Document Type : Original Article
Department of Physics, College of science, University of Baghdad, Baghdad, Iraq
In this paper, pure and Cr-doped SiO2 was prepared by sol-gel method and deposited on glass substrates by dip-coating and calcination of the resulting thin films at 500 °C. An X-ray diffraction examination was performed for the pure and doped films, where we note the appearance of a spectrum in the form of a wide band at the equivalent Bragg angle at 2θ = 22.6°, which indicates that the obtained material is amorphous silica. After that, field emission scanning electron microscopy was conducted to study the morphology of the pure SiO2 thin layer doped with chromium, as it appears that the prepared film is of good quality and free of cracks and holes, as we note that the crystals adopted certain geometric shapes with different sizes and shapes. Likewise, the particle distribution is uniform with the spherical shape. By examining the atomic force microscope to study the surface topography and roughness, we notice that the surface roughness increases with the increase in the doping percentage, where the surface roughness values of the samples vary between (2.94-5.22) nm, and we will get the highest surface roughness values at a concentration of 9%. After preparing the SiO2 nanoparticles, they are mixed with the pigment and deposited on the glass substrates using the dip-coating method to measure the contact angles and study the duration of the effect of SiO2 particles on the pigment. In addition, by measuring the contact angle between a drop of water and the coating surface, we notice that the contact angle increases after adjusting the coating by adding pure SiO2 doped with chromium and mixed with the pigment, as the contact angle increases from (95.71°) to (107.17°) depending on the surface roughness. The surface roughness and its energy are important factors affecting the contact angle.
The creation of nanoparticles with sizes between 1 and 100 nm using various synthetic techniques is the subject of nanotechnology. Today, there has been an unexpected rise in fields including molecular biology, physics, organic and inorganic chemistry, medicine, and materials science that are not seen in particles larger than bulk. Nanoparticles have exotic and enhanced features, such as particle size distribution and shape, until they are reduced to the nanoscale. Overall, it is these unique qualities of nanoparticles that give them their multifunctional capabilities and stimulate interest in their use in other industries . To produce nanoparticles, several methods are used such as vapor deposition and micro emulsion treatment chemistry, combustion synthesis, hydrothermal techniques, plasma synthesis, and sol-gel regardless of the synthesis method, the main focus was how to control the nanoparticle size as well, processing particle surface interaction, and morphology. The most common technology for fabricating nanoparticles is sol-gel. The sol-gel method has some advantages such as low temperature and controlled synthesis the course of the reaction by changing the chemical structure using the sol-gel method . As the production conditions can improve the shape of the nanocomposites, where the grain size and shape can be controlled to produce a highly efficient nanostructure . Silicon dioxide (SiO2) is an important material and has many applications in various fields because of its unique properties such as electrical insulation, thermal stability, and chemical stability . Due to its potential use in several industrial applications as self-cleaning, antireflective, protective, or photocatalytic coatings, SiO2 composite sol-gel coatings have attracted a lot of interest recently . These days, there is a lot of interest in hydrophobic surface preparation, which includes widely used self-cleaning , corrosion protection [7, 8], and antifreeze . According to the reports, several techniques, including sol-gel, were used to produce the hydrophobic surfaces . More coatings with water-repellent qualities are employed as protective coatings for long-term service life and efficient self-cleaning. These coatings have to be very resilient and mechanically robust over time. Sol-gel technology is the desired method for creating these coatings because the formulation process makes it simple to change the coating's desired properties, such as hardness, surface roughness, transparency, and surface energy to achieve highly water-resistant surfaces akin to those of lotus leaves. You should concurrently regulate surface morphology and surface chemistry. Self-cleaning may be accomplished using the lotus leaf-like surfaces, waterproofing, and non-stick qualities . In general, lowering surface energy and creating a rough structure can produce a hydrophobic