Surfactant-free Hydrothermal Process Research Articles

Structural, optical and magnetic properties of SnO2 nanoparticles synthesized via surfactant-free hydrothermal process

Tin oxide (SnO2) nanoparticles are synthesized at different molar concentrations by employing a surfactant-free hydrothermal process. The structural properties of the samples investigated using XRD analysis indicated that the synthesized SnO2 nanoparticles have pure crystalline phase with tetragonal crystal structure of P42/mnm space group. The surface morphology analyzed using SEM micrographs showed that the nanoparticles get agglomerated without any specific structure. Optical characterization using UV–visible and PL spectroscopic techniques indicated that the synthesized SnO2 nanoparticles have absorption in the ultraviolet region and emission in the visible region due to defect states. A weak room-temperature ferromagnetism (RTFM) exhibited by the nanoparticles has been observed from the analysis of magnetic parameters.

Sol-Gel Derived Tertiary Bioactive Glass-Ceramic Nanorods Prepared via Hydrothermal Process and Their Composites with Poly(Vinylpyrrolidone-Co-Vinylsilane).

Bioactive glass (BG) nanoparticles have wide applications in bone repair due to their bone-bonding and biodegradable nature. In this work, nanometric rod-shaped ternary SiO2-CaO-P2O5 bioactive glass particles were prepared through sol-gel chemistry followed by a base-induced hydrothermal process at 130 °C and 170 °C for various times up to 36 h. This facile, low-temperature and surfactant-free hydrothermal process has shown to be capable of producing uniform nanorods and nanowires. One-dimensional growth of nanorods and the characteristics of siloxane bridging networks were dependent on the hydrothermal temperature and time. Hardened bioactive composites were prepared from BG nanorods and cryo-milled poly(vinylpyrrolidone-co-triethoxyvinylsilane) in the presence of ammonium phosphate as potential bone graft biomaterials. Covalent crosslinking has been observed between the organic and inorganic components within these composites. The ultimate compressive strength and modulus values increased with increasing co-polymer content, reaching 27 MPa and 500 MPa respectively with 30% co-polymer incorporation. The materials degraded in a controlled non-linear manner when incubated in phosphate-buffered saline from 6 h to 14 days. Fibroblast cell attachment and spreading on the composite were not as good as the positive control surfaces and suggested that they may require protein coating in order to promote favorable cell interactions.

Platinum nanoparticle decorated rutile titania synthesized by surfactant free hydrothermal method for visible light catalysis for dye degradation and hydrogen production study

Rutile phase of titanium oxide and platinum nanoparticle decorated rutile titania is prepared by a surfactant free hydrothermal process in acidic condition. The pure rutile phase of TiO2 particle is forms the specific cauliflower morphology. Hydrothermal process in presence of specific acid addition led to the formation of cauli-flower shaped rutile phase of titania. The efficiency and solar assisted photo catalytic ability of these materials are tested for methylene blue degradation as well as hydrogen generation by methanol reforming process. The X-ray diffraction of pattern of pure rutile phase formation is confirmed by reported JCPDS data. The surface physico-chemical property of prepared rutile Titania is further characterized by BET, Raman and SEM analysis. The HR-TEM of the prepared samples show the reduced particle size for rutile titania and studied their morphology in detail. Solar light assisted methylene blue degradation reaction was carrying out to study the catalytic efficiency towards dye degradation and kinetic activity of the same for prepared commercial titania and pure rutile TiO2. The platinum loaded and photo deposited rutile Titania is further analyzed for hydrogen production reaction by methanol reforming process. The rate of hydrogen evolution on platinum nanoparticle photo deposited on reforming process shows more than 900 μmol/g compared to pristine rutile titania catalysts.

