TiO2 photocatalysis: Design and applications

Mechanistical investigation on the self-enhanced photocatalytic activity of CuO/Cu2O hybrid nanostructures by density functional theory calculations

The photocatalytic mechanism of a Cu₂O/CuO hybrid system is disclosed in detail by density functional theory (DFT) calculations. The synergistic relationship of the two counterparts is confirmed by hydrogen peroxide (H₂O₂) formation on the CuO nanowires and dissociation on the Cu₂O nanoparticles; this enables the system to self-sufficiently produce hydroxyl radicals, which is highly efficient in the photocatalytic degradation of methyl orange. The exposed surfaces are found to be crucial in the cooperative photocatalytic system, especially the Cu₂O(111) surface, in the dissociation of H₂O₂. The distinct positions of the conduction band minimum and valence band maximum for CuO and Cu₂O and synergic surface reactions enable the effective utilization of the electrons and holes generated by visible-light irradiation. Our results will contribute to a greater understanding of the specific mechanism of photodegradation catalyzed by Cu₂O/CuO heterostructures, which may lead to promising directions in structure optimization for photocatalysts with high photocatalytic efficiency.

A Semi-Pilot Photocatalytic Rotating Reactor (RFR) with Supported TiO₂/Ag Catalysts for Water Treatment

A four stage semi-pilot scale RFR reactor with ceramic disks as support for TiO₂ modified with silver particles was developed for the removal of organic pollutants. The design presented in this article is an adaptation of the rotating biological reactors (RBR) and its coupling with the modified catalyst provides additional advantages to designs where a catalyst in suspension is used. The optimal parameter of rotation was 54 rpm and the submerged surface of the disks offer a total contact area of 387 M². The modified solid showed a decrease in the value of its bandgap compared to commercial titanium. The system has a semi-automatic operation with a maximum reaction time of 50 h. Photo-activity tests show high conversion rates at low concentrations. The results conform to the Langmuir heterogeneous catalysis model.

Adsorption and photocatalytic performance of bentonite-titanium dioxide composites for methylene blue and rhodamine B decoloration

Bentonite − TiO₂ composites were prepared by impregnation of TiO₂ and bentonite, followed by microwave irradiation processes. The composites were characterized using FTIR, SEM, XRD, and nitrogen sorption methods. Anatase phase of TiO₂ in all composites are observed through XRD diffraction peaks and surface morphology of the composites. The adsorption and photocatalytic capabilities of the composites were tested in liquid phase adsorption of methylene blue and Rhodamine B. The adsorption and photocatalytic degradation experiments were conducted in the presence or absence of UV light irradiation. Langmuir and Freundlich models were employed to correlate the experimental adsorption data, and it was found that Langmuir gave better performance in correlating the experimental data. Modification of Langmuir equation to accommodate photocatalytic degradation process was conducted, and the model could represent the experimental results very well.

Efficient photodegradation of chlorophenols by BiOBr/NaBiO3 heterojunctioned composites under visible light

Forming heterojunctioned composites is an effective way to develop visible-light-driven photocatalysts. A series of BiOBr/NaBiO₃ composites were synthesized by surface transformation of NaBiO₃ with hydrobromic acid. Commensurate planes of BiOBr and NaBiO₃ enabled the formation of a closely bound interface. Composites with <20wt.% BiOBr exhibited excellent photocatalytic activity towards the degradation of chlorophenols under low intensity visible light (λ>400nm). The best photocatalyst was 9% BiOBr/NaBiO₃ with a quantum yield of 0.365. No photocorrosion was observed after three cycles. Using radical scavengers and inert atmosphere, holes, superoxide and hydroxyl radicals were found to be involved in the photoactivity of the BiOBr/NaBiO₃ composite. Hydroxylated and open-ring diacid molecules were identified as intermediates in the mineralization of 4-chlorophenol.

Syntheses and structural understanding of a Ti–Ta alloy-based nanotubular oxide photocatalyst

Ti–Ta mixed oxide freestanding nanotubes obtained by means of anodic oxidation of a homemade Ti–Ta (50–50 at%) alloy.

Influence of fluorine on the synthesis of anatase TiO2 for photocatalytic partial oxidation: are exposed facets the main actors?

The influence of fluorine on TiO₂ exposed facets and on the physico-chemical properties was evaluated and a synergetic effect of the presence of fluorine and the facets' distribution was observed.

