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.     

Initial stages of plasma electrolytic oxidation of titanium

The initial stages of oxide growth on titanium are examined in a recently developed commercial alkaline pyrophosphate/aluminate electrolyte of interest for plasma electrolytic oxidation of light metal alloys. Constant current anodizing was employed, with resultant films examined by scanning and transmission electron microscopies and Rutherford backscattering spectroscopy. The initial film is relatively uniform and composed of TiO₂, with low concentrations of aluminium and phosphorus species incorporated from the electrolyte. With increase in voltage the film breaks down locally, and regions of original and modified film develop simultaneously, with the latter occupying more of the surface as the voltage rises. Porous regions due to dielectric breakdown also become increasingly evident. At 240 V, sparking commences, and the surface reveals extensive, relatively uniform porosity, with the coating now containing much enhanced concentrations of aluminium and phosphorus species compared with the coating at lower voltages. The films develop at low efficiency due to generation of oxygen. The oxygen is produced within the original film material and at sites of dielectric breakdown. The former type of film develops a two-layered morphology, with an outer layer of amorphous TiO₂ and an inner layer with numerous fine and course cavities. The cavities are due to the generation of oxygen that may be associated with the formation of anatase in the inner layer.     

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.     

Characterization of the surface layers formed on titanium by plasma electrolytic oxidation

This paper is concerned with the surface modification of titanium by the PEO method (plasma electrolytic oxidation) in the solutions which contain Ca, P, Si and Na. The chemical composition of the thus formed surface layers was examined by XPS and EDS. The morphology of the surface was observed by SEM. The phase composition was determined by X-ray diffraction (XRD). The adhesive strength of the oxide layers was evaluated by the scratch-test. The corrosion resistance was determined in a simulated body fluid (SBF) at a temperature of 37 °C by electrochemical methods for various exposure times.The oxide layers obtained were porous and enriched with Ca, P, Si and Na and their properties depended on the electrolyte solution and the parameters of the oxidation process. The results of the electrochemical examinations show that the surface modification by PEO does not worsen the corrosion resistance of titanium after a 13 h exposure in SBF. The electrochemical impedance spectroscopy (EIS) results indicate that the surface layers have a complex structure and that their electric properties undergo changes during long-term exposures in SBF.     

Characterization of titanium nitride films deposited by cathodic arc plasma technique on copper substrates

TiN films were deposited directly on Cu substrates by a cathodic arc plasma deposition technique. The films were then characterized by X-ray diffraction (XRD), grazing incidence X-ray diffraction (GID), (TEM), Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS). The preferred orientation of the film changed from (200) to (111) with increasing film thickness. Analyses of both the XRD and GID results showed that in the highly (111) textured grains, the (111) plane was approximately parallel to the film surface, while in the (200) textured grains, the (200) plane was tilted away from the film surface. Small-elongated crystallites with a large aspect ratio and textured grains were found on the TiN surface. AES, which was employed to examine the concentration depth profile, showed no apparent interdiffusion between Cu and TiN during the growth of the film. XPS results showed that amorphous TiO₂, as well as titanium oxynitride, was present on the TiN surface. The spectra of Ti-2p, N-1s, O-1s and Cu-2p before and after the film being sputter etched through the entire film region were also discussed.     

Application of optical transmission interferometry for in-situ structural investigations of titanium dioxide sputter-deposited coatings

Titanium dioxide coatings (from 0.1 to 1.5 μm thick) have been dc sputter-deposited on glass slides from titanium targets in various Ar-O₂ reactive gas mixtures. Deposition rate and optical properties were controlled in-situ by optical transmission interferometry (OTI) with an optical fibre located behind the glass substrate in order to perform a real-time control of transmittance of the growing film. Thus, it is possible to determine in-situ the optical indices (n, k) and the thickness of the as-deposited film by using a simple simulation, developed on Matlab software. The optical properties of the films were investigated in relation to their structure, which depends on the sputtering conditions adopted. In particular, the effects of the sputtering pressure (working pressure and oxygen partial pressure), the discharge power and the substrate location into the reactor are investigated in detail. Films structure is assessed by standard grazing incidence X-ray diffraction (XRD).     