Development of pure rutile $$\hbox {TiO}_{2}$$ TiO 2 and Magneli titanium sub-oxide microstructures over titanium oxide-seeded glass substrates using surfactant-free hydrothermal process

Titanium oxide seed layer was coated over glass substrates by sol–gel spin coating method. Single-step surfactant-free hydrothermal process developed titanium oxide microstructures over the titanium oxide-seeded glass substrates. Various morphologies of titanium oxide microstructures, varying from coral reefs to flower-like morphology were evolved by changing HCl concentration (1 and 3 M) in the precursor solution and growth temperature (GT) (120, 150 and \(180{^{\circ }}\hbox {C}\)) of the hydrothermal process. Structural studies confirmed the formation of rutile \(\hbox {TiO}_{2,} \hbox { TiO}_{2}+\hbox {Ti}_{2}\hbox {O}_{3}\) and \(\hbox {Ti}_{4}\hbox {O}_{7}\) microstructures due to varying the concentration of HCl (1 and 3 M) in the solution and GTs. Morphological studies revealed the possibility of engineering coral reefs, cauliflower, flowers, ball, cactus and jasmine flower-shaped microstructures of titanium oxide films by tuning the growth parameters. The indirect band gap of the titanium oxide microstructures derived from UV–Vis absorption spectra lies in the range of 2.9–3.02 eV.

Flexible electrode materials based on WO3 nanotube bundles for high performance energy storage devices

The demand for high performance energy storage devices such as supercapacitors and lithium-ion batteries has prompted people to design and develop new electrode materials. In this work, WO3 nanotube bundles as the electrode materials are prepared by a template- and surfactant-free hydrothermal process. As anode materials for supercapacitor, WO3 nanotube bundles possess an area capacitance of 2575mF/cm2 at a current density of 3mA/cm2 and cyclic durability of 85.1% retention after 6000 cycles and a specific capacitance of 615.7F/g at a current density of 1A/g. The as-fabricated asymmetric supercapacitor presents a high energy density and cycle abilities after 10,000 cycles. The as-synthesized products are also applied as binder-free Li-ion batteries anode, revealing high discharge capacity at the first discharge and a stable capacity after 200 cycles. Excellent electrochemical performance of the as-synthesized WO3 products for supercapacitors and lithium ion batteries could be ascribed to their unique tube-like structures.

Surfactant-free synthesis of NiPd nanoalloy/graphene bifunctional nanocomposite for fuel cell

Abstract Nickel-palladium alloy nanoparticles over graphene nanosheets (NiPd alloy NPs/GNS) are synthesized using ammonia-hydrazine method via a surfactant-free hydrothermal process. NiPd alloy NPs/GNS represent a new class of electrocatalyst with enhanced activity and stability for formic acid and ethanol oxidation reactions. The as-prepared catalysts are characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), STEM-elemental mapping, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Cyclic voltammetry (CV) and chronoamperometry (CA) studies reveal that bimetallic alloy nanoparticles exhibit higher catalytic activity than monometallic catalysts toward electro-oxidation reactions. The higher activity is mainly attributed to the strong assembly of bimetallic NiPd alloy NPs over graphene sheets. The novel NiPd alloy NPs/GNS catalysts provide a new strategy for superior electro-catalytic performance in next-generation fuel cells.

Synthesis, luminescence properties and EPR investigation of hydrothermally derived uniform ZnO hexagonal rods

One-dimensional (1D) zinc oxide (ZnO) hexagonal rods have been successfully synthesized by surfactant free hydrothermal process at different temperatures. It can be found that the reaction temperature play a crucial role in the formation of ZnO uniform hexagonal rods. The possible formation processes of 1-D ZnO hexagonal rods were investigated. The zinc hydroxide acts as the morphology-formative intermediate for the formation of ZnO nanorods. Upon excitation at 325nm, the sample prepared at 180°C show several emission bands at 400nm (∼3.10eV), 420nm (∼2.95eV), 482nm (∼2.57eV) and 524nm (∼2.36eV) corresponding to different kind of defects. TL studies were carried out by pre-irradiating samples with γ-rays ranging from 1 to 7kGy at room temperature. A well resolved glow peak at ∼354°C was recorded which can be ascribed to deep traps. Furthermore, the defects associated with surface states in ZnO nano-structures are characterized by electron paramagnetic resonance.

