Flame Annealing Research Articles

Electrochemistry of Glycerol Oxidaton at Low Index Crystal Planes of Platinum

Introduction: The energy crisis related to the consumption of fossil fuels has raised concern in many countries, thus the use of biofuels is an alternative to those fuels. In Brazil, in recent years has increased biodiesel production in line with current legislation, introducing biofuels in their energy mix. Biodiesel is a renewable fuel produced from transesterification of a vegetable or animal’s oil and an alcohol (methanol or ethanol), the process produces as a co-product glycerol. Glycerol has many industrial applications in industries: cosmetics, pharmaceutical, food and others, but the large amount produced is not absorbed by the industry, which has lost its market value and in a few years could pose damage to the environment. The glycerol electrooxidation in platinum low index single crystal electrodes in alkaline medium is investigated in this work using techniques such as cyclic voltammetry and chronoamperometry. Experimental: Glycerol electrooxidation experiments were performed in a 0.1M NaOH and 0.1M glycerol solution. Electrochemical cell potential were controlled with a potentiostat (autolab). The platinum single crystals work electrodes ((111 ) , (100 ) and (110)) used were prepared by Clavillier’s method [1]. Previous to each electrochemistry experiment the work electrode was cleaned by flame annealing in an oxygen gas flame, cooled in H2/Ar atmosphere and protected with water in equilibrium with this gas mixture to prevent contamination before immersion in the electrochemical cell. Results and Discussion: Figure 1 displays the cyclic voltammograms recorded at 50 mV/s for Pt(hkl) in 0.1 M NaOH. The dotted lines are blank voltammograms profiles of Pt(111), Pt(100) and Pt(110) electrodes in the absence of glycerol in the support electrolyte which are in good agreement with previously published results. In presence of glycerol in the electrolyte, the electrochemical responses show the strongly reaction dependence to the electrode surface arrangement. As already documented for other small alcohols (methanol, ethanol, etc. ), at low potentials the surface is completed blocked by the adsorbates which are only oxidized at higher potentials ( < 0.45 V). The electrooxidation glycerol voltammograms expose the different reaction mechanisms for the three electrodes. These are not surprising results since the presence of alcohol group in each carbon in the glycerol molecule enables many possibilities of adsorbate species following by their oxidation: producing soluble products or adsorbed intermediates. It is interesting to note the hysteresis displayed for Pt(111) and Pt(110). For both surfaces, the positive and negative scans differ only by current density. The same was observed for the electrooxidation of ethanol in Pt(111) in alkaline media[2]. In the other hand, the Pt(100) shows superior electroactivity in the negative scan, characterized by a sharp peak centered at 0.65V. Although not shown in Figure 1, we investigated the successive cycles for the glycerol oxidation. We observed a continuous surface poisoning cycle after cycle evidence by the decrease of density current. This is evidence that some of the adsorbates formed during the cycles remains at the surface even at higher potentials. Some works [2,3] observed the presence of CO during the glycerol electrooxidation but there is a lack of information in relation of extensive electrolysis. Conclusion: The glycerol electrooxidation is a field of research still to be explored since the reaction complexity is evidenced in this work. The main observation is the reaction surface sensitivity which can lead to produce different kind of products for different surfaces. It is particularly important if we want to use the glycerol as feedstock to produce more valuable products.

Cyclic voltammetry and oxygen reduction activity of the Pt{1 1 0}-(1 × 1) surface

A Pt{110}-(1×1) single-crystal electrode surface was created by flame annealing and cooling of the electrode in gaseous CO. For the first time, the voltammetry of this unreconstructed surface is reported using aqueous perchloric acid and sodium hydroxide electrolytes. The voltammetric response for Pt{110}-(1×1) produces marked differences when compared with the reconstructed, H2- and N2-cooled, disordered Pt{110}-(1×2) surface phases. Pt{110}-(1×1) exhibits as many as 6 individual peaks in the low potential region (0–0.25V vs. Pd/H), a singular sharp oxidation peak at 0.95V (Pd/H) corresponding to the electrosorption of oxide and almost zero current associated with OH formation between 0.6V (Pd/H) and 0.9V (Pd/H). Charge density curves indicate that the total charge passed between 0V (Pd/H) and 1.5V (Pd/H) to be almost identical for both the (1×1) and the disordered (1×2) phases in perchloric acid, sulphuric acid and sodium hydroxide electrolyte. The oxygen reduction reaction (ORR) activity of the (1×1) surface phase was also examined using rotating disc electrode voltammetry in the hanging meniscus configuration. The half-wave ORR peak potential was found to be ∼30mV more negative than for the disordered reconstructed surface. This leads to the conclusion that the activity of the unreconstructed basal planes of platinum towards the ORR follows the order {100}<{110}<{111} when E1/2 is used as a measure of activity and that the higher activity usually ascribed to Pt{110} over Pt{111} is actually a manifestation of the disordered (1×2) surface phase in which step sites and defects promote ORR.

