Sensitivity to humidity of TiO2 thin films obtained by reactive magnetron sputtering

This study presents results on the humidity-sensing properties of titanium dioxide thin films measured by a quartz microbalance. A novel two-layer structure, consisting of a polymer sub-layer and a sensing titanium dioxide layer, was fabricated on a quartz resonator. The polymer sub-layer was synthesized by a plasma process from hexamethyldisiloxane to protect the resonator's surface during the deposition of the titanium dioxide film by magnetron sputtering. The TiO₂ films were characterized by X-ray diffraction and Auger Electron Spectroscopy. The film composition was determined to be close to that of stoichiometric TiO₂. The sensitivity to humidity varied from 5 Hz/%RH to 7 Hz/%RH for TiO₂ film thickness lying in the range of 18-70 nm. An increase of film thickness in this interval led to a slight decrease in sensitivity, which is explained by water sorption occurring principally at the surface of the titanium dioxide film and a change of the morphology to a higher surface smoothness for thicker films. It was found that 30-60 min of sorption time is necessary to completely eliminate hysteresis, which suggests that the process is reversible.These results are promising for the development of sensor devices for measuring the relative humidity of air.     

Deposition of titanium dioxide from TTIP by plasma enhanced and remote plasma enhanced chemical vapor deposition

Photocatalytic materials, and especially titanium dioxide, have gained wide popularity in recent years in reason of their interesting properties which can be useful in various fields of application, in particular as self-cleaning materials. When submitted to an illumination (in most cases UV-light), the surface of a photocatalytic material becomes chemically active and simultaneously displays a photo-generated hydrophilic activity (PSH effect).Classically, photocatalytic properties are displayed by crystalline titanium dioxide. In this paper we detail the chemical vapor deposition of TiO₂ carried out in two experimental set-ups: one is plasma enhanced reactor and the other is remote plasma enhanced reactor. Both experiments were carried out with titanium(IV) isopropoxide (TTIP). Titanium(IV) tetrachloride (TiCl₄) and titanium(IV) ethoxide (TEOT) were also used in the first setup.In RF-diode plasma reactor, titanium dioxide films were deposited in amorphous form and crystallization of amorphous films was obtained with the help of thermal post-treatments, since seeding and under-layers appeared unable to trigger the crystallization of films. In the remote plasma CVD reactor, allowing high plasma density and ion energy, deposition of crystalline anatase titanium dioxide was achieved at a deposition temperature of 400 °C.Conclusions are presented and suggest control mechanisms for the stoichiometry of titanium dioxide films.     

Development of a novel cathodic plasma/electrolytic deposition technique: Part 2: Physico-chemical analysis of the plasma discharge

A novel plasma system has been developed recently for the deposition of carbon and titanium thin films on metal and metal alloy substrates. Unlike other deposition techniques, the process occurs in liquid precursors and a plasma discharge is created and confined around the cathode in a superheated vapour sheath surrounded by the liquid phase. This paper presents a detailed analysis of the physico-chemical mechanisms underlying this process. A correlation has then been carried out between the voltage/current characteristics and the consecutive physical phenomena occurring during the process (vapour phase formation, plasma discharge initiation and evolution). The structure and composition of the produced TiO₂ films have been compared with the composition and physical characteristics of the plasma discharge. This analysis allowed the construction of a first dissociation and deposition mechanism for this new plasma system.     

Conversion of amorphous TiO2 coatings into their crystalline form using a novel microwave plasma treatment

Crystalline titanium dioxide (TiO₂) coatings have been widely used in photo-electrochemical solar cell applications. In this study, TiO₂ and carbon-doped TiO₂ coatings were deposited onto unheated titanium and silicon wafer substrates using a DC closed-field magnetron sputtering system. The resultant coatings had an amorphous structure and a post-deposition heat treatment is required to convert this amorphous structure into the photoactive crystalline phase(s) of TiO₂. This study investigates the use of a microwave plasma heat treatment as a means of achieving this crystalline conversion. The treatment involved placing the sputtered coatings into a 2.45 GHz microwave-induced nitrogen plasma where they were heated to approximately 550 °C. It was observed that for treatment times as short as 1 min, the 0.25-μm thick coatings were converted into the anatase crystalline phase of TiO₂. The coatings were further transformed into the rutile crystalline phase after treatments at higher temperatures. The doping of TiO₂ with carbon was found to result in a reduction in this phase transformation temperature, with higher level of doping (up to 5.8% in this study) leading to lower anatase-to-rutile transition temperature. The photoactivity performance of both doped and un-doped coatings heat-treated using both furnace and microwave plasma was compared. The carbon-doped TiO₂ exhibited a 29% increase in photocurrent density compared to that observed for the un-doped coating. Comparing carbon-doped coatings heat-treated using the furnace and microwave plasma, it was observed that the latter yielded a 19% increase in photocurrent density. This enhanced performance may be correlated to the differences in the coatings' surface morphology and band gap energy, both of which influence the coatings' photoabsorption efficiency.     

