Role of reactive oxygen species on the activity of noble metal-doped TiO2 photocatalysts

Unravelling the Mechanisms that Drive the Performance of Photocatalytic Hydrogen Production

The increasing interest and applications of photocatalysis, namely hydrogen production, artificial photosynthesis, and water remediation and disinfection, still face several drawbacks that prevent this technology from being fully implemented at the industrial level. The need to improve the performance of photocatalytic processes and extend their potential working under visible light has boosted the synthesis of new and more efficient semiconductor materials. Thus far, semiconductor–semiconductor heterojunction is the most remarkable alternative. Not only are the characteristics of the new materials relevant to the process performance, but also a deep understanding of the charge transfer mechanisms and the relationship with the process variables and nature of the semiconductors. However, there are several different charge transfer mechanisms responsible for the activity of the composites regardless the synthesis materials. In fact, different mechanisms can be carried out for the same junction. Focusing primarily on the photocatalytic generation of hydrogen, the objective of this review is to unravel the charge transfer mechanisms after the in-depth analyses of already reported literature and establish the guidelines for future research.

Hierarchical TiO2 Nanoflower Photocatalysts with Remarkable Activity for Aqueous Methylene Blue Photo-Oxidation

This study systematically evaluates the performance of a series of TiO₂ nanoflower (TNF) photocatalysts for aqueous methylene blue photo-oxidation under UV irradiation. TNF nanoflowers were synthesized from Ti(IV) butoxide by a hydrothermal method and then calcined at different temperatures (T = 400-800 °C) for specific periods of time (t = 1-5 h). By varying the calcination conditions, TNF-T-t photocatalysts with diverse physicochemical properties and anatase/rutile ratios were obtained. Many of the TNF-T-1 photocatalysts demonstrated remarkable activity for aqueous methylene blue photo-oxidation at pH 6 under UV excitation (365 nm), with activities following the order TNF-700-1 > TNF-600-1 > TNF-500-1 > TNF-400-1 ∼ P25 TiO₂ ≫ TNF-800-1. The activity of the TNF-700-1 photocatalyst (99% anatase, 1% rutile) was 2.3 times that of P25 TiO₂ at pH 6 and 14.4 times that of P25 TiO₂ at pH 4. Prolonged calcination of the TNFs at 700 °C proved detrimental to dye degradation performance due to excessive rutile formation, which reduced the photocatalyst surface area and suppressed OH^• generation. The outstanding activities of TNF-700-1 and TNF-600-1 are attributed to their hierarchical nanoflower morphology which benefitted UV absorption, a near-ideal anatase crystallite size for efficient charge separation, and their unusually low isoelectric point (IEP = 4.3-4.5).

Visible light activation of persulfate by magnetic hydrochar for bacterial inactivation: Efficiency, recyclability and mechanisms

The development of "green" water disinfection technology utilizing solar energy is highly desired but remains challenging. In this study, sulfate radical (•SO₄^-)-mediated bacterial inactivation was first attempted by using Fe₃O₄-based magnetic hydrochar (MHC) as a recyclable catalyst for persulfate (PS) activation under visible light (VL) irradiation. Complete treatment of 8.0 log E. coli cells was reached within 40 min in VL/PS/MHC system, compared with that of only 2.0 log-reduction was obtained in the PS/MHC system under the same conditions. The system was applicable in wide range of pH (3.0-9.0), and increasing dissolved O₂ could further promote the efficiency. A three-route mechanism was proposed, in which the PS activation by ≡Fe(II) of Fe₃O₄ and photo-generated electron captured by PS were the major processes. The bacterial cell lesion process was found to be triggered directly via •SO₄^-, which caused the damage of outer membrane, followed by up-regulation of intracellular ROSs and destroy of chromosomal DNA, finally leading to irreversible cell death. Moreover, the VL/PS/MHC system is also effective to inactivate versatile pathogenic bacteria including P. aeruginosa and S. aureus. As a proof-of-concept, our study provides meaningful information to advance the areas of "green" water disinfection technology which can be realized by recyclable photocatalytic systems using solar energy.

