Quickest single-step one pot mechanosynthesis and characterization of ZnTe quantum dots

ZnTe quantum dots (QDs) are synthesized at room temperature in a single step by mechanical alloying the stoichiometric equimolar mixture (1:1 mol) of Zn and Te powders under Ar within 1 h of milling. Both XRD and HRTEM characterizations reveal that these QDs having size ∼5 nm contain stacking faults of different kinds. A distinct blue-shift in absorption spectra with decreasing particle size of QDs confirms the quantum size confinement effect (QSCE). It is observed for first time that the QDs with considerable amount of faults can also show the QSCE. Optical band gaps of these QDs increase with increasing milling time and their band gaps can be fine-tuned easily by varying milling time of QDs.     

A facile method to synthesize high-quality ZnS(Se) quantum dots for photoluminescence

Colloidal zinc sulfide (ZnS) quantum dots are synthesized by a solvothermal route from Zn(Ac)₂·2H₂O, sulfur powder and oleylamine at 120-240 °C. Microstructural, morphological, and optical properties of the as-synthesized ZnS quantum dots are characterized by X-ray diffraction analysis, transmission electron microscopy, UV-vis absorption spectroscopy, and photoluminescence spectroscopy. Results indicate that the obtained ZnS quantum dots distribute uniformly, the particle size is in the range between 1.7 nm and 3.1 nm, and the band gap decreases from 4.16 eV to 3.90 eV with an increase of the particle size. The size-dependent photoluminescence exhibits a strongly broadened peak accompanied by a pronounced blue-shift. It is also found that the size of the ZnS nanocrystals can be effectively controlled by adjusting synthesis temperature. It is shown that the present method is also applicable to synthesize other binary II-VI semiconductor materials, such as ZnSe quantum dots.     

Synthesis, structural and magnetic characterization of nanocrystalline nickel ferrite—NiFe2O4 obtained by reactive milling

Nanocrystalline nickel ferrite (NiFe₂O₄) has been synthesized from a stoichiometric mixture of oxides NiO and α-Fe₂O₃ in a high energy planetary mill. An annealing at 350 °C, after milling, was used to improve the solid state reaction. The obtained powders were investigated by X-ray diffraction, magnetic measurements, scanning electron microscopy, X-ray microanalysis and differential scanning calorimetry. The particles size distribution was analyzed using a laser particle size analyser. The nickel ferrite begins to form after 4 h of milling and continuously form up to 16 h of milling. The obtained nickel ferrite has many inhomogeneities and a distorted spinel structure. The mean crystallites size at the final time of milling is 9 ± 2 nm and the lattice parameter increases with increase the milling time. DSC measurements revealed a large exothermic peak associated with cations reordering in the crystalline structure. The magnetization of the obtained powder depends on the milling time and annealing. After the complete reaction between the starting oxides the milling reduces the magnetization of the samples. The magnetization increases after annealing, due to the reorganization of the cations into the spinel structure.     

Mechanochemical solid state synthesis of (Cd0.8Zn0.2)S quantum dots: Microstructure and optical characterizations

(Cd_0.8Zn_0.2)S quantum dots with a mixture of both cubic (Zinc-blende) and hexagonal (Wurtzite) phases have been prepared within 75 min by mechanical alloying the stoichiometric mixture of Cd, Zn and S powders at room temperature in a planetary ball mill under Ar. The Rietveld analysis of X-ray powder diffraction data reveals relative phase abundances of both cubic and hexagonal phases and several microstructure parameters like lattice parameters, particle sizes, lattice strains, concentrations of different kinds of stacking faults, etc. in both the phases. At the time of formation, hexagonal phase dominates over the cubic phase (molar ratio ∼0.6:0.4), but in course of milling up to 15 h, the hexagonal phase partially transforms to cubic phase and the molar ratio becomes ∼0.4:0.6. Particle sizes of hexagonal and cubic phases reduce to ∼4.5 nm and 12.5 nm, respectively, after 15 h of milling. The hexagonal phase contains a significant amount of lattice strain in comparison to cubic phase. The presence of different kinds of stacking faults is revealed clearly from the high resolution transmission electron microscope (HRTEM) images.     

