Journal of Materials Research and Technology Journal of Materials Research and Technology
Original Article
Synthesis and enhanced visible-light activity of N-doped TiO2 nano-additives applied over cotton textiles
Elham Katouei Zadeh, Seyed Mojtaba Zebarjad, , Kamal Janghorban
Department of Materials Science and Engineering, Engineering Faculty, Shiraz University, Shiraz 7194685115, Iran
Received 04 December 2016, Accepted 20 May 2017
Abstract

To provide photocatalytic textiles, application of TiO2 nanoparticles by surface modifications during the manufacturing process is known as a reliable choice. In this study, nitrogen-doped TiO2 nanoparticles were synthesized by sol–gel route at two different water/triethylamine ratios and applied to the textiles which provides photocatalytic properties that unlike the conventional photocatalytic textiles, does not necessarily need UV radiations of high energy photons. N-doped TiO2 nanoparticles were coated over textiles during the synthesizing process. Microstructure and morphology of synthesized N-doped TiO2 nanoparticles were evaluated by XRD, PSA and SEM/EDS analysis. The results of XRD analysis indicated that the amorphous phase transformed slightly into an anatase crystallite without calcination at high temperature. The morphology confirmed that doping process had significant effect on the appearance of the synthesized nanoparticles and implied the effect of the presence of the N-doped source material on the morphology. The PSA analysis showed narrow distribution of about 15nm for diameter of synthesized N-doped TiO2 nanoparticles. According to UV–vis spectra, the band gap energy was measured 2.98eV which exhibits high absorption in visible light range due to its low band gap energy. The results show that adding nitrogen increases the absorption wavelength of the N-doped TiO2 nanoparticles and N-doped TiO2 coated textiles shows super hydrophilic behavior examined by DSA analysis. The photodegradation of methylene blue (MB) over textiles was investigated under UV-radiation, visible light and dark conditions. Super-hydrophilicity and methylene blue photodegradation properties with the most homogenous nanoparticle distribution over textiles were achieved without utilization of UV radiation.

Keywords
Sol–gel method, N-doped TiO2, Visible-light, Cotton textiles
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1Introduction

Nowadays, environmental pollution and shortage of energy resources has motivated researchers to pay much more attention to the materials with self-cleaning and photocatalytic behaviors [1–3]. For woven natural fibers, photocatalytic characteristics are usually applied by surface modifications during the manufacturing process. This property of cotton fabrics is widely being developed using novel sciences such as nanotechnology [4–6]. Among different potential materials, application of TiO2 nanoparticles is known as a reliable choice to provide photocatalytic properties considering its non-toxicity, extensive domain of performance, chemical and photo stability, lower cost, higher efficiency, etc. [7].

Photocatalytic behavior of TiO2 was discovered about four decades ago by Honda and Fujishima [8]. Later on, the mechanism of this behavior was investigated by other researchers. They found that the produced free radicals react with electrons and electron holes, showing photocatalytic activity as illustrated in Eq. (1)[9–11].

TiO2+=TiO2 (e, h+)
h++OH=°OH
e+O2=°O2

In fact, photocatalytic activity depends on the band gap energy of materials, particularly the formed free radicals. TiO2 with anatase and rutile phases has a band gap value of 3.2eV, and 3.0eV, respectively [12]. It is also clear that addition of different elements into TiO2 structure changes the material properties [13]. For example, nitrogen decreases the band gap energy, resulting in less energy to produce free radicals and enabling the photocatalyst to operate under visible light [14,15]. More recently, band-gap narrowing nitrogen doped TiO2 nanoparticles have been presented which provide improved photocatalytic properties in comparison with the conventional TiO2 nanoparticles [10]. In fact, the mechanism of band gap narrowing is due to the formation of intra bands from mixing of the 2p states of oxygen from titania with the 2p states of the dopant, which leads to band gap narrowing and facilitates the visible light absorption [16].

The structure, size, interaction between vacancies and interstitial or substitutional sites and photocatalytic activity of TiO2 depend strongly on the selected procedure and variable parameters during synthesis. Based on the literature survey conducted by the authors, TiO2 can be synthesized by several methods such as sol–gel, ion implantation, magnetron sputtering, oxidation of titanium alkoxides, etc.

In the last decade, sol–gel method was used for the synthesis and incorporation of nanoparticles over non-conductive textiles particularly at low temperatures [17,18]. This is because at high temperature the performance of natural fibers decreases drastically. Nitrogen can be doped by usage of urea [15], triethylamine (TEA) [19], NH4OH [20], aqueous solution of NH3[21] or various nitrogen sources like NH4Cl, NH3, NH4NO3, and N2H4[22].