Structural characterisation of High Velocity Suspension Flame Sprayed (HVSFS) TiO2 coatings

The microstructural features of TiO₂ coatings, deposited by High Velocity Suspension Flame Spraying (HVSFS) from a suspension of titania nanoparticles, were investigated by Focused Ion Beam (FIB) + Scanning Electron Microscopy (SEM) techniques, by Transmission Electron Microscopy (TEM) and by micro-Raman spectroscopy, and were compared to those of conventional HVOF-sprayed TiO₂. Proper selection of the HVSFS deposition parameters results in coatings consisting of a dense matrix, made up by the efficient superposition of well-flattened micrometric lamellae, with homogeneously distributed porosity containing sub-micrometric re-solidified spherical particles. Unlike conventional HVOF coatings, lamella boundaries are hardly discernible, no intralamellar cracking occurs and equiaxed crystals appear instead of columnar ones. A homogeneous distribution of anatase and rutile is also found. Modifications to the spray parameters can give rise to large, unmelted agglomerates, scattered throughout the coating and having poor cohesion to the surrounding material. These agglomerates retain the original phase composition of the nanopowder.     

TiOxNy coatings grown by atmospheric pressure metal organic chemical vapor deposition

Titanium oxynitride coatings were deposited on various substrates by an original atmospheric pressure metal organic chemical vapor deposition (MOCVD) process using titanium tetra-iso-propoxide as titanium and oxygen precursors and hydrazine as a nitrogen source. The films composition was monitored by controlling the N₂H₄ mole fraction in the initial reactive gas phase. The variation of the N content in the films results in significant changes in morphological, structural and mechanical properties. When a large excess of the nitrogen source is used the resulting film contains ca 17  at % of nitrogen and forms dense and amorphous TiO_xN_y films. Growth rates of these amorphous TiO_1.5N_0.5 coatings as high as 14 μm/h were obtained under atmospheric pressure. The influence of the deposition conditions on the morphology, the structure, the composition and the growth rate of the films is presented. For the particular conditions leading to the growth of amorphous TiO_1.5N_0.5 coatings, first studies on the mechanical properties of samples grown on stainless steel have revealed a high hardness, a low friction coefficient, and a good wear resistance in unlubricated sliding experiments against alumina which make them very attractive as protective metallurgical coatings.     

Combinatorial approach to the growth of α-(Al1 − x,Crx)2O3 solid solution strengthened thin films by reactive r.f. magnetron sputtering

The development of new coatings with superior functionalities for high performance cutting tools is a key challenge in manufacturing. In this context, the synthesis of aluminium oxide and derivative oxide thin films is attracting large scientific and technical interests. The present paper addresses fundamental materials science-based aspects of the physical vapour deposition (PVD) growth of Al-Cr-O thin films at a substrate temperature of 500 °C. A combinatorial experimental approach was chosen to describe the growth and microstructure evolution of Al-Cr-O thin films by means of reactive r.f. magnetron sputtering. A segmented target consisting of two half plates of Al and Cr was used for the deposition carried out under stationary conditions in a laboratory-scale PVD coater. Opposite to the cathode five substrate samples were placed in a line. The r.f. cathode power was set to 500 W and the r.f. substrate bias was set to − 100 V. The total gas pressure was kept constant at 0.4 Pa for all experiments with a fixed ratio of oxygen to argon gas flow. Detailed results on the coatings composition, constitution, microstructure and properties as a function of the elemental composition are presented. X-Ray Diffraction (XRD), X-Ray Reflection (XRR), Transmission Electron Microscopy (TEM) and Electron Probe Microanalysis (EPMA) studies prove the growth of nanocrystalline, stoichiometric, metastable corundum-like solid solution strengthened α-(Al_1 − x,Cr_x)₂O₃ thin films with a high degree of crystallinity, grain sizes between 27 ± 6 nm (in the case of Al-rich coatings) and 44 ± 17 nm (in the case of Cr-rich coatings), Vickers micro hardness values up to 2620 ± 80 HV0.05 and thin film densities between 4.00 g/cm³ (in the case of Al-rich coatings) and 4.86 g/cm³ (in the case of Cr-rich coatings).     

