Preparation of silica doped titania nanoparticles with thermal stability and photocatalytic properties and their application for leather surface functionalization

2.2.1 Inductively coupled plasma - optical emission spectrometry (ICP-OES)

Analytical method for determining Si dopant was based on inductively coupled plasma-optical emission spectrometry (ICP–OES), according to ASTM E 1479-99(2011) standard. Agilent 725 ICP-OES system (Agilent technologies, USA) was used. Plasma temperature in analytic area was ∼8.000 K and the wavelength used by ICP-OES for Si quantification was 251.611 nm.

2.2.2 X-ray diffraction (XRD)

The X-ray diffraction (XRD) patterns of nanostructured powders were obtained using BRUKER D8 ADVANCE diffractometer with monochromatic Cu Kα radiation, Bragg-Brentano diffraction method, assisted by DIFFRACplus XRD Commender software. Scans were obtained in the 2θ range 4–74°, with a step size of 0.02° every 2 s. The crystallite size was calculated by Scherrer method for hydrothermal synthesized TiO₂ and SiO₂ doped TiO₂NPs.

2.2.3 High Resolution Transmission Electron Microscopy (HRTEM)

Morphostructural characterization was performed by High Resolution Transmission Electron Microscopy (HRTEM) with a Tecnai F30, G2S Twin (1 Å line resolution) device of FEI Company with 50–300 kV electron source and 230,000 magnification. The nanopowders were dispersed in ethanol by mechanical stirring for 15 min and sprayed onto copper EM grid percoated with carbon. Particle size measurements were performed through Digital Micrograph™, Gatan Inc., image acquisition and processing software.

2.2.4 Dynamic Light Scattering (DLS) technique

Zetasizer Nano ZS equipment (Malvern Instruments, UK) was used for particle size and Zeta potential measurements of selected nanopowders dispersed in aqueous suspensions, stabilized with 0.03% PEG 600.

2.2.5 FT-IR spectroscopy

Molecular structures of SiO₂ doped TiO₂NPs for selected nanoparticles were characterized using FT-IR spectroscopy. FT-IR spectra were recorded in the transmission mode on a ABB MB 3000 spectrometer equipped with EasiDiff device for powder analyses, between 4000 and 550 cm⁻¹ wave numbers, and consisted of 64 scans at 4 cm⁻¹ resolution.

2.2.6 UV-vis spectroscopy

UV-vis diffuse reflectance spectra (DRS) were measured with a JASCO V 570 spectrophotometer, equipped with integrating sphere accessory with the wavelength ranging from 300 to 600 nm. As standard for these measurements, BaSO₄ was used. DRS were performed in order to evaluate the shifting of absorption peak from UV to Vis range for selected doped nanoparticles as compared to the undoped nanoparticles.

2.3.1 Leather finishing with nanoparticle-doped composites

The sheepskin white crust leathers were finished by integrating the nanoparticles in the base coat mixture using polyethylene glycol (PEG 600, Romaqua SA) (Bitlisi and Yumurtas, 2008), by mechanical and ultrasound stirring (Petica et al., 2015) to get a good dispersion and by leather surface spraying. The control sample was processed with the finishing composite without nanoparticles using the same classical formula for base coat layer followed by nitrocellulose lacquer emulsion top coating (Table 1).

2.3.2 Characterization of leather properties

1. Self-cleaning properties under UV-vis light

The leathers were stained with 2 μL of methylene blue (C₁₆H₁₈ClN₃S·3H₂O) dye (MB) and were subjected to UV (VL 204 with irradiation at 365 nm) and visible (500 W halogen lamp) light exposure. The stain color modification was assessed over time by photography and color parameter measurement using DATA Color Check Plus II portable device assisted by CIELab color management software.

• Scanning electron microscopy (SEM) coupled with Energy Dispersive X-ray spectroscopy (EDX)

SEM-EDX with Quanta 200 FEI instrument was used to evaluate the leather coating appearance and nanoparticle uniform dispersion. A supplementary 3D image for the selected leather sample as compared to control sample was captured with a SEM type Phenom ProX device.

