CdWO4 nanocrystals with controlled particle size and crystallinity were successfully synthesized via a simple hydrothermal method using citric acid as the capping agent. By systematic sample characterization using X-ray powder diffraction, transmission electron microscope, selected area electron diffraction, Barret−Emmett−Teller technique, Fourier transformed infrared spectra, UV−visible diffuse reflectance spectra, and photoluminescence spectra, all as-prepared CdWO4 samples were demonstrated to crystallize in a pure-phase of monoclinic wolframite structure. With varying the reaction temperature from 160 to 220 °C, particle size was controlled to grow from 11 to 21 nm. With particle size reduction, CdWO4 nanostructure showed a lattice expansion, as is followed by a surprisingly lowered lattice symmetry, band gap broadening, and red-shift of Au vibration mode. Photocatalytic activity of CdWO4 nanocrystals was examined by monitoring the degradation of methyl orange dye in an aqueous solution under UV radiation of 254 nm. High crystallinity CdWO4 nanostructures with relatively small particle size showed an optimum photocatalytic performance. Consequently, systematic control over semiconductor nanostructures is proved to be useful, in some cases likely general, in achieving the advanced photocatalytic properties for technological uses.

Ultrathin cation-exchanged layered metal oxides are promising for many applications, while such substances are barely successfully synthesized to show several atomic layer thickness, owing to the strong electrostatic force between the adjacent layers. Herein, we took LiCoO2, a prototype cation-exchanged layered metal oxide, as an example to study. By developing a simple synthetic route, we synthesized LiCoO2 nanosheets with 5–6 cobalt oxide layers, which are the thinnest ever reported. Ultrathin nanosheets thus prepared showed a surprising coexistence of increased oxidation state of cobalt ions and oxygen vacancy, as demonstrated by magnetic susceptibility, X-ray photoelectron, electron paramagnetic resonance, and X-ray absorption fine spectra. This unique feature enables a higher electronic conduction and electrophilicity to the adsorbed oxygen than the bulk. Consequently ultrathin LiCoO2 nanosheets provided a current density of 10 mA cm–2 at a small overpotential of a mere 0.41 V and a small Tafel slope of ∼88 mV/decade, which is strikingly followed by an excellent cycle life. The findings reported in this work suggest that ultrathin cation-exchanged layered metal oxides could be a next generation of advanced catalysts for oxygen evolution reaction.Keywords: electrocatalysis; LiCoO2; mixed state; nanosheets; oxygen evolution reaction;

Series of perovskite PrCo1−xNixO3−δ (x = 0–0.4) were prepared and carefully investigated to understand the spin state transition driven by hole doping and further to reveal the effect of spin state transition on electronic conduction. It is shown that with increasing doping level, the transition temperature Ts for Co3+ ions from low-spin (LS) to intermediate-spin (IS) reduces from 211.9 K for x = 0 to 190.5 K for x = 0.4. XPS and FT-IR spectra demonstrate that hole doping promoted this transition due to a larger Jahn–Teller distortion. Moreover, a thermal activation of spin disorder caused by thermal population of the spin states for Co ions has a great impact on the electrical transport of these perovskite samples. This work may shed light on the comprehension of spin transition in cobalt oxides through hole doping, which is promising for finding new strategies of enhancing electronic conduction, especially for energy and catalysis applications.

Electron competitive migration between the conduction band and charge trap centre is the key in governing the catalytic activity and the relevant applications of semiconductor nanomaterials, which is however poorly understood yet. Herein, we systematically studied the electron competitive migration in defective SnO2 nanoparticles through hybridizing with a polymer electron donor, graphitic carbon nitride (g-C3N4). When varying the mass ratio of defective SnO2 from 5% to 70%, an increase of surface-charge trapping centres (oxygen vacancies) in SnO2 effectively regulated the electron competitive migration. As a consequence, dual catalytic activity maxima were observed in hydrogen generation from water splitting under visible light irradiation (λ > 420 nm). For instance, the relative mass ratios at 10% and 40% yielded maximum hydrogen generation rates of 54.3 μmol h−1 g−1 and 44.3 μmol h−1 g−1, respectively, far beyond that of 27.9 μmol h−1 g−1 for pure g-C3N4. Strikingly, the photon–hydrogen conversion efficiency also showed dual maxima values as SnO2 mass ratio changes. These abnormal observations were comparatively investigated via XPS, EPR and photoluminescence spectra in solid state and aqueous environments. It is demonstrated that electron competitive migration was primarily caused by oxygen vacancies on the SnO2 surface, which plays a key role in creating the dual catalytic activity maxima in water splitting.

•Heat capacity data and thermodynamic properties for brookite TiO2 are presented.•Thermodynamic data for brookite are different from those for rutile and anatase TiO2.•Thermal stability for brookite, anatase, and rutile TiO2 are discussed.This work represents the first report on the heat capacity of brookite TiO2, which was measured using a Quantum Design physical property measurement system (PPMS) in the temperature range of 2 K to 300 K. The experimental data were fitted using a series of theoretical functions for low temperatures (T < 15 K), orthogonal polynomials for mid temperatures (15 < T < 75 K), and a combination of Debye and Einstein functions for high temperatures (T > 75 K), from which the thermodynamic functions of brookite TiO2 were calculated. The standard molar entropy and molar enthalpy for brookite TiO2 at T = 298.15 K were determined to be 52.61 ± 0.53 J · K−1 · mol−1 and 9.03 ± 0.09 kJ · mol−1, respectively, leading to a Gibbs free energy of −6.65 ± 0.18 kJ · mol−1. Strikingly, this Gibbs free energy was found to be slightly less than those reported for rutile TiO2 (−6.377 kJ · mol−1) and anatase TiO2 (−6.198 kJ · mol−1) (Smith et al., 2009). Further data analyses demonstrated that the thermal stability of TiO2 polymorphs follows a sequence of anatase < rutile < brookite at T = 298.15 K.

Brookite-TiO2 is a promising next-generation semiconductor material for solar energy conversion, but it suffers from difficulty in achieving high quality and phase purity due to its metastable characteristics. Long-chain fatty acid modification or surfactant assisted methods could orient the growth of brookite; however, purifying the products is complicated and the surface reactivity is invariably undermined. Herein, we demonstrate the design and tuneable synthesis of brookite nanostructures with geometric features of quasi-octahedral (QO), ellipsoid-tipped (ET) and wedge-tipped (WT) nanorods that are exposed primarily with {210} facet via water-soluble titanium precursors. When tested as a photocatalyst for hydrogen evolution from water or for the degradation of organic pollutants, QO brookite nanocrystals exhibited the highest catalytic activity compared to ET and WT nanorod counterparts. This observation could be due to the redox facets that form a “surface-heterojunction” and promote the separation of photogenerated carriers. The precursor-directed method reported here may usher in a new phase for the synthesis of novel metastable nanocrystals with specific facet exposure that are highly useful for applications in energy conversion and environment protection.

Catalysts are urgently needed to remove the residual CO in hydrogen feeds through selective oxidation for large-scale applications of hydrogen proton exchange membrane fuel cells. We herein propose a new methodology that anchors high concentration oxygen vacancies at interface by designing a CeO2–x/Cu hybrid catalyst with enhanced preferential CO oxidation activity. This hybrid catalyst, with more than 6.1% oxygen vacancies fixed at the favorable interfacial sites, displays nearly 100% CO conversion efficiency in H2-rich streams over a broad temperature window from 120 to 210 °C, strikingly 5-fold wider than that of conventional CeO2/Cu (i.e., CeO2 supported on Cu) catalyst. Moreover, the catalyst exhibits a highest cycling stability ever reported, showing no deterioration after five cycling tests, and a super long-time stability beyond 100 h in the simulated operation environment that involves CO2 and H2O. On the basis of an arsenal of characterization techniques, we clearly show that the anchored oxygen vacancies are generated as a consequence of electron donation from metal copper atoms to CeO2 acceptor and the subsequent reverse spillover of oxygen induced by electron transfer in well controlled nanoheterojunction. The anchored oxygen vacancies play a bridging role in electron capture or transfer and drive molecule oxygen into active oxygen species to interact with the CO molecules adsorbed at interfaces, thus leading to an excellent preferential CO oxidation performance. This study opens a window to design a vast number of high-performance metal-oxide hybrid catalysts via the concept of anchoring oxygen vacancies at interfaces.Keywords: anchoring oxygen vacancies; cycling-stability; electron transfer; enhanced activity; interface; preferential CO oxidation

The synthesis of anionic-modified nanostructures with specific properties is often hindered by difficulty in tuning the material compositions without sacrificing phase purity and sample uniformity. Here, we present a novel methodology using NH4F as fluorine source and a high-temperature ionic diffusion that facilitate surface fluorination of TiO2 without changes of phase structure. Using this method, we prepared fluorinated TiO2 nanostructures of different phases (brookite, anatase and rutile) and investigated their structural and photocatalytic redox properties. Photocatalytic selective oxidation of MO and selective reduction of water in producing hydrogen under UV-light irradiation are employed as probe reactions to test the redox properties of the fluorinated TiO2 of different phases. The results show that the photocatalytic redox properties of TiO2 are highly dependent on phase structure and surface fluorination. Among all the phases, the oxidation activity of fluorinated TiO2 can be ranked as brookite > anatase > rutile. Strikingly, the photocatalytic reduction activity for these fluorinated TiO2 followed the same sequence. The variations of photocatalytic redox activity are most likely due to the synergism of phase composition control and surface fluorination. This could effectively delay electron–hole recombination, and generate more free ˙OH radicals but less free electrons. The methodology reported here is simple and general, which might be used to design and control the structures and properties of anionic-modified nanostructures for a variety of applications in environmental cleaning and energy conversion.

