Impact of reaction temperature,stirring and cosolvent on the solvothermal synthesis of anatase TiO2 and TiO2/titanate hybrid nanostructures: Elucidating the growth mechanism

One-dimensional anatase TiO₂ and hybrid TiO₂/titanate nanostructures are synthesized by a simple low temperature solvothermal route followed by the Na⁺/H⁺ ion-exchange and final calcination process. We investigated the impact of reaction temperature, stirring conditions and cosolvent on the morphologies of the as-prepared nanostructures. Nanotubes and nanorods are formed in alkaline solution, while nanorods/nanowires and nanoporous nanoribbons are formed in alkaline water–ethanol and alkaline water–ethylene glycol mixed solvents, respectively. X-ray diffraction, Raman scattering and high-resolution transmission electron microscopy studies are employed to identify the structure and phase composition. The formation of different morphologies of the as-synthesized nanostructures is investigated by field emission scanning electron microscopy and transmission electron microscopy. The growth mechanism and reaction process of the as-prepared nanostructures are explained based on the experimental observations. The photoluminescence, optical absorption and the tuning of band gap of the prepared samples are also studied. This work will be valuable for understanding the growth mechanism of various nanostructured TiO₂ and to explore the commercial applications of nanoporous nanoribbons of TiO₂.     

A biomimetic hierarchical scaffold: natural growth of nanotitanates on three-dimensional microporous Ti-based metals

Nanophase materials are promising alternative implant materials in tissue engineering. Here we report for the first time the large-scale direct growth of nanostructured bioactive titanates on three-dimensional (3D) microporous Ti-based metal (NiTi and Ti) scaffolds via a facile low temperature hydrothermal treatment. The nanostructured titanates show characteristics of 1D nanobelts/nanowires on a nanoskeleton layer. Besides resembling cancelous bone structure on the micro/macroscale, the 1D nanostructured titanate on the exposed surface is similar to the lowest level of hierarchical organization of collagen and hydroxyapatite. The resulting surface displays superhydrophilicity and favors deposition of hydroxyapatite and accelerates cell attachment and proliferation. The remarkable simplicity of this process makes it widely accessible as an enabling technique for applications from engineering materials treatment including energy-absorption materials and pollution-treatment materials to biotechnology.     

Insight into the structural,electrical and photoresponse properties of individual Fe:SrTiO3 nanotubes

Titanates are suitable for many applications such as oxygen sensing and tunable HTS (high temperature superconducting) microwave filters. The potential advantages of the nanostructured forms have been however scarcely explored compared to other oxides. In this work, the structural and electrical properties of individual iron-doped strontium titanate nanotubes (Fe:SrTiO₃) grown by electrophoretic deposition (EPD) were assessed for the first time, showing high stability and reproducibility. This result paves the way to further development of more complex titanate-based devices, as for instance nanostructured oxygen STFO sensors. From experimental data, it was concluded that the polycrystalline form of Fe:SrTiO₃ nanotubes is the major limitation to attain high photoconductivity gains when exposed to UV-light.     

Hydrothermal synthesis of highly pure brookite-type titanium oxide powder from aqueous sols of titanate nanosheets

Hydrothermal synthesis of brookite-type titanium dioxide was examined using aqueous sols of titanate nanosheets, which were prepared with Ti[OCH(CH₃)₂]₄ and N(CH₃)₄OH by a bottom-up process in aqueous solutions. Highly pure brookite powders were yielded at hydrothermal temperatures as low as 120 °C and N(CH₃)₄OH/Ti[OCH(CH₃)₂]₄ molar ratios greater than 2. Although titanate nanosheets are formed at N(CH₃)₄OH/Ti[OCH(CH₃)₂]₄ ≥ 0.4, the addition of excess N(CH₃)₄OH was critical in forming highly pure brookite powders. Excess N(CH₃)₄⁺ caused the precipitation of layered titanates, which were formed by stacking titanate nanosheets. Additionally, the crystal nucleation of brookite occurred in the precipitates. Then, the adsorption of titanate nanosheets on brookite particles caused the particles to transform into rod-like shapes elongated along the c-axis. The adsorbed titanate nanosheets were structurally converted into brookite under the influence of the underlying brookite crystal. Despite having a rough surface, the resulting brookite particles were single crystal-like. Moreover, the use of any tetraalkylammonium hydroxides other than N(CH₃)₄OH did not yield brookite. The addition of N(CH₃)₄OH to the reaction sols yielded precipitates of small titanate nanosheets. The morphology of the titanate nanosheets could have a significant influence on brookite crystallization.     

