Surfactant-free Hydrothermal Method Research Articles

BiVO4 microstructures with various morphologies: Synthesis and characterization

Bismuth vanadate (BiVO4) microstructures with dumbbell, rod, ellipsoid, sphere, and cake-like morphologies have been successfully fabricated by using a surfactant-free hydrothermal method, in which the morphology of the BiVO4 microstructures can be tuned by simply varying the molar ratio of Bi(NO)3·5H2O to NaVO3 in the starting materials. Based on a series of contrast experiments, the probable formation mechanism of the BiVO4 microstructures with multiple shapes have been proposed. The photocatalytic performances of the as-prepared BiVO4 microstructures have been evaluated by studying the degradation of Rhodamine B solutions under visible light irradiation. The results reveal that the cake-like BiVO4 microstructures exhibit the higher photocatalytic activity than other BiVO4 microstructures due to its high surface area and unique morphology.

Surfactant-free hydrothermal synthesis of flower-like BiOBr hierarchical structure and its visible light-driven catalytic activity towards the degradation of sunset yellow

Flower-like BiOBr hierarchical structures were successfully prepared by a facile and surfactant-free hydrothermal method. The as-synthesized products were characterized by X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and UV–Visible diffuse reflectance spectroscopy (UV–Vis DRS). The characterization results showed that the as-synthesized products were tetragonal phase pure BiOBr and demonstrated well-crystalline with good optical properties. It was also observed that the as-synthesized products were accumulated by large amount of interleaving nanosheets and formed an open porous structure through oriented aggregation. Under visible light irradiation, the as-synthesized products exhibited much higher photocatalytic activity compared to commercial TiO2-P25 for sunset yellow degradation. Such enhancement was attributed to the unique hierarchical porous surface structure of flower-like BiOBr with good visible light absorption ability which can enhance the generation and separation of electron–hole pairs and led to high yield of hydroxyl radicals quantities as evidenced by the photoluminescence spectra. Moreover, the photocatalytic studies showed that various effects of parameters exerted their individual influence on the degradation of sunset yellow. Furthermore, the as-synthesized products were reused several times without appreciable loss of activity, showing great potential for practical applications in environmental remediation.

Growth Mechanism of Surfactant-Free Size-Controlled Luminescent Hydroxyapatite Nanocrystallites

Herein, a facile surfactant-free hydrothermal method assisted with coprecipitation has been explored in order to prepare size-controlled HAP nanorods, and the growth mechanism of HAP nanostructures has also been proposed. In this article, the size of the HAP crystallites, ranging from 40 to 1500 nm, were controlled by varying the reaction conditions. Likewise, the experimental conditions can be changed to alter the shapes of the HAP nanostructures from nanorods to nanobelts. Additionally, interesting optically properties were induced in nano-HAP by doping with rare earth elements. The samples doped with terbium and europium generated green and red orange emissions, respectively, under irradiation by blue light, which is popularly used for wound healing and skin disease treatment. Thus, the doped HAP nanocrystals through the surfactant-free method have potential applications in blue phototherapy and photodynamic therapy.

Visible Light Active Single-Crystal Nanorod/Needle-like α-MnO2@RGO Nanocomposites for Efficient Photoreduction of Cr(VI)

Herein, we report a highly efficient template or surfactant free hydrothermal method to fabricate a graphene based single-crystal tetragonal needle- and nanorod-like α-MnO2 composite, a kind of semiconductor photocatalyst for the reduction of hexavalent chromium [Cr(VI)] under visible irradiation. Hydrothermal reaction time and in situ introduction of graphene played a important role in tuning the morphology of α-MnO2. The as-synthesized reduced graphene oxide (RGO)/α-MnO2 nanorod composite exhibited outstanding photoreduction ability as compared to RGO/α-MnO2 needle composite. The higher activity of RGO/α-MnO2 nanorod composite was successfully derived from photoluminescence (PL) and photocurrent measurements. The low PL intensity and high photocurrent density of RGO/α-MnO2 nanorod composite concludes that change in aspect ratio as well as the presence of RGO favors intimate strong interaction of a two-dimensional layer of graphene and one-dimensional α-MnO2 nanorod which facilitates for enhanced photoex...

