Surface modification of TiO2 photocatalyst for environmental applications

Quantum dot sensitized electrospun mesoporous titanium dioxide hollow nanofibers for photocatalytic applications

TiO₂ hollow nanofibers prepared by coaxial electrospinning were sensitized with CdS QDs by SILAR method and their photocatalytic dye degradation performance was investigated.

A convenient approach of MIP/Co–TiO2 nanocomposites with highly enhanced photocatalytic activity and selectivity under visible light irradiation

MIP/Co–TiO₂ nanocomposites were synthesized. Their mechanisms of preferable photocatalytic activity and good selectivity for target contaminants were identified and discussed.

Hierarchical photocatalysts

The design, fabrication, performance and applications of hierarchical semiconductor photocatalysts are thoroughly reviewed and apprised.

ZnO rods rooted on manifold carbon nanofiber paper as a scalable photocatalyst platform: the effects of ZnO morphology

Crystalline ZnO rods were grown and tailored on manifold carbon nanofiber paper via electrodeposition and their photocatalytic activities were examined.

Titania modification with a ruthenium(ii) complex and gold nanoparticles for photocatalytic degradation of organic compounds

Interaction between two kinds of titania modifiers, i.e., a ruthenium complex and gold nanoparticles, influenced the resultant properties and photocatalytic activities of hybrid photocatalysts under UV and/or vis light irradiation.

Harnessing and storing visible light using a heterojunction of WO3 and CdS for sunlight-free catalysis

A heterojunction of CdS/WO₃ is a suitable candidate to convert sunlight to electrons and store them for use during periods of unavailable sunlight.

Synthesis of (Ag,F)-modified anatase TiO2 nanosheets and their enhanced photocatalytic activity

The strong interactions between Ag nanoparticles and TiO₂ nanosheets can improve the photocatalytic activity of TiO₂ nanosheets.

A novel & effective visible light-driven TiO2/magnetic porous graphene oxide nanocomposite for the degradation of dye pollutants

A novel & effective visible light-driven TiO₂/magnetic porous graphene oxide nanocomposite synthesized and the nanocatalyst was applied for degrading dye pollutant.

Visible light induced photocatalytic inactivation of bacteria by modified titanium dioxide films on organic polymers

Commercially available polypropylene foil was pretreated with a low temperature oxygen plasma and covered with a thin film of nanocrystalline titanium dioxide by dip coating. The films were then photosensitized by titanium(IV) surface charge transfer complexes formed by impregnation with catechol. The photoactivity of the coatings up to 460 nm was confirmed by photoelectrochemical measurements. The photoinactivation of Escherichia coli and Staphylococcus aureus was evaluated by a glass adhesion test based on ISO 27447:2009(E) in the presence of visible light. The coating showed good antimicrobial activity induced by light from a light-emitting diode (405 nm), in particular towards E. coli ATCC 25922 strain. Adaptation of ISO 27447:2009(E) to assess bacterial photoinactivation by photocatalytic coatings will allow this procedure to be applied for the comparison of photoactivity under a range of irradiation conditions.

Photocatalytic activity of TiO₂ nanoparticles synthesized in presence of ammonium hexafluorosilicate

A series of silicon incorporated TiO2 with anatase crystalline phase were prepared via sol-gel using ammonium hexafluorosilicate (AHFS). The synthesized products were characterized using X-ray diffraction, FTIR, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), UV-vis diffuse reflectance spectrophotometry (DRS), energy dispersive X-ray analysis (EDX) and BET surface area measurement. Presence of Si-O-Ti bonds is revealed based on the XPS results. The products were tested for their activity as photocatalysts for degradation of congo red as a prob molecule. The 20% AHFS incorporation displayed the highest photocatalytic activity. This adequate amount of AHFS was beneficial for transferring photogenerated electron and reducing the recombination of electron-holes.