Rutile phase n- and p-type anodic titania nanotube arrays with square-shaped pore morphologies.

Rutile-phase TiO2 nanotube arrays without broken walls were formed by annealing of anodically formed nanotubes in a propane flame at 650 °C and in air at 750 °C. An unusual morphological transformation was observed from the ellipsoidal pore-shapes of titania nanotubes grown in aqueous electrolyte to a square-shaped pore structure subsequent to the anneals. 750 °C annealed nanotubes were found to be lightly p-type, rare in TiO2.

A study on preparation and characterization of carbon doped TiO2 nanotubes

The present study is directed to clarify the influence of carbon doping on the degradation of methyl orange. TiO2 nanotubes were prepared by anodizing titanium foils in a two electrode configuration bath with titanium foil as the anode and platinum as the counter electrode. The electrochemical bathconsists of 1 M Na2SO4 with 0.7 g ammonium fluoride, NH4F. The nanotubes obtained were further doped with carbon via in-situ and ex-situ method. Incorporation of carbon on TiO2 via in-situ method is accomplished during the anodization process by introducing oxalic acid into electrolyte while theex-situ doping involves carbon incorporation into pre-fabricated TiO2 nanotube via flame annealing using carbon blackN330. Characterization such as Scanning Electron Microscope (SEM), Energy Dispersive X-ray Analysis (EDX), and X-Ray Diffraction (XRD) are used to determine the surfacemorphology, composition of dopants, and phases exists. Well ordered nanotube with good adherence and smooth surface was obtained for both methods. When the oxide was annealed, X-ray diffraction analysis revealed the presence of anatase and rutile phase. The photocatalytic properties of thepure TiO2 and carbon doped TiO2 were tested for methyl orange degradation and the result indicated that the in-situ doped TiO2 has much better degradation than the ex-situ and pure TiO2. The percentage of methyl orange degradation for in-situ was 20% and 41% higher than ex-situ doped TiO2 and pure TiO2, respectively.

A comparative study of CO adsorption on tetrahexahedral Pt nanocrystals and interrelated Pt single crystal electrodes by using cyclic voltammetry and in situ FTIR spectroscopy.

This study focuses on CO adsorption at tetrahexahedral Pt nanocrystals (THH Pt NCs) by using cyclic voltammetry and in situ FTIR spectroscopy. Since the electrochemically prepared THH Pt NCs in this study are enclosed by {730} facets which could be considered by a subfacet configuration of 2{210} + {310}, we have also studied CO adsorption on the interrelated Pt(310) and Pt(210) single crystal electrodes as a comparison. Cyclic voltammetry results demonstrated that CO adsorbs dominantly on the (100) sites of THH Pt NCs at low CO coverage (θ(CO)≤ 0.135), while on both (100) and (110) sites at higher CO coverage. On ordered Pt(310) and Pt(210), i.e. they were flame annealed and then cooled in H(2) + Ar, CO adsorption also illustrates relative priority on (100) sites at low CO coverage; while at high CO coverage or on oxygen-disordered Pt(310) and Pt(210) when they were cooled in air after flame annealing, the adsorption of CO presents a weak preference on (100) sites of Pt(310) and even no preference at all on (100) sites of Pt(210). In situ FTIR spectroscopic studies illustrated that CO adsorption on THH Pt NCs yields anomalous infrared effects (AIREs), which are depicted by the Fano-like IR feature on a dense distribution (60 μm(-2)) and the enhancement of abnormal IR absorption on a sparse distribution (22 μm(-2)) of THH Pt NCs on glassy carbon substrate. Systematic investigation of CO coverage dependence of IR features revealed that, on THH Pt NCs, the IR band center (ν(COL)) of linearly bonded CO (COL) is rapidly shifted to higher wavenumbers along with the increase of CO coverage to 0.184, yielding a fast linear increase rate with a high slope (dν(COL)/dθ(IR)(CO) = 219 cm(-1)); when θ > 0.184, the increase of ν(COL) with θCO slows down and deviates drastically from linearity. In contrast, the ν(COL) on the ordered Pt(310) electrode maintains a linear increase with θ(IR)(CO) for the whole range of θ(IR)(CO) variation, and gives a much smaller increase rate of slope 74.3 cm(-1). The significant differences in CO adsorption behavior on THH Pt NCs and on interrelated Pt single crystal planes demonstrated clearly the unique properties of nanoparticles enclosed by high-index facets.