surface. Therefore, creating a suitable hydrophobic surface on a smooth surface is difficult . Water vapor molecules can interact significantly with hydrophobic moieties when an active photocatalyst is properly constructed and a large proportion of its surface is covered with hydrophobic materials. The surface can repel water droplets . Recently, self-cleaning surfaces that exhibit both hydrophobicity and photocatalytic behavior have been brought to the attention of researchers such as Ding et al. . Therefore, a highly water-resistant surface can be produced more effectively by obtaining a rough surface. In addition, the fabrication of rough surfaces for hydrophobic surfaces has far involved techniques including etching, bundling, electro spinning, sol-gel, curing chemical vapor deposition, electro deposition, etc. . In the self-cleaning process, the hydrophobic surfaces showed a distinctive behavior so that the water molecules could wash away the dust and dirt present in the surface, as well as showed good resistance to the corrosion in an aqueous medium . The contact point between the surface and the water droplet is an important factor in self-cleaning . Hydrophobicity is measured by examining the contact angle with water. When using the sessile drop technique, a liquid drop is placed on a horizontal surface, and the contact angle is then measured . In this work, the pure and Cr-doped SiO2 nanoparticles were prepared by sol-gel technology, where an X-ray diffraction (XRD) test was performed, as well as field emission Scanning electron microscopy images (FE-SEM) to clearly determine the particle size, as well as an atomic force microscope (AFM) examination to determine the surface roughness. The aim of this research is to study the effect of mixing pure and Cr-doped SiO2 nanoparticles with pigments by measuring the contact angle between a drop of water and the paint surface to improve the pigments properties in the self-cleaning process and to preserve the coatings appearance from external pollutants.
Materials and Methods
Tetraethyl orthosilicate (TEOS) and Si (OC2H5)4 were purchased from Sigma-Aldrich (purity 99% percent). Isopropanol alcohol (CH3)2CHOH (assay 99.8%) was purchased from GCC. Chromic nitrate (pure 97%), was purchased from GPS, and deionized water was also used. Black pigments were purchased from Al-Tabieaa Company for the production of dyes.
Preparation of pure and Cr -doped SiO2
The SiO2 was prepared using sol-gel technique, in which tetraethyl orthosilicate (4.4 mL) was mixed with isopropanol alcohol (15.6 mL) and mixed for half an hour representing (Sol A), after which isopropanol alcohol (15.4 mL) was mixed with deionized water (1 mL) and nitric acid (HNO3) were used to adjust the pH and this represents (Sol B). Then, sol B was added dropwise to sol A. The solution was placed on a magnetic stirrer for 2 hours. The solution was left for 24 hours at a temperature of 55 °C. After that, Cr-doped SiO2 is prepared in the same way as above, only chromium is added at the following concentration (1, 3, 5, 7, and 9)% to the (Sol A) solution, and then sol B is added to sol A by drop and placed on a magnetic stirrer for two hours. The solution was left for 24 hours at 55 °C.
Thin film preparation
The glass substrates are initially cleaned with water and detergents and placed in an oven at 400 °C for 15 minutes to remove organic pollutants. To deposit thin films on glass substrates, a dip-coating technique is used, whereby the glass substrates are placed in a pure and Cr-doped SiO2 solution that was prepared in the above paragraph, where the glass substrates are immersed in a solution for one minute, and then they were withdrawn at a constant speed (3 mm/min). The substrates are dried at 55 °C for 15 minutes, and finally calcination was carried out for two hours at 500 °C .
SiO2 pure and Cr-doped mixing with pigments
To study the effect of mixing nanoparticles with pigment at this stage, pure SiO2 particles doped with (1 g) of chromium were prepared in advance, added, and mixed with (99 g) pigment where it is mixed for one hour using a mixing device. After mixing the pigment with the nanoparticles, the glass films are prepared and deposited in the pigment using the dip-coating method.
To evaluate the hydrophobic property resulting from the images by measuring the water contact angle of the coating films prepared in the above step using a contact angle measuring device, and the measurement was taken at a fixed time for all samples with a water drop of a fixed size (5 µL) and this process requires a fixed camera with high accuracy high function to capture the image of the droplets in contact with the coating surface and a computer to accurately control the size of the water droplet, the computer was used to analyze the image data.