Surface Properties and Photocatalytic Activities of the Colloidal ZnS:Mn Nanocrystals Prepared at Various pH Conditions

Water-dispersible ZnS:Mn nanocrystals (NC) were synthesized by capping the surface with mercaptoacetic acid (MAA) molecules at three different pH conditions. The obtained ZnS:Mn-MAA NC products were physically and optically characterized by corresponding spectroscopic methods. The UV-Visible absorption spectra and PL emission spectra showed broad peaks at 310 and 590 nm, respectively. The average particle sizes measured from the HR-TEM images were 5 nm, which were also supported by the Debye-Scherrer calculations using the X-ray diffraction (XRD) data. Moreover, the surface charges and the degrees of aggregation of the ZnS:Mn-MAA NCs were determined by electrophoretic and hydrodynamic light scattering methods, indicating formation of agglomerates in water with various sizes (50–440 nm) and different surface charge values accordingly the preparation conditions of the NCs (−7.59 to −24.98 mV). Finally, the relative photocatalytic activities of the ZnS:Mn-MAA NCs were evaluated by measuring the degradation rate of methylene blue (MB) molecule in a pseudo first-order reaction condition under the UV-visible light irradiation. As a result, the ZnS:Mn-MAA NC prepared at the pH 7 showed the best photo-degradation efficiency of the MB molecule with the first-order rate constant (k_obs) of 2.0 × 10⁻³·min⁻¹.

Titania-Silica Materials for Enhanced Photocatalysis

Mesoporous titania-organosilica nanoparticles comprised of anatase nanocrystals crosslinked with organosilica moieties have been prepared by direct co-condensation of a titania precursor, tetrabuthylortotitanate (TBOT), with two organosilica precursors, 1,4-bis(triethoxysilyl) benzene (BTEB) and 1,2-bis(triethoxysilyl) ethane (BTEE), in mild conditions and in the absence of surfactant. These hybrid materials show both high surface areas (200-360 m(2)  g(-1) ) and pore volumes (0.3 cm(3)  g(-1) ) even after calcination, and excellent photoactivity in the degradation of rhodamine 6G and in the partial oxidation of propene under UV irradiation, especially after the calcination of the samples. During calcination, there is a change in the Ti(IV) coordination and an increase in the content of SiOTi moieties in comparison with the uncalcined materials, which seems to be responsible for the enhanced photocatalytic activity of hybrid titania-silica materials as compared to both uncalcined samples and the control TiO2 .

The influence of Ce doping of titania on the photodegradation of phenol

Pure and cerium-doped [0.05, 0.1, 0.3, 0.5, and 1.0 Ce nominal atomic % (at.%)] TiO2 was synthesized by the sol-gel method. The obtained catalysts were characterized by X-ray diffraction (XRD), UV-visible diffused reflectance spectroscopy (DRS), Raman, and BET surface area measurement. The photocatalytic activity of synthesized samples for the oxidative degradation of phenol in aqueous suspension was investigated. The content of Ce in the catalysts increases both the transition temperature for anatase to rutile phase transformation and the specific surface area, and decreases the crystallite size of anatase phase, the crystallinity, and the band gap energy value. The material with higher efficiency corresponds to 0.1 Ce nominal at.%. Under irradiation with 350 nm lamps, the degradation of phenol could be described as an exponential trend, with an apparent rate constant of (9.1 ± 0.6) 10(-3) s(-1) (r(2) = 0.98). Hydroquinone was identified as the main intermediate.

Doping of TiO2 for sensitized solar cells

This review gives a detailed summary and evaluation of the use of TiO2 doping to improve the performance of dye sensitized solar cells. Doping has a major effect on the band structure and trap states of TiO2, which in turn affect important properties such as the conduction band energy, charge transport, recombination and collection. The defect states of TiO2 are highly dependent on the synthesis method and thus the effect of doping may vary for different synthesis techniques, making it difficult to compare the suitability of different dopants. High-throughput methods may be employed to achieve a rough prediction on the suitability of dopants for a specific synthesis method. It was however found that nearly every employed dopant can be used to increase device performance, indicating that the improvement is not so much caused by the dopant itself, as by the defects it eliminates from TiO2. Furthermore, with the field shifting from dye sensitized solar cells to perovskite solar cells, the role doping can play to further advance this emerging field is also discussed.