Hydrothermal growth of ZnO nanostructures

Abstract

One-dimensional nanostructures exhibit interesting electronic and optical properties due to their low dimensionality leading to quantum confinement effects. ZnO has received lot of attention as a nanostructured material because of unique properties rendering it suitable for various applications. Amongst the different methods of synthesis of ZnO nanostructures, the hydrothermal method is attractive for its simplicity and environment friendly conditions. This review summarizes the conditions leading to the growth of different ZnO nanostructures using hydrothermal technique. Doping of ZnO nanostructures through hydrothermal method are also highlighted.     

A simple synthesis of long nanostructured arrays of crystalline strontium titanates at low-temperatures

Long aligned arrays of crystalline strontium titanate (SrTiO₃) nanostructures were synthesized by using simple low-temperature processes that incorporate strontium into titanium oxides. Tubular nanostructures are often confine energy carriers that result in extraordinary transport behaviors in various semiconductors including strontium titanates, which are promising for developing efficient thermoelectric energy conversion materials. However, synthesizing a micron-to-milimeter scale array of one-dimensional ternary nanostructures has been difficult. Moreover, ternary compounds are often obtained as disordered cubic-shape particles at the end of complicated and/or long reactions. In this study, a two-step process—anodization for preparing amorphous titanium oxides and a subsequent thermal annealing process in a mixture of strontium hydroxide, ammonia, and water—was employed. Typical diameter and length of the tubes are ~150 nm and ~160 μm, respectively. It has been found that the amorphous structure of titanium oxides plays an important role in obtaining high-purity long strontium titanate nanotubes at low temperatures (90 and 180 °C) with short reaction times. Comparative and systematic studies with different sample pre-treatments, etching times, temperatures, reaction times, and strontium concentrations revealed reaction mechanisms and key synthesis parameters, which may be utilized to obtain other ternary or quaternary nanostructured compounds such as barium or lead titanates.     

Synthesis of TiO2-based nanotube on Ti substrate by hydrothermal treatment

TiO2-based titanate nanotube film was directly synthesized by hydrothermal treatment of Ti substrate in NaOH solution. The prepared high aspect ratio nanotubes have diameter of 10 nm and pore size of 5 nm with length of several microns. The nanotubes show the same structure and component characteristics as the nanotubes prepared through hydrothermal treatment of TiO2. Other nanostructured titanate as oriented nanofiber film and translucent film were also prepared by adjusting the hydrothermal conditions. The formation mechanism of nanostructured titanate was discussed.     

Hydrothermal growth of ZnO nanostructures

One-dimensional nanostructures exhibit interesting electronic and optical properties due to their low dimensionality leading to quantum confinement effects. ZnO has received lot of attention as a nanostructured material because of unique properties rendering it suitable for various applications. Amongst the different methods of synthesis of ZnO nanostructures, the hydrothermal method is attractive for its simplicity and environment friendly conditions. This review summarizes the conditions leading to the growth of different ZnO nanostructures using hydrothermal technique. Doping of ZnO nanostructures through hydrothermal method are also highlighted.     

Influence of microstructure on dielectric properties of Nd-doped barium titanate synthesized by hydrothermal method

Nd-doped barium titanates were successfully synthesized via a hydrothermal route. The as-prepared barium titanate was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), fourier transformation infrared spectroscopy (FTIR), and Vis–NIR spectroscopy respectively. The results show that pure and Nd-doped barium titanate powders have cubic perovskite structure. After sintering at a temperature of 1,250 °C for 2 h, the phase compositions of all barium titanate are tetragonal phase structure. Vis–NIR spectra well confirmed that Nd³⁺ have been doped into barium titanate. The particle diameters of Nd-doped barium titanate powders and ceramics become samller with the increase of Nd³⁺ content. When Nd/Ba molar ratio is 0.02, the dielectric loss (0.0008) of the powder measured at 1 MHz and room temperature dramatically decreases by 99 % comparing with pure barium titanate (0.083) and shows frequency independence with the frequency increasing from 40 Hz to 1 MHz. The dielectric constant and dielectric loss are 436 and 0.09 after sintering. The Nd-doped BaTiO₃ show an improvement in the dielectric quality which possess a decreased sensitivity to frequency for both the dielectric constant and dielectric loss. Such improvements are of potential importance for high energy density and low loss.     

Facile tailoring of titanate nanostructures at low alkaline concentration by a solvothermal route

Nanostructured titanates with different morphologies such as nanoflakes, nanotubes, and nanofibers have been selectively synthesized by a simple solvothermal treatment of commercial anatase TiO₂ using the mixed water–ethanol cosolvent at low alkaline concentration. The effects of solvothermal temperature, volume ratio of H₂O to C₂H₅OH, amount of NaOH and solvents on the formation of titanate nanostructures have been systematically studied through X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). At low concentration of NaOH solution (the actual concentration of OH⁻ in the solution is only 0.58 M), different titanate nanostructures are achieved by simply changing the volume ratio of H₂O to C₂H₅OH at 180 °C and titanate nanotubes can be synthesized between 100 and 180 °C. A probable formation mechanism is proposed based on XRD, SEM and TEM analysis. The influence of cosolvent on the transformation from anatase TiO₂ to titanate is also investigated.     