Critical review on the mechanistic photolytic and photocatalytic degradation of triclosan

The environmentally extended presence of triclosan, TCS, component of many pharmaceutical and personal care products, and its known persistent character have awoke the scientific and social concern leading to the study of effective remediation techniques. Advanced oxidation techniques stand out for the effectiveness in degrading many persistent compounds, and as a result, they have been addressed by many researchers. However, the powerful oxidation media might lead to the formation of undesirable by-products, concern that has also been widely addressed. With regard to the presence of TCS, photolytic and photocatalytic processes provide a very effective degradation yield and rate, with a large number of reports addressing its removal from different environmental matrices. But currently, there is no clear understanding of the mechanisms involved and the routes responsible for the formation of degradation products. Thus, this work presents an exhaustive and critical analysis of the state of the art related to the photo-degradation of TCS, with special focus on the formation of oxidation by-products, on the phenomena responsible and on the influence of operation variables. This report aims at offering valuable information to researchers dealing with this environmentally relevant problem.

Highly Intensified Molecular Oxygen Activation on Bi@Bi2MoO6 via a Metallic Bi-Coordinated Facet-Dependent Effect

Construction of the semimetal/semiconductor composite interface is widely used to promote the O₂ molecule adsorption and charge transfer for boosting solar-driven molecular oxygen activation (MOA). Herein, a Bi@Bi₂MoO₆ heterostructure is fabricated via a two-step wet chemical method as a typical photocatalyst to investigate the underlying mechanism of Bi-coordinated facet-dependent MOA under visible-light illumination. Density functional theory and systematical characterization methods reveal the distinct charge transfer and O₂ activation processes on the surface of Bi nanoparticle-deposited Bi₂MoO₆ nanosheets with different facets exposed. By virtue of a particular and efficient [Bi₂O₂]²⁺ → Bi → MoO₄^2- interfacial charge-transfer channel, Bi deposited on the (001) facet of Bi₂MoO₆ can observably intensify MOA, thereby giving birth to more generation of reactive oxygen species and endowing the Bi@Bi₂MoO₆ with excellent photocatalytic performance in sodium pentachlorophenate (NaPCP) removal. The decomposition pathway of NaPCP is also proposed based on the intermediate determination and mineralization analysis. This work provides deep insights into the mechanism of facet-dependent MOA over a semimetal-semiconductor system and also sheds light on designing effective molecular oxygen-activated interface for environmental remediation.

Hierarchical Z-scheme 1D/2D architecture with TiO2 nanowires decorated by MnO2 nanosheets for efficient adsorption and full spectrum photocatalytic degradation of organic pollutants

A novel hierarchical 1D/2D TiO₂/MnO₂ composite shows superior adsorption, full spectrum photocatalytic activity and excellent stability for organic pollutant removal.

Vertically aligned Pt/TiO2 nanobelt films on Ti sheets for efficient degradation of a refractory ethyl thionocarbamate collector

Noble metal modified TiO2 nanostructures on a substrate featuring a two-dimensional (2D) morphology are of great interest in wastewater remediation due to high photocatalytic activity and avoidance of separating powder catalysts from water. In this work, vertically aligned Pt/TiO2 nanobelt films (Pt/TNFs) on Ti sheets were fabricated via a synthesis strategy including an alkaline hydrothermal treatment and electrostatic self-assembly. The Pt/TNFs had a BET specific surface area of 93.35 m2 g-1, showing high adsorption capacity in removing an ethyl thionocarbamate (ETC) flotation collector. After the deposition with Pt nanoparticles, the photocatalytic activity of the TNFs increased by 94.98% with the enhanced mineralization of the ETC collector. Moreover, the Pt/TNFs on Ti sheets exhibited strong substrate adhesion enabling superior photocatalytic stability in the cyclic degradation of ETC. The solid phase extraction and gas chromatography-mass spectrometry (SPE/GC-MS) analysis revealed that seven byproducts still remained even when 100% of ETC was degraded, showing the difficulty in the complete mineralization of the ETC collector. The Pt/TNF can serve as a promising photocatalyst to treat mineral flotation wastewaters containing organic reagents.

