TiO2 photocatalysis: Design and applications

Photocatalytic removal of gaseous nitrogen oxides using WO3/TiO2 particles under visible light irradiation: Effect of surface modification

Photocatalytic nanoparticles have been receiving considerable attention for their potential use in many environmental management applications, including urban air quality control. This paper investigates the performance of surface modified WO₃/TiO₂ composite particles in removing gaseous nitrogen oxides (NO_x) under visible light irradiation. The WO₃/TiO₂ composite particles were synthesized using a modified wet chemical method with different concentrations of NaOH solution used as a surface modification agent for the host TiO₂ particles. The NO_x removal efficiency of the WO₃/TiO₂ particles was evaluated using a lab-scale continuous gas flow photo-reactor with a gas contact time of 1 min. Results showed that surface modification using NaOH can enhance the photocatalytic activity of the WO₃/TiO₂ particles. The NO_x removal efficiency of the surface modified WO₃/TiO₂ was greater than 90%, while that of WO₃/TiO₂ particles prepared by the conventional wet chemical method was ∼75%. The enhanced removal efficiency might be attributed to the formation of oxygen vacancies on the TiO₂ surface, providing sites for WO₃ particles to effectively bind with TiO₂. However, excess amount of NaOH >3 M deteriorated the photocatalytic performance due to the increased agglomeration of the host TiO₂ particles.

Interfacial Electron Injection Probed by a Substrate-Specific Excitonic Signature

Ultrafast interfacial electron transfer in sensitized solar cells has mostly been probed by visible-to-terahertz radiation, which is sensitive to the free carriers in the conduction band of the semiconductor substrate. Here, we demonstrate the use of deep-ultraviolet continuum pulses to probe the interfacial electron transfer, by detecting a specific excitonic transition in both N719-sensitized anatase TiO₂ and wurtzite ZnO nanoparticles. Our results are compared to those obtained on bare nanoparticles upon above-gap excitation. We show that the signal upon electron injection from the N719 dye into TiO₂ is dominated by long-range Coulomb screening of the final states of the excitonic transitions, whereas in sensitized ZnO it is dominated by phase-space filling. The present approach offers a possible route to detecting interfacial electron transfer in a broad class of systems, including other transition metal oxides or sensitizers.

Encapsulation of Single Nanoparticle in Fast-Evaporating Micro-droplets Prevents Particle Agglomeration in Nanocomposites

This work describes the use of fast-evaporating micro-droplets to finely disperse nanoparticles (NPs) in a polymer matrix for the fabrication of nanocomposites. Agglomeration of particles is a key obstacle for broad applications of nanocomposites. The classical approach to ensure the dispersibility of NPs is to modify the surface chemistry of NPs with ligands. The surface properties of NPs are inevitably altered, however. To overcome the trade-off between dispersibility and surface-functionality of NPs, we develop a new approach by dispersing NPs in a volatile solvent, followed by mixing with uncured polymer precursors to form micro-droplet emulsions. Most of these micro-droplets contain no more than one NP per drop, and they evaporate rapidly to prevent the agglomeration of NPs during the polymer curing process. As a proof of concept, we demonstrate the design and fabrication of TiO₂ NP@PDMS nanocomposites for solar fuel generation reactions with high photocatalytic efficiency and recyclability arising from the fine dispersion of TiO₂. Our simple method eliminates the need for surface functionalization of NPs. Our approach is applicable to prepare nanocomposites comprising a wide range of polymers embedded with NPs of different composition, sizes, and shapes. It has the potential for creating nanocomposites with novel functions.