Hydroxyapatite coating on titanium substrate by electrophoretic deposition method: Effects of titanium dioxide inner layer on adhesion strength and hydroxyapatite decomposition

Nanosized hydroxyapatite (HA) powders were prepared by a chemical precipitation method and electrophoretically deposited on Ti6Al4V substrates. The powders were calcined before the deposition process in order to obtain crack-free coating surfaces. As an inner layer between Ti6Al4V substrate and HA coating, nanosized titanium dioxide (TiO₂) powders were deposited, using different coating voltages, in order to connect substrate and HA tightly. Moreover, this layer is considered to be acting as a diffusion barrier, reducing the HA decomposition due to ion migration from the metal substrate into the HA. After the sintering stage, adhesion strengths of coatings were measured by shear testing, phase changes were studied by X-ray diffraction, and coating morphology was analyzed through scanning electron microscopy observations. Results showed that usage of the TiO₂ inner layer prevented HA decomposition. Furthermore, decreasing the voltage used in TiO₂ deposition resulted in crack-free surfaces and increased adhesion strength of the overall coating.     

Preparation of ceramic coatings on inner surface of steel tubes using a combined technique of hot-dipping and plasma electrolytic oxidation

A new method for inner surface modification of steel tubes, named a combined technique of hot-dipping and plasma electrolytic oxidation (PEO) was proposed and demonstrated in this paper. In this work, metallurgically bonded ceramic coatings on inner surface of steel tubes were obtained using this method. In the combined process, aluminum coatings on steel were firstly prepared by the hot-dip process and then metallurgically bonded ceramic coatings were obtained on the aluminum coatings by PEO. The element distribution, phase composition and morphology of the aluminide layer and the ceramic coatings were characterized by SEM/EDX and XRD. The corrosion resistance of the ceramic coatings were also studied. The results show that, after hot-dip treatment, the coating layers consist of two layers, where Al, Fe_xAl _(1−x) were detected from external topcoat to the aluminide/steel substrate. Then after PEO process, uniform Al₂O₃ ceramic coatings have been deposited on inner surface of steel tubes. The ceramic coatings are mainly composed of -Al₂O₃ and γ-Al₂O₃ phase. The compound coatings show favorable corrosion resistance property. The investigations indicate that the combination of hot-dipping and plasma electrolytic oxidation proves a promising technique for inner surface modification of steel tubes for protective purposes.     

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.     

O2 plasma treatment of mesoporous and compact TiO2 photovoltaic films: Revealing and eliminating effects of Si incorporation

Radio frequency (13.56 MHz) O₂ plasmas were used to modify the surface of mesoporous and compact TiO₂ films. The effects of substrate location in the plasma, applied rf power, and plasma mode (pulsed or continuous wave) were explored. X-ray photoelectron spectroscopy, contact angle measurements, and scanning electron microscopy were used to characterize changes to the TiO₂ films. For mesoporous materials, O₂ plasma treatment was found to increase oxygen content in the films, but Si content increased with applied rf power as a result of sputtering and redeposition of Si species from the reactor walls. XPS depth profiling using ion sputtering as well as O₂ plasma treatment of dyed materials revealed that Si was deposited throughout the mesoporous network, not as a surface SiO₂ layer. Pulsing the plasma with pulse duty cycles < 40% resulted in the elimination of Si and a reduction of damage in the modified films.     

The effect of oxygen concentration on the low temperature deposition of TiO2 thin films

The aim of this study was to investigate the influence of oxygen concentration in Ar/O₂ gas mixture on the crystalline properties of TiO₂ thin films obtained at low temperature by reactive magnetron sputtering technique. Mass spectrometry of plasma medium provides, in conjunction with XRD and AFM measurements, a guide for attainment of good quality anatase TiO₂ films.     