New Evidence of the Enhanced Elimination of a Persistent Drug Used as a Lipid Absorption Inhibitor by Advanced Oxidation with UV-A and Nanosized Catalysts

This work demonstrates new evidence of the efficient destruction and mineralization of an emergent organic pollutant using UV-A and titanium nanosized catalysts. The target compound considered in this work is the primary metabolite of a lipid regulator drug, clofibrate, identified in many studies as refractory during conventional wastewater treatment. The photocatalytic performance study was carried out in batch mode at laboratory scale, in aqueous suspension. Kinetic data showed that titanium dioxide P25 Aeroxide® exhibits the highest photocatalytic efficiency compared to the other investigated catalysts. Pollutant degradation and mineralization efficiencies strongly increased when decreasing the initial substrate concentration. Target molecules oxidized faster when the catalyst load increased, and the mineralization was enhanced under acidic conditions: 92% of mineralization was achieved at pH 4 after 190 min of reaction. Radical quenching assays confirmed that HO• and ( h vb + ) were the reactive oxygen species involved in the photocatalytic oxidation of the considered pollutant. In addition, further results revealed that the removal efficiency decreased in real water matrices. Finally, data collected through a series of phytotoxicity tests demonstrated that the photocatalytic process considerably reduces the toxicity of the treated solutions, confirming the process’s effectiveness in the removal of persistent and biorefractory emergent organic water pollutants.

Solar Photocatalytic Degradation of Sulfamethoxazole by TiO2 Modified with Noble Metals

Application of solar photocatalysis for water treatment is intensively studied. In this work, we investigated TiO2 modified with platinum (Pt/TiO2) and palladium (Pd/TiO2) using sulfamethoxazole (SMX) as the model contaminant. We considered the following parameters: (i) level of TiO2 modification with Pt/Pd, (ii) initial concentration of photocatalysts, (iii) geographic location where processes were conducted, and (iv) natural water matrix. The catalysts characterized by SEM, EDX, DRS, and XRD techniques showed successful deposition of Pd and Pt atoms on TiO2 surface that enabled light absorption in the visible (Vis) range, and therefore caused efficient SMX removal in all tested conditions. A comparison of the rate constants of SMX degradation in various conditions revealed that modification with Pd gave better results than modification with Pt, which was explained by the better optical properties of Pd/TiO2. The removal of SMX was higher with Pd/TiO2 than with Pt/TiO2, independent of the modification level. In the experiments with the same modification level, similar rate constants were achieved when four times the lower concentration of Pd/TiO2 was used as compared with Pt/TiO2. Formation of four SMX transformation products was confirmed, in which both amine groups are involved in photocatalytic oxidation. No toxic effect of post-reaction solutions towards Lepidium sativum was observed.

Titanium dioxide modified with silver by two methods for bactericidal applications

“Titanium dioxide (TiO₂) is a semiconductor material that exhibits antibacterial activity due to its photocatalytic properties under ultraviolet light. On the other hand, silver also exhibits strong antibacterial activity towards a wide range of microorganisms and TiO₂ with silver addition exhibits more efficient photocatalytic properties than unmodified TiO₂. In this work, TiO₂ nanoparticles were synthesized by the hydrothermal method and modified with silver by two different methods: wet impregnation (Ex situ) and In situ incorporation. The antimicrobial activity of TiO₂ nanoparticles synthesized and modified by both methods was evaluated against Escherichia coli and Staphylococcus aureus. The results showed that TiO₂ nanoparticles have anatase phase. Also, spherical morphology with a mean particle size around 10.6 nm was obtained. The presence of silver in the modified TiO₂ nanoparticles was confirmed by EDS and XPS. TiO₂ particles modified by the Ex situ method, showed a better bactericidal activity compared to the particles modified by In situ incorporation method and TiO₂ unmodified nanoparticles. This study demonstrated that both methods used to modify the titanium dioxide nanoparticles are effective as bactericidal materials and better results were found for the Ex situ method.”