For almost all catalysts, doping with cheaper transition metal ions could reduce the application price necessary, however usually a catalytic activity degradation occurs. This work reports on the preparation of Ce–M–O solid solutions (M = transition metal) with the aim to achieve cheaper catalysts with high activity and stability. Systematic sample characterizations indicate that the existence of more transition metal hydroxides is beneficial for the formation of Ce–M–O solid solutions and that the formation reaction involves a complicated process (e.g., nucleation, dehydration, Ostwald attachment, doping in ceria lattice, and oriented growth). When tested as the catalysts for CO oxidation, the solid solutions of M = Fe maintained excellent catalytic performance, much higher than ever reported, even when Fe doping levels reach 15%. No sign of deactivation was detected for M = Fe after 100 h reaction, which compares to the apparent decrease in catalytic activity for M = Co or Ni. By studying the doping effect on structure, reducibility, oxygen storage capacity and catalytic performance, it is demonstrated that Fe doping led to lattice crystal shrinking, lattice distortion and restrained grain growth of CeO2, yielding a synergistic effect in promoting oxygen storage capacity and catalytic activity. The results reported herein may provide some direction for exploring advanced catalysts at lower prices.

Noble-metal-supported metal oxides (e.g., Au/CeO2, Pd/Al2O3, etc.) are popular as efficient catalysts for many important catalytic reactions, while their high price and easy deactivation restrict their broad application. Herein, we initiate a double substitution methodology to acquire catalysts with merits of excellent catalytic activity, high stability, and relatively low cost. For this purpose, a Au/CeO2 catalyst was used as reference, and the noble metal Au was completely replaced by the cheaper Ag, meanwhile the rare earth ion, Ce4+ of the CeO2 support was partially substituted by the transition metal ion, Fe3+. This inversely supported catalyst, Ce0.9Fe0.1O1.97/Ag, was prepared by a two-step method based on co-precipitation and a subsequent liquid-phase reduction. Systematic characterizations demonstrate that Ag nanoparticles with a diameter of around 50 nm were wrapped by Ce0.9Fe0.1O1.97 layers. Between the Ag nanoparticles and Ce0.9Fe0.1O1.97 layer, there existed a strong interaction. When this sample was tested as a catalyst for CO oxidation, a full conversion temperature of 150 °C was achieved at a space velocity of 24000 ml h−1 g−1cat, showing a catalytic activity comparable to that previously reported in the literature on the known catalyst Au@CeO2 measured at a lower space velocity of 15000 ml h−1 g−1cat. More strikingly, this catalyst showed a superb catalytic stability and enhanced oxygen storage capacity. The introduction of smaller Fe3+ into the CeO2 lattice and the strong interactions between Ag and Ce0.9Fe0.1O1.97 strongly improved the stability and catalytic performance.

Bismuth phosphate represents one promising class of luminescent host materials, while its application is challenged by structural instability. In this work, a series of nanocrystals Bi1−xLnxPO4 (x = 0.02–0.09; Ln = La, Ce, Nd, Eu, Er) were prepared with an aim to find new routes in improving the structural stability of BiPO4 nanocrystals. Systematic sample characterization by XRD, SEM, TEM, and EDX indicates that doping a small amount of Ln3+ ions ≤9% could significantly stabilize the low temperature monoclinic phase (LTMP) by retarding the structure transition to the high temperature monoclinic phase (HTMP). For instance, after annealing at 900 °C, the relative contents of LTMP for co-doping of 5% La3+, 5% Nd3+, 5% Ce3+ with 2% Eu3+ were as high as 89%, 87%, and 96%, respectively, which are totally different from the un-doped BiPO4 at this temperature where LTMP is absent completely. This phenomenon has never been reported before for improving the stability of BiPO4, which has been explained in terms of lattice disorder due to the preference of the linking ways of rare earth–oxygen polyhedra and PO4 tetrahedra. With this abnormal structural stabilization, the luminescent properties of the Ln3+ doped LTMP, including emission intensity, lifetime, and quantum efficiency, were further tailored through simply tuning the calcination temperatures.

In this work, a two-step method that involves a hydrothermal reaction and a subsequent calcination was initiated to prepare a novel heterostructure (1 − x)CeO2·xNiO. All samples were systematically characterized by X-ray diffraction, Raman spectra, Transmission electron microscopy, In situ DRIFT spectra, Brunauer–Emmett–Teller technique, H2-temperature-programmed-reduction, and O2-temperature-programmed-desorption. Distinct from the conventional sol–gel or co-precipitation methods, the present methodology allows Ni2+ to be simultaneously doped in the CeO2 lattice and dispersed onto the surfaces of CeO2 nanoparticles by simply varying the calcination temperature from 400 to 700 °C. When calcined at an optimized temperature of 500 °C, the heterostructure at x = 0.1 showed a mean grain size of 18 nm, specific surface area of 69.3 m2 g−1, the smallest lattice parameter, and the largest amount of surface adsorbed and desorped oxygen species, which has led to an excellent catalytic performance towards CO and CH4 oxidation. Such a superior performance benefits from the unique synergistic effects of the finely dispersed NiO and Ni–O–Ce solid solution species detected in the heterostructure.

Activation of the C–H bonds in CH4 is relatively difficult, since CH4 is a quite a stable hydrocarbon with an extremely high ignition temperature (>1600 °C), which usually causes sintering and deactivation of catalysts. It remains a challenge to find catalysts that can show both good catalytic activity and thermal stability. In this work, Fe3+ doped CeO2 nanoparticles were initially prepared and tested for catalytic activity towards CH4 combustion. Systematic sample characterizations indicate that our nanoparticles were exposed by highly energetic facets (200), which yielded an excellent catalytic performance and thermal stability. The total conversion of CH4 at a specific velocity of 60000 ml g−1 h−1 appeared at T100 = 520 °C, about 100 °C lower than that under similar test conditions previously reported for the best Ce–Fe–O solid solution catalysts. Strikingly, the present nanoparticles were also merited by an ultra-high thermal stability, since no particle growth or agglomeration is detected after high-temperature treatment, and since both the structure and oxygen species of the catalysts did not change at all before and after catalytic tests. The catalysts did not show any sign of deactivation, even when the test time was beyond 100 h. The surface oxygen species on the exposed (200) plane of the nanoparticles could be beneficial for the excellent catalytic performance towards CH4 combustion.

A novel heterostructure was first synthesized by directly depositing photocatalytic inert ZnO2 onto facet {201} of brookite nanorods. The heterostructure thus obtained was found to show a superior photocatalytic activity under UV-light irradiation. The exceptional photocatalytic performance was due to the band-structure match between ZnO2 and brookite as well as synergic charge accumulation by different facets of the brookite nanorods.

A facile strategy was initiated to fabricate large-scale uniform brookite TiO2 nanospindles preferentially grown along the [001] direction, which were highly thermally stable and exhibited superior electrical conductivity, about two orders of magnitude higher than those of anatase and rutile counterparts.

Metal phosphates have been popularly regarded as excellent luminescence hosts of lanthanide ions, while such an issue is challenged by the ignorance about the structural stability that may originate from the different chemical nature between the framework and the dopant lanthanide ions. Here, we choose BiPO4 as a model compound to study. A detailed investigation of the effects of Eu doping and annealing on the structures and related luminescence properties of Bi1–xPO4:Eux (x = 0–0.199) has been carried out. A monoclinic phase (denoted as LTMP) was obtained for the undoped sample, which gradually transformed to a hexagonal phase (HP) with increasing doping level of Eu3+ to x = 0.068. Further, it is also found that annealing had an obvious impact on the structures of the resulted samples. With increasing the annealing temperature up to 400 °C a phase transformation from HP to LTMP happens, which is opposite to that with doping. The above phase transformation behaviors were further confirmed by performing structural studies of doping with Dy3+ ions and annealing undoped BiPO4. The structural evolution had a great influence on the luminescent properties. Initially a significant decrease in Eu3+ luminescence intensity and quantum efficiency was observed when LTMP transformed to HP. Afterward, a converse situation, increasingly enhanced luminescence performance, appeared when HP transformed to LTMP. Therefore, whether metal phosphates could be taken as better luminescence hosts must take into account their structural changes caused by the different chemical natures between framework and dopant ions, which may provide an important reference for designing new luminescent materials.

