Nanostructured photocatalytic titania coatings formed by suspension plasma spraying

This paper describes formation of titanium dioxide coatings designed for photocatalytic applications, obtained by suspension plasma spraying (SPS), an alternative of the atmospheric plasma spraying (APS) technique in which the material feedstock is a suspension of the material to be sprayed. Two different TiO₂ powders were dispersed in distilled water and ethanol and injected in Ar-H₂ or Ar-H₂-He plasma under atmospheric conditions. Scanning electron microscopy (SEM) and x-ray diffraction (XRD) analyses were performed to study the microstructure of the titania coatings. Photocatalytic efficiency of the elaborated samples was evaluated from the conversion ratio of different air pollutants: nitrogen oxides (NOx) and sulfur dioxide (SO₂). The morphology and crystalline structure of the deposits depended mainly on the nature of the solvent (water or alcohol) used in the preparation of the slurries. Dense coatings were obtained starting from aqueous suspensions and porous deposits were elaborated by plasma spraying of a PC105 alcoholic suspension. A significant phase transformation from anatase to rutile occurred when ethanol was used as a solvent. Different photocatalytic performances were observed as a function of the nature of the liquid material feed-stock, the spraying parameters, and the nature of the pollutant. This article was originally published inBuilding on 100 Years of Success, Proceedings of the 2006 International Thermal Spray Conference (Seattle, WA), May 15–18, 2006, B.R. Marple, M.M. Hyland, Y.-Ch. Lau, R.S. Lima, and J. Voyer, Ed., ASM International, Materials Park, OH, 2006.     

Microstructural Analysis and Photocatalytic Activity of Plasma-Sprayed Titania-Hydroxyapatite Coatings

Hydroxyapatite (HAp Ca₁₀(PO₄)₆(OH)₂) is known to be a biomaterial and an adsorbent for chromatography. In this study, HAp was agglomerated with anatase TiO₂ to manufacture thermal-spray powders to improve the adsorption activity of TiO₂, and then to improve its photocatalytic activity. The microstructures, compositions and photocatalytic activity of plasma-sprayed TiO₂, TiO₂-10%HAp, TiO₂-30%HAp, and HAp coatings were investigated. Due to the low thermal conductivity of HAp compound, not all HAp particles fully melted even under the arc current of 800 A. The addition of HAp inhibited the phase transformation of anatase TiO₂ to rutile. Under the arc current of 600 A, the anatase content in the TiO₂, TiO₂-10%HAp and TiO₂-30%HAp coatings was 11, 20 and 42%, respectively. With the increasing of the spraying distance from 70 to 110 mm, the anatase content in the TiO₂-30%HAp coatings decreased from 34 to 17% under arc current of 700 A. Furthermore, a slight decomposition of HAp to α-Ca₃(PO₄)₂ was found in the TiO₂-30%HAp coatings, it did not decompose to CaO and P₂O₅ according to the XRD and EDAX analysis. The addition of the secondary gas of helium had no significant influence on the melting state of the TiO₂-HAp feedstock powders. Moreover, the HAp in the TiO₂-10%HAp and TiO₂-30%HAp coatings had adsorption characteristic to acetaldehyde. The photocatalytic activity of TiO₂-10%HAp coating was highest among TiO₂, TiO₂-10%HAp, and TiO₂-30%HAp coatings sprayed under the arc current of 600 A for the optimum adsorption property and anatase content. This article is an invited paper selected from presentations at the 2007 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Global Coating Solutions, Proceedings of the 2007 International Thermal Spray Conference, Beijing, China, May 14-16, 2007, Basil R. Marple, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2007.     