水热法合成四方相超细BaTiO3粉体的研究进展

近年来，水热法合成四方相钛酸钡是一个研究热点问题，主要综述了水热法合成四方相钛酸钡粉体的研究进展，指出目前关于四方相和立方相相变的机理尚不完全清楚，临界尺寸还有待于进一步证实，四方相及其含量的测定还依赖于更为先进的设备和测试方法。     

Effects of thermal treatment of nanostructured trititanates on their crystallographic and textural properties

Nanostructured titanates (TTNT) with general formula Na_xH_2−xTi₃O₇·nH₂O were synthesized by hydrothermally reacting different TiO₂ anatase (distinct crystal sizes) with NaOH at 120 °C followed by washing with water or diluted acid and drying of the precipitate. The resulting powders with different sodium contents were submitted to various calcination temperatures up to 800 °C and each calcined product was characterized as for its phase structure, composition, crystallite size and textural properties, namely BET surface area, mesopore volume and pore size distribution. Thermal transformations of TTNT samples were investigated by monitoring the modifications on crystallographic (X-ray diffraction) and textural (N₂ desorption isotherms) properties, revealing the influence of the type of starting anatase and sodium content over the stability of TTNT. Moreover, a detailed study on the reduction of the interlayer distance in TTNT samples upon thermal treatment allowed corroborating the formation of an intermediate nanostructured hexatitanate, just before phase transformation into the corresponding TiO₂ polymorphs and/or titanate crystals, depending on the sodium content and calcination temperature.     

An aqueous, alkaline route to titanate nanotubes under atmospheric pressure conditions

A significant reduction in the temperature for alkaline synthesis of titanate nanotubes is reported. Nanotubular titanates have been synthesized from TiO(2) at 100?°C in a mixture of concentrated, aqueous solutions of KOH and NaOH. After 48?h of refluxing, essentially complete conversion was achieved. For a given temperature, the composition of KOH and NaOH in the binary mixture was chosen to achieve a particular concentration of Ti(IV), which favoured the formation of nanotubes. The morphology, surface area and crystal structure of the titanate nanotubes produced were similar to those produced in a solution of pure NaOH at a higher temperature (≈140?°C).     

Preparation and photocatalytic activity of alkali titanate nano materials A2TinO2n+1 (A = Li, Na and K)

Photocatalysts nano A₂Ti_nO_2n+1 (A = Li, Na, K) were prepared successfully by novel hydrothermal synthesis process. Powders were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), ultraviolet-visible (UV-vis) absorption spectra and field-emission scanning electron microscope (FE-SEM) measurements. These results showed that the compositions of lithium, sodium and potassium titanates were Li₂TiO₃, Na₂Ti₃O₇ and K₂Ti₈O₁₇, respectively. The nano crystals of Li₂TiO₃ were self-assembled as snowflakes while that of Na₂Ti₃O₇ and K₂Ti₈O₁₇ were nanorods. Photocatalytic properties of alkali titanates were also investigated. The results indicated that alkali titanates as prepared have higher photocatalytic activities compared with P25 TiO₂ in the degradation of chloroform under UV light irradiation. A combination of K₂Ti₈O₁₇ and NiO produces a photocatalyst effective for the degradation of chloroform in aqueous solution. The framework of the tunnel structure was suitable for accommodating cocatalysts such as NiO to induce a strong interaction between the active species and cocatalysts. Na₂Ti₃O₇ has high photocatalytic activity under visible-light irradiation due to its strong absorption in the visible light region. The photocatalytic properties of Li₂TiO₃ are inferior to that of Na₂Ti₃O₇ and K₂Ti₈O₁₇ due to its mono-perovskite structure.     

钛酸盐一维纳米材料向TiO2纳米材料的水热转化研究

采用强碱性水热处理法分别控制第一次水热反应为160℃和200℃,制备出一维纳米管和纳米棒2种形貌的产物,将其作为第二次水热反应的前驱体,考察了第二次水热体系中pH值和温度对TiO2纳米材料的晶相组成及其微观形貌的影响;采用XRD、TEM以及HRTEM对样品进行了分析.结果表明,当以纳米管为前驱体时,除在pH=0的体系中得到了以金红石相为主的单晶纳米棒外,在pH值为2、4和7的条件下均得到了单晶纯锐钛矿相TiO2纳米颗粒.当以纳米棒为前驱体时,在pH=0的体系中得到了金红石相与板钛矿相共存的纳米棒和纳米颗粒混合产物;在pH值为2、4和7的条件下均得到了纯锐钛矿相TiO2纳米棒;当二次水热温度低于180℃时,前驱体没有转化完全,所得产物为前驱体与锐钛矿相TiO2共存的纳米棒;当水热温度为180℃和210℃时,产物为纯锐钛矿相纳米棒.     

