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Doped Ti[O.sub.2] nanophotocatalysts for leather surface finishing with self-cleaning properties.

Abstract Ti[O.sub.2] nanoparticles (Ti[O.sub.2]NPs) doped with N and Fe, exhibiting improved photocatalytic activity in the visible light region, were prepared and then applied onto leather surfaces. Diffuse reflectance spectra measurements evidenced the shifting of the absorption peak toward visible region and the enlargement of absorption band (the so-called "tail" of the band). Absorbance spectra for a 20 ppm methylene blue (MB) solution, in the presence of 0.1% (wt%) doped Ti[O.sub.2]NPs after 1 h under visible light irradiation, showed the high efficiency of the doping. Doped Ti[O.sub.2]NPs were embedded in composites material based on acrylic binder, nitrocellulose lacquer, and pigmented paste and were used for leather surface finishing. Self-cleaning properties were confirmed by spectrophotometrical measurements and CIELab color coordinates recording for the degradation of MB spots applied to the treated leather surface exposed to UVA (A = 365 nm) and visible light irradiation. Leather treated with Fe-N co-doped Ti[O.sub.2]NPs presents the highest activity for MB spot degradation under visible light irradiation. The photographic images of MB dye spots and ball pen ink lines indicate a complete degradation after 30 h of exposure to visible light. Water contact angle measurements in dynamic conditions evidenced the photo-induced hydrophilicity process on functionalized leather surface, supporting the degradation phenomena through water and oxidative species generation.

Keywords Doped Ti[O.sub.2] nanoparticles, Photocatalyst, Self-cleaning leather, Contact angle, Photo-induced hydrophilicity

Introduction

Titanium dioxide, a very stable material under illumination, has been extensively studied for its application in water and air purification, surface self-cleaning, and self-sterilizing. (1,2) The most important shortcoming limiting the use of Ti[O.sub.2]NPs as a photoactive material for many applications is the requirement of UV photons for excitation. Due to its large band gap (3.0-3.2 eV) only the UV fraction of solar light, which represents about 4-6%, can be used. The modification of Ti[O.sub.2] to render it sensitive to visible light is one of the most important goals to enable the increased utility of Ti[O.sub.2]. (3) Highly active radical species produced on its surface under UV/visible light irradiation participate in oxidation reactions that facilitate the destruction of organic contaminants and also cause microorganism inactivation. (4) The presence of the doping ions within the titania structures produces electronic mid-gap states and determines a significant absorption shift to the visible region as compared to undoped Ti[O.sub.2]NPs. Doping with nitrogen is attributed to the new electronic states within the band gap that can be excited to the conduction band by means of visible photons. (5) Investigation of the N-doped Ti[O.sub.2]NPs showed that during the doping process, O-Ti-N bond formation took place, and therefore, this substitutional doping is held accountable for the significant increase in photo catalytic activity in N-doped Ti[O.sub.2]NPs. (6) Additionally, Fe doping generates impurity states and the band gap of titania red shifts by substitution of [Fe.sup.3+] for [Ti.sup.4+] sites, due to their similar size. Introducing iron ions into the lattice of Ti[O.sub.2] provides photocatalysts not only with lower electron-hole recombination rate but also with excitability by visible light. (7) Enhanced photocatalytic activity was noticed for iron-doped photocatalyst under UV8 and also for visible light irradiation in several publications. (9)

Currently, the practical use of Ti[O.sub.2]NPs as self-cleaning material in commercial applications is already a reality. (3) While textile materials with enhanced functionalities such as antibacterial, antistatic, stain resistant, self-cleaning, or UV protection are extensively studied with good reported results, (10) little information is available in the case of leathers with multiple advanced characteristics.