Raman Microspectroscopic Mapping with Multivariate Curve Resolution-Alternating Least Squares (MCR-ALS) Applied to the High-Pressure Polymorph of Titanium Dioxide, TiO2-II

The high-pressure, α-PbO₂-structured polymorph of titanium dioxide (TiO₂-II) was recently identified in micrometer-sized grains recovered from four Neoarchean spherule layers deposited between ∼2.65 and ∼2.54 billion years ago. Several lines of evidence support the interpretation that these layers represent distal impact ejecta layers. The presence of shock-induced TiO₂-II provides physical evidence to further support an impact origin for these spherule layers. Detailed characterization of the distribution of TiO₂-II in these grains may be useful for correlating the layers, estimating the paleodistances of the layers from their source craters, and providing insight into the formation of the TiO₂-II. Here we report the investigation of TiO₂-II-bearing grains from these four spherule layers using multivariate curve resolution-alternating least squares (MCR-ALS) applied to Raman microspectroscopic mapping. Raman spectra provide evidence of grains consisting primarily of rutile (TiO₂) and TiO₂-II, as shown by Raman bands at 174 cm^-1 (TiO₂-II), 426 cm^-1 (TiO₂-II), 443 cm^-1 (rutile), and 610 cm^-1 (rutile). Principal component analysis (PCA) yielded a predominantly three-phase system comprised of rutile, TiO₂-II, and substrate-adhesive epoxy. Scanning electron microscopy (SEM) suggests heterogeneous grains containing polydispersed micrometer- and submicrometer-sized particles. Multivariate curve resolution-alternating least squares applied to the Raman microspectroscopic mapping yielded up to five distinct chemical components: three phases of TiO₂ (rutile, TiO₂-II, and anatase), quartz (SiO₂), and substrate-adhesive epoxy. Spectral profiles and spatially resolved chemical maps of the pure chemical components were generated using MCR-ALS applied to the Raman microspectroscopic maps. The spatial resolution of the Raman microspectroscopic maps was enhanced in comparable, cost-effective analysis times by limiting spectral resolution and optimizing spectral acquisition parameters. Using the resolved spectra of TiO₂-II generated from MCR-ALS analysis, a Raman spectrum for pure TiO₂-II was estimated to further facilitate its identification.

Investigation on H-containing shallow trap of hydrogenated TiO2 with in situ Fourier transform infrared diffuse reflection spectroscopy

A novel technique, high temperature high pressure in situ Fourier transform infrared diffuse reflection spectroscopy, was successfully used to investigate the formation and stability of shallow trap states in P25 TiO₂ nanoparticles. Two types of shallow traps (with and without H atoms) were identified. The H-containing shallow trap can be easily generated by heating in H₂ atmosphere. However, the trap is unstable in vacuum at 600 °C. In contrast, the H-free shallow trap, which can be formed by heating in vacuum, is stable even at 600 °C. The energy gaps between shallow trap states and the conduction band are 0.09 eV for H-containing shallow trap and 0.13 eV for H-free shallow trap, indicating that the H-containing shallow trap state is closer to the conduction band than that without H.

Sulfur-Doped TiO2: Structure and Surface Properties

A comprehensive study on the sulfur doping of TiO2, by means of H2S treatment at 673 K, has been performed in order to highlight the role of sulfur in affecting the properties of the system, as compared to the native TiO2. The focus of this study is to find a relationship among the surface, structure, and morphology properties, by means of a detailed chemical and physical characterization of the samples. In particular, transmission electron microscopy images provide a simple tool to have a direct and immediate evidence of the effects of H2S action on the TiO2 particles structure and surface defects. Furthermore, from spectroscopy analyses, the peculiar surface, optical properties, and methylene blue photodegradation test of S-doped TiO2 samples, as compared to pure TiO2, have been investigated and explained by the effects caused by the exchange of S species with O species and by the surface defects induced by the strong H2S treatment.

In Situ Determination of the Water Condensation Mechanisms on Superhydrophobic and Superhydrophilic Titanium Dioxide Nanotubes