• Thermal properties

The thermal analysis of leathers treated with TiO₂-SiO₂NPs was assessed by Differential Scanning Calorimetry-thermal gravimetry (DSC-TG) with Setaram Setsys Evolution device in air atmosphere and alumina crucible. The heating rate was 20 °C/min in the temperature range 25–750 °C. Data analysis was performed with the Calisto software.

• Hydrophilic properties

The contact angle of water drop on leather surface was measured after 1 h, 2 h and 1 h rest after the UV and Vis exposure, with VGA Optima XE device.

3.1.1 Dopant composition

Dopant composition of TiO₂-SiO₂NPs expressed as Si concentration is presented in Table 2 and showed a range of hydrothermal nanopowders with theoretical concentrations of 2%, 5%, 10% and 20% Si reported to whole sample weight and practical values of 1.8%, 4.3%, 9.2% and 17.2% Si. The used hydrothermal method allowed the synthesis of TiO₂NPs and the successful loading of different quantities of Si in a single step, with controlled morphology and sizes.

3.1.2 X-ray diffraction (XRD)

The investigation of hydrothermal synthesized TiO₂-SiO₂NPs allowed identification of the crystallite structure and sizes and the results are shown in Table 2. The anatase structures were generated through hydrothermal synthesis route with crystallite sizes of 16.2–16.7 nm for TiO₂-2SiO₂NPs and TiO₂-5SiO₂ NPs, as compared to 15 nm, the size of TiO₂NPs. Crystallite sizes of TiO₂-10SiO₂NPs and TiO₂-20SiO₂NPs were of 16.8 nm and 18.1 nm showing a significant modification of crystallite phase as compared to the other samples, due to the influence of the optimum dopant concentration able to suppress phase transformation, in agreement with the literature data (Cheng et al., 2003).

X-ray diffraction spectra of TiO₂, TiO₂-2SiO₂NPs and TiO₂-5SiO₂NPs nanostructured powders, respectively, are depicted in Fig. 1a–c, and showed a predominantly anatase structure (91.5–90.1%) with low rate of brookite crystalline phase (7.1–8.5%).

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Figure 1

XRD spectra of (a) TiO₂; (b) TiO₂-2SiO₂; (c) TiO₂-5SiO₂; (d) TiO₂-10SiO₂; (e) TiO₂-20SiO₂ nanostructured powders.

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XRD spectra of TiO₂-10SiO₂NPs and TiO₂-20SiO₂NPs samples are depicted in Fig. 1d–e and showed a major anatase phase and trace amounts of halite (NaCl) as a side product of the hydrothermal reaction. The crystallite sizes of hydrothermally synthesized TiO₂-SiO₂NPs were similar to sol-gel silica-doped titania photocatalyst after the thermal treatment at 800 °C with values of 12 nm and with values of 23 nm after the treatment at 950 °C, but with different compositions in silica concentration and crystallite structures (Cheng et al., 2003).

3.1.3 High-resolution transmission electron microscopy (HRTEM)

Small anatase crystallites demonstrated by interplanar distances of 3.52 Å and Miller indices (101) were identified in the case of TiO₂ and TiO₂-10SiO₂NP samples (Fig. 2a and b). The crystallite sizes of hydrothermally synthesized TiO₂NP were in the range 8.8–27 nm, while in the TiO₂-20SiO₂NP sample they were between 2.7 nm and 3.5 nm (Fig. 2c) due to the constraints of the SiO₂-based amorphous matrix. The other explanation of smaller particle sizes is the role of silica in TiO₂ crystallization process delay with effect on lowering the crystal size (Calleja et al., 2008). Well-developed morphology of the primary nanocrystallites under hydrothermal conditions can explain the enhanced photocatalytic properties according to other authors (Cho et al., 2003).

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Figure 2

HRTEM image of (a) TiO₂NPs; (b) TiO₂-10SiO₂NPs; (c) TiO₂-20SiO₂NPs.

