Synthesis of Titania Nanosheets with a High Percentage of Exposed (001) Facets and Related Photocatalytic Properties

Visible Light Photocatalytic Activity of C, N, S-Tridoped Anatase TiO2 Nanosheets

The C, N, S-tridoped TiO2nanosheets are synthesized via hydrothermal synthesis and post-treatment, and characterized by X-ray diffraction (XRD), Transmission electron microscopy (TEM), X-ray photoelectron spectra (XPS), and UV–vis diffuse reflectance spectroscopy. The UV-diffuse reflectance spectra of all the C, N, S-tridoped TiO2nanosheets greatly extends the absorption edge into the visible light region, which can be attributed to C, N, S-tridoping in the form of cation C-doping, interstitial N-doping, anionic S-doping, and adsorbed SO42-ions states. The C, N, S-tridoped TiO2nanosheets exhibits high visible light photocatalytic activity, and the xanthate concentration reduce as much as 100% in 120 min. All these indicate that C, N, S-tridoped TiO2nanosheets photocatalysts are promising visible light photocatalyst for the practical application.

Catalysis based on nanocrystals with well-defined facets

Using bottom-up chemistry techniques, the composition, size, and shape in particular can now be controlled uniformly for each and every nanocrystal (NC). Research into shape-controlled NCs have shown that the catalytic properties of a material are sensitive not only to the size but also to the shape of the NCs as a consequence of well-defined facets. These findings are of great importance for modern heterogeneous catalysis research. First, a rational synthesis of catalysts might be achieved, since desired activity and selectivity would be acquired by simply tuning the shape, that is, the exposed crystal facets, of a NC catalyst. Second, shape-controlled NCs are relatively simple systems, in contrast to traditional complex solids, suggesting that they may serve as novel model catalysts to bridge the gap between model surfaces and real catalysts.

Photogenerated defects in shape-controlled TiO2 anatase nanocrystals: a probe to evaluate the role of crystal facets in photocatalytic processes

The promising properties of anatase TiO(2) nanocrystals exposing specific surfaces have been investigated in depth both theoretically and experimentally. However, a clear assessment of the role of the crystal faces in photocatalytic processes is still under debate. In order to clarify this issue, we have comprehensively explored the properties of the photogenerated defects and in particular their dependence on the exposed crystal faces in shape-controlled anatase. Nanocrystals were synthesized by solvothermal reaction of titanium butoxide in the presence of oleic acid and oleylamine as morphology-directing agents, and their photocatalytic performances were evaluated in the phenol mineralization in aqueous media, using O(2) as the oxidizing agent. The charge-trapping centers, Ti(3+), O(-), and O(2)(-), formed by UV irradiation of the catalyst were detected by electron spin resonance, and their abundance and reactivity were related to the exposed crystal faces and to the photoefficiency of the nanocrystals. In vacuum conditions, the concentration of trapped holes (O(-) centers) increases with increasing {001} surface area and photoactivity, while the amount of Ti(3+) centers increases with the specific surface area of {101} facets, and the highest value occurs for the sample with the worst photooxidative efficacy. These results suggest that {001} surfaces can be considered essentially as oxidation sites with a key role in the photoxidation, while {101} surfaces provide reductive sites which do not directly assist the oxidative processes. Photoexcitation experiments in O(2) atmosphere led to the formation of Ti(4+)-O(2)(-) oxidant species mainly located on {101} faces, confirming the indirect contribution of these surfaces to the photooxidative processes. Although this work focuses on the properties of TiO(2), we expect that the presented quantitative investigation may provide a new methodological tool for a more effective evaluation of the role of metal oxide crystal faces in photocatalytic processes.

Tuning the relative concentration ratio of bulk defects to surface defects in TiO2 nanocrystals leads to high photocatalytic efficiency

TiO(2) nanocrystals with tunable bulk/surface defects were synthesized and characterized with TEM, XRD, BET, positron annihilation, and photocurrent measurements. The effect of defects on photocatalytic activity was studied. It was found for the first time that decreasing the relative concentration ratio of bulk defects to surface defects in TiO(2) nanocrystals could significantly improve the separation efficiency of photogenerated electrons and holes, thus significantly enhancing the photocatalytic efficiency.

SnO₂ nanosheet hollow spheres with improved lithium storage capabilities

In this work, we employ new chemistry to grow tin oxide nanosheets in the gel matrix of sulfonated polystyrene hollow spheres. After calcination in air, hierarchical hollow spheres assembled from SnO(2) nanosheets can be obtained. In virtue of the porous shell structure and internal voids, these SnO(2) hierarchical nanosheet hollow spheres exhibit improved lithium storage capability.