One-dimensional (1D) nanostructured surfaces based on high-density arrays of nanowires and nanotubes of photoactive titanium dioxide (TiO₂) present a tunable wetting behavior from superhydrophobic to superhydrophilic states. These situations are depicted in a reversible way by simply irradiating with ultraviolet light (superhydrophobic to superhydrophilic) and storage in dark. In this article, we combine in situ environmental scanning electron microscopy (ESEM) and near ambient pressure photoemission analysis (NAPP) to understand this transition. These experiments reveal complementary information at microscopic and atomic level reflecting the surface wettability and chemical state modifications experienced by these 1D surfaces upon irradiation. We pay special attention to the role of the water condensation mechanisms and try to elucidate the relationship between apparent water contact angles of sessile drops under ambient conditions at the macroscale with the formation of droplets by water condensation at low temperature and increasing humidity on the nanotubes' surfaces. Thus, for the as-grown nanotubes, we reveal a metastable and superhydrophobic Cassie state for sessile drops that tunes toward water dropwise condensation at the microscale compatible with a partial hydrophobic Wenzel state. For the UV-irradiated surfaces, a filmwise wetting behavior is observed for both condensed water and sessile droplets. NAPP analyses show a hydroxyl accumulation on the as-grown nanotubes surfaces during the exposure to water condensation conditions, whereas the water filmwise condensation on a previously hydroxyl enriched surface is proved for the superhydrophilic counterpart.

Metal-Free Photocatalyst with Visible-Light-Driven Post-Illumination Catalytic Memory

A novel metal-free photocatalyst with post-illumination catalytic memory was fabricated by the graphitic carbon nitride (g-C₃N₄), carbon nanotubes (CNTs), and graphene (Gr), in which g-C₃N₄ acts as an efficient photocatalyst and the CNTs and Gr act as supercapacitors. The removal of phenol was achieved in the dark by post-illumination catalytic memory because the photocatalyst could store a portion of its photoactivity via photogenerated electrons in the CNTs and Gr under visible-light illumination and then release the electrons again in the dark. Therefore, this metal-free photocatalyst is capable of operation in the dark for a broad range of applications.

CdS quantum dots confined in mesoporous TiO2 with exceptional photocatalytic performance for degradation of organic polutants

In this paper, the mesoporous TiO₂ with different concentration of CdS quantum dots (i.e., x% CdS/TiO₂) was successfully fabricated by the sol-gel method. The composition, structure and morphology of the nanocomposites were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), UV-vis diffuse reflectance spectroscopy (UV-Vis/DRS) and nitrogen physical adsorption test and so on. The proportion of CdS and TiO₂ is very important for the photocatalytic performance. As a result, the photocatalytic degradation performance from the most to the least is in the order of 2% CdS/TiO₂, 4% CdS/TiO₂, 8% CdS/TiO₂, pure TiO₂ and 1% CdS/TiO₂. The photocatalytic (PC) activity of the 2% CdSTiO₂ is characterized by photocatalytic degradation of methyl orange, which can be completely degraded within 45 min better than 60 min TiO₂ takes. It is also much better than CdS. Moreover, other four organic pollutants, such as methylthionine chloride, bisphenol A, rhodamine B, malachite green can also be degraded quickly on the condition of 2% CdS/TiO₂. What's more, the chemical stability and cycling capability of 2% CdS/TiO₂ are reflected by five cyclic degradation of methyl orange.

Photocatalytic hollow TiO2 and ZnO nanospheres prepared by atomic layer deposition

Carbon nanospheres (CNSs) were prepared by hydrothermal synthesis, and coated with TiO₂ and ZnO nanofilms by atomic layer deposition. Subsequently, through burning out the carbon core templates hollow metal oxide nanospheres were obtained. The substrates, the carbon-metal oxide composites and the hollow nanospheres were characterized with TG/DTA-MS, FTIR, Raman, XRD, SEM-EDX, TEM-SAED and their photocatalytic activity was also investigated. The results indicate that CNSs are not beneficial for photocatalysis, but the crystalline hollow metal oxide nanospheres have considerable photocatalytic activity.

