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Article

Photocatalytic Applications of SnO2 and Ag2O-Decorated SnO2 Coatings on Cement Paste

by
Danilo da Silva Vendramini
1,
Victoria Gabriela Benatto
1,
Alireza Mohebi Ashtiani
1 and
Felipe de Almeida La Porta
1,2,*
1
Nanotechnology and Computational Chemistry Laboratory, Federal University of Technology—Paraná, Avenida dos Pioneiros 3131, Londrina 86036-370, PR, Brazil
2
Post-Graduation Program in Chemistry, State University of Londrina, Londrina 86057-970, PR, Brazil
*
Author to whom correspondence should be addressed.
Catalysts 2023, 13(12), 1479; https://doi.org/10.3390/catal13121479
Submission received: 20 September 2023 / Revised: 21 October 2023 / Accepted: 29 October 2023 / Published: 28 November 2023
(This article belongs to the Special Issue Theoretical and Experimental Investigation of Catalytic Materials)
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Abstract

:
Recently, the production of new photocatalytic materials has attracted considerable attention as a promising strategy to mitigate anthropogenic environmental degradation. In this study, cement paste composites (water/cement ratio = 0.5) were prepared using a coating based on nanoparticles of SnO2 (SnO2/cement paste) and SnO2 decorated with Ag2O (Ag2O-decorated SnO2/cement paste) for photocatalytic applications. These coatings were prepared in this study by using the hydrothermal method as the strategy. Thus, photocatalyst efficiency was evaluated through the degradation of methylene blue (MB) and methyl red (MR) as cationic and anionic dyes, respectively, and the simultaneous degradation of MB/MR (1:1 v/v) dyes. Moreover, the photocatalytic mechanism was investigated in the presence of scavengers. Notably, an increase in pH in the range of 2–6 resulted in selective degradation of the MB/MR dye mixtures. Overall, the photocatalytic performance of these materials provides a novel platform technology focused on advanced civil engineering applications, which consequently facilitates the mitigation of various environmental problems.

1. Introduction

Nowadays, one of the most significant global problems has been the contamination of the waters of rivers, lakes, seas, etc. [1,2,3,4]. This happens due to the improper disposal of partially treated or even untreated water from factories and industries that use various chemical products on a large scale. Contaminated water contains various pollutants, such as dyes, heavy metals, pharmaceutical residues, pesticides, and so on [5,6,7,8,9,10]. These pollutants affect microorganisms and aquatic life and, in addition, can also be extremely harmful to humans [1,2].
There are many widely used techniques for water treatment, such as filtration, flocculation, adsorption, biological degradation, advanced oxidation processes (AOPs), and even combinations between them [2,11,12,13,14,15]. However, some of these techniques may have limitations, including the need to use expensive equipment, slow processes, and low degradation efficiency [2,12,14,15]. Therefore, treatment techniques involving AOPs are interesting in solving such environmental issues because they generate reactive oxygen species and are known for being highly reactive and non-selective [1,15,16,17,18]. AOPs are widely studied and commonly use ozone, hydrogen peroxide, Fenton’s reagent, and irradiation in the UV region [18,19,20].
For photocatalytic activity on the surface of target materials, the chosen catalyst needs to be irradiated with light with an energy equal to or greater than its band gap, generating electron/hole pairs (e−/h+) that will act as carriers to promote the desired reactions [21]. Irradiated semiconductor materials are considered promising photocatalysts due to their ability to generate an efficient separation of e−/h+ pairs [22]. The photophysical properties of semiconductor materials are strongly related to their compositions, morphologies, sizes, structure, as well as imperfections in these parameters [9,23,24,25,26]. Therefore, studies evaluate ways to improve the efficiency of the photocatalytic activity of these semiconductors. For this proposal, materials with relevant valence and conduction band positions are needed to promote several photocatalytic reactions of interest, and because of this, we have other semiconductors that are even less reported, such as SnO2 [27,28,29]. Despite SnO2 being more investigated in other research fields, interest in this system as a catalyst has grown considerably over the last decade. Particularly, this is likely due to its non-toxicity, low cost, long-term stability, optical transparency, and high electron mobility [30,31].
SnO2 is a versatile n-type semiconductor material with high thermal stability and easy preparation [30,31,32,33]. Nowadays, it is already used as a catalyst in several organic syntheses, as a solid electrolyte, and as a gas sensor [22,34,35,36,37]. However, SnO2 may have certain limitations for solar applications. This happens because of its ~3.6 eV band gap and its fast recombination of the e−/h+ pairs [31,38]. Recent studies have combined SnO2 with other semiconductors such as SnO2/rGO [39], BiPO4/SnO2 [40], g-C3N4/SnO2 [41,42], TiO2/SnO2 [43], and SnO2/Ag2O [44] to overcome this intrinsic limitation of the material and further improve its desired high-performance applications.
Knowing this, different processing methods have been widely developed in recent decades to obtain SnO2 and its related desired physical characteristics [29,31]. Among them, the hydrothermal method stands out, which is a simple, inexpensive, and environmentally friendly strategy for obtaining a wide variety of materials [45,46,47]. Other than that, synthesizing a catalyst through the hydrothermal method has some advantages, such as high particle purity, good dispersion, and uniform particle sizes [48]; these reactions usually occur in a closed system using water [49] at high temperature and high pressure [50,51].
At the same time, it has been observed that the development of Portland cement-based materials, such as concrete and mortar, has a strong relationship with the progress in civil engineering. These Portland cement composites are widely used because they have a low cost and are adaptable to different use situations [52,53,54,55,56,57]. The research involving cementitious materials is advancing toward the production of lighter, stronger, and more durable structures [58,59]. To corroborate the improvement in the performance of cementitious materials, nanoparticles of various oxides are being tested, such as SnO2 [60,61,62], TiO2 [63,64], and Al2O3 [58]. In addition, the coating of these materials with nanoparticles can promote a high photocatalytic activity, which is also largely responsible for the self-cleaning characteristic observed in these new materials developed for advanced applications in civil engineering [63,65,66,67]. This self-cleaning characteristic that accompanies the photocatalytic property brings an increase in the lifetime of cementitious material, due to its ability to reduce the presence of pollutants on its surface in the presence of light [63,65,67,68].
In this work, nanostructured SnO2 and Ag2O-decorated SnO2 as coatings of the cement paste composite were prepared by using the hydrothermal method as a strategy. As is well-known, silver oxide (Ag2O) materials are stable and have broad-spectrum microbial properties. Therefore, our choice is based on the objective of including this functionality in the prepared SnO2-based coatings. Then, the photocatalytic activity of the SnO2/cement paste composite was investigated through the degradation of methylene blue (MB) and methyl red (MR) dyes under ultraviolet light irradiation. The simple chemical structure of these dyes and well-understood degradation pathways makes them convenient model compounds for developing and optimizing new catalysts. In addition, the pH conditions on the Fenton-like process were investigated for the nanostructured SnO2 and Ag2O-decorated SnO2 as a coating of the cement paste composite. Our results showed that the increasing pH (using a variation between 2 and 6) could lead to selective degradation in MB/MR dye mixtures. This work offers a new perspective for the use of photocatalytic materials in civil construction. As far as we know, this is the first study in the literature that uses SnO2 and SnO2 decorated with Ag2O as a coating on cementitious materials (cement paste) as a strategy to mitigate environmental problems, especially those related to the treatment of effluents with complex characteristics. Finally, the need to develop new advanced cementitious materials encourages study in this area.