Metastable materials usually possess unique properties. How to acquire these properties is still a great challenge. In this work, we explored the electrical properties of metastable BiPO4 through switchable phase transitions. A metastable monoclinic phase (denoted as HTMP) was synthesized by a heat-treatment over a hexagonal phase (HP). It is found that there is a reversible phase transformation between HTMP and a low-temperature monoclinic phase (LTMP). Namely, HTMP gradually transformed to LTMP by a simple hand-grinding or ball milling, while LTMP transformed back to HTMP upon a heat treatment. Accompanying the transformation from HTMP to LTMP, the morphology varied from the cobblestone-like to spherical-like, and particle sizes changed from micrometre scale to several tens of nanometres, as followed by a decrease in the symmetry of tetragonal PO43− groups from Cs to C1. The reversible transformation was understood by taking into account several structural factors like the arrangement of PO4 tetrahedra and BiO8 polyhedra, unit cell volume (V/Z), and the symmetry of PO4. Finally, the electrical properties of the metastable HTMP were successfully acquired through a pellet-pressing technique. The temperature dependence of bulk conductivity indicates that the conductivities deviated from the Arrhenius law, but followed a simple temperature dependence, T−1/4, obeying a Mott variable range hopping conduction mechanism. The corresponding characteristic temperature of the bulk conduction for HTMP is estimated to be 2.3 × 109 K. These findings are fundamentally important, as they enable one to acquire the properties of many other metastable phases, meanwhile they pave a way for solving the controversies presented in metastable systems.

A novel nontoxic g-C3N4/WO3 composite photocatalyst with an increased surface area but a reduced defect concentration was synthesized, which shows an excellent catalytic performance under visible light irradiation.

Ionic-liquid-assisted solution chemistry was initiated to prepare FeTiO3 nanosheets with {0001} polar facets exposed predominantly, which yielded improved electrochemical performance and excellent catalytic activity towards thermal decomposition of ammonium perchlorate.

The brookite phase of TiO2 is hardly prepared and rarely studied in comparison with the common anatase and rutile phases. In addition, there exist immense controversies over the cognition of the light-induced liveliness of this material. Here, a novel, low-basicity solution chemistry method was first used to prepare homogeneous high-quality brookite TiO2 single-crystalline nanosheets surrounded with four {210}, two {101}, and two {201} facets. These nanosheets exhibited outstanding activity toward the catalytic degradation of organic contaminants superior even to that of Degussa P25, due to the exposure of high-energy facets and the effective suppression of recombination rates of photogenerated electrons and holes by these facets as the oxidative and reductive sites. In contrast, irregularly faceted phase-pure brookite nanoflowers and nanospindles were inactive in catalytic reactions. These results demonstrate that the photocatalytic activity of brookite TiO2 is highly dependent upon its exposed facets, which offers a strategy for tuning the catalysts from inert to highly active through tailoring of the morphology and surface structure.

Interfacial interactions are often found in human medical devices, hybrid solar cells, and catalysis. However, there is a lack of control of these interactions when tailoring the materials properties for many technological applications. As a case study, we reported on the synthesis of Ag–CeO2 core–shell nanospheres with the aim of strengthening the interfacial interactions to give enhanced catalytic performance. All core–shell nanospheres were synthesized by a surfactant-free method with a subsequent annealing redox reaction. Systematic sample characterizations indicate that metallic Ag cores with a diameter of 50–100 nm were wrapped by assembled nanoparticles of CeO2 with a shell thickness of 30–50 nm to form a nano-scale core–shell structure. The interfacial interactions between the Ag core and CeO2 shell were strengthened by annealing, surprisingly, as followed by generation of oxygen vacancies to provide abundant of absorption sites for oxygen species. As a consequence, the temperature for oxygen spilling was lowered to 79 °C, and the catalytic performance was abnormally enhanced, as indicated by complete CO oxidation at 120 °C with no sign of deactivation, even when the reaction time is beyond 100 h. The reaction products were desorbed quickly from the surfaces of the core–shell nanospheres, which accounts for their superior stability during catalytic reactions.

Exposure of highly-energetic facets challenges one's capabilities of designing new substances at the atomic level and of exploiting novel physicochemical properties. We report herein on TiO2 microspheres with a maximum exposure of the highly-energetic facet (001). Intriguingly, these microspheres were fabricated by bundles of adjacent nanowires that grow roughly parallel along the c-axis from sphere centres to outward surfaces. In between these nanowires, there existed nanoscale boundary cavities. Reducing the nanowire diameter led to a lattice expansion, and meanwhile nanoscale boundary cavities in between nanowires were tailored to possess an optimum charge storage at a nanowire diameter of 6.2 nm. This charge storage could suppress the combination of photo-generated holes and electrons. Furthermore, owing to the lattice expansion, photo-generated holes were promoted to transfer along the c-axial to the highly-energetic facet (001) to produce reactive hydroxyl radicals. As a consequence, under UV-light irradiation, microspheres with a nanowire diameter of 6.2 nm showed a maximum photocatalytic activity among all nanowire diameters. When the microspheres were broken into segments, the catalytic activities were further enhanced and even superior to commercial P25, because of sufficient utilization of incident light. The methodology reported in this work is fundamentally important, and may offer opportunities for exploring highly-energetic facets of micro-architectures that interplay with spatial charge storage to active novel surface activities, potentially useful in various catalytic applications.

This work explores the size-induced lattice modification and its relevance to photoluminescence properties of tetragonal zircon-type GdVO4:Eu3+ nanostructures. GdVO4:Eu3+ nanoparticles with crystallite sizes ranging from 14.4 to 24.7 nm were synthesized by a hydrothermal method using sodium citrate as a capping agent. Regardless of the reaction temperatures, all samples retained an ellipsoidal-like morphology. Nevertheless, as the crystallite size reduces, there appears a tensile strain and lattice distortion, which is accompanied by a lattice expansion and a decreased symmetry of structural units. These lattice modifications could be associated with the changes in the interior chemical bonding due to the interactions of surface defect dipoles that have imposed an increased negative pressure with crystallite size reduction. Furthermore, crystallite size reduction also led to a significant increase in the amounts of surface hydroxyl groups and citric species, as well as the concentration of the surface Eu3+ ions. When Eu3+ was taken as a structural probe, it was found that the asymmetric ratio (I02/I01) of Eu3+ gradually declined to show a remarkable decrease in color chromaticity as crystallite size reduces, which could be interpreted as due to the change of local environments of Eu3+ ions from the interior to the surface of the nanoparticles.

High purity SbPO4 hollow spheres assembled by nanoparticles were successfully fabricated by a template-free solvothermal reaction in a mixed solvent of water, ethanol, and oleic acid. The synthetic conditions for the hollow sphere SbPO4 were monitored by a series of time-resolved experiments and further optimized by adjusting the solvent ratio and pH value in the reaction system. Based on XRD and SEM characterization, it is interesting to reveal a phase conversion process from Sb4O5Cl2 to SbPO4 as well as morphology evolution. The phase and morphology transition mechanism was discussed from the chemical equilibrium perspective. Size distributions of Sb4O5Cl2 and SbPO4 samples were in a narrow range of 7 to 13 μm. The Sb4O5Cl2 spheres were constructed in situ by nanosheets, which act as self-sacrificed templates for the SbPO4 hollow spheres. Room-temperature alternative current impedance measurements indicate that the permittivity for the SbPO4 hollow spheres is as high as 130 at 40 Hz, which obviously originates from the interfacial effects with numerous carriers blocked. The synthetic methodology reported in this work can be applied to a broad class of assembled nanostructures for optimization of dielectric performance.

Chemical composition directly determines the structure and properties of almost all bulk inorganic solids, which are however popularly dismissed in the literature as a cause of property changes when studying multi-component oxide nanostructures by solution chemistries. The current work focuses on this subject through a systematic case study on CaWO4 nanocrystals. CaWO4 nanocrystals were prepared using room-temperature solution chemistry, in which a capping agent of citric acid was employed for kinetic grain size control. Sample characterizations by a set of techniques indicated that 5–7 nm CaWO4 was obtained at room temperature, showing a pure-phase of tetrahedral scheelite structure. The molar ratio of Ca2+ to W6+ was found to be 1.2:1, apparently deviating from the unity expected for the stoichiometric CaWO4. Such nonstoichiometry was further modulated via iso-valent incorporation of smaller Zn2+ to the Ca2+-sites in CaWO4. It is found that with increasing the Zn2+ content, there appeared transformation from high to low nonstoichiometry, though a pure scheelite-typed structure was retained. Such a nonstoichiometry was primarily represented by excessive cations like Zn2+ and/or Ca2+ within the surface disorder layers, which in turn showed a great impact on the structure and properties as demonstrated by a lattice contraction, band-gap narrowing, luminescence quenching, as well as improved conductivity. The property changes were rationalized in terms of surface structural disorder, electro-negativity discrepancy, and effective activation on the mobile protons. Consequently, systematic control over the non-stoichiometry for single-phase multi-component oxide nanostructures by solution chemistry is proven fundamentally important, which may help to achieve quantitatively the structure–property relationship for materials design and performance optimization.

