Abstract
In this work, a SiO2-TiO2 coating, composed of different numbers of TiO2 and SiO2 layers, was fabricated by a spray-coating technique. The films were deposited onto ignimbrite rock and divided into two groups according to the number of SiO2 layers applied, 10 and 15 layers of SiO2 and 5 layers of TiO2 for each group. The morphology and chemical composition of the synthesized samples were characterized by field emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray spectrometer (EDS), which reveal the successful SiO2-TiO2 coating on ignimbrite. The photocatalytic activities of samples obtained were evaluated toward the decomposition of 3 ppm of methyl orange (MO). Finally, NOx gas degradation was studied. The obtained results evidenced that the SiO2 and TiO2 coating improved the photocatalytic activity of ignimbrite.
1. Introduction
In Peru, there are few studies on mitigating damage to rocks belonging to architectural monuments. However, in the city of Lima, Gallarday [1] studied the deterioration of various churches in the historic center of the city and made a financing proposal for the preservation and restoration of the main ornamental rocks. In Arequipa, the most important study was carried out in 2006 [2] by the Ministry of the Environment, in which the main agents and mechanisms of alteration of the rocks (ignimbrite) belonging to the historical monuments of Arequipa were determined. Thanks to this study, the concern for the improvement of the historic center of Arequipa city began, seeking effective and simple solutions or alternative that allows the protection, self-cleaning, and preservation of the cultural heritage. In this aspect, the use of nanomaterials begins to gain importance to achieve the desired improvement. Zornosa-Indart et al. [3, 4] used silica-based inorganic hybrid nanomaterials that improve robustness, hydrophobicity, and resistance and consolidate limestone rocks significantly, in order to achieve the conservation of the cultural heritage. Additionally, the use of titanium dioxide (TiO2) photocatalyst in the improvement of the facades of historic buildings has been studied with good results [5, 6] and in combination with cementitious and other construction materials has shown a favorable synergetic effect in the removal of air pollutants [7, 8]. Thus, the researches continued and led to the implementation of a system based on silica as a support material and TiO2 nanoparticles as a photocatalyst material [9–11]. Nevertheless, the use of a silicon-titanium hybrid system in the degradation of methyl orange (MO) dye and NOx gases on ignimbrite surfaces has not yet been reported. Therefore, the aim of this work is to carry out a study on a TiO2-SiO2 coating system, composed of different numbers of layers of SiO2 and TiO2 achieved with the spraying coating technique. These coatings could protect the cultural heritage of the city of Arequipa, Peru, from organic and air pollutants. Detailed morphological characterization of samples was investigated by field emission scanning electron microscopy (FE-SEM). Then, methyl orange dye degradation measurements will be previously performed in order to determine the effectiveness of the coating systems. Finally, we will evaluate the NOx gas elimination capacity in a laboratory gas analyzer.
2. Experimental
2.1. Synthesis of the Sols
All reagents used in the experiments were of analytical grade and used without any further purification. The TiO2 and SiO2 sols were prepared, respectively, according to Arconada [12] and Reyes et al. [13]. The TiO2 sol was obtained from the mixture of 49.5426 g of ethanol, component used as solvent of the sol-gel process with 1.6140 g of acetic acid and 7.8758 g of titanium isopropoxide (TTIP), where TTIP is the main component of titanium precursor. Then, 0.9676 g of water acidified with hydrochloric acid (HCl, 0.1 N), that acts as the catalyst for the solution, was added dropwise, and the whole mixture was stirring for 1 hour until dissolved. Meanwhile, the preparation of the silica sol (SiO2) was prepared from the precursor tetraethylorthosilicate (TEOS), dissolving 40.602222 g of TEOS in 105.63776 g of ethanol. Then, 3.4384 g of water acidified with 0.1 N hydrochloric acid was added dropwise; the mixture was stirring at 60°C in a cooling bath with glycerin at 11°C for a period of 90 minutes. After that time, the temperature was reduced to 40°C and 10.31536 g of acidified water was added dropwise again. Finally, the solution was left under stirring for a period of 60 minutes in the cooling bath.