2. Results and Discussion

The appearance of the synthesized materials is shown in Figure 1a–f, with Figure 1a showing a reference cement paste block for comparison. A visible change in the appearance of the cement paste blocks was observed at each step of the process. As shown in Figure 1b, the nucleating layer was deposited with a slightly whitish color. Moreover, the block darkened in color because of the extended period in the muffle furnace during the preparation process. As shown in Figure 1c, the surface layer became more visible with the growth of the SnO2 film. The white layer of the film caused more contrast than the dark color of the substrate; thus, the defects in the film caused by the porosity of the cement paste surface were more evident. Another noteworthy characteristic is the defects at the interface between the film and block. These defects were mainly observed at the edges of the surface where the film was absent. These defects occurred during the deposition of the nucleating layer because the surface of the block is not uniformly flat, which complicates the deposition process. As shown in Figure 1d–f, samples decorated with Ag2O exhibited identical defects because they underwent the same preparation process before being decorated. When the material was decorated, a reddish color appeared, which became slightly stronger with increasing concentration during the decoration process. This is owing to the dark brown color of Ag2O [69].
The wettability of a solid surface can be evaluated based on its contact angle. Generally, a high contact angle value indicates a relatively low surface wettability [70]. The contact angle results for the analyzed samples are listed in Table 1. Consequently, the high contact angle observed for the cement paste sample was probably owing to its surface roughness. The surface roughness decreased with the addition of the nucleating layer and film; consequently, the contact angle decreased.
Notably, the sample with the smallest contact angle possessed the film layer. Therefore, the objective of the study was achieved because the samples that had the smallest contact angle were those that underwent hydrothermal growth. Furthermore, the obtained contact angle of approximately 11° for the SnO2 film is similar to that reported in the literature; however, the reported value was for a different substrate [71].
The XRD patterns of the as-prepared samples are shown in Figure 2a,b. In Figure 2a, a new peak was observed at approximately 27° when the nucleating layer and SnO2 film were added to the samples, confirming the presence of SnO2. The crystalline structure obtained for the SnO2 phase was identified as tetragonal cassiterite (space group P42/mnm) according to the inorganic crystal structure database (ICDD) card no. 00-211-250, whereas the cement paste exhibited the main hydrated cement products: C-S-H (ICDD card no. 00-029-0373), calcium hydroxide (ICDD card no. 00-050-0008), and hydrated calcium sulfoaluminates (ICDD card no. 00-720-646), as well as the presence of calcite (ICDD card no. 00-471-743). Figure 2b shows a new peak in the region between 40° and 50° for the decorated samples, which confirms the formation of Ag2O (ICDD card no. 00-420-874). In addition to the presence of diffraction peaks, the crystalline fraction of the material increased from 44% in the sample with the SnO2 film to 66% in the Ag2O decorated samples. After the hydrothermal process, a powder was obtained and characterized by XRD. As shown in Figure 2c, XRD confirmed the presence of the SnO2 phase in the powder samples, and the unindexed peaks are attributed to cement paste substrate residues. Furthermore, as listed in Table 2, the crystallite size of the SnO2 and Ag2O phases in all the prepared samples was calculated using the Scherrer equation [72].
Figure 3a shows the SEM images of the cement paste in the presence of C-S-H. As shown in Figure 3b, the deposition of the nucleating layer indicated the presence of SnO2. The block surface was analyzed, indicating that these points were present at a certain distance from each other. After the process of nucleation layer deposition and hydrothermal growth, the appearance of SnO2 nanoparticles covering the sample surface was observed, as shown in Figure 3c. For the decorated samples, the presence of the decorator material (Ag2O) on the surface was investigated because only some evidence of its presence was observed by the reddish color of the blocks and a minor XRD peak. Therefore, as shown in Figure 3d–f, the presence of Ag2O was confirmed for the decorated samples. For all the decorated samples (Ag1, Ag2, and Ag3), the Ag2O exhibited a cubic morphology and agglomerated in regions with a relatively higher concentration of the material.
In general, cementitious materials are formed by particles in different size scales. For this reason, e.g., when analyzing the roughness of such systems based on traditional methods, there are generally significant deviations in these estimations, representing a great challenge for researchers. Therefore, aiming to improve the accuracy in describing the roughness of such samples, this study developed a code (see Supplementary Material) to analyze and extract this information directly from SEM data. In sequence, Figure 3a–f are analyzed again, and some parts of them (the demarcated regions in yellow) are selected for the next steps of the process. In GNU Octave [73], the region is read and a matrix is created as the first return, and, then, a morphological surface plot is reconstructed by the 3D surface function, as shown in Figure 4, which, in its turn, has solid faces and border colors. Then, the fractal dimension is calculated. Also, by means of the program’s regression function, the curve fitting curve is plotted, which enables a direct estimation of the roughness of this sample. To do this, as the images obtained through the scanning electron microscope are not RGB, they do not need previous treatment, but it is important that the demarcated regions be almost 1:1 of 2s size (for some s) because, otherwise, the image needs to be filled with zeros for the image to be 1:1. Then, the traditional box-counting method (BCM) is employed to determine the fractional properties of our demarcated 2D images. Regarding the yellow sample zone, as the used BCM is based on counting the number of the non-zero submatrices of its corresponding binary image (equivalent to those boxes covering the fractal), its change may imply some different results but not as significant. In other words, all analyses used the same selected area, meaning the values obtained are statistically reproducible.