In addition to photocatalytic properties of Ti[O.sub.2]NPs-based materials, their wettability should also be taken into consideration, acting in the same direction even if they are governed by different mechanisms. (11) Wang et al. (12) showed for the first time that UV light generates hydrophilic Ti[O.sub.2] surfaces and then an increased interest in this particular property was evidenced, being related to various applications. (12-14) Some studies have reported that nitrogen-doped titania surfaces transform from partially hydrophobic to fully hydrophilic upon visible light irradiation. (5,15) Studies regarding the effect of visible light on the water contact angles (WCA) of semiconductors oxides showed that the kinetics of the decrease in WCA induced under visible light are much faster than the decrease evidenced under UV irradiation and involve different surface species. (16) The most widely accepted explanation consists in the hydroxylation of the surface induced by a photocatalytic process. (10) The electrons tend to reduce the Ti(IV) cations to the Ti(III) state, and the holes oxidize the [O.sub.2.sup.-] anions. In the process, oxygen atoms are ejected, creating oxygen vacancies, and water molecules can then occupy these oxygen vacancies, producing adsorbed OFU groups, which tend to make the surface hydrophilic. (17) The longer the surface is illuminated, the smaller the contact angle for water becomes, meaning that water has a tendency to spread perfectly across the surface. (18) WCA depends not only on the chemical composition of the solid surface, e.g., density of surface hydroxyls, but also on macroscopic structure, e.g., surface roughness and also of porosity, which may enhance both hydrophilicity and hydrophobicity. (10,19) Porosity of Ti[O.sub.2] coating films enhances hydrophilicity and it was found that the adsorbed hydroxyl content in these films increases due to the larger size and number of pores, and as a result, the contact angle for water becomes smaller. (20) Enhancing the UV-induced hydrophilicity of Ti[O.sub.2] thin film involving Fe ions dopants was described by other authors, too. The undoped films could be entirely wetted by water after 5 h UV illumination, while 5 mol% Fe ions-doped films could show hydrophilicity after 2 h UV illumination and be entirely wetted after 4 h UV illumination. (21) Also, MB discoloration on transparent PE-Ti[O.sub.2] sputtered films, under sunlight irradiation, was studied. (22)

In this article, N- and Fe-doped Ti[O.sub.2]NPs preparation with improved phototocatalytic activity in visible range and the investigations on self-cleaning properties on treated leather surface under visible and UV light exposure are reported as an innovative step forward for smart leather product development.

Experimental

Synthesis of doped titania nanopowders

All used chemicals, namely, urea (C[H.sub.4]O[N.sub.2]), hydrazine hydrate ([N.sub.2][H.sub.4]x[H.sub.2]O]), iron chloride (Fe[Cl.lsub.3] x 6[H.sub.2]0), and methylene blue (Ci6H18C1N3S-3H20), were of analytical grade and supplied by Sigma-Aldrich. Ti[O.sub.2] nanopowder of 99.5% purity was supplied by TitanPE Technologies, Inc., China (particle size of max. 20 nm, surface area of 115 [m.sup.2]/g and average pore size of 130 Å).

N-doped Ti[O.sub.2] was prepared using a two-step process involving (i) a thermal treatment of a mixture of Ti[O.sub.2] and urea in 1:2 molar ratio at 450°C for 4 h, (23) followed by (ii) a treatment of the resulted nanopowder through immersion in hydrazine hydrate 60% solution for 12 h. The dispersion was then subjected to centrifugation and the solid product was air dried at 110°C. Finally, yellowish nitrogen-doped Ti[O.sub.2] powders were formed. (24)

Fe-doped Ti[O.sub.2] and Fe-N-doped Ti[O.sub.2] were obtained through addition of 5% Ti[O.sub.2] and N-doped Ti[O.sub.2], respectively, in a solution of 1 M Fe[Cl.lsub.3] x 6[H.sub.2]0, followed by stirring on a magnetic plate for 2 h, at a temperature of 70°C. Then the dispersion was subjected to centrifugation and the obtained powder was washed with deionized water up to neutral pH and then dried for 6 h.

Leather surface treatment

Sheepskin leather surfaces were finished according to the classical technologies (25) by spraying a base coat of compact acrylic polymers and a topcoat of nitrocellulose lacquer emulsion (Table 1). The 4% Ti[O.sub.2]NPs were integrated by mechanical stirring with 1:1 polyethylene glycol (PEG 400) as dispersing agent followed by 10 min ultrasound mixing with 100 mL base coat composite based on acrylic film-forming polymers and white pigment paste based on macro-Ti[O.sub.2] (commercial rutile-based Ti[O.sub.2]). The control sample was manufactured with the same composition and technology without Ti[O.sub.2]NPs. The simulation of leather surface soiling with organic materials was done by using MB dye and ball pen ink. As compared to the previous research, (25) the nanoparticles were embedded in the base coat (Table 1), not in the fixing coat, and the concentration of doped nanoparticles was significantly reduced (4% doped NPs reported to the base coat weight as compared to up to 100% Ti[O.sub.2]NPs reported to nitrocellulose lacquer, wt%).

