Effect of amorphous fluorinated coatings on photocatalytic properties of anodized titanium surfaces

doi:10.1016/j.tsf.2013.08.004

Thin Solid Films

Volume 545, 31 October 2013, Pages 210-216

https://doi.org/10.1016/j.tsf.2013.08.004 Get rights and content

Highlights

•
Coated anodized titanium surfaces show a decreased wettability.
•
Evaluation of the stability of perfluorinated coatings towards photocatalysis.
•
Amorphous perfluorinated coatings do not hinder photocatalytic activity.

Abstract

The photocatalytic activity promoted by anodized titanium surfaces coated with different amorphous perfluoropolymers was evaluated. A copolymer between tetrafluoroethylene and perfluoro-4-trifluoromethoxy-1,3-dioxole and two perfluoropolyethers containing ammonium phosphate and triethoxysilane functionalities, respectively, were tested as coating materials. These coatings revealed good adhesion to the anodized titanium substrate and conferred to it both hydrophobicity and oleophobicity. The photocatalytic activity of the coating on anodized titanium was evaluated by monitoring the degradation of stearic acid via Infrared spectroscopy. The degradation rate of stearic acid was reduced but not set to zero by the presence of the fluorinated coatings, leading to the development of advanced functional coatings. The morphological variations of the coatings as a result of photocatalysis were also determined by atomic force microscopy.

Introduction

In the last years, titanium has received considerable attention because of its unique characteristics, like exceptional resistance to corrosive environments, high strength-to-weight ratio, high resistance to fatigue and excellent biocompatibility [1], [2], [3], [4], [5], [6]. As a consequence, it has been widely applied in several fields of engineering and architecture. Moreover, anodic oxidation can be used to form on the surface of titanium a partially crystalline oxide of titanium dioxide characterized by photocatalytic activity. Different crystal phases of titanium dioxide can be obtained depending on the anodizing procedure, namely, rutile, anatase and brookite: owing to its superior photoactivity, the greater the content of anatase in the titanium dioxide layer, the more effective the photocatalysis [2], [3], [7], [8], [9]. In addition, aesthetically attractive colors, due to interference phenomena between the oxide film and the underlying metal surface, can be conferred to the surface of anodized titanium [6], [10]. Titanium dioxide in the anatase phase is a semiconductor photocatalyst, and it offers convenient routes to the decontamination of air and water from gaseous, dissolved or solid compounds, even if present in low concentrations [11], [12]. In fact, such materials are able to photocatalyze the complete mineralization of many organic and inorganic compounds, including aromatics, insecticides, pesticides, dyes, surfactants, hormones and halohydrocarbons [3], through a process that can be schematically summarized as follows: $Organic pollutant + O_{2} \to mineralized products$

Further applications of titanium dioxide as photocatalyst are in the fields of solar energy conversion [13], [14], [15], [16], [17], [18], [19], electrochromic devices [20], self-cleaning [21] antimicrobial coatings [22] and antitumor treatments [2]. Photocatalytic reactions take place on the surface of the titanium-based photocatalyst, where electron-hole pairs are generated by the exposure to ultra-band gap light. Each electron-hole pair can either recombine or react with species adsorbed on the surface [12], [21]. The photogenerated electrons can reduce surface oxygen to water, while the photogenerated holes are able to directly mineralize organic compounds [12], [21]. The oxidation process is probably due to the initial oxidation of surface OH⁻ groups to OH ∙ which subsequently oxidize the adsorbed organic species [2], [3], [23]. However, one of the major drawbacks of anodized titanium is the easy soiling of this material [24]. In fact, its photocatalytic activity is significantly reduced whenever its surface is in contact with greasy or oily compounds [25]. Thus, the protection with a polymeric coating, which limits the soiling but does not inhibit the photocatalytic properties, can be an important improvement towards a more widespread use of anodized titanium. The polymers that can generate this synergic effect should present some fundamental characteristics: high chemical and thermal resistance to guarantee stability during photocatalysis, high transparency to radiations to allow the photoactivation of the oxide film, high permeability to the compounds required for the photocatalytic reactions and low wettability to achieve a self-cleaning effect.