According to the BCM, to analyze a complex pattern, a sequence of boxes of various smaller and smaller sizes is extracted from the original pattern, in which the size of boxes is always a power of two. Thus, one counts how many boxes overlap the image of interest. It is worth saying that since the size of the boxes varies, the number of boxes overlapping the image also varies. Therefore, by decreasing the size of the boxes more and more, one can observe how this relationship varies and so the fractional dimension is calculated. In fact, the box-counting dimension is estimated by the least squares method through a linear function (see [74] for some discussion). In regard to the quantity of the displayed submatrices, for a yellow sample zone of order n × n pixels, the developed BCM is of a complexity of 0( n 2 [ log 2 ⁡ n ⌉ ) , according to lines 76 to 87 of the code available in the Supplementary Material.
In addition, the bandgap energy provides critical information for a deeper understanding of novel advanced civil engineering materials. Figure 4a shows the experimental direct bandgap values estimated based on the Tauc plots for the diffuse reflectance measurement of Ag2O-decorated SnO2 coatings of the cement paste composites. The peak associated with Ag2O becomes more evident with an increase in the composition of this phase. As expected, a reduction in the bandgap of the Ag2O phase was observed in Figure 4, which is associated with the growth of this material on the SnO2 phase. However, the bandgap of the SnO2 phase decreased by increasing the Ag2O phase, which is mainly associated with a large structural polarization, i.e., due to lattice mismatch that leads to the formation of complex defects in the Ag2O/SnO2 interface.
Figure 5 shows the photocatalytic degradation of the MB solution by the SnO2/cement paste composite in the presence of different scavengers, and the degradation results without the addition of a scavenger (denoted here as WS). Reactive oxygen species are known to promote photodegradation reactions [31,47,75]. They can react with micropollutants, bacteria, or other target species (depending on the specific application) that come into contact with the photocatalyst surface. Based on the observed results, all scavengers play an important role in the photocatalytic mechanism. The photocatalytic reduction of MB in the presence of scavengers was significant in all cases, resulting in MB degradation of approximately 31%, 46%, 42%, and 37% for Ag, AO, ISO, and p-BQ, respectively, compared with the reference value (WS sample). Table 3 lists the kinetic constants obtained from the photocatalytic performance of these samples. Particularly, the cement paste substrate exhibited a kinetic constant (k) of approximately 0.0059 during the photocatalysis process, which is higher than that obtained for dye degradation by films based on titanium [76] and glass [71]. These results are likely owing to the large surface area and porosity of the cement paste sample. Notably, it has a kinetic constant similar to that of SnO2 films coated on polyetherimide [77] confirming that cement paste can be used as an alternative substrate. Moreover, after the photocatalytic process, the cementitious materials exhibited a different surface color, indicating that these samples have a large absorption capacity (see Figure 5h). Also, in Figure 5i, the samples displayed their self-cleaning properties after being exposed again to UV light. The samples visibly had decreased the strong blue color, characteristic of MB, on their surface, after a period of 120 min of exposure to UV light.
The MB for photo-Fenton was evaluated similarly to the previous test; however, for the photo-Fenton process was added H2O2. The reference photocatalysis (WI) showed a degradation rate of 64%; in contrast, the reference sample (Ref) for photo-Fenton degraded 82% of the MB due to the combination of photocatalysis and Fenton reactions.
Furthermore, in Figure 6, it is also possible to notice the increase in degradation with the presence of Ag2O. The best result obtained was a degradation of 93% in the Ag3 sample, in which we had an 11% improvement in efficiency compared to the Ref sample. This result is corroborated in Table 3, in which we can observe the comparison between the kinetic constants. A similar improvement is seen in the literature for the degradation of methyl orange using 1 mMol and 3 mMol of AgNO3 as a precursor of the decorator material [29]. However, this result was obtained using SnO2 in powder form. For comparative purposes, Table 4 presents the kinetic constants and the respective degradation values at the end of the experiment for the samples submitted to photo-Fenton, which used only MB as a dye. Figure 6 illustrates the photo-Fenton degradation results of MB solutions by Ag2O-decorated SnO2/cement paste composites.