One of the main reasons that have limited the previous studies on the functionalization of textiles using nitrogen-doped titania is the requirement of a high temperature, preferably above 300°C, for creating a coating layer of N–TiO2[23,14,24,25]. In most of the previous studies, the commercial titania (usually Degussa P25) was used which requires relatively high temperature for the diffusion of nitrogen to its crystal structure [24,25]. Clearly, this temperature can severely degrade the properties of the cotton textiles. In the current study, this problem was tackled by adjusting the synthesis specifications to produce nano-sized N-doped TiO2 at low temperature. This will make it possible to apply the N-doped TiO2 nanoparticles on the cotton textiles. It should be noted that in the current method nitrogen is added to the titania crystal structure during the titania synthesis process.

The aim of this research was to produce textiles with photocatalytic properties. The activation of this property needs UV radiations of high energy photons; therefore, the authors decided to synthesize N-doped TiO2 with also some modifications to lower its band gap energy. This enables the photocatalytic property under visible light radiations. Also as discussed previously, in order not to degrade the physical property of textiles the synthesis were conducted at low temperatures. Furthermore, the hydrophilicity and methylene blue degradation properties (photocatalysis) of the coated N-doped TiO2 nanoparticles over textiles under visible light were studied.

2Experimental procedure

For synthesizing nitrogen-doped titanium dioxide, titanium (IV) isopropoxide (TTIP) Ti(OCH(CH3)2)4 (Merck 98%), triethylamine N(CH2CH3)3 (Merck 99%), diluted nitric acid (Merck 25%), n-propanol alcohol (Merck 99.5%) and deionized water were used. Cotton textiles (100%) were decontaminated 20min in ammonia solution and were washed by ethanol and deionized water. Then, they were nitrogen purged (99.99%) to be dried completely.

The aqueous solutions were synthesized in multiple batches in accordance with different valid synthesis routes [26–28]. The solution of n-propanol alcohol to TTIP (95:5) was prepared and added drop wise to distilled water adjusted to pH=2 by nitric acid. After stirring the solution for 12h at low temperature (i.e. 2°C), triethylamine was added to the transparent solution. Consequently, the solution tends to be yellow in color. The investigated volume ratios of the tiethylamine:TTIP:water were 1.35:1:200 and 1.35:1:4. The final solution was dispersed by an ultrasonic homogenizer for few hours. Then, the textiles were dipped over petri dishes containing the synthesized solution for coating of nitrogen-doped titanium dioxide over them without using any binder. Dehydration was performed for 10h at 50°C and finally cured at 150°C for 5min in a preheated curing oven. TiO2 nanoparticles were also synthesized for comparison by the same route without addition of triethylamine. After dehydration of the synthesized solution and prior to X-ray analysis, the powders were washed by acetone and deionized water, filtered and grinded. The powder phase of synthesized nanoparticles, including N-doped and undoped titanium dioxide, were evaluated by X-ray diffraction (XRD) pattern with the scan rate of 0.05°/min (λ=1.54056Å) Cu Kα line radiation, 40kV and 40mA.

Elemental compositions of the N-TiO2 precursor nanoparticles were investigated using an in situ energy dispersive spectroscopy unit coupled with scanning electron microscopy (EDS/SEM VEGA-TESCAN) for a coated textile sample.

The ultraviolet–visible spectrophotometer (UV–vis) was conducted for plotting the absorbance spectrum. The spectrometer was calibrated by the field solution before each measurement to define the base line. The band gap energy (Eg) of TiO2 nanoparticles was calculated from Tauc equation and UV–vis spectra [29]. The absorption coefficient (α) depends on the film thickness (length of the absorption media) and absorbance, as given in Eq. (2):

where A is the absorbance and d is the thickness. The energy gap was estimated by the Tauc equation as shown in Eq. (3):
where B is a constant, α is the absorption coefficient, h is the Planck's constant and is the incident photon energy in eV [30].

The solutions were diluted using the base disturbant component and were sonicated for 10min before particle size analysis, performed by PSA LS-550.

For macroscopic evaluation, the textiles were coated by gold in sputtering chamber at low pressure (about 10−2MPa) to avoid accumulation of negative charge over the surface of the textiles. The micrographs of the synthesized nanoparticles and coated textile were obtained by scanning electron microscope (Leica Cambridge-S360) and stereographic microscope (Union UKZ-TR).

The hydrophilicity and the surface contact angle of textiles for self-cleaning activity were measured by a drop shape analyser (DSA100-Krüss). A needle with an approximate diameter of 1mm was adjusted over textile and the deionized water droplet of 0.5–1μL was dropped to the surface. The pictures were taken at the first moment of surface contact under visible light with atmosphere pressure and temperature.