Interfacial Electron Transfer Barrier at Compact TiO2 /CH3 NH3 PbI3 Heterojunction

Low-temperature solution-processed CH3 NH3 PbI3 interfaced with TiO2 has recently been demonstrated as a highly successful type-II light harvesting heterojunction with ≈20% efficiency. Therefore, an efficient ultrafast photoexcited electron transfer from CH3 NH3 PbI3 to TiO2 is expected. However, by probing the photoexcited charge carrier dynamics in CH3 NH3 PbI3 /quartz, CH3 NH3 PbI3 /TiO2 (compact), and CH3 NH3 PbI3 /PCBM in a comparative study, an electron transfer potential barrier between CH3 NH3 PbI3 and the compact TiO2 (prepared with the spray pyrolysis method) formed by surface states is uncovered. Consequently, the CH3 NH3 PbI3 photoluminescence intensity and lifetime is enhanced when interfaced to compact TiO2 . The electron accumulation within CH3 NH3 PbI3 needed to overcome this interfacial potential barrier results in the undesirable large current-voltage hysteresis observed for CH3 NH3 PbI3 /TiO2 planar heterojunctions. The findings in this study indicate that careful surface engineering to reduce this potential barrier is key to pushing perovskite solar cell efficiencies toward the theoretical limit.

Why is Anatase a Better Photocatalyst than Rutile? The Importance of Free Hydroxyl Radicals

A tale of two polymorphs: Anatase is typically a more effective photocatalyst than rutile, however no consensus exists upon the reasons for this difference. Highlighted is a recent development in this area, which gives new insight into the desorption of reactive species from the surface of anatase and rutile. Following presentation of key methods and results of the highlighted work, the wider implications to other similar semiconductor photocatalysts are considered.

Controllable Synthesis and Tunable Photocatalytic Properties of Ti(3+)-doped TiO2

Photocatalysts show great potential in environmental remediation and water splitting using either artificial or natural light. Titanium dioxide (TiO2)-based photocatalysts are studied most frequently because they are stable, non-toxic, readily available, and highly efficient. However, the relatively wide band gap of TiO2 significantly limits its use under visible light or solar light. We herein report a facile route for controllable synthesis of Ti(3+)-doped TiO2 with tunable photocatalytic properties using a hydrothermal method with varying amounts of reductant, i.e., sodium borohydride (NaBH4). The resulting TiO2 showed color changes from light yellow, light grey, to dark grey with the increasing amount of NaBH4. The present method can controllably and effectively reduce Ti(4+) on the surface of TiO2 and induce partial transformation of anatase TiO2 to rutile TiO2, with the evolution of nanoparticles into hierarchical structures attributable to a high pressure and strong alkali environment in the synthesis atmosphere; in this way, the photocatalytic activity of Ti(3+)-doped TiO2 under visible-light can be tuned. The as-developed strategy may open up a new avenue for designing and functionalizing TiO2 materials for enhancing visible light absorption, narrowing band gap, and improving photocatalytic activity.

One-dimensional hybrid nanostructures for heterogeneous photocatalysis and photoelectrocatalysis

Semiconductor-based photocatalysis and photoelectrocatalysis have received considerable attention as alternative approaches for solar energy harvesting and storage. The photocatalytic or photoelectrocatalytic performance of a semiconductor is closely related to the design of the semiconductor at the nanoscale. Among various nanostructures, one-dimensional (1D) nanostructured photocatalysts and photoelectrodes have attracted increasing interest owing to their unique optical, structural, and electronic advantages. In this article, a comprehensive review of the current research efforts towards the development of 1D semiconductor nanomaterials for heterogeneous photocatalysis and photoelectrocatalysis is provided and, in particular, a discussion of how to overcome the challenges for achieving full potential of 1D nanostructures is presented. It is anticipated that this review will afford enriched information on the rational exploration of the structural and electronic properties of 1D semiconductor nanostructures for achieving more efficient 1D nanostructure-based photocatalysts and photoelectrodes for high-efficiency solar energy conversion.

Iron-functionalized titanium dioxide on flexible glass fibers for photocatalysis of benzene, toluene, ethylbenzene, and o-xylene (BTEX) under visible- or ultraviolet-light irradiation