The MR is an anionic dye that also works very well as a pH indicator. For example, this compound is red for pH values below 4.4 and becomes yellow for pH values above 6.2; this characteristic also causes changes in its UV–Vis absorption spectrum [78,79,80]. Therefore, pH control is a variable that needs to be controlled during the process. Figure 7 shows the result of MR degradation at pH 6, the same pH used for solutions with MB. Hence, the maximum absorbance of MR for pH 6 occurred at a wavelength of about 426 nm, as shown in Figure 7c [79,81]. Regarding the degradation of the MR dye, its behavior differs from that observed in MB. The first aspect to be analyzed is the reference sample, where the degradation is 19% lower than that observed for the cationic dye.
Thus, the second aspect to be analyzed refers to the decorated material with Ag2O; the respective absorption spectra are shown in Figure 7. With the increase in Ag2O in the composition of the SnO2 coating, we can clearly observe a displacement of the absorption region to a wavelength close to 400 nm; i.e., this movement indicates the possible presence of Ag2O nanoparticles in the collected aliquots [82]. In addition to the possible presence of Ag2O nanoparticles, the decorated samples presented results lower than the reference, reaching a reduction of 51% when compared to the SnO2 coating for the lowest degradation result obtained. A possible explanation for this behavior can relate to the electronic charges of the surface of these materials, which, in turn, can lead to the repulsion of dye molecules due to similar surface charges [81,83,84,85]. The values of the kinetic constant for MR are lower than those obtained for MB; this is due to the low degradation of the dye during the photocatalysis process. Table 5 presents these kinetic constant and degradation values obtained for the samples. This decrease in MR degradation with Ag2O-decorated SnO2 raised the possibility of the coating being selective in relation to pollutants. Due to the possibility of having a selectivity, it was necessary to verify the capacity of the material to degrade different dyes simultaneously. Then, for this purpose, the two dyes were mixed and subjected to the photo-Fenton process, similar to the analysis of the individual dyes.
Figure 8 displays the photocatalytic selective degradation of the MB/MR dye mixtures by the Ag2O-decorated SnO2 coating of the cement paste composite. Since the dyes have different absorption regions in the spectrum, as shown in Figure 8, the different graphs refer to the different wavelengths analyzed for each dye. Despite noting the difference in degradation efficiency, the behavior of MB in the mixture is similar to when analyzed individually. Table 6 presents the kinetic constant values, and it can be observed that the kinetic constant of Ag3 is lower. However, for the mixture, the behavior of MR follows the MB trend with a reduction in the degradation efficiency in Ag1 and an increase in Ag3, when compared to the reference. Therefore, the combination of anionic and cationic dyes presents a favorable characteristic regarding the degradation of MR, once the literature shows that solutions of individual dyes may give better results than when there is more than one dye in the same solution [85,86,87].
Another characteristic of photocatalysis is that it is sensitive to pH variations. In general, the pH of the aqueous medium can influence photocatalysis through absorption between the dyes and the catalyst surface, because the change in pH can affect the charges of the catalyst surface [88]. Therefore, until this moment, all samples that underwent the photocatalysis process were close to neutral with pH ~6. However, to verify the impact of pH in relation to the material produced, it was decided to choose the Ag3 sample that presented the best degradation results so far. Figure 9 shows the comparison of the pH effect on the photo-Fenton process for the Ag3 sample. At different pH levels, the surface of the photocatalyst may become positively or negatively charged. Hence, from this perspective, the surface charge of as-prepared materials can influence the adsorption of reactants and micropollutants onto the catalyst surface. Furthermore, it is well-known that the different pH conditions can also lead to variations in the energy levels of the VB and CB. This, in turn, affects the generation and separation of electron-hole pairs when the photocatalyst is irradiated with UV light.
In an acidic environment, the charge surface of the catalyst becomes positive; because of this, there is a better interaction with the anionic dye, improving the photocatalytic activity [64]. These results are presented in Table 7, where we see that the highest degradation results were obtained in acidic environments. It is also noted that MR is the most affected dye with pH change, thus showing that the coating prefers MB degradation under neutral pH conditions. The MR still showed a change in absorption when the pH of the solution became acidic. Figure 8h shows that the absorption region previously identified at a wavelength of 425 nm for pH 6, moves slightly to 533 nm in solutions with pH 2 and 4, respectively [78,79,80,81].
The acidic pH, in addition to totally degrading the dyes, also degraded it before the 180 min point of the experiment. Thus, the new coating for cementitious materials proposed in this work proved to be efficient for the degradation of pollutants, with this being demonstrated by the complete degradation of both MB and MR dyes. Additionally, the band edge alignment of SnO2 and Ag2O exhibits a specific configuration of type-I straddling gap photocatalysts. Figure 9g displays the charge transfer mechanism occurring in the Ag2O/SnO2 under UV-light irradiation, making them well-suited for photocatalytic applications. Therefore, optimizing the photocatalyst and reaction conditions is crucial for effective and selective photodegradation reactions.

