Electron Microscopy of 2D/3D ZnAl/ZnSn(OH) Hydrotalcite/Zinc Tin Composite Nanophotocatalyst

This composite material is developed as a photocatalytic heterojunction with activity to degrade the emerging contaminants (EC) in water. The EC include pharmaceutical compounds as antibiotics, anti-inflammatories, analgesics, regulatory blood lipids and natural and synthetic hormones. These contaminants are quite alarming due its low biodegradability, high persistence, ease of bioaccumulation, and depending upon absolute concentration, they can cause different damages to the environment. The main phase in the composite material is the hydrotalcite, also known as layered double hydroxides (LDHs). Hydrotalcite-type materials (either as pure systems, composites or in heterojunctions) can be applied in a variety of photocatalytic processes for the degradation of different contaminants in water such as reduction of heavy metals [1], degradation of organic compounds [2] including antibiotics [3] among others. This investigation deals in the direction of synthesizing a heterostructure that can contribute to elimination of paracetamol from water. The coprecipitation method is used to synthesize the ZnAl/ZnSn(OH) composite using urea as a reducing agent. Sodium hydroxide is later added as pH controller. A full description of the synthesis procedure is given elsewhere [4]. Electron microscopy is performed in low dose conditions in a Cs corrected electron microscope (Titan) and by using an accelerating voltage of 200 KeV. Image focal series (40 images) are taken in low dose conditions of around 20 e-/Å2 s, they are processed to determine phase and amplitude images in the software MacTempass®. Different image filters are used to as indicated, they are in all cases carefully chosen so as not to introduce new un-existing features. The material under consideration is based upon ZnAl/ZnSn(OH). It is synthesized to produce basically two compounds as shown in Fig. 1. Here the result of X-Ray Diffraction is given showing two phases, i.e, Zinc/Tin hydroxide and Zn-Al carbonate hydroxide hydrate (Zn6Al2(OH)16CO3.4H2O). Scanning electron microscopy shows these two phases as cuboidal particles impregnated by a continuous phase (see Fig. 2). The cubes have a relatively small size of approximately 2.5 mm and the laminar compound shows small pieces of ZnAl sheet. The phase distribution helps improving the photocatalityc properties as shown elsewhere [4]. Transmission electron microscopy gives details at a molecular and atomic level in Figures 3 and 4. Transmission electron microscopy is performed in low dose conditions so as to avoid destructive beam sample interactions. A dose of approximately 20 e-/Ås is used to acquire focal series from +20 nm to -20 nm in a focus step of 1 nm. As a result, single images lack appropriate contrast to be visible but after an exit wave reconstruction procedure, both phase and amplitude image can be developed. The phase image shows atomic contrast and will be here preferentially presented to describe the atomic distribution of the hydroxide sample. X ray diffraction pattern of the ZnAl/ZnSn(OH) sample. Two different phases are identified Zinc/Tin hydroxide and Zn-Al carbonate hydroxide hydrate (Zn6Al2(OH)16CO3.4H2O). The PDF chart numbers are 38-0486 and 73-2384. Scanning electron image of the the ZnAl/ZnSn(OH) sample. Cuboidal particles of the ZnSn hydroxide in the micrometer range covered by a laminar compound. Fig. 3 shows a phase image and the corresponding Normal Background Subtracted Phase filtered image. As shown, the image has a visible contrast with some crystalline parts that can be related to the ZnSn hydroxide. They are most likely small parts that result from fracturing of the larger cuboidal particles shown in Fig. 2. Other crystalline small regions can also be observed and they are most likely related to the improvement of the photocatalytic properties of the material. The Background subtraction filter has no effect of the characteristics of the image and it only provides a more direct interpretation. Fig. 3 also shows regions near the edge of the sample with a wavy contrast, this is most likely related to fibers or a laminar part of the sample as expected for the Zn-Al carbonate hydroxide hydrate phase. Fig. 4 shows electron microscopy images with atomic contrast after EWR procedure. Fig. 4a gives a view of a relatively larger area with laminar and nanoregions with the characteristics of metallic hydroxides (ZnSn in this case). Here both the laminar characteristics of the composite material can be clearly observed in relatively large regions around the edge of the sample. Phase images of ZnAl/ZnSn(OH) composite nanophotocatalyst after exit wave reconstruction. (a) Phase image showing nanocrystalline regions (ZnSn hydroxides) and laminar components. (b) Background subtracted phase image showing nanocrystalline spatial distribution of composite phases. See text for explanation of electron microscopy procedure. Fig. 4b shows a sample region with a relatively higher magnification, the lamellar and nanocrystalline phases are visible. The corresponding filtering through a selective mask involving only the crystalline arrangement is shown in Fig. 4c. This is the same area as in Fig. 4b. Once the mask is applied, an Inverse Fourier Transform is (IFFT) used to recover the contrast in the regions included in the mask. Both, the FFT and the selective mask are applied with the MacTempass software. Fig. 4c shows the IFFT of the phase image in Fig. 4b, it shows crystalline nanoregions in the sample. Nanocrystalline regions can also be seen directly in the phase images and a clearer example is given in Fig. 4d. Here a relatively small region is magnified to show the crystalline components of the sample [4]. Electron microscope phase images after EW reconstruction. (a) Phase image Showing lamellar and crystalline regions particularly at the edge of the sample. (b) Phase image after background subtraction filtering showing lamellar and crystalline regions. (c) IFFT of image Fig. 4b, only crystalline reflections are filtered by using the corresponding mask. (d) Crystalline region in the sample in a direct phase image after EWR.