Defective TiO2 with oxygen vacancies: synthesis, properties and photocatalytic applications

Synthesis and characterization of BN/Bi2WO6 composite photocatalysts with enhanced visible-light photocatalytic activity

Bi₂WO₆ modified with few-layer BN was synthesized by an impregnation method. The as-prepared BN/Bi₂WO₆ photocatalysts exhibited a higher photocatalytic activity for the degradation of Rhodamine B (RhB) than the pure Bi₂WO₆.

1D CdS nanowire–2D BiVO4 nanosheet heterostructures toward photocatalytic selective fine-chemical synthesis

The CdS–BiVO₄ 1D/2D composite is fabricated by a simple electrostatic self-assembly methodology, which exhibits a significantly enhanced photocatalytic performance as compared to bare 1D CdS and 2D BiVO₄.

The hierarchical structure of cubic K0.5La0.5TiO3 layers and enhanced photocatalytic hydrogen evolution after surface acidification

A puzzle-like 3D hierarchical structure constructed with K_0.5La_0.5TiO₃ nanosheets was first synthesized. After a modest acidification, the 3D hierarchical structure transforms to a unique core–shell nanostructure, which exhibits remarkably enhanced photocatalytic H₂ production.

Transformation of hydrogen titanate nanoribbons to TiO2 nanoribbons and the influence of the transformation strategies on the photocatalytic performance

The influence of the reaction conditions during the transformation of hydrogen titanate nanoribbons to TiO2 nanoribbons on the phase composition, the morphology, the appearance of the nanoribbon surfaces and their optical properties was investigated. The transformations were performed (i) through a heat treatment in oxidative and reductive atmospheres in the temperature range of 400-650 °C, (ii) through a hydrothermal treatment in neutral and basic environments at 160 °C, and (iii) through a microwave-assisted hydrothermal treatment in a neutral environment at 200 °C. Scanning electron microscopy investigations showed that the hydrothermal processing significantly affected the nanoribbon surfaces, which became rougher, while the transformations based on calcination in either oxidative or reductive atmospheres had no effect on the morphology or on the surface appearance of the nanoribbons. The transformations performed in the reductive atmosphere, an NH3(g)/Ar(g) flow, and in the ammonia solution led to nitrogen doping. The nitrogen content increased with an increasing calcination temperature, as was determined by X-ray photoelectron spectroscopy. According to electron paramagnetic resonance measurements the calcination in the reductive atmosphere also resulted in a partial reduction of Ti(4+) to Ti(3+). The photocatalytic performance of the derived TiO2 NRs was estimated on the basis of the photocatalytic oxidation of isopropanol. After calcinating in air, the photocatalytic performance of the investigated TiO2 NRs increased with an increased content of anatase. In contrast, the photocatalytic performance of the N-doped TiO2 NRs showed no dependence on the calcination temperature. An additional comparison showed that the N-doping significantly suppressed the photocatalytic performance of the TiO2 NRs, i.e., by 3 to almost 10 times, in comparison with the TiO2 NRs derived by calcination in air. On the other hand, the photocatalytic performance of the hydrothermally derived TiO2 NRs was additionally improved by a subsequent heat treatment in air.

Efficient visible light photocatalytic activity and enhanced stability of BiOBr/Cd(OH)2 heterostructures

The BiOBr/Cd(OH)₂ heterojunction formation decreased the charge recombination phenomenally and imparted significant visible light response.

Synthesis of CuS flowers exhibiting versatile photo-catalyst response

A controlled synthesis of CuS microflowers through sonochemical technique is carried out and their excellent and versatile natural light driven photodegradation capability has been witnessed.

Improved light absorption and photocatalytic activity of Zn,N-TiO2−x rich in oxygen vacancies synthesized by nitridation and hydrogenation

Compared with N-TiO₂ and Zn,N-TiO₂ samples, Zn,N-TiO_2−x rich in oxygen vacancies demonstrates high photocatalytic activity.

A novel photocatalyst CeF3: facile fabrication and photocatalytic performance

The synthesized CeF₃ nanoparticles can effectively decompose organic contaminants in aqueous solution and show high stability.