Furthermore, the photoactivity of textiles coated by TiO2 and N-TiO2 nanoparticles was evaluated by measuring the photodegradation of methylene blue under UV, visible light radiation and dark condition. The degradation of methylene blue dye over textiles was monitored over time.

3Results and discussion3.1X-ray diffraction analysis

The structures of undoped TiO2 and N-doped TiO2 nanoparticles were characterized by XRD spectra as shown in Fig. 1. The intensity of diffraction peaks indicates the crystalline phase over the amorphous background. The most intensive diffraction peaks in the XRD patterns can be ascribed to the anatase crystal structure which was previously reported and confirmed by different researchers [26,31,32]. The results suggest that the amorphous phase transforms slightly into an anatase crystallite only by addition of nitrogen without calcination at high temperature. The slight peaks associated with the crystalline phase imply a mixture of amorphous and anatase phases of N-Ti-O in the structure. In contrast, TiO2 nanoparticles produced by the same heat treatment route as N-doped TiO2 are pure amorphous. This result proved that addition of nitrogen to the TiO2 decreases the crystallization temperature.

Fig. 1.
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X-ray diffraction patterns of TiO2 and N-TiO2 synthesized powders.

3.2Energy dispersive spectroscopy

Elemental compositions of the N-TiO2 precursor nanoparticles were investigated using an in situ energy dispersive spectroscopy unit coupled with scanning electron microscopy (EDS/SEM) for a coated textile sample. EDS microanalysis spectrum of the coated textile by N-doped TiO2 nanoparticles is shown in Fig. 2 and the percent of each elements is presented in Table 1. This reveals the presence of 0.35at.% nitrogen on the surface.

Fig. 2.
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EDS microanalysis spectra of the fibers coated by N-TiO2 nanoparticles.

Table 1.

The percentage of each elements in the EDS microanalysis.

Element  Series  Atom. C [at.%] 
Carbon  K series  45.23 
Nitrogen  K series  0.35 
Oxygen  K series  52.65 
Titanium  K series  1.77 
Total: 100.0%
3.3Ultraviolet–visible spectrophotometery

The ultraviolet–visible spectrophotometer (UV–vis) was conducted for plotting the absorbance spectrum. The UV–vis absorption results presented in Fig. 3 show that TiO2 nanoparticles have no absorption in the visible light range (400–700nm) while adding nitrogen to the TiO2 nanoparticles enhances the absorption in this range. This is in accordance with the results reported by Asahi et al. [10]. Excitations of electrons from valence band to conduction band of TiO2 needs high energies (UV range) while the absorption changes toward lower energies (visible range) in the case of N-doped TiO2 with respect to the pure TiO2 (UV range). This is due to the formation of intra band energy levels that require lower energies for excitation of electrons, which leads to band gap narrowing and facilitates the visible light absorption [16]. This confirms that the photo oxidation by N-TiO2 coated textiles increases their applicability.

Fig. 3.
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UV–Vis spectra of TiO2 and N-TiO2 solutions.

According to the UV–vis spectra, at 0.5 absorbance unit the absorption of TiO2 is about 350nm, covering the wavelength range of ultraviolent radiation (200–400nm), whereas the absorption of N-TiO2 is 500nm, which covers the visible light range (400–700nm). This is because activation of photocatalytic behavior is conducted without any need for high energy photons and the required band gap energy used for the formation of free radicals is provided by a conventional visible light lamp and adsorbed by the coated textiles.

The value of the energy gap (Eg) of TiO2 nanoparticles is calculated from Tauc equation [29]. Fig. 4 shows the plot of (αhν)2 vs in which Eg is determined by extrapolating the straight line portion of the spectrum to αhν=0. It can be also seen that the band gap energy of TiO2 is decreased from 3.2eV to 2.98eV after nitrogen doping.

Fig. 4.
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Determination of band gaps for TiO2 (a) and N-TiO2 nanoparticles from UV–Vis spectra.

3.4Particle size analysis

The particle size distribution (number vs. diameter) of the synthesized nanoparticles dispersed in the synthesized solutions is presented in Fig. 5. The results show that N-doped TiO2 synthesized solution with water/TTIP ratio of 4, contains nanoparticles with almost narrow distribution of mean diameter (about 15nm), whereas mean particle size diameter of N-doped TiO2 synthesized solution with water/TTIP ratio of 200 is about 25nm with wide distribution size. Moreover, the mean particle size diameter of undoped TiO2 solution is about 40nm.

Fig. 5.
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Particle size analysis of TiO2 and N-TiO2 solutions at different water/TTIP ratios.

3.5Scanning electron microscopy

Fig. 6a–f shows the SEM micrographs of the bare and coated fibers with TiO2 and N-TiO2 nanoparticles at different conditions. It can be seen that nanoparticles with spherical shape and colloidal average size of about 100nm are well coated over textile fibers.

