Preparation of a novel TiO2-based p-n junction nanotube photocatalyst

A novel enhanced visible-light-driven photocatalyst via hybridization of nanosized BiOCl and graphitic C3N4

Nanosized C₃N₄/BiOCl composite photocatalysts were synthesized via a facile solvothermal method in the presence of arabic gum. A possible photocatalytic mechanism over C₃N₄/BiOCl composite photocatalysts is proposed by combining photosensitization and photocatalysis.

UV-absorption--the primary process in photocatalysis and some practical consequences

TiO₂ photochemistry studies generally address reactions of photogenerated charge-carriers at the oxide surface or the recombination reactions which control the proportion of charge carriers that reach the surface. By contrast, this review focuses on UV absorption, the first photochemical step in semiconductor photocatalysis. The influence of particle size on absorption and scattering of light by small TiO₂ particles is summarized and the importance of considering, the particle size in the application, not the BET or X-ray line broadening size, is emphasized. Three different consequences of UV absorption are then considered. First, two commercially important systems, pigmented polymer films and paints, are used to show that TiO₂ can protect from direct photochemical degradation. Then the effect of UV absorption on the measured photocatalytic degradation of aqueous solutions of organics is considered for two separate cases. Firstly, the consequences of UV absorption by TiO₂ on the generation of hydroxyl radicals from H₂O₂ are considered in the context of the claimed synergy between H₂O₂ and TiO₂. Secondly, the effect of altered UV absorption, caused by changed effective particle size of the catalyst, is demonstrated for photocatalysis of propan-2-ol oxidation and salicylic acid degradation.

Are TiO2 nanotubes worth using in photocatalytic purification of air and water?

Titanium dioxide nanotubes (TNT) have mainly been used in dye sensitized solar cells, essentially because of a higher transport rate of electrons from the adsorbed photo-excited dye to the Ti electrode onto which TNT instead of TiO₂ nanoparticles (TNP) are attached. The dimension ranges and the two main synthesis methods of TNT are briefly indicated here. Not surprisingly, the particular and regular texture of TNT was also expected to improve the photocatalytic efficacy for pollutant removal in air and water with respect to TNP. In this short review, the validity of this expectation is checked using the regrettably small number of literature comparisons between TNT and commercialized TNP referring to films of similar thickness and layers or slurries containing an equal TiO₂ mass. Although the irradiated geometrical area differed for each study, it was identical for each comparison considered here. For the removal of toluene (methylbenzene) or acetaldehyde (ethanal) in air, the average ratio of the efficacy of TNT over that of TiO₂ P25 was about 1.5, and for the removal of dyes in water, it was around 1. This lack of major improvement with TNT compared to TNP could partially be due to TNT texture disorders as seems to be suggested by the better average performance of anodic oxidation-prepared TNT. It could also come from the fact that the properties influencing the efficacy are more numerous, their interrelations more complex and their effects more important for pollutant removal than for dye sensitized solar cells and photoelectrocatalysis where the electron transport rate is the crucial parameter.

Highly-dispersed boron-doped graphene nanosheets loaded with TiO2 nanoparticles for enhancing CO2 photoreduction

Boron doped graphene nanosheets (B-GR) as a p-type semiconductor, provides much more edges to facilitate the loading of TiO2 nanoparticles (P25). Highly-dispersed P25/B-GR nanosheets with the size of 20-50 nm, are successfully synthesized by the vacuum activation and ultraphonic method. The nanosized morphology can decrease the local density of defects which are induced by the boron substitutional doping, and make the B-GR keeping excellent conductivity and p-type transport property. Ti-O-C bonds are formed during the mixing process, which could efficiently transfer the electrons from TiO2 to B-GR and the holes from B-GR to TiO2. The tunable bandgap of B-GR determines the large potential application of P25/B-GR in the photoreduction of CO2 and other gaseous organic pollutants.