Ultrathin tungsten oxide nanowires: oleylamine assisted nonhydrolytic growth, oxygen vacancies and good photocatalytic properties

Ultrathin tungsten oxide nanowires with the diameter of around 1.1 nm have been fabricated through controlling the amount of oleylamine in a ​nonhydrolytic process.

The critical role of intragap states in the energy transfer from gold nanoparticles to TiO2

Cathodoluminescence spectroscopy allows the elucidation of energy transfer mechanisms between gold nanoparticles and semiconductors occurring during hydrogen production by photo-steam reforming.

Biotemplated synthesis of hierarchically nanostructured TiO2 using cellulose and its applications in photocatalysis

Hierarchically nanostructured TiO₂ has been hydrothermally synthesized using cellulose as a biotemplate involving various types of acids.

A novel preparation of Ag-doped TiO2 nanofibers with enhanced stability of photocatalytic activity

A mechanism to form Ag/TiO₂ heterostructures by a solvothermal reducing reaction in HMTA solvent is reported.

Photocatalytic reduction of CO2 with H2O to CH4 on Cu(i) supported TiO2 nanosheets with defective {001} facets

A series of Cu–TiO₂-x has been reasonably designed and synthesized as efficient photocatalysts for the reduction of CO₂ with H₂O to CH₄. It is found that the high activity may be contributed by the co-existence of surface oxygen vacancies and Cu(i) species on {001} facets of TiO₂.

Efficient improvement of photoelectrochemical activity for multiple semiconductor (CdS/PbS/ZnS) co-sensitized TiO2 photoelectrodes by hydrogen treatment

In the present work we report a simple and viable approach to improve the photoelectrochemical activity of TiO₂ photoelectrodes.

Mechanical control of magnetism in oxygen deficient perovskite SrTiO3

Magnetism changes with pressure for deficient SrTiO₃. A negative pressure enhances the magnetic moment while a positive pressure eliminates it.

New insights into self-modification of mesoporous titania nanoparticles for enhanced photoactivity: effect of microwave power density on formation of oxygen vacancies and Ti3+ defects

Mesoporous titania nanoparticles (MTN) were successfully prepared by a microwave (MW)-assisted method under various power densities.

Facile electrochemical assisted synthesis of ZnO/graphene nanosheets with enhanced photocatalytic activity

In the present study, an electrochemical exfoliation technique was used to synthesize a series of zinc oxide/graphene (ZnO/Gr) nanocomposites using different concentrations of zinc nitrate in an electrolyte solution.

Hydrothermal synthesis of defective TiO2nanoparticles for long-wavelength visible light-photocatalytic killing of cancer cells

Newly fabricatedd-TiO₂NPs were demonstrated to be efficient in long wavelength visible light-triggered killing of cancer cells.

The positive effect of anatase and rutile on the brookite-photocatalyzed degradation of phenol

In this study, the biphase effect of brookite with anatase or rutile has been reinvestigated by using phenol degradation in aqueous solution as a model reaction.

Electrically conductive polyaniline sensitized defective-TiO2 for improved visible light photocatalytic and photoelectrochemical performance: a synergistic effect

A proposed scheme for the synthesis of s-Pani@m-TiO₂ nanocomposites.

Highly-efficient cocatalyst-free H2-evolution over silica-supported CdS nanoparticle photocatalysts under visible light

A silica-supported CdS nanoparticle photocatalyst exhibits excellent visible-light driven H₂evolution activity without the use of a cocatalyst. The apparent quantum yield can reach 42% under 420 nm light illumination.

Xylene gas sensor based on Ni doped TiO2 bowl-like submicron particles with enhanced sensing performance

Bowl-like TiO₂ submicron particles prepared by electrospray technique were used to detect xylene gas and Ni element was added into TiO₂ to improve the gas sensing performances.

The Rational Design of a Single-Component Photocatalyst for Gas-Phase CO2 Reduction Using Both UV and Visible Light

The solar-to-chemical energy conversion of greenhouse gas CO2 into carbon-based fuels is a very important research challenge, with implications for both climate change and energy security. Herein, the key attributes of hydroxides and oxygen vacancies are experimentally identified in non-stoichiometric indium oxide nanoparticles, In2O3-x(OH)y, that function in concert to reduce CO2 to CO under simulated solar irradiation.