Characterization methods

Characterization of doped Ti[O.sub.2]

To obtain information on the composition and structure of the nanosized-doped Ti[O.sub.2] powders, X-ray diffraction (XRD) was carried out involving a Bruker AXS D8 ADVANCE diffractometer. From the full width at half-maximum of the diffraction pattern, the crystal sizes can be calculated by using Scherrer's equation:

d = 0.9 ?/ß cos ?'

where ? is X-ray wavelength, ? the diffraction angle, and ß is the half width at half height for the diffraction peak. The morphology and the composition were evidenced by SEM/EDX measurements, with Quanta 200 FEI equipment. UV-Vis diffuse reflectance absorbance spectra (DRS) were measured with a JASCO V 570 spectrophotometer, equipped with integrating sphere accessory with the wavelength ranging from 300 to 600 nm. The y-axis in the spectrum was shown in terms of absorbance. BaS[O.sub.4] was used as standard for these measurements.

Photocatalytic activity of the doped and undoped Ti[O.sub.2]NPs

Photocatalytic activity of doped and undoped Ti[O.sub.2] was evaluated by investigating degradation of a dye, namely methylene blue (MB) ([?.sub.max] = 668 nm) as model pollutant. As UV light, a lamp-type VL 204, E 2116, with irradiation at 365 nm and light intensity of 1.35 mW/c[m.sup.2] was used and as visible light, a 500 W halogen lamp with average light intensity of 8 mW/ c[m.sup.2]. The distance between the light and the suspension was kept at 20 cm.

The experiments were carried out in a Berzelius glass with 50 mL of 20 ppm MB solution and a catalyst content of 0.1%, under magnetic stirring. The suspensions were first stirred in the dark for 30 min to reach equilibrium sorption of the dye and the MB-recorded visible and UV absorptions represent the initial reference values. Aliquot samples of 3 mL were taken at determined time intervals (1 h) and filtered through 0.1-pm membrane filters to remove the TiQ2 particles.

Photocatalytic and self-cleaning testing on leather surface

Leather surface of samples and control were stained with 0.01 mL MB solution of 200 ppm concentration and were marked with a ball pen ink line. The Ti[O.sub.2]NPs dispersion on leather surface and surface morphology were evaluated by SEM-EDX analyses (FEI Quanta 200) in comparison to the control sample. The leather surfaces were subjected to UV and visible light exposure at 20 cm and were monitored for selfcleaning properties with Datacolor Check II Instrument with color management software interface (Datacolor), based on CIELab color coordinates. The ball pen ink lines were visually analyzed over time and photographic images were collected.

The surface hydrophilicity was determined by dynamic contact angle measurements using a contact angle analyzer (VGA Optima XE). A sessile drop method was used for contact angle measurements, and the volume of a water droplet in each measurement was 3 µL. The contact angles of water droplet on treated and untreated leather surface after 1 and 2 h of UV and Vis irradiation and after 1-h rest in the dark were recorded.

Results and discussion

Structural and morphological characterization

As previously shown, to enhance photocatalytic activity of titania nanopowders, doping and co-doping them with nonmetallic elements and/or transitional metals are usually recommended.

Figure 1 presents the recorded XRD patterns of N-doped Ti[O.sub.2], Fe-doped Ti[O.sub.2], and Fe-N-doped Ti[O.sub.2], respectively, as compared to undoped Ti[O.sub.2]. It can be seen that the pattern of the prepared samples can be indexed to Ti[O.sub.2] in the anatase phase only (JCPDS 211272). The strongest peak at 2? = 25.3° is representative for (10 1) anatase phase reflections. The ascalculated crystal sizes were between 15.7 and 24.7 nm. The difference in crystal size may be ascribed to the different preparation conditions which affect the crystal development. (26)

SEM micrographs of the doped and undoped Ti[O.sub.2]NPs (Fig. 2) suggested that the nanoparticles are approaching spherical shape, with particle agglomeration in the case of N-Ti[O.sub.2]NPs and Fe-N-Ti[O.sub.2]NPs.