Study of bactericidal efficiency of magnetron sputtered TiO2 films deposited at varying oxygen partial pressure

TiO₂ thin films have been deposited at different Ar:O₂ gas ratios (20:80,70:30,50:50,and 40:60 in sccm) by rf reactive magnetron sputtering at a constant power of 200 W. The formation of TiO₂ was confirmed by X-ray photoelectron spectroscopy (XPS). The oxygen percentage in the films was found to increase with an increase in oxygen partial pressure during deposition. The oxygen content in the film was estimated from XPS measurement. Band gap of the films was calculated from the UV-Visible transmittance spectra. Increase in oxygen content in the films showed substantial increase in optical band gap from 2.8 eV to 3.78 eV. The Ar:O₂ gas ratio was found to affect the particle size of the films determined by a transmission electron microscope (TEM). The particle size was found to be varying between 10 and 25 nm. The bactericidal efficiency of the deposited films was investigated using Escherichia coli (E. coli) cells under 1 h UV irradiation. The growth of E. coli cells was estimated through the Optical Density measurement by UV-Visible absorbance spectra. The qualitative analysis of the bactericidal efficiency of the deposited films after UV irradiation was observed through SEM. A correlation between the optical band gap, particle size and bactericidal efficiency of the TiO₂ films at different argon:oxygen gas ratio has been studied.     

Cu2ZnSnS4 synthesized through a green and economic process

Quaternary chalcogenide Cu₂SnZnS₄ (CZTS), a promising absorber material in solar cells, was demonstrated synthesizable through self-sustaining reactions by using an environmental friendly and cost-effective mechanochemical (ball milling) process from elemental Cu, Zn, Sn, and S, without using either polluting chemicals or expensive vacuum facilities. Transmission electron microscopy, X-ray diffraction, and Raman spectroscopy and X-ray photoelectron spectroscopy confirmed CZTS formation. CZTS could be formed after planetary ball milling the starting mixture of Cu, Zn, Sn, and S for 20 h. The grain size of CZTS decreased with a further increase in ball milling time.     

Formation of ZnO nanorod arrays on polytetraflouroethylene (PTFE) via a seeded growth low temperature hydrothermal reaction

ZnO nanorod arrays were formed by a low temperature hydrothermal process on seeded polytetraflouroethylene (PTFE) sheets. The seed layer was formed using thermal oxidation of a thin evaporated Zn film on the PTFE sheet at 300 °C in air for 10 min. The formation of ZnO nanorod arrays in the hydrothermal reactive bath consisting of hexamethylamine (HMT) and Zn ions occurred via the reaction of hydroxyl ions released during the thermal degradation of HMT with the Zn ions. The seed layer provided a template for the nucleation of the ZnO and HMT which also acted as a chelating agent that promoted growth of the ZnO along the c-axis, leading to the formation of exclusively (0 0 2) ZnO nanorods. The effect of exposure time of the seeded PTFE to the reactive solution on the formation of the nanorods was investigated. Well aligned, relatively uniform tapered 300 nm long nanorods can be formed after 8 h of exposure. Longer exposure times to 24 h resulted in the formation of more uniform nanorods with base diameter averaged of ∼100 nm and the tip diameter of ∼50 nm. XRD analysis showed that the ZnO nanorod array had a hexagonal wurtzite structure. This result is in agreement with HR-TEM observations and Raman scattering analysis. Photoluminescence study showed that a strong UV emission peak was obtained at 380 nm and a small peak at 560 nm, which is associated with green emission. The optical band gap measured from these plots was at 3.2 eV on average.     

Spark plasma sintering consolidation of nanostructured TiC prepared by mechanical alloying

Spark plasma sintering technique was used for the consolidation of nanostructured titanium carbide synthesized by mechanical alloying in order to avoid any important grain growth of the compact materials. The TiC phase was obtained after about 2 h of mechanical alloying. Towards the end of the milling process (20 h), the nanocrystalline powders reached a critical size value of less than 5 nm. Some physical and mechanical properties of the consolidated carbide were reported as a function of the starting grain size powders obtained after different mechanical alloying durations. The crystalline grain size of the bulk samples was found to be increased to a maximum of 120 nm and 91 nm for carbides mechanically alloyed for 2 h and 20 h respectively. The Vickers hardness showed to be improved to about 2700 Hv for a maximum density of 95.1% of the bulk material.     