Tailored synthesis of hierarchical spinous hollow titania hexagonal prisms via a self-template route

Novel hierarchical spinous hollow titania hexagonal prisms are prepared through a facile fluorine-free self-template route using Ti2O3(H2O)2(C2O4)·H2O (TC) hexagonal prisms as a precursor. The hollowing transformation can be elucidated by the template-free Kirkendall effect, and diverse nanostructures can also be synthesized during the conversion process, such as the spinous core-shell and yolk-shell nanocomposites. The hierarchical hollow microparticles are composed of ultrathin nanobelts of 50-100 nm in length and about 10 nm in thickness, and possess a higher surface area of up to 163 m(2) g(-1) compared with solid microparticles (49 m(2) g(-1)). This type of morphology is of great interest for lithium-ion batteries because of its shorter length for Li(+) transport and better electrode-electrolyte contact.

Polymorphic phase transition among the titania crystal structures using a solution-based approach: from precursor chemistry to nucleation process

Nanocrystalline titania are a robust candidate for various functional applications owing to its non-toxicity, cheap availability, ease of preparation and exceptional photochemical as well as thermal stability. The uniqueness in each lattice structure of titania leads to multifaceted physico-chemical and opto-electronic properties, which yield different functionalities and thus influence their performances in various green energy applications. The high temperature treatment for crystallizing titania triggers inevitable particle growth and the destruction of delicate nanostructural features. Thus, the preparation of crystalline titania with tunable phase/particle size/morphology at low to moderate temperatures using a solution-based approach has paved the way for further exciting areas of research. In this focused review, titania synthesis from hydrothermal/solvothermal method, conventional sol-gel method and sol-gel-assisted method via ultrasonication, photoillumination and ILs, thermolysis and microemulsion routes are discussed. These wet chemical methods have broader visibility, since multiple reaction parameters, such as precursor chemistry, surfactants, chelating agents, solvents, mineralizer, pH of the solution, aging time, reaction temperature/time, inorganic electrolytes, can be easily manipulated to tune the final physical structure. This review sheds light on the stabilization/phase transformation pathways of titania polymorphs like anatase, rutile, brookite and TiO2(B) under a variety of reaction conditions. The driving force for crystallization arising from complex species in solution coupled with pH of the solution and ion species facilitating the orientation of octahedral resulting in a crystalline phase are reviewed in detail. In addition to titanium halide/alkoxide, the nucleation of titania from other precursors like peroxo and layered titanates are also discussed. The non-aqueous route and ball milling-induced titania transformation is briefly outlined; moreover, the lacunae in understanding the concepts and future prospects in this exciting field are suggested.

Hydrogenated anatase: strong photocatalytic dihydrogen evolution without the use of a co-catalyst

The high-pressure hydrogenation of commercially available anatase or anatase/rutile TiO2 powder can create a photocatalyst for H2 evolution that is highly effective and stable without the need for any additional co-catalyst. This activation effect cannot be observed for rutile; however, for anatase/rutile mixtures, a strong synergistic effect can be found (similar to results commonly observed for noble-metal-decorated TiO2). EPR and PL measurements indicated the intrinsic co-catalytic activation of anatase TiO2 to be due to specific defect centers formed during hydrogenation. These active centers can be observed specifically for high-pressure hydrogenation; other common reduction treatments do not result in this effect.

Surface properties and photocatalytic activity of KTaO3, CdS, MoS2 semiconductors and their binary and ternary semiconductor composites

Single semiconductors such as KTaO3, CdS MoS2 or their precursor solutions were combined to form novel binary and ternary semiconductor nanocomposites by the calcination or by the hydro/solvothermal mixed solutions methods, respectively. The aim of this work was to study the influence of preparation method as well as type and amount of the composite components on the surface properties and photocatalytic activity of the new semiconducting photoactive materials. We presented different binary and ternary combinations of the above semiconductors for phenol and toluene photocatalytic degradation and characterized by X-ray powder diffraction (XRD), UV-Vis diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) specific surface area and porosity. The results showed that loading MoS2 onto CdS as well as loading CdS onto KTaO3 significantly enhanced absorption properties as compared with single semiconductors. The highest photocatalytic activity in phenol degradation reaction under both UV-Vis and visible light irradiation and very good stability in toluene removal was observed for ternary hybrid obtained by calcination of KTaO3, CdS, MoS2 powders at the 10:5:1 molar ratio. Enhanced photoactivity could be related to the two-photon excitation in KTaO3-CdS-MoS2 composite under UV-Vis and/or to additional presence of CdMoO4 working as co-catalyst.

