Synthesis and photocatalytic activity for TiO2 nanoparticles as air purification

In the present work, titanium dioxide (TiO2) nanoparticles (NP’s) were prepared using sol-gel process from Titanium Tetrachloride (TiCl4) as a precursor with calcinations at two temperatures (500 and 900) oC. The effect of calcinations temperatures on the structural, optical, morphological and Root Mean Square (roughness) properties were investigated by means of Scanning Electron Microscopy, X-ray Diffraction (XRD), and Atomic Force Microscopy (AFM). Bacterial inactivation was evaluated using TiO2-coated Petri dishes. A thin layer of photocatalytic TiO2 powder was deposited on glass substrate in order to investigate the selfcleaning effect of TiO2 nanoparticles in indoor and outdoor applications. Ultra-hydrophilicity was assessed by measuring the contact angle and it evaluated photolysis properties through the degradation of potassium permanganate (KMnO4) under direct sunlight. XRD analysis indicated that the structure of TiO2 was anatase at 500 oC and rutile at 900 oC calcination temperatures. As the calcination temperature increases, the crystallinity is improved and the crystallite size becomes larger. Coated films of TiO2 made the has permeability, low water contact angle and good optical activity. These are properties essential for the application of the surface of the self-cleaning. The final results illustrate that titanium dioxide can be used in the build materials to produce coated surfaces in order to minimize air pollutants that are placed in microbiologically sensitive circumference like hospitals and the food factory.


Introduction
Ecological pollution, such as dirty water or contaminated air, has become a universal problem that affects human health. Typical polluting sources are toxic organic molecules or exhausted gas compounds that are released from household waste, livestock waste and local industries [1,2,3,4,5]. Growth and deposition of microbes on surfaces such as walls, windows and tables also pose a substantial threat to human health. Continual exposure to these pollutants can lead to varied health troubles like irritations and toxic effects, and other various respiratory or skin diseases. Most studies on photocatalysts show that it is a better technique for biodegradable organic pollutants using clean solar energy without generating any harmful products [6]. Photocatalytic technique is an effective purification process capable of disrupting a wide range of harmful microorganisms in various media. It is safe, non-toxic, and relatively inexpensive method of disinfection which allows the ability to adapt to use in many applications such as air purification, water purification, and treatment of hazardous waste, water purification [7,8,9]. This technique can be envisaged as one of the most promising Advanced Oxidation Process (AOPs) due to its specific advantages, such as bland reaction conditions, the possibility of using molecular oxygen as oxidant species, the total mineralization of pollutants into substances innocuous to the environment. Photocatalysis is based on the interaction between semiconductor materials and light by considering that 'free' light from the sun, electron will be excited from the valance band to the conduction band thus creating an electronhole pair, the holes (h+) and hydroxyl radicals (OH•) generated in the valence band, also electrons and superoxide anions (O2 •-) generated in the conduction band, irradiated TiO2 photocatalysts can decompose and mineralize organic compounds by a series of oxidation reactions leading to carbon dioxide [10,11]. The idea of using solar light energy as resource to clean up the environment is an ideal and promising approach. Among various oxide photocatalysts, the preparation, characterization and applications of semiconductor titanium dioxide (TiO2) have become a research hot spot. The photocatalytic oxidation (PCO) using titanium dioxide is an efficient technology for the degradation of many pollutants [12,13,14,15]. It has also been known that the TiO2 shows the hydrophilicity due to the photocatalytic nature, the water tends to sheet across this surface rather than forming droplets [16]. This makes the surface anti-fogging and easily washable. TiO2 photocatalyst has good chemical stability, relative low price, wide availability, non-toxicity and it has strong oxidizing power [17,18,19,20,21,22].
Super-hydrophilicity is a phenomenon that occurs when TiO2 coated surface is illuminated by UV radiation. The hydrophilic surface has a very small contact angle (θ ≤ 5o) that allows water to sheet across this surface rather than forming droplets. This makes the surface anti-fogging and easily washable [23].
In this work, the synthetic method of nanostructure, a Titanium dioxide (TiO2) based on TiCl4, has been synthesized by sol-gel method and thermally treated at two temperatures. The effect of thermal treatment on the structural and morphological properties of the material was investigated by means of SEM, XRD and AFM and measuring the contact angle of water drop on the films by spin coating technique to evaluate the hydrophilicity property.