Construction of a novel BiSI/MoS2 nanocomposite with enhanced visible-light driven photocatalytic performance

A BiSI/MoS₂ nanocomposite was synthesized for the first time and used as a new efficient and stable visible light driven photocatalyst.

Controlled morphology of ZnSe nanostructures by varying Zn/Se molar ratio: the effects of different morphologies on optical properties and photocatalytic performance

Synthesis of ZnSe nanostructures with different morphologies by changing the Zn/Se ratio in a co-precipitation method.

Efficient photocatalytic degradation of gaseous acetaldehyde over ground Rh–Sb co-doped SrTiO3 under visible light irradiation

A visible-light-responsive Rh–Sb co-doped SrTiO₃ photocatalyst (STO:Rh,Sb) via a solid-state reaction was successfully developed, following pulverization by using ball-milling. The prepared STO:Rh,Sb exhibited a large surface area and showed efficient photocatalytic degradation of acetaldehyde. The photocatalytic activity of STO:Rh,Sb ground for 60 min exceeded that of STO:Rh ground for 60 min (photocatalyst doped without antimony), indicating that doped antimony plays an important role in suppressing the Rh⁴⁺, which works as a recombination center, in STO:Rh,Sb. Furthermore, the photocatalytic performance of STO:Rh,Sb ground for 60 min was sustained over 3 cycles, confirming the chemical stability of the photocatalyst. Therefore, ground STO:Rh,Sb has the potential to be applied to environmental remediation under visible light irradiation.

A visible-light-responsive Rh–Sb co-doped SrTiO₃ photocatalyst (STO:Rh,Sb) via a solid-state reaction was successfully developed, following pulverization by using ball-milling.

CO2 Reduction by Photocatalysis on TiO2

This chapter focuses on the recent research progress on TiO2-based photocatalysts for CO2 reduction. The scope of this chapter for photoreduction of CO2 is set to focus on the most widely studied TiO2-based photocatalysts, composites, and systems since 1979. In addition, several important kinds of other related photocatalysts will be introduced briefly.

Oxygen vacancies of the TiO2nano-based composite photocatalysts in visible light responsive photocatalysis

The TiO₂nano-based composite photocatalyst is best known for application in solving the recent issues related to energy and environmental purification.

Review of Water-Assisted Crystallization for TiO2 Nanotubes

TiO₂ nanotubes (TNTs) have drawn tremendous attention owing to their unique architectural and physical properties. Anodizing of titanium foil has proven to be the most efficient method to fabricate well-aligned TNTs, which, however, usually produces amorphous TNTs and needs further thermal annealing. Recently, a water-assisted crystallization strategy has been proposed and investigated by both science and engineering communities. This method is very efficient and energy saving, and it circumvents the drawbacks of thermal sintering approach. In this paper, we review the recent research progress in this kind of low-temperature crystallization approach. Here, various synthetic methods are summarized, and the mechanisms of the amorphous-crystalline transformation are analyzed. The fundamental properties and applications of the low-temperature products are also discussed. Furthermore, it is proved that the water-assisted crystallization approach is not only applicable to TNTs but also to crystallizing other metal oxides.

UPS and UV spectroscopies combined to position the energy levels of TiO2 anatase and rutile nanopowders

Positioning of absolute energy levels and the quantitative description of occupied levels obtained for TiO₂ nanopowders, combining UPS and UV-Vis spectroscopies.

Preparation of MoS2/TiO2 based nanocomposites for photocatalysis and rechargeable batteries: progress, challenges, and perspective

The rapidly increasing severity of the energy crisis and environmental degradation are stimulating the rapid development of photocatalysts and rechargeable lithium/sodium ion batteries. In particular, MoS₂/TiO₂ based nanocomposites show great potential and have been widely studied in the areas of both photocatalysis and rechargeable lithium/sodium ion batteries due to their superior combination properties. In addition to the low-cost, abundance, and high chemical stability of both MoS₂ and TiO₂, MoS₂/TiO₂ composites also show complementary advantages. These include the strong optical absorption of TiO₂vs. the high catalytic activity of MoS₂, which is promising for photocatalysis; and excellent safety and superior structural stability of TiO₂vs. the high theoretic specific capacity and unique layered structure of MoS₂, thus, these composites are exciting as anode materials. In this review, we first summarize the recent progress in MoS₂/TiO₂-based nanomaterials for applications in photocatalysis and rechargeable batteries. We highlight the synthesis, structure and mechanism of MoS₂/TiO₂-based nanomaterials. Then, advancements and strategies for improving the performance of these composites in photocatalytic degradation, hydrogen evolution, CO₂ reduction, LIBs and SIBs are critically discussed. Finally, perspectives on existing challenges and probable opportunities for future exploration of MoS₂/TiO₂-based composites towards photocatalysis and rechargeable batteries are presented. We believe the present review would provide enriched information for a deeper understanding of MoS₂/TiO₂ composites and open avenues for the rational design of MoS₂/TiO₂ based composites for energy and environment-related applications.

