biblio.ugent.be - Ghent University Academic Bibliography · Web viewThe formulations, also prepared by high-throughput equipment, are found to be stable in the operational pH range

High-throughput analysis for preparation, processing and analysis of TiO2 coatings on steel by chemical solution deposition.

Marcos Cuadrado Gila, Isabel Van Driesschea, Sake Van Gilsb, Petra Lommensa, Pieter Casteleinc, Klaartje De Buyssera,*

a SCRIPTS – Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281 (S3), Belgium

b OCAS –ArcelorMittal Gent R&D Centre, Pres. J.F. Kennedylaan 3, B-9060, Zelzate, Belgium

c Flamac - a division of SIM, Technologiepark 903, 9052 Zwijnaarde

*Corresponding author at: SCRiPTS - Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281 (S3), Belgium – Tel: 0032 9 264 44 41 – Fax: 0032 9 264 49 83

E-mail addresses: [email protected] (Marcos Cuadrado Gil), [email protected] (Isabel Van Driessche), [email protected] (Sake Van Gils), [email protected] (Petra Lommens), [email protected] (Pieter Castelein), [email protected] (Klaartje De Buysser)

mailto:[email protected]






Abstract

A high-throughput preparation, processing and analysis of titania coatings prepared by chemical solution deposition from water-based precursors at low temperature (≈250°C) on two different types of steel substrates (Aluzinc® and bright annealed) is presented. The use of the high-throughput equipment allows fast preparation of multiple samples saving time, energy and material; and helps to test the scalability of the process. The process itself includes the use of IR curing for aqueous ceramic precursors and possibilities of using UV irradiation before the final sintering step. The IR curing method permits a much faster curing step compared to normal high temperature treatments in traditional convection devices (i.e. tube furnaces).

The formulations, also prepared by high-throughput equipment, are found to be stable in the operational pH range of the substrates (6.5 – 8.5). Titanium alkoxides itself lack stability in pure water-based environments, but the presence of the different organic complexing agents prevents it from hydrolysis and precipitation reactions. The wetting interaction between the substrates and the various formulations is studied by the determination of the surface free energy of the substrates and the polar and dispersive components of the surface tension of the solutions. The mild temperature program used for preparation of the coatings however does not lead to the formation of pure crystalline material, necessary for the desired photocatalytic and super-hydrophilic behaviour of these coatings. Nevertheless, some activity can be reported for these amorphous coatings by monitoring the discoloration of methylene blue in water under UV irradiation.

Keywords

Semiconductors, sol-gel processes, thin films, titanium oxide, photocatalysis

1. Introduction

Titanium dioxide, which is a ceramic semiconductor, can be obtained as three main crystalline phases when synthesized: anatase, rutile and brookite. In nature, this material can also be found in other crystalline structures that are mainly formed at higher pressures [1-7]. The semiconductivity of bulk titania can be activated using electromagnetic radiation with energies equal or higher than its band gap energy which is around 3.2 eV. This energy corresponds to radiation in the ultraviolet (UV) region. When activated, an electron hole pair is created: the electron is promoted from the valence band to the conduction band while the hole stays in the valence band. Due to this phenomenon, titania can act as a photo-catalyst for oxidation of organic matter that react with the very oxidizing free radicals formed by reaction with the charge carriers in presence of water and/or oxygen molecules [8-16].

For practical applications, the brittle nature of bulk titania ceramics has some important disadvantages. The development of thin films is therefore an interesting way of improving the mechanical properties of titania without losing the photocatalytic nature of the bulk material and consuming less material for the same photocatalytic activity. Several studies have been carried out to synthesize titania thin films on different substrates and at different temperatures [17-22].

Up to now, organic solvent-based routes been the most common way of preparing these TiO 2 thin films [18, 19, 21, 23-25]. Alcohols are used as solvent and small amounts of water are added to start the hydrolysis and condensation reactions. The current demands of nowadays research urge to move towards the so-called “green chemistry”: an environmentally friendly way of understanding, studying and performing materials science that consists in the invention, design and application of chemical products and processes in order to reduce or to eliminate the use and generation of hazardous substances [26-27].

Water-based synthesis, as used in this work, are a relatively novel field [7, 9, 17, 28] . Instead of working in pure alcohol, the amount of organics used is reduced to a minimum and is replaced by water as a main solvent which adds the “green” character to this approach. The instability of titanium salts or alkoxides in water, causing a rapid hydrolysis to form hydroxides, has been the major impediment for the preparation of these type of formulations. Peptization of acidic titanium hydroxide dispersions and synthesis via the peroxide method are examples of the preparation of titania in aqueous solutions [22, 29-31]. Recently, a new path to obtain stable water-based titania formulations was published by SCRiPTS using complexed aqueous titanium solutions. This synthetic route has been found to be efficient not only for the synthesis of titania, but also for other ceramic materials like ZrW2O8 , CeO2, La2Zr2O7 or YBa2Cu3O7 [32-37].

In this research, the development of titanium oxide thin films on Aluzinc® (AZ) or Glavalume steel for self-cleaning and corrosion protection starting from water-based precursor formulations was aimed by using mild temperature programs. Another steel substrate, bright annealed steel (BA), was used as a smooth reference substrate. Three different chelating agents were used: citric acid, acetylacetone and triethanolamine. To speed up the development of novel sol-gel solutions based upon the synthetic route, we opted for high-throughput. The scalability of the process was demonstrated by the use of high-throughput equipment both in the preparation of the precursor formulations as in the coating application. These high throughput techniques move towards minimizing the use of chemicals, energy and time. It brings the possibility of fast and reliable

experiments for screening, testing and optimizing the formulation preparation and the coating procedure. The specific software of the equipment permits the design for larger numbers of samples and conditions compared to classical experiments. In this research, a full range of the titanium concentration, complexing agent type and concentration were modified in the formulation step allowing a wide range of compositions to be analyzed.