Synthesis of TiO 2 nanoparticles
TiO 2 Nanaparticles was prepared by Sol-Gel method using Titanium Tetrachloride (TiCl 4 ) as precursor of TiO 2 as described in (Adawyia et al. 2017).

Characterization
The prepared nanoparticles were characterized for the crystalline phase by X-ray diffraction using (XRD-6000 2kw type), at room temperature, operating at 40 kV and 30 mA, Cu (1.54060 A). The morphological changes of prepared TiO 2 nanoparticles at different calcination temperatures were analyzed using Scanning Electron Microscopy (SEM, Tescan VEGA3 SB). Zeta potential (ZetaPlus) was used to measure Zeta potential of the particles in liquid suspension using Electrophoretic Light Scattering (ELS) technique with the range between -250 mV to 250 mV (sample dependent). The size of the prepared TiO 2 nanoparticles was measured by using (Particle size analyzer, 90Plus). Fourier Transform Infrared spectroscopy (FTIR) has been extensively used for identifying the various functional groups.

X-ray diffraction (XRD)
Precise metal composition and grain size of TiO 2 NP's were determined by XRD.  (1) Where K is a constant that depends on the crystallite shape (0.9, with the assumption of spherical particles) [24], λ is the X-ray wavelength, β is the full width at half maximum of the selected peak and θ is the Bragg's angle of diffraction for the peak. The XRD patterns results indicate that the structure of TiO 2 was anatase at calcination temperatures at 500 ᵒC, rutile at 900 ᵒC. The anatase phase was identified at 2θ values of 25.

Scanning Electron Microscopy (SEM)
The micrographs SEM images of TiO2 nanoparticles were prepared at different calcination temperatures as shown in Figure 2. (Figure 2a) shows TiO2 nanoparticles calcined at 500ᵒC have roughly spherical spongy shape with small size nanoparticles around the range of less than 10 nm. When the temperature increases, the sizes become bigger and the agglomeration becomes significant. Further increase in temperature up to 900ᵒC, TiO 2 nanoparticles exhibited nonuniform particles shape due to the agglomeration of initial particles with increase in crystalline size. These results are in agreement with XRD results that showed that the particles size of the anatase phase is smaller than rutile phase due to aggregation of nanoparticles with increased calcination temperature.

Zeta potential surface charge for TiO2 nanoparticles
Zeta potential surface charge for TiO2 nanoparticles is illustrated in Figure (3). According to ASTM Standard (D 4187-82), illustrated in Table (3), TiO2 collides prepared at 500ᵒC have good stability. An excellent stability has been found with TiO2 collides prepared at 900ᵒC.

Particle Size Analyzer
Particle size analysis illustrated in Table 4 shows that when the calcination temperature was increased the crystallites tend to agglomerate leading to the formation of bigger particles. This result is in agreement with XRD and SEM results.

Atomic force microscopy (AFM)
The surface morphology was studied by means of AFM (Atomic force microscopy). Figure 3 shows two-and threedimensional AFM images of the TiO 2 coatings calcined at 500 and 900 °C, respectively. It was observed that, at different calcination treatment temperatures, the surface morphologies and roughness of the TiO 2 surface are obviously different. As it can be seen from Figure 3a (coating treated at 500 °C), a granular microstructure and a flat texture, with the lowest surface roughness, and is composed of large particles diameter of spherical shapes. According to previous XRD results, these range on the surface of TiO 2 coating are aggregates of many small TiO 2 crystallites [25]. Give images AFM analysis also surface roughness values, as shown in Table 2. means the root of the square roughness (RQ) of the surface of less than (1 nm) and increases with increasing temperature calcinations , which shows the grain of normal forms on the surface. When the temperature increases, the changes in topography and roughness of the paint TIO 2 are largely due to the phase shift of titanium Anatase to rutile and the growth of the crystal TiO 2 [27,28]. The table shows (1) that the roughness values increase with increasing temperature calcination. After calcined up to 900 ° C, it observed a significant increase in grain size and RQ to 1.22 nm. It is because the shift from titanium Anatase phase peak toward Rutile phase peak went through a total of smaller particles and the formation of larger particles (J. Yu Yu, is, Jiang, 2002), which then increased its grain size. This result is in agreement with Jimmy C Yu, et al. [29] and Jiaguo Yu, et al. [30] references.

