The effect of metal dissolution on carbon production by high-temperature molten salt electrolysis

High-temperature molten salt electrolysis is suitable for the production of carbon morphologies such as carbon nanotubes and nano-onions. In this study, CO2 was electrochemically reduced to solid carbon by molten lithium carbonate electrolysis in an Inconel 625 vessel at a fixed temperature of 750°C. Four different cathodes (clean nickel, used nickel, stainless steel, and galvanized steel) were used to determine the effect of the electrode material on the morphology produced. The carbonaceous products obtained were analyzed with scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), Raman microscopy, and X-ray diffraction (XRD). With nickel cathodes, the dominant forms of carbon were spherical, whereas tubular structures dominated with steel-based cathodes. Nano-onion was the structure of carbon with the least metal impurities. Iron was discovered to promote carbon nanotube growth. In the presence of iron, nanotube wool was also found. A greater number of different morphologies were observed when the amount of metal impurities increased. The correlation found between XRD results and sample masses suggests that the amount of metal impurities in the sample varied more than the carbon content. Thus, the yield of the process can be expected to be fairly similar between parallel experiments.publishedVersionPeer reviewe

Silver nanoparticle coatings with adjustable extinction spectra produced with liquid flame spray, and their role in photocatalytic enhancement of TiO2

Silver nanoparticles deposits were produced with liquid flame spray (LFS) on glass and TiO2 substrates to study their optical response and photocatalytic enhancement. The correlation between extinction spectrum of the nanoparticle coating and the LFS process parameters was studied. The spectra consisted of two partly overlapping peaks: one centered in the UV region and the other in the visible light region. The visible light peak redshifted as either the silver mass concentration in the precursor solution or the precursor solution feed rate was increased, which also correlated with growing primary particle size. However, simultaneous correlation with photocatalytic activity of the decorated TiO2 surfaces was not observed, which was attributed to particle sintering on the surface. Instead, the photocatalytic activity was seen to change as the surface coverage of silver nanoparticles was varied. When the surface coverage was raised from ∼10 % to roughly 30 %, the activity, and then decreased as the loading was further raised. The increase was assumed to originate from plasmonic activation, and the decrease was attributed to the excessive amount of silver either blocking reactive area of the TiO2 or absorbing/scattering too much of the incoming light, which hindered the photocatalytic activity.Peer reviewe

Crystallographic Phase Control of Iron Oxide Particles in Liquid Flame Spray and Utilization of Nanoparticles in Applications

Nanotechnology has become a very important branch of research and industry especially during this millenium, and it keeps growing constantly. Simultaneously, demand for large volumes of quality nanomaterial is increasing. Flame spray pyrolysis (FSP) is one of the most promising fabrication methods for inexpensive and simple large-scale production. However, the details of the synthesis process still remain reasonably poorly understood. Liquid flame spray (LFS), a certain type FSP method, was chosen as the focal point of this dissertation. Because both the knowledge on the synthesis process and utilization of LFS in applications are essential for capitalizing on its the strengths in the long term, half of this dissertation was dedicated to improving the understanding of the process, and the other half for exploring its potential in two distinct applications. In the first part, factors determining the crystallographic phase composition of iron oxide particles in LFS synthesis was studied. The synthesis yielded maghemite (γ-Fe2O3) and hematite (α-Fe2O3), and the equivalence ratio was found as the best measure for predicting their ratio. However, the correlation between the phase ratio and the equivalence ratio was observed to differ between different experimental setups. Modifying the precursor solution through mixed solvents or additives also had a clear effect on the phase composition, which indicates that the solution chemistry likely has unknown effects on the process. The first of the two applications was bidisperse magnetorheological (MR) fluids, where LFS-made nanoparticles were added to a more traditional MR fluid to counteract sedimentation. The addition reduced the sedimentation rate considerably without impairing its performance. The second application was a liquid-repellent nanocoating consisting of a nanoparticle layer produced with LFS, an ALD mid layer, and a topmost silane layer. The ultrathin final coating repelled several test liquids effectively, but possessed lower stability than initially hoped