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3.1.4 Dynamic light scattering (DLS)

The aqueous dispersions of TiO₂-10SiO₂NPs and TiO₂-20SiO₂NPs made in the same conditions showed higher particle sizes for the major percent of nanoparticles of 64 nm (Fig. 3a) and 72 nm (Fig. 3b), respectively. The stabilities of TiO₂-10SiO₂NPs and TiO₂-20SiO₂NPs dispersions were high, with Zeta potentials of −53.6 mV (Fig. 4a) and −52.9 mV (Fig. 4b) which represent the premises for well dispersed NPs and stable leather finishing composite preparation. The influence of silica in hindering the agglomeration of TiO₂ domains in TiO₂-SiO₂ materials was explained through silica role in titania interface stabilizations (Anderson and Bard, 1997).

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Figure 3

Size distribution of (a) TiO₂-10SiO₂NPs in aqueous dispersion and (b) TiO₂-20SiO₂NPs in aqueous dispersion.

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Figure 4

Zeta potential of (a) TiO₂-10SiO₂NPs in aqueous dispersion and (b) TiO₂-20SiO₂NPs in aqueous dispersion.

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3.1.5 FT-IR spectroscopy

The comparison of FT-IR spectra of TiO₂-SiO₂NPs showed that for higher concentration of SiO₂ the intensity of the bands corresponding to the deformation vibration of Si-O-Si (1050–1066 cm⁻¹, 750–718 cm⁻¹) and Si-O-Ti (950–910 cm⁻¹) increased, which means that Si penetrated the TiO₂ network and a complex compound of TiO₂ and Si was generated (Fig. 5a and b). The increase of Si-O-Ti bonds with Si dopant concentration acts as efficient barriers against sinterization and growth of TiO₂ particles and explains the low particle size identified in TEM analyses as compared to undoped TiO₂NPs (Jung et al., 2001). The other studies showed that silica promotes the Si-O-Ti bonds and oxygen vacancies in titania structure with effect on enhancement of photocatalytic properties (Cheng et al., 2003). The inclusion of silica in the titanium crystalline network creates active defects with influence on catalytic photoactivity (Yamashita et al., 1998).

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Figure 5

FT-IR spectra for (a) TiO₂-10SiO₂NPs and (b) TiO₂-20SiO₂NPs.

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3.1.6 UV-vis diffuse reflectance spectra (DRS) of doped TiO₂ nanoparticles

10% and 20% silica doped TiO₂NPs along with undoped TiO₂NPs were subjected to DRS measurements in order to compare the photocatalytic efficiency of these samples. The absorption shift in visible range in the case of 10% silica dopant concentration can be attributed to the lowering of recombination rate of exited electron-hole pairs of TiO₂NPs, with effect on the higher photoadsorption of O₂ and the possibility to generate improved self-cleaning properties (Aguado et al., 2006). In our study we found that the concentration of silica is important for photoactivity enhancement of TiO₂NPs and 20% silica dopant can cover the reactive sites with no improved effect in visible light in a similar manner for other reported sol-gel binary oxides prepared for organic compound photo decomposition (Jung et al., 2001).

Fig. 6 presents the enhancement of absorbance recorded for TiO₂-10SiO₂NPs, which suggests the highest photocatalytic reactivity upon UV-vis light (Wei et al., 2008).

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Figure 6

DRS of TiO₂NPs, TiO₂-10SiO₂NPs and TiO₂-20SiO₂NPs.

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3.2.1 Self-cleaning properties under UV-vis light exposure

Based on the results obtained from DRS analysis, TiO₂-SiO₂NPs were further selected for testing self-cleaning properties. The MB color modification under UV-vis exposure was surveyed over time as compared to the control sample by photography (Table 3) and photocolorimetry measurement (Fig. 7). The lightness difference (ΔL^∗) of MB stain measured at initial time and after UV and visible light exposure was higher for leather surface treated with TiO₂-SiO₂NPs and the self-cleaning effect was faster under visible light exposure. Table 3 shows that after 60 h of exposure at UV light, the MB stains of the leather surfaces treated with TiO₂-10SiO₂NPs were the most discolored.