3. Materials and Methods

3.1. Materials

Portland Cement, CP-II-Z (Votorantim Co., Sao Paulo, Brazil), produced in accordance with ABNT NBR 16697:2018 was used. Tin chloride (SnCl2, Sigma-Aldrich, St. Louis, MO, USA) and silver nitrate (AgNO3, Synth, Sao Paulo, Brazil) were used as precursors, and sodium hydroxide (NaOH, Neon, Suzano, SP, Brazil) as mineralizing agent.

3.2. Cemen Paste Mixture

For the samples’ production, we used a water/cement ratio of 0.5 in a silicone mold (25 mm × 25 mm × 4 mm) greased with demolding agent. For the mixing process: (1) cement was added; (2) then water; (3) then, the materials were mixed by hand until completely homogenized. All samples were demolded after 24 h and cured in water until needed for testing.

3.3. SnO2 Coatings of Cement Paste Prepared by Hydrothermal Approaches

It is well-known that there is more growth of nanostructures on a nucleation layer than when grown directly on various substrates [89]. Hence, in this study, the nucleation layer was produced as follows: (1) adding 10 mg of SnCl2 for every 1 mL of acetone; (2) mixing for 16 min with a magnetic stirrer; (3) applying layers of 100 µL (0.1 mL) on the cement samples surface, totaling 5 layers at the end, and always waiting for the previous layer to dry before applying a new one; (4) holding at 350 °C for 30 min inside the muffle furnace, using a heating rate of 10 °C/min.
In parallel, after this procedure described above, the 0.003 mol of SnCl2 was mixed at room temperature with 20 mL of deionized water until the solution turned to a homogeneous yellow color; then, 30 mL of 1 M NaOH solution was added under vigorous agitation. After 10 min, the resulting mixture reaction was added to the cement paste samples with the nucleation layer inside a Teflon-coated stainless-steel autoclave to be heated at 180 °C for 8 h. At the end of the process, the samples were collected at room temperature, then washed with deionized water and dried at the same temperature.

3.4. Ag2O-Decored SnO2/Cement Paste Composites

After hydrothermal growth of nanostructured SnO2 film, a solution containing AgNO3 and deionized water was prepared. The solution was added to the samples inside another autoclave to be heated at 180 °C for 30 min. This procedure was performed using 50 mL of deionized water as a standard and changing the concentrations of AgNO3 (Ag1: 0.01 mmol, Ag2: 0.02 mmol, and Ag3: 0.04 mmol). A schematic diagram of this fabrication process based on one or two-step hydrothermal approach is illustrated in Figure 1.

3.5. Photocatalytic Activity Evaluation

The coated cement paste samples were placed vertically in contact with 100 mL of MB solution (10 mg/L) under magnetic stirring inside a wooden box containing 3 UVC lamps (15 W, G15T8/OF, 254 nm, OSRAM, München, Germany) [64]. The solution was stirred continuously for 60 min in the dark to establish an adsorption–desorption equilibrium, then lights were turned on. These results in the dark were then used to calculate the adsorption of these samples. After lights were turned on for 180 min (3 h), 1 mL aliquots were collected at 20 min intervals. It is well known that photocatalytic reactions are mainly driven by intrinsic reactive oxygen species, such as holes (h•), electrons (e−), superoxide (•O2−), and hydroxyl (•OH) radicals [64,65,66]. Here, for a better understanding, the generation mechanisms of reactive oxygen species involved in photocatalytic degradation was investigated by used of different radical scavengers, such as isopropanol (ISO as •OH scavenger), AgNO3 (Ag as e− scavenger), ammonium oxalate (AO as h• scavenger), and p-benzoquinone (p-BQ as a •O2− scavenger), in similar intervals of time to the photocatalytic process carried out without the scavenger [64,65,66].