Effects of swift copper (Cu2+) ion irradiation on structural,optical and electrochemical properties of Co3O4-CuO-MnO2/GO nanocomposites powder

The properties of conjugated and irradiated Co₃O₄-CuO-MnO₂/GO nanostructured nanocomposites powder for possible applications in various sphere of life were studied in this research work. The hydrothermal technique was employed to the synthesis of high quality nanocomposite powder and dr. blades technique was employed to construct thin films known as electrodes. The properties of Co₃O₄-CuO-MnO₂/GO nanostructured composite powder (electrodes) were evaluated using SEM, EDS, XRD, FTIR and DRS. The electrochemical properties were studied using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in a 1.0 M solution of Na₂SO₄ electrolytic solution with a triple electrodes arrangement. The electrode irradiated with 2.25 × 10¹⁵ ions/cm² offered the highest specific capacitance of 1590 F/g at a scan rate of 10 mV/s. The results of electrochemical studies indicate that the combination of three transition metal oxides with GO irradiated by moderate ions is a noble contender for electrode applications.     

Facile and template-free preparation of α-MnO2 nanostructures and their enhanced adsorbability

In this paper, nanostructured MnO₂ materials were successfully prepared through a simple and template-free hydrothermal method. X-ray diffraction pattern indicates that the as-prepared nanomaterials are α-MnO₂. Transmission Electron Microscopy and Scanning Electron Microscopy images demonstrate that nanostructured rod-clusters α-MnO₂ could be evolved from the nanorods. Brunauer-Emmett-Teller (BET) surface area measurement was employed to characterize the surface property. Moreover, the as-obtained (α-MnO₂) nanomaterials could act as an efficient adsorbent to remove Congo Red and Methlylene Blue. More significantly, the nanomaterials are nontoxic and environmentally friendly via biological methylthiazolyldiphenyltetrazoliumbromide assay experiments. Its nontoxic and enhanced adsorbability properties guarantee their safe applications in environmental protection and industrial aspects.     

锆钛酸铅纳米陶瓷粉体的水热合成技术

阐述了水热合成法制备锆钛酸铅纳米陶瓷粉多晶体的原理、工艺过程及其特点,重点分析了水热法制备锆钛酸铅纳米陶瓷粉多晶体的溶解/沉淀和原位合成两种形成机制,以及工艺过程中的影响因素,如反应的温度、碱度、时间、粉体的洗涤、干燥等影响因素。通过分析得出水热法制备锆钛酸铅纳米陶瓷粉多晶体适宜的工艺参数为:反应温度150～220℃,碱度pH7～11.5,反应时间2～4h,采用冷冻干燥法可获得分散性好、粒度均匀的锆钛酸铅纳米粉体。     

On the luminescence of SrTiO3 and related titanates

A comparison is made between the recently published luminescence of some semiconducting titanates (TiO₂, SrTiO₃, BaTiO₃) and the luminescence of mixed metal oxides containing isolated titanate groups. This comparison shows that part of the assignments of the emissions of the former group of compounds is doubtful. Alternative proposals are made.     

Lead-free piezoelectric ceramics and thin films

Recent progress in lead-free piezoelectric ceramics and thin films with special emphasis on alkaline niobatebased and bismuth sodium titanate-based systems is reviewed concisely. Modifications of potassium sodium niobate (KNN) ceramics are presented and subsequent improvements in the electrical properties are summarized. Special attention is devoted to the phase diagram of the KNN system when a solid solution is formed with other perovskite niobates and titanates. Impact of A-site and B-site dopants on the electromechanical properties of KNN ceramics are distinguished in view of transition temperatures. It is shown that the addition of most A-site and B-site dopants reduces the transition temperatures and improves the piezoactivity at room temperature. This is attributed to the shift of polymorphic transition from tetragonal to orthorhombic phase in the vicinity of room temperature. In contrast, formation of a solid solution of KNN with 18 mol% AgNbO? revealed a significant enhancement of properties without a notable change in the transition temperatures. Also, a bismuth sodium titanate (BNT) composition is introduced with particular emphasis on its binary and ternary derivatives. Moderate piezoelectric properties reported at the morphotropic phase boundaries, formed in BNT-based solid solutions are also represented. Advances on thin films based on these two compositions are evaluated and challenges involved with development of stoichiometric thin films with low leakage current are discussed.