2. Materials and methods

2.1 The high-throughput equipment (hte): Symyx® platform for formulation preparation

The high-throughput formulation equipment consists of several stations: the Symyx® Powder Dosing Station for gravimetric powder dosing, the Symyx® Liquid Handling Robot Platform for volumetric liquid dispensing of the chemicals and the high-throughput formulation workflow from hte AG® for gravimetric dosing of both powders and (high) viscous liquids, capping and de-capping of the vials, magnetic and overhead stirring, barcode reading and the possibility of using vials with volumes from 8 to 100 mL. This equipment can be seen in figure 1. The Symyx stations are controlled by using specific software from Symyx: Discovery Tools TM. This software package has four main actions: experiment design, execution of actions, analysis and reporting of the data. The high-throughput formulation workflow from the AG® uses software packages Mythe® that is used for experimental design, data management, data analysis and workflow integration and hte Control TM that enables the operation to control, schedule and monitor the process. Design Expert® from Stat-Ease® is a software package available for designing experiments using statistical methods.

Fig. 1: Left: High-throughput formulation preparation equipment: powder dosing station from Symyx®. Middle: High-throughput formulation preparation equipment: Dual-arm liquid handling robot platform from Symyx®. Right: High-throughput formulation preparation equipment: high –throughput formulation workflow from hte AG®.

2.2 Solution preparation

Two different sets of precursor formulations were prepared with titanium isopropoxide [Panreac, 97% PS) as metal source. The organic solvents used to modify the titanium alkoxides were ethanol [Panreac, absolute P.A.], isopropanol [Fiers, 99.7%] and acetic acid [Carl Roth®, ROTIPURAN® 100% P.A.]; all of them used without further purification. Three different complexing agents were used in order to stabilize the titanium species in solution: citric acid monohydrate (CA) [Carl Roth®, ≥99.5% Ph. Eur.], acetylacetone (acacH) [Merck, ≥99%] and triethanolamine (TEA) [Acros Organics, 99.7%]. The solutions containing acetylacetone and citric acid were mixed with small amounts of concentrated nitric acid [Carl Roth®, ROTIPURAN® 65% p.a.] in order to stabilize the precursors. The general process for the preparation of the formulations was the same for all of them. First, water-based stock formulations with variable amounts of the different reagents were prepared by dropwise

addition of a mixture of titanium alkoxide and organic modifier to pure distilled water (for TEA-based formulations) or to water acidified with HNO3 (pH ≈ 2) for acacH and citric acid-based formulations. These highly concentrated Ti4+ formulations contain relatively high amounts of organic solvents in comparison with the final precursor solutions. The concentrated stock solutions wee diluted using the high-throughput formulation and powder dosing station. The device was also used to add the desired extra amounts of complexing agent in order to vary the Ti:complexing agent ratios. The different formulations were tested in stability vs time and pH, contact angle measurements and corrosive activity on both AZ and BA steel substrates. A solution was considered stable at a certain pH value when it remained without visible precipitation for 18 – 24 h.

The first set of precursor formulations were mainly low concentrated solutions. The concentration of titanium alkoxide varied from 0.020 to 0.067 M with ratios of complexing agent vs titanium from 0.5 to 3 (see formulations from 101 to 716 in appendix table A1). The second set of precursor formulations (M103 – M823 in appendix table A1) consisted out of a series of more concentrated titanium solutions (0.200 – 0.500 M) with ratios of titanium vs complexing agent from 1 to 3. A graphical overview of all different solutions prepared is given in figure 2.

For predicting the wetting behavior of the different formulations over the substrates, the surface tension of the solutions was measured via pendant droplet method and the polar and dispersive components were obtained via captive bubble measurements in a completely non-polar solvent, n-pentane (DSA30 drop shape analysis – Krüss).

Fig. 2: Schematic overview of the different formulations. The graphs indicate the type of solvent used, complexing agent, vol% of solvent and Ti concentrations. The ratio Ti to complexing agent are indicated pictographically: (0.5 - ) (1 - ) (2 - ) (3 - )

2.3 High throughput coating procedure

The thin organic coating (TOC) platform (the, Heidelberg, Germany) (see figure 3) allows all steps of a coating process via high-throughput: liquid and substrate handling and deposition, coating applications (by use of a barcoater) and curing of the films with IR and or UV irradiation. It permits the variation of parameters such as volume of formulation that will be applied, barcoater type (rubber or steel with different wire thicknesses), barcoater pressure, application speed, dose of curing radiation or speed of each of the steps. The whole process is controlled by a central computer.

Fig. 3: TOC platform for high-throughput coating application (left) and curing in FLAMAC and close-up image of the coating after (right) the application of the wet film

The volume of liquid dispended, the pressure at which the barcoater works and the application speed are important parameters for the thickness and aspect of the coatings. The IR dose and speed of the IR drying step will determine the curing temperature while the influence of the presence or absence of UV light during drying is examined on some properties of the coatings as, for example, crystallinity. The curing IR radiation could be applied directly over the wet film or as back-heat.

Before coating, the substrates must be cleaned in order to eliminate fatty residues that interfere with the interactions between substrate and wet film and to activate the surface of the substrates by creating hydroxyl groups that favor those interactions and allow the wet film to be applied more homogeneously. The cleaning formulations were water-based solutions of Ridoline C60 or C72 detergents (Henkel, Germany), two commercially available alkaline-based powder cleaning agents with surfactants. Substrates were immersed in the solution for 10 s, followed by hot water (60 °C) and cold water (20 °C) rinsing and finally blown by pressurized air to obtain a dry surface.

The following scheme (figure 4) depicts the whole coating and curing process: Pre-heating of the substrate, application of the formulation, coating, first step of drying with IR (directly over the “wet” film), application of UV (over the still “wet” film), resting period with cooling step and final drying using back-heat IR radiation followed by cooling with pressurized air. The application or absence of UV light during drying was the variable parameter in the coating platform in order to study its possible influence on the properties of the final coatings. The high-throughput equipment also allows a fast and reproducible optimization of the coating procedure varying curing temperature via IR irradiation, volume of formulation applied, pressure of the bar-coater over the substrate and its speed. The parameters selected after optimization are given in table 1.

The set of more concentrated solutions prepared (formulations from M101 to M823 in supporting info: table 1) were applied and cured using the TOC platform. The parameters to be used during the procedure were adjusted using the more viscous stock formulations: 301, 601 and 801 (η ≈ 2.9 cP).

The homogeneity of the coatings, in order to perform a reproducible and valid procedure, was analysed first visually and afterwards using Gloss Discharge Optical Emission Spectroscopy (GDOES).

Bare substrate

IR lamps UV lamps

IR lamps (backheat)

Preheated substrate Substrate + wet film

Substrate + wet film

Coated substrateSubstrate + wet filmFinal drying

Coating

CoolingCooling

Preheating

Drying

UV application

Fig. 4: Schematic representation of the coating procedure on the TOC high-throughput platform.