Nanobeads of zinc oxide with rhodamine B dye as a sensitizer for dye sensitized solar cell application

Cost effective, ruthenium metal free rhodamine B dye has been chemically adsorbed on ZnO films consisting of nanobeads to serve as a photo anode in dye sensitized solar cells. These ZnO films were chemically synthesized at room temperature (27 °C) on to fluorine doped tin oxide (FTO) coated glass substrates followed by annealing at 200 °C. These films consisting of inter connected nanobeads (20-40 nm) which are due to the agglomeration of very small size particles (3-5 nm) leading to high surface area. The film shows wurtzite structure having high crystallinity with optical direct band gap of 3.3 eV. Optical absorbance measurements for rhodamine B dye covered ZnO film revealed the good coverage in the visible region (460-590 nm) of the solar spectrum. With poly-iodide liquid as an electrolyte, device exhibits photon to electric energy conversion efficiency (η) of 1.26% under AM 1.5G illumination at 100 mW/cm².     

Effect of Co substitution on the structural and optical properties of ZnO nanoparticles synthesized by sol-gel route

Co doped ZnO nanoparticles were synthesized by sol-gel method and characterized by X-ray diffraction (XRD), Transmission electron microscopy (TEM), Energy dispersive X-ray analysis (EDAX), UV-Visible absorption spectroscopy and Fourier transform infrared spectroscopy (FTIR). XRD analysis revealed the formation of single phase structure of all samples which was further supported by FTIR data. With the increase in Co concentration from 0% to 5%, crystallite size was observed to vary from 27.1 to 21.3 nm. It suggests the prevention of crystal growth as a result of Co doping in ZnO. It was also evident from the absorption spectra that the absorbance tends to increase with the increase in dopant concentration. Optical band gap was found to increase slightly with the increase in Co content, confirming the size reduction as a result of Co doping.     

Optical and Raman scattering studies on SnS nanoparticles

Tin sulfide nanoparticles were synthesized through wet chemical route. Structure and phase purity were confirmed by powder XRD. Morphology and size were identified from TEM and AFM. The room temperature photoluminescence spectrum shows the band edge emission at 1.57 eV. The direct and indirect band gaps are estimated from UV-vis-NIR absorption spectrum as 1.78 and 1.2 eV, respectively. Blue shift of 0.48 eV observed for direct transition and 0.2 eV for indirect transition as compared to bulk band gap is due to quantum confinement effect. The Raman spectrum of SnS nanoparticles shows all the predicted Raman modes which show shift towards lower wave number side in comparison with those of the SnS single crystal. This is attributed to phonon confinement.     

Synthesis behavior of nanocrystalline Al-Al2O3 composite during low time mechanical milling process

In this work, four different volume fractions of Al₂O₃ (10, 20, 30 and 40 vol.%) were mixed with the fine Al powder and the powder blends were milled for 5 h. Scanning electron microscopy analysis, particle size analysis and bulk density measurements were used to investigate the morphological changes and achieving the steady state conditions. The results showed that increasing the Al₂O₃ content can provide the steady state particle size in 5 h milling process. It was found that increasing the volume fraction of Al₂O₃ leads to increasing the uniformity of Al₂O₃. Standard deviations of microhardness measurements confirmed this result. The XRD pattern and XRF investigations depicted that increasing the Al₂O₃ content causes an increase in the crystal defects, micro-strain and Fe contamination during 5 h milling process of nanocrystalline composite powders while the grain size is decreased. To investigate the effect of milling time, Al-30 vol.% Al₂O₃ (which achieved steady state during 5 h milling process) was milled for 1-4 h. The results depicted that the milling time lower than 5 h, do not achieve to steady state conditions.     

A novel technique for production of nano-crystalline mono tungsten carbide single phase via mechanical alloying

Due to simultaneous synthesis of WC and W₂C phases in most of the synthesis processes and lower mechanical properties of W₂C than WC, in this work the possibility of production of nano-crystalline WC single phase as a useful refractory ceramic by means of mechanical alloying has been investigated. The raw materials containing W and C with WC were milled in a planetary ball mill. The sampling has been done in different times. As it was expected, XRD studies showed that after 75 h of milling the WC with W₂C were produced. By adding WC to the raw materials in the beginning of the process it led to the fact that after 50 h of milling WC was synthesized only without any other phases which remained stable at the higher times while milling. During broadening of XRD peaks, the size of synthesized crystalline WC was estimated in the order of nano-meter. Crystalline size and mean strain of synthesized WC in the system without additive were higher and lower than the system containing WC, respectively.     