CdSe nanorod/TiO2 nanoparticle heterojunctions with enhanced solar- and visible-light photocatalytic activity

CdSe nanorods (NRs) with an average length of ≈120 nm were prepared by a solvothermal process and associated to TiO2 nanoparticles (Aeroxide® P25) by annealing at 300 °C for 1 h. The content of CdSe NRs in CdSe/TiO2 composites was varied from 0.5 to 5 wt %. The CdSe/TiO2 heterostructured materials were characterized by XRD, TEM, SEM, XPS, UV-visible spectroscopy and Raman spectroscopy. TEM images and XRD patterns show that CdSe NRs with wurtzite structure are associated to TiO2 particles. The UV-visible spectra demonstrate that the narrow bandgap of CdSe NRs serves to increase the photoresponse of CdSe/TiO2 composites until ≈725 nm. The CdSe (2 wt %)/TiO2 composite exhibits the highest photocatalytic activity for the degradation of rhodamine B in aqueous solution under simulated sunlight or visible light irradiation. The enhancement in photocatalytic activity likely originates from CdSe sensitization of TiO2 and the heterojunction between these materials which facilitates electron transfer from CdSe to TiO2. Due to its high stability (up to ten reuses without any significant loss in activity), the CdSe/TiO2 heterostructured catalysts show high potential for real water decontamination.

JAK/STAT and TGF-ß activation as potential adverse outcome pathway of TiO2NPs phototoxicity in Caenorhabditis elegans

Titanium dioxide nanoparticles (TiO₂NPs) are widely used nanoparticles, whose catalytic activity is mainly due to photoactivation. In this study, the toxicity of TiO₂NPs was investigated on the nematode Caenorhabditis elegans, with and without UV activation. Comparative analyses across the four treatments revealed that UV-activated TiO₂NPs led to significant reproductive toxicity through oxidative stress. To understand the underlying molecular mechanism, transcriptomics and metabolomics analyses were conducted, followed by whole-genome network-based pathway analyses. Differential expression analysis from microarray data revealed only 4 DEGs by exposure to TiO₂NPs alone, compared to 3,625 and 3,286 DEGs by UV alone and UV-activated TiO₂NPs, respectively. Pathway analyses suggested the possible involvement of the JAK/STAT and TGF-ß pathways in the phototoxicity of TiO₂NPs, which correlated with the observation of increased gene expression of those pathways. Comparative analysis of C. elegans response across UV activation and TiO₂NPs exposure was performed using loss-of-function mutants of genes in these pathways. Results indicated that the JAK/STAT pathway was specific to TiO₂NPs, whereas the TGF-ß pathway was specific to UV. Interestingly, crosstalk between these pathways was confirmed by further mutant analysis. We consider that these findings will contribute to understand the molecular mechanisms of toxicity of TiO₂NPs in the natural environment.

Construction of highly efficient and stable ternary AgBr/Ag/PbBiO2Br Z-scheme photocatalyst under visible light irradiation: Performance and mechanism insight

In this work, the novel ternary AgBr/Ag/PbBiO₂Br Z-scheme photocatalysts were synthesized via a CTAB-assisted calcination process. The AgBr/Ag/PbBiO₂Br composites were employed for the degradation of rhodamine B (RhB) and antibiotic bisphenol A (BPA) under visible light irradiation. Results showed that the obtained AgBr/Ag-3/PbBiO₂Br displayed optimal photocatalytic performance, which could remove almost all RhB within 25 min and effectively decompose 82.3% of BPA in 120 min. Three-dimensional excitation-emission matrix fluorescence spectra (3D EEMs) were utilized for the purposes of fully grasping the behaviors of RhB molecules during the reaction process. Meanwhile, the effects of initial RhB concentration and co-existent electrolytes were investigated from the viewpoint of practical application. In addition, there was no obvious loss in degradation efficiency even after four cycles. The enhanced photocatalytic performances of AgBr/Ag/PbBiO₂Br could be credited to the accelerated interfacial charge transfer process and the improved separation of the photogenerated electron-hole pairs. The existence of a small amount of metallic Ag played a significant role in preventing AgBr from being further photocorroded, resulting in the formation of a stable Z-scheme photocatalyst system. This study demonstrated that using metallic Ag as an electron mediator to construct Z-scheme photocatalytic system provided a feasible strategy in promoting the stability of Ag-based semiconductors.