Table 1: Coating and curing parameters after optimization

Coating parameters Curing parameters Barcoater type: Rubber bar. Pressure on barcoater: 0.5 bar. Speed of application: 50 mm s-1. Total volume (4 droplets): 0.120 mL.

Preheat: IR (100% for 2 seconds, 40 mm above substrate). Rest period = 0.5 seconds. Substrate was 100°C when drops are dispensed.

Drying: IR (100% for 0.5 seconds, 40 mm above substrate). Rest period = 5 seconds. Substrate goes up to 200°C.

UV: Hg-bulb, 100%, speed of application 0.1 m/s. Cooling: 30 seconds. IR: 100% (2 seconds back-heat), 40mm under substrate, rest

period = 5 seconds. Cooling: 30 seconds.

2.4 Substrate and coating characterization

The roughness and surface aspect of the substrates were examined by Atomic Force Microscopy (AFM) (PicoplusTM Agilent 5500) and Scanning Electron Microscopy (SEM) (FEI Quanta-FEG 200). The wettability of the substrates and the wetting envelopes were determined by contact angle measurements (DSA30 drop shape analysis – Krüss). The thicknesses of the coatings were quantified using ellipsometry (SE850 ellipsometer – Sentech). GDOES was used as a complementary technique for the thickness determination (GDA 750 – Spectruma). The presence of crystalline titania was evaluated using X-Ray Diffraction (XRD) (ACL-X´tra diffractometer - Thermo Scientific).

The discoloration of methylene blue in water under UV irradiation was chosen to determine the photocatalytic activity of the coatings. The setup consists out of a reactor, located inside a PVC box with a stirring plate and a cooling system. The vessel used as reactor was a double sided glass beaker that allowed the circulation of a mixture of water and ethylene glycol at 15°C to ensure a thermostatic environment. 30 mL of a water-based solution of methylene blue (approximately 1.5 x 10-5 M) were placed in the reactor together with the sample in a circular holder that limited the

surface exposed to the UV radiation to 3.14 cm2. The lamps used were VILBER LOURMAT VL-315BLB blacklight blue fluorescent light tubes with a maximum emission at 365 nm and emit 10W/m2. The

average distance from the substrate to the lamp was approximately 6 cm and the surface of the sample was covered with 2 – 3 mm of solution during the catalytic experiment.

The discoloration of methylene blue versus time was determined using UV-Vis spectroscopy, based on the absorption of the organic dye at 665 nm (Cary 50 Conc UV-Vis spectrophotometer-Varian).

3. Results and discussion

3.1 Stability vs pH of formulations with low concentration (101 to 716) and corrosivity tests

Table 2 summarizes the stability vs pH tests performed for the first set of formulations (low concentrations). The initial pH is the one at which the solution remained after preparation. All the solutions were tested from this initial pH value until pH 6.5 – 8.5, close to neutrality and where the Aluzinc® substrate is chemically more stable [38]. The alkalinisation experiments were performed by addition of 10-3 M aqueous NaOH solution and for the acidification experiments 10 -3 M HNO3 is added. An ageing period of 24 h was used between each addition that corresponds to, approximately, one pH unit. Formulations that showed no precipitation after this period were considered to be stable at this pH value and were retained for the next step in the stability tests. Those that presented precipitation were considered non stable at this pH value and discarded for the next pH modification. This last measured pH value was then taken as stability limit and the value is noted in table 3. Non-stable formulations at a certain pH have a dark grey background with an ‘X’ in the corresponding box.

The formulations based on ethanol + citric acid are not listed because none of them were stable 24 h after their preparation despite part of the precipitate could be re-dissolved due to peptization after some resting time.

Table 2: Summary of stability vs pH tests performed over the low concentrated formulations prepared via high throughput experiments in FLAMAC.

Composition Sample Initial pH pH1,5 3 4 5 6 7 8 9

Ethanol + acetylacetone

202 4,5 - - 6,5 X X X203 4,5 - - 6,5 X X X204 5,0 - - 6,5 X X X206 4,0 - - 6,5 X X X207 5,0 - - 7,0 X X208 5,0 - - - 7,0 X X210 4,5 - - 6,5 X X X211 4,5 - - 7,0 X X212 4,5 - - 7,0 X X214 4,0 - - 6,5 X X X215 4,0 - - 7,5 X X216 5,0 - - - 7,5 X X

Ethanol + triethanolamine

302 9,0 - - - - - - 8,5 303 8,5 - - - - - - 8,5 -304 8,0 - - - - - - 8,0 -306 9,0 - - - - - - 8,5 307 9,0 - - - - - - 8,5 308 9,0 - - - - - - 8,5 310 9,0 - - - - - - 8,5 311 9,0 - - - - - - 8,5 312 9,0 - - - - - - 8,5 314 9,0 - - - - - - 8,5 315 9,0 - - - - - - 8,5 316 9,0 - - - - - - 8,5

Composition Sample Initial pH pH1,5 3 4 5 6 7 8 9

Isopropanol + citric acid

402 2,0 6,5 7,5 - -403 2,5 6,5 7,5 - -404 2,5 6,5 7,5 - -406 2,0 6,5 7,5 - -407 2,0 6,5 7,5 - -408 2,5 6,5 7,5 - -410 1,5 6,5 7,5 - -411 2,0 6,5 7,5 - -412 2,5 6,5 7,5 - -414 1,5 6,5 7,5 - -415 2,0 6,5 7,5 - -416 2,0 6,5 7,5 - -

Isopropanol + acetylacetone

502 4,5 - - 6,5 X X X503 4,5 - - 6,5 X X -504 5,0 - - - 6,5 X X -506 4,5 - - 6,5 X - -507 5,0 - - 7 X -508 5,0 - - - 7 X -510 4,5 - - 6,5 X X X511 4,5 - - 6,5 X X X512 4,5 - - 8 X514 4,5 - - 6,5 X X X515 5,0 - - 8 X516 5,0 - - - 8 X

