Dongjiang Yang

An Efficient Photocatalyst Structure: TiO2(B) Nanofibers with a Shell of Anatase Nanocrystals

A new efficient photocatalyst structure, a shell of anatase nanocrystals on the fibril core of a single TiO(2)(B) crystal, was obtained via two consecutive partial phase transition processes. In the first stage of the process, titanate nanofibers reacted with dilute acid solution under moderate hydrothermal conditions, yielding the anatase nanocrystals on the fiber. In the subsequent heating process, the fibril core of titanate was converted into a TiO(2)(B) single crystal while the anatase crystals in the shell remained unchanged. The anatase nanocrystals do not attach to the TiO(2)(B) core randomly but coherently with a close crystallographic registry to the core to form a stable phase interface. For instance, (001) planes in anatase and (100) planes of TiO(2)(B) join together to form a stable interface. Such a unique structure has several features that enhance the photocatalytic activity of these fibers. First, the differences in the band edges of the two phases promote migration of the photogenerated holes from anatase shell to the TiO(2)(B) core. Second, the well-matched phase interfaces allow photogenerated electrons and holes to readily migrate across the interfaces because the holes migrate much faster than excited electrons, more holes than electrons migrate to TiO(2)(B) and this reduces the recombination of the photogenerated charges in anatase shell. Third, the surface of the anatase shell has both a strong ability to regenerate surface hydroxyl groups and adsorb O(2), the oxidant of the reaction, to yield reactive hydroxyl radicals (OH(.)) through reaction between photogenerated holes and surface hydroxyl groups. The adsorbed O(2) molecules can capture the excited electrons on the surface, forming reactive O(2)(-) species. The more reactive species generated on the external surface, the higher the photocatalytic activity will be, and generation of the reactive species also contributes to reducing recombination of the photogenerated charges. Indeed, the mixed-phase nanofibers exhibited superior photocatalytic activity for degradation of sulforhodamine B under UV light to the nanofibers of either pure phase alone or mechanical mixtures of the pure phase nanofibers with a similar phase composition. Finally, the nanofibril morphology has an additional advantage that they can be separated readily after reaction for reuse by sedimentation. This is very important because the high cost for separating the catalyst nanocrystals has seriously impeded the applications of TiO(2) photocatalysts on an industrial scale.

Titanate Nanofibers as Intelligent Absorbents for the Removal of Radioactive Ions from Water

Layered titanate nanofibers can absorb bivalent ions from waste water via an ion exchange process. The sorption induces a considerable deformation of the layered structure, thus trapping the cations in the fibers permanently. Therefore, the fibers are desirable sorbents for the removal of toxic, radioactive Ra(2+) and Sr(2+) ions from water and subsequent safe disposal thereof.

Capture of Radioactive Cesium and Iodide Ions from Water by Using Titanate Nanofibers and Nanotubes