Perfluorinated polymers are characterized by exceptional chemical and thermal stability, low surface energy and low wettability [24], [25], [26], [27], [28]. All these properties are strictly related to the presence of highly energetic C–F bonds into their polymeric chains. In addition, amorphous perfluoropolymers are completely transparent to a wide spectrum of radiations and show very low refractive indexes [29], [30], [31], [32], [33], [34]. Highly amorphous fluoropolymers also have a good permeability towards elementary gases, such as O₂ [35], [36] and water vapor [37]. In particular, this property can allow the photocatalytic process to take place even if the organic pollutant is not in direct contact with the surface of the photocatalyst.

Three different coatings were applied on anodized titanium to test their effect on the photocatalytic activity: HYFLON® AD60, a high molecular weight copolymer of tetrafluoroethylene (TFE) and 2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxole (TTD) [38], [39] and two perfluoropolyethers (PFPEs), FLUOROLINK® F10 and FLUOROLINK® S10, containing ammonium phosphate and triethoxysilane functionalities, respectively. The photocatalytic activity of uncoated anodized titanium and that of the samples coated with fluoropolymers were compared in this study. The photocatalytic activity was assessed by evaluating the photodegradation rate of stearic acid, via the following reaction [40], [41], [42], [43], [44]: ${CH}_{3} {({CH}_{2})}_{16} {CO}_{2} H + 26 O_{2} \overset{hv \geq E_{band gap}}{\to} 18 {CO}_{2} + 18 H_{2} O$

Since stearic acid is a solid compound, it can be considered as a molecular model of the solid organic films, which are usually deposited on surfaces such kitchen glasses or light covers in tunnel roads. The photodegradation of stearic acid was evaluated with Fourier transform infrared (FT-IR) analysis by observing specific absorbance peaks: the asymmetric in-plane C–H stretching mode of CH₃ group at 2958 cm^− 1 and the asymmetric and symmetric C–H stretching modes of CH₂ groups at 2912 cm^− 1 and 2853 cm^− 1 [44], [45]. FT-IR analyses for the determination of photodegradation rate were focused on the 2912-cm^− 1 peak because it is more evident and sharp than the others. The morphological changes of fluorinated coatings exposed to UV radiations were also determined by atomic force microscopy (AFM).

Section snippets

Materials

Commercially pure titanium (ASTM Grade 2) was employed as metallic substrate and amorphous perfluoropolymers HYFLON® AD60, FLUOROLINK® F10 and FLUOROLINK® S10 (from now on briefly indicated as AD60, F10 and S10, respectively) were used as protective coatings (Fig. 1).

These fluoropolymers are produced and commercialized by Solvay Specialty Polymers. HYFLON® AD60 is a random copolymer of tetrafluoroethylene (TFE) and 2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxole (TTD), characterized by a low

Photocatalysis

A widely used technique to evaluate the photocatalytic activity of materials involves the determination of the removal rate of stearic acid [40], [41], [42], [43], [49]. Stearic acid is highly stable under UV illumination in the absence of a photocatalyst and the photodegradation of this organic compound usually does not produce any component that may interfere with its own FT-IR peaks [50]. Moreover, only carbon dioxide and water are generated as gaseous products during the photocatalytic

Conclusions

In this work, AD60, S10 and F10 amorphous perfluorinated coatings were applied on anodized titanium surfaces and tested in the photodegradation of stearic acid: the coatings did not block photocatalytic reactions, although a reduction of photoefficiency was observed. Moreover, these amorphous fluorinated coatings decreased the wettability of the treated surfaces, which helps maintaining their aesthetic characteristics. Among the three applied perfluoropolymeric materials, only the plate treated

References (53)

S.R. Seagle
A. Fujishima et al.
J. Photochem. Photobiol., C
(2000)
A. Mills et al.
J. Photochem. Photobiol., A
(1997)
N. Serpone
J. Photochem. Photobiol., A
(1997)
T. Ohtsuka et al.
Corros. Sci.
(1998)
M.V. Diamanti et al.
J. Exp. Nanosci.
(2009)
J.L. Delplancke et al.
Surf. Technol.
(1982)
W. Navarrini et al.
Prog. Org. Coat.
(2012)
M.E. Sibert
J. Electrochem. Soc.
(1963)
M.F. Brunella et al.
Thin Solid Films
(2007)

A. Russo et al.

J. Polym. Sci., Part A: Polym. Chem.

(2005)

K. Sato et al.

Appl. Catal., B

(2011)

M. Grätzel

Nature

(2001)

M. Grätzel

J. Photochem. Photobiol., C

(2003)

A. Hagfeldt et al.

Chem. Rev.