Isopropanol + triethanolamine

602 9,0 - - - - - - 8,5 603 8,5 - - - - - - 8,5 -604 8,0 - - - - - - 8,0 -606 9,0 - - - - - - 8,5 607 8,5 - - - - - - 8,5 -608 9,0 - - - - - - 8,5 610 9,0 - - - - - - 8,5 611 9,0 - - - - - - 8,5 612 9,0 - - - - - - 8,5 614 9,0 - - - - - - 8,5 615 9,0 - - - - - - 8,5 616 9,0 - - - - - - 8,5

Acetic acid + citric acid

702 2,5 6,5 - - -703 2,5 6,5 - - -704 3,0 6,5 - - -706 2,0 6,5 - - -707 2,5 6,5 - - -708 2,5 6,5 - - -710 1,5 6,5 - - -711 2,0 6,5 - - -712 2,5 6,5 - - -714 1,5 6,5 - - -715 2,0 6,5 - - -

Based on the observations in table 2, we conclude that isopropanol based formulations are more stable than ethanol based ones. Ethanol is a slightly stronger acid than isopropanol (pKa, ethanol = 15.5 ; pKa isopropanol = 16.5 [39]). This, together with the smaller electron donor character of the ethoxy group compared to the isopropoxy, makes the ethoxy-titanium complexes, formed by ligand exchange of the titanium isopropoxide in ethanol-based media, easier to be attacked by H+ and OH- , breaking down the bonds and generating hydrolyzed titanium species and ethanol.

All the solutions prepared with acetylacetone and TEA were stable in the operational pH range desired for coating. TEA formulations, prepared without initial HNO3, had a starting pH value closer to the operational range making them more suitable to be used without further modification after

preparation. The TEA based formulations were also more viscous than the other ones, what facilitated the application over the steel substrates.

Corrosivity was tested by deposition of approximately 10 µL of each formulation on the substrate. The samples were placed in a furnace at 60°C for 24 h to allow gelification of the formulations. The presence or absence of corrosion was attested by visual inspection after removal of the dry residues. None of the formulations seemed to attack the substrates independently of their composition or pH.

3.2 Stability of formulations with high concentration (M101 to M823)

Only the TEA based formulations were stable after preparation, most likely by the presence of three terminal alcohols and a free electron pair over the nitrogen in the TEA molecule while acetylacetone only has two (the di-ketone oxygen atoms). Acetylacetone complexes metal ions via its enol tautomeric form [40]. This specie is stable due to the conjugation of one ketone double bond with the carbon-carbon double bond formed. At pH values lower than 8.95, below its pKa [41], the acetylacetone tautomer keeps the secondary alcohol group protonated and its complexing strength is reduced. The citric acid is neither a good stabilizer because the de-protonated species that would complex the metal more efficiently are not present at pH values lower than 6.4 (pKa 3, citric acid = 6.40 [41]). As the formulations were prepared in water-based solutions of these two complexing agents, the pH values will always be lower than the pKa´s, while TEA-based ones (pKa = 7.76 [42]) were prepared at a pH where the alcohol groups are partially or totally de-protonated.

Indeed, the acetylacetone based formulations were not stable after preparation. This wass determined by visual inspection due to a big amount of precipitated hydroxide species. Citric acid based samples showed precipitation after preparation but this precipitation re-dissolved after some time. For the less concentrated solutions this process was faster and more efficient than for the more concentrated ones. The higher the amount of complexing agent present, the faster the precipitated species re-dissolved. This effect is attributed to the peptization of titanium hydroxides [29]. For some highly concentrated citric acid based formulations with small ratios of complexing agent vs titanium it was not possible to re-dissolve the precipitate completely. The peptization effect could be also observed in acacH based samples after much longer periods.

The stability of the complexed solutions at values of pH close to 7 depends on different factors such as titanium concentration and complexing agent ratio. This goes into agreement with other studies that determine that the isoelectric point of titania is located around 5.5 – 6 [43]. Lower titanium concentrations and higher ratios Ti:complexing agent, logically, improve stability.

3.3 Contact angle and pendant droplet measurements

After preparation of the formulations using the high throughput equipment, the interaction of the different solutions with the substrates was evaluated. First, the wetting behaviour of both substrates was quantified by measuring the contact angle of water (polar solvent) and di-iodomethane (non polar) on the cleaned substrates. An arbitrary contact angle of 10º is assumed to be necessary for a good wettability of the substrate. Subsequently, the contact angle of different formulations on both Aluzinc® and bright annealed substrates were determined. The total surface tension of the different

formulations, measured by the pendant droplet method; and the dispersive component of it, extracted by the captive bubble method in pentane; were used to calculate the polar component of the surface tension [44-45]. These components can be plotted as coordinates in a graph together with the wetting envelopes of the substrates in order to determine the wetting of the formulations over these substrates. The formulations whose coordinates are contained within the limits of the wetting envelopes theoretically wet with an angle lower than the one used for the calculation of the wetting envelope (in our specific case, 10°).

From figure 5, it can be concluded that the wettability of bright annealed steel is much lower than for Aluzinc® , mainly due to the low polar component of the first one. This can be explained by the formation of hydroxyl groups over the surface of aluminum and zinc in the Aluzinc® after alkaline cleaning while for bright annealed a much stronger cleaning method was needed. Figure 5 shows that all the formulations would wet AZ while half of the solutions represented in the graph (700 series: acetic acid + citric acid based formulations; and the ones with higher water composition for 300, 400 and 600 series) wouldn´t wet BA. The coordinates of water and ethanol are also shown for comparison.

Fig. 5: Wetting envelopes of Aluzinc® (AZ) and bright annealed (BA) substrates of series 600 – isopropanol + triethanolamine

(); 400 - isopropanol + citric acid (); 300 – ethanol + triethanolamine () and 700 – acetic acid + citric acid (), water (x)

and ethanol (+)

The dependence of the surface tension of different formulations with their composition was analyzed as can be seen in figures 6 and 7. A detailed description can be found in appendix A1. The total surface tension and the polar component are plotted versus the molar fraction of water, as the most abundant component in all the solutions. An increase in total surface tension upon increasing water content can clearly be observed. Extracting the polar component percentage of it shows the same trend. On the other hand, the dispersive component of all the formulations stays more or less constant for formulations composed by the same species, independently of the amounts of them.