In this work, hierarchical CuO hollow microspheres were hydrothermally prepared without use of any surfactants or templates. By controlling the formation reaction conditions and monitoring the relevant reaction processes using time-dependent experiments, it is demonstrated that hierarchical CuO microspheres with hollow interiors were formed through self-wrapping of a single layer of radically oriented CuO nanorods, and that hierarchical spheres could be tuned to show different morphologies and microstructures. As a consequence, the formation mechanism was proposed to proceed via a combined process of self-assembly and Ostwald's ripening. Further, these hollow microspheres were initiated as the anode material in lithium ion batteries, which showed excellent cycle performance and enhanced lithium storage capacity, most likely because of the synergetic effect of small diffusion lengths in building blocks of nanorods and proper void space that buffers the volume expansion. The strategy reported in this work is reproducible, which may help to significantly improve the electrochemical performance of transition metal oxide-based anode materials via designing the hollow structures necessary for developing lithium ion batteries and the relevant technologies.Graphical abstractHierarchical CuO microspheres with hollow interiors were formed through self-wrapping of a single layer of radically oriented CuO nanorods, and these microspheres showed excellent cycle performance and enhanced lithium storage capacity.Research highlights▶ Hierarchical CuO hollow microspheres were prepared by a hydrothermal method. ▶ The CuO hollow microspheres were assembled from radically oriented nanorods. ▶ The growth mechanism was proposed to proceed via self-assembly and Ostwald's ripening. ▶ The microspheres showed good cycle performance and enhanced lithium storage capacity. ▶ Hierarchical microstructures with hollow interiors promote electrochemical property.

Hierarchical SnIn4S8 microspheres were successfully fabricated via a solvothermal method without the assistance of any templates or surfactants. These microspheres were of a new tetragonal polymorph, distinct from the cubic counterpart popularly reported in the literature. Systematic characterization indicated that these microspheres exhibited a diameter of 2–4 μm and were assembled by numerous interconnected nanosheets of 40 nm thickness. The formation mechanism of hierarchical SnIn4S8 microspheres was proposed by taking into account the influences of experimental conditions on the morphology and structure. Hierarchical microspheres possessed a large specific surface area of 197 m2 g−1 and an accessible porous configuration, which gave rise to an excellent visible light-driven photocatalytic efficiency for methyl orange degradation in aqueous solutions, superior to that for the known N-doped TiO2 photocatalyst.

This work presents a critical review on the studies of defect chemistry of oxide nanoparticles for creating new functionalities pertinent to energy applications including dilute-magnetic semiconductors, giant-dielectrics, or white light generation. Emphasis is placed on the relationships between the internal structure and defective surfaces of oxide nanoparticles and their synergy in tailoring the materials properties. This review is arranged in a sequence: (1) structural fundamentals of bulk oxides, using TiO2 as a model simple oxide to highlight the importance of polymorphs in tuning the electronic structures; (2) structural features of simple oxide nanoparticles distinct from the bulk, which show that nanoparticles can be considered as a special solid under the compression as originated from the surface defect dipole-dipole interactions; and (3) new functions achieved through extending the defect chemistry concept to the assembled architectures or multi-component oxide nanoparticles, in which defect surfaces enable the localized electrons or intermediate levels to produce giant dielectric performance or tunable light generation. It is concluded that understandings of defect chemistry provide diverse possibilities to manipulate electrons in oxide nanoparticles for functionalities in energy-relevant applications.

This work reports on a two-stage strategy toward the selective Pt deposition onto the face (110) of TiO2 assembled microspheres for excellent photocatalytic activities. This approach includes the assembly of nanowires with exposed faces (110) and tips (001) to form microspheres, which is followed by Pt deposition onto the face (110) under photoreduction. Systematic sample characterizations indicate that there exist abundant microcavities in between the roughly parallel nanowires of the microspheres which act as the microcapacitors for electronic storage, while Pt deposition featured by surface plasmon resonance under irradiation may function to donate or accept electrons. Therefore, under ultraviolet-light irradiation, Pt deposition accelerated the electronic migration from the conduction band of the microspheres to Pt, while under visible-light irradiation, electrons from Pt nanoparticles would transfer to the conduction band of the microspheres with a simultaneous transfer of the compensable electrons from a donor in reaction solution to the deposited Pt nanoparticles. As a consequence, Pt deposition onto TiO2 assembled microspheres is proved to yield an excellent photocatalytic performance superior over that without Pt deposition under ultraviolet or visible light irradiation.

Rutile TiO2 hierarchical microspheres were prepared by a facile solution chemistry method with an aim to achieve nanoscale boundary cavities (NBCs) that can be tailored for optimum giant dielectric performance. The formation of these microspheres proceeded via a supersaturated spontaneous nucleation and a subsequent radial growth to develop into well-defined 3D hierarchical structures. All microspheres showed a diameter of about 8–10 μm and were constructed by small bundles that consisted of smaller nanowires with a diameter of 8–10 nm. These nanowires are characterized by a preferential growth along the [001] direction which eventually led to the externally exposed (110) planes for hierarchical microspheres. Strikingly, in between these constituent nanowires, there existed plenty of NBCs that created a great number of surface defect dipoles. The NBCs were further tailored by subsequent annealing of the microspheres, as clearly indicated by lattice contraction, linear increase of axis ratio, and red-shift of band-gap energy. As a consequence, rutile TiO2 hierarchical microspheres showed an optimum giant permittivity of approximately 104 level till 500 Hz at room temperature, compatible to the known giant-dielectric multicomponent materials such as CaCu3Ti4O12. These findings were rationalized in terms of NBCs and the resulting surface defect dipoles. As a reproductive prototype, tailoring of the NBCs in rutile hierarchical microspheres as reported in this work can be applied to a broad class of assembled nanostructures and probably film systems to modulate the dielectric performances for advanced electronic device aspects.

In this work, we report on white light generation by chemical modification of red phosphor LaPO4:Eu3+ nanorods. The method includes an initial hydrothermal crystallization of single hexagonal phase LaPO4:Eu3+ nanorods with surface hydration layers, followed by the deprotonation of oleic acid when reacted with surface hydroxyls adjacent to the oxygen vacancies for chemical bonding of oleic acid species. The resulting oleic acid/LaPO4:Eu3+ surface complexes exhibited a synergy of the relevant interface mid-gap states and red emission of Eu3+ which led to surprisingly tunable colors from purplish pink through green-blue to white when simply varying the excitation lines to longer wavelengths. As a consequence of the tunable colors, an intense white light emission was readily achieved at optimum excitation wavelengths of 380 and 395 nm. The findings reported in this work may open up new avenues for simplified white light generation and its relevant technologies.

A facile supersaturated spontaneous nucleation method was initiated to fabricate novel hierarchical microspheres assembled from roughly parallel rutile TiO2nanowires, which showed a dielectric constant of ∼104 level, about one order of magnitude larger than those for all other polymorphic TiO2.

What's the difference when water molecules are confined in a rather limited space? This work addresses this question by decorating water molecules within the scrolled titanate nanotubes. Both scrolled nanotubes Na0.96H1.04Ti3O7·nH2O and Na0.036H1.964Ti3O7·nH2O were first prepared to show large specific areas around 200 m2 g−1, within which quantities of water molecules were confined to form H2O tubes that are alternatively arranged with the titanate nanotubes. This unique double-tube structure exhibited remarkable polarization and dielectric performance, yielding a huge dielectric constant around ε = 14000, comparable to some known giant-dielectric-constant ceramics. Depending on the measurement frequency and temperature, the dielectric relaxation peaks were monitored by the content of the water molecules confined within the nanotubes. A two-layer dielectric model that involves the distinct anisotropy and confinement effect of the double-tube structure was proposed to explain this dielectric behavior. The findings reported in this work may pave the way for optimizing many subtle hydrated nanostructures in nature that could create an abundance of confined water molecules for a broad class of applications.

This work initiated a systematic study on the chemical nature of organic coating for monodispersed nanoparticles and its impact on the defect chemistry and the relevant properties. Monodispersed TiO2 nanoparticles were prepared by a nonhydrolytic sol–gel reaction, which showed features of uniform diameter distribution around 5.2 nm, high crystallinity, and single anatase structure. These nanoparticles were terminated by oleate-related molecules, which stabilized the surface oxygen vacancies and further generated intense photoluminescence and co-existence of ferromagnetism and diamagnetism. After removal of organic coating, the nanoparticles became highly aggregated with no apparent changes in particle size, while the oxygen vacancy concentration was significantly reduced, as followed by energy position shift towards the deeper-levels which promoted the separation of photogenerated electrons and holes for improved photocatalytic activity. The results reported here are fundamentally important, which may be extended to comprehend the size-dependent defects and structure–property correlations of monodispersed nanoparticles for applications.