2.2. Deposition of the Coatings
The coating of ignimbrite employing TiO2 and SiO2 sols was carried out by a spray-coating technique, using a set of airbrushes with fluid control at a distance of 5 cm from the ignimbrite. SiO2 and TiO2 layers were obtained by calcination at 450°C for 30 minutes and 60 minutes, respectively, at a heating rate of 10°C/min. Two groups of samples were fabricated according to the number of layers of TiO2 and SiO2 applied. The first group of samples was fabricated with 10 layers of SiO2 and 5 of TiO2 and labelled as 10TEOS-5TiO2, whereas the second group was fabricated with 15 layers of SiO2 and 5 of TiO2 and labelled as 15TEOS-5TiO2.
2.3. Characterization of the Samples
The morphologies of obtained samples were visualized by a field emission scanning electron microscope (FESEM, Hitachi Regulus 8230) equipped with an energy dispersive X-ray spectrometer (EDX). The photocatalytic activities of the fabricated nanostructures were evaluated by the degradation of methyl orange (MO) under UV light irradiation, using a light source which simulates solar radiation (Newport 50-500 W). The next step was NOx gas mitigation monitoring (NO+NO2) performed with a chemiluminescence analyzer AC-32 M, Environment S.A., following the guidelines of ISO 22197-1: 2007. The NOx degradation efficiency was calculated using where is the initial NOx concentration (before turning on the UV source) and is the concentration at the end of the illumination period.
The photocatalytic activity and NOx degradation of 10TEOS-5TiO2 and 15TEOS-5TiO2 samples fabricated by a spray-coating technique were compared.
3. Results and Discussion
Figure 1 shows a photograph of ignimbrite, 10TEOS-5TiO2, and 15TEOS-5TiO2 samples. Figure 1(a) is clearly to see the variety of porosity and minerals that conform the ignimbrite, unlike in Figure 1(b) where a whitish color is visualized covering the surface of the rock. In Figure 1(c), the intensity of this color increases and we obtained better compaction of the minerals present in the ignimbrite, an important requirement to be applied on rocks of historical monuments for restoration [14].
(a)
(b)
(c)
The FE-SEM images obtained by field emission scanning electron microscopy of 15TEOS-5TiO2 samples fabricated by spray coating at magnifications of 10 KX and 80 KX are show in Figures 2(a) and 2(b), respectively. As shown in the figures, TiO2 layers formed by nanoparticles can been seen in the top of the ignimbrite. Moreover, the EDS element mapping image in Figure 2(c) reveals the presence of titanium (Ti, green), silicon (Si, red), and oxygen (O, blue) elements, proving the coating of SiO2 and TiO2 layers on the surface of the ignimbrite.
(a)
(b)
(c)
In order to elucidate the effects of coating with TiO2 and SiO2 layers on the ignimbrite, the photocatalytic dye degradation performances of the 10TEOS-5TiO2 and 15TEOS-5TiO2 were evaluated in aqueous solution of methyl orange dye under UV-A irradiation. Figure 3(a) shows the change in the methyl orange concentration in aqueous solution in the presence of all samples. As shown, methyl orange molecules were not completely decomposed during 150 min of photocatalytic reaction. However, it was noted that 15TEOS-5TiO2 photocatalyst shows the highest photocatalytic activity compared with 10TEOS-5TiO2 photocatalyst. The degradation efficiency of the 15TEOS-5TiO2 photocatalyst shows a maximum degradation of ~55.73% at 150 min, whereas the degradation efficiency of 10TEOS-5TiO2 was ~20.91% at the same irradiation time. The enhancement of photocatalytic efficiency must be attributed to the increase in the number of SiO2 layers, which achieve better support for the coating of TiO2 [15]. Thus, having more TiO2 exposed directly the photocatalytic efficiency which could be improved, having a better performance in the degradation of the methyl orange dye.