(1995)

A.C. Arango et al.

Adv. Mater.

(2000)

K. Hara et al.

Chem. Commun.

(2001)

M.E. Butler et al.

J. Mater. Sci.

(1980)

K. Zhu et al.

J. Nano Lett.

(2007)

A. Ghicov et al.

Chem. Phys. Lett.

(2007)

A. Zaggia et al.

Curr. Opin. Colloid Interface Sci.

(2012)

Y. Kikuchi et al.

J. Photochem. Photobiol. A

(1997)

A. Mills et al.

J. Photochem. Photobiol., A

(2002)

W. Navarrini et al.

J. Coat. Technol. Res.

(2011)

W. Navarrini et al.

Diamond Relat. Mater.

(2010)

M. Sansotera et al.

Carbon

(2010)

Cited by (13)

Unraveling the role of surface molecular structure on vacuum flashover for fluorinated copolymers
2020, Applied Surface Science
Citation Excerpt :
The variance in the nm-scale (see Fig. 5(c) and (d)) is ascribed to the high mobility of the PFPE chains. The amorphous fluorinated monomers (PFPE) tend to fill the holes, peaks, and valleys present on the surface, thereby reducing the surface roughness remarkably [41]. The influence of the doping concentration on the surface roughness is summarized in Table 1.
To investigate the influence of surface molecular structure on the electrical performance of fluorinated copolymers, ultraviolet (UV) curable resin containing different types of fluorinated monomers (ethers and alkyls) were prepared by photopolymerization. The X-ray photoelectron spectra indicated the presence of fluorinated monomers on the surface after UV radiation. Furthermore, the surface morphology measurement revealed that these monomers cause a slight change to the surface roughness. Subsequently, the electrical properties were examined through a pulsed flashover test in vacuum. The results showed an improvement in the electrical strength of the specimen doped with dodecafluoroheptyl methacrylate (alkyls). In contrast, the sample doped with perfluoropolyether chains (ethers) exhibited no alleviation in flashover performance, despite the fact that its surface F 1s/C 1s atomic ratio is 10 times higher than that of the former. A coarse-grained model based on quantum chemical calculations reveals that electron traps are introduced at the surface because of the grafted fluorinated chains. Further explanation based on the free volume theory indicates that the higher gas absorption arising from the amorphous structure during the electron-induced degassing process accounts for the deterioration of the insulation system, which was verified by surface charge behavior before and after the flashover.
Absorption and photocatalytic degradation of VOCs by perfluorinated ionomeric coating with TiO<inf>2</inf> nanopowders for air purification
2019, Chemical Engineering Journal
Citation Excerpt :
The compatibility of fluorinated materials in the presence of TiO2 was also studied and perfluoropolymers resulted extraordinarily stable toward UV-activated TiO2 photodegradation [38]. In particular, amorphous perfluoropolymers showed the highest chemical stability and, due to their high UV transparency and high gas permeability, emerged as best candidate class of materials for the fabrication of TiO2-embedding matrices [39,40]. Recently, a multilayered perfluorinated coating based on amorphous and low-crystalline fluoropolymers with immobilized TiO2 was applied in a continuous photocatalytic apparatus for water remediation [41].
In this work, we propose a transparent multilayered perfluoropolymeric coating as immobilization method for TiO₂ nanoparticles, and evaluate its suitability in the gas phase photocatalytic degradation of six different volatile organic compounds. The coating was made of a layer of TiO₂-containing perfluorosulfonic acid polymer on a layer of perfluorinated amorphous polymer. The chemical stability of perfluoropolymeric materials to UV radiation and UV-activated TiO₂ overcomes the possible degradation of the polymeric immobilization system which is typical of more traditional polymeric coatings. Moreover, the TiO₂-containing ionomeric perfluorosulfonic layer worked as selective membrane for pollutants absorption and catalyst preservation, depending on the interactions between the superacidic polar heads of the ionomer and the pollutants, in particular those capable of hydrogen bonding. Gas-phase photocatalytic degradation tests were performed using pentane, methanol, 2-propanol, toluene, dichloromethane