This work reports on the kinetic control of MnWO4 nanoparticles with an aim to achieve the tailored structural properties. Using citric acid as the capping agent, MnWO4 nanocrystals were prepared to show particle sizes ranging from 8 to 29 nm under hydrothermal conditions. The grain-growth kinetics for MnWO4 nanoparticles was determined to quantitatively follow the equation, D5 = (4.59 × 1020)te−17.0/T, where D is the particle size at given reaction time, t, and reaction temperature, T. Systematic sample characterization using combined techniques of X-ray diffraction, transmission electron microscope, selected area electron diffraction, Barret−Emmett−Teller technique, Fourier transformed infrared spectra, UV−visible diffuse reflectance spectra, and Raman spectra indicates that particle size reduction led to an apparent lattice expansion, which is followed by lowering of lattice symmetry and band gap broadening. Different from the bulk counterparts, new vibration modes were observed in both Infrared and Raman spectra at about 913 and 930 cm−1, respectively, which intensified monotonously with particle size reduction, leading to a picture that MnWO4 nanoparticles were terminated by surface disordered layers. All these size-dependent physical properties were closely related to the surface disorder and the relevant absorbates.

In this work, highly active, low Au-loaded CeO2 nanoparticle catalysts were prepared by several solution chemistries and studied for low-temperature CO oxidation applications. Structures of the CeO2 nanoparticles and the resulting Au/CeO2 catalysts were characterized by X-ray diffraction, transmission electron microscope, X-ray photoelectron spectroscopy, and UV−vis diffuse reflectance spectra. The CeO2 support had a surface area of 120 m2/g and had particle sizes that ranged from 9 to 15 nm. The large surface area of the particles and their fine sizes allowed high dispersion of Au species over the CeO2 support. Conversion of CO to CO2 with the catalyst was significant (ca. 80%) at 0 °C and complete (100%) at 20 °C for a Au loading of 0.52 wt %, demonstrating efficient low-temperature oxidation. Chemical species present in the Au/CeO2 catalyst were Au0, Au+, and Au3+, in which Au+ acted as the dominant active species. During the catalytic reactions, Au3+ was probably reduced to Au+, which enhanced the catalytic oxidation of CO.

The optimum luminescence and its relevant applications of phosphors are always challenged by the generation of multicolour lights in a single solid with a single-wavelength excitation. In this work, we initiated the preparation of novel core-shell CaWO4 microspheres co-doped with Na+ and Ln3+ (Ln = Tb, Sm, Dy, Eu) and systematically studied their tunable wavelength lights. The core and shell of the microspheres are based on the same materials that crystallized in a tetragonal scheelite structure. The primary particle size for the un-doped microspheres was about 36 nm, while upon doping with Ln3+/Na+, the primary particle became as small as 14–19 nm. This core-shell structure is proved unique in significantly suppressing the energy-loss processes occurring at the nanoparticle surfaces. As a consequence, the un-doped microspheres exhibited an intense blue luminescence with a lifetime of 8.46 µs and a chromaticity coordinate of (0.16, 0.14), while with increasing the Ln3+ concentration, the blue emission disappeared and the emissions belonging to Ln3+ were significantly enhanced as is followed by an apparent variation of the chromaticity coordinates. By simply varying the dopant concentration of Ln3+, tunable wavelength lights were successfully achieved in the core-shell CaWO4 microspheres using a single-wavelength excitation light, which is a consequence of the modulated relative intensity of the WO42− emission and Ln3+ emission.

Ag–Ni bi-metal nanocrystals were prepared by a novel solution method, in which ethanol was first taken as a green solvent with no use of any external toxic reducing agents. The as-prepared bi-metal nanocrystals were spherical and constructed by an aggregation of tiny crystals with particle size of about 12 nm. Infrared data indicated that the surfaces of the as-prepared nanocrystals were free of organic contaminants. The obtained bi-metal nanocrystals showed great potential as the additive in promoting the decomposition of ammonium perchlorate (AP), the key component of composite solid propellants. They were also initiated as the anode material of solid oxide fuel cells (SOFCs) which showed a maximum power density of 52.34 mW cm−2 for single cell at 800 °C.

High-quality brookite flowers were fabricated via a facile solution chemistry technique. The synthetic conditions to the flower-like brookite were monitored by a series of time-resolved experiments and further optimized by adjusting the concentrations of the Na+ and OH− species involved in the reaction system. Careful sample characterizations by the combined techniques of X-ray diffraction, Raman, high resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and electron paramagnetic resonance spectra indicate the formation of highly phase-pure and well-crystallized brookite with an extremely low defect concentration. Different from the natural brookite mineral with an indirect transition (Zallen, R.; Moret, M. P. Solid State Commun. 2006, 137, 154), the present high-quality brookite flowers showed a direct transition with a bandgap energy of 3.4 ± 0.1 eV, which is larger than those of its two other polymorphs, that is, a direct band gap of 3.0 ± 0.1 eV for rutile and indirect band gap of 3.2 ± 0.1 eV for anatase. Room-temperature alternative current impedance measurements indicate that the permittivity for the brookite flowers is 93 at 40 Hz, which is much higher than that for anatase but slightly lower than rutile as opposed to what is theoretically predicted in the literature. Strikingly, the flower shape also enables high quality brookite TiO2 with a high structural stability up to 900 °C in air, impossibly accessible when using other preparation methods. These observations pave the way for high-quality brookite flowers to find a broad class of technological uses.

Structural tailoring for dimensionally confined electrical properties is fundamentally important for nanodevices and the relevant technologies. Titanate-based nanotubes were taken as a prototype one-dimensional material to study. First, Na0.96H1.04Ti3O7·3.42H2O nanotubes were prepared by a simple hydrothermal condition, which converted into Na0.036H1.964Ti3O7·3.52H2O nanotubes by a subsequent acidic rinsing. Systematic sample characterization using combined techniques of X-ray diffraction, field emission scanning electron microscopy, high resolution transmission electron microscopy, electron paramagnetic resonance, Fourier transform infrared spectroscopy, elemental analyses, and alternative current impedance indicated that both nanotubes possessed a scrolled trititanate-type structure with the (200) crystal face predominant on the tube surface. With increasing temperature, both nanotubes underwent a continuous dehydration process, which however imposed different impacts on the structures and electrical properties, depending on the types of the nanotubes: (1) the Na0.036H1.964Ti3O7·3.52H2O nanotube converted into anatase-type structure TiO2 nanotubes, while the Na0.96H1.04Ti3O7·3.42H2O nanotube kept the trititanate-type tube structure after calcination at 300 °C; (2) both nanotubes exhibited a maximum conductivity at high temperatures, in which Na0.96H1.04Ti3O7·3.42H2O nanotubes are relatively highly conductive, showing a grain conductivity approximately 2 orders of magnitude larger than that of Na0.036H1.964Ti3O7·3.52H2O nanotubes. These observations were interpreted in terms of the defect chemistry, hydration, and the triple conductive species that were confined in the one-dimensional nanostructures. The findings reported in this work may pave the way for titanate-based nanotubes to find a broad class of technological uses for nanodevices.

Scheelite nanostructures Ca1−2x(Eu,Na)2xWO4 (0 < x ≤ 0.135) were prepared from 5 nm Ca0.968(Eu,Na)0.032WO4 by hydrothermal treatment. The preparation of 5 nm Ca0.968(Eu,Na)0.032WO4 at room temperature and subsequent hydrothermal treatment allow control over chemical compositions and particle size of CaWO4-based red phosphors that has not yet possible when using traditional preparation methods. By careful structural and electronic characterization, it is shown that simultaneous substitutions of Eu3+ and Na+ at Ca2+ sites were possible using this methodology, which allows one to vary the local symmetry surrounding Eu3+ and moreover the energy transfer from O2− to Eu3+ and tungstate groups to Eu3+ for optimum luminescence. As a consequence, the obtained CaWO4-based nanocrystals displayed excellent luminescence properties as demonstrated by luminescence lifetimes of milliseconds, abnormally narrowed emissions, and maximum quantum efficiencies of 92%. The results reported in this work show that it is possible to control chemical composition of oxide nanostructures for structural decoration and luminescence property tailoring via codoping aliovalent ions.

One-dimensional alpha manganese dioxide (α-MnO2) nanorods synthesized by a hydrothermal route were explored as the starting material for preparing lithium manganese spinel LiMn2O4. Pure and highly crystalline spinel LiMn2O4 was easily obtained from α-MnO2 nanorods through a low-temperature solid-state reaction route, while Mn2O3 impurity was present along with the spinel phase when commercial MnO2 was used as starting material. The particle size of LiMn2O4 prepared from α-MnO2 nanorods was about 100 nm with a homogenous distribution. Electrochemical tests demonstrated that the LiMn2O4 thus prepared exhibited a higher capacity than that prepared from commercial MnO2. Therefore, α-MnO2 nanorods are proved to be a promising starting material for the preparation of high quality LiMn2O4.

This work studied the acidic surface modifications on the conduction properties of yttrium-stabilized zirconia (YSZ) nanocrystals using sulfuric and phosphoric acid protonation techniques with an aim to discover suitable additives of the proton-conducting membranes for intermediate temperature operation. All YSZ nanostructures were hydrothermally prepared and the sample surfaces were modified with sulfate or phosphate groups by subsequently immersing the as-prepared samples into sulfuric or phosphoric acid. The obtained samples were characterized by X-ray diffraction, high-resolution transmission electron microscope, element analysis, infrared spectra, N2 adsorption/desorption, and impedance spectroscopy. It was found that the samples exhibited a cubic fluorite structure with a grain size of 4.5 nm and a high surface area of 205 m2 g−1. Proton conduction measurements showed that sulfate-modified samples had an apparently low proton conductivity, while the phosphate-modified ones exhibited a significant proton conductivity and improved thermal stability. These observations were explained by taking into account the grafted species and the amount of hydrated water.