Enhancing the Water Splitting Efficiency of Sn‐Doped Hematite Nanoflakes by Flame Annealing

The effect of flame annealing on the water-splitting properties of Sn decorated hematite (α-Fe2O3) nanoflakes has been investigated. It is shown that flame annealing can yield a considerable enhancement in the maximum photocurrent under AM 1.5 (100 mW cm(-2)) conditions compared to classic furnace annealing treatments. Optimizing the annealing time (10 s at 1000 °C) leads to a photocurrent of 1.1 mA cm(-2) at 1.23 V (vs. RHE) with a maximum value 1.6 mA cm(-2) at 1.6 V (vs. RHE) in 1 M KOH. The improvement in photocurrent can be attributed to the fast direct heating that maintains the nanoscale morphology, leads to optimized Sn decoration, and minimizes detrimental substrate effects.

Codoping titanium dioxide nanowires with tungsten and carbon for enhanced photoelectrochemical performance

Recent density-functional theory calculations suggest that codoping TiO2 with donor-acceptor pairs is more effective than monodoping for improving photoelectrochemical water-splitting performance because codoping can reduce charge recombination, improve material quality, enhance light absorption and increase solubility limits of dopants. Here we report a novel ex-situ method to codope TiO2 with tungsten and carbon (W, C) by sequentially annealing W-precursor-coated TiO2 nanowires in flame and carbon monoxide gas. The unique advantages of flame annealing are that the high temperature (>1,000 °C) and fast heating rate of flame enable rapid diffusion of W into TiO2 without damaging the nanowire morphology and crystallinity. This is the first experimental demonstration that codoped TiO2:(W, C) nanowires outperform monodoped TiO2:W and TiO2:C and double the saturation photocurrent of undoped TiO2 for photoelectrochemical water splitting. Such significant performance enhancement originates from a greatly improved electrical conductivity and activity for oxygen-evolution reaction due to the synergistic effects of codoping.

The nature and stability of the Au(110)/electrochemical interface produced by flame annealing

It is shown using reflection anisotropy spectroscopy (RAS) that following flame annealing and immersion in pure water, the Au(110) surface adopts a (1×1) structure and that this structure is preserved in a 0.1 M H2SO4 environment. The surface transforms to the (1×2) reconstruction following the application of a potential of 0.0 V versus SCE (a saturated calomel electrode). This surface is unstable and the RAS profile changes over periods of 15 min and 1 h in a manner which suggests that changes are occurring in the structure and distribution of steps. Over longer periods the RAS transforms towards a profile attributed to a surface associated with the specific adsorption of anions.

Fabrication of porous silicon by metal-assisted etching using highly ordered gold nanoparticle arrays

A simple method for the fabrication of porous silicon (Si) by metal-assisted etching was developed using gold nanoparticles as catalytic sites. The etching masks were prepared by spin-coating of colloidal gold nanoparticles onto Si. An appropriate functionalization of the gold nanoparticle surface prior to the deposition step enabled the formation of quasi-hexagonally ordered arrays by self-assembly which were translated into an array of pores by subsequent etching in HF solution containing H2O2. The quality of the pattern transfer depended on the chosen preparation conditions for the gold nanoparticle etching mask. The influence of the Si surface properties was investigated by using either hydrophilic or hydrophobic Si substrates resulting from piranha solution or HF treatment, respectively. The polymer-coated gold nanoparticles had to be thermally treated in order to provide a direct contact at the metal/Si interface which is required for the following metal-assisted etching. Plasma treatment as well as flame annealing was successfully applied. The best results were obtained for Si substrates which were flame annealed in order to remove the polymer matrix - independent of the substrate surface properties prior to spin-coating (hydrophilic or hydrophobic). The presented method opens up new resources for the fabrication of porous silicon by metal-assisted etching. Here, a vast variety of metal nanoparticles accessible by well-established wet-chemical synthesis can be employed for the fabrication of the etching masks.