Iron-functionalized titanium dioxide (TiO2) composites with various Fe-to-Ti ratios were prepared on flexible glass fibers (GF-Fe-TiO2) via a sol-gel method, followed by a dip-coating process. The photocatalytic ability of these composites in degrading selected volatile organic compounds (VOCs; benzene, toluene, ethylbenzene, and o-xylene [BTEX]) at indoor concentration levels was examined. The GF-Fe-TiO2 composites were characterized using scanning electron microscopy, energy-dispersive X-ray elemental analysis, ultraviolet (UV)-visible spectroscopy, and X-ray diffraction. The GF-Fe-TiO2 composites showed superior photocatalytic performance to that of a reference glass fiber-supported TiO2 photocatalyst for the treatment of BTEX under visible light. However, this trend was reversed under UV irradiation. Specifically, the average BTEX photocatalytic efficiencies of the 0.01-GF-Fe-TiO2 composite in a 3-hr visible-light photocatalytic process were 4%, 33%, 51%, and 74%, respectively. Conversely, the average BTEX photocatalytic efficiencies obtained for GF-TiO2 were close to 0%, 5%, 16%, and 29%, respectively. These findings demonstrated that the GF-Fe-TiO2 composites could be applied to photocatalytically purify BTEX, especially under visible-light exposure. Moreover, the GF-Fe-TiO2 composites prepared with different Fe-to-Ti ratios displayed different BTEX photocatalytic decomposition efficiencies under visible or UV light, allowing for optimization of the Fe-to-Ti ratio (which was found to be 0.01).

IMPLICATIONS

The application of nanomaterials for air purification necessitates a supporting material to stabilize them while in contact with the treated air in the photocatalytic chamber. Glass fibers have an obvious advantage over other supporting materials mainly because of its flexibility, which makes it much easier to handle. However, the applications of glass fiber-supported, visible light-activated photocatalysts to the treatment of air pollutants are rarely reported in literature. Accordingly, this study aimed to investigate the applicability of glass fiber-supported Fe-TiO2 for the purification of VOCs under visible- as well as UV-light exposure.

TiO2 nanotube array photoelectrocatalyst and Ni-Sb-SnO2 electrocatalyst bifacial electrodes: a new type of bifunctional hybrid platform for water treatment

Bifunctional hybrid electrodes capable of generating various reactive oxygen species (ROS) over a wide range of potentials were developed by coupling electrocatalysts and photoelectrocatalysts. To achieve this, Ni-doped Sb-SnO2 (NSS) was deposited on one side of a titanium (Ti) foil while the other side was anodized to grow a TiO2 nanotube array (TNA) for electrochemical ozone generation and photoelectrochemical hydroxyl radical generation, respectively. Surface characterization indicated that NSS and TNA were formed and spatially separated yet electrically connected through the Ti substrate. While each catalyst possessed unique electrochemical properties, the coupling of both catalysts resulted in mixed electrochemical properties that drove electrocatalysis at high potentials and photoelectrocatalysis at low potentials. The performance of the NSS/TNA electrode for phenol decomposition was ∼3 times greater than that of single-layer catalysts and ∼1.5 times greater than the combined catalytic performances of the individual NSS and TNA catalysts. This synergistic effect was attributed partly to the simultaneous generation of hydroxyl radicals and ozone, followed by the production of other ROS. A mechanism for the generation of ROS was discussed.

Black titanium dioxide (TiO2) nanomaterials

Recent progress in the preparation, properties and applications of black TiO₂nanomaterials is reviewed.

Patterning of polymer brushes made easy using titanium dioxide: direct and remote photocatalytic lithography

Titanium dioxide photocatalytic lithography of initiator functionalized surfaces is proved for the realization of micropatterned polymer brushes.

Arsenite oxidation-enhanced photocatalytic degradation of phenolic pollutants on platinized TiO2

The effect of As(III) on the photocatalytic degradation of phenolic pollutants such as 4-chlorophenol (4-CP) and bisphenol A (BPA) in a suspension of platinized TiO2 (Pt/TiO2) was investigated. In the presence of As(III), the photocatalytic degradation of 4-CP and BPA was significantly enhanced, and the simultaneous oxidation of As(III) to As(V) was also achieved. This positive effect of As(III) on the degradation of phenolic pollutants is attributed to the adsorption of As(V) (generated from As(III) oxidation) on the surface of Pt/TiO2, which facilitates the production of free OH radicals ((•)OHf) that are more reactive than surface-bound OH radicals ((•)OHs) toward phenolic pollutants. The generation of (•)OHf was indirectly verified by using coumarin as an OH radical trapper and comparing the yields of coumarin--OH adduct (i.e., 7-hydroxycoumarin) formed in the absence and presence of As(V). In repeated cycles of 4-CP degradation, the degradation efficiency of 4-CP gradually decreased in the absence of As(III), whereas it was mostly maintained in the presence of As(III), which was either initially present or repeatedly injected at the beginning of each cycle. The positive effect of As(III) on 4-CP degradation was observed over a wide range of As(III) concentrations (up to mM levels) with Pt/TiO2. However, a high concentration of As(III) (hundreds of μM) inhibited the degradation of 4-CP with bare TiO2. Therefore, Pt/TiO2 can be proposed as a practical photocatalyst for the simultaneous oxidation of phenolic pollutants and As(III) in industrial wastewaters.