Recent Progress in Two-Dimensional Oxide Photocatalysts for Water Splitting

This Perspective focuses on the photocatalytic activity of two-dimensional (2D) oxide and nitrogen-doped oxide crystals and the effective use of 2D photocatalysts for understanding the mechanism of the water splitting reaction. Strategies for improving the activities of 2D photocatalysts are slightly different from those of bulk photocatalysts. Although it is well-known that a photocatalyst without co-catalyst loading has low activity for hydrogen production from water, a certain type of 2D oxide nanosheet shows high activity without co-catalyst loading. It is difficult to determine what factors contribute to this separation of oxidation and reduction sites of water because there are many factors on the reaction surface. A nanosheet p-n junction surface is an ideal surface for understanding the carrier transfer during the photocatalytic reaction. In this system, the driving force of the carrier transfer to the reaction sites was found to be the potential gradient generated by the nanosheet junction.

Structural improvement of CaFe₂O₄ by metal doping toward enhanced cathodic photocurrent

Various metal-doped p-type CaFe2O4 photocathodes were prepared in an attempt to improve the low quantum efficiency for photoreaction. CuO and Au doping enhanced the photocurrent by expansion of the absorption wavelength region and plasmon resonance, respectively. X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS) analysis showed that doping with these metals further disturbed the originally distorted crystal structure of CaFe2O4. In contrast, doping with Ag relaxed the distorted crystal structure around the Fe center toward symmetry. Ag doping resulted in improvement of the carrier mobility together with a red-shift of photoabsorption with Ag-doped CaFe2O4 having a 23-fold higher photocurrent than undoped CaFe2O4.

Engineering BiOX (X = Cl, Br, I) nanostructures for highly efficient photocatalytic applications

Heterogeneous photocatalysis that employs photo-excited semiconductor materials to reduce water and oxidize toxic pollutants upon solar light irradiation holds great prospects for renewable energy substitutes and environmental protection. To utilize solar light effectively, the quest for highly active photocatalysts working under visible light has always been the research focus. Layered BiOX (X = Cl, Br, I) are a kind of newly exploited efficient photocatalysts, and their light response can be tuned from UV to visible light range. The properties of semiconductors are dependent on their morphologies and compositions as well as structures, and this also offers the guidelines for design of highly-efficient photocatalysts. In this review, recent advances and emerging strategies in tailoring BiOX (X = Cl, Br, I) nanostructures to boost their photocatalytic properties are surveyed.

Recent progress in highly efficient Ag-based visible-light photocatalysts

This review summarizes the recent progress in Ag-compound-based semiconductor photocatalysts, which will provide insights for future photocatalytic designs.

Bi4Ti3O12 nanofibers-BiOI nanosheets p-n junction: facile synthesis and enhanced visible-light photocatalytic activity

A novel p-n junction photocatalyst of Bi4Ti3O12 nanofibers-BiOI nanosheets has been fabricated through a simple and economical technique of electrospinning combined with a successive ionic layer adsorption and reaction (SILAR) process. The products are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), UV-visible diffuse reflectance spectra (DRS), and photoluminescence (PL) spectroscopy. The as-formed Bi4Ti3O12 nanofibers are composed of inter-linked nanoparticles of 50-80 nm in size. The thickness of the as-grown BiOI nanosheets is about 10 nm and the size of the BiOI nanosheets increases with the SILAR cycles. In particular, many {001} facets of BiOI nanosheets are exposed, which is favorable to enhance the visible-light photocatalytic activity. The p-n junction photocatalyst exhibits enhanced visible-light-driven photocatalytic activity for decomposition of rhodamine B (RhB) and phenol. The enhanced photocatalytic activity can be attributed to the extended absorption in the visible light region resulting from the BiOI nanosheets and the effective separation of photogenerated carriers driven by the photo-induced potential difference generated at the Bi4Ti3O12-BiOI p-n junction interface.

Nano-/microstructure improved photocatalytic activities of semiconductors

Photocatalysis has emerged as a promising technique owing to its valuable applications in environmental purification. With the demand of building effective photocatalyst materials, semiconductor investigation experienced a developing process from simple chemical modification to complicated morphology design. In this review, the general relationship between morphology structures and photocatalytic properties is mainly discussed. Various nano-/microsized structures from zero- to three-dimensional are discussed, and the photocatalytic efficiency correspon- ding to the structures is analysed. The results showed that simple structures can be easily obtained and can facilitate chemical modification, whereas one- or three-dimensional structures can provide structure-enhanced properties such as surface area increase, multiple reflections of UV light, etc. Those principles of structure-related photocatalytic properties will afford basic ideology in designing new photocatalytic materials with more effective catalytic properties.