An influence of pretreatment conditions on surface structure and reactivity of Pt(100) towards CO oxidation reaction

We present a combined electrochemical and in situ STM study of the surface structure of Pt(100) single crystal electrodes in dependence on the cooling atmosphere after flame annealing. The following cooling conditions were applied: Ar/H2 and Ar/CO mixtures (reductive atmosphere), argon (inert gas) and air (oxidative atmosphere). Surface characterization by in-situ STM allows deriving direct correlations between surface structure and macroscopic electrochemical behavior of the respective platinum electrodes. We investigated the influence of defect type and density as well as long range surface order on the kinetics of the CO electro-oxidation reaction. The defect-rich Pt(100) electrodes as cooled in air or Ar, and followed by immersion in the hydrogen adsorption region display higher activities as compared to the rather smooth Pt(100)-(1 × 1) electrode cooled in an Ar/H2-atmosphere.

Flame annealing effects on self-organized TiO2 nanotubes

In the present work we study the modification of ordered TiO2 nanotube layers by flame annealing. We investigate different type of nanotubes (with variation in tube length, diameter, and grown in different electrolytes). We vary flame annealing parameters and analyze the morphology, phase composition and the resulting photoelectrochemical properties of the modified TiO2 nanotubes. The results show that flame annealing allows the crystallization of TiO2 nanotubes to anatase or rutile within seconds. The crystallization times strongly depend on the nanotube length and flame parameters. Samples from the outer cone show typically carbon burn off, while in the inner cone carbon accumulation occurs. The resulting photo electrochemical properties are to a large extent determined by the phase composition of the nanotubes (and not by the carbon content). Additionally, it may be noted that although the flame annealing process is fast and cheap, we could not achieve a maximum UV photoelectrochemical response comparable to classic furnace annealing.

A quick method for the preparation of Pt(111)-like thin films

A simple and quick method for forming Pt(111)-like thin films on Si/Ti substrates for electrochemical and/or electrocatalytic experiments is reported. This method involves physical vapour deposition followed by flame annealing, electrochemical cleaning and a short heat treatment under a controlled atmosphere. Careful selection of the substrate, surface preparation and cooling atmosphere allows production of Pt thin films which show voltammetry features typical of large Pt(111) single crystal electrodes in 0.1M HClO4. This technique promises a method for the production of Pt(111) type surfaces on a larger scale.

Improved water-splitting behaviour of flame annealed TiO 2 nanotubes

In the present work we investigate the effect of flame annealing on the water-splitting properties of anodic TiO 2 nanotubes. It is shown that flame annealed (FA) nanotubes can yield a two-fold increase in the maximum water-splitting efficiency under standard AM 1.5 conditions compared to classic annealing treatments. The improved efficiency is mainly the result of the fact that using FA, a higher rutile content can be established in the tube walls without a thermal degradation of the tube morphology. A minor, secondary effect is carbon doping which is caused mainly by organic remnants from the tube formation electrolyte.

Thirty years of platinum single crystal electrochemistry

The electrochemistry of platinum single crystals is historically reviewed. After a brief revision of historical results dating before the publication of the landmark experiment by J. Clavilier of the flame annealing in 1980, the controversy introduced by this experiment into the surface electrochemistry community is described. Questions about the structure and composition of the platinum surface after the flame annealing and their implications on the characteristic voltammetry of platinum single crystal electrodes were slowly answered in the years that followed the first introduction of this methodology. One of the last questions to be solved was that about the nature of the chemical species responsible for the charge transfer process that leads to the so-called unusual features in the voltammogram. This was solved with the charge displacement experiment. Nowadays, a great deal of knowledge has been gathered about the structure of the interphase between platinum electrodes and electrolytic solutions and also about the electrocatalytic behaviour of platinum surfaces. State-of-the-art information about platinum electrochemistry is provided, with emphasis on results from our group, especially those obtained with a thermodynamic analysis, involving either constant or variable temperatures and with the laser-induced temperature jump method.