Figure 6: Total surface tension vs molar fraction of water of 24 of the more stable solutions. From left to right for series 600 – isopropanol + triethanolamine () 614-610-606-602-603-604; for series 400 - isopropanol + citric acid () 414-410-406-402-403-404; for series 300 – ethanol + triethanolamine () 314-310-306-302-303-304 and for series 700 – acetic acid + citric acid () 717-710-706-702-703-704

Figure 7: Polar component percentage over the total surface tension of the 24 more stable formulations. From left to right for series 600 – isopropanol + triethanolamine () 614-610-606-602-603-604; for series 400 - isopropanol + citric acid () 414-410-406-402-403-404; for series 300 – ethanol + triethanolamine () 314-310-306-302-303-304 and for series 700 – acetic acid + citric acid () 717-710-706-702-703-704

The formulations show differences in their surface tension depending on their composition. The acetic acid samples (700 series) have, in general, the highest values followed by the ethanol based (300 series) samples and, finally, those prepared with isopropanol (400 and 600 series). Differences by changing the type of complexing agents is not so visible as they are present in lesser amounts than the co-solvents. In the first 4 data points measured for all 4 systems the v/v% of solvent is kept fixed at 10 % and is then lowered to 5 and finally to 3 %. In this 4 data points no remarkable trend could be addressed to changes in concentration of complexing agent.

Figure 8 shows the contact angle change versus time for three TEA based formulations, stable in the pH range 6.5 – 8.5, where the formulations are most likely to be used, for long term (more than six months). Formulation 604 contains the higher water content while 602 presents the lowest.

Fig. 8: Contact angle versus time graph of three TEA based formulations over AZ.

3.4 Coating characterization

Coatings obtained from stable TEA based formulations were homogeneous, transparent and colourless. Citric acid-based formulations also led to transparent and homogeneous coatings despite the fact that the precursor formulations as such are not stable. On the other hand, coatings prepared with the non-stable acetone-based formulations looked more like a powder dispersion on the surface of the steel and presented a yellowish colour that most likely is due to the type of complexing agent. These powder-like layers could be easily removed by rubbing the surface with a tissue. After curing, TEA and citric acid-based coatings did not dry completely and a tacky feeling remains, mainly those obtained from citric acid-based formulations.

No reliable technique has been found to measure the thickness of the coatings. The rough nature of the Aluzinc® panels (figure 9) did not allow to perform ellipsometry or to build up a theoretical model for the system consistent with the results. GDOES could only be used based on reference samples.

Fig 9: SEM micrographs of Aluzinc® substrates after cleaning.

No crystalline material was found after the curing process of the different layers (see figure 10). The temperatures reached during coating (240 – 250°C) were not high enough to completely remove the organic materials present in the formulation and to transform the amorphous film into a crystalline layer. Some bright annealed panels, more resistant to higher temperatures, were annealed in a furnace at 400 °C under air atmosphere and some weak reflections corresponding to anatase phase are detected as it can be seen in figure 11. The reflection at 2θ ≈ 25° corresponds to the anatase (101) while the rest of the reflections present correspond to the substrate(chromium-iron-nickel-carbon austenitic steel). This heat treatment at 400°C caused some oxidation reactions seen by a clear colour change of the panels and a drastic loss of the mechanical properties of the substrates.

Fig. 10: XRD pattern of coating on Aluzinc® steel from formulation M601 after synthesis. Reflections by Zn (+) and Al (*) are indicated

Fig. 11: XRD pattern of coating on bright annealed steel from formulation M301 after heat treatment at 400°C for 1 h. Substrate reflections are indicated by an asterix (*)

The lack of crystalline material will strongly affect the photo-catalytic activity of the coating. Figure 12 presents a comparison between 2 different samples, a non coated substrate used as blank measurement and a commercially available self-cleaning Saint Gobain glass sample. The amorphous character of the samples are the main reason for this low activity. An improved synthesis at lower temperature could overcome this problem.

Fig. 12: Discoloration of methylene blue vs time in the presence of different substrates and coatings and under different conditions compared to a commercial available glass sample.

4. Conclusion

A method is given to prepare stable complexed water-based TiO2 precursor formulations. When TEA was used as the complexing agent it was possible to obtain concentrated solutions (up to 0.5 M of metal ions) while citric acid and acetylacetone were only capable to prevent titanium from hydrolysis when the concentration is lower than 0.07 M. In addition, TEA worked better in the range of pH = 6.5 – 8.5 which correlates to the stable region in the Pourbaix diagrams of aluminium and zinc. These precursor solutions were, therefore, applicable as coatings on Aluzinc® substrates.

The selected formulations for the preparation of the coatings were the more concentrated ones in order to obtain higher loads and thicker films with the minimum number of coating steps possible.

The interactions of the substrate with the formulations are favoured by alkaline cleaning of the surface of the steel. The cleaning agents create hydroxyl groups on the surface of the metal and these polar groups interact favourably with water and other hydroxyl groups or polar species present in the solution. An increase in the organic content of the solutions (organic solvent and/or complexing agent) also lead to an expected decrease in the contact angle due to the lower surface energy of the organic solvents compared to water.

The formulations prepared via high-throughput had a higher polar than dispersive component for their surface tension as expected as water is the major component of all of them. This made them all laying within the wetting envelope of Aluzinc® for a contact angle of 10° indicating good wetting of the substrate. On the other hand, the polar and dispersive coordinates of some of these formulations wee outside the wetting envelope for bright annealed, predicting a bad wetting of these substrate which was in agreement with the experimental observations.

The coatings obtained via the high-throughput experiment were not completely dry and organic components were still present because of the low sintering temperature. This too mild temperature programs resulted in the formation of non crystalline coatings that presented poor photocatalytic activity. The use of crystalline TiO2 particles dispersions could be a possible way of improving the photocatalytic properties of the coatings allowing mild temperature programs below the limiting working temperature of the Aluzinc®.

Acknowledgments

The authors are very grateful for the outstanding help of the FLAMAC personnel with the high-throughput experiments. This research was carried out under the Interuniversity Attraction Poles Program IAP/VI-17 (INANOMAT) financed by the Belgian State, Federal Science Policy Office.

References

1 G. Madras, B.J. McCoy, A. Navrotsky, Kinetic model for TiO2 polymorphic transformation from anatase to rutile, J. Am. Ceram. Soc. 90 (2007) 250-255

2 K. Yamamoto, H. Shimoita, K. Tomita, K. Fujita, M. Kobayashi, V. Petrykin, M. Kakihana, Photocatalytic activity of nanocrystalline TiO2 synthesized from titanium glycolate complex by hydrothermal method, J. Ceram. Soc. Jpn. 117 (2009) 347-350.