Morphologically tuned 3D/1D rutile TiO2 hierarchical hybrid microarchitectures engineered by one-step surfactant free hydrothermal method

Present investigation reports on the surfactant free hydrothermal synthesize of the morphologically tuned hierarchical hybrid rutile titanium oxide (TiO2) microarchitectures showing three dimensional microflower structures and cook pine tree like structures on the one dimensional nanorods formed over TiO2 seed layer coated glass substrates by tuning growth temperature. TiO2 seed layer of ∼100nm thick was coated on the glass substrates employing sol–gel spin coating method and then rutile TiO2 microarchitectures were synthesized on the TiO2 seed layer by one-step surfactant free hydrothermal method. Deposited samples were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, UV–vis spectroscopy and photoluminescence spectroscopy techniques. Influence of the growth temperature on the crystallinity, morphology and optical properties along with the growth mechanism to achieve hierarchical microarchitectures was investigated. Present work revealed that the structural, morphological and optical properties of the TiO2 hierarchical microarchitectures strongly depend on the growth temperature. Further we proposed a model for the cause to effect possible morphological changes of rutile TiO2 microarchitectures as a function of growth temperatures on the TiO2 seeded glass substrates.

Improving Surface Adsorption via Shape Control of Hematite α-Fe2O3 Nanoparticles for Sensitive Dopamine Sensors.

α-Fe2O3 nanoparticles (NPs) with morphologies varying from shuttle to drum were synthesized through an anion-assisted and surfactant-free hydrothermal method by simply varying the ratios of ethanol and water in solvent. Control experiments show that the structural evolution can be attributed to a small-molecular-induced anisotropic growth mechanism in which the growth rate of α-Fe2O3 NPs along the a-, b-, or c-axis was well-controlled. The detailed structural analysis through the high-resolution transmission electron microscope (HRTEM) indicated that shuttle-like Fe2O3 NP surface was covered by high-density atomic steps, which endowed them with the enhanced adsorption and sensor ability toward dopamine (DA). The XPS characterizations indicated that the percentages of the OC component follow the order of shuttle-like Fe2O3 (S-Fe2O3 for short) > pseudoshuttle-like Fe2O3 (Ps-Fe2O3 for short) > polyhedron-like Fe2O3 (Ph-Fe2O3 for short) > drum-like Fe2O3 (D-Fe2O3 for short). Benefits from these structural advantages, the S-Fe2O3 NPs-Nafion composite electrode exhibited remarkable electrochemical detection ability with a wide liner range from 0.2 μM to 0.107 mM and a low detection limit of 31.25 nM toward DA in the presence of interferents.

Spinel ZnCo2O4 nanosheets as carbon and binder free electrode material for energy storage and electroreduction of H2O2

We present simple surfactant free hydrothermal method followed by calcination process to in situ fabricate ZnCo2O4 material on Ni foam with two distinctly different morphologies. The interconnected nanosheets (ZnCo2O4-H) and bundle-like nanosheets (ZnCo2O4-U) morphologies of ZnCo2O4 material are seeded on Ni foam using hexamethylenetetramine and urea as precipitating agents. These nanosheets on Ni foams consist of large number of mesopores with BET surface areas varying between 88 and 112 m2 g−1. The Ni foams seeded with ZnCo2O4 nanosheets are directly used as electrodes to evaluate their charge storage performance and electrocatalytic activity for hydrogen peroxide reduction in alkaline medium. The interconnected nanosheets exhibit high specific capacitance value of 1102 F g−1 compared to 653 F g−1 of bundle-like nanosheets at a current density of 1 A g−1. The bundle-like nanosheets of zinc cobalt oxide material shows excellent rate capability with 77% capacitance retention at 16 A g−1. Interestingly both showed excellent capacitance retention over 5000 galvanostatic charge-discharge cycles at very high current densities. Furthermore, both the materials are very effective for hydrogen peroxide reduction. In the presence of 0.5 M H2O2 the cathodic current increased by about 100 times for interconnected nanosheets electrode and 30 times for bundle-like nanosheets electrode compared to H2O2 free alkaline solutions. These results consistently show strong correlation between the morphology related properties and electrochemical activity of oxide materials.

Facile synthesis of porous SnO2 quasi-nanospheres for photocatalytic degradation of Rhodamine B

Porous SnO2 quasi-nanospheres were successfully synthesized by a template- and surfactant-free hydrothermal method. The structure, morphology and optical properties were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high solution TEM (HRTEM) and UV–vis diffuse reflectance spectra (DRS). The results showed that the uniform SnO2 quasi-nanospheres with a diameter of ~400nm were prepared on a large scale. Moreover, it was found that such quasi-nanospheres consisted of numerous nanoparticles 4–6nm in diameters with a tetragonal crystal structure. The photocatalytic activity of SnO2 quasi-nanospheres was investigated by degradation Rhodamine B (RhB) under UV–vis light irradiation, which exhibited excellent photocatalytic activity (the degradation of RhB even reached 82% within 60min).