and pyridine as reference volatile organic pollutants, thus ranging on different polarity properties. Results indicate performances comparable to other approaches reported in the literature and show a strong influence of both atmospheric conditions (namely, humidity) and pollutant nature – polarity, proticity – on the actual kinetics of photodegradation, also depending on the interactions regulating the affinity between the ionomeric layer of the coating and pollutants. The high potential of the coating in the photodegradation was confirmed by the observed values of the photoabatement rates: all approximatively above 10⁻⁵ s⁻¹ and maximum for alcohols (1.4 × 10⁻⁴ and 1.7 × 10⁻⁴ s⁻¹ in dry and humid conditions, respectively).
Thermodynamics of aqueous perfluorooctanoic acid (PFOA) and 4,8-dioxa-3H-perfluorononanoic acid (DONA) from DFT calculations: Insights into degradation initiation
2018, Chemosphere
Citation Excerpt :
In this context, PFOA was thoroughly adopted as the prototypical perfluoroalkyl emulsifier. Chemical, thermal, and photolytic stability of perfluorinated compounds are directly related to the high dissociation energy of CF bonds (Amii and Uneyama, 2009; Avataneo et al., 2009; Lemal, 2004; Persico et al., 2013), which makes conventional degradation techniques rather inefficient. Advanced oxidation processes are thus required.
Modern fluorosurfactants introduced during and after perfluoroalkyl carboxylates/sulfonates phase-out present chemical features designed to facilitate abatement, hence reducing persistence. However, the implications of such features on environmental partitioning and stability are yet to be fully appreciated, partly due to experimental difficulties inherent to the handling of their (diluted) aqueous solutions. In this work, rigorous quantum chemistry calculations were carried out in order to provide theoretical insights into the thermodynamics of hydroperfluorosurfactants in aqueous medium. Estimates of acid dissociation constant (pK_a), standard reduction potential (E⁰), and bond dissociation enthalpy (BDE) and free energy (BDFE) were computed for perfluorooctanoic acid (PFOA), 4,8-dioxa-3H-perfluorononanoic acid (DONA) and their anionic forms via ensemble averaging at density functional theory level with implicit solvent models. A ‹pK_a› in the neighborhood of zero and a E⁰ of about 2.2 V were obtained for PFOA. Predictions for the acidic function of DONA compare well with PFOA's, with a pK_a of 0.8–1.5 and a E⁰ of 2.07–2.15 V. Deprotonation thus represents the dominant phenomenon at environmental conditions. Calculations indicate that H-abstraction of the aliphatic proton of DONA by a hydroxyl radical is the thermodynamically favored reaction path in oxidative media, whereas hydrolysis is not a realistic scenario due to the high dissociation constant. Short intramolecular interactions available to the peculiar hydrophobic tail of DONA were also reviewed, and the relevance of the full conformational space of the fluorinated side chain discussed.
Superhydrophilicity and durability of fluoropolymer-TiO<inf>2</inf> coatings
2016, Progress in Organic Coatings
Self-cleaning fluoropolymer-TiO₂ coatings have been in focus as a promising group of materials for a number of years due to their favorable combination of photo-activated hydrophilicity and chemical resistivity of the Teflon-based material. The technology incorporates photocatalysis, which results in oxidation of organic compounds absorbed to a surface, with its superhydrophilic nature, which hinders further absorption of any non-polar substances. The main practical challenge concerning this subject is optimization of the coatings composition without compromising its stability and durability. The aim of this study was to characterize the superhydrophilicity (SHP) of a fluoropolymer-TiO₂ coatings as a function of their composition, namely the amounts of polymer, type of the applied curing agent, and the amounts of the titania particles. The hydrophilicity of the coatings was compared to their surface structure and durability to withstand UV-light exposure. In this study, 24 combinations of coatings were studied with contact angle measurements, Fourier transform infra red spectroscopy (FTIR), and scanning electron microscopy (SEM). The results were analyzed statistically with general linear models (GLM) and multivariate data analysis (MDA). The contact angle measurements have shown that time was the most important variable influencing the surface properties of self-cleaning coatings. In terms of composition, the amount of polymer was of significance. Neither the curing agent, nor the amount of titanium dioxide seemed to have any lasting effect on SHP. The analysis of surface structure indicated that illumination influenced the organic polymer, and at long exposure times it led to the degradation of the polymer. This was confirmed by SEM analysis.