3 L. Andronic, D. Andrasi, A. Enesca, M. Visa, A. Duta, The influence of titanium dioxide phase composition on dyes photocatalysis, J. Sol-Gel Sci. Technol. 58 (2011) 201-208.

4 S. Bakardjieva, V. Stengl, L. Szatmary, J. Subrt, J. Lukac, N. Murafa, D. Niznansky, K. Cizek, J. Jirkovsky, N. Petrova, Transformation of brookite-type TiO2 nanocrystals to rutile: correlation between microstructure and photoactivity, J. Mater. Chem. 16 (2006) 1709-1716.

5 L. Gerward, J.S. Olsen, Post-rutile high-pressure phases in TiO2, J. Appl. Crystallogr. 30 (1997) 259-264.

6 J.G. Li, T. Ishigaki, Brookite to rutile phase transformation of TiO2 studied with monodispersed particles, Acta Mater. 52 (2004) 5143-5150.

7 M. Sakanoue, Y. Kinoshita, Y. Otsuka, H. Imai, Photocatalytic activities of rutile and anatase nanoparticles selectively prepared from an aqueous solution, J. Ceram. Soc. Jpn. 115 (2007) 821-825.

8 B.N. Lee, W.D. Liaw, J.C. Lou, Photocatalytic decolorization of methylene blue in aqueous TiO2 suspension, Environ. Eng. Sci. 16 (1999) 165-175.

9 G. Balasubramanian, D.D. Dionysiou, M.T. Suidan, Y. Subramanian, I. Baudin, J.M. Laine, Titania powder modified sol-gel process for photocatalytic applications, J. Mater. Sci. 38 (2003) 823-831.

10 A. Langlet, S. Permpoon, D. Riassetto, G. Berthome, E. Pernot, J.C. Joud, Photocatalytic activity and photo-induced superhydrophilicity of sol-gel derived TiO2 films, J. Photochem. Photobiol. A-Chem. 181 (2006) 203-214.

11 W. Ho, J.C. Yu, S. Lee, Photocatalytic activity and photo-induced hydrophilicity of mesoporous TiO2 thin films coated on aluminum substrate, Appl. Catal. B-Environ. 73 (2007) 135-143.

12 F. Sayilkan, M. Asiltuerk, P. Tatar, N. Kiraz, E. Arpac, H. Sayilkan, Preparation of re-usable photocatalytic filter for degradation of Malachite Green dye under UV and vis-irradiation, J. Hazard. Mater. 148 (2007) 735-744.

13 X. Zhao, M. Liu, Y. Zhu, Fabrication of porous TiO2 film via hydrothermal method and its photocatalytic performances, Thin Solid Films 515 (2007) 7127-7134.

14 F. Wei, H. Zeng, P. Cui, S. Peng, T. Cheng, Various TiO 2 microcrystals: Controlled synthesis and enhanced photocatalytic activities, Chem. Eng. J. 144 (2008) 119-123.

15 X.-M. Song, J.-M. Wu, M. Yan, Photocatalytic degradation of selected dyes by titania thin films with various nanostructures, Thin Solid Films 517 (2009) 4341-4347.

16 W. Choi, S. Kim, S. Cho, H.I. Yoo, M.H. Kim, Photocatalytic reactivity and diffusing OH radicals in the reaction medium containing TiO2 particles, Korean J. Chem. Eng. 18 (2001) 898-902.

17 M. Arin, P. Lommens, N. Avci, S.C. Hopkins, K. De Buysser, I.M. Arabatzis, I. Fasaki, D. Poelman, I. Van Driessche, Inkjet printing of photocatalytically active TiO2 thin films from water based precursor solutions, J. Eur. Ceram. Soc. 31 (2010) 1067-1074.

18 P. Bouras, P. Lianos, Photodegradation of dyes in aqueous solutions catalyzed by highly efficient nanocrystalline titania films, J. Appl. Electrochem. 35 (2005) 831-836.

19 W.C. Du, H.T. Wang, W. Zhong, L. Shen, Q.G. Du, High refractive index films prepared from titanium chloride and methyl methacrylate via a non-aqueous sol-gel route, J. Sol-Gel Sci. Technol. 34 (2005) 227-231.

20 M.H. Habibi, N. Talebian, J.-H. Choi, The effect of annealing on photocatalytic properties of nanostructured titanium dioxide thin films, Dyes Pigment. 73 (2007) 103-110.

21 A.M. Ramirez, K. Demeestere, N. De Belie, T. Mantyla, E. Levanen, Titanium dioxide coated cementitious materials for air purifying purposes: Preparation, characterization and toluene removal potential, Build. Environ. 45 (2010) 832-838.

22 N. Van de Velde, M. Arin, P. Lommens, D. Poelman, I. Van Driessche, Characterization of the aqueous peroxomethod for the synthesis of transparent TiO2 thin films, Thin Solid Films 519 (2011) 3475-3479.

23 H.J. Zhu, R.H. Hill, The photochemistry of thin films of titanium diacetylacetonate diisopropoxide on silicon surfaces, J. Photochem. Photobiol. A-Chem. 147 (2002) 127-133.

24 N. Avci, P.F. Smet, H. Poelman, N. Van de Velde, K. De Buysser, I. Van Driessche, D. Poelman, Characterization of TiO2 powders and thin films prepared by non-aqueous sol-gel techniques, J. Sol-Gel Sci. Technol. 52 (2009) 424-431.

25 S.-Y. Lin, Y.-C. Chen, C.-M. Wang, C.-C. Liu, Effect of heat treatment on electrochromic properties of TiO2 thin films, J. Solid State Electrochem. 12 (2008) 1481-1486.

26 J.H. Clark, Green Chemistry: challenges and opportunities, Green Chem. 1 (1999) 1-8.27 J.H. Clark, Green chemistry: today (and tomorrow), Green Chem. 8 (2006) 17-21.28 S. Music, M. Gotic, M. Ivanda, S. Popovic, A. Turkovic, R. Trojko, A. Sekulic, K. Furic, Chemical

and microstructural properties of TiO2 synthesized by sol-gel procedure, Mater. Sci. Eng. B-Solid State Mater. Adv. Technol. 47 (1997) 33-40.