Comparison of the photocatalytic behavior of TiO2 coatings elaborated by different thermal spraying processes

This paper proposes a comparative study on the microstructure and photocatalytic performances of titanium dioxide coatings elaborated by various thermal spraying methods (plasma spraying in atmospheric conditions, suspension plasma spraying, and high-velocity oxyfuel spraying). Agglomerated spray dried anatase TiO₂ powder was used as feedstock material for spraying. Morphology and microstructural characteristics of the coatings were studied mainly by scanning electron microscopy and x-ray diffraction. The photocatalytic behavior of the TiO₂-base surfaces was evaluated from the conversion rate of gaseous nitrogen oxides (NOx). It was found that the crystalline structure depended strongly on the technique of thermal spraying deposition. Moreover, a high amount of anatase was suitable for the photocatalytic degradation of the pollutants. Suspension plasma spraying has allowed retention of the original anatase phase and for very reactive TiO₂ surfaces to be obtained for the removal of nitrogen oxides. 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.     

Investigation of novel low temperature atmospheric pressure plasma system for deposition photo-catalytic TiO2 thin film

The purpose of this study was to develop a novel low-temperature atmospheric pressure (AP) plasma system and to use the system to deposit photo-catalytic titanium dioxide (TiO₂) thin film. In this study, titanium tetraisopropoxide (TTIP) was used as a precursor for TiO₂ thin film deposition. The precursor was vaporized by ultrasonic oscillator and introduced into an atmospheric plasma system by argon (Ar) carrier gas. The main plasma working gas was Ar mixed with O₂. Microstructure evolutions of TiO₂ thin film were investigated by low-angle grazing-incidence x-ray diffraction (GID), x-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), and transmission electron microscope (TEM). The photo-catalytic properties were determined by contact angle and methylene orange de-coloration testing. In this study, the substrate temperature, the precursor flow rate and the O₂ flow rate were varied. TiO₂ thin film grown at a temperature of 350 °C, with precursor and O₂ flow rates of 20 sccm and 200 sccm, respectively, revealed the optimum photo-catalytic properties. It was also found that titanium dioxide thin films synthesized by the AP plasma method possess reasonable photo-catalytic characteristics like other deposition techniques.     

A Comparison of the Potential Capability of SFS,SPS and HVSFS for the Production of Photocatalytic Titania Coatings

The photocatalytic capabilities of titanium dioxide are widely published. Reported applications of titania coatings include air purification, water purification and self-cleaning. Suspension spray has been highlighted as a possible route for the deposition of highly active nanostructured TiO₂ coatings. Published work has demonstrated the capabilities of suspension plasma spray and high-velocity suspension flame spray; however, little work exists for suspension flame spray (SFS). Herein, these three suspension spray processes are compared as regards their capability to produce photocatalytic TiO₂ coatings and their potential for industrial scale-up. A range of coatings were produced using each process, manipulating coating parameters in order to vary phase composition and other coating characteristics to modify the activity. The coatings produced varied significantly between the processes with SFS being the most effective technique as regards future scale-up and coating photoactivity. SFS coatings were found to be up to nine times more active than analogous coating produced by CVD.     

Investigations on composition and morphology of electrochemical conversion layer/titanium dioxide deposit on stainless steel

In this study, the formation and characterization of conversion coatings modified by a sol-gel TiO₂ deposit were investigated as a way to develop a new photocatalyst for water and air depollution. The conversion coating, characterised by strong interfacial adhesion, high roughness and high surface area facilitates the sol-gel deposition of titania and enhances its adhesion to the substrate. The conversion treatment is carried out in an acid solution. Observation by Scanning Electron Microscopy (SEM) reveals a rough surface with pores and cavities. According to SIMS measurements, the thickness of the initial conversion layer is evaluated at about 1.5 μm. On this pre-functionalised support, the titanium dioxide was deposited by the sol-gel method. The roughness measurements coupled with SIMS analysis allowed a precise evaluation of the surface state of the final layers. The coating consists of two layers: a TiO₂ outer layer and an inner layer containing iron chromium oxides. Characterization by X-ray diffraction (XRD) showed the existence of the TiO₂ anatase structure as the main compound.     