Combination of photocatalytic and membrane distillation hybrid processes for reactive dyes treatment

In this study, the degradation of azo dye solutions (Reactive Red 180 and Reactive Orange 16) of textile industry wastewater was investigated for using innovative hybrid process of photocatalytic and membrane distillation (MD) processes. Photocatalytic oxidation was conducted with semiconductor catalysts (ZnO and TiO2) and their mixture under UVA and UVC irradiation. More effective results were obtained under UVA at the initial stages of the reactions for both dye solutions. ZnO and TiO2 catalysts have given similar efficient results, but results with ZnO were better at initial stages. For the next stage, hybrid design of MD and photocatalytic processes was performed sequentially. Initially, the photocatalytic process was conducted for at least 1 h at initial values of 100 mg/L RR-180 dye solutions and 1 g/L ZnO catalyst loading under UVA irradiation and then treated solution was run through the distillation module at different temperatures (30°C and 40°C) and flow rates (210, 425, and 665 mL/min). Three types of membranes (polypropylene, polytetrafluoroethylene, and polyvinylidene fluoride) with different pore sizes (0.45 and 0.22 μm) were used in the module. Increasing temperature on the side of treated solution and decreasing the temperature on the other side has increased the distillate efficiency.

Charge migration and charge transfer in molecular systems

The transfer of charge at the molecular level plays a fundamental role in many areas of chemistry, physics, biology and materials science. Today, more than 60 years after the seminal work of R. A. Marcus, charge transfer is still a very active field of research. An important recent impetus comes from the ability to resolve ever faster temporal events, down to the attosecond time scale. Such a high temporal resolution now offers the possibility to unravel the most elementary quantum dynamics of both electrons and nuclei that participate in the complex process of charge transfer. This review covers recent research that addresses the following questions. Can we reconstruct the migration of charge across a molecule on the atomic length and electronic time scales? Can we use strong laser fields to control charge migration? Can we temporally resolve and understand intramolecular charge transfer in dissociative ionization of small molecules, in transition-metal complexes and in conjugated polymers? Can we tailor molecular systems towards specific charge-transfer processes? What are the time scales of the elementary steps of charge transfer in liquids and nanoparticles? Important new insights into each of these topics, obtained from state-of-the-art ultrafast spectroscopy and/or theoretical methods, are summarized in this review.

Solar photocatalytic application of NbO2OH as alternative photocatalyst for water treatment

Water recycling and industrial effluents remediation are a hot topic of research to reduce the environmental impact of the human activity. Persistent organic pollutants are highly recalcitrant compounds with hazardous effects associated to their fate in water bodies. Several novel technologies have been developed during the last decades to deal with this novel contamination. However, the natural sources and idiosyncrasy of each country lead to the potential application of different technologies. In this context, we have focused on the development of phocotalytic treatment of solutions containing dyes using a novel photocatalytic material, the NbO₂OH. The NbO₂OH was synthesized and characterized with different techniques. Several assays demonstrated the solar photoactivity of this novel oxyhydroxide catalyst, achieving complete decolorizations after 10min of treatment under optimal conditions of 1.0gL^-1 NbO₂OH photocatalyst loading, 0.1M of H₂O₂ as electron scavenger, pH4.0 and methyl orange concentrations up to 15mgL^-1. Also, the catalyst recuperation demonstrated the potential reuse of this photocatalyst without losing catalytic response after five cycles. This work is of significant importance because niobium is a natural resource, mainly extracted in Brazil and the annual global sunlight irradiation in the near-equatorial region of northeast Brazil is over the average solar irradiation of the planet. Thus, the solar photocatalytic treatment using NbO₂OH in northeast Brazil appears as a highly potential environmental-friendly nanotechnology to mitigate the water pollution.