Table 3 The self-cleaning properties of leather surface under UV light exposure (L-leather).

Control	L-TiO2NPs	L-TiO2-5SiO2	L-TiO2-10SiO2	L-TiO2-20SiO2
Initial stained
After 60 h of UV light exposure

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Figure 7

Self-cleaning of leather surface under UV exposure.

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Under visible light, the degradation of MB stain started after 3 h of exposure, and became evident in 5 h and after 30 h of exposure all leather surfaces treated with doped nanoparticles were cleaned as compared to control sample and to the sample treated with undoped TiO₂ nanopowder (Table 4 and Fig. 8). The influence of silica doping concentration showed that neither the low concentration (TiO₂-5SiO₂NPs), nor the too high silica concentration (TiO₂-20SiO₂NPs) is efficient, probably due to the low influence on titania electron-gap transfer at low concentration and to the phase separation at high concentration (Cheng et al., 2003). The covering of TiO₂NPs with SiO₂ in the case of TiO₂-20SiO₂NPs can hinder the accessibility of the MB stain with the active site of the titanium oxide species, resulting in a decrease in the photocatalytic reactivity of the catalysts (Yamashita et al., 1998).

Table 4 The self-cleaning properties of leather surface under visible light exposure (L-leather).

Control	L-TiO2	L-TiO2-5SiO2	L-TiO2-10SiO2	L-TiO2-20SiO2
Initial stained
After 3 h of visible light exposure
After 30 h of visible light exposure

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Figure 8

Self-cleaning of leather surface under visible light exposure.

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3.2.2 Thermal behavior of leather surface treated with doped TiO₂ nanoparticles

It is known that doping of TiO₂ with SiO₂ improves flame retardant properties; therefore, thermal resistance of leather treated with TiO₂-SiO₂NPs was investigated using DSC-TG analysis up to 750 °C.

The investigation of thermal resistance of leathers treated with TiO₂-SiO₂NPs clearly showed a delay in leather sample decomposition (higher temperature for the first two stages and last stage), a supplementary stage of decomposition (corresponding to the TiO₂-SiO₂ compound formation assisted by oxy and hydroxy species) and a reduced total mass loss (Table 5).

3.2.3 Degradation kinetics study of leather treated with doped TiO₂ nanoparticles

Thermal degradation kinetics study of leather samples treated with TiO₂-SiO₂NPs was performed by isoconversional analysis using Kissinger method, because the method is considered to be independent of the type of analyzed process (Brown et al., 2001). This method is based on the following equations: $$f(\alpha )={(1-\alpha )}^n$$ $$d\alpha /\mathit{dT}=A/\beta \text{exp}(-E/\mathit{RT}){(1-\alpha )}^n$$ $$\frac{A}{\beta }\text{exp}\left(-\frac{E}{{\mathit{RT}}_{\text{max}}}\right)n{(1-\alpha )}^{(n-1)}=\frac{E}{{\mathit{RT}}_{\text{max}}^2}$$ where α – transformation degree; n – reaction degree; E – activation energy; R – Gas constant; T – absolute temperature (K); β – constant heating rate.

A plot of ln (β/T²max) versus 1/Tmax for a series of experiments at different heating rates is a line with slope –E/R, so we can calculate the activation energy E of the thermal process taking place. A Mathcad subroutine was used.

The DSC-TG curves for leather treated with TiO₂-SiO₂NPs compared with untreated leather are presented in Figs. 9a–9e.

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Figure 9a

DSC-TG curve of control sample (untreated leather).

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Figure 9b

DSC-TG curve of L-TiO₂-2SiO₂.

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Figure 9c

DSC-TG curve of L-TiO₂-5SiO₂.

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Figure 9d

DSC-TG curve of L-TiO₂-10SiO₂.