Doping of wide-bandgap titanium-dioxide nanotubes: optical, electronic and magnetic properties

Doping semiconductors is an important step for their technological application. While doping bulk semiconductors can be easily achieved, incorporating dopants in semiconductor nanostructures has proven difficult. Here, we report a facile synthesis method for doping titanium-dioxide (TiO₂) nanotubes that was enabled by a new electrochemical cell design. A variety of optical, electronic and magnetic dopants were incorporated into the hollow nanotubes, and from detailed studies it is shown that the doping level can be easily tuned from low to heavily-doped semiconductors. Using desired dopants - electronic (p- or n-doped), optical (ultraviolet bandgap to infrared absorption in co-doped nanotubes), and magnetic (from paramagnetic to ferromagnetic) properties can be tailored, and these technologically important nanotubes can be useful for a variety of applications in photovoltaics, display technologies, photocatalysis, and spintronic applications.

Large-scale synthesis of TiO2 microspheres with hierarchical nanostructure for highly efficient photodriven reduction of CO2 to CH4

In this study, a simple and reproducible synthesis strategy was employed to fabricate TiO2 microspheres with hierarchical nanostructure. The microspheres are macroscopic in the bulk particle size (several hundreds to more than 1000 μm), but they are actually composed of P25 nanoparticles as the building units. Although it is simple in the assembly of P25 nanoparticles, the structure of the as-prepared TiO2 microspheres becomes unique because a hierarchical porosity composed of macropores, larger mesopores (ca. 12.4 nm), and smaller mesopores (ca. 2.3 nm) has been developed. The interconnected macropores and larger mesopores can be utilized as fast paths for mass transport. In addition, this hierarchical nanostructure may also contribute to some extent to the enhanced photocatalytic activity due to increased multilight reflection/scattering. Compared with the state-of-the-art photocatalyst, commercial Degussa P25 TiO2, the as-prepared TiO2 microsphere catalyst has demonstrated significant enhancement in photodriven conversion of CO2 into the end product CH4. Further enhancement in photodriven conversion of CO2 into CH4 can be easily achieved by the incorporation of metals such as Pt. The preliminary experiments with Pt loading reveal that there is still much potential for considerable improvement in TiO2 microsphere based photocatalysts. Most interestingly and significantly, the synthesis strategy is simple and large quantity of TiO2 microspheres (i.e., several hundred grams) can be easily prepared at one time in the lab, which makes large-scale industrial synthesis of TiO2 microspheres feasible and less expensive.

Role of zinc interstitials and oxygen vacancies of ZnO in photocatalysis: a bottom-up approach to control defect density

Oxygen vacancies (V(O)s) in ZnO are well-known to enhance photocatalytic activity (PCA) despite various other intrinsic crystal defects. In this study, we aim to elucidate the effect of zinc interstitials (Zn(i)) and V(O)s on PCA, which has applied as well as fundamental interest. To achieve this, the major hurdle of fabricating ZnO with controlled defect density requires to be overcome, where it is acknowledged that defect level control in ZnO is significantly difficult. In the present context, we fabricated nanostructures and thoroughly characterized their morphological (SEM, TEM), structural (XRD, TEM), chemical (XPS) and optical (photoluminescence, PL) properties. To fabricate the nanostructures, we adopted atomic layer deposition (ALD), which is a powerful bottom-up approach. However, to control defects, we chose polysulfone electrospun nanofibers as a substrate on which the non-uniform adsorption of ALD precursors is inevitable because of the differences in the hydrophilic nature of the functional groups. For the first 100 cycles, Zn(i)s were predominant in ZnO quantum dots (QDs), while the presence of V(O)s was negligible. As the ALD cycle number increased, V(O)s were introduced, whereas the density of Zn(i) remained unchanged. We employed PL spectra to identify and quantify the density of each defect for all the samples. PCA was performed on all the samples, and the percent change in the decay constant for each sample was juxtaposed with the relative densities of Zn(i)s and V(O)s. A logical comparison of the relative defect densities of Zn(i)s and V(O)s suggested that the former are less efficient than the latter because of the differences in the intrinsic nature and the physical accessibility of the defects. Other reasons for the efficiency differences were elaborated.