Use of a theoretical model to investigate RF and DC reactive sputtering of titanium and chromium oxide coatings

An analysis of RF and DC reactive sputtering techniques is presented. The transition between a metal sputtering mode and a compound sputtering mode is usually noticed with a metallic target and an argon+oxygen gas mixture. The so-called hysteresis effect often observed for small amounts of reactive gas is explained in recent models. By considering gas kinetics parameters, it is possible to evaluate quite simple relationships between the main processing parameters. These theoretical calculations enable the prediction and aid the understanding of instability phenomena observed in reactive sputtering. In this paper, the effects of some parameters on the position and size of instability regions are discussed, and the difference between DC and RF reactive deposition is investigated. Simulations and experimental results are compared for the case of titanium and chromium oxide thin films prepared by DC and RF reactive sputtering. The influence of sputtering power on the position of the hysteresis loop is analysed theoretically and experimentally, and the changes observed between the reactive sputtering of titanium and chromium oxide materials are also discussed.     

Gas sensing properties of layer-by-layer self-assembled ultrathin film of polyaniline/titanium dioxide

Electrostatic layer-by-layer (LbL) self-assembly (SA) was realized with processable polyaniline prepared with polystyrene sulfonic acid as a template (PANI-PSSA) and titanium dioxide sol as the starting materials, resulting in an ultrathin film of PANI-PSSA/TiO₂. The SA process was confirmed by UV–vis spectra and quartz crystal microbalance measurements. The composite ultrathin film was characterized by X-ray photoelectron spectroscopy, X-ray diffraction patterns, field-emission scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. It was revealed that the content of TiO₂ in the composite film was much higher than that of PANI-PSSA, and the doping level of PANI-PSSA was as high as 19.1%. Gas sensors were fabricated by depositing self-assembled ultrathin film of PANI-PSSA/TiO₂ on interdigital gold electrodes, then covering another layer of PANI-PSSA via dip-coating. Electrical response to NH₃ of the sensor was investigated at room temperature. Gas sensitivity of the sensor was found to closely relate to the number of self-assembled bilayers. Under optimal conditions, the sensor displayed high sensitivity (26.5% and 81.2% towards NH₃ of 10 and 100 ppm, respectively), fast response (response time ∼ 1 min; recovery time ∼ 2 min), good reversibility and repeatability.     

The effects of thermal spray technique and post-deposition treatment on the photocatalytic activity of TiO2 coatings

Titanium dioxide coatings were deposited by two thermal spray methods: high velocity oxygen fuel and atmospheric plasma spray. The coatings were characterized by XRD, SEM, Raman and UV-vis spectroscopy. The photocatalytic activities of the samples towards decomposition of methylene blue were also determined. As-prepared samples showed a very limited photocatalytic activity. In contrast, post-deposition oxidation of the samples in air resulted in significantly improved catalytic performance of the coatings. It appears that the oxidation state of titania played a critical role in photocatalytic activity given that partially reduced coatings resulted a very low activity. This conclusion was consistent with the observation of the Ti₈O₁₅ phase in the as-prepared samples. However, when oxidized, parameters such as the anatase content became a dominant factor in catalytic performance of the samples. The coating sprayed by high velocity oxygen fuel resulted in much higher activity than the atmospheric plasma sprayed coating, which can be attributed to higher anatase content of the former.     

Production of LaPO4 coatings using a novel ultrasonically-assisted plating technique

Dense LaPO₄ and LaPO₄-Al₂O₃ coating films were produced on the surface of stainless steel substrates at room temperature and ambient pressure with the aid of a novel ultrasonic-based mechanical coating method, which we call UMCA. The main emphasis was on examining the conditions necessary for the successful coating operation and characterizing the as-deposited coatings for the thickness, uniformity and surface morphology. The experimental results suggested that hardness and thermal conductivity of balls and substrate are key parameters influencing the coating efficiency. The coated samples showed an improved hot corrosion resistant in Na₂SO₄-NaCl molten salts at 950 °C.     