Orthorhombic structured LiMnPO4 was synthesized by a hydrothermal method. The possibility of manganese disorder in LiMnPO4 was studied using powder X-ray diffraction and X-ray absorption fine structure analysis. A manganese-rich model was proposed for the hydrothermally synthesized LiMnPO4. It is found that the extent of Mn2+ disorder on the Li+ sites was suppressed by increasing the reaction temperature, which led to an enhanced electrochemical activity. These observations are explained on the basis of the manganese-rich model, in which the disordered Mn2+ on the Li+ sites may act as a blockage in one-dimensional lithium ion transport pathway, thus reducing the electrochemical activity of the LiMnPO4 prepared at low temperatures.

ZnO twin-cones, a new member to the ZnO family, were prepared directly by a solvothermal method using a mixed solution of zinc nitrate and ethanol. The reaction and growth mechanisms of ZnO twin-cones were investigated by X-ray diffraction, UV–visible spectra, infrared and ion trap mass spectra, and transmission electron microscopy. All as-prepared ZnO cones consisted of tiny single crystals with lengths of several micrometers. With prolonging of the reaction time from 1.5 h to 7 days, the twin-cone shape did not change at all, while the lattice parameters increased slightly and the emission peak of photoluminescence shifted from the green region to the near orange region. ZnO twin-cones are also explored as an additive to promote the thermal decomposition of ammonium perchlorate. The variations of photoluminescence spectra and catalytic roles in ammonium perchlorate decomposition were discussed in terms of the defect structure of ZnO twin-cones.

This work addresses the chemical nature of the catalytic activity of X-ray “pure” CoO nanocrystals. All samples were prepared by a solvothermal reaction route. X-ray diffraction indicates the formation of CoO in a cubic rock-salt structure, while infrared spectra and magnetic measurements demonstrate the coexistence of CoO and Co3O4. Therefore, X-ray “pure” CoO nanocrystals are a unique composite structure with a CoO core surrounded by an extremely thin Co3O4 surface layer, which is likely a consequence of the surface passivation of CoO nanocrystals from the air oxidation at room temperature. The CoO core shows a particle size of 22 or 280 nm, depending on the types of the precursors used. This composite nanostructure was initiated as a catalytic additive to promote the thermal decomposition of ammonium perchlorate (AP). Our preliminary investigations indicate that the maximum decomposition temperature of AP is significantly reduced in the presence of CoO/Co3O4 composite nanocrystals and that the maximum decomposition peak shifts toward lower temperatures as the loading amount of the composite nanocrystals increases. These findings are different from the literature reports when using many nanoscale oxide additives. Finally, the decomposition heat for the low-temperature decomposition stages of AP was calculated and correlated to the chemical nature of the CoO/Co3O4 composite nanostructures.

High-purity anatase TiO2 nanoparticles were prepared using a low-temperature sol–gel route. The as-prepared sample was characterized by X-ray diffraction, transmission electron microscopy, infrared spectroscopy, thermogravimetric analysis, UV–vis spectroscopy, and photoluminescence. It is shown that the as-prepared sample crystallized in a pure anatase phase with an average crystallite size of about 7 nm, and the surfaces were highly hydrated. These nanoparticles were stabilized as a water suspension via the cooperation of DLVO force and surface hydration force. These suspensions showed characteristic band-gap emission at 397±1.5 nm397±1.5 nm, which is a little red-shifted compared with the band-gap energy of indirect electronic transition measured in the UV–vis absorption spectrum. These observations were explained by the light-induced relaxation of polar water molecules in the surface hydration layer.Surface hydration effect on the stern layers formation process.

In this work, NaYF4:Eu2+ microcrystals with an intense blue luminescence were successfully fabricated via a solution-based route. During the sample preparation, oleic acid and cetyltrimethylammonuim bromide were used as the surfactants to tune the morphology, while citric acid was taken as a ligand to stabilize the β-phase NaYF4 and to completely reduce Eu3+ to Eu2+ for activation of blue luminescence. As confirmed by transmission electron microscopy and scanning electron microscopy measurements, all as-prepared samples crystallized in hexagonal rods or tubes, depending on the types of surfactants used. The rod samples had a diameter of about 1.7 µm and a length in the range of 2.6−3 µm, while the tube samples showed an external diameter in the range of 0.8−1 µm and a length in the range of 2.5−3.5 µm with an internal diameter of about 0.5 µm. Both rods and tubes were capped with surfactants, which enabled them to stably disperse in glycol to form transparent solutions. This work seems to be the first example of the successful preparation of stable dispersion of micron luminescence solids. These transparent solutions showed an intense blue luminescent emission of Eu2+ with a quantum yield of about 14%. Finally, the mechanism for the fabrication of the rods and tubes as well as the Eu3+ reduction was discussed.

This work addresses the origin of the poor electrochemical activity of triclinic LiVOPO4. Sample characterization using X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, and magnetic measurements indicated the formation of a pure triclinic phase of LiVOPO4. Diffuse reflectance spectra analysis along with a density functional theory (DFT) calculation showed that triclinic LiVOPO4 is a wide bandgap semiconductor which is confirmed by a very large resistance measured using AC impedance technique. All these experimental characterization and theoretical investigations suggest that the poor electrochemical performance of the triclinic LiVOPO4 is primarily associated with its extremely low intrinsic electronic conductivity. The poor electrochemical performance of triclinic LiVOPO4 as cathode for lithium ion batteries may be caused by the very low electronic conductivity of this material at room temperature.

Low-temperatures impedance and dielectric properties of NiO nanocrystals as a function of particle size were investigated by alternative current impedance spectra. It is found that NiO nanocrystals showed distinct bulk and grain boundary conductions at low temperatures, which were almost the same within the experimental error during the decreasing and the subsequent increasing temperature processes. This result indicated that no further protons are formed during these temperature cycles. The conductivities are also highly dependent on the particle size. As the particle size reduced, the conductivities of NiO nanocrystals increased to show a maximum. Further particle size reduction less than 8.9 nm led to a sudden decrease in conductivity. The corresponding activation energies showed a minimum with varying the particle size. These observations were most likely due to the balance between the concentration of charge carriers, holes and protons.NiO nanocrystals showed distinct low-temperature bulk and grain boundary conductions that are highly dependent on the particle sizes.

In previous calculations about the electronic transport properties through a gated region in graphene, a step-like potential is frequently adopted to mimic the gated barrier in two different theoretical models, i.e. the tight-binding model and the Dirac equation description. Our theoretical investigation indicates that the same step-like potential but used in the two distinct models are not equivalent. In the Dirac equation, a step-like potential can not reflect the normally incident electron due to Klein paradox. However, in tight-binding model, such a potential includes the scattering terms which can destroy the pseudo-spin conservation to cause a finite reflection. Thus, the electron transmission is not perfect.

A series of Zn1−xMgxO samples with dopant content ranging from x = 0 to 0.10 were prepared by a novel rheological phase reaction route. All Zn1−xMgxO samples were investigated by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, inductive coupled plasma optical emission spectroscopy, infrared and UV−vis absorption spectroscopy, and the Barrett−Emmett−-Teller technique. The effects of Mg2+ doping in ZnO on the electronic structures and photogradation of methylene blue dye solution were investigated experimentally and theoretically. All Zn1−xMgxO samples exhibit high photoactivities comparable to Degussa P-25, which first increased with the Mg doing content up to x = 0.05, and then slightly decreased with further doping of Mg to x = 0.10. Density function theory calculations revealed that the substitutions of Mg for Zn ions in the wurtzite ZnO structure largely affected the conduction band, but left the valence band nearly unchanged. The bottom of the conduction band shifted toward higher energies and the contribution of Mg 3s orbitals to the conduction band became more pronounced with increasing Mg content, which explains the enhanced photocatalytic activities. The state density of interstitial Mg in ZnO showed a set of shallow acceptor levels above the valence band. These shallow levels could act as the trapping or recombination centers for photoinduced electrons and holes, accounting for the slightly decreased photodegradation efficiency with further increasing Mg content to x = 0.10. The optimal doping content for photocatalytic performance was determined to be x = 0.05, which is the consequence of the balance of two competing doping effects from lattice substitution and interstitial occupations on the electronic structures.