Enhanced photocatalytic H₂-production activity of graphene-modified titania nanosheets

Graphene-modified TiO(2) nanosheets with exposed (001) facets (graphene/TiO(2)) were prepared by microwave-hydrothermal treatment of graphene oxide (GO) and hydrothermally synthesized TiO(2) nanosheets with exposed (001) facets in an ethanol-water solvent. These nanocomposite samples showed high photocatalytic H(2)-production activity in aqueous solutions containing methanol, as sacrificial reagent, even without Pt co-catalyst. The optimal graphene content was found to be ∼1.0 wt%, giving a H(2)-production rate of 736 μmol h(-1) g(-1) with a quantum efficiency (QE) of 3.1%, which exceeded the rate observed on pure TiO(2) nanosheets by more than 41 times. This high photocatalytic H(2)-production activity is due to the deposition of TiO(2) nanosheets on graphene sheets, which act as an electron acceptor to efficiently separate the photogenerated charge carriers. The observed enhancement in the photocatalytic activity is due to the lower absolute potential of graphene/graphene˙(-) (-0.08 V vs. SHE, pH = 0) in comparison to the conduction band (-0.24 V) of anatase TiO(2), meanwhile the aforementioned absolute value is higher than the reduction potential of H(+) (0 V), which favors the electron transfer from the conduction band (CB) of TiO(2) to graphene sheets and the reduction of H(+), thus enhancing photocatalytic H(2)-production activity. The proposed mechanism for the observed photocatalytic performance of TiO(2) nanosheets, modified with a small amount of graphene, was further confirmed by photoluminescence spectroscopy and transient photocurrent response. This work not only shows a possibility for the utilization of low cost graphene sheets as a substitute for noble metals (such as Pt) in the photocatalytic H(2)-production but also for the first time shows a significant enhancement in the H(2)-production activity by using metal-free carbon material as an effective co-catalyst.

One-pot synthesis of N-F-Cr-doped anatase TiO₂ microspheres with nearly all-(001) surface for enhanced solar absorption

The synthesis and clarifications in structure-property relationship for anatase TiO(2) crystals exposing (001) facets have attracted much attention. In this paper, a novel titania microsphere with nearly all-(001) surface was synthesized by hydrothermal treatment of a thermal sprayed TiN/Ti coating with HF aqueous solution containing chromium powders. Unlike the conventional (001)-facet exposed anatase crystals, which are highly truncated bipyramids, the crystal achieved in the current investigation is drum-like with a round cross section. The formation of the drum-like crystals was contributed to a balance between the erosion and precipitation of anatase single crystallites. The Cr-doping in anatase was believed to increase the surface deficiency which enhanced the erosion procedure, leading to the drum-like crystals. The XPS analysis confirmed the incorporations of N, F and Cr in the microsized anatase crystals through the one-pot reaction, which led to a significantly enhanced solar absorption. The UV-Vis diffuse reflectance revealed a band-to-band red-shift of the band gap of the anatase crystals to 1.60 eV, which is contributed mainly to the homogeneous Cr-doping.

Formation of large 2D nanosheets via PVP-assisted assembly of anatase TiO2 nanomosaics

We demonstrate an unusual formation of large 2D nanosheets from nanomosaic building blocks of anatase TiO(2) nanosheets with exposed (001) facets. It is proposed that large PVP molecules adsorbed on the (001) facets serve as the linker that brings building blocks together, at the same time prevents them from stacking along the c-axis.

Synergistic effect of nickel ions on the coupled dechlorination of trichloroethylene and 2,4-dichlorophenol by Fe/TiO₂ nanocomposites in the presence of UV light under anoxic conditions

The coupled removal of priority pollutants by nanocomposite materials has recently been receiving much attention. In this study, trichloroethylene (TCE) and 2,4-dichlorophenol (DCP) in aqueous solutions were simultaneously removed by Fe/TiO₂ nanocomposites under anoxic conditions in the presence of nickel ions and UV light at 365 nm. Both TCE and DCP were effectively dechlorinated by Fe/TiO₂ nanocomposites, and the pseudo-first-order rate constants (k(obs)) for TCE and DCP dechlorination were (1.39 ± 0.05)×10⁻² and (1.08 ± 0.05)×10⁻² h⁻¹, respectively, which were higher than that by nanoscale zerovalent iron alone. In addition, the k(obs) for DCP dechlorination was enhanced by a factor of 77 when Fe/TiO₂ was illuminated with UV light for 2 h. Hydrodechlorination was found to be the major reaction pathway for TCE dechlorination, while DCP could undergo reductive dechlorination or react with hydroxyl radicals to produce 1,4-benzoquinone and phenol. TCE was a stronger electron acceptor than DCP, which could inhibit the dechlorination efficiency and rate of DCP during simultaneous removal processes. The addition of nickel ions significantly enhanced the simultaneous photodechlorination efficiency of TCE and DCP under the illumination of UV light. The k(obs) values for DCP and TCE photodechlorination by Fe/TiO₂ in the presence of 20-100 μM Ni(II) were 30.4-136 and 13.2-192 times greater, respectively, when compared with those in the dark. Electron spin resonance analysis showed that the photo-generated electron-hole pairs could be effectively separated through Ni ions cycling, leading to the improvement of electron transfer efficiency of TCE and DCP by Fe/TiO₂.