Comparative study on the photocatalytic behaviour of titanium oxide thermal sprayed coatings from powders and suspensions

This work presents a study of the microstructures and photocatalytic behaviour of titanium oxide coatings obtained by thermal spraying of agglomerated nanopowders and suspensions. Fine TiO₂ Degussa P25 nanopowder, generally considered as the reference material in photocatalytic applications, was used as the material feedstock. HVOF process and suspension thermal spraying were used to prepare photocatalytic titania coatings. The coatings were mainly characterised by means of SEM and X-ray diffraction. The photocatalytic performance was evaluated based on decolouration of the pink dye Rhodamine B and degradation of gaseous acetaldehyde. A lower degree of pollutant degradation was found for deposits prepared by HVOF spraying of granules due principally to the low content of the photocatalytically active phase, i.e. anatase. Complete photocatalytic degradation of the organic compounds was recorded for the suspension-sprayed coatings. Based on the current results, suspension thermal spraying appears to be the better choice for preparing photocatalytically active titanium oxide surfaces for the removal of organic pollutants.     

Nanoindentation study of the mechanical and damage behaviour of suspension plasma sprayed TiO2 coatings

TiO₂ coatings can be used as self-cleaning surfaces owing to their photocatalytic and hydrophilic properties. Suspension plasma spray (SPS) has proven to be a feasible and cheap technique for producing self-cleaning surfaces with acceptable photo-activity. This paper presents a nanoindentation study of the mechanical properties (hardness, Young's modulus and scratch resistance) of photoactive layers of suspension plasma sprayed TiO₂ coatings applied on to glass substrates. Microstructure observation showed that the rutile grains were surrounded by fine anatase crystals. Under the same spraying conditions, the resulting anatase/rutile concentrations varied depending on the cooling rate (the substrate being either cooled with water or in air). The results showed that higher concentrations of anatase, which is softer than rutile, reduced the scratch damage and increased the friction coefficient.     

Suspension plasma spraying of nanostructured WC-12Co coatings

Nanostructured WC-12% Co coatings were deposited by suspension plasma spraying of submicron feedstock powders, using an internal injection plasma torch. The liquid carrier used in this approach allows for controlled injection of much finer particles than in conventional thermal spraying, leading to thin coatings with a fine surface finish. A polyethylene-imine (PEI) dispersant was used to stabilize the colloidal suspension in an ethanol carrier. In-flight particle states were measured for a number of operating conditions of varying plasma gas flow rates, feed rates, and standoff distances and were related to the resulting microstructure, phase composition (EDS, SEM, XRD), and Vickers hardness. High in-flight particle velocities (>800 m/s) were generated, leading to dense coatings. It was observed that the coating quality was generally compromised by the high temperature and reactivity of the small particles. To compensate for this shortcoming, the suspension feed rate was adjusted, thereby varying the thermal load on the plasma. Results showed that a slightly larger agglomerate size, in conjunction with low particle jet temperatures, could somewhat limit the decomposition of WC into brittle W₂C/W₃C and amorphous cobalt containing binder phases. This article was originally published inBuilding on 100 Years of Success, Proceedings of the 2006 International Thermal Spray Conference (Seattle, WA), May 15–18, 2006, B.R. Marple, M.M. Hyland, Y.-Ch. Lau, R.S. Lima, and J. Voyer, Ed., ASM International, Materials Park, OH, 2006.     

Influence of Silver Doping on Photocatalytic Activity of Liquid-Flame-Sprayed-Nanostructured TiO2 Coating

Silver ion was added to liquid feedstock to deposit Ag⁺-doped-nanostructured TiO₂ photocatalytic coatings through liquid-flame spraying. The coating microstructure was characterized by x-ray diffraction (XRD). The photocatalytic performance of coatings was examined by photodegradation of acetaldehyde. The XRD analysis showed that the phase structure of coatings was not significantly influenced by the silver ion doping. However, a shift was found for XRD peaks of anatase TiO₂. The photocatalytic activity of the TiO₂ coatings increased and then decreased with the increase of dopant concentration. The photocatalytic activity of doped coatings was higher than that of pure TiO₂ coating, regardless of the dopant concentration. The enhancement of photocatalytic performance of doped coatings is attributed to co-doping of Ag⁺ ion and metallic Ag. This article is an invited paper selected from presentations at the 2007 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Global Coating Solutions, Proceedings of the 2007 International Thermal Spray Conference, Beijing, China, May 14-16, 2007, Basil R. Marple, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2007.     