Antimicrobial and antibiofilm efficacy of self-cleaning surfaces functionalized by TiO2 photocatalytic nanoparticles against Staphylococcus aureus and Pseudomonas putida

A photocatalytic sol of TiO₂ nanoparticles has been used for creating self-cleaning antimicrobial flat and porous glass surfaces. The substrates were irradiated to study their photocatalytic properties and behavior in the presence of biofilm-forming bacteria. Smooth glass surfaces and glass microfiber filters were covered with 1.98×10^-3±1.5×10^-4gcm^-2 and 8.55×10^-3±3.0×10^-4gcm^-2 densities, respectively. Self-cleaning properties were analyzed using the methylene blue 365nm UV-A photodegradation test. TiO₂-coated filters achieved rapid and complete photodegradation of methylene blue because of the better TiO₂ dispersion with respect to the glass slides. The effect of functionalized surfaces on the growth and viability of bacteria was studied using the strains Staphylococcus aureus and Pseudomonas putida. After irradiation (2h, 11.2Wm^-2, 290-400nm), the initially hydrophobic surface turned hydrophilic. The antibacterial effect led to extensive membrane damage and significant production of intracellular reactive oxygen species in all TiO₂-loaded irradiated specimens. The reduction of cell viability was over 99.9% (>3-log) for TiO₂ on glass surfaces. However, the polymeric extracellular matrix formed before the irradiation treatment was not removed. This study highlights the importance of bacterial colonization during dark periods and the difficulty of removing the structure of biofilms.

Selective Inactivation of Bacteriophage in the Presence of Bacteria by Use of Ground Rh-Doped SrTiO3 Photocatalyst and Visible Light

Bacteriophage (denoted as phage) infection in the bacterial fermentation industry is a major problem, leading to the loss of fermented products such as alcohol and lactic acid. Currently, the prevention of phage infection is limited to biological approaches, which are difficult to apply in an industrial setting. Herein, we report an alternative chemical approach using ground Rh-doped SrTiO₃ (denoted as g-STO:Rh) as a visible-light-driven photocatalyst. The g-STO:Rh showed selective inactivation of phage without bactericidal activity when irradiated with visible light (λ > 440 nm). After inactivation, the color of g-STO:Rh changed from gray to purple, suggesting that the Rh valence state partially changed from 3+ to 4+ induced by photocatalysis, as confirmed by diffuse reflectance spectroscopy. To study the effect of the Rh⁴⁺ ion on phage inactivation under visible-light irradiation, the survival rate of phage for g-STO:Rh was compared to that for ground Rh,Sb-codoped SrTiO₃ (denoted as g-STO:Rh,Sb), where the change of Rh valence state from 3+ to 4+ is almost suppressed under visible-light irradiation due to charge compensation by the Sb⁵⁺ ion. Only g-STO:Rh effectively inactivated phage, which indicated that Rh⁴⁺ ion induced by photocatalysis particularly contributed to phage inactivation under visible-light irradiation. These results suggested that g-STO:Rh has potential as an antiphage material in bacterial fermentation.

Detailed modeling of oxalic acid degradation by UV-TiO2 nanoparticles: Importance of light scattering and photoreactor scale-up

A detailed computational fluid dynamics model is presented that integrates reactor hydrodynamics with advanced light models and UV-TiO₂ advanced oxidation kinetics to yield the degradation of oxalic acid in a dispersed-phase photoreactor. Model predictions were first compared against experimental data obtained from the literature and subsequently used in a parametric study for investigating scale-up effects associated with both process and photoreactor variables. Investigated variables included: TiO₂ concentration (5-400 mg L^-1), initial oxalic acid concentration (0.9-32 mg L^-1), lamp irradiance (100-10,000 W m^-2), background fluid absorbance (0-30 m^-1), reactor size (1/4-4 as relative scaling factor), lamp orientation (0-360°) and flowrate (2.5-10 m³ h^-1). The analysis revealed that an optimum in oxalic acid degradation is observed when the TiO₂ concentration was controlled in the 20-40 mg L^-1 range (depending on lamp irradiance). While lamp orientation showed minimal impact, reactor size and flowrate emerged as key variables for photoreactor design. Moreover, an increase in initial oxalic acid concentration substantially reduced oxalic acid degradation performance observed at high loadings. Also, TiO₂ activation and photoreactor degradation performance were impacted negatively by light competition with background fluid absorbance.