3.6. Photo-Fenton Activity Evaluation

The nanostructured SnO2 and Ag2O-decorated SnO2 as a coating of cement paste composite prepared were evaluated for photo-Fenton activity through the decomposition of individual and mixed MB and MR dyes. Again, the cement paste samples were placed in 100 mL of solution; however, the solution now has a 9:1 ratio of peroxide (H2O2), with 90 mL made up of dye solution (10 mg/L) and the remaining 10 mL is H2O2 in a 100 mL solution, using the same time and photocatalytic reactor described above. For the mixture scenario the MB and MR dyes were mixed in a 1:1 ratio, so within 100 mL of solution, we have 45 mL of MB, 45 mL of MR, and 10 mL of H2O2. Furthermore, the highest degradation result for the dye mixture was evaluated under different pH conditions (2, 4, and 6). The collected aliquots were analyzed by the UV–Vis spectrometer in the absorbance mode using a Biochrom spectrometer, model Libra S60. All measures were performed at room temperature with quartz cuvettes.

3.7. Material Characterizations

To characterize the properties of materials from a structural and morphological point of view we used a diffractometer (XRD), D2 Phaser (Bruker, Karlsruhe, Germany) with an X-ray source with a copper anode tube (Cu-Kα), 30 kV, and 10 mA, with a step of 0.02°/s and 60 s for the angular step. The crystalline fraction was obtained by a simple ratio between the crystalline regions (sum of areas corresponding to the peaks in the diffractogram) divided by all-spectrum area (area total of diffractogram). Then, scanning electron microscopy (SEM), Tescan Vega 4 (Brno, Czech Republic), was used to obtain images to verify the presence of the SnO2 coating, as well as its modification with Ag2O, based on the analysis of their respective morphologies; also, the contact angle measurement was performed using the goniometric method, characterized by the direct measurement of the contact angle. Then, the bandgap of these coatings was determined by means of the diffuse reflectance measurements using UV-3600i Plus UV–Vis–NIR spectrophotometer (Shimadzu Corporation, Kyoto, Japan).

4. Conclusions

In summary, we have investigated the processes of hydrothermal synthesis, characterization, and evaluation of the photocatalytic properties of SnO2 and Ag2O-decorated SnO2 as a coating of cement paste composites. The high porosity of the cement paste composites favors the adsorption process of MB and MR dyes on the surface of the photocatalyst, contributing to maximizing its catalytic performance. The degradation of MR showed a lower efficiency when compared to MB for all analyzed samples. The mechanism was studied using different radical scavengers, confirming the formation of reactive oxygen species. For tests carried out in the presence of these scavengers, we observed a reduction in the efficiency of this process between 31% and 46%. The highlight goes to the electron scavenger that had a higher effect on the dye degradation mechanism. On the other hand, we have demonstrated that the Ag2O-decorated SnO2 coating of the cement paste composite exhibits photocatalytic selective degradation of the MB/MR dye mixtures. Using pH values 2 and 4, the degradation of the dyes took place completely; a result that had not been obtained by any of the previous compositions. Furthermore, complete degradation for both dyes took place before 180 min for the two acidic pH values. With pH 6, the degradation of MB stands out compared to MR with a difference of 34% between them. Highlighting the preference of MB degradation, and indicating a selectivity in the process. Hence, this involves considering the material properties, the target pollutants, and the reaction kinetics. On the other hand, it is important to highlight that the application of SnO2 as a catalyst is not as popular as other semiconductors. These findings are elucidated by a structural and morphological analysis of such materials, and may provide new clues to an in-depth understanding of their catalytic nanoscale behavior.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal13121479/s1, The code used in SEM analysis (see Figure 3) can be downloaded.

Author Contributions

D.d.S.V., V.G.B. and A.M.A. Conceptualization, methodology, software, validation, formal analysis, investigation, data curation, and writing—original draft preparation. F.d.A.L.P. Conceptualization, resources, formal analysis, investigation, data curation, writing—review and editing, supervision, project administration, and funding acquisition. All authors have read and agreed to the published version of the manuscript.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Data Availability Statement

Data are contained within the article.