Nanomaterials for Water Remediation

The term nanotechnology is employed to describe the creation and exploitation of materials with structural features in between those of atoms and bulk materials, with at least one dimension in the nanometer range (1 nm to 100 nm). This chapter presents a brief introduction to nanotechnology and provides a broad context to its narrower focus on water remediation. The introduction also addresses some pollutants presents in water and their adverse effects on health and environment. The intent is to give reader an overview of the current state of art as well as a look into the future use of nanomaterials in water remediation.

Nanoscale materials and their use in water contaminants removal-a review

Water scarcity is being recognized as a present and future threat to human activity and as a consequence water purification technologies are gaining major attention worldwide. Nanotechnology has many successful applications in different fields but recently its application for water and wastewater treatment has emerged as a fast-developing, promising area. This review highlights the recent advances on the development of nanoscale materials and processes for treatment of surface water, groundwater and industrial wastewater that are contaminated by toxic metals, organic and inorganic compounds, bacteria and viruses. In addition, the toxic potential of engineered nanomaterials for human health and the environment will also be discussed.

Visible light-driven α-Fe₂O₃ nanorod/graphene/BiV₁-xMoxO₄ core/shell heterojunction array for efficient photoelectrochemical water splitting

We report the design, synthesis, and characterization of a novel heterojunction array of α-Fe(2)O(3)/graphene/BiV(1-x)Mo(x)O(4) core/shell nanorod for photoelectrochemical water splitting. The heterojunction array was prepared by hydrothermal deposition of α-Fe(2)O(3) nanorods onto Ti substrate, with subsequent coating of graphene interlayer and BiV(1-x)Mo(x)O(4) shell by photocatalytic reduction and a spin-coating approach, respectively. The heterojunction yielded a pronounced photocurrent density of ∼1.97 mA/cm(2) at 1.0 V vs Ag/AgCl and a high photoconversion efficiency of ∼0.53% at -0.04 V vs Ag/AgCl under the irradiation of a Xe lamp. The improved photoelectrochemical properties benefited from (1) the enhanced light absorption due to behavior of the "window effect" between the α-Fe(2)O(3) cores and BiV(1-x)Mo(x)O(4) shells, and (2) the improved separation of photogenerated carriers at the α-Fe(2)O(3) nanorod/graphene/BiV(1-x)Mo(x)O(4) interfaces. Our results demonstrate the advantages of the novel graphene-mediated core/shell heterojunction array and provide a valuable insight for the further development of such materials.

Development of titanium-dioxide-based aerogel catalyst with tunable nanoporosity and photocatalytic activity

Nanostructured highly porous TiO(2)/WO(3)/Fe(3+) aerogel composite photocatalysts are prepared, characterized and tested for model photocatalytic reactions. The catalyst structure is tailored to capture environmental pollutants and enable their decomposition in situ under both ultraviolet (UV) and visible light through oxidation to smaller benign molecules. A novel and green method is utilized to prepare the unique surfactant-templated aerogel composite photocatalyst that has a highly accessible porous nanostructure with high surface area and tailored pore size distribution. The sol-gel process is combined with supercritical extraction and drying. Supercritical drying with heat treatment results in titanium dioxide with anatase crystal form. Templates used further enable retention and tuning of the nanopore structure and surface properties. The synthesized catalysts were characterized using SEM, FIB, XRD and porosimetry prior to post-evaluation in model reactions. The bandgap of the catalyst particles was also determined using diffuse reflectance. The resulting aerogel TiO(2)/WO(3)/Fe(3+) has similar photocatalytic capability compared to highly optimized commercial Degussa P25 under UV exposure and offers much superior photocatalytic capability under visible light exposure. The model reaction utilized employed methylene blue (MB) photooxidation under visible and UV light.

