Highly Porous SnO2/TiO2 Heterojunction Thin-Film Photocatalyst Using Gas-Flow Thermal Evaporation and Atomic Layer Deposition

The Influence of Synthesis Methods and Experimental Conditions on the Photocatalytic Properties of SnO2: A Review

Semiconductors based on transition metal oxides represent an important class of materials used in emerging technologies. For this, the performance of these materials strongly depends on the size and morphology of particles, surface charge characteristics, and the presence of bulk and surface defects that are influenced by the synthesis method and the experimental conditions the materials are prepared. In this context, the present review aims to report the importance of choosing the synthesis methods and experimental conditions to modify structural, morphological, and electronic characteristics of semiconductors, more specifically, tin oxide (SnO2), since these parameters may be a determinant for better performance in various applications, including photocatalysis. SnO2 is an n-type semiconductor with a band gap between 3.6 and 4.0 eV, whose intrinsic characteristics are responsible for its electrical conductivity, good optical characteristics, high thermal stability, and other qualities. Such characteristics have provided excellent results in advanced oxidative processes, i.e., heterogeneous photocatalysis applications. This process involves semiconductors in the production of hydroxyl radicals via activation by light absorption, and it is considered as an emerging and promising technology for domestic-industrial wastewater treatment. In our review article, we focused on the photodegradation of different organic dyes and types of persistent organic pollutants using SnO2-based photocatalysts, and how the efficiency of these materials can be impacted by synthesis methods and experimental conditions employed to prepare them.

Photocatalytic Activity in the In-Flow Degradation of NO on Porous TiO2–Coated Glasses from Hybrid Inorganic–Organic Thin Films Prepared by a Combined ALD/MLD Deposition Strategy

A combined ALD/MLD (where ALD and MLD stand for atomic and molecular layer deposition, respectively) deposition strategy using TiCl4, H2O and HQ (hydroquinone) as precursors has been applied for the preparation of inorganic–organic thin films on soda-lime glasses. The alternate deposition of TiO2 layers, by pulsing TiCl4/H2O (ALD), and hybrid layers, using TiCl4/HQ (MLD), results in the formation of thin films that are precursors for porous TiO2-coatings after removal of the HQ template by annealing. The coated-glassed show good photocatalytic activity in the degradation of NO with up to 15% reduction of NO concentration in three successive photocatalytic cycles of 5 h each. Surface Scanning Electron Microscopy (SEM) images show that the TiO2-coating is composed of large grains that are made up of finer subgrains resulting in a porous structure with an average pore size of 3–4 nm. Transmission Electron Microscopy (TEM) images show two regions, a porous columnar structure on top and a denser region over the glass substrate. Energy Dispersive X-Ray (EDX) analysis, nanocrystal electron diffraction and Raman spectroscopy confirm the presence of the anatase phase, which, together with the porosity of the material, accounts for the observed photocatalytic activity.

Removal of Paracetamol from Aqueous Solutions by Photocatalytic Ozonation over TiO2-MexOy Thin Films

Analgesics and nonsteroidal anti-inflammatory drugs (NSAIDs) such as paracetamol, diclofenac, and ibuprofen are frequently encountered in surface and ground water, thereby posing a significant risk to aquatic ecosystems. Our study reports the catalytic performances of nanosystems TiO2-MexOy (Me = Ce, Sn) prepared by the sol-gel method and deposited onto glass slides by a dip-coating approach in the removal of paracetamol from aqueous solutions by catalytic ozonation. The effect of catalyst type and operation parameters on oxidation efficiency was assessed. In addition to improving this process, the present work simplifies it by avoiding the difficult step of catalyst separation. It was found that the thin films were capable of removing all pollutants from target compounds to the oxidation products.

Graphene Family Nanomaterials (GFN)-TiO2 for the Photocatalytic Removal of Water and Air Pollutants: Synthesis, Characterization, and Applications

Given the industrial revolutions and resource scarcity, the development of green technologies which aims to conserve resources and reduce the negative impacts of technology on the environment has become a critical issue of concern. One example is heterogeneous photocatalytic degradation. Titanium dioxide (TiO2) has been intensively researched given its low toxicity and photocatalytic effects under ultraviolet (UV) light irradiation. The advantages conferred by the physical and electrochemical properties of graphene family nanomaterials (GFN) have contributed to the combination of GFN and TiO2 as well as the current variety of GFN-TiO2 catalysts that have exhibited improved characteristics such as greater electron transfer and narrower bandgaps for more potential applications, including those under visible light irradiation. In this review, points of view on the intrinsic properties of TiO2, GFNs (pristine graphene, graphene oxide (GO), reduced GO, and graphene quantum dots (GQDs)), and GFN-TiO2 are presented. This review also explains practical synthesis techniques along with perspective characteristics of these TiO2- and/or graphene-based materials. The enhancement of the photocatalytic activity by using GFN-TiO2 and its improved photocatalytic reactions for the treatment of organic, inorganic, and biological pollutants in water and air phases are reported. It is expected that this review can provide insights into the key to optimizing the photocatalytic activity of GFN-TiO2 and possible directions for future development in these fields.

Oxygen-Deficient WO3/TiO2/CC Nanorod Arrays for Visible-Light Photocatalytic Degradation of Methylene Blue

At present, TiO2 is one of the most widely used photocatalytic materials. However, the narrow response range to light limits the photocatalytic performance. Herein, we reported a successful construction of self-doped R-WO3/R-TiO2/CC nanocomposites on flexible carbon cloth (CC) via electrochemical reduction to increase the oxygen vacancies (Ovs), resulting in an enhanced separation efficiency of photo-induced charge carriers. The photocurrent of R-WO3/R-TiO2/CC at −1.6 V (vs. SCE) was 2.6 times higher than that of WO3/TiO2/CC, which suggested that Ovs could improve the response to sunlight. Moreover, the photocatalytic activity of R-WO3/TiO2/CC was explored using methylene blue (MB). The degradation rate of MB could reach 68%, which was 1.3 times and 3.8 times higher than that of WO3/TiO2/CC and TiO2/CC, respectively. Furthermore, the solution resistance and charge transfer resistance of R-WO3/R-TiO2/CC were obviously decreased. Therefore, the electrochemical reduction of nanomaterials enabled a promoted separation of photogenerated electron–hole pairs, leading to high photocatalytic activity.