Platinum nanoparticles on electrospun titania nanofibers as hydrogen sensing materials working at room temperature

Platinum nanoparticles (PtNPs), with diameters of 3-10 nm, were synthesized by water phase reduction, using 3-mercapto-1-propanesulfonate (3MPS) as a hydrophilic capping agent. PtNPs were deposited by a dipcoating technique on titania nanofibers (TiO2NFs), obtained by electrospinning. The investigated properties of the Pt-TiO2 hybrid at room temperature show that this material combines the properties of photoconduction of titania and the photocatalytic activity of the hybrid. To assess the best performance of Pt-TiO2, different measurements were performed at room temperature, comparing hydrogen response under UV of the uncoated TiO2NFs, compared with the Pt-TiO2 system prepared with two different amounts of PtNPs. During the sensing tests toward hydrogen an enhancement of photoconductivity (150%), an increase in response (400%) and an overall improvement of their dynamic behaviour were observed.

In situ surface hydrogenation synthesis of Ti3+ self-doped TiO2 with enhanced visible light photoactivity

A novel one-step, vapor-fed aerosol flame synthetic process (VAFS) has been developed to prepare Ti(3+) self-doped titanium dioxide (TiO2). The freshly formed TiO2 was in situ surface hydrogenated during the condensation stage by introducing H2 above the flame, and Ti(3+) ions were created near the surface of TiO2. The relative content of Ti(3+) ions near the surface of TiO2 is estimated to be 8%. Because of the high absorption of visible light and suppression of charge recombination, the photocurrent density and decomposition of MB under visible light irradiation were remarkably enhanced. This study demonstrates a simple, potential method to produce Ti(3+) self-doped TiO2 with effective photoactivity in visible light.

Bismuth oxyhalide nanomaterials: layered structures meet photocatalysis

In recent years, layered bismuth oxyhalide nanomaterials have received more and more interest as promising photocatalysts because their unique layered structures endow them with fascinating physicochemical properties; thus, they have great potential photocatalytic applications for environment remediation and energy harvesting. In this article, we explore the synthesis strategies and growth mechanisms of layered bismuth oxyhalide nanomaterials, and propose design principles of tailoring a layered configuration to control the nanoarchitectures for high efficient photocatalysis. Subsequently, we focus on their layered structure dependent properties, including pH-related crystal facet exposure and phase transformation, facet-dependent photoactivity and molecular oxygen activation pathways, so as to clarify the origin of the layered structure dependent photoreactivity. Furthermore, we summarize various strategies for modulating the composition and arrangement of layered structures to enhance the photoactivity of nanostructured bismuth oxyhalides via internal electric field tuning, dehalogenation effect, surface functionalization, doping, plasmon modification, and heterojunction construction, which may offer efficient guidance for the design and construction of high-performance bismuth oxyhalide-based photocatalysis systems. Finally, we highlight some crucial issues in engineering the layered-structure mediated properties of bismuth oxyhalide photocatalysts and provide tentative suggestions for future research on increasing their photocatalytic performance.