Elemental identification of the prepared doped Ti[O.sub.2] samples was performed using energy-dispersive X-ray spectroscopy (EDX). The spectra, shown in Fig. 3, revealed the presence of Ti, O, and N in N-Ti[O.sub.2]NPs (a) and of Ti. O, N, and Fe in Fe-N-Ti[O.sub.2]NPs (b).

Optical properties of doped Ti[O.sub.2] nanopowders

Doping and co-doping with N and Fe were performed to decrease the band gap energy ([E.sub.g]) by red shifting with respect to the band gap for bulk Ti[O.sub.2] anatase of 3.23 eV. In order to dope Ti[O.sub.2]NPs with N, a two-step process was selected, ensuring a higher content of interstitial and substitutional N.

In our investigations, Fe doping was done through a chemical route by adsorption from 1 M FeCl3 solution at a temperature of 70°C, when a content of around 2.3% Fe was obtained. Data from the literature showed that higher visible light efficiency was achieved at lower concentrations of Fe in Ti[O.sub.2] due to the fact that [Fe.sup.3+] ions actively trap and transfer both photogenerated electrons and holes to the surface of titania at lower level of doping, while at higher concentration these ions serve as recombination centers.7-8

A relevant characterization of doped Ti[O.sub.2]NPs is provided by UV-Vis diffuse reflectance spectra (DRS) recording. Figure 4 presents UV-Vis DRS for the prepared N- and Fe-doped Ti[O.sub.2] and Fe-N co-doped Ti[O.sub.2]. It should be noted that the shifting of the absorption peak toward visible region and the enlargement of absorption band (the so-called "tail" of the band) for all doped Ti[O.sub.2]NPs samples suggest an increase of photocatalytic activity in the visible range. The most pronounced increase of the absorption peak in visible domain is evidenced for Fe-N-Ti[O.sub.2]NPs, showing synergic effect of the doping elements. The presence of Fe is found to have a significant contribution, both for doped Fe-Ti[O.sub.2] and co-doped Fe-NTi[O.sub.2] nanopowders.

Photocatalytic activity of the doped Ti[O.sub.2]NPs

In order to assess the photocatalytic performance of the doped and undoped Ti[O.sub.2] samples under visible and UV light irradiation, the photo-degradation of MB as a model dye has been performed. The recorded absorbance spectra during photocatalytic degradation of a 20 ppm MB solution in the presence of 0.1% doped and undoped Ti[O.sub.2] under visible light and under UV light irradiation, respectively, are shown in Figs. 5a and 5b.

As shown in Fig. 5a, the highest decomposition rate is evidenced in the presence of N-Ti[O.sub.2]NPs, probably due to the N-doping induced intraband. As expected, undoped Ti[O.sub.2] presented the lowest decomposition rate in comparison with Fe-doped Ti[O.sub.2] and Fe-N codoped Ti[O.sub.2] which showed intermediate values.

It was reported that [Fe.sup.3+] is present in a substitutional position in the Ti[O.sub.2] lattice, introducing a dopant energy level into the Ti[O.sub.2] band gap so that the light absorption of Ti[O.sub.2] can be extended into the visible region. (27) [Fe.sup.3+] impurity level formed under the conduction band (CB) induces the high photocatalytic activities of iron-doped Ti[O.sub.2]. In the case of N-doped Ti[O.sub.2], the localized dopant levels near the valence band (VB) are responsible for the enhancement of photoactivity. In the case of co-doped catalysts, the N 2p acceptor states contribute to the band gap narrowing by mixing with O 2p states combined with the overlapping of the conduction band by the iron "d" orbital, resulting in improvement of the photo-performance under visible light irradiation. (28)