The chemiluminescence analysis for the variation of the concentration of NO and NO2 in parts per micromolar is shown in Figure 4. An appropriate amount of the SiO2-TiO2-coated ignimbrite was loaded into the reactor, and then, the reactor was carefully sealed. Afterwards, the NO containing nitrogen gas and the purified air were allowed to flow into the reactor at flow rates of 3 L/min each, until equilibrium NOx concentration in the inflow was achieved (1000 ppb). The evaluation of photocatalytic activity was 1 hour at 10 W/m2 with 35 minutes of saturation of the rock in the dark. In the first 30 minutes in the dark, the peaks observed in the graphs are the flow of gas entering the chamber, so it has no influence on the measurements. After the one hour of irradiation, the light source was turned off and then the gas valves were closed. All experiments were conducted at ambient temperature (). The detailed experimental procedure can be referred to published literatures [7, 8] and the ISO 22197-1 : 2007 standard of air purification performance of semiconductor photocatalytic materials [7]. Figure 4(a) shows a slight increase in NO2 production for the 10TEOS-5TiO2 substrate, which causes a greater amount of NOx removal. In the case of the gaseous medium, a better degradation result of 0.80 μmol was obtained, which translates into 15.95%, whereas the obtained degradation for 15TEOS-5TiO2 substrate, shown in Figure 4(b), was 0.63 μmol, which is equivalent to 10.56%. According to the reports [16, 17], the NOx gas degradation phenomena are mainly due to the presence of TiO2 in our samples.
(a)
(b)
It is important to know the mechanism of heterogeneous photocatalytic degradation of NOx gases by TiO2. These processes are summarized in the following reactions for the 10TEOS-5TiO2 and 15TEOS-5TiO2 samples as photocatalytic materials. When the nanostructure is irradiated from the light source, the electrons (e-) in the valence band (VB) are excited to the conduction band (CB) with generation simultaneous of the same number of holes (h+) in the VB (Equation (2)).
The reaction of the H+ with the OH- is dissociated from the water to form the OH [18–20] and the reaction with the electrons with the O2 to form a superoxide anion O2- [20, 21].
Then, the reaction of with from water produces radicals; NO diffusion occurs on the surface of TiO2 and forms NO2. Finally, NO2 reacts with hydroxyl radicals forming nitric acid:
4. Conclusions
In summary, SiO2 and TiO2 coatings on ignimbrite were achieved by a spray-coating technique, varying the number of SiO2 layers in 10 and 15 layers, while the number of TiO2 layers remained constant at 5 layers. The photocatalytic activities of the samples obtained were evaluated toward the decomposition of methyl orange (MO) and NOx gas degradation. The obtained results evidenced that the SiO2 improved the porosity of ignimbrite, whereas the TiO2 coating improved the photocatalytic activity. The enhancement in the photocatalytic activity of the SiO2-TiO2 hybrid system is attributed to the high efficiency in both light utilizations, the higher transfer rate of photogenerated electrons from SiO2 to TiO2 and repressed recombination of the photoinduced hole-electron pairs of TiO2, which is closely related to the chemical interaction between TiO2 and SiO2.
Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This work was funded by the Universidad Nacional de San Agustín de Arequipa, with the topic: “Evaluación de Materiales Fotocataliticos y sus Aplicaciones en la Restauracion y Mantenimiento del Patrimonio Cultural de la Ciudad de Arequipa” (No. Contrato IBA 0021-2016) and the collaboration of Eva Jimenez Relinque for her technical assistance in the analysis of NOx degradation of the Institute of Building Sciences E. Torroja-Madrid. P.G.R.A and J.M.R.R wants to thank the project 32-2019-FONDECYT-BM-INC.INV.