29 M.T. Colomer, J. Guzman, R. Moreno, Peptization of Nanoparticulate Titania Sols Prepared Under Different Water-Alkoxide Molar Ratios, J. Am. Ceram. Soc. 93 (2010) 59-64.

30 M.T. Colomer, J. Guzman, R. Moreno, Determination of peptization time of particulate sols using optical techniques: Titania as a case study, Chem. Mat. 20 (2008) 4161-4165.

31 M. Hirano, K. Ota, T. Ito, Anatase-type TiO2 and ZrO2-doped TiO2 directly formed from titanium(III) sulfate solution by thermal hydrolysis: Effect of the presence of ammonium peroxodisulfate on their formation and properties, J. Am. Ceram. Soc. 88 (2005) 3303-3310.

32 V. Cloet, J. Feys, R. Huehne, S. Engel, B. Holzapfel, S. Hoste, I. Van Driessche, A Water-Based Sol-Gel Precursor for Deposition of Thin La2Zr2O7 Layers on Ni-W Substrates, IEEE Trans. Appl. Supercond. 19 (2009) 3467-3470.

33 V. Cloet, J. Feys, R. Huehne, S. Hoste, I. Van Driessche, Thin La 2Zr2O7 films made from a water-based solution, J. Solid State Chem. 182 (2009) 37-42.

34 I. Van Driessche, G. Penneman, J.S. Abell, E. Bruneel, S. Hoste, Chemical approach to the deposition of textured CeO2 buffer layers based on sol gel dip coating, Thermec'2003, Pts 1-5 426-4 (2003) 3517-3522.

35 G. Penneman, I. Van Driessche, E. Bruneel, S. Hoste, Deposition of CeO2 buffer layers and YBa2Cu3O7-sigma superconducting layers using an aqueous sol-gel method, Euro Ceramics Viii, Pts 1-3 264-268 (2004) 501-504.

36 K. De Buysser, P.F. Smet, B. Schoofs, E. Bruneel, D. Poelman, S. Hoste, I. Van Driessche, Aqueous sol-gel processing of precursor oxides for ZrW2O8 synthesis, J. Sol-Gel Sci. Technol. 43 (2007) 347-353.

37 N. Van de Velde, D. Van de Vyver, O. Brunkahl, S. Hoste, E. Bruneel, I. Van Driessche, CeO 2

Buffer Layers for HTSC by an Aqueous Sol-Gel Method - Chemistry and Microstructure, Eur. J. Inorg. Chem. (2010) 233-241.

38 M. Pourbaix, Atlas of electrochemical equilibria in aqueous solutions, first ed., Pergamon Press, New York, 1966.

39 G.M. Loudon, Organic Chemistry, fourth ed., Oxford University Press, New York, 2002.40 N.C. Pramanik, S.I. Seok, B.Y. Ahn, Wet-chemical synthesis of crystalline BaTiO3 from stable

chelated titanium complex: Formation mechanism and dispersibility in organic solvents, J. Colloid Interface Sci. 300 (2006) 569-576.

41 A.E. Martell, R.M. Smith, Critical Stability Constants - Volume 3: Other Organic Ligands, Plenum Press, New York, 1977.

42 A.E. Martell, R.M. Smith, Critical Stability Constants - Volume 2: Amines, Plenum Press, New York, 1977.

43 S. Fazio, J. Guzman, M. Colomer, A. Salomoni, R. Moreno, Colloidal stability of nanosized titania aqueous suspensions, J. Eur. Ceram. Soc. 28 (2008) 2171-2176.

44 M. Feigl, M. Nofz, R. Sojref, A. Kohl, Improved wetting of bare and pre-coated steels by aqueous alumina sols for optimum coating success, J. Sol-Gel Sci. Technol. 55 (2010) 191-198.

45 D. Janssen, R. De Palma, S. Verlaak, P. Heremans, W. Dehaen, Static solvent contact angle measurements, surface free energy and wettability determination of various self-assembled monolayers on silicon dioxide, Thin Solid Films 515 (2006) 1433-1438.

Appendix table A1

Table A 1: Low concentrated and stock formulations prepared using the high throughput equipment. The stock formulations are indicated with an asterisk (*).

Solution Solvent Solvent (v/v %) [Ti] (M) Complexing agent Ratio Ti vs compl. ag.* 101 Ethanol 22.23 0.148 Citric acid 0.5

102 Ethanol 10.00 0.066 Citric acid 0.5103 Ethanol 5.00 0.033 Citric acid 0.5104 Ethanol 3.00 0.020 Citric acid 0.5

* 105 Ethanol 22.23 0.148 Citric acid 1.0106 Ethanol 10.00 0.066 Citric acid 1.0107 Ethanol 5.00 0.033 Citric acid 1.0108 Ethanol 3.00 0.020 Citric acid 1.0



* 201 Ethanol 45.00 0.300 acacH 0.5202 Ethanol 10.00 0.067 acacH 0.5203 Ethanol 5.00 0.033 acacH 0.5204 Ethanol 3.00 0.020 acacH 0.5




* 301 Ethanol 11.43 0.076 TEA 0.5302 Ethanol 10.00 0.067 TEA 0.5303 Ethanol 5.00 0.033 TEA 0.5304 Ethanol 3.00 0.020 TEA 0.5306 Ethanol 10.00 0.067 TEA 1.0307 Ethanol 5.00 0.033 TEA 1.0308 Ethanol 3.00 0.020 TEA 1.0310 Ethanol 10.00 0.067 TEA 2.0311 Ethanol 5.00 0.033 TEA 2.0312 Ethanol 3.00 0.020 TEA 2.0314 Ethanol 10.00 0.067 TEA 3.0315 Ethanol 5.00 0.033 TEA 3.0

316 Ethanol 3.00 0.020 TEA 3.0* 401 Isopropanol 16.26 0.108 Citric acid 0.5

402 Isopropanol 10.00 0.066 Citric acid 0.5403 Isopropanol 5.00 0.033 Citric acid 0.5404 Isopropanol 3.00 0.020 Citric acid 0.5

* 405 Isopropanol 16.26 0.108 Citric acid 1.0406 Isopropanol 10.00 0.066 Citric acid 1.0407 Isopropanol 5.00 0.033 Citric acid 1.0408 Isopropanol 3.00 0.020 Citric acid 1.0