Functionalization of tungsten oxide into MWCNT and its application for sunlight-induced degradation of rhodamine B

A composite of multi-walled carbon nanotube/tungsten oxide (MWCNT/WO(3)) has been successfully synthesized. The prepared composite was characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), high-resolution transmission electron microscopy (HRTEM) and Fourier transform infrared spectroscopy (FTIR). The catalytic activity was investigated by rhodamine B degradation under solar irradiation. The influence of various degradation parameters such as solar illumination time, initial dye concentration, dosage and pH was investigated. It was found that the composite exhibits an enhanced photocatalytic activity as compared with WO(3) and a mechanical mixture of MWCNTs and WO(3). The enhancement in photocatalytic performance of the MWCNT/WO(3) composite has been explained based on adsorption ability and electron transportation as a result of a strong interaction between WO(3) and MWCNTs. Besides, MWCNTs acts as dispersing agent preventing WO(3) from agglomerating during the catalytic process, providing a high active surface area of the catalyst. A reasonable mechanism for the enhanced reactivity was proposed.

ZnO nanorod array/CuAlO2 nanofiber heterojunction on Ni substrate: synthesis and photoelectrochemical properties

A novel ZnO nanorod array (NR)/CuAlO(2) nanofiber (NF) heterojunction nanostructure was grown on a substrate of Ni plates using sol-gel synthesis for the NFs and hydrothermal reaction for the NRs. Compared with a traditional ZnO/CuAlO(2) laminar film nanostructure, the photocurrent of this fibrous network heterojunction is significantly increased. A significant blue-shift of the absorption edge and a favorable forward current to reverse current ratio at applied voltages of -2 to +2 V were observed in this heterojunction with the increase of Zn(2+) ion concentration in the hydrothermal reaction. Furthermore, the photoelectrochemical properties were investigated and the highest photocurrent of 3.1 mA cm(-2) was obtained under AM 1.5 illumination with 100 mW cm(-2) light intensity at 0.71 V (versus Ag/AgCl). This novel 3D fibrous network nanostructure plays an important role in the optoelectronic field and can be extended to other binary or ternary oxide compositions for various applications.

Super-long aligned TiO2/carbon nanotube arrays

5 mm long aligned titanium oxide/carbon nanotube (TiO(2)/CNT) coaxial nanowire arrays have been prepared by electrochemically coating the constituent CNTs with a uniform layer of highly crystalline anatase TiO(2) nanoparticles. While the presence of the TiO(2) coating was confirmed by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy and x-ray diffraction, the resultant TiO(2)/CNT coaxial arrays were demonstrated to exhibit minimized recombination of photoinduced electron-hole pairs and fast electron transfer from the long TiO(2)/CNT arrays to external circuits. This, in conjunction with the aligned macrostructure, facilitates the fabrication of TiO(2)/CNT arrays for various device applications, ranging from photodetectors to photocatalytic systems. Thus, the millimeter long TiO(2)/CNT arrays represent a significant advance in the development of new macroscopic photoelectronic nanomaterials attractive for a variety of device applications beyond those demonstrated in this study.

High efficient photocatalytic degradation of p-nitrophenol on a unique Cu2O/TiO2 p-n heterojunction network catalyst

p-Nitrophenol (PNP) is a difficultly decomposed organic pollutant under solar light in the absence of strong oxidants. This study shows that under artificial solar light PNP can be effectively degraded by a Cu(2)O/TiO(2) p-n junction network which is fabricated by anodizing Cu(0) particles-loaded TiO(2) nanotubes (NTs). The network is composed of p-type Cu(2)O nanowires on the top surface and Cu(2)O nanoparticles on the inner walls of the n-type TiO(2) NT arrays. The Cu(2)O/TiO(2) network shows much higher degradation rate (1.97 μg/min cm(2)) than the unmodified TiO(2) NTs (0.85 μg/min cm(2)). The enhanced photocatalytic acitivity can be attributed to the extended absorption in the visible resulting from the Cu(2)O nanowire networks and the effective separation of photogenerated carriers driven by the photoinduced potential difference generated at the Cu(2)O/TiO(2) p-n junction interface.