Biotemplated synthesis of anatase titanium dioxide nanoparticles via lignocellulosic waste material

Anatase titanium dioxide nanoparticles (TiO₂-NPs) were synthesized by sol-gel method using rice straw as a soft biotemplate. Rice straw, as a lignocellulosic waste material, is a biomass feedstock which is globally produced in high rate and could be utilized in an innovative approach to manufacture a value-added product. Rice straw as a reliable biotemplate has been used in the sol-gel method to synthesize ultrasmall sizes of TiO₂-NPs with high potential application in photocatalysis. The physicochemical properties of titanium dioxide nanoparticles were investigated by a number of techniques such as X-ray diffraction analysis (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, thermogravimetric analysis (TGA), ultraviolet visible spectra (UV-Vis), and surface area and pore size analysis. All results consensually confirmed that particle sizes of synthesized titanium dioxide were template-dependent, representing decrease in the nanoparticles sizes with increase of biotemplate concentration. Titanium dioxide nanoparticles as small as 13.0 ± 3.3 nm were obtained under our experimental conditions. Additionally, surface area and porosity of synthesized TiO₂-NPs have been enhanced by increasing rice straw amount which results in surface modification of nanoparticles and potential application in photocatalysis.

Observation of defect state in highly ordered titanium dioxide nanotube arrays

For the first time, a conductive-substrate induced electro-deposition approach is employed to achieve highly ordered TiO2 nanotube arrays based on an anodic aluminum oxide template. Different from other methods, the morphology and parameters of arrays can be adjusted easily through changing the exposure area of a conductive layer. All these arrays are used as matrixes to explore the defect state emission by photoluminescence (PL) spectra. Interestingly, we find that the emission from blue edge to red edge (∼450 nm-600 nm) are apparently quenched in the ordered nanotube arrays, especially when compared to the PL spectra of nanowire arrays, single nanotube and nanoparticles. This distinct result originates from passivation of oxygen vacancies residing along the tube walls when the tubes are interconnected, which is further evidenced by the observation of PL spectra with crystalline phase and sintering. The passivation of defects suggests valuable charge transport perpendicular to the long axis of the tubes in the ordered arrays. This point is particularly significant to the design of highly efficient devices and the applications in various energy-related fields.

A nanotree-like CdS/ZnO nanocomposite with spatially branched hierarchical structure for photocatalytic fine-chemical synthesis

Branched hierarchical CdS/ZnO nanocomposites have been synthesized for application toward photocatalytic fine-chemical synthesis. Growing ZnO nanorods on the surface of CdS nanowires boosts the light harvesting efficiency and charge separation as well as fast charge transport and collection. A Z-scheme mechanism under artificial solar light is also proposed.

Selective isolation of the electron or hole in photocatalysis: ZnO-TiO2 and TiO2-ZnO core-shell structured heterojunction nanofibers via electrospinning and atomic layer deposition

Heterojunctions are a well-studied material combination in photocatalysis studies, the majority of which aim to improve the efficacy of the catalysts. Developing novel catalysts begs the question of which photo-generated charge carrier is more efficient in the process of catalysis and the associated mechanism. To address this issue we have fabricated core-shell heterojunction (CSHJ) nanofibers from ZnO and TiO2 in two combinations where only the 'shell' part of the heterojunction is exposed to the environment to participate in the photocatalysis. Core and shell structures were fabricated via electrospinning and atomic layer deposition, respectively which were then subjected to calcination. These CSHJs were characterized and studied for photocatalytic activity (PCA). These two combinations expose electrons or holes selectively to the environment. Under suitable illumination of the ZnO-TiO2 CSHJ, e/h pairs are created mainly in TiO2 and the electrons take part in catalysis (i.e. reduce the organic dye) at the conduction band or oxygen vacancy sites of the 'shell', while holes migrate to the core of the structure. Conversely, holes take part in catalysis and electrons diffuse to the core in the case of a TiO2-ZnO CSHJ. The results further revealed that the TiO2-ZnO CSHJ shows ∼1.6 times faster PCA when compared to the ZnO-TiO2 CSHJ because of efficient hole capture by oxygen vacancies, and the lower mobility of holes.

Nanoporous TiO2 nanoparticle assemblies with mesoscale morphologies: nano-cabbage versus sea-anemone

We report the novel synthesis of nanoporous TiO2 nanoparticle ensembles with unique mesoscale morphologies. Constituent nanoparticles evolved into multifaceted assemblies, exhibiting excellent crystallinity and enhanced photocatalytic activity compared with commercial TiO2. Such materials could be exploited for applications, like organic pollutant degradation.