Synthesis of Superior Photocatalytic Nanomaterials Using Specially-Designed ALD Rotary Reactor

Abstract

With the advent of nanomaterials and nanotechnology, the modern scientific world has inclined its technological progress towards the development of nanoscale materials. On one hand, newer engineering applications such as solar cells, batteries, cell phones, etc. are involving a huge portion of nanofabrication, while newer applications such as water splitting, etc. are emerged owing to the expertise we have developed in the field of nanomaterials, on the other. If we analyze precisely, almost all the conventional science and technology fields are influenced by nanomaterials. Starting from the drug delivery, we can make a long list including agronomical fertilizers, industrial catalysts, antimicrobial agents, thermal barrier coatings, etc. those have recently been linked with nanomaterials and quite an affirmative output is obtained in research and development of commercialized products. Without any reluctance, we can say that nanomaterials have laid the stones of the modern technological world which will keep on emerging in years to come. So far, the development of nanomaterials has been dependent on wet-chemical methods such as chemical-bath deposition (CBD), sol-gel and successive ionic layer adsorption and reaction (SILAR) etc. But with the passage of time, high-tech technologies have also been associated with the performance, optimization and development of nanomaterials. For example, chemical vapor deposition (CVD) and atomic layer deposition are directly and enormously involved in the deposition of nanoscale films for semiconducting applications which are least possible otherwise. However, when considering the deposition of thin films on individual nanoparticles, not only the conventional wet-chemical deposition techniques become least effectual; the high-tech techniques also face a challenge. Having a conformal, nanoscale, structurally complex deposition of materials on individual nanoparticles with required characteristics is yet a challenge. For example, a wet-chemically deposited passivation layer on nanoparticles can improve its photostability in quantum-dot sensitized solar cells, however, it can create a charge transfer problem at the same time, which is entirely dependent on its thickness and can be controlled only by having a nanoscale (sometimes sub-nanoscale) deposition of the passivation layer. Alongside, wet-chemical methods are less effectual for realizing the full coverage of nanomaterials, particularly 1D and 2D nanomaterials such as carbon nanótubes (ÇNT) and graphène, etc. To meet these challenges, our laboratory has developed a specialized atomic layer deposition set-up coupled with a rotary reactor, which ensues effectual deposition of thin films on individual nanoparticles. Unlike, conventional wet-chemical methods, it provides us control on nanoscale deposition and the complexity of the substrate particles. While at the same time, unlike conventional ALD, it is specialized for nanomaterials and not for films on planar substrates. Pretty newly, I have employed the same specially-designed ALD rotary reactor for dealing with the above-stated challenges, while the superior photocatalytic performance of synthesized nanomaterials is also realized concomitantly. In the first case, which is totally a new concept in surface treatment of individual nanoparticles, an H2S-treatment was introduced for TiO2 NPs. Owing to the rotational organization of TiO2 nanoparticles, H2S gas introduced to the reactor develops apposite physico-chemical bonding with TiO2 and enables us to enhance the deposition of quantum-dots (QDs) via psudo-sucçessive ionic layer adsorption and reàction (p-SILAR) process. In this study, the main purpose was to introduce a modernized surface treatment for nanoparticles which enhances the performance of conventional wet-chemical processes, indirectly improving the photocatalytic performance of nanomaterials. TiO2-PbS and TiO2-CdS photocatalysts were chosen for affirming the appropriateness of modernized H2S-treatment in a specialized ALD rotary reactor. Resultantly, a higher amount of PbS as well as CdS QDs was deposited on TiO2 and ~12% improvement in the photocatalytic performance under visible light (λ>400nm) was obtained against toxic dye (Rhodamine B ~RhB). The resultant nanomaterials were in-depth characterized using SEM, XPS, UV-Vis spectroscopy, while their qualitative features were affirmed using electrochemical measurements as well. More importantly, we employed the same reactor for depositing MoS2 on graphitic carbon nitride (GCN). GCN being one of the most attractive 2D photo-electro-chemical nanomaterials for various optoelectronic applications, needs to be coupled with low bandgap materials for extending its absorbance range to higher wave lengths of solar irradiation so that the performance of GCN is realistically improved. Owing to its high surface area and porous nature, GCN is least supportive for high deposition of MoS2 by conventional wet-chemical methods. In parallel, the stoichiometric diversity of MoS2, which enables it for superior performance in energy applications such as hydrógen evolution rèaction (HER), is compromised due to poor control on deposition of MoS2. It is to reiterate that MoS2 developed in tetragonal structure or with deficiencies at its structural ends is more proficient photoenergy applications, however, such control is more plausibly achieved using precursor control by ALD. This was why, deposition of MoSX on GCN was optimized first, which is quite novel itself. And then, its performance was realized as GCN-MoSX nanocomposite, ensuing a significant increase in HER and photocatalytic degradation of RhB dye. Detailed SEM, TEM, XPS, PL, and EDS-based material characterization was done to support the characteristics appositeness of MoSX deposited on GCN using specialized ALD rotary reactor. Herein, it is summarized that specialized ALD rotary reactor was successfully used for modernized H2S-treatment of TiO2 nanoparticles as well as for deposition of MoSX on GCN, which in turn realized the superior performance of conventionally developed TiO2-QDs nanocomposites and GCN. It affirms the multipurpose efficacy of specialized ALD rotary reactor for the development of superior photocatalysts