, which contributed to electric double layer capacitance in aqueous lithium-ion electrolyte solution. The specific capacitance was increased from 69.05mFcm−2 for TiN NTA to 121.50mFcm−2 for MoNx/TiN NTA at 0.3mAcm−2, presenting the improved capacitance performance. MoNx exhibited the capacitance of 174.83Fg−1 at 1.5Ag−1 and slightly declined to 109.13Fg−1 at 30Ag−1, presenting high rate capability. MoNx/TiN NTA exhibited the capacitance retention ratio of 93.8% at 3.0mAcm−2 after 1000 cycles, presenting high cycling stability. MoNx/TiN NTA could act as a promising electrode material of supercapacitor.

Direct correlations between XPS analyses and growth film by chronopotentiometry on InP in liquid ammonia (−55 °C)

This paper is based on the understanding of the formation of a reproducible polyphosphazene-like film ([(H2N)PN]n) obtained on InP by anodic treatment in liquid ammonia. The approach is innovative as it combines indications from the coulometric charges and the related chemical information from XPS analyses. Anodic charges are accurately monitored by galvanostatic treatment between 0.05mCcm−2 and 12.5mCcm−2. XPS investigation of the treated surfaces demonstrates the presence of an anodic film on InP. Whatever the spent charge, the specific P2p and N1s signals agree with the growth of an ultrathin phosphazene layer. From 0.25mCcm−2 to 12.5mCcm−2, a quasi constant XPS response is revealed without thickening of the film. However a gradual chemical evolution of the modified surface is clearly observed for the lower anodic charges (from 0.04mCcm−2 to 0.5mCcm−2). In this case, the surface is entirely recovered by the film as soon as 0.25mCcm−2 is consumed at the interface. Same atomic surface ratios are indeed revealed indicating that a constant chemical composition is consistent with a polyphosphazene film. On the basis of atomic surface ratios evolutions determined by XPS, a mechanism of the film growth is deduced. It requires a nucleation step which is followed by a phosphazene coalescence phenomenon in the two dimensions of the surface. A final phosphazene monolayer film is suggested if a sufficient anodic charge spent at the interface is considered, allowing a quantitative discussion related to electrochemical and XPS data.

Mn doped GaN nanoparticles synthesized by rapid thermal treatment in ammonia

We present a novel route for the synthesis of manganese doped GaN nanoparticles. Nanoparticles in the form of hexagonal discs were synthesized by rapid thermal treatment of manganese doped ammonium hexafluorogallate in ammonium atmosphere. The morphology of GaN:Mn nanoparticles was investigated using scanning electron microscopy. A concentration over 0.7 wt.% of Mn was observed by X-ray fluorescence and electron microprobe. Structural and electronic properties were investigated using X-ray diffraction, Raman spectroscopy and micro-photoluminescence with excitation wavelength of 325 nm and 532 nm. The magnetic properties between 4.5 K and 300 K were investigated by a superconducting quantum interference device (SQUID) magnetometer. GaN:Mn nanoparticles show a purely paramagnetic behavior which can be interpreted in terms of Mn2+ ions exhibiting an antiferromagnetic interaction.