Radioactive Cs and I ions are the products of uranium fission, and can be easily dissolved in water during an accident at a nuclear reactor, such as those that occurred at Chernobyl in 1986, at Three Mile Island in Pennsylvania in 1979, and in 2011 at Fukushima, Japan. In 2009, leaks of radioactive materials such as Cs and I isotopes also occurred during minor accidents at nuclear power stations in Britain, Germany, and the U.S. These leaks have raised concerns about exposure levels in the nearby communities because it is feared that these fission products could make their way into the food chain when present in waste water. Radioactive iodine is also used in the treatment of thyroid cancer, and, as a result, radioactive wastewater is discharged by a large number of medical research institutions. The wide use of radioisotopes requires effective methods to manage radioactive waste, and methods currently used are complex and extremely costly. Herein we demonstrate a potentially cost-effective method to remediate Cs and I ions from contaminated water by using the unique chemistry of titanate nanotubes and nanofibers, which can not only chemisorb these ions but efficiently trap them for safe disposal. Inorganic cation exchangers, such as crystalline silicotitanates, zeolites, clay minerals, layered Zr phosphates, and layered sulfide frameworks, have been studied for separation of Cs ions from nuclear wastewater and safe disposal of the exchanged cations because of the ability of these exchangers to withstand intense radiation and elevated temperatures, in addition to their high ion-exchange capacity. Because ion exchange in materials is usually a reversible process, except in micas, the radioactive ions in the exchanger may be released to water. Titanates are refractory mineral substances that are very stable with respect to radiation and chemical, thermal, and mechanical changes. Titanate nanofibers and nanotubes (with chemical formula Na2Ti3O7) can be easily synthesized at low cost under hydrothermal conditions. These materials possess a layered structure in which TiO6 octahedra are the basic structural units (Figure S1 in the Supporting Information). These layers carry negative charges and are approximately two oxygen atoms thick. Na ions are situated between the layers and can be exchanged with other cations. In the present study, we show how trititanate nanofibers (T3NF) and nanotubes (T3NT) can be used to efficiently remove radioactive Cs ions from aqueous solution by cation exchange. Figure 1a shows that the nanotubular T3NT can remove 80% of Cs ions from solutions with Cs concentrations up to 250 ppm. The ions can be completely removed when the Cs ion concentration is below 80 ppm. In contrast, the fibril T3NF has a comparatively lower absorption capacity than

Egg-Box Structure in Cobalt Alginate: A New Approach to Multifunctional Hierarchical Mesoporous N-Doped Carbon Nanofibers for Efficient Catalysis and Energy Storage

Carbon nanomaterials with both doped heteroatom and porous structure represent a new class of carbon nanostructures for boosting electrochemical application, particularly sustainable electrochemical energy conversion and storage applications. We herein demonstrate a unique large-scale sustainable biomass conversion strategy for the synthesis of earth-abundant multifunctional carbon nanomaterials with well-defined doped heteroatom level and multimodal pores through pyrolyzing electrospinning renewable natural alginate. The key part for our chemical synthesis is that we found that the egg-box structure in cobalt alginate nanofiber can offer new opportunity to create large mesopores (∼10–40 nm) on the surface of nitrogen-doped carbon nanofibers. The as-prepared hierarchical carbon nanofibers with three-dimensional pathway for electron and ion transport are conceptually new as high-performance multifunctional electrochemical materials for boosting the performance of oxygen reduction reaction (ORR), lithium ion batteries (LIBs), and supercapacitors (SCs). In particular, they show amazingly the same ORR activity as commercial Pt/C catalyst and much better long-term stability and methanol tolerance for ORR than Pt/C via a four-electron pathway in alkaline electrolyte. They also exhibit a large reversible capacity of 625 mAh g–1 at 1 A g–1, good rate capability, and excellent cycling performance for LIBs, making them among the best in all the reported carbon nanomaterials. They also represent highly efficient carbon nanomaterials for SCs with excellent capacitive behavior of 197 F g–1 at 1 A g–1 and superior stability. The present work highlights the importance of biomass-derived multifunctional mesoporous carbon nanomaterials in enhancing electrochemical catalysis and energy storage.

Alumina nanofibers grafted with functional groups: A new design in efficient sorbents for removal of toxic contaminants from water

A new design in efficient sorbents for the removal of trace pollutants from water was proposed: grafting the external surface of gamma-alumina (gamma-Al(2)O(3)) nanofibers with functional groups that have a strong affinity to the contaminants. This new grafting strategy greatly improves the accessibility of these sorption sites to adsorbates and thus efficiency of the fibrous sorbents. The product sorbents could capture the pollutants selectively even when the concentration of the contaminants is extremely low. Two types of gamma-Al(2)O(3) nanofibers with different size were prepared via facile hydrothermal methods. Thiol groups were then grafted on the gamma-Al(2)O(3) fibers by refluxing the toluene solution of 3-mercaptopropyltrimethoxysilane (MPTMS). The thiol group modified fibers not only can efficiently remove heavy metal ions (Pb(2+) and Cd(2+)) from water at a high flux, but also display high sorption capacity under sorption equilibrium conditions. Similar result was obtained from the nanofibers grafted with octyl groups which are employed to selectively adsorb highly diluted hydrophobic 4-nonylphenol molecules from water. This study demonstrates that grafting nanofibers is a new and effective strategy for developing efficient sorbents.

