BiOCl Photocatalyst at Different Temperatures: TEM Insights into Structural Modifications and Photocatalytic Efficiency

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In recent times, contamination of aquatic environments by emerging contaminants has become a critical environmental problem due to their persistence, toxicity, and potential risks to human health and ecosystems. These contaminants include pesticides, drugs, and antibiotics [1]. Pesticides are of particular concern, as they are widely used in agricultural activities and can accumulate in aquatic environments. One of the pesticides most used is 2,4-dichlorophenoxyacetic acid (2,4-D), a synthetic herbicide used to control weeds and pests. Due to its high solubility and resistance to conventional water treatment methods, the removal of 2,4-D from aqueous systems requires the development of advanced and efficient degradation techniques [2]. For the removal of these emerging pollutants in aquatic environments, several advanced oxidation processes (AOPs) have been developed as effective treatment strategies. AOPs are based on the generation of highly reactive species, hydroxyl radicals (•OH), which can degrade complex organic molecules into smaller and less toxic compounds [3]. Among them, heterogeneous photocatalysis has emerged as a particularly promising approach due to its ability to operate in diverse media, to be inexpensive, and easy to implement. Recently, bismuth oxyhalides (BiOX, X = Cl, Br, I) have gained significant attention as efficient photocatalysts due to their unique layered structure, efficient charge separation, and improved photocatalytic performance [4]. Among them, bismuth oxychloride (BiOCl) has shown promising activity under ultraviolet irradiation. The structural and morphological features of BiOCl play a crucial role in its photocatalytic efficiency, and temperature variations during synthesis or treatment can significantly affect its crystalline structure, surface area, and defect density. In this study, we investigated the structural evolution of the BiOCl photocatalyst at two different temperatures using transmission electron microscopy (TEM) and analyzed the impact of these structural changes on its photocatalytic performance for 2,4-D degradation. The synthesis of the materials was carried out by the solvothermal process. Briefly, bismuth nitrate pentahydrate (Bi(NO3)3·5H2O) and sodium chloride (NaCl) were dissolved in equal amounts in 20 mL of an ethanol/ethylene glycol solution. After 60 minutes of stirring magnetic, the suspension was transferred into a Teflon-lined autoclave and heated at two different temperatures 120°C and 200°C, each for 12 hours. The photocatalytic properties were evaluated in the degradation of a 20 ppm solution of 2,4-D under UV irradiation at 254 nm, using a low intensity lamp (4.4 mW/cm2). Figure 1a shows the X-ray diffraction study performed on the samples, where it can be observed that both materials present the characteristic peaks of the BiOCl phase. However, the sample at a lower temperature exhibits a shift in the peaks and broader peaks, suggesting lower crystallinity compared to the BiOCl 200 sample. Figure 1b presents the adsorption and photocatalysis results of 2,4-D on both materials. Both materials present almost no adsorption of 2,4-D, however, when performing the photocatalysis experiments, both materials managed to degrade the contaminant, with the BiOCl 200 material being better, reaching a degradation of 85%, while the BiOCl 120 sample reached approximately 60% degradation. Figure 2 shows transmission electron microscope images. Fig. 2a shows a low magnification image with several of the characteristic distribution of heterostructures in a material synthesized at 120°C. A higher magnification image is shown in Fig. 2b. The interplanar spacing is characteristic of the BiOCl phase. Fig 2c gives a higher magnification view of the BiOCl phase with a Wiener filtering and Fig. 2d increases the visibility in an inverse Fourier transform image where the characteristic atomic layer are more visible. Figure 3 shows similar results for the heterostructure containing the BiOCl phase synthesized at 200°C. Fig. 3a shows an image at relatively low magnification. In this case the morphology of the synthesized heterostructures has changed as well as the phase distribution. Fig. 3b shows a magnified view of the heterostructure where the projected lattice spacings can be related to the BiOCl phase but now with small surface irregularities. Figures 4 c and d show different sections of the heterostructure with similar characteristics. Summarizing, electron microscopy is used for localized phase identification as well as to complete characterization of the properties of the BiOCl heterostructures after different synthesizing times. The observed properties can be explained on the basis of the achieved characterization [5]. (a) X-ray diffraction analysis of BiOCl 120 and BiOCl 200 materials. (b) Adsorption and photocatalysis results of 2,4-D using BiOCl 120 and BiOCl 200 materials. Transmission electron images of the BiOCl heterostructure synthesized at 120°C. (a) Low magnification view of the heterostructure. (b) Magnified section of (a) see indicated area. (c) Wiener filtering of image in (b). (d) IFFT of selected area showing lattice more clearly. Transmission electron images of the BiOCl heterostructure synthesized at 200°C. (a) Low magnification view of the heterostructure. (b) Magnified section of (a) see indicated area. (c) Image from a different area and phase identification is confirmed. (d) Magnified area of image in (c) and showing planar lattice spacings more clearly.