Cold Spraying of TiO<Subscript>2</Subscript> Photocatalyst Coating With Nitrogen Process Gas

Titanium dioxide (TiO₂) is a promising material for photocatalyst coatings. However, it is difficult to fabricate a TiO₂ coating with anatase phase by conventional thermal spray processes due to a thermal transformation to rutile phase. In this paper, anatase TiO₂ coatings were fabricated by the cold spray process. To understand the influence of process gas conditions on the fabrication of the coatings, the gas nature (helium or nitrogen) and the gas temperature are investigated. It was possible to fabricate TiO₂ coatings with an anatase phase in all spraying conditions. The process gas used is not an important factor to fabricate TiO₂ coatings. The thickness of the coatings increased with the process gas temperature increasing. It indicates that the deposition efficiency of the sprayed particles can be enhanced by controlling the spray conditions. The photocatalytic activity of the coatings is similar or better than the feedstock powder due to the formation of a large reaction area. Concludingly, cold spraying is an ideal process for the fabrication of a TiO₂ photocatalyst coating.     

Plasma spray synthesis from precursors: Progress, issues, and considerations

Precursor plasma spray synthesis is an innovative and rapid method for making functional oxide ceramic coatings by starting from solution precursors and directly producing inorganic films. This emerging method utilizes molecularly mixed precursor liquids, which essentially avoids the handling and selection of powders, opening up new avenues for developing compositionally complex functional oxide coatings. Precursor plasma spray also offers excellent opportunities for exploring the nonequilibrium phase evolution during plasma spraying of multicomponent oxides from inorganic precursors. Although there have been efforts in this area since the 1980s and early 1990s with the goal of synthesizing nanoparticles, only recently has the work progressed in the area of functional systems. At the Center for Thermal Spray Research an integrated investigative strategy has been used to explore the benefits and limits of this synthesis strategy. Water- and alcohol-based sol/solution precursors derived from various chemical synthesis methods were used as feedstocks to deposit thin/thick films of spherical and nanostructured coatings of yttrium aluminum garnet (YAG), yttrium iron garnet, lanthanum strontium manganate and Zr-substituted yttrium titanates, and compositions of Y₂O₃-Al₂O₃ and their microstructural space centered around stoichiometric YAG. A detailed discussion of the salient features of the radiofrequency induction plasma spraying approach, the results obtained in the investigations to develop various functional oxide coatings, and process issues and challenges are presented. This article was originally published inBuilding on 100 Years of Success: Proceedings of the 2006 International Thermal Spray Conference (Seattle, WA), May 15–18, 2006, B.R. Marple, M.M. Hyland, Y.-Ch. Lau, R.S. Lima, and J. Voyer, Ed., ASM International, Materials Park, OH, 2006.     

Effect of Cu2+ doping on photocatalytic performance of liquid flame sprayed TiO2 coatings

Cu²⁺ was added to liquid feedstock to deposit ion doping TiO₂ photocatalytic coatings through liquid flame spraying. The coating microstructure was characterized by x-ray diffraction (XRD), transmission electron microscopy, and x-ray photoelectron spectroscopy (XPS). The photocatalytic performance of coatings was examined by photodegradation of acetaldehyde. The XRD analysis shows that the crystalline structure of coatings is not significantly influenced by Cu²⁺ doping. The photocatalytic activity of the TiO₂ coatings is enhanced by Cu²⁺ doping. It is found that a high concentration of Cu²⁺ doping decreases the activity. The XPS analysis shows that the adsorbed oxygen concentration is increased with the increase of Cu²⁺ dopant concentration and decreases with a further increase of dopant concentration. The enhancement of photocatalytic activity can be attributed to the adsorption ability of oxygen and other reactants on the surface of doping TiO₂ coatings. This article was originally published inBuilding on 100 Years of Success, Proceedings of the 2006 International Thermal Spray Conference (Seattle, WA), May 15–18, 2006, B.R. Marple, M.M. Hyland, Y.-Ch. Lau, R.S. Lima, and J. Voyer, Ed., ASM International, Materials Park, OH, 2006.     