Results and Discussion
Figure 1a,b displays an X-ray diffraction data of SiO2 pure and doped thin film the precipitated on glass substrate by dip-coating method. It should be noted that the film amorphous SiO2 pure and doped with 1% Cr. The spectrum appears as a wide band with the equivalent Bragg angle at 2θ=22.6°, which indicates that the obtained material is amorphous silica . The results is also compared with the JCPDS data (card No. 01-086-1561) for SiO2 and it reveals no impurities peak for SiO2 [21, 22]. Figure 1c, d, e and f depicts the recorded XRD pattern of SiO2 doped with 3, 5, 7, and 9% Cr in the range of (10-90). SiO2 and Cr2O3 peaks were observed at 3, 5, and 7% Cr indexed by comparing the reported JCPDS data file no. 72-3533 [23, 24]. However, no peaks appeared in SiO2 doped at 9% Cr, but the presence of hill at 2θ =22° returns for SiO2 film amorphous. The peak width at half maximum is used to evaluate the crystallite size (D) by using the following Debby-Scherrer Formula for SiO2 pure and doped 1, 3, 5, 7, and 9% Cr, where the crystallite size were 12, 11, 35, 16.6, 20.25, and 11 nm, respectively.
Field emission scanning electron microscope
The field emission scanning electron microscopy is a suitable method to study morphology of SiO2 thin film pure and doped with ratio (1, 3, 5, 7, and 9)% Cr surface morphology of the film take an important role in many applications. Therefore, the film morphology should be of good quality and free from cracks and pinholes. Figure 2a, b, c, d, e and f represents the FE-SEM images of SiO2 that the prepared by chemical method and deposited on glass substrate by dip-coating for pure and doped with 1, 3, 5, 7, and 9% Cr, respectively. From these figures, we can clearly notice that these samples possess distinct crystals with different shapes and sizes. It is clear that crystals adopted certain geometric shapes. Where in sample pure and doped with ratio 1% Cr, the distribution of nanoparticles relatively uniform with spherical shape and diameter in the range of 17.54-20.71 nm and 43.32-88.63 nm, respectively, as shown in inner Figure 2a and b. Moreover, no voids appear between the film and substrate. Therefore, the film has good adhesion to the substrate. While, Figures 2c, d, e, and f, display the FE-SEM images of SiO2 with doped ratio 3, 5, 7, and 9% Cr, respectively, also it prepared by chemical method and deposited on glass substrate by dip-coating. These figures reveal that films have uniform surface features with different shapes grains with diameters in the range of 27.84-38.8 nm and 18.94-35.84 nm, 41.24-68.50 nm, and 48.24-51.74 nm, respectively. The surfaces of films became more porosity with increase of doping content, where the porous surface increasing area of absorbance by improving the photons trapping ability [25-28].
Atomic force microscopy
To study the surface topography, roughness of thin films and the effect of doped ratios used the atomic force microscope (AFM), which has the ability to obtain very precise statistical values about the grain size and surface roughness values depending on the root mean square (r.m.s). Figure 3a, b, c, d, e and f demonstrates AFM images in three-dimensional forms for SiO2 thin film pure and doped. The film surface exhibited no apparent cracking. The grains illustrated large nicely separated conical columnar growth combined grains throughout the surface with coalescence of some columnar grains at few places. The results of the average grain size, root mean square, and roughness surface are listed in Table 1. It was found that the grain size and surface roughness differ according to the percentage of dope value, and thus affect the surface properties of the film, which leads to a change in the optical properties of the material. We find that the lowest roughness value when doping with chromium at a rate of 1% Cr, and that the highest value of the surface roughness when doping with chromium was at the rate of 9% Cr. We also found that the measured surface roughness average values were large values, which indicates the possibility of using these films in solar cells or using them to increase the absorption of film to photons of energy. This change in the structural properties of the film surface roughness value has an impact on the optical and electrical properties of the film material and selection of the appropriate application in the electronic device industry .