Femtosecond Laser Fabrication of Anatase TiO2 Micro-nanostructures with Chemical Oxidation and Annealing

The fabrication of nanoporous anatase TiO₂ on a microstructured Ti base is achieved through an innovative hybrid fabrication method involving femtosecond laser ablation coupled with H₂O₂ oxidation and annealing. The anatase TiO₂ micro-nanostructures have superior photocatalytic degradation of methyl orange due to enhanced light harvesting capacity and surface area. The photodegradation efficiency increases by a maximum of 80% compared to the nanoporous anatase TiO₂ fabricated through H₂O₂ oxidation and annealing only (without femtosecond laser ablation). Meanwhile, The anatase TiO₂ micro-nanostructures show good cyclic performance, indicating a great potential for practical application. The proposed hybrid method can easily tune the morphology and size of microstructure by simply adjusting the femtosecond laser parameters, showing advantage in fabricating of micro-nanostructures with a rich variety of morphologies.

Photoactive Hybrid Catalysts Based on Natural and Synthetic Polymers: A Comparative Overview

In the present review, we would like to draw the reader's attention to the polymer-based hybrid materials used in photocatalytic processes for efficient degradation of organic pollutants in water. These inorganic-organic materials exhibit unique physicochemical properties due to the synergistic effect originating from the combination of individual elements, i.e., photosensitive metal oxides and polymeric supports. The possibility of merging the structural elements of hybrid materials allows for improving photocatalytic performance through (1) an increase in the light-harvesting ability; (2) a reduction in charge carrier recombination; and (3) prolongation of the photoelectron lifetime. Additionally, the great majority of polymer materials exhibit a high level of resistance against ultraviolet irradiation and improved corrosion resistance. Taking into account that the chemical and environmental stability of the hybrid catalyst depends, to a great extent, on the functional support, we highlight benefits and drawbacks of natural and synthetic polymer-based photocatalytic materials and pay special attention to the fact that the accessibility of synthetic polymeric materials derived from petroleum may be impeded due to decreasing amounts of crude oil. Thus, it is necessary to look for cheap and easily available raw materials like natural polymers that come from, for instance, lignocellulosic wastes or crustacean residues to meet the demand of the "plastic" market.

Impact of paint matrix composition and thickness of paint layer on the activity of photocatalytic paints

Silicate, acrylic and latex photocatalytic paints were analyzed in regards to impact of paint matrix composition and paint layer’s thickness on performance in two photocatalytic tests. These included performances in photocatalytic decomposition of benzo[a]pyrene (BaP) and assessment of photocatalytic activity through use of smart ink test. Silicate photocatalytic paints displayed lower photocatalytic activity in comparison to acrylic and latex photocatalytic paints in both tests, despite the similar content of nanocrystalline TiO2. Measurements of depth of UV light penetration through the paints layer were performed and it appeared, that more porous structure of coating resulted in deeper penetration of UV light. In the case of acrylic paint, the thickness of the photocatalytic layer was around 9 μm, but for silicate paint DR this thickness was higher, around 21 μm.

Immobilization of Pt Nanoparticles via Rapid and Reusable Electropolymerization of Dopamine on TiO2 Nanotube Arrays for Reversible SERS Substrates and Nonenzymatic Glucose Sensors

Inspired by mussel-adhesion phenomena in nature, polydopamine (PDA) coatings are a promising route to multifunctional platforms for decorating various materials. The typical self-polymerization process of dopamine is time-consuming and the coatings of PDA are not reusable. Herein, a reusable and time-saving strategy for the electrochemical polymerization of dopamine (EPD) is reported. The PDA layer is deposited on vertically aligned TiO₂ nanotube arrays (NTAs). Owing to the abundant catechol and amine groups in the PDA layer, uniform Pt nanoparticles (NPs) are deposited onto the TiO₂ NTAs and can effectively prevent the recombination of electron-hole pairs generated from photo-electrocatalysis and transfer the captured electrons to participate in the photo-electrocatalytic reaction process. Compared with pristine TiO₂ NTAs, the as-prepared Pt@TiO₂ NTA composites exhibit surface-enhanced Raman scattering sensitivity for detecting rhodamine 6G and display excellent UV-assisted self-cleaning ability, and also show promise as a nonenzymatic glucose biosensor. Furthermore, the mussel-inspired electropolymerization strategy and the fast EPD-reduced nanoparticle decorating process presented herein can be readily extended to various functional substrates, such as conductive glass, metallic oxides, and semiconductors. It is the adaptation of the established PDA system for a selective, robust, and generalizable sensing system that is the emphasis of this work.