Suspension plasma spraying of TiO2 for the manufacture of photovoltaic cells

Highly porous TiO₂ coatings have been produced by suspension plasma spraying on ITO coated glass substrates. The deposition process could be optimized so that fine/nano grained highly porous coatings were obtained. Mean crystallite sizes well below 50 nm could be achieved in the coatings for the anatase phase.Special emphasis was on the establishment of a high volume fraction of the desired anatase phase. The best coatings showed values of about 90% of anatase phase.With these coatings photovoltaic cells could be produced. Different levels of layer thickness and porosity as well as contents of anatase phase have been investigated.In the paper details on the microstructure of the cells as well as the photovoltaic performance will be given.     

Influence of Suspension Plasma Spraying Process Parameters on TiO2 Coatings Microstructure

The study aimed at optimizing the suspension plasma spraying of TiO₂ coatings obtained using different suspensions of fine rutile particles in water solution onto aluminum substrates. The experiments of spraying were designed using a 2³ full factorial plan. The plan enabled to find the effects of three principal parameters, i.e. electric power input to plasma, spray distance, and suspension feed rate onto microstructure of coatings, content of anatase phase and size of anatase crystals in the coatings. The microstructure of deposits was observed with scanning electron microscope (SEM) and optical microscope and their composition was characterized using energy dispersive spectrometry (EDS). The observations were made on the coatings surface and their cross-sections. The latter made it possible to determine the coatings thicknesses to be in the range from 8 to 33 μm.

Phase formation of nano-TiO<Subscript>2</Subscript> particles during flame spraying with liquid feedstock

The nanostructured TiO₂ photocatalytic coatings were synthesized through flame spraying with liquid feedstock under different conditions. The nanostructured TiO₂ deposit of substantial anatase phase was annealed at different temperatures. X-ray diffraction analysis showed that significant transformation from anatase to rutile occurred at a temperature above 600 °C. However, thermal analysis suggested that the phase transformation from anatase to rutile started at a temperature from 400 to 500°C. It was found that the grain size of rutile phase was larger than that of anatase. The deposits annealed at temperatures lower than 450°C were photocatalytically active. However, the deposit annealed at 500°C, which contained 95% anatase crystalline, became photocatalytically inactive. Based on the experimental findings, a model is proposed to explain the phase transformation of the nano-TiO₂ particles and the phase formation in flame-spraying of nanostructured TiO₂ deposit with liquid feedstock. The original version of this paper was published as part of the DVS Proceedings: “Thermal Spray Solutions: Advances in Technology and Application,” International Thermal Spray Conference, Osaka, Japan, 10–12 May 2004, CD-Rom, DVS-Verlag GmbH, Düsseldorf, Germany.     

A new application field of plasma spraying technique to environmental depollution

To expand the application of plasma spraying technique, TiO2 coatings were prepared using agglomerated anatase TiO2 powder to solve the environmental problems. The composition and photocatalytic activity of plasma sprayed TiO2 coatings were investigated systematically. The content of anatase TiO2 in the sprayed coatings was approximate to 7%- 15%, which was influenced by the melting state of TiO2 powder in plasma spraying process. The surface of sprayed coating was very rough and the arithmetical mean deviation of the surface profiles (Ra) was in the range of 5.7 - 8.8 μm. Under lower arc current, the surface of the coating became rougher. The anatase to rutile phase transformation temperature of agglomerated anatase TiO2 powder was approximate to 1 173 K. The TiO2 coating sprayed under the arc current of 400 A had good photocatalytic activity for the relative high content of anatase phase in it. It is concluded that the application of plasma spraying technique to environmental field has been developed.     

Characterization of mechanical properties of suspension plasma sprayed TiO2 coatings using scratch test

The study aimed at characterizing mechanical properties of TiO₂ coatings obtained by the use of aqueous suspensions of fine rutile and anatase particles onto metal substrates. Thickness of the coatings was found with the use of optical microscope observations of metallographical cross-sections. The coatings morphology was found with scanning electron microscope (SEM) and their phase composition was determined by X-ray diffraction method. The mechanical properties were characterized using scratch test. The test enabled to characterize the adhesion of coatings by determination of critical force necessary to peel off the coatings from the substrate and, on the other hand, to estimate their cohesion by the measurement of the scratch hardness. The hardness was calculated using the width of the scratch and the value of force applied. The design of experiments (DOE) of spraying with the use of a 2³ full factorial plan was applied to rutile powder. The coatings were sprayed onto aluminium substrate. The DOE enabled to find the effects of three principal parameters, i.e. electric power input to plasma, spray distance and suspension feed rate onto coating adhesion expressed by critical force. A preliminary data for anatase powder sprayed with one set of operational parameters onto stainless steel substrate are also shown.     