Solvent-regulated solvothermal synthesis and morphology-dependent gas-sensing performance of low-dimensional tungsten oxide nanocrystals

This paper reports a solvent-regulated, surfactant-free hydrothermal process at 180°C for 24h to synthesize tungsten oxide (WO3) nanocrystals with various morphologies (urchin-like, rod-like and plate-like WO3) using WCl6 as the W source and just changing the solvents (from ethanol, ethanol/water (1:1 in v/v) to water); then those WO3 nanocrystals are used as the active materials to fabricate gas sensors. The results of XRD, SEM, TEM and N2 adsorption-desorption isotherms indicate that the sample obtained in ethanol takes on a urchin-like morphology (U-WO3) with apparent diameters of 1-2 μm derived by radially aggregating WO3 nanorods (∼5nm in diameter), different from the sample obtained in the mixture of ethanol/water (1:1 in v/v), showing a rod-like morphology (R-WO3 with diameters of ∼5nm), and the product synthesized in water, showing a plate-like shape (P-WO3) with dimensions of (100–200nm)×(100–200nm)×(20–30nm). Common volatile organic compounds (acetone, methanol, methanal, ethanol and benzene) and some gases (H2, CO and H2S) are used as the target substances to evaluate the gas-sensing performance of the WO3 sensors obtained. The U-WO3 sensor is highly sensitive to acetone vapors, whereas the R-WO3 and P-WO3 sensors are highly sensitive to H2S. The morphology-dependent gas-sensing performance can be attributed to the difference in microstructures and crystalline states of the WO3 nanocrystals obtained in different solvents.

Controllable synthesis of CaTi 2 O 4 (OH) 2 nanoflakes by a facile template-free process and its properties

Abstract Serrated leaf-like CaTi 2 O 4 (OH) 2 nanoflake crystals were synthesized via a template-free and surfactant-free hydrothermal process. The samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The growth process for CaTi 2 O 4 (OH) 2 nanoflakes was dominated by a crystallization–dissolution–recrystallization growth mechanism. BET analysis showed that CaTi 2 O 4 (OH) 2 nanoflakes had mesoporous structure with an average pore size of 8.7 nm, and a large surface area of 88.4 m 2 g −1 . Cyclic voltammetry and galvanostatic charge–discharge tests revealed that the electrode synthesized from CaTi 2 O 4 (OH) 2 nanoflakes reached specific capacitances of 162 F g −1 at the discharge current of 2 mA cm −2 , and also exhibited excellent electrochemical stability.

Surfactant-free scalable synthesis of hierarchically spherical Co3O4 superstructures and their enhanced lithium-ion storage performances

Unique hierarchically porous spherical Co3O4 superstructures were synthesized via a surfactant-free hydrothermal process followed by a calcination treatment, in which the concentration of reactant cobalt (II) nitrate hexahydrate is a key factor affecting the morphology of products. X-ray powder diffraction, electron microscopies (TEM and SEM), and thermogravimetric analysis were employed to investigate the formation of Co3O4 spherical superstructures. Our results suggest that they formed from numerous cubic Co3O4 nanocrystals via an oriented attachment mechanism. These superstructures exhibit a high specific capacity of 1750 mA h g−1 after the first charge–discharge cycle, and the capacity retention remains at a constant of 1600 mA h g−1 at 0.2 C after 50 cycles. The facile, scalable, energy-efficient and environmentally friendly nature of the presented approach renders it particularly attractive from a technological standpoint. In addition, this scalable and facile synthesis method could be extended to the preparation of other transition metal oxides with specific morphologies and surface textures.