Crystallization and surface morphology of Au/SiO 2 thin films following furnace and flame annealing

A crystallization and surface evolution study of Au thin film on SiO 2 substrates following annealing at different temperatures above the eutectic point of the Au/Si system are reported. Samples were prepared by conventional evaporation of gold in a high vacuum (10 −7 mbar) environment on substrates at room temperature. Thermal treatments were performed by both furnace and flame annealing techniques. Au thin films can be crystallized on SiO 2 substrates by both furnace and flame annealing. Annealing arranges the Au crystallites in the (1 1 1) plane direction and changes the morphology of the surface. Both, slow and rapid annealing result in a good background in the XRD spectra and hence clean and complete crystallization which depends more on the temperature than on the time of annealing. The epitaxial temperature for the Au/SiO 2 system decreases in the range of 350–400 °C. Furnace and flame annealing also form crystallized gold islands over the Au/SiO 2 surface. Relaxation at high temperatures of the strained Au layer, obtained after deposition, should be responsible for the initial stages of clusters formation. Gold nucleation sites may be formed at disordered points on the surface and they become islands when the temperature and time of annealing are increased. The growth rate of crystallites is highest around 360 °C. Above this temperature, the layer melts and gold diffuses from the substrate to the nucleation sites to increase the distance between islands and modify their shapes. Well above the eutectic temperature, the relaxed islands have hexagonally shaped borders. The mean crystallite diameters grow up to a maximum mean size of around 90 nm. The free activation energy for grain boundary migration above 360 °C is 0.2 eV. Therefore the type of the silicon substrate changes the mechanism of diffusion and growth of crystallites during annealing of the Au/Si system. Epitaxial Au(1 1 1) layers without formation of islands can be prepared by furnace annealing in the range of 300–310 °C and by flame annealing of a few seconds and up to 0.5 min.

Anti-Fogging Nanofibrous SiO2 and Nanostructured SiO2−TiO2 Films Made by Rapid Flame Deposition and In Situ Annealing

Transparent, pure SiO(2), TiO(2), and mixed silica-titania films were (stochastically) deposited directly onto glass substrates by flame spray pyrolysis of organometallic solutions (hexamethyldisiloxane or tetraethyl orthosilicate and/or titanium tetra isopropoxide in xylene) and stabilized by in situ flame annealing. Silicon dioxide films consisted of a network of interwoven nanofibers or nanowires several hundred nm long and 10-15 nm thick, as determined by microscopy. These nanowire or nanofibrous films were formed by chemical vapor deposition (surface growth) on bare glass substrates during scalable combustion of precursor solutions at ambient conditions, for the first time to our knowledge, as determined by thermophoretic sampling of the flame aerosol and microscopy. In contrast, titanium dioxide films consisted of nanoparticles 3-5 nm in diameter that were formed in the flame and deposited onto the glass substrate, resulting in highly porous, lace-like nanostructures. Mixed SiO(2)-TiO(2) films (40 mol % SiO(2)) had similar morphology to pure TiO(2) films. Under normal solar radiation, all such films having a minimal thickness of about 300 nm completely prevented fogging of the glass substrates. These anti-fogging properties were attributed to inhibition of water droplet formation by such super-hydrophilic coatings as determined by wetting angle measurements. Deactivated (without UV radiation) pure TiO(2) coatings lost their super-hydrophilicity and anti-fogging properties even though their wetting angle was reduced by their nanowicking. In contrast, SiO(2)-TiO(2) coatings exhibited the best anti-fogging performance at all conditions taking advantage of the high surface coverage by TiO(2) nanoparticles and the super-hydrophilic properties of SiO(2) on their surface.