Photocatalytic activity of TiO<inf>2</inf>-embedded fluorinated transparent coating for oxidation of hydrosoluble pollutants in turbid suspensions
2015, Applied Catalysis B: Environmental
Citation Excerpt :
It is well known that soiling on TiO2 thin layers (e.g., TiO2 nanotubes by titanium anodizing) can easily deactivate the catalyst [21]. Perfluoropolymeric materials, due to the massive presence of highly stable CF bonds, as high as 544 kJ/mol [22], are characterized by thermal and chemical stability [22,23], in particular toward UV radiation and UV-activated TiO2 [24], low surface energy and, if amorphous, also high transparency [14,25]. The interaction between the embedded TiO2 catalyst and the aqueous polluted solution can be optimized by the use of ionomeric materials.
The photodegradative activity of titanium dioxide immobilized into a multilayered transparent fluoropolymeric matrix was studied. The photoactive coating was developed by applying a TiO₂containing fluorinated ionomeric dispersion and a perfluorinated amorphous polymer in an appropriate sequence directly on the UV source. The multilayered coating photocatalytic activity toward hydrosoluble organic pollutants was evaluated in clear as well as in highly turbid conditions obtained by dispersing barium sulfate microparticles in the polluted solution. Rhodamine B-base was chosen as reference organic pollutant. The photoabatement rates with TiO₂-embedded fluorinated coating in clear solution and turbid conditions were 0.0923 min⁻¹ and 0.0546 min⁻¹, respectively. In both clear and turbid conditions, TiO₂-embedded transparent coating revealed higher photocatalytic activity than merely dispersed TiO₂. This behavior was particularly evident at low pollutant concentrations. In addition, catalyst immobilization prevented TiO₂ separation and catalyst losses, allowing the development of a simple and efficient continuous apparatus.
Surface fluorination on TiO<inf>2</inf> catalyst induced by photodegradation of perfluorooctanoic acid
2015, Catalysis Today
The photoabatement of perfluorooctanoic acid in aqueous solution has been performed with a commercial nano-sized TiO₂-based photocatalyst content of 0.66 g/L under an UV irradiation of 95 W/m². PFOA degradation intermediates were investigated by HPLC–MS and ¹⁹F-NMR analysis. Evidences of a degradation mechanism based on two competitive pathways are discussed: photo-redox and β-scission pathways. Shorter perfluorinated carboxylic acids, C_nF_2n+1COOH (n = 1–6), as expected degradation intermediates, were identified and their concentration trends over time were determined. The apparent pseudo-first order kinetic constant expressed as rate of PFOA disappearance was also measured: k_app 0.1296 h⁻¹. The influence of fluoride ions on TiO₂ surface was analyzed by XPS technique, revealing a surface modification that affects the performances of the catalyst.

View all citing articles on Scopus

View full text

Effect of amorphous fluorinated coatings on photocatalytic properties of anodized titanium surfaces

Highlights

Abstract

Introduction

Section snippets

Materials

Photocatalysis

Conclusions

J. Photochem. Photobiol., C

J. Photochem. Photobiol., A

J. Photochem. Photobiol., A

Corros. Sci.

J. Exp. Nanosci.

Surf. Technol.

Prog. Org. Coat.

J. Electrochem. Soc.

Thin Solid Films

J. Polym. Sci., Part A: Polym. Chem.

Appl. Catal., B

Nature

J. Photochem. Photobiol., C

Chem. Rev.

Adv. Mater.

Chem. Commun.

J. Mater. Sci.

J. Nano Lett.

Chem. Phys. Lett.

Curr. Opin. Colloid Interface Sci.

J. Photochem. Photobiol. A

J. Photochem. Photobiol., A

J. Coat. Technol. Res.

Diamond Relat. Mater.

Carbon