Under UV illumination, strongly oxidizing holes of Ti[O.sub.2] and hydroxyl radicals (HO*) act as very good oxidizing agents, as seen in Fig. 5b, where Ti[O.sub.2] has the highest decomposition rate. Ultraviolet absorption in Ti[O.sub.2] is due to its wide band gap of 3.2 eV and the band narrowing enables it to extend the absorption edge into the visible region, and for the N-doped Ti[O.sub.2]-x[N.sub.x], increased photocatalytic activity is due to the mixing of N-2p and 0-2p orbitals, as considered by Asahi et al. (29) On the other hand, Ihara et al. suggested that not only nitrogen in the lattice but also the oxygen vacancies and electronic levels slightly below the conduction band edge were responsible for the visible light response. (30) Even if some authors have shown that doping with [Fe.sup.3+] may lead to an increase in the rate of recombination between photogenerated electrons and hole, (31) the majority of research papers exhibited that iron and nitrogen are together incorporated into Ti[O.sub.2] to more effectively narrow the band gap, to promote the separation of the photogenerated electrons and holes and to accelerate the transmission of photocurrent carrier. (32) The photocatalytic reaction efficiency depends on the competition between the surface charge carrier transfer rate and the electron-hole recombination rate. It was found that [Fe.sup.3+] doped can enhance photocatalytic activity, which is because [Fe.sup.3+] ions can act as traps for the photogenerated electrons and holes, and inhibit the electron-hole recombination. (33) In our experiments, all the doping catalysts show higher photoactivities than undoped Ti[O.sub.2]NPs under visible light irradiation and the synergistic effects of the co-doping contribute to the enhancement of the photocatalytic activity.

Self-cleaning characterization of the treated leather

Leather surface characterization

The treated leather surface samples were analyzed with SEM-EDX to detect the presence of doped Ti[O.sub.2]NPs in the binder composite of surface finishing, as illustrated in Fig. 6. EDX spectra evidenced the presence of Ti and O (the other elements in the spectrum are attributed to the other binder finishing components). The doping elements, e.g., N and Fe, were not evidenced due to the small amount of the used doped Ti[O.sub.2]NPs, so that their presence was below the detection limit of the equipment.

Leather surface with self-cleaning properties induced by doped Ti[O.sub.2]NPs

Surface of leather samples, functionalized with NPs, and control, stained with 0.01 mL MB solution of 200 ppm concentration and marked with ball pen ink line, irradiated with UV and visible light, were analyzed to evidence their self-cleaning properties.

The ball pen ink lines were visually analyzed in time and photographic images were collected.

MB STAINS PHOTO-DEGRADATION MEASURED BY [CIEL.sub.AB] technique: To analyze the degradation of the MB dye on leather surface, under UV and visible light irradiation, by watching the color change, L* (lightness), a* (red-green color), b* (yellow-blue), C* (chroma), and h* (hue angle) parameters were recorded. The CIELab system enables an approach to the changes of colors and the perceptible and overall color impression of a color depends on the relative amount of red and yellow color, which is expressed as an angle of hue in the CIELab color space system. (34)

From the experimental data, it appeared that L* and h* parameters are more appropriate to describe MB color degradation on the leather surface. Comparative representations of the ?L* evolution in time, measuring the discoloring of MB stain, are represented in Fig. 7a under visible light and Fig. 7b under UV light. The color nuance variations, expressed in terms of hue angle, in degrees, for the two types of light exposure are illustrated in Figs. 7c and 7d.

According to Fig. 7a, the higher degradation of the MB under visible light irradiation is evidenced for the treated leather using Fe-N-Ti[O.sub.2] and Fe-Ti[O.sub.2] photocatalysts. This behavior suggests that titania doping and especially co-doping enhances the photocatalytic response under visible light exposure; N-Ti[O.sub.2]NPs also provide a better photoactivity as compared to undoped Ti[O.sub.2]NPs. Under UV light irradiation (A = 365 nm), as shown in Fig. 7b, Ti[O.sub.2]NPs and Fe-N co-doped Ti[O.sub.2]NPs exhibit the highest photo-degradation activity, followed by Fe-Ti[O.sub.2]NPs and N-TiOzNPs. These results are in accordance with the theory that the energy of the visible light is not sufficient to excite electrons from the valence band of the Ti[O.sub.2] anatase, due to its large band gap, but under UV light, the energy of the photons is high enough for Ti[O.sub.2] activation. The higher lightness (L*) values for FeN-Ti[O.sub.2]NPs as compared to those of undoped Ti[O.sub.2]NPs, under visible light, is due to the increased absorption, also evidenced in the recorded UV and visible spectra (Fig. 4). After 60 h of exposure under visible light, the lightness (L*) of MB spot increased from 52.95 to 85.61 in the case of leather surface treated with Fe-N/Ti[O.sub.2]NPs, close to the initial L* value of the substrate (85.15), and only from 53.17 to 70.5 for the leather treated with undoped Ti[O.sub.2]NPs. Under UV light, all L* values are lower than those recorded in visible light which represents an important practical advantage for consumer goods.