Oxidation and wear behaviors of Ti-based thin films

The degradation of Ti-based coatings is known to be due to the formation of titanium oxide (TiO₂) at their surfaces. In this study, wear and thermal oxidation behaviors of various magnetron sputtered Ti-based thin films were studied after static oxidation and sliding wear. The oxidized surfaces after the static oxidation and the wear debris generated from pin-on-disc wear tests with alumina ball were characterized to identify the compounds, particularly titanium oxides, to gain a better understanding of the tribochemical reactions. The coatings that were examined include TiN, TiCN (N rich), TiCN (C rich), TiAlN, AlTiN, TiSiN, and TiCNO thin films. These coatings were characterized using Raman spectroscopy, scanning electron microscopy, and X-Ray diffractometer. The results show that TiSiN and AlTiN have the highest oxidation resistance, comparing with other coatings. As for the analyses of wear debris, all of the Ti-based coatings are worn by the mechanism of forming TiO₂, except AlTiN. AlTiN is worn by ploughing wear.     

Low temperature growth of nanocrystalline TiO2 films with Ar/O2 low-field helicon plasma

TiO₂ thin films were deposited on silicon wafer substrates by low-field (1 < B < 5 mT) helicon plasma assisted reactive sputtering in a mixture of pure argon and oxygen. The influence of the positive ion density on the substrate and the post-annealing treatment on the films density, refractive index, chemical composition and crystalline structure was analysed by reflectometry, Rutherford backscattering spectroscopy (RBS) and X-ray diffraction (XRD). Amorphous TiO₂ was obtained for ion density on the substrate below 7 × 10¹⁶ m^− 3. Increasing the ion density over 7 × 10¹⁶ m^− 3 led to the formation of nanocrystalline (~ 15 nm) rutile phase TiO₂. The post-annealing treatment of the films in air at 300 °C induced the complete crystallisation of the amorphous films to nanocrystals of anatase (~ 40 nm) while the rutile films shows no significant change meaning that they were already fully crystallised by the plasma process. All these results show an efficient process by low-field helicon plasma sputtering process to fabricate stoichiometric TiO₂ thin films with amorphous or nanocrystalline rutile structure directly from low temperature plasma processing conditions and nanocrystalline anatase structure with a moderate annealing treatment.     

Plasma spraying of lanthanum silicate electrolytes for intermediate temperature solid oxide fuel cells (ITSOFCs)

A new challenge in the field of solid oxide fuel cells (SOFCs) concerns reducing their operating temperature to 973 K. Apatite ceramics are interesting candidates for SOFC electrolytes due to their high ionic conductivity at this temperature. The present work reports on the fabrication and characterization of La₉SrSi₆O_26.5 coatings obtained by atmospheric plasma spraying with two different plasma spray powers. The microstructure and the composition of the as-sprayed and heat-treated coatings were investigated by several techniques including X-Ray Diffraction, Inductively Coupled Plasma-Atomic Emission Spectroscopy and Scanning Electron Microscopy. The open porosity of the coatings was evaluated by the Archimedean method. It was found that the as-sprayed apatite coatings were composed of an amorphous phase as well as of a crystalline apatite phase, and that they contained chemical heterogeneities resulting from Si volatilization in the high-temperature plasma. Furthermore, a heat treatment rendered it possible to obtain denser, fully crystallized apatite coatings. Ionic conductivity measurements carried out with impedance spectroscopy demonstrated that the conductivity of the apatite coatings - depending on the spraying conditions - increased with sintering.     

Synthesis of titanium nano-particles via chemical vapor condensation processing

In the present study, titanium nano-particles have been synthesized using chemical vapor condensation (CVC) process. Reaction of sodium and titanium tetrachloride vapors in the tube furnace resulted in the production of titanium nano-particles that were encapsulated in sodium chloride. Dried Argon gas was employed as a carrying agent. Titanium nano-particles were contained in an ethanol bath. Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD) were employed for analysis and characterization of nano-particles. The size of primary particles was smaller than 100 nm and secondary particles were submicron agglomerations.     

Amorphous Boron containing silicon carbo-nitrides created by ion sputtering

Silicon carbo-nitride films with Boron were deposited onto Silicon, glass and SS304 Stainless Steel substrates using the ion beam assisted deposition (IBAD) method. The coating composition, rate of ion-assistance and substrate temperature were varied. Films were examined by X-Ray Diffraction, Scanning Electron microscopy, Energy Dispersive X-Ray analysis, Cathodoluminescence, Atomic Force Microscopy and Nano-indentation. The composition and chemical bonding variation was found to be dependent on deposition conditions. All coatings were amorphous, fully dense and showed high hardness up to 33 GPa. It is suggested that the low friction coefficient of about 0.3, measured against Al₂O₃ using the pin-on-disc method, may be the result of the presence of C nanoclusters which are formed under the low energy deposition conditions. Films deposited on Stainless Steel had an onset of rapid thermal oxidation at 1150 °C in air as determined by thermogravimetric analysis. The films have a Tauc bandgap between 2.2 and 2.8 eV and were also exceptionally high electrical resistive which may indicate the presence of localised states.     