Supported and unsupported nanomaterials for water and soil remediation: are they a useful solution for worldwide pollution?

Remediation technologies for wastes generated by industrial processes include coagulation, reverse osmosis, electrochemistry, photoelectrochemistry, advanced oxidation processes, and biological methods, among others. Adsorption onto activated carbon, sewage sludge, zeolites, chitosan, silica, and agricultural wastes has shown potential for pollutants' removal from aqueous media. Recently, nanoscale systems [nanoparticles (NPs) supported on different inorganic adsorbents] have shown additional benefits for the removal/degradation of several contaminants. According to the literature, NPs enhance the adsorption capacity of adsorbent materials and facilitate degradation of pollutants through redox reactions. In this review we analyzed relevant literature from 2011 to 2013, dealing with water and soil remediation by nanomaterials (NMs), either unsupported or supported upon inorganic adsorbents. Despite the outstanding reported results for some NMs, the analysis of the literature makes clear the necessity of more studies. There is lack of information about NMs regeneration and reusability, their large-scale application, and their efficiency in actual industrial wastewaters and contaminated soils. Additionally, little is known about NMs' life cycle, release of metal ions, disposal of pollutant loaded NMs, and their impacts on different ecosystems.

Photocatalytic degradation of 17α-ethinylestradiol (EE2) in the presence of TiO2-doped zeolite

Current design limitations and ineffective remediation techniques in wastewater treatment plants have led to concerns about the prevalence of pharmaceutical and personal care products (PPCPs) in receiving waters. A novel photocatalyst, TiO2-doped low-silica X zeolite (TiO2-LSX), was used to study the degradation of the pharmaceutical compound, 17α-ethinylestradiol (EE2). The catalyst was synthesized and characterized using XRD, BET surface analysis, SEM-EDAX, and ICP-OES. The effects of different UV light intensities, initial EE2 concentrations, and catalyst dosages on the EE2 removal efficiency were studied. A higher EE2 removal efficiency was attained with UV-TiO2-LSX when compared with UV-TiO2 or UV alone. The EE2 degradation process followed pseudo-first-order kinetics. A comprehensive empirical model was developed to describe the EE2 degradation kinetics under different conditions using multiple linear regression analysis. The EE2 degradation mechanism was proposed based on molecular calculations, identification of photoproducts using HPLC-MS/MS, and reactive species quenching experiments; the results showed that oxidative degradation pathways initiated by hydroxyl radicals were predominant. This novel TiO2-doped zeolite system provides a promising application for the UV disinfection process in wastewater treatment plants.

Metal/Semiconductor hybrid nanostructures for plasmon-enhanced applications

Hybrid nanostructures composed of semiconductor and plasmonic metal components are receiving extensive attention. They display extraordinary optical characteristics that are derived from the simultaneous existence and close conjunction of localized surface plasmon resonance and semiconduction, as well as the synergistic interactions between the two components. They have been widely studied for photocatalysis, plasmon-enhanced spectroscopy, biotechnology, and solar cells. In this review, the developments in the field of (plasmonic metal)/semiconductor hybrid nanostructures are comprehensively described. The preparation of the hybrid nanostructures is first presented according to the semiconductor type, as well as the nanostructure morphology. The plasmonic properties and the enabled applications of the hybrid nanostructures are then elucidated. Lastly, possible future research in this burgeoning field is discussed.