Inactivation of Staphylococcus aureus and Escherichia coli under various light sources on photocatalytic titanium dioxide thin film

Although photocatalytic TiO₂ thin film has been considered for antimicrobial applications and is sensitive to light sources, its inactivation mechanism under various light sources is relatively unknown. This work elucidates the mechanism by which photocatalytic TiO₂ thin film inactivates to Staphylococcus aureus and Escherichia coli under various light sources using a bacterial cell culture and the microscopic observation of bacterial cells. Experimental results indicate that antimicrobial behavior can only be activated when the light source exhibits emissions with a threshold energy that exceeds the band gap energy of anatase TiO₂. The black light source with a stronger UV emission than the fluorescent lamp, is associated with an earlier bacterial growth inhibition phase. The bacterial inactivation mechanism of Staphylococcus aureus differs from that of Escherichia coli. The former suffers the detachment of the cell wall from cell membrane and the later undergoes a nucleoid pattern change from features of relaxation toward features of condensation.     

Roles of titanium dioxide and ion-doped titanium dioxide on photocatalytic degradation of organic pollutants (phenolic compounds and dyes) in aqueous solutions: A review

Water pollution by organic pollutants is an ever increasing problem for the global concerns. This paper presents a critical review on the abatement of organic pollutants, dyes and phenolic compounds in particular, using photocatalytic reaction by titanium dioxide (TiO₂). Mechanism of photocatalytic reaction is briefly discussed. A detailed search of published reports on the advancement in photocatalytic degradation of organic pollutants in wastewater by doping titanium dioxide with foreign species such as metal and non-metal component has also been carried out and analyzed in this paper.     

Atmospheric pressure barrier torch discharge and its optimization for flexible deposition of TiO2 thin coatings on various surfaces

This paper reports on the preparation of titanium (IV) oxide films via the improved atmospheric pressure barrier torch discharge (BTD) deposition. This approach is a modification of the atmospheric pressure glow discharges (APGDs) ranking among plasma enhanced chemical vapor deposition (PECVD) techniques. These methods are based on plasma–chemical reactions of the precursors' vapors occurring in the active plasma environment. The layers are produced in terms of heterogeneous recombination reactions of the high active species on the supporting surface. The major treated topic comprises the influence of the used support on the physical properties of the layers. A set of three different supports was used including quartz slides (non-conducting, dielectric), silicon discs (semi-conducting) and polished Ni sheets (conducting). Crystallographic structure, surface roughness, surface wettability and the film thickness were assessed and used as a set of physical properties to be discussed and compared for each of the films and mutually. In parallel the qualitative analysis of the emission spectra of the barrier torch discharge during the deposition process was also presented. Different conductive connection of plasma stream with the substrate crucially influences the temperature of this substrate. It has a direct effect mainly on the crystallinity and morphology of the films and also on the plasma parameters. This knowledge might be used as a tool for the optimization of deposition conditions. Photocatalytic functionality of the layers was quantified in a simple test based on the photocatalytic oxidation of Rhodamine B (C₂₈H₃₁ClN₂O₃) under UV radiation.     

Effects of addition of Al(NO3)3 to electrolytes on alumina coatings by plasma electrolytic oxidation

Alumina coatings were prepared by plasma electrolytic oxidation (PEO) on Al alloys using NaAlO₂ electrolytes with added Al(NO₃)₃ to enhance the mechanical properties of the coatings. Electrolytes (0.1 M NaAlO₂) with small amounts of Al(NO₃)₃ ranging from 1.0 to 5.3 mM were employed at a fixed current density (10 A/dm²). With the added Al(NO₃)₃, the onset of spark discharges was delayed, which is associated with the dissolution and oxidation of Al alloys in the nitrates. X-ray diffraction results showed that the obtained coatings consisted of both the α-Al₂O₃ and γ-Al₂O₃ phases. The maximum α-Al₂O₃ content of the coatings was obtained with the addition of 1.6 mM Al(NO₃)₃ to the electrolytes. The hardness of the coatings was significantly enhanced by increasing their α-Al₂O₃ content, which could be achieved by adding suitable amounts of Al(NO₃)₃ to the electrolytes.     

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.