Characterization of microstructure of Nano-TiO2 coating deposited by vacuum cold spraying

Control of the microstructure of TiO₂ coatings through preparation methods significantly influences the coating performance. In this study, a vacuum cold-spray process, as a new coating technology, is used to deposit nanocrystalline TiO₂ coatings on conducting glass and stainless steel substrates. TiO₂ deposits were formed using two types of nanocrystalline TiO₂ powders with mean particle diameters of 200 and 25 nm. Coating microstructures were characterized by scanning electron microscopy and x-ray diffraction analysis. Results demonstrate that a thick nanocrystalline TiO₂ coating can be deposited by the vacuum cold-spray process. The coating was found to consist of particles stacked as agglomerates that build up to several hundred nanometers. The coating also presents a mesoporous microstructure that could be effective in such applications as photocatalytic degradation and dye-sensitized solar cells. This article was originally published inBuilding on 100 Years of Success, Proceedings of the 2006 International Thermal Spray Conference (Seattle, WA), May 15–18, 2006, B.R. Marple, M.M. Hyland, Y.-Ch. Lau, R.S. Lima, and J. Voyer, Ed., ASM International, Materials Park, OH, 2006.     

Properties of alumina-based coatings deposited by plasma spray and detonation gun spray processes

Alumina, Al₂O₃ + 3 to 40 wt% TiO₂, and Al₂O₃ + 40 wt% ZrO₂ coatings were deposited by atmospheric plasma spraying (APS) and detonation gun spraying (DGS). The coatings were evaluated by optical microscopy, microhardness measurements, and X- ray diffraction. Wear resistance of the coatings was evaluated by rubber wheel sand abrasion and particle erosion test methods. Detonation gun- sprayed coatings exhibited more homogeneous microstructures and somewhat higher microhardness than corresponding plasma- sprayed coatings. Small additions of TiO₂ (3 wt%) improved both the abrasion and erosion wear resistance, whereas 40 wt% TiO₂ significantly decreased the erosion wear resistance of both APS and DGS coatings. Alumina + 40% ZrO₂ coatings exhibited the best abrasion wear resistance of both APS and DGS coatings, but the erosion wear resistance of these coatings was lower than that of the Al₂O₃ and Al₂O₃ + 3 wt% TiO₂ coatings. The best abrasion wear resistance of the coatings studied was obtained with DGS Al₂O₃ + 40 wt% ZrO₂ and Al₂O₃ + 3 to 40 wt% TiCh coatings. These coatings exhibited lower wear rates than bulk Al₂O₃. The best erosion wear resistance was obtained with the DGS Al₂O₃ + 3 wt% TiO₂ coating; however, it exhibited a higher wear rate than bulk Al₂O₃. In general, detonation gun- sprayed coatings showed significantly enhanced abrasion and erosion wear resistance than the corresponding plasma- sprayed coatings.     

Alumina coatings obtained by thermal spraying and plasma anodising — A comparison

Thermally sprayed alumina coatings are widely used in a range of industrial applications to improve wear and erosion resistance, corrosion protection and thermal insulation of metallic surfaces. These properties are required for many components to be used for production processes in the paper and printing industry.Another appropriate method to produce ceramic coatings is the plasma electrolytic oxidation (PEO). However PEO can only be applied on self-passivating metals like aluminium, titanium, magnesium and their alloys. The present paper concerns a combination of cost-efficient arc spraying and flame spraying of Al coatings (Al99.5, AlCu4Mg1) on steel substrates and post-treatment by plasma-electrolytic oxidation (PEO). The microstructure and phase composition of generated oxide coatings are examined and discussed. The created Al₂O₃ layers show outstanding hardness up to 1600 HV0.1, good bonding strength and excellent abrasion resistance compared to atmospheric plasma-sprayed Al₂O₃-coatings. The results show the superior performance of PEO-coatings and demonstrate their applicability for technical components in extreme operating conditions.     