Facile and surfactant-free hydrothermal synthesis of PbS nanoparticles: the role of hydrothermal reaction time

PbS nanoparticles having the suitability for power semiconductor devices, were synthesized by facile, effective, and surfactant-free hydrothermal method. The effect of reaction time on the morphology, microstructure and optical properties of PbS nanoparticles was investigated. The methods of XRD, TEM, HRTEM, EDX, FTIR and UV–VIS photometry measurements were used for PbS nanoparticles characterization. The reaction time was found to have an effective role in controlling the morphology, crystallinity, crystallite size, microstrain and optical band gap of the prepared samples.

Porous thin layered nanosheets assembled ZnCo2O4 grown on Ni-foam as an efficient electrode material for hybrid supercapacitor applications

Thin layered nanosheets of ZnCo2O4 with desirable porous nanoarchitecture grown on Ni-foam were prepared using a simple surfactant free hydrothermal method. The Ni-foam supported ZnCo2O4 nanostructure was characterized using X-ray diffraction (XRD) analysis, field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). FESEM clearly revealed thin nanosheets which were randomly connected together to form a porous structure. The binder-free Ni-foam supported ZnCo2O4 electrode was applied directly to hybrid supercapacitor analysis. In three electrode measurement, thin layered nanosheets of ZnCo2O4 exhibited a maximum specific capacity of 886 C g−1 at a current density of 2 mA cm−2. When the current density was increased from 2 to 50 mA cm−2, the specific capacity value was 362.5 C g−1. After 2000 continuous charge–discharge cycles, about 94% of the maximum specific capacity was retained. This high specific capacity, better rate capacity, and excellent cyclic stability of thin layered nanosheets of ZnCo2O4 suggest that the prepared electrode is a promising candidate for hybrid supercapacitor applications.

Facile synthesis of α-Fe2O3 micro-ellipsoids by surfactant-free hydrothermal method for sub-ppm level H2S detection

The α-Fe2O3 micro-ellipsoids were prepared using a facile hydrothermal process without any surfactant or template, and their morphological, structural and H2S sensing properties were investigated. The α-Fe2O3 showed uniform micro-ellipsoids with a long axis diameter of 1.7μm and a short axis diameter of 1.2μm. Detailed structural analysis confirmed that the synthesized α-Fe2O3 micro-ellipsoids were compact particles with a hexagonal structure. Gas sensor base on the α-Fe2O3 micro-ellipsoids showed excellent response, short response/recovery time (<90s and 30s, respectively), low detection concentration (~0.5ppm), good long-term stability and excellent selectivity towards H2S gas at the optimized operating temperature of 350°C. The sensing mechanism of the sensor based on the α-Fe2O3 micro-ellipsoids towards H2S was discussed.

Hierarchical WO3 nanostructures assembled by nanosheets and their applications in wastewater purification

In this work, WO3 hierarchical structures assembled by many nanosheets have been successfully fabricated through a facile and surfactant-free hydrothermal method. Morphologies and microstructures of the as-synthesized products are examined by scanning electron microscopy (SEM), X-ray powder diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). A possible growth mechanism of the as-obtained WO3 hierarchical structures is proposed by adjusting reaction time and the dosage of Na2SO4. The as-prepared WO3 products are further regarded as a kind of photocatalyst for the photocatalytic degradation of three toxic organic dyes, Methylene Blue (MB), Eosin Red (ER) and Congo Red (CR). In addition, degradation efficiencies of WO3 products with different morphologies towards MB are also investigated, revealing that hierarchical WO3 structures possess preferable photocatalytic performance. It demonstrated that the as-prepared products may be promising application in wastewater purification.

Photocatalytic Escherichia coli inactivation by means of trivalent Er3+, Y3+ doping of BiVO4 system

BiVO4 samples doped with different contents of Er3+ and Y3+ were prepared by a simple surfactant free hydrothermal method. X-ray diffraction reveals that the doped materials consist of a heterogeneous structure formed by a mixture of tetragonal and monoclinic phases, being found Er3+ and Y3+ co-doping clearly stabilize the tetragonal structure of BiVO4. The monoclinic BiVO4 samples shows a strong absorption in the visible light region leading to band-gap values of around 2.4eV while the tetragonal BiVO4 displays higher band-gap values of 2.9eV. The photocatalytic activity of the catalysts was investigated for the oxidation of methanol and inactivation of Escherichia coli showing that all the BiVO4 catalysts are photocatalytically active in the oxidation of methanol and are able to inactivate more than 99.99% of bacteria not only under UV light but also under visible light irradiation. The results revealed that the co-doping of Er3+ and Y3+ into BiVO4 exhibited enhanced photocatalytic activity for methanol oxidation under simulated solar light irradiation. The inactivation of E.coli show similar results for the doped systems although in relative terms of activity the Er3+,Y3+-BiVO4 sample show a better use of the visible light, leading to a higher activity than P25-TiO2.