Photocatalytic Activity of Nanostructured Anatase Coatings Obtained by Cold Gas Spray

This article describes a photocatalytic nanostructured anatase coating deposited by cold gas spray (CGS) supported on titanium sub-oxide (TiO_2?x ) coatings obtained by atmospheric plasma spray (APS) onto stainless steel cylinders. The photocatalytic coating was homogeneous and preserved the composition and nanostructure of the starting powder. The inner titanium sub-oxide coating favored the deposition of anatase particles in the solid state. Agglomerated nano-TiO₂ particles fragmented when impacting onto the hard surface of the APS TiO_2?x bond coat. The rough surface provided by APS provided an ideal scenario for entrapping the nanostructured particles, which may be adhered onto the bond coat due to chemical bonding; a possible bonding mechanism is described. Photocatalytic experiments showed that CGS nano-TiO₂ coating was active for photodegrading phenol and formic acid under aqueous conditions. The results were similar to the performance obtained by competitor technologies and materials such as dip-coating P25^® photocatalysts. Disparity in the final performance of the photoactive materials may have been caused by differences in grain size and the crystalline composition of titanium dioxide.     

Plasma polymerized allylamine films deposited on 316L stainless steel for cardiovascular stent coatings

Coronary stents are metallic (316L stainless steel) medical devices used during balloon angioplasty to scaffold diseased arteries and prevent their reblockage. To reduce the restenosis rate, bare metal stent coating is a promising solution. The coating can protect the metallic surface of the stent from corrosion attack caused by the biological environment. In addition, according to Food and Drug Administration (FDA) the coating properties must be guaranteed even after stent expansion. The aim of this study was to develop a dry process to coat the metallic surface from the biological environment by depositing an ultra-thin, stable, cohesive and adhesive plasma polymerized allylamine (CH₂=CH―CH₂―NH₂) coating with high selectivity towards primary amine groups. Plasma polymerized allylamine (PPAA) coatings were deposited on electropolished 316L stainless steel (316L SS) samples using a low pressure plasma reactor (70 kHz). XPS (X-Ray Photoemission Spectroscopy) and FTIR-ATR (Fourier Transform Infrared-Attenuated Total Reflectance) spectroscopy measurements were used to investigate the chemical composition of the coatings. A chemical derivatization technique was employed in order to quantify the amine retention rate of the deposited films. Morphology of the films was evaluated by FE-SEM (Field Effect-Scanning Electron Microscopy) imaging. Furthermore, special attention was devoted to study the stability of the coating and its adhesion properties after plastic deformation up to 25%. The effect of the power discharge and treatment time on these properties was also investigated. Our results showed that coatings present the required adhesion and cohesion properties to be stable upon deionised (D.I.) water immersion and to resist to a stent expansion.     

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.     

Preparation of Sn4+-Doped Titanium Dioxide Photocatalytic Films on 304 Stainless Steel by Duplex Treatment

Sn⁴⁺-doped titanium dioxide photocatalytic films were synthesized on 304 stainless steel (SS) by a duplex treatment. The SS substrates were alloyed with titanium (Ti) through cathodic-arc ion plating followed by a microarc oxidation (MAO) treatment in different electrolytes. Field-emission scanning electron microscopy, x-ray diffraction, and energy dispersive spectroscopy were used to characterize the films surface morphology, crystalline phase, and composition, respectively. Photocatalytic activity was measured using an UV-Vis spectrophotometer. It was found that the films with a porous structure are mainly composed of TiO₂, which exists in an anatase and rutile state. Furthermore, small quantities of SnO₂ have been found in the Sn⁴⁺-doped titanium dioxide films. The fraction of anatase varies with the MAO time and electrolytes, whereas the pore size remains the similar with the same MAO current intensity and density and the surface roughness increases slightly with increasing MAO time. It was also found that the photocatalytic activity of the Sn⁴⁺-doped porous film improved, and the film synthesized with a shorter MAO time in a lower Na₂SnO₃-containing electrolyte is superior to the films with longer MAO times and higher Na₂SnO₃ concentrations.