Fourier Transform Infrared (FTIR) Analysis
FTIR spectrum shows a wide band range (3400-4000) cm -1 which is contribution to the H-O-H stretching mode of the surface adsorbed H 2 O molecule and also due to the reaction between the O-H groups of TiO 2 [31]. This OHadsorbate plays an important role in photoreceptor stimulation by trapping charge carriers to produce OH reaction radicals, which are the driving force behind the process. Furthermore, they act as adsorbents and activating sites for reacting molecules or contaminants [32]. Another band of around (1620-1640) cm -1 is attributed to the H-O-H bending mode. These results agreed with Huang et al. [33] and Thangavelu et. al. [34]. FTIR results of TiO 2 nanoparticles calcined at (400, 500 and 600) ᵒC showed a broad band at 3419 cm -1 and 1634 cm -1 due to the stretching and deformation vibrations of surface adsorbed water and hydroxyl groups in the surface of TiO 2. Whereas, TiO 2 calcined at 900 ᵒC did not show any aforesaid two broad peaks and it completely disappeared at high calcination temperature. That led to reduced surface area, pore volume and adsorbed water molecule at high calcinations temperature.
All the samples showed the foremost vibration bands around (500-900) cm -1 due to the Ti-O-Ti bond-stretching mode, which indicates the formation of a Titanium oxide [35].

Photocatalysis activity
The photocatalytic activity was evaluated by photocatalytic oxidation of Potassium permanganate (KMnO 4 ) under direct sunlight irradiation, by measuring the degradation of Potassium permanganate (KMnO 4 ) under direct sunlight irradiation. UV-Visible Spectrophotometry demonstrated the absorbance value for different film samples with Potassium permanganate after exposure to UV-irradiation from sunlight. Spectrum shown in Figure 5 demonstrates the relationship between the absorbance values and exposure time of irradiation in (hr.). The color of Potassium permanganate dye gradually reduces and finally it completely disappears which means the dye is completely degraded . The results illustrate calcination temperate effect on the degradation of the dye [36]. It demonstrates that TiO 2 with pure anatase phase exhibit higher photocatalytic activity compared with TiO 2 with pure rutile that needs more time to degrade the dye. This perhaps is due to the higher adsorptive affinity of anatase surface and lower recombination rate of electron-hole pairs as compared to rutile [37]. At high calcination temperatures (900ᵒC), the rutile crystallite size increased due to aggregation under direct sunlight irradiation. The electron generated was permanently trapped in the larger rutile crystallites aggregates, which results in increasing electron-hole recombination rate and decreasing photocatalytic activity. In addition, the growth of TiO 2 crystallites at higher calcination temperature, results in a significant decrease in the surface area. TiO 2 calcined at 800ᵒC shows the highest photocatalytic activity. This is because its' composed of both anatase and rutile phases. The photogenerated electrons transfered from the anatase to the rutile phase may have reduced the rate of electron-hole combination and thus enhanced the photoactivity. Fig-5: Illustrates the relationship between absorbance and wavelength for TiO2 nanoparticle calcined at a) 500 ˚C, b) 900 ᵒC, measured befor irradation by sunlight (0 min) and after 30, 60, and 120 min exposure to direct sunlight.

Conclusions
TiO 2 NP's were synthesis by solgel technique at various calcinations temperatures, and then deposited on a glass substrate by spin coating method. The XRD pattern of calcined TiO 2 nanoparticles shows the sharp picks that reveals the anatase crystalline phase at 500 ᵒC, and rutile phase at 900 ᵒC. Self-cleaning properties of the prepared nanoparticles were characterized by Water Contact Angle (WCA) and photocatalysis activity. WCA show that the anatase and mixed phase are super-hydrophobic surfaces with WCA of zero value, while rutile phase shows good hydrophilicity with WCA of 7.775.