Optimal Doping for Enhanced SnO2 Sensitivity and Thermal Stability

AbstractTin oxide nanocrystals (5–10 nm) doped with silica (0–15 wt %) were made by flame‐spray‐pyrolysis direct deposition onto the sensing electrodes and in situ stabilization by rapid flame annealing. Although increased SiO2‐doping reduced the SnO2 crystal and grain size, its sensing performance to ethanol vapor (0.1–50 ppm) exhibited an optimum with respect to SiO2 content. The thermal stability and morphology of SiO2‐doped SnO2 nanoparticles were evaluated by sintering at 200–900 °C for 4–24 h in air. At low SiO2 content, sintering of SnO2 was prevented only partially resulting in small sinter necks (bottlenecks) between SnO2 primary particles (smaller than twice the Debye length). This morphology drastically enhanced the sensitivity toward the analyte by maintaining a thermally stable high surface area and fully depleted connections at the primary particle necks. This enhancement is attributed mostly to the decreasing neck size of the SnO2SiO2 heterojunctions rather than the decreasing SnO2 crystallite and grain sizes with increasing SiO2 doping. At high SiO2 contents, SnO2 sintering was inhibited as its grains were separated effectively by dielectric SiO2; this resulted in isolated SnO2 nanocrystals with drastically reduced sensitivity, thereby effectively being insulators.

A study of the growth and CO electrooxidation behaviour of PtRh alloys on Pt{1 0 0} single crystals

The formation of quasi-crystalline PtRh two dimensional films supported on Pt{1 0 0} is described. For the first time, the voltammetry of PtRh{1 0 0} single crystal alloys covering the whole range of alloy composition is reported. Synthesis follows a similar procedure to that described previously for the formation of PtPd alloys supported on Pt{ h k l} but with some important provisos concerning the final annealing step. CO electrooxidation was used as a probe reaction and for certain PtRh{1 0 0} surface alloys, unusually high electrocatalytic activity was observed relative to monometallic Pt and Rh electrodes. The flame annealing of a “PtRhPt sandwich” precursor structure was found to be the best method of forming the PtRh alloy surfaces. For PtRh films annealed under nitrogen, significant phase separation was observed in agreement with previous surface science studies of PtRh adlayers on Pt{1 0 0} annealed in the absence of oxygen. In addition, an excess of Rh in the “sandwich structure” from which the alloy was formed tended to preclude good alloy formation. It is suggested that the protocols for thin film formation described may prove useful in many other important electrocatalytic systems.

A new cleaning methodology for efficient Au-SAM removal

Abstract The desorption of a self-assembled monolayer (SAM) of 11-amino-1-undecanethiol (AUT) formed on an Au polycrystalline electrode was investigated with the purpose of establishing the most efficient method for SAM removal prior to electrode re-use. The cleanliness of the surface was evaluated by assessing the characteristics (and their reproducibility) of a newly prepared SAM (on a freshly cleaned electrode) in the presence of Ru(NH3)63+/2+ probe species. The simple flame annealing of the modified electrodes showed poor reproducibility. Later, cleaning treatments based on previous reports about the electrochemical desorption of SAMs, were investigated in NaOH, HClO4, and KCl solutions. The anodic removal in alkaline and chloride solutions was not efficient enough due to the effect of the Au surface oxidation and dissolution. The commonly used reductive desorption in alkaline solution did also not offer a high efficiency (in contrast with the behavior described in those reports) probably due to the low stability of the thiolate molecules at high-pH values. In addition, such procedures did not provide a way to visualize the evolution of the process. The oxidative desorption in perchloric acid showed a higher efficiency at each single desorption cycle, and allowed to monitor the extension of the removal by comparing the cyclic voltammograms obtained in that medium with the expected fingerprint for the bare Au. However the voltammetric characterization of new AUT films, prepared on such electrochemically treated surfaces by re-incubation in the AUT solution, showed that the conditions of substrate cleanliness and smoothness necessaries to reproduce a close-packed and compact AUT monolayer were not reached. A new cleaning strategy based on the coupled use of oxidative removal in HClO4 0.1 M and flame annealing showed to be efficient and reproducible, providing the proper substrate precursors for the formation of highly ordered AUT films. The results also showed that the methodology works well in the removal of other SAMs like the one formed by the neutral 1-dodecanethiol (1-DT).

Formation of Highly Ordered Arrays of Dimples on Tantalum at the Nanoscale

We show that electropolishing of tantalum metal in a single step of about 5 min can reproducibly lead to dimples tens of nanometers in diameter, regular in shape, monodispersed in size, and arranged in highly ordered arrays which even transverse grain boundaries. Dimpled tantalum is ductile, high melting, and chemically inert, which makes it suitable for nanostructure synthesis even under extreme conditions, as demonstrated with a simple sputter coating and flame annealing procedure for gold nanoparticles.