Fabrication of BiVO4: Effect of structure and morphology on photocatalytic activity and its methylene blue decomposition mechanism

BiVO4 photocatalysts were synthesized by a surfactant free hydrothermal method without any further treatments, and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS), Raman spectroscopy, and Brunauer-Emmett-Teller (BET) surface area techniques. The photocatalytic activity was evaluated for the degradation of the methylene blue (MB) under visible light irradiation. Seen from the structural and morphological characterization, it is stated that the obtained samples present monoclinic phase, and the pH value has significant influence on the morphologies. The enhanced photocatalytic performance was associated with its crystallinity, unique morphology, band gap energy, BET specific surface area, surface charge and adsorption capacity. The recycle experiments results show that the BiVO4 photocatalysts have excellent photo-stability, and we deduced a possible mechanism by examining the effects of the active species involved in the photocatalytic process for MB photocatalytic degradation.

Hydrothermally synthesized Copper Oxide (CuO) superstructures for ammonia sensing

According to environmental protection agencies (EPA), the emission threshold of NH3 in air is 1000kg/yr which is now about 20Tg/yr. Hence, there is a rapid increase in need of NH3 sensors to timely detect and control NH3 emissions. Metal oxide nanostructures such as CuO with special features are potential candidates for NH3 sensing. In the present study, morphology controlled 3-dimensional CuO superstructures were synthesized by surfactant-free hydrothermal method for NH3 detection. In addition to conventional hydrothermal method where water as solvent, a modified approach using a mixture of water and ethylene glycol (EG) was used as solvent to control the growth process. Hierarchical superstructures namely, snowflake-like, flower-like, hollow-sphere-like and urchin-like feature with particle dimensions ranging from 0.3 to 1μm were obtained by varying water/EG ratio and reaction temperature. The synthesized nanostructures exhibited morphology dependent luminescence and gas sensing properties. The surface area and pore distribution determined by BET surface analysis also largely influenced by the presence of EG in the reaction system. The average pore diameter enhanced from 6nm to 14nm by the addition of 10ml EG as solvent. The room temperature ammonia sensing behavior of all samples was studied using an indigenous gas sensing set-up. It was found that hollow-sphere like CuO nanostructures showed a maximum sensitivity of 150% towards 600ppm ammonia with a response and recovery time of 6min. The hydrothermal synthesis strategy reported here has the advantage of producing shape controlled hierarchical materials are highly suitable for various technological applications.

Ultrafast breathing humidity sensing properties of low-dimensional Fe-doped SnO2 flower-like spheres

It is demonstrated that the low-dimensional Fe-doped SnO2 flower-like spheres with enhanced breathing sensing property are synthesized by a simple template- and surfactant-free hydrothermal method.

Core-shell nanoparticles by silica coating of metal oxides in a dual-stage hydrothermal flow reactor.

γ-Fe2O3@SiO2, TiO2@SiO2, and α-Fe2O3@SiO2 core-shell nanoparticles were synthesized by a surfactant-free hydrothermal method in a continuous flow dual-stage reactor. Uniform silica shells of 2 nm thickness were obtained when grown on γ-Fe2O3 and TiO2 nanoparticles while thicker and more irregular shells were deposited on α-Fe2O3 due to low pH.

Controllable synthesis of CeO2 nanoparticles with different sizes and shapes and their application in NO oxidation

This study describes the synthesis of ceria catalysts with octahedron and nanosphere shapes via a facile and surfactant-free hydrothermal method. The morphology of the products could be controlled by adjusting the proportions of the solvent.

Synthesis of hierarchical flower-like SnO2 nanostructures and their photocatalytic properties

SnO2 nanostructures were prepared successfully through a template-free and surfactant-free hydrothermal method. The prepared SnO2 nanostructures were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), Raman spectra and UV–vis absorption spectra. Photocatalytic activity was evaluated using the degradation of organic dye methyl orange. Furthermore, the possible growth and photodegradation mechanism of the as-prepared sample were also briefly discussed.

Self-assembled growth of BiFeO3 meso-octahedral particles synthesized by a facile surfactant-free hydrothermal method

Meso-scale BiFeO3 octahedral particles were synthesized by a hydrothermal method through modulating KOH concentration in the precursor solution. The morphologies of BiFeO3 particles were observed by scanning electron microscopy (SEM), displaying a morphological transformation from sphere to regular octahedron with the increase of KOH concentration. The time-dependent particle growth was also studied by SEM and the final products, octahedral BiFeO3 particles, were characterized by electron backscatter diffraction (EBSD) for understanding the crystalline growth. EBSD results show that the rhombohedral BiFeO3 octahedron is covered by eight crystal plane (001)hex, which is different from the previous report. The possible mechanism of crystal growth is discussed.