Photocatalytical Properties of TiO2 Nanotubes

Titanium dioxide (TiO2) nanotubes have been reported one decade ago and have proven to be of a great interest in photocatalytic water splitting, as well as gas sensing and anti-bacterial/cancer treatment. This paper presents an overview on general preparation approaches (chemical treatment, template method and anodic oxidation) of tubular TiO2 nanoarchitectures and their characterization. Current applications of the nanotubes as photocatalysts are also reviewed.

A facile route to n-type TiO(2)-nanotube/p-type boron-doped-diamond heterojunction for highly efficient photocatalysts

Anatase TiO(2) nanotube (TiNT) arrays have been fabricated on a p-type boron-doped diamond substrate by a liquid phase deposition method using a ZnO nanorod template. The n-type TiNT/p-type diamond heterojunction structures which are realized show significantly enhanced photocatalytic activities with good recyclable behavior, with respect to the cases of sole TiNTs.

Engineered nanoparticles in wastewater and wastewater sludge--evidence and impacts

Nanotechnology has widespread application in agricultural, environmental and industrial sectors ranging from fabrication of molecular assemblies to microbial array chips. Despite the booming application of nanotechnology, there have been serious implications which are coming into light in the recent years within different environmental compartments, namely air, water and soil and its likely impact on the human health. Health and environmental effects of common metals and materials are well-known, however, when the metals and materials take the form of nanoparticles--consequential hazards based on shape and size are yet to be explored. The nanoparticles released from different nanomaterials used in our household and industrial commodities find their way through waste disposal routes into the wastewater treatment facilities and end up in wastewater sludge. Further escape of these nanoparticles into the effluent will contaminate the aquatic and soil environment. Hence, an understanding of the presence, behavior and impact of these nanoparticles in wastewater and wastewater sludge is necessary and timely. Despite the lack of sufficient literature, the present review attempts to link various compartmentalization aspects of the nanoparticles, their physical properties and toxicity in wastewater and wastewater sludge through simile drawn from other environmental streams.

Structuring a TiO2-based photonic crystal photocatalyst with Schottky junction for efficient photocatalysis

Facile and effective approaches were developed to fabricate the inverse TiO2/Pt opals Schottky structures on the Ti substrate. The as-prepared samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and diffuse reflectance UV-vis spectra (DRS), respectively. The results indicate that these samples were of ordered network, which was built by the Pt skeleton frame and the outer TiO2 layer. The TiO2 layer was identified as anatase with the preferential orientation of (101) plane. The experiments of short-circuit photocurrent (SCPC) and photocatalytic degradation of phenol were also conducted under the UV irradiation in order to evaluate the photoactivity of the samples. By tuning the red edge of photonic stop-band overlapping the absorption maximum of anatase (at 360 nm), both the UV absorption and the carrier separation of the samples were improved. The kinetic constant using the optimal inverse TiO2/Pt opals (0.992 h(-1)) was about 1.5 times as great as that of the disordered inverse TiO2/Pt opals (TiO2/Pt-mix) and was 3.3 times as great as that of pristine TiO2 nanocrystalline film (TiO2-nc) on Ti substrate.

Hierarchically macro-/mesoporous Ti-Si oxides photonic crystal with highly efficient photocatalytic capability

Hierarchically macro-/mesoporous Ti-Si oxides photonic crystal (i-Ti-Si PC) with highly efficient photocatalytic activity has been synthesized by combining colloidal crystal template and amphiphilic triblock copolymer. It was found that the thermal stability of mesoporous structures in the composite matrix were improved due to the introduction of silica acting as glue and linking anatase nanoparticles together, and the photocatalytic activity of the i-Ti-Si PCs was affected by the calcination conditions. The influences of photonic and structural effect of the i-Ti-Si PCs on photocatalytic activity were investigated. Photodegradation efficiency of the i-Ti-Si PCs was 2.1 times higher than that of TiO(2) photonic crystals (i-TiO(2) PCs) in the photodegradation of Rhodamine B (RB) dye as a result of higher surface area. When the energy of slow photon (SP) was optimized to the abosorption region of TiO(2), a maximum enhanced factor of 15.6 was achieved in comparison to nanocrystalline TiO(2) films (nc-TiO(2)), which originated from the synergetic effect of SP enhancement and high surface area.