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Figure 9e

DSC-TG curve of L-TiO₂-20SiO₂.

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Thermal analysis has highlighted several endothermic peaks which could be assigned to the following: dehydration of leather and oxide (peak 1), melting/softening of crystalline phase from collagen/leather (peak 2), decomposition of organic compounds from leather/collagen (peak 3), accompanied by a phase transformation probably due to the formation of complex compounds based on Ti and Si oxy-hydroxides (peak 4). The last peak appears only in the case of doped nanooxides.

Samples based on TiO₂NPs or TiO₂-SiO₂NPs lead to a slight increase in the temperatures corresponding to thermal effects assigned to the first two peaks. Also, a shift of peak 1 to higher temperatures is observed, which could be explained by the increase in thermal stability of the samples treated with TiO₂NPs or TiO₂-SiO₂NPs. Also, total mass loss diminishes in the case of leather treated with oxide materials (TiO₂-SiO₂) compared with the control (untreated leather).

Fig. 10 represents Kissinger lines of peak 3, assigned to decomposition of organic compounds from leather (collagen and organic chemical auxiliary materials used for leather processing). Calculated activation energies are presented in Table 6. An increase of about 3 times can be seen for the activation energy of the decomposition process for the treated leather when Si concentration increases from 5% to 10–20%. This effect can be explained by the increasing absorbed energy with SiO₂ transformation from doped nanooxides.

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Figure 10

Kissinger graphs: (a) untreated leather; leather treated with: (b) L-TiO₂-2SiO₂; (c) L-TiO₂-5SiO₂; (d) L-TiO₂-10SiO₂ and (e) L-TiO₂-20SiO₂.

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3.2.4 Dynamic contact angle of water on leather surface

The photocatalytic processes under UV and visible light were identified in a dynamic test regarding the generation of hydroxyl species and hydrophilic surface after 1 h, 2 h and 1 h rest of UV and visible exposure. The water contact angle measured for leather surface treated with TiO₂-10SiO₂NPs in comparison with leather surface treated with TiO₂NPs presented in Fig. 11 showed a hydrophilic surface generation after 1 h exposure to visible and UV light. The hydrophilicity was more evident after 1 h of visible light exposure with preservation of the characteristics after 2 h and 1 h rest as compared to initial state and to untreated leather surface. This behavior explains the photocatalytic generation of reactive species such as hydroxyl radicals (^•OH) and superoxide anions (^•O₂⁻) with self-cleaning properties against organic stains on leather surface.

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Figure 11

Contact angle of water on leather surface exposed to UV and visible light.

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The efficient self-cleaning properties on leather surfaces are connected to the photoreactivity of nanoparticles and to the surface morphology which was investigated by SEM-EDX.

3.2.5 Scanning electron microscopy (SEM) coupled with energy dispersive X-ray (EDX)

The doping element of TiO₂-10SiO₂ and TiO₂-20SiO₂ NPs was identified by EDX on leather surface (Fig. 12a and b). The uniform distribution of nanoparticles on leather surface is a key factor for both photocatalytic self-cleaning properties and thermal resistant surface and can be seen in Fig. 13 in comparison with the control sample.

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Figure 12

EDX analyses of Si doping element for (a) TiO₂-10SiO₂NPs and (b) TiO₂-20SiO₂NPs.

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Figure 13

SEM image of leather surface treated with film forming polymers and TiO₂-10SiO₂NPs (b and c) and TiO₂-20SiO₂NPs (d) as compared to the control sample (a and e).

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The visible light photoreactivity on doped TiO₂NPs surface and the appropriate silica concentration for both functions, photocatalytic reactivity enhancement and thermal resistance improving were achieved through TiO₂-10SiO₂NPs prepared by hydrothermal method. Leather surface coating with silica doped TiO₂NPs represents a progress in NPs application due to the development of self-cleaning and thermal resistance properties as compared to the results reported in this field (Bitlisi and Yumurtas, 2008; Petica et al., 2015; Xu et al., 2015).