Producing omniphobicity on steel surface by a multilayered nanocoating

Funktionaalisten nanopinnoitteiden tutkimus ja kehittäminen on kiihtynyt jatkuvasti viimeisen vuosikymmenen aikana. Kuitenkin etenkin kovat ja vaikeasti muokattavat substraatit, kuten ruostumaton teräs, tuottavat edelleen paljon haasteita. Omnifobiset eli nesteitä hylkivät pinnat ovat yksi nykyään mielenkiintoa herättävistä sovelluksista. Useampien toiminnallisten kerrosten yhdistäminen avaa mahdollisuuksia entistä parempien ja monipuolisempien pinnoitteiden valmistamiseen. Tämän työn tarkoituksena oli etsiä uusia ratkaisuja tuottaa ruostumattomalle teräkselle voimakkaasti omnifobinen ja mahdollisesti kestävä ohutpinnoite hyödyntäen kerrosrakennetta. Valmistettiin kolmesta kerroksesta koostuva rakenne. Ensimmäisenä substraatille tuotettiin nesteliekkiruiskutuksella (Liquid Flame Spray, LFS) TiO2-nanohiukkasia sopivan pintatopografian luomiseksi. Seuraavaksi hiukkaskerrosta vahvistettiin pinnoittamalla päälle ohut Al2O3-kerros atomikerroskasvatuksella (Atomic Layer Deposition, ALD). Päällimmäiseksi valittiin silaanikerros pintaenergian laskemiseksi, vahvistaen näin pinnan omnifobisuutta. Pinnan rakennetta ja kemiallista koostumusta tutkittiin pyyhkäisyelektronimikroskopian (Scanning Electron Microscopy, SEM) ja fotoelektronispektroskopian (X-ray Photoelectron Spectroscopy, XPS) avulla. Pinnan hylkivyyttä eri nesteille tutkittiin kontaktikulmamittauksilla ja mekaanista kestävyyttä yksinkertaisen teippitestin sekä naarmutustestin avulla. LFS tuotti pinnalle nanorakenteen, jonka ALD-kerros peitti tasaisesti muodostaen silaanikerroksen kanssa tiettävästi ohuimman (paksuimmillaan n. 140 nm) kirjallisuudessa raportoidun kerrosrakenteisen, omnifobisen pinnoitteen. Superomnifobisuus eli ’täydellinen’ hylkivyys saavutettiin vedelle, dijodimetaanille ja etyleeniglykolille, joten tavoitteeseen päästiin tältä osin suhteellisen hyvin. Pinnoitteen mekaaninesta kestävyydestä on vaikea sanoa mitään varmaa, mutta pinnan nanorakenne luultavasti häviää melko helposti olosuhteissa, joissa pinta altistuu mekaaniselle kulumiselle. Sintrauksella ei ollut suurta vaikutusta pinnoitteen ominaisuuksiin

Crystallographic phase formation of iron oxide particles produced from iron nitrate by liquid flame spray with a dual oxygen flow

We fabricated iron oxide particles from iron(III) nitrate in a liquid flame spray synthesis. Unlike in most liquid flame spray studies, we implemented a secondary oxygen flow. The effect of the gas flow setup and two additives to the precursor solution, oxalic acid and citric acid, on the resulting particles was studied, with the focus on crystallographic phase composition. The synthesis yielded either pure maghemite or maghemite/hematite mixed phase powders. For solutions without additives, the maghemite fraction was almost linearly dependent on the equivalence ratio. The specific surface area was highest for the smallest equivalence ratios, then decreased, and increased again for the highest values. Some variation was observed between samples with equal equivalence ratios but the total oxygen flow divided differently between the two oxygen channels, a higher atomization flow promoting larger hematite fraction and higher specific surface area. Both additives reduced the amount of hematite in the powder samples, citric acid being the more efficient one. Citric acid slightly raised the specific surface area, whereas oxalic acid dropped it in half.publishedVersionPeer reviewe

Controlling the phase of iron oxide nanoparticles fabricated from iron(III) nitrate by liquid flame spray

Iron oxide nanoparticles were synthesized in a liquid flame spray process from iron(III) nitrate. The choice of chemicals and all other process parameters affects the crystallographic phase composition and the quality of the material. Adjustment of the solvent composition and the gas flow rates was used to control the phase composition of the produced particles. All samples consisted of pure maghemite (γ‐Fe2O3) or a mixture of maghemite and hematite (α‐Fe2O3). When using pure alcohols as solvents, the maghemite/hematite phase ratio could be adjusted by changing the equivalence ratio that describes the oxidation conditions in the flame zone. A large residual particle mode formed in the size range of ~20‐700 nm along with a dominant very fine particle mode (2‐8 nm). Both phases seemed to contain large particles. A partial substitution of methanol with carboxylic acids turned the hematite phase into maghemite completely, even though some of particles were possibly not fully crystallized. Residual particles were still present, but their size and number could be decreased by raising the heat of combustion of the precursor solution. 30 vol‐% substitution of methanol with 2‐ethylhexanoic acid was adequate to mostly erase the large particles.publishedVersionPeer reviewe

Fabrication of ultrathin multilayered superomniphobic nanocoatings by liquid flame spray, atomic layer deposition, and silanization

Superomniphobic, i.e. liquid-repellent, surfaces have been an interesting area of research during recent years due to their various potential applications. However, producing such surfaces, especially on hard and resilient substrates like stainless steel, still remains challenging. We present a stepwise fabrication process of a multilayered nanocoating on a stainless steel substrate, consisting of a nanoparticle layer, a nanofilm, and a layer of silane molecules. Liquid flame spray was used to deposit a TiO2 nanoparticle layer as the bottom layer for producing a suitable surface structure. The interstitial Al2O3 nanofilm, fabricated by atomic layer deposition (ALD), stabilized the nanoparticle layer, and the topmost fluorosilane layer lowered the surface energy of the coating for enhanced omniphobicity. The coating was characterized with field emission scanning electron microscopy, focused ion beam scanning electron microscopy, x-ray photoelectron spectroscopy, contact angle (CA) and sliding angle (SA) measurements, and microscratch testing. The widely recognized requirements for superrepellency, i.e. CA > 150° and SA < 10°, were achieved for deioinized water, diiodomethane, and ethylene glycol. The mechanical stability of the coating could be varied by tuning the thickness of the ALD layer at the expense of repellency. To our knowledge, this is the thinnest superomniphobic coating reported so far, with the average thickness of about 70 nm.acceptedVersionPeer reviewe