Strategical Metal Oxide-Based Nanocomposites for the Effective Removal of Toxic Pollutants

Abstract

With the dawn of nanotechnology and materials science at the nanoscale, newer applications have been emerging on yearly basis. During the last few decades, a lot of advancement has taken place in the field of nanomaterials that evolved interesting applications such as photocatalytic degradation of various toxic reagents associated with water pollution. At the same time, analogous materials have shown promising candidacy for water splitting reaction that is one of the most encouraging fields of science these days. Despite a little difference between these two applications, there are few common characteristics in-between various nanomaterials proving proficient for both applications such as harvesting of solar light. Enhanced solar light absorbance has been a key factor for the development of newer nanomaterials including but not limited to nanocomposites. As we know that the development of nanocomposites is a straightforward way to combine two or more than two materials with a different set of optical and/or electronic characteristics including but not limited to the harvesting of solar light, a lot of research methodologies have been reported to realize smarter and efficient materials with superior photocatalytic properties. In the same context, the methods associated with the development of nanocomposites can be divided into two types; (a) High-tech deposition methods such as atomic layer deposition (ALD) and (b) Simplistic wet-chemical methods such as successive ionic layer adsorption and reaction (SILAR) method. Both these methods are pretty much similar in a way that they involve exposure of reactants in distinct cycles and a suitable cleaning step in-between the ionic/molecular exposures. ALD is a more efficient process than SILAR as it deals with the reactants in gaseous states while wet/liquid precursors are used in the SILAR process which limits its proficiency for many materials. Interestingly, both SILAR and ALD have been recently employed for deposition of various materials on the substrate of particles nature (i.e. TiO2 and ZnO, etc.). Therefore, many challenges are there which need to be addressed. For example, it is quite hard to deposit a uniform, crystalline ZnO shell on substrate particles (i.e. nano- and micro-particles) including but not limited to SiO2. This is so because the crystallization of wet-chemically deposited ZnO often needs thermal treatment which originates the formation of an undesirable oxide of Zn and Si both on the one hand, while on the other hand, the deposition of ZnO is non-uniform which makes it least possible to develop SiO2@ZnO core-shell particles for various applications such as scattering of light inside the matrix of emerging solar cells, etc. In such a situation ALD proves as an efficient alternate to wet-chemical methods including SILAR. Similarly, SILAR has its own advantages over ALD which includes cost-effectiveness mainly. However, a lot of optimization and reaction control is needed to balance the advantageous deposition trend of ALD. In this work, it is shown that both these processes can be efficiently tailored and provide a diverse option to prepare proficient photocatalytic nanocomposites for harvesting/absorbing ultraviolet (UV) and/or visible irradiation from the solar spectrum of light. ALD is used where the conventional techniques for the development of nanocomposites fail and provided highly controlled and proficiently designed nanocomposites for any functional application (e.g. SiO2@ZnO), while SILAR (i.e. pseudo-SILAR for particle depositions) is a good option for preparing cost-effective nanocomposites (e.g. TiO2-SnO) where ALD has its limitation of yield. In the first strategic approach, we developed SiO2@ZnO core-shell particles by atomic layer deposition with conformal and crystalline ZnO layer in the form of a shell on SiO2 particles. A 20-nm thick ZnO shell on SiO2@ZnO core-shell structure evolved a UV-light dependent photocatalytic behavior in it which resulted in effective degradation of toxic Rhodamine B dye in aqueous form. The dye degradation test proficiently proved that the resultant core-shell particles had enhanced UV-light harvesting tendency and capable of performing well for other photonic applications too. An extensive microscopic investigation was carried out on the designed SiO2@ZnO structure involving SEM and TEM, additionally supported by energy dispersive spectroscopy in both cases. Quite a few other characterizations including PL, UV-Visible absorbance spectroscopy, XRD, Raman, and FTIR, etc. were are utilized to affirm the core-shell nature of SiO2@ZnO nanocomposites, which we reported as the novelty of our work. In the second strategy, nano-sized SnO is deposited on TiO2 nanoparticles using a very simple approach that involved half-of the conventional pseudo-SILAR process. By simply going through the adsorption of Sn-ions, TiO2 particles show the tendency to get transformed into TiO2-SnO nanocomposites upon thermal treatment under an ambient environment. Being visible light active, the presence of SnO over TiO2 enable it to harvest visible light significantly and thus to degrade Rhodamine B dye at a higher rate than commercially available TiO2. Such a novel approach can though be applied for the development of various SnO and/or SnO2 based nanocomposites, however, we focused on TiO2-SnO to devise a strategic route basically. Meaningful results were obtained from XRD, reflecting that it is important to measure the extent of transformation of Sn to SnO, as Sn also co-exists in TiO2-SnO nanocomposites until its complete thermal transformation to SnO. XRF, SEM coupled with EDS, UV-Vis absorbance, and PL spectroscopy were associated with the development and performance of TiO2-SnO nanocomposites. We believe that these strategies will not only grab the attention for the development of more nanocomposites associated with the deposition of ZnO shell and SnO particles separately, the exploration of newer applications for SiO2@ZnO and TiO2-SnO will also be attempted. Upon further revelation and optimization, there can be a good opportunity to develop TiO2-SnO2 and complex oxides of SiO2 and ZnO, TiO2, and SnO. Our results also show that ALD and SILAR are suitable for developing efficient nanocomposites where other techniques fail or prove less effectual.