Characterization of YSZ solid oxide fuel cells electrolyte deposited by atmospheric plasma spraying and low pressure plasma spraying

Yttria doped zirconia has been widely used as electrolyte materials for solid oxide fuel cells (SOFC). Plasma spraying is a cost-effective process to deposit YSZ electrolyte. In this study, the 8 mol% Y₂O₃ stabilized ZrO₂ (YSZ) layer was deposited by low pressure plasma spraying (LPPS) and atmospheric plasma spraying (APS) with fused-crushed and agglomerated powders to examine the effect of spray method and particle size on the electrical conductivity and gas permeability of YSZ coating. The microstructure of YSZ coating was characterized by scanning electron microscopy and x-ray diffraction analysis. The results showed that the gas permeability was significantly influenced by powder structure. The gas permeability of YSZ coating deposited by fused-crushed powder is one order lower in magnitude than that by agglomerated powder. Moreover, the gas permeability of YSZ deposited by LPPS is lower than that of APS YSZ. The electrical conductivity of the deposits through thickness direction was measured by potentiostat/galvanostat based on three-electrode assembly approach. The electrical conductivity of YSZ coating deposited by low pressure plasma spraying with fused-crushed powder of small particle size was 0.043 S cm⁻¹ at 100 °C, which is about 20% higher than that of atmospheric plasma spraying YSZ with the same powder. This article was originally published inBuilding on 100 Years of Success, Proceedings of the 2006 International Thermal Spray Conference (Seattle, WA), May 15–18, 2006, B.R. Marple, M.M. Hyland, Y.-Ch. Lau, R.S. Lima, and J. Voyer, Ed., ASM International, Materials Park, OH, 2006.     

Nanostructured Photocatalytic TiO<Subscript>2</Subscript> Coating Deposited by Suspension Plasma Spraying with Different Injection Positions

High plasma power is beneficial for the deposition efficiency and adhesive strength of suspension-sprayed photocatalytic TiO₂ coatings, but it confronts two challenges: one is the reduced activity due to the critical phase transformation of anatase into rutile, and the other is fragmented droplets which cannot be easily injected into the plasma core. Here, TiO₂ coatings were deposited at high plasma power and the position of suspension injection was varied with the guidance of numerical simulation. The simulation was based on a realistic three-dimensional time-dependent numerical model that included the inside and outside of torch regions. Scanning electron microscopy was performed to study the microstructure of the TiO₂ coatings, whereas x-ray diffraction was adopted to analyze phase composition. Meanwhile, photocatalytic activities of the manufactured TiO₂ coatings were evaluated by the degradation of an aqueous solution of methylene blue dye. Fragmented droplets were uniformly injected into the plasma jet, and the solidification pathway of melting particles was modified by varying the position of suspension injection. A nanostructured TiO₂ coating with 93.9% anatase content was obtained at high plasma power (48.1 kW), and the adhesive coating bonding to stainless steel exhibited the desired photocatalytic activity.     

Properties of High Velocity Suspension Flame Sprayed (HVSFS) TiO2 coatings

TiO₂ coatings were manufactured by the High Velocity Suspension Flame Spraying (HVSFS) technique using a nanopowder suspension. Their microstructure, nanohardness, tribological properties and photocatalytic activity were studied and compared to conventional atmospheric plasma sprayed (APS) and HVOF-sprayed TiO₂ coatings manufactured using commercially available feedstock. The HVSFS process leaves a fairly large freedom to adjust coating properties (thickness, porosity, anatase content, hardness, etc…) according to the desired objective. Layers with higher anatase content and higher porosity can be produced to achieve higher photocatalytic efficiency, better than conventional APS and HVOF TiO₂. Alternatively, dense protective layers can be deposited, possessing lower porosity and pore interconnectivity and better wear resistance than as-deposited APS and HVOF layers. In all cases, HVSFS-deposited layers are thinner (20 µm-60 µm) than those which can be obtained by conventional spraying processes.