Acknowledgments

The authors gratefully acknowledge the support from the Brazilian agencies CNPq and CAPES. We are also grateful to the LabMulti-LD equipment infrastructure at UTFPR.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 13 01479 g001
Figure 1. Samples with dimensions of 25 mm × 25 mm × 4 mm of cement paste: (a) Reference; (b) with nucleating layer; (c) with SnO2 film; (d) film decorated with 0.01 mMol of AgNO3 (Ag1); (e) film decorated with 0.02 mMol of AgNO3 (Ag2); and (f) film decorated with 0.04 mmol of AgNO3 (Ag3). Illustration of the (g) SnO2 and (h) Ag2O unit cells and (i) schematic diagram for the fabrication process of SnO2 and Ag2O-decorated SnO2 coatings of the cement paste composites.
Figure 1. Samples with dimensions of 25 mm × 25 mm × 4 mm of cement paste: (a) Reference; (b) with nucleating layer; (c) with SnO2 film; (d) film decorated with 0.01 mMol of AgNO3 (Ag1); (e) film decorated with 0.02 mMol of AgNO3 (Ag2); and (f) film decorated with 0.04 mmol of AgNO3 (Ag3). Illustration of the (g) SnO2 and (h) Ag2O unit cells and (i) schematic diagram for the fabrication process of SnO2 and Ag2O-decorated SnO2 coatings of the cement paste composites.
Catalysts 13 01479 g001
Catalysts 13 01479 g002
Figure 2. Result of XRD analysis for the preparation steps: (a) SnO2 and (b) Ag2O-decorated SnO2 coatings of the cement paste composites and (c) SnO2 powder formed in the hydrothermal processing. The peaks identified in Figure (b) are, respectively, (2) Ag2O; (3) CaOH2; (4) C-S-H; (5) Ettringite; (6) Calcite.
Figure 2. Result of XRD analysis for the preparation steps: (a) SnO2 and (b) Ag2O-decorated SnO2 coatings of the cement paste composites and (c) SnO2 powder formed in the hydrothermal processing. The peaks identified in Figure (b) are, respectively, (2) Ag2O; (3) CaOH2; (4) C-S-H; (5) Ettringite; (6) Calcite.
Catalysts 13 01479 g002
Catalysts 13 01479 g003aCatalysts 13 01479 g003b
Figure 3. SEM images of (a) Cement paste; (b) Nucleation layer; (c) SnO2 Film; (d) Ag1; (e) Ag2; (f) Ag3. Red arrows as inset in SEM indicate the corresponding phase of the analyzed samples.
Figure 3. SEM images of (a) Cement paste; (b) Nucleation layer; (c) SnO2 Film; (d) Ag1; (e) Ag2; (f) Ag3. Red arrows as inset in SEM indicate the corresponding phase of the analyzed samples.
Catalysts 13 01479 g003aCatalysts 13 01479 g003b
Catalysts 13 01479 g004
Figure 4. (a–d) Tauc plot for the UV–Vis diffuse reflectance spectra of the Ag2O-decorated SnO2 coatings of the cement paste composites.
Figure 4. (a–d) Tauc plot for the UV–Vis diffuse reflectance spectra of the Ag2O-decorated SnO2 coatings of the cement paste composites.
Catalysts 13 01479 g004
Catalysts 13 01479 g005
Figure 5. (a) Degradation of the MB dye during the photocatalysis process; (b) Kinetic constant; Absorbance spectra for (c) WS, (d) Ag, (e) ISO, (f) AO, and (g) p-BQ; (h) Sliced WS sample after MB photocatalysis process; (i) Samples after self-cleaning process.
Figure 5. (a) Degradation of the MB dye during the photocatalysis process; (b) Kinetic constant; Absorbance spectra for (c) WS, (d) Ag, (e) ISO, (f) AO, and (g) p-BQ; (h) Sliced WS sample after MB photocatalysis process; (i) Samples after self-cleaning process.
Catalysts 13 01479 g005
Catalysts 13 01479 g006
Figure 6. (a) Degradation of MB dye during the photo-Fenton process; (b) Kinetic constant; Absorbance spectra for (c) Ref, (d) Ag1, (e) Ag2 and (f) Ag3.
Figure 6. (a) Degradation of MB dye during the photo-Fenton process; (b) Kinetic constant; Absorbance spectra for (c) Ref, (d) Ag1, (e) Ag2 and (f) Ag3.
Catalysts 13 01479 g006
Catalysts 13 01479 g007
Figure 7. (a) Degradation of the MR dye during the photo-Fenton process; (b) Kinetic constant; Absorbance spectra for (c) Ref, (d) Ag1, (e) Ag2, and (f) Ag3.
Figure 7. (a) Degradation of the MR dye during the photo-Fenton process; (b) Kinetic constant; Absorbance spectra for (c) Ref, (d) Ag1, (e) Ag2, and (f) Ag3.
Catalysts 13 01479 g007
Catalysts 13 01479 g008
Figure 8. Degradation of (a) MB and (b) MR dyes during the photo-Fenton process; Kinetic constant for (c) MB and (d) MR solutions; Absorbance spectra for degraded MB/MR dye mixtures of composites: (e) Ref, (f) Ag1, (g) Ag2, and (h) Ag3.
Figure 8. Degradation of (a) MB and (b) MR dyes during the photo-Fenton process; Kinetic constant for (c) MB and (d) MR solutions; Absorbance spectra for degraded MB/MR dye mixtures of composites: (e) Ref, (f) Ag1, (g) Ag2, and (h) Ag3.
Catalysts 13 01479 g008