The h parameter, showing the change of color shades (in this case from dark blue, due to MB, to the leather tone), decreased from 232.88° to 72.78° for FeN/Ti[O.sub.2]NPs under visible light and suggests complete dye degradation (see Figs. 7c and 8). Under UV light (see Fig. 7d), all h values are higher than in visible light, leather surfaces treated with Fe-N/Ti[O.sub.2]NPs and Ti[O.sub.2]NPs showing better degradation of MB spots. It may be concluded that the leather surface treatment using Fe-N co-dopants provides the highest photoactivity, both under visible and UV radiation, due to their synergistic effect.

Photographic images: The surface properties of the treated leather stained with 0.01 mL of 200 ppm MB and ball pen ink lines were analyzed visually, over time. Figures 8, 9, 10, and 11 present photographic images of the leather samples treated with Fe-NTi[O.sub.2]NPs, Fe-Ti[O.sub.2]NPs, N-Ti[O.sub.2]NPs, and Ti[O.sub.2]NPs embedded in film-forming polymeric composite, subjected to visible and UV light illumination. The experiments confirmed the photocatalytic activity increasing under visible light exposure in the case of doped NPs on leather surface as compared to undoped NPs.

As shown in Fig. 8, after 30 h under visible light exposure, both MB dye and ball pen ink line are completely degraded for the leather sample treated with Fe-N-Ti[O.sub.2]NPs-based composite, showing a very efficient photocatalytic activity. Under UV light irradiation, the degradation is incomplete, even after 60 and 100 h. Leather sample treated with Fe-Ti[O.sub.2]NPs-based composite (Fig. 9), after 30 h. under visible light illumination, presents a total degradation for MB spot and a partial degradation for the ball pen ink line. In UV light, a good degradation is observed after 100 h. The results are in good agreement with DRS presented in Fig. 4, where the absorption peaks for Fe-TiCLNPs and more for Fe-N-Ti[O.sub.2]NPs are shifting in visible area of spectrum. According to Fig. 10, the leather treated with N-Ti[O.sub.2]NPs-based composite, after 30 h of visible light illumination, showed a good degradation for MB spot, but not for the ball pen ink line. Under UV light, a better degradation can be observed for the ball pen ink line, in agreement with Fig. 4, where N-Ti[O.sub.2]NPs have a higher absorption than Ti[O.sub.2]NPs both in UV and in visible light. In the case of the leather sample treated with Ti[O.sub.2]NPs-based composite (Fig. 11), in visible light, after 30 h, MB spot is faded, but not the ball pen ink line; in UV light, after 100 h, MB spot is almost degraded, but not the ball pen ink line. It can be noticed that on the leather surface, in a thin layer, the photocatalytical behavior of doped Ti[O.sub.2] is slightly different than that in solution (Figs. 5a and 5b). Control samples consisting of classical finished leather surface without NPs (Fig. 12) have shown very small modifications over time of the MB spot and no change in ball pen ink line.

Photo-induced hydrophilicity of treated leather surface

The hydrophilic/hydrophobic behavior of the treated leather surface was assessed taking into consideration the evolution of WCA against the type of doped Ti[O.sub.2]NPs under UV and visible light illumination in dynamic conditions (Figs. 13 and 14).

As shown in Fig. 13, in the case of treated leather involving Fe-N-Ti[O.sub.2]NPs subjected to UV radiation, a decrease of WCA after 1 h is noticed, from 30° toward 10°. Then, the WCA value remained quite constant. The same tendency may be evidenced when NTi[O.sub.2]NPs or Fe-Ti[O.sub.2]NPs were applied, but with slightly higher values of WCA.