Highly crystalline one-dimensional (1D) α-MnO2 nanostructures were synthesized by a hydrothermal method. All samples were characterized by X-ray diffraction, transmission electron microscope, thermogravimetric and differential scanning calorimeter, and infrared spectroscopy. During the formation reactions, the tunnel structure of 1D α-MnO2 was simultaneously modified by NH4+ species and water molecules. The amount of NH4+ species that were trapped in the tunnels is almost independent on the reaction temperature, while the total water content increased with the reaction temperature. The average diameter of α-MnO2 nanorods increased from 9.2 to 16.5 nm when the reaction temperature increased from 140 to 220 °C. 1D α-MnO2 was destabilized by a subsequent high-temperature treatment in air, which is accompanied by a structural transformation to 1D Mn2O3 of a cubic structure. At low temperatures, all 1D α-MnO2 nanorods showed two magnetic transitions that were characterized by a decreased Néel temperature with rod diameter reduction. According to the effective magnetic moments experimentally measured, Mn ions presented in the nanorods were determined to be in a mixed valency of high spin state Mn4+/Mn3+.Highly crystalline one-dimensional (1D) α-MnO2 nanostructures were achieved to have tunnel structures modified by NH4+ species and water molecules. By tuning the diameters. 1D α-MnO2 showed two magnetic transition as indicated by hump and kink peaks at low temperatures. Mn ions presented in 1D α-MnO2 were determined to be in a mixed valency of high spin state Mn4+/Mn3+.

A facile strategy was initiated to fabricate large-scale uniform brookite TiO2 nanospindles preferentially grown along the [001] direction, which were highly thermally stable and exhibited superior electrical conductivity, about two orders of magnitude higher than those of anatase and rutile counterparts.

A novel heterostructure was first synthesized by directly depositing photocatalytic inert ZnO2 onto facet {201} of brookite nanorods. The heterostructure thus obtained was found to show a superior photocatalytic activity under UV-light irradiation. The exceptional photocatalytic performance was due to the band-structure match between ZnO2 and brookite as well as synergic charge accumulation by different facets of the brookite nanorods.

Series of perovskite PrCo1−xNixO3−δ (x = 0–0.4) were prepared and carefully investigated to understand the spin state transition driven by hole doping and further to reveal the effect of spin state transition on electronic conduction. It is shown that with increasing doping level, the transition temperature Ts for Co3+ ions from low-spin (LS) to intermediate-spin (IS) reduces from 211.9 K for x = 0 to 190.5 K for x = 0.4. XPS and FT-IR spectra demonstrate that hole doping promoted this transition due to a larger Jahn–Teller distortion. Moreover, a thermal activation of spin disorder caused by thermal population of the spin states for Co ions has a great impact on the electrical transport of these perovskite samples. This work may shed light on the comprehension of spin transition in cobalt oxides through hole doping, which is promising for finding new strategies of enhancing electronic conduction, especially for energy and catalysis applications.

Rutile TiO2 hierarchical microspheres were prepared by a facile solution chemistry method with an aim to achieve nanoscale boundary cavities (NBCs) that can be tailored for optimum giant dielectric performance. The formation of these microspheres proceeded via a supersaturated spontaneous nucleation and a subsequent radial growth to develop into well-defined 3D hierarchical structures. All microspheres showed a diameter of about 8–10 μm and were constructed by small bundles that consisted of smaller nanowires with a diameter of 8–10 nm. These nanowires are characterized by a preferential growth along the [001] direction which eventually led to the externally exposed (110) planes for hierarchical microspheres. Strikingly, in between these constituent nanowires, there existed plenty of NBCs that created a great number of surface defect dipoles. The NBCs were further tailored by subsequent annealing of the microspheres, as clearly indicated by lattice contraction, linear increase of axis ratio, and red-shift of band-gap energy. As a consequence, rutile TiO2 hierarchical microspheres showed an optimum giant permittivity of approximately 104 level till 500 Hz at room temperature, compatible to the known giant-dielectric multicomponent materials such as CaCu3Ti4O12. These findings were rationalized in terms of NBCs and the resulting surface defect dipoles. As a reproductive prototype, tailoring of the NBCs in rutile hierarchical microspheres as reported in this work can be applied to a broad class of assembled nanostructures and probably film systems to modulate the dielectric performances for advanced electronic device aspects.

The optimum luminescence and its relevant applications of phosphors are always challenged by the generation of multicolour lights in a single solid with a single-wavelength excitation. In this work, we initiated the preparation of novel core-shell CaWO4 microspheres co-doped with Na+ and Ln3+ (Ln = Tb, Sm, Dy, Eu) and systematically studied their tunable wavelength lights. The core and shell of the microspheres are based on the same materials that crystallized in a tetragonal scheelite structure. The primary particle size for the un-doped microspheres was about 36 nm, while upon doping with Ln3+/Na+, the primary particle became as small as 14–19 nm. This core-shell structure is proved unique in significantly suppressing the energy-loss processes occurring at the nanoparticle surfaces. As a consequence, the un-doped microspheres exhibited an intense blue luminescence with a lifetime of 8.46 µs and a chromaticity coordinate of (0.16, 0.14), while with increasing the Ln3+ concentration, the blue emission disappeared and the emissions belonging to Ln3+ were significantly enhanced as is followed by an apparent variation of the chromaticity coordinates. By simply varying the dopant concentration of Ln3+, tunable wavelength lights were successfully achieved in the core-shell CaWO4 microspheres using a single-wavelength excitation light, which is a consequence of the modulated relative intensity of the WO42− emission and Ln3+ emission.

What's the difference when water molecules are confined in a rather limited space? This work addresses this question by decorating water molecules within the scrolled titanate nanotubes. Both scrolled nanotubes Na0.96H1.04Ti3O7·nH2O and Na0.036H1.964Ti3O7·nH2O were first prepared to show large specific areas around 200 m2 g−1, within which quantities of water molecules were confined to form H2O tubes that are alternatively arranged with the titanate nanotubes. This unique double-tube structure exhibited remarkable polarization and dielectric performance, yielding a huge dielectric constant around ε = 14000, comparable to some known giant-dielectric-constant ceramics. Depending on the measurement frequency and temperature, the dielectric relaxation peaks were monitored by the content of the water molecules confined within the nanotubes. A two-layer dielectric model that involves the distinct anisotropy and confinement effect of the double-tube structure was proposed to explain this dielectric behavior. The findings reported in this work may pave the way for optimizing many subtle hydrated nanostructures in nature that could create an abundance of confined water molecules for a broad class of applications.

This work initiated a systematic study on the chemical nature of organic coating for monodispersed nanoparticles and its impact on the defect chemistry and the relevant properties. Monodispersed TiO2 nanoparticles were prepared by a nonhydrolytic sol–gel reaction, which showed features of uniform diameter distribution around 5.2 nm, high crystallinity, and single anatase structure. These nanoparticles were terminated by oleate-related molecules, which stabilized the surface oxygen vacancies and further generated intense photoluminescence and co-existence of ferromagnetism and diamagnetism. After removal of organic coating, the nanoparticles became highly aggregated with no apparent changes in particle size, while the oxygen vacancy concentration was significantly reduced, as followed by energy position shift towards the deeper-levels which promoted the separation of photogenerated electrons and holes for improved photocatalytic activity. The results reported here are fundamentally important, which may be extended to comprehend the size-dependent defects and structure–property correlations of monodispersed nanoparticles for applications.

Chemical composition directly determines the structure and properties of almost all bulk inorganic solids, which are however popularly dismissed in the literature as a cause of property changes when studying multi-component oxide nanostructures by solution chemistries. The current work focuses on this subject through a systematic case study on CaWO4 nanocrystals. CaWO4 nanocrystals were prepared using room-temperature solution chemistry, in which a capping agent of citric acid was employed for kinetic grain size control. Sample characterizations by a set of techniques indicated that 5–7 nm CaWO4 was obtained at room temperature, showing a pure-phase of tetrahedral scheelite structure. The molar ratio of Ca2+ to W6+ was found to be 1.2:1, apparently deviating from the unity expected for the stoichiometric CaWO4. Such nonstoichiometry was further modulated via iso-valent incorporation of smaller Zn2+ to the Ca2+-sites in CaWO4. It is found that with increasing the Zn2+ content, there appeared transformation from high to low nonstoichiometry, though a pure scheelite-typed structure was retained. Such a nonstoichiometry was primarily represented by excessive cations like Zn2+ and/or Ca2+ within the surface disorder layers, which in turn showed a great impact on the structure and properties as demonstrated by a lattice contraction, band-gap narrowing, luminescence quenching, as well as improved conductivity. The property changes were rationalized in terms of surface structural disorder, electro-negativity discrepancy, and effective activation on the mobile protons. Consequently, systematic control over the non-stoichiometry for single-phase multi-component oxide nanostructures by solution chemistry is proven fundamentally important, which may help to achieve quantitatively the structure–property relationship for materials design and performance optimization.