Catalysts 13 01479 g009
Figure 9. Ag3 sample submitted to photo-Fenton process with pH ranging from 2 to 6: (a) MB degradation; (b) MR degradation, analyzed at wavelength 426 nm for pH 6 and at wavelength 533 nm for pH 2 and 4; (c) Kinetic constant for MB; (d) Kinetic constant for the MR; (e) Absorbance spectrum for pH 2; (f) Absorbance spectrum for pH 4. (g) Schematic diagram of charge carrier transfer process for the Ag2O-decorated SnO2 coating of cement paste composites during UV light illumination.
Figure 9. Ag3 sample submitted to photo-Fenton process with pH ranging from 2 to 6: (a) MB degradation; (b) MR degradation, analyzed at wavelength 426 nm for pH 6 and at wavelength 533 nm for pH 2 and 4; (c) Kinetic constant for MB; (d) Kinetic constant for the MR; (e) Absorbance spectrum for pH 2; (f) Absorbance spectrum for pH 4. (g) Schematic diagram of charge carrier transfer process for the Ag2O-decorated SnO2 coating of cement paste composites during UV light illumination.
Catalysts 13 01479 g009
Table 1. Results of contact angle measurement for each sample.
Table 1. Results of contact angle measurement for each sample.
Sample Contact Angle (θ)
Cement paste126°
Nucleation layer30°
Film11°
Ag123°
Ag223°
Ag323°
Table 2. Crystallite size for SnO2 and Ag2O at different samples.
Table 2. Crystallite size for SnO2 and Ag2O at different samples.
Samples Crystallite Size (tc)
SnO2—Powder6 nm
SnO2—Film33 nm
SnO2—Ag126 nm
SnO2—Ag232 nm
SnO2—Ag329 nm
Ag2O—Ag137 nm
Ag2O—Ag235 nm
Ag2O—Ag333 nm
Table 3. Photocatalysis parameters with and without scavengers for MB degradation.
Table 3. Photocatalysis parameters with and without scavengers for MB degradation.
Scavenger Kinetic Constant (k) (min−1)Degradation
(%)		Adsorption (%)R2
WS0.005964270.9865
Ag0.002331210.9271
AO 0.003546220.9925
ISO0.003342270.9947
p-BQ0.002637180.9941
Table 4. Kinetic constant and degradation values at 180 min for photo-Fenton processes for MB.
Table 4. Kinetic constant and degradation values at 180 min for photo-Fenton processes for MB.
Sample k (min−1)Degradation
(%)		Adsorption (%)R2
Ref0.009082280.9808
Ag1 (0.01 mMol)0.007976280.9954
Ag2 (0.02 mMol)0.014291340.9873
Ag3 (0.04 mMol)0.015493220.9929
Table 5. Kinetic constant and degradation values at 180 min for photo-Fenton processes for MR.
Table 5. Kinetic constant and degradation values at 180 min for photo-Fenton processes for MR.
Sample k (min−1)Degradation
(%)		Adsorption (%)R2
Ref0.006063310.9491
Ag1 (0.01 mMol)0.001929280.9463
Ag2 (0.02 mMol)0.001427180.8895
Ag3 (0.04 mMol)0.000712220.9020
Table 6. Kinetic constant and degradation values at 180 min for photo-Fenton processes for the MB/MR dye mixtures.
Table 6. Kinetic constant and degradation values at 180 min for photo-Fenton processes for the MB/MR dye mixtures.
Samplesk (min−1)Degradation
(%)		Adsorption (%)R2
Photo-Fenton—Wavelength 665 nm (MB)
Ref0.009279250.9301
Ag1 (0.01 mMol)0.005863190.9730
Ag2 (0.02 mMol)0.007573140.9993
Ag3 (0.04 mMol)0.008479300.9712
Photo-Fenton—Wavelength 425 nm (MR)
Ref0.002134290.9298
Ag1 (0.01 mMol)0.001219310.9220
Ag2 (0.02 mMol)0.002131220.9735
Ag3 (0.04 mMol)0.003245230.9282
Table 7. Kinetic constant and degradation values at 180 min for the Ag3 sample for the mixture of MB and MR under different pH conditions.
Table 7. Kinetic constant and degradation values at 180 min for the Ag3 sample for the mixture of MB and MR under different pH conditions.
pH k (min−1)Degradation
(%)		Adsorption (%)R2
Wavelength 665 nm (MB)
20.0322100220.9509
40.0205100190.9292
60.008479300.9712
Wavelength 425 nm (pH 6) and 533 nm (pH 2 and 4) (MR)
20.0440100160.9427
40.1106100180.9008
60.003245230.9282
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MDPI and ACS Style

Vendramini, D.d.S.; Benatto, V.G.; Ashtiani, A.M.; La Porta, F.d.A. Photocatalytic Applications of SnO2 and Ag2O-Decorated SnO2 Coatings on Cement Paste. Catalysts 2023, 13, 1479. https://doi.org/10.3390/catal13121479

AMA Style

Vendramini DdS, Benatto VG, Ashtiani AM, La Porta FdA. Photocatalytic Applications of SnO2 and Ag2O-Decorated SnO2 Coatings on Cement Paste. Catalysts. 2023; 13(12):1479. https://doi.org/10.3390/catal13121479

Chicago/Turabian Style

Vendramini, Danilo da Silva, Victoria Gabriela Benatto, Alireza Mohebi Ashtiani, and Felipe de Almeida La Porta. 2023. "Photocatalytic Applications of SnO2 and Ag2O-Decorated SnO2 Coatings on Cement Paste" Catalysts 13, no. 12: 1479. https://doi.org/10.3390/catal13121479

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