Contact angle measurements
In this test, the addition of nanoparticles with pigment was studied where pure and Cr-doped SiO2 was added with pigment in a fixed ratio and mixed for an hour. Glass substrates are prepared and pigment deposited using the dip-coating method. The contact angles of the prepared samples were measured using the low-projection method.
In this work, the water resistance of the prepared samples was evaluated and the contact angle was measured at fixed time constant. According to Figure 4a, the contact angle of the prepared thin films was measured as pigment only (without the SiO2 addition) was measured where this sample showed a contact angle of 95.71° meaning that this sample is hydrophobic. After that, the pigment were modified by adding pure SiO2 to the pigment, and the contact angle was measured, which is equal to 102.26°, where we notice an increase in the contact angle, as depicted in Figure 4b. Then, 7% Cr-doped SiO2 was added and mixed with pigment, and the contact angle was measured at 105.53°, as illustrated in Figure 4c, and we notice an increase in the hydrophobic property with the increase in surface roughness. When 9% Cr-doped SiO2 was added, and mixed with pigment, the contact angle was 107.17°, as indicated in Figure 4d. We also notice an increase in the contact angle with the increase in surface roughness. We notice through the measurements of the contact angles that when the surface roughness increases, the contact angle increases, and also that the sol-gel solution with the catalyst increases the surface roughness, which affects the increase in contact angles of the water droplets . Surfaces with a contact angle of less than 90° are hydrophilic surfaces, and surfaces greater than 90° are hydrophobic surfaces. By reviewing our research, pure and doped SiO2 is expected to increase surface roughness and water properties, which means that surfaces approach water resistance, as suggested by Wang et al. and Bravo et al. [31, 32]. Surfaces with different roughness levels produce hydrophobic surfaces due to different surface energies. Hydrophobic surfaces are characterized by high surface roughness and low surface energy, as the water droplets falling on the rough surface come into contact with the air trapped inside the uneven surface. This will increase the hydrophobic properties . The increase in the hydrophobic state of the coating surface with the addition of SiO2 particles depending on the roughness is consistent with Ali Ansari et al. .
Pure and Cr-doped SiO2 nanoparticles were prepared by sol -gel technique, which was deposited by dip-coating method. It is shown by performing an X-ray diffraction examination of pure and doped films; where we note that the obtained material is amorphous silica. It was concluded by examining field emission scanning electron microscopy that the nanoparticles distribution is relatively uniform with spherical shape. Moreover, no voids appear between the film and the substrate, so the film has good adhesion to the substrate. We noticed that the surface roughness increases with the increase in doping ratios, and this is what the atomic force microscope examination shows. The pigment was improved by mixing pure and doped SiO2 particles. The contact angle measurement was taken for all samples, where we notice an increase in the hydrophobic state, where the contact angle of the pigment was before. The SiO2 addition is equal to 95.71°. After mixing SiO2 with the pigment, we noticed an increase in the contact angle, which reached 107.17°. This means an increase in the hydrophobic state. The reason for the increase in the contact angle is due to an increase in the surface roughness. The roughness can trap the air-water interaction so that the interaction can decrease on the surface which leads to a larger contact angle. Likewise, silica particles have non-polar properties. This property will help reduce the surface energy, resulting in a large contact angle between the water droplet and the surface. Hydrophobic coatings can be controlled by surface roughness and surface energy reduction.
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
All authors contributed to data analysis, drafting, and revising of the paper and agreed to be responsible for all the aspects of this work.
Conflict of Interest
We have no conflicts of interest to disclose.
HOW TO CITE THIS ARTICLE
Mustafa M. Mohsin, Falah H. Ali. Enhancement of Pigments Hydrophobicity by Mixing with Cr Doped SiO2 Nanoparticles. Chem. Methodol., 2023, 7(5) 335-347