Recent advances in heterogeneous photocatalytic decolorization of synthetic dyes

During the process and operation of the dyes, the wastes produced were commonly found to contain organic and inorganic impurities leading to risks in the ecosystem and biodiversity with the resultant impact on the environment. Improper effluent disposal in aqueous ecosystems leads to reduction of sunlight penetration which in turn diminishes photosynthetic activity, resulting in acute toxic effects on the aquatic flora/fauna and dissolved oxygen concentration. Recently, photodegradation of various synthetic dyes has been studied in terms of their absorbance and the reduction of oxygen content by changes in the concentration of the dye. The advantages that make photocatalytic techniques superior to traditional methods are the ability to remove contaminates in the range of ppb, no generation of polycyclic compounds, higher speed, and lower cost. Semiconductor metal oxides, typically TiO₂, ZnO, SnO, NiO, Cu₂O, Fe₃O₄, and also CdS have been utilized as photocatalyst for their nontoxic nature, high photosensitivity, wide band gap and high stability. Various process parameters like photocatalyst dose, pH and initial dye concentrations have been varied and highlighted. Research focused on surface modification of semiconductors and mixed oxide semiconductors by doping them with noble metals (Pt, Pd, Au, and Ag) and organic matter (C, N, Cl, and F) showed enhanced dye degradation compared to corresponding native semiconductors. This paper reviews recent advances in heterogeneous photocatalytic decolorization for the removal of synthetic dyes from water and wastewater. Thus, the main core highlighted in this paper is the critical selection of semiconductors for photocatalysis based on the chemical, physical, and selective nature of the poisoning dyes.

Photocatalytic decomposition of bromate ion by the UV/P25-Graphene processes

The photocatalysis of bromate (BrO3(-)) attracts much attention as BrO3(-) is a carcinogenic and genotoxic contaminant in drinking water. In this work, TiO2-graphene composite (P25-GR) photocatalyst for BrO3(-) reduction were prepared by a facile one-step hydrothermal method, which exhibited a higher capacity of BrO3(-) removal than P25 or GR did. The maximum removal of BrO3(-) was observed in the optimal conductions of 1% GR doping and at pH 6.8. Compared with that without UV, the higher decreasing of BrO3(-) on the composite indicates that BrO3(-) decomposition was predominantly contributed to photo-reduction with UV rather than adsorption. This hypothesis was supported by the decreasing of [BrO3(-)] with the synchronous increasing of [Br(-)] at nearly constant amount of total Bromine ([BrO3(-)] + [Br(-)]). Furthermore, the improvement of BrO3(-) reduction on P25-GR was observed in the treatment of a tap water. However, the efficiency of BrO3(-) removal was less than that in deionized water, probably due to the consumption of photo-generated electrons and the adsorption of natural organic matters (NOM) on graphene.

An in vivo study on the photo-enhanced toxicities of S-doped TiO2 nanoparticles to zebrafish embryos (Danio rerio) in terms of malformation, mortality, rheotaxis dysfunction, and DNA damage

The role of light on the acute toxicities of S-doped and Sigma TiO2 nanoparticles in zebrafish was studied. Metrics included mortality for both, and rheotaxis dysfunction and DNA damage for S-doped only. It was found that the acute toxicity of S-TiO2 nanoparticles was enhanced by simulated sunlight (SSL) irradiation (96-h LC50 of 116.56 ppm) and exceeded that of Sigma TiO2, which was essentially non-toxic. Behavioral disorder, in terms of rheotaxis, was significantly increased by treatment with S-TiO2 nanoparticles under SSL irradiation. In order to further understand its toxicity mechanism, we investigated hair cells in neuromasts of the posterior lateral line (PLL) using DASPEI staining. Significant hair cell damage was observed in the treated larvae. The Comet assay was employed to investigate the DNA damage, which might be responsible for the loss of hair cells. Production of the superoxide anion ([Formula: see text]), a major ROS generated by TiO2 nanoparticles, was assayed and used to postulate causative factors to account for these damages. Oxidative effects were most severe in the liver, heart, intestine, pancreatic duct, and pancreatic islet - results consistent with our earlier findings in the investigation of embryonic malformation. TEM micrographs, used to further investigate the fate of S-TiO2 nanoparticles at the cellular level, suggested receptor-mediated autophagy and vacuolization. Our findings validate the benefit of using the transparent zebrafish embryo as an in vivo model for evaluating photo-induced nanotoxicity. These results highlight the importance of conducting a systematic risk assessment in connection with the use of doped TiO2 nanoparticles in aquatic ecosystems.