Exposure of highly-energetic facets challenges one's capabilities of designing new substances at the atomic level and of exploiting novel physicochemical properties. We report herein on TiO2 microspheres with a maximum exposure of the highly-energetic facet (001). Intriguingly, these microspheres were fabricated by bundles of adjacent nanowires that grow roughly parallel along the c-axis from sphere centres to outward surfaces. In between these nanowires, there existed nanoscale boundary cavities. Reducing the nanowire diameter led to a lattice expansion, and meanwhile nanoscale boundary cavities in between nanowires were tailored to possess an optimum charge storage at a nanowire diameter of 6.2 nm. This charge storage could suppress the combination of photo-generated holes and electrons. Furthermore, owing to the lattice expansion, photo-generated holes were promoted to transfer along the c-axial to the highly-energetic facet (001) to produce reactive hydroxyl radicals. As a consequence, under UV-light irradiation, microspheres with a nanowire diameter of 6.2 nm showed a maximum photocatalytic activity among all nanowire diameters. When the microspheres were broken into segments, the catalytic activities were further enhanced and even superior to commercial P25, because of sufficient utilization of incident light. The methodology reported in this work is fundamentally important, and may offer opportunities for exploring highly-energetic facets of micro-architectures that interplay with spatial charge storage to active novel surface activities, potentially useful in various catalytic applications.

This work explores the size-induced lattice modification and its relevance to photoluminescence properties of tetragonal zircon-type GdVO4:Eu3+ nanostructures. GdVO4:Eu3+ nanoparticles with crystallite sizes ranging from 14.4 to 24.7 nm were synthesized by a hydrothermal method using sodium citrate as a capping agent. Regardless of the reaction temperatures, all samples retained an ellipsoidal-like morphology. Nevertheless, as the crystallite size reduces, there appears a tensile strain and lattice distortion, which is accompanied by a lattice expansion and a decreased symmetry of structural units. These lattice modifications could be associated with the changes in the interior chemical bonding due to the interactions of surface defect dipoles that have imposed an increased negative pressure with crystallite size reduction. Furthermore, crystallite size reduction also led to a significant increase in the amounts of surface hydroxyl groups and citric species, as well as the concentration of the surface Eu3+ ions. When Eu3+ was taken as a structural probe, it was found that the asymmetric ratio (I02/I01) of Eu3+ gradually declined to show a remarkable decrease in color chromaticity as crystallite size reduces, which could be interpreted as due to the change of local environments of Eu3+ ions from the interior to the surface of the nanoparticles.

In this work, we report on white light generation by chemical modification of red phosphor LaPO4:Eu3+ nanorods. The method includes an initial hydrothermal crystallization of single hexagonal phase LaPO4:Eu3+ nanorods with surface hydration layers, followed by the deprotonation of oleic acid when reacted with surface hydroxyls adjacent to the oxygen vacancies for chemical bonding of oleic acid species. The resulting oleic acid/LaPO4:Eu3+ surface complexes exhibited a synergy of the relevant interface mid-gap states and red emission of Eu3+ which led to surprisingly tunable colors from purplish pink through green-blue to white when simply varying the excitation lines to longer wavelengths. As a consequence of the tunable colors, an intense white light emission was readily achieved at optimum excitation wavelengths of 380 and 395 nm. The findings reported in this work may open up new avenues for simplified white light generation and its relevant technologies.

Interfacial interactions are often found in human medical devices, hybrid solar cells, and catalysis. However, there is a lack of control of these interactions when tailoring the materials properties for many technological applications. As a case study, we reported on the synthesis of Ag–CeO2 core–shell nanospheres with the aim of strengthening the interfacial interactions to give enhanced catalytic performance. All core–shell nanospheres were synthesized by a surfactant-free method with a subsequent annealing redox reaction. Systematic sample characterizations indicate that metallic Ag cores with a diameter of 50–100 nm were wrapped by assembled nanoparticles of CeO2 with a shell thickness of 30–50 nm to form a nano-scale core–shell structure. The interfacial interactions between the Ag core and CeO2 shell were strengthened by annealing, surprisingly, as followed by generation of oxygen vacancies to provide abundant of absorption sites for oxygen species. As a consequence, the temperature for oxygen spilling was lowered to 79 °C, and the catalytic performance was abnormally enhanced, as indicated by complete CO oxidation at 120 °C with no sign of deactivation, even when the reaction time is beyond 100 h. The reaction products were desorbed quickly from the surfaces of the core–shell nanospheres, which accounts for their superior stability during catalytic reactions.

Activation of the C–H bonds in CH4 is relatively difficult, since CH4 is a quite a stable hydrocarbon with an extremely high ignition temperature (>1600 °C), which usually causes sintering and deactivation of catalysts. It remains a challenge to find catalysts that can show both good catalytic activity and thermal stability. In this work, Fe3+ doped CeO2 nanoparticles were initially prepared and tested for catalytic activity towards CH4 combustion. Systematic sample characterizations indicate that our nanoparticles were exposed by highly energetic facets (200), which yielded an excellent catalytic performance and thermal stability. The total conversion of CH4 at a specific velocity of 60000 ml g−1 h−1 appeared at T100 = 520 °C, about 100 °C lower than that under similar test conditions previously reported for the best Ce–Fe–O solid solution catalysts. Strikingly, the present nanoparticles were also merited by an ultra-high thermal stability, since no particle growth or agglomeration is detected after high-temperature treatment, and since both the structure and oxygen species of the catalysts did not change at all before and after catalytic tests. The catalysts did not show any sign of deactivation, even when the test time was beyond 100 h. The surface oxygen species on the exposed (200) plane of the nanoparticles could be beneficial for the excellent catalytic performance towards CH4 combustion.

Brookite-TiO2 is a promising next-generation semiconductor material for solar energy conversion, but it suffers from difficulty in achieving high quality and phase purity due to its metastable characteristics. Long-chain fatty acid modification or surfactant assisted methods could orient the growth of brookite; however, purifying the products is complicated and the surface reactivity is invariably undermined. Herein, we demonstrate the design and tuneable synthesis of brookite nanostructures with geometric features of quasi-octahedral (QO), ellipsoid-tipped (ET) and wedge-tipped (WT) nanorods that are exposed primarily with {210} facet via water-soluble titanium precursors. When tested as a photocatalyst for hydrogen evolution from water or for the degradation of organic pollutants, QO brookite nanocrystals exhibited the highest catalytic activity compared to ET and WT nanorod counterparts. This observation could be due to the redox facets that form a “surface-heterojunction” and promote the separation of photogenerated carriers. The precursor-directed method reported here may usher in a new phase for the synthesis of novel metastable nanocrystals with specific facet exposure that are highly useful for applications in energy conversion and environment protection.

A facile supersaturated spontaneous nucleation method was initiated to fabricate novel hierarchical microspheres assembled from roughly parallel rutile TiO2nanowires, which showed a dielectric constant of ∼104 level, about one order of magnitude larger than those for all other polymorphic TiO2.

For almost all catalysts, doping with cheaper transition metal ions could reduce the application price necessary, however usually a catalytic activity degradation occurs. This work reports on the preparation of Ce–M–O solid solutions (M = transition metal) with the aim to achieve cheaper catalysts with high activity and stability. Systematic sample characterizations indicate that the existence of more transition metal hydroxides is beneficial for the formation of Ce–M–O solid solutions and that the formation reaction involves a complicated process (e.g., nucleation, dehydration, Ostwald attachment, doping in ceria lattice, and oriented growth). When tested as the catalysts for CO oxidation, the solid solutions of M = Fe maintained excellent catalytic performance, much higher than ever reported, even when Fe doping levels reach 15%. No sign of deactivation was detected for M = Fe after 100 h reaction, which compares to the apparent decrease in catalytic activity for M = Co or Ni. By studying the doping effect on structure, reducibility, oxygen storage capacity and catalytic performance, it is demonstrated that Fe doping led to lattice crystal shrinking, lattice distortion and restrained grain growth of CeO2, yielding a synergistic effect in promoting oxygen storage capacity and catalytic activity. The results reported herein may provide some direction for exploring advanced catalysts at lower prices.

Noble-metal-supported metal oxides (e.g., Au/CeO2, Pd/Al2O3, etc.) are popular as efficient catalysts for many important catalytic reactions, while their high price and easy deactivation restrict their broad application. Herein, we initiate a double substitution methodology to acquire catalysts with merits of excellent catalytic activity, high stability, and relatively low cost. For this purpose, a Au/CeO2 catalyst was used as reference, and the noble metal Au was completely replaced by the cheaper Ag, meanwhile the rare earth ion, Ce4+ of the CeO2 support was partially substituted by the transition metal ion, Fe3+. This inversely supported catalyst, Ce0.9Fe0.1O1.97/Ag, was prepared by a two-step method based on co-precipitation and a subsequent liquid-phase reduction. Systematic characterizations demonstrate that Ag nanoparticles with a diameter of around 50 nm were wrapped by Ce0.9Fe0.1O1.97 layers. Between the Ag nanoparticles and Ce0.9Fe0.1O1.97 layer, there existed a strong interaction. When this sample was tested as a catalyst for CO oxidation, a full conversion temperature of 150 °C was achieved at a space velocity of 24000 ml h−1 g−1cat, showing a catalytic activity comparable to that previously reported in the literature on the known catalyst Au@CeO2 measured at a lower space velocity of 15000 ml h−1 g−1cat. More strikingly, this catalyst showed a superb catalytic stability and enhanced oxygen storage capacity. The introduction of smaller Fe3+ into the CeO2 lattice and the strong interactions between Ag and Ce0.9Fe0.1O1.97 strongly improved the stability and catalytic performance.