Enhancing Photoactivity of TiO2(B)/Anatase Core–Shell Nanofibers by Selectively Doping Cerium Ions into the TiO2(B) Core

Cerium ions (Ce(3+)) can be selectively doped into the TiO2(B) core of TiO2(B)/anatase core-shell nanofibers by means of a simple one-pot hydrothermal treatment of a starting material of hydrogen trititanate (H2Ti3O7) nanofibers. These Ce(3+) ions (≈0.202 nm) are located on the (110) lattice planes of the TiO2(B) core in tunnels (width≈0.297 nm). The introduction of Ce(3+) ions reduces the defects of the TiO2(B) core by inhibiting the faster growth of (110) lattice planes. More importantly, the redox potential of the Ce(3+)/Ce(4+) couple (E°(Ce(3+)/Ce(4+))=1.715 V versus the normal hydrogen electrode) is more negative than the valence band of TiO2(B). Therefore, once the Ce(3+)-doped nanofibers are irradiated by UV light, the doped Ce(3+) ions--in close vicinity to the interface between the TiO2(B) core and anatase nanoshell--can efficiently trap the photogenerated holes. This facilitates the migration of holes from the anatase shell and leaves more photogenerated electrons in the anatase nanoshell, which results in a highly efficient separation of photogenerated charges in the anatase nanoshell. Hence, this enhanced charge-separation mechanism accelerates dye degradation and alcohol oxidation processes. The one-pot treatment doping strategy is also used to selectively dope other metal ions with variable oxidation states such as Co(2+/3+) and Cu(+/2+) ions. The doping substantially improves the photocatalytic activity of the mixed-phase nanofibers. In contrast, the doping of ions with an invariable oxidation state, such as Zn(2+), Ca(2+), or Mg(2+), does not enhance the photoactivity of the mixed-phase nanofibers as the ions could not trap the photogenerated holes.

Selective Capture of Iodide from Solutions by Microrosette-like δ-Bi2O3

Radioactive iodine isotopes that are produced in nuclear power plants and used in medical research institutes could be a serious threat to the health of many people if accidentally released to the environment because the thyroid gland can absorb and concentrate them from a liquid. For this reason, uptake of iodide anions was investigated on microrosette-like δ-Bi2O3 (MR-δ-Bi2O3). The MR-δ-Bi2O3 adsorbent showed a very high uptake capacity of 1.44 mmol g(-1) by forming insoluble Bi4I2O5 phase. The MR-δ-Bi2O3 also displayed fast uptake kinetics and could be easily separated from a liquid after use because of its novel morphology. In addition, the adsorbent showed excellent selectivity for I(-) anions in the presence of large concentrations of competitive anions such as Cl(-) and CO3(2-), and could work in a wide pH range of 4-11. This study led to a new and highly efficient Bi-based adsorbent for iodide capture from solutions.