Under visible light irradiation (Fig. 14), contact angle for the treated leather surface using Fe-NTi[O.sub.2]NPs drops to 0° after 1 h, remains constant after 2 h, and increases slightly after 1-h rest in the dark. It should be remarked that Fe-N-Ti[O.sup.2]NPs gives hydrophilicity to the leather surface both in UV light and visible light. Results are in accordance with other studies that highlight the presence of Fe-dopant which increases hydroxyl groups content on the surface and which, in the case of thin films, can serve not only as a recombination center but also as a mediator of interfacial charge transfer. (21) The results showed a synergistic effect of Fe and N co-dopants on increasing photo-induced hydrophilicity on the leather surface which is in accordance with the literature data. (32,34,35)

The leather surfaces coated with doped NPs integrated in base coat polymer composites showed higher hydrophilicity as compared to undoped NPs under UV and visible exposure. The hydrophilicity surface exposed to the visible light was increased in the case of doped NPs which represents a major practical advantage taking into consideration that the UV light represents just 4-6% of visible light. The better activity of undoped nano-Ti[O.sub.2] under UV light (contact angles drop to 22°, as compared to visible light exposure, when WCA decreased to 34°) is attributed to the nitrocellulosic topcoat which can shield the photocatalytic activity under visible light. The use of reduced quantities of doped NPs in finishing composites with photocatalytic activity in visible light is the main advantage of the present research.

The study of smart leather surfaces through self-cleaning properties under the influence of doped NPs opens the way for further applications and innovation in the area of consumer goods. Leather products with self-cleaning properties have improved durability and allow the reduction of carcinogenic solvents used for dry cleaning and maintenance with favorable ecological and economical impact.

Conclusions

The experimental results showed that Ti[O.sub.2]NPs doping using N and Fe determined an improved photocatalytic activity in visible light region. The recorded DRS for the doped Ti[O.sub.2] powders involving N and Fe evidenced the shifting of the absorption peak toward visible region and the enlargement of absorption band. The synergistic effects in co-doped catalysts contribute to the enhancement of photocatalytic activity. All the doping catalysts showed higher photoactivity as compared to undoped Ti[O.sub.2]NPs under visible light irradiation. Photocatalytic self-cleaning leather has been prepared using doped TiO?NPs-based polymeric composites. The self-cleaning properties were confirmed by colorimetric measurements using CIELab parameters for the methylene blue spots applied to the treated leather surface exposed to UVA (? = 365 nm) and visible light irradiation. Leather treated with Fe-N co-doped Ti[O.sub.2]NPs presents the highest activity for methylene blue stains degradation under visible light irradiation. Water contact angle measurements showed the photo-induced hydrophilicity phenomena for the leather surface. This aspect is important in maintaining humidity and, therefore, the presence of oxidative species, which is responsible for the self-cleaning effect.

The original finishing leather surface with smart properties was developed by integrating in the base coat the doped and co-doped Ti[O.sub.2]NPs with selfcleaning properties under visible light exposure opening the way for further research with applicative potential for large consumer products.

DOI 10.1007/s11998-015-9711-2

A. Petica, C. Gaidau, M. Ignat (53), C. Sendrea INCDTP-Leather and Footwear Research Institute Division, 93, Ion Minulescu, 031215 Bucharest, Romania e-mail: madalina.fleancu@yahoo.com

A. Petica e-mail: aura_p25@yahoo.com C. Gaidau

e-mail: carmen_gaidau@hotmail.com L. Anicai Center of Surface Science and Nanotechnology, POLITEHNICA University of Bucharest, Splaiul Independentei nr. 313, 060042 Bucharest, Romania

Acknowledgments The present work was supported by a Grant of the Romanian National Authority for Scientific Research, CNDI-UEFISCDI, Project Number 167--SELFPROPIEL.

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Table 1: Classical technology for leather surface finsihing with
film-forming polymers

Leather Chemical product Concentration Application
surface
coatings
and
products

Base coat

Roda base Water emulsion 250 mLA 4 coatings by gun
5786 of acrylic resins spraying with
 and filler intermediary free
 drying

Roda Casi Water-based 110 g/L Pressing at
white casein pigment 110°C and 20
pigment kg with leather
 ironing press 2
 coatings by gun
 spraying with
 intermediary free
 drying

Topcoat

Roda lac Water-based 850 mL/L 2 spray coatings with
93 nitrocellulose intermediary free
 drying and final
 pressing at
 110°C and 20
 kg with leather
 ironing press


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Author:Petica, Aurora; Gaidau, Carmen; Ignat, Madalina; Sendrea, Claudiu; Anicai, Liana
Publication:Journal of Coatings Technology and Research
Geographic Code:1USA
Date:Nov 1, 2015
Words:5676
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