One-Step, Surfactant-Free Hydrothermal Method for Syntheses of Mesoporous TiO2Nanoparticle Aggregates and Their Applications in High Efficiency Dye-Sensitized Solar Cells

A facile, one-step, template-less, surfactant-free hydrothermal process, using a metal salt as the precursor, is developed to prepare submicrometer sized mesoporous TiO2 nanoparticle aggregates (NPGs). The as-prepared TiO2 NPGs are crystalline of the anatase phase, with a high specific surface area of 166 m2/g, an average pore size of 8.9 nm, and an average NPG size of 840 nm. With these NPGs, a new form of composite photoanode, consisting of the mesoporous TiO2 NPGs and xerogels, is proposed for high efficiency dye-sensitized solar cells (DSSCs). TiO2 xerogels are incorporated into the TiO2 NPGs layer with an impregnation process to form the TiO2 NPGs/xerogels composite. A high power conversion efficiency of 8.41% is achieved for DSSCs based on the TiO2 NPGs/xerogels composite photoanode, representing a 38% efficiency boost over the efficiency of 6.11% achieved with a P25 TiO2 based cell. The success of the present composite TiO2 nanostructure can be attributed to the effective utilization of the inter-N...

Gram-scale synthesis of highly crystalline, 0-D and 1-D SnO2 nanostructures through surfactant-free hydrothermal process

We report the synthesis of highly crystalline SnO2 nanoparticle and nanorod structures with average diameters well within quantum confinement limit (3.5−6.4 nm), through surfactant-free hydrothermal synthesis. The size and shape of the nanostructures could be controlled by controlling the pH (4.5–13.0) of the reaction mixture and the temperature of hydrothermal treatment. Probable mechanisms for the variation of particle size and growth of one-dimensional structures are presented considering the size-dependent crystal solubility at lower pH values of the reaction solution and Ostwald ripening of the quasi-spherical nanoparticles at higher pH values, respectively. Variation of optical band gap energy and hence the effects of quantum confinement in the nanostructures have been studied.

Mineralizer Mediated Self-Assembly of PbTiO<sub>3</sub> Crystals

PbTiO3 crystals with different morphologies, such as flower-like and spherical shape, which was constructed by oriented nanoparticles have been obtained by a simple surfactant-free hydrothermal process assisted by K2CO3 as a mineralizer. Powder X-ray diffraction (XRD) analysis was applied to examine the purity and crystallinity of the as-prepared samples. The morphologies have been characterized by scanning electron microscopy (SEM). The results indicated that the self-assembly structures of PbTiO3 crystals were dependent on the addition amount of K2CO3 and the formation mechanism of the products was briefly discussed.

Synthesis of PZT Disks by a Hydrothermal Method

The organization of nanostructures across extended length scales is a key challenge in the design of integrated materials with advanced functions. PbZr0.52Ti0.48O3multilayer disks which were constructed by oriented rectangle nanoparticles were easily prepared by a simple surfactant-free hydrothermal process. The as-prepared powders were characterized by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). The results indicated that the as-prepared PZT disks were constructed by self-assembly of rectangle nanoparticles by a perfect manner. The formation mechanism of the products was discussed.

Controllable synthesis of ZnO architectures by a surfactant-free hydrothermal process

Various ZnO architectures like novel flowerlike structures radially assembled by rods, nails or towers and novel radialized bundled tubular structures with diverse diameter in entire length were controllably synthesized with different amine precursors by a simple surfactant-free hydrothermal process. It suggests that diverse amine sources, which possibly have different hydroxyl ion releasing rate contributing to different reaction rates, probably play an importance role in controlling the assembly of different ZnO architectures, besides temperature and time. X-ray powder diffraction (XRD) results prove the ZnO belonging to wurtzite structure and room temperature photoluminescence (PL) demonstrates a high quality of the products.

Surfactant-free solution phase synthesis of monodispersed SnO2 hierarchical nanostructures and gas sensing properties

In this work, SnO2 hierarchical nanostructures were successfully prepared via a simple and surfactant-free hydrothermal process starting from stannous sulfate (SnSO4) and trisodium citrate dihydrate (Na3C6H5O7·2H2O) in a suitable ethanol–water system. TEM and HRTEM images showed that the obtained SnO2 products are uniform, well-dispersed, and have spherical architectures, composed of tiny primary nanocrystals, and the diameters are about 50 nm. It was found that the amount of Na3C6H5O7·2H2O and the volume ratio of ethanol and water played important roles in determining the final morphologies of the products. The gas sensing results indicated that the sensor made from porous SnO2 nanostructures calcined at 400 °C exhibited excellent gas sensing performance to butanol at the low temperature compared with those under higher calcination temperature and commercial SnO2. The SnO2 hierarchical nanostructures possess uniform size and large surface areas, making them ideal candidates for more potential applications such as battery electrodes and as opto-electronic devices.