Coherent interfaces between crystals in nanocrystal composites

Numerous materials are polycrystalline or consist with crystals of different phases. However, materials consisting of crystals on the nanometer scale (nanocrystals) are not simply aggregates of randomly oriented crystals as is generally regarded. We found, that in four different materials that consist of nanocrystals of two different phases and were obtained by different approaches, the nanocrystals of different phases are combined coherently forming interfaces with a close crystallographic registry between adjacent crystals (coherent interfaces). The four materials were fabricated by (i) depositing Ag(2)O nanoparticles on titanate nanofibers, (ii) phase transition from TiO(2)(B) nanofibers to the nanofibers of mixed TiO(2)(B) and anatase phases, (iii) dehydration of the single crystal fibril titanate core coated with anatase nanocrystals, and (iv) attaching zeolite Y nanocrystals on the surface of titanate nanofibers. The finding suggests that preferred orientations and coherent interfaces generally exist in nanocrystal systems, and according to our results, they are largely unaffected by the fabrication process that was used. This is because the preferred orientations require that the engaged crystal planes from two connected crystals have the same basal spacing and that the crystals can interlock tightly at the atomic level to form thermodynamically stable interfaces. Hence it is rational that the preferred orientations and coherent interfaces dominant the nanostructures formed between the different nanocrystals and play a key role in assembling the composite nanostructures. The orientation and interfaces between crystals of different phases in mixed-phase materials are extremely difficult to determine. Nonetheless, the thermodynamic stability of the coherent interfaces allows us to apply phase-transformation invariant line strain theory to predict the preferred orientation (and thus the structure of the coherent interfaces). The theoretical predications agree remarkably with the transmission electron microscopy (TEM) analysis. This implies that we may acquire knowledge of the orientation and the interface structures in the mixed-phase materials without TEM measurement, and the knowledge is essential for comprehensively understanding properties of the many materials and processes that depend on the interfaces.

Grafting silica species on anatase surface for visible light photocatalytic activity

The grafting of optimal trace silica on TiO2 (anatase) substrates possessing a large specific surface area can significantly enhance their photocatalytic activity for decomposing organic contaminants, such as chlorophenols and phenol in water under visible-light illumination. The silica grafting produces surface electronic states within the TiO2 band gap, which are responsible for the visible-light activity. Different from the bulk doping, SiO2 was grafted on the anatase surface in two possible modes: (i) formation of SiO2 clusters on the surface; and (ii) substitution of Si atoms for Ti atoms of the outmost layer at the surface. Thus, the surface electronic states originate mainly from the 2p orbitals of coordination-unsaturated oxygen atoms bound to Si atoms. The location of these states depends on the forms by which the introduced silica species exist and the type of the grafted facet. Nonetheless, the attempt of grafting the same substrate with alumina failed. Also a greater enhancement in the visible-light activity was achieved when TiO2 surfaces with higher surface energy, such as the surfaces of anatase (010) and (116) planes, were grafted. Advantages of the grafted approach include: (i) different from bulk-doping, the surface-grafting does not change the crystalline and electronic structure in the bulk of the photocatalysts; (ii) and consequently the electron mobility in the photocatalysts and their photocatalytic activity under UV irradiation are not affected, (iii) the grafted photocatalysts have high chemical stability and repeatable photocatalytic activity.

Preparation of Nitrogen-Doped TiO2/Graphene Nanohybrids and Application as Counter Electrode for Dye-Sensitized Solar Cells

The preparation of nitrogen-doped TiO2/graphene nanohybrids and their application as counter electrode for dye-sensitized solar cell (DSSC) are presented. These nanohybrids are prepared by self-assembly of pyrene modified H2Ti3O7 nanosheets and graphene in aqueous medium via π-π stacking interactions, followed by thermal calcination at different temperatures in ammonia atmosphere to afford nitrogen-doped TiO2/graphene nanohybrids. H2Ti3O7 nanosheets were synthesized from TiOSO4·xH2O by a hydrothermal reaction at 150 °C for 48 h. The microstructure of the obtained mixed-phase nanohybrids was characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Fourier transforms infrared spectroscopy (FTIR). Moreover, the performances of the as-prepared nanohybrids as counter electrode materials for DSSC was investigated, and the results indicated that the nanohybrids prepared at higher nitridation temperature would lead to higher short-circuit current density than those prepared at lower nitridation temperature, indicating that it can be utilized as a low-cost alternative to Pt for DSSCs and other applications.