Sculpturing patterns of plasmonic silver nanoprisms by means of photocatalytic lithography

The controlled shaping of nanoparticles' morphology is one of the pillars of nanotechnology. Here, we demonstrate that photocatalytic lithography, a technique already proved to be useful in materials science, can act as a dry etching technique for noble metal nanoparticles. Triangular silver nanoprisms are self-assembled on titanium dioxide films and photocatalytically shaped into discoidal particles upon irradiation with near-UV light. The obtained patterned surfaces show a dramatically different surface-enhanced Raman scattering response, suggesting the utility of our approach for the development of sensors. The photocatalytic nature of the particle shaping is demonstrated and a plausible mechanism drawn by performing photocatalysis in different configurations (direct and remote) and by irradiating in different solvents.

Effects of spatial topologies and electron Fermi-level gradient on the photocatalytic efficiency of nano-particulate semiconductors

Nanocrystalline (nc) semiconductor materials are important in photocatalysis. The nanoparticle (NP) topologies and electron Fermi-level (E_F) gradient along the interconnected NPs affect the photocatalytic efficiency (η) of the nc-materials because of the charge carrier interparticle transport (IPT). However, the detailed physiochemical kinetic mechanism remains unclear. Based on the kinetic analysis and the numerical Monte-Carlo simulation of random walks, the statistical probability distributions p_Rec(t) and p_it(t) for the recombination time and interfacial transfer (IT) time have been proposed in this study. The recombination lifetime (τ_Red) and IT lifetime (τ_IT) were calculated by averaging p_Rec(t) and p_it(t). The characteristic time τ_e of the entire electron kinetics was defined using τ_Re and τ_IT, and η was calculated by dividing τ_e by τ_IT. The simulation results show that the p_Rec(t) clearly shows the IPT of electrons. Both the kinetic factors (NP spatial topologies and boundary barrier) and the thermodynamic factor (electron E_F gradient) can affect the IPT. It was observed that the increase in IPT cannot lead to a monotonous increase in η although it can prohibit recombination. Whether the IPT can increase the η is dependent on ratio of the back IPT for recombination and the forward IPT for IT. The existence of an electron E_F gradient from the electron generation site to the active site can increase η by promoting the forward IPT.

On the Charge State of Titanium in Titanium Dioxide

The oxidation state of titanium in titanium dioxide is commonly assumed to be +4. This assignment is based on the ionic approximation and is used ubiquitously to rationalize phenomena observed with TiO₂. It implies a charge state +4 and that no further oxidation of the metal center is possible. We present a comprehensive electronic structure investigation of Ti ions, TiO₂ molecules, and TiO₂ bulk crystals using different density functional theory and wave function-based approaches, which shows that the charge state of Ti is +3. Specifically, there is evidence of a significant remaining contribution from valence s and d electrons of Ti, including the presence of a nuclear cusp around the Ti core. The charge corresponding to valence s and d states of Ti amounts to 1 e. This suggests the possibility of further oxidation of Ti in TiO₂ compounds and challenges the commonly assumed picture of assigning the oxidation state of Ti in titania to +4.

Synthesis, properties, and applications of black titanium dioxide nanomaterials

Photocatalysis has been regarded as one of best solutions to using the sunlight to produce hydrogen from water and to removing organic pollutants from the environment, and titanium dioxide (TiO₂) nanomaterials have been treated as the primary photocatalyst for these purposes. However, their large band gap has largely limited the activity to the UV region of the solar spectrum. The discovery of black TiO₂ in 2011 has triggered world-wide research interests with new hope to overcome this problem. This review briefly summarizes the recent progresses of black TiO₂ nanomaterials, including their synthesis, properties and applications, to provide a timely update and to inspire more ideas in the related research.