Mesoporous TiO2 core-shell spheres composed of nanocrystals with exposed high-energy facets: facile synthesis and formation mechanism

A facile new method that combines electrospray and hydrothermal treatment is used to prepare mesoporous core-shell TiO(2) spheres with high specific surface areas and high pore volumes. Interestingly, the resulting TiO(2) spheres are composed of anatase TiO(2) nanocrystals with exposed step-like {001} and smooth {010} facets. The percentage of exposed {001} facets can be adjusted by changing the experimental parameters used in the electrospray and hydrothermal treatment processes, such as the contents of poly(N-vinyl-2-pyrrolidone) and acetic acid. The combination of high specific surface area (>100 m(2) g(-1)), high pore volume (>0.30 cm(3) g(-1)), useful pore size (10-15 nm), spherical core-shell structure, and exposed high energy facets makes these TiO(2) spheres an important candidate for use in many photoelectrochemical applications. The formation mechanism of the mesoporous TiO(2) spheres is also studied. The great advantage of this method is that interesting and complicated mesoporous superstructures can be prepared using electrospray technology.

Anatase TiO₂ crystal facet growth: mechanistic role of hydrofluoric acid and photoelectrocatalytic activity

This work reports a facile hydrothermal approach to directly grow anatase TiO(2) crystals with exposed {001} facets on titanium foil substrate by controlling pH of HF solution. The mechanistic role of HF for control growth of the crystal facet of anatase TiO(2) crystals has been investigated. The results demonstrate that controlling solution pH controls the extent of surface fluorination of anatase TiO(2), hence the size, shape, morphology, and {001} faceted surface area of TiO(2) crystals. The theoretical calculations reveal that {001} faceted surface fluorination of anatase TiO(2) can merely occur via dissociative adsorption of HF molecules under acidic conditions while the adsorption of Na(+)F(-) is thermodynamically prohibited. This confirms that the presence of molecular form of HF but not F(-) is essential for preservation of exposed {001} facets of anatase TiO(2). Anatase TiO(2) crystals with exposed {001} facets can be directly fabricated on titanium foil by controlling the solution pH ≤ 5.8. When pH is increased to near neutral and beyond (e.g., pH ≥ 6.6), the insufficient concentration of HF ([HF] ≤ 0.04%) dramatically reduces the extent of surface fluorination, leading to the formation of anatase TiO(2) crystals with {101} facets and titanate nanorods/nanosheets. The anatase TiO(2) nanocrystals with exposed {001} facets exhibits a superior photoelectrocatalytic activity toward water oxidation. The findings of this work clarify the mechanistic role of HF for controlling the crystal facet growth, providing a facile means for massive production of desired nanostructures with high reactive facets on solid substrates for other metal oxides.

Hydrothermal growth of nanometer- to micrometer-size anatase single crystals with exposed (001) facets and their ability to assist photodegradation of rhodamine B in water

Anatase single crystals with exposed (001) facets have been the focus of many researches in recent years. This paper reports the hydrothermal synthesis of (001)-exposed anatase single crystals through reactions of Ti plates in aqueous HF solutions with mass concentrations of 0.15-0.80%, in an autoclave at 180°C for 2-12h. The size of the achieved anatase single crystals varied from 0.4 to 13.6 μm, exposing 15-49% (001) facets. The crystal size and the (001) fraction increased with increasing HF concentrations. For a prolonged reaction time, anatase crystals with larger sizes and reduced fractions of (001) facets were achieved. The activity of the anatase crystals to assist photodegradation of rhodamine B in water increased with decreasing sizes and increasing fractions of (001) facets. Selective erosion of the anatase single crystals along the high-energy (001) facets was noted, for the first time, which resulted in cone-shaped walls with a thickness ranging from several to hundreds of nanometers. The selective erosion contributed to the photocatalytic activity of the (001)-exposed anatase single crystals.