* 501 Isopropanol 38.00 0.250 acacH 0.5502 Isopropanol 10.00 0.066 acacH 0.5503 Isopropanol 5.00 0.033 acacH 0.5504 Isopropanol 3.00 0.020 acacH 0.5




* 601 Isopropanol 11.41 0.076 TEA 0.5602 Isopropanol 10.00 0.067 TEA 0.5603 Isopropanol 5.00 0.034 TEA 0.5604 Isopropanol 3.00 0.020 TEA 0.5606 Isopropanol 10.00 0.067 TEA 1.0607 Isopropanol 5.00 0.034 TEA 1.0608 Isopropanol 3.00 0.020 TEA 1.0610 Isopropanol 10.00 0.067 TEA 2.0611 Isopropanol 5.00 0.034 TEA 2.0612 Isopropanol 3.00 0.020 TEA 2.0614 Isopropanol 10.00 0.067 TEA 3.0615 Isopropanol 5.00 0.034 TEA 3.0616 Isopropanol 3.00 0.020 TEA 3.0

* 701 Acetic acid 23.82 0.158 Citric acid 0.5702 Acetic acid 10.00 0.066 Citric acid 0.5703 Acetic acid 5.00 0.033 Citric acid 0.5

704 Acetic acid 3.00 0.020 Citric acid 0.5* 705 Acetic acid 23.82 0.158 Citric acid 1.0

706 Acetic acid 10.00 0.066 Citric acid 1.0707 Acetic acid 5.00 0.033 Citric acid 1.0708 Acetic acid 3.00 0.020 Citric acid 1.0

* 709 Acetic acid 23.82 0.158 Citric acid 2.0710 Acetic acid 10.00 0.066 Citric acid 2.0711 Acetic acid 5.00 0.033 Citric acid 2.0712 Acetic acid 3.00 0.020 Citric acid 2.0

* 713 Acetic acid 23.82 0.158 Citric acid 3.0714 Acetic acid 10.00 0.066 Citric acid 3.0715 Acetic acid 5.00 0.033 Citric acid 3.0716 Acetic acid 3.00 0.020 Citric acid 3.0

* M101 Ethanol 10.00 0.400 Citric acid 1M102 Ethanol 10.00 0.400 Citric acid 2M103 Ethanol 10.00 0.400 Citric acid 3M111 Ethanol 6.77 0.277 Citric acid 1M112 Ethanol 6.77 0.277 Citric acid 2M113 Ethanol 6.77 0.277 Citric acid 3M121 Ethanol 5.00 0.200 Citric acid 1M122 Ethanol 5.00 0.200 Citric acid 2M123 Ethanol 5.00 0.200 Citric acid 3

* M201 Ethanol 10.00 0.400 acacH 1M202 Ethanol 10.00 0.400 acacH 2M203 Ethanol 10.00 0.400 acacH 3M211 Ethanol 6.77 0.277 acacH 1M212 Ethanol 6.77 0.277 acacH 2M213 Ethanol 6.77 0.277 acacH 3M221 Ethanol 5.00 0.200 acacH 1M222 Ethanol 5.00 0.200 acacH 2M223 Ethanol 5.00 0.200 acacH 3

* M301 Ethanol 10.00 0.500 TEA 1M302 Ethanol 10.00 0.500 TEA 2M303 Ethanol 10.00 0.500 TEA 3M311 Ethanol 6.77 0.346 TEA 1M312 Ethanol 6.77 0.346 TEA 2M313 Ethanol 6.77 0.346 TEA 3M321 Ethanol 5.00 0.250 TEA 1M322 Ethanol 5.00 0.250 TEA 2M323 Ethanol 5.00 0.250 TEA 3

* M401 Isopropanol 10.00 0.400 Citric acid 1M402 Isopropanol 10.00 0.400 Citric acid 2M403 Isopropanol 10.00 0.400 Citric acid 3M411 Isopropanol 6.77 0.277 Citric acid 1M412 Isopropanol 6.77 0.277 Citric acid 2M413 Isopropanol 6.77 0.277 Citric acid 3M421 Isopropanol 5.00 0.200 Citric acid 1M422 Isopropanol 5.00 0.200 Citric acid 2M423 Isopropanol 5.00 0.200 Citric acid 3

* M501 Isopropanol 10.00 0.400 acacH 1M502 Isopropanol 10.00 0.400 acacH 2M503 Isopropanol 10.00 0.400 acacH 3M511 Isopropanol 6.77 0.277 acacH 1M512 Isopropanol 6.77 0.277 acacH 2M513 Isopropanol 6.77 0.277 acacH 3M521 Isopropanol 5.00 0.200 acacH 1M522 Isopropanol 5.00 0.200 acacH 2M523 Isopropanol 5.00 0.200 acacH 3

* M601 Isopropanol 10.00 0.500 TEA 1M602 Isopropanol 10.00 0.500 TEA 2M603 Isopropanol 10.00 0.500 TEA 3M511 Isopropanol 6.77 0.346 TEA 1M612 Isopropanol 6.77 0.346 TEA 2M613 Isopropanol 6.77 0.346 TEA 3M621 Isopropanol 5.00 0.250 TEA 1M622 Isopropanol 5.00 0.250 TEA 2M623 Isopropanol 5.00 0.250 TEA 3

* M701 Acetic acid 10.00 0.400 Citric acid 1M702 Acetic acid 10.00 0.400 Citric acid 2M703 Acetic acid 10.00 0.400 Citric acid 3M711 Acetic acid 6.77 0.277 Citric acid 1M712 Acetic acid 6.77 0.277 Citric acid 2M713 Acetic acid 6.77 0.277 Citric acid 3M721 Acetic acid 5.00 0.200 Citric acid 1M722 Acetic acid 5.00 0.200 Citric acid 2M723 Acetic acid 5.00 0.200 Citric acid 3

* M801 - 10.00 0.500 TEA 1M802 - 10.00 0.500 TEA 2M803 - 10.00 0.500 TEA 3M811 - 6.77 0.346 TEA 1M812 - 6.77 0.346 TEA 2M813 - 6.77 0.346 TEA 3M821 - 5.00 0.250 TEA 1M822 - 5.00 0.250 TEA 2M823 - 5.00 0.250 TEA 3

Documents

biblio.ugent.be - Ghent University Academic Bibliography · Web viewThe formulations, also prepared by high-throughput equipment, are found to be stable in the operational pH range