Polymorphological LaVO<sub> 4</sub>:Dy <sup>3+</sup> Phosphors: Hydrothermal Synthesis and Luminescent Properties

Tetragonal LaVO4:Dy3+ crystals with novel shapes of rod, rod-bundles and needle-like flower were successfully synthesized via a facile and surfactant-free hydrothermal process. The products were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and photoluminescence spectroscopy (PL).The experimental results showed that the concentration of the precursor and the reaction temperature played important roles in morphological evolution of LaVO4:Dy3+ crystals (NCs). It was found that needle-like flower LaVO4:Dy3+ exhibited the highest B/Y value compared with rod and rod-bundle samples. Possible growth mechanisms of polymorphological LaVO4:Dy3+ crystals were briefly discussed.

Efficiency Enhancement Achieved with Elongated Titania Nanocrystals for Dye Sensitized Solar Cells

A significant efficiency enhancement of 28%, from 5.82 to 7.44%, is achieved by simply blending a minor amount of elongated titania nanocrystals to the photoanode of dye sensitized solar cells. With the addition of 10 wt % elongated titania nanocrystals of aspect ratio of 5, an increase of 24% in the short-circuit current density and a non-negligible increase of the open-circuit voltage from 0.71 to 0.74 V are obtained, leading to the observed efficiency enhancement. These improvements can be attributed to the enhanced light utilization and reduced electron-hole recombination, enabled by the extra light scattering and enhanced electron transport, respectively offered by the incorporated elongated titania nanocrystals. These nanocrystals are prepared at desired aspect ratios ranging from 1 to 5 with a template-free and surfactant-free hydrothermal process through adjusting the pH value of the reaction solution. This work demonstrates that the critical merits of enhancements in light scattering and electron transport can be realized through elongated titania nanocrystals, without the necessity of using nanowires, nanotubes, or nanorods of high aspect ratios.

Surfactant-free preparation of NiO nanoflowers and their lithium storage properties

Well-defined 3D Ni(OH)2 nanoflowers were synthesized in high yield via a simple surfactant-free hydrothermal process. On the basis of a series of contrast experiments, the probable growth mechanism and fabrication process of the products were proposed. NiO nanoflowers have also been obtained by thermal decomposition of Ni(OH)2 nanoflowers in air at 500 °C. N2 adsorption–desorption measurements indicate the BET surface area is 198 m2 g−1 and the average pore size of the NiO nanoflowers is 15.1 nm. The electrochemical properties of the NiO electrodes in a lithium ion battery and magnetic performance were also investigated. The first discharge capacity of the NiO nanoflowers could reach about 1300 mA h g−1. The unique flower-like structure played a critical role in the morphology requirement to serve as a transport path for lithium ions in lithium batteries.

Hydrothermal synthesis of YBO3:Tb3+ microflowers and their luminescence properties

Three-dimensional flowerlike YBO3:Tb3+ phosphors have been successfully prepared by an efficient surfactant-free hydrothermal process directly without further sintering treatment. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray (EDS) spectrometry, selected area electron diffraction (SAED), photoluminescence (PL) spectra were used to characterize the samples. The as-obtained samples present flowerlike agglomerates composed of nanoflakes with thickness of 20nm and high crystallinity in spite of the moderate reaction temperature of 180°C. The reaction mechanism has been considered as a dissolution/precipitation mechanism; the self-assembly evolution process has been proposed on homocentric layer-by-layer growth style. The different luminescent intensity with different molar ratio of Y–Tb [Y:Tb=8:2; 7:3; 6:4; 5:5; 4:6], YBO3:Tb3+ phosphors exhibit different light (white, red, green) under ultraviolet excitation, which might find potential applications in the fields such as light display systems and optoelectronic devices.