Fig. 6.
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SEM micrographs taken from the textiles: (a) without coating, (b) coated with TiO2, (c) coated with N-TiO2 (water/TTIP: 200), (d–f) coated with N-TiO2 (water/TTIP: 4).

The observed morphology confirms that doping process has significant effect on the appearance of the synthesized nanoparticles which indicates the effect of nitrogen doping on the morphology. The N-TiO2 nanoparticles are homogenously spherical in shape and size, in comparison with the flat or plate structure of the coated TiO2 nanoparticles over textiles.

3.6Stereographic microscopy

The stereographic microscope evaluations are presented in Fig. 7a–c which shows that after all treatments, the pore sizes are decreased but the pores are not totally blocked. The coating process does not also change the physical and visual properties of textiles at macro scale.

Fig. 7.
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Stereographic pictures taken from textiles: (a) without coating, (b) coated with N-TiO2 (water/TTIP: 200), (c) coated with N-TiO2 (water/TTIP: 4).

3.7Drop shape analysis

The hydrophilicity and the surface contact angle of textiles for self-cleaning activity were measured, as shown in Fig. 8. Since the N-doped nano TiO2 coated textile is super hydrophilic, the contact angle cannot be measured immediately after the deionized water is dropped over the textile. It can be also seen that the hydrophilicity of the coated TiO2 textile is improved which is in accordance with the findings of Yamamoto et al. [33]. In comparison with the previous works which needed UV radiation or passing of time [34], this experiment shows the instant superhydrophilic behavior of these textiles which improves the shortcomings of previous researches.

Fig. 8.
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Drop shape analysis (DSA): (a) droplet before contact, (b) textile without coating, (c) textile coated with TiO2 and (d) textile coated with N-TiO2.

3.8Photodegradation of methylene blue

To clarify the photocatalytic behavior of the coated cotton textiles, photodegradation of methylene blue was performed. The discoloration of methylene blue was monitored by passing of time after UV and visible light irradiation as well as dark condition. The results of these photoreactions are presented in Table 2 for comparison. It can be seen that photodegradation of methylene blue over N-doped TiO2 textiles is drastically changed and improved in comparison with the case of bare TiO2, even by the exposure of visible light. Actually as mentioned previously, the N doping causes the absorption edge of TiO2 to be shifted into the lower energy region. Subsequently, methylene blue can be decomposed by visible light radiation by the N-doped TiO2 coated textiles. Consequently, superhydrophilicity and methylene blue photodegradation characteristics of the textiles are achieved. Existence of high surface wetting and visible light oxidation properties provides the high efficiency self-cleaning surfaces. In fact, this high surface wetting property spreads the droplet over the surface and in contact with the photocatalytic surface. This behavior highly enhances the photocatalytic reaction.

Table 2.

Photodegradation of methylene blue over the textiles by passing of time under different radiation conditions.

4Conclusions

In this study, textiles with photocatalytic properties were produced that unlike the conventional photocatalytic textiles, does not necessarily need UV radiations of high energy photons. To achieve this goal, N-doped TiO2 nanoparticles were synthesized by sol–gel route which enables the photocatalytic property under visible light radiations. Results of the experiments including X-ray diffraction analysis, energy dispersive spectroscopy, ultraviolet–visible spectrophotometery, particle size analysis, scanning electron microscopy, stereographic microscopy, drop shape analysis and photodegradation of methylene blue led to the following conclusions:

  • The N-doped TiO2 synthesized solution with water/TTIP ratio of 4 contains nanoparticles with almost narrow distribution of about 15nm mean diameter.

  • The observed morphology confirms that doping process has significant effect on the appearance of the synthesized nanoparticles and verifies the effect of the presence of the N-source material on the morphology. The synthesized N-TiO2 nanoparticles are homogenously spherical in shape and size, in comparison with the bed or plate structure of the coated TiO2 nanoparticles over textiles.

  • According to the UV–vis spectra, the band gap energy of N-TiO2 is decreased to 2.98eV, which increases its photocatalytic activity induced by visible light.

  • Existence of high surface wetting and visible light absorption properties provides the high efficiency self-cleaning surfaces. In fact, this high surface wetting property spreads the droplet over the surface and in contact with the photocatalytic surface. This behavior highly enhances the photocatalytic reaction.

  • Photodegradation of methylene blue was also performed to clarify the photocatalytic behavior of the coated cotton textiles. The results show that photodegradation of methylene blue over N-doped TiO2 textiles is drastically changed and improved in comparison with the case of bare TiO2, even by the exposure of visible light.

Conflicts of interest

The authors declare no conflicts of interest.

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Corresponding author. (Seyed Mojtaba Zebarjad Mojtabazebarjad@shirazu.ac.ir)
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