Application of ultrasound-aided method for the synthesis of CdS-incorporated three-dimensional TiO2 photocatalysts with enhanced performance

In this study, an ultrasound-aided hydrothermal-impregnation method was used to synthesize three-dimensional (3D) urchin-like CdS-TiO₂ nanostructures (UCTs) with variable CdS content. The photocatalytic efficiencies (for degrading limonene and toluene vapor) of UCTs synthesized using the ultrasound-aided process were greater than those of UCTs fabricated without ultrasound treatment. In addition, the photocatalytic efficiencies of ultrasound-treated UCTs were greater than those of zero-dimensional ultrasound-treated CdS-TiO₂ particles, which, in turn, were greater than those of untreated 3D TiO₂. These results indicate that ultrasonication is an amicable process for the synthesis of UCTs with high photocatalytic activity. The enhanced activity of ultrasound-treated photocatalysts is ascribed to the greater charge carrier efficiency, adsorption capacity, and light absorption efficiency of these materials. The photocatalytic efficiencies of ultrasound-treated UCTs increased as the CdS loading was increased from 0.1% to 0.3%, gradually dropping as the loading was further increased to 3.0%, which indicated the existence of an optimum CdS loading. UCT photocatalytic efficiencies depended on the input concentration of target pollutants, relative humidity, and air flow rate. The photocatalytic efficiency for the decomposition of limonene mixed with 2-propanol was lower than that for limonene alone, likely due to the radical scavenging properties of 2-propanol. However, the photocatalytic degradation efficiency of the latter alcohol was not changed upon admixture. Toluene exhibited the same behavior. The mineralization ratios of both target compounds were lower than their decomposition ratios, indicating formation of byproducts due to incomplete oxidation. In addition to CO, three organic compounds were observed as photocatalytic decomposition byproducts of limonene (acetic acid, limonene oxide, and methacrolein) and toluene (benzene, benzaldehyde, and p-xylene). UCTs synthesized by the ultrasound-aided hydrothermal-impregnation method could be used to decompose organic vapors with an efficiency of up to 98%, depending on operating conditions.

Performance of ZnO synthesized by sol-gel as photocatalyst in the photooxidation reaction of NO

ZnO samples were prepared by sol-gel method applying a factorial design in order to improve the photocatalytic properties of the semiconductor oxide in the NO photooxidation reaction. The concentrations of zinc acetate and ammonium hydroxide were selected as critical variables in the synthesis of ZnO. Nine samples of ZnO were obtained as product of the factorial design and were characterized by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, diffuse reflectance spectroscopy, and N₂ adsorption-desorption isotherms. The photocatalytic activity of ZnO samples was associated with the physical properties developed by each sample according to its respective conditions of synthesis. Some photocatalytic reaction parameters, such as mass of photocatalyst, irradiance, and relative humidity, were modified in order to evaluate its effect in the photocatalytic conversion of NO. As a relevant point, the relative humidity played an important role in the increase of the selectivity of the NO photooxidation reaction to innocuous nitrate ions when ZnO was used as photocatalyst.

Significant changes in the photo-reactivity of TiO2 in the presence of a capped natural dissolved organic matter layer

Natural dissolved organic matter (NDOM) in surface waters has a high sorption affinity for TiO₂ during long contact. An attached NDOM layer can act as a conduction band electron and/or valance band hole acceptor, and NDOM can also decrease the concentration of hydroxyl radicals (OH) in the bulk phase. Therefore, the degradation kinetics and mechanism for degradation of acetaminophen on NDOM capped TiO₂ (NDOM-TiO₂) are significantly different from those on raw TiO₂. Quantum calculation results suggest that hydroxylation to the ortho position in relation to the acetamide group is more favorable. Although OH induced hydroxylation is the predominant pathway for degradation of acetaminophen on TiO₂, one-electron oxidation of acetaminophen by a valance band hole, excited triplet NDOM or NDOM radical cation is the major degradation pathway on NDOM-TiO₂. This study is the first to detect and confirm APAP oligomers as intermediates during the degradation of acetaminophen by TiO₂ photocatalysis, especially when using NDOM-TiO₂ as a catalyst. The results suggest the reactivity of TiO₂ could change significantly after long exposure to natural water, which need to be concerned about for removal of micropollutants in surface water by TiO₂ photocatalysis.