Phase transition of Ni-Mn-Ga alloy powders prepared by vibration ball milling

This study investigated the phase transformation of the flaky shaped Ni-Mn-Ga powder particles with thickness around 1 μm prepared by vibration ball milling and post-annealing. The SEM, XRD, DSC and ac magnetic susceptibility measurement techniques were used to characterize the Ni-Mn-Ga powders. The structural transition of Heusler → disordered fcc occurred in the powders prepared by vibration ball milling (high milling energy) for 4 h, which was different from the structural transition of Heusler → disordered fct of the powders fabricated by planetary ball milling (low milling energy) for 4 h. The two different structures after ball milling should be due to the larger lattice distortion occurred in the vibration ball milling process than in the planetary ball milling process. The structural transition of disordered fcc → disordered bcc took place at ∼320 °C during heating the as-milled Ni-Mn-Ga powders, which was attributed to the elimination of lattice distortion caused by ball milling. The activation energy for this transition was 209 ± 8 kJ/mol. The Ni-Mn-Ga powder annealed at 800 °C mainly contained Heusler austenite phase at room temperature and showed a low volume of martensitic transformation upon cooling. The inhibition of martensitic transformation might be attributed to the reduction of grain size in the annealed Ni-Mn-Ga particles.     

Microstructural and mechanical properties of Al-Mg/Al2O3 nanocomposite prepared by mechanical alloying

The effect of milling time on the microstructure and mechanical properties of Al and Al-10 wt.% Mg matrix nanocomposites reinforced with 5 wt.% Al₂O₃ during mechanical alloying was investigated. Steady-state situation was occurred in Al-10Mg/5Al₂O₃ nanocomposite after 20 h, due to solution of Mg into Al matrix, while the situation was not observed in Al/5Al₂O₃ nanocomposite at the same time. For the binary Al-Mg matrix, after 10 h, the predominant phase was an Al-Mg solid solution with an average crystallite size 34 nm. Up to 10 h, the lattice strain increased to about 0.4 and 0.66% for Al and Al-Mg matrix, respectively. The increasing of lattice parameter due to dissolution of Mg atom into Al lattice during milling was significant. By milling for 10 h the dramatic increase in microhardness (155 HV) for Al-Mg matrix nanocomposite was caused by grain refinement and solid solution formation. From 10 to 20 h, slower rate of increasing in microhardness may be attributed to the completion of alloying process, and dynamic and static recovery of powders.     

Structural, optical and transport properties of Al doped BiFeO3 nanopowder synthesized by solution combustion method

Aluminum doped Bismuth ferrite (BFO) nanopowders (grain size 13-20 nm) having composition Bi_1−xAl_2xFe_1−xO₃ (x = 0.00, 0.025, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30) were successfully synthesized by solution combustion method using citric acid as fuel at a temperature as low as 200 °C. As-prepared samples were examined by powder XRD for phase identification and crystallite size determination. The d.c. resistivity as a function of temperature was measured by standard two probe setup which exhibits clear metal to insulator transition for all samples. FTIR analysis was carried out to identify the chemical bonds present in the system. The optical band gap was calculated from the UV-vis absorbance spectra using classical Tauc relation which was found to vary from 2.78 eV to 2.93 eV for different Al³⁺ concentrations. The activation energies calculated from the slopes of ln(ρ) versus 10³/T plots are in the range 0.54-0.73 eV.     

Mechanochemical synthesis of nanocrystalline ZnWO4 at room temperature

Single nanocrystalline ZnWO₄ powders were successfully synthesized by ball milling at room temperature. A stoichiometric mixture of ZnO and WO₃ in a 1:1 molar ratio was subjected to intense mechanical treatment in air using a planetary ball mill (Fritsch - Premium line - Pulversette No. 7) for a period varying from 5 to 300 min. The influence of the four different milling conditions was investigated on the formation of ZnWO₄. The products obtained were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Brunauer-Emmer-Teller (BET) surface area, infrared (IR) spectroscopy and X-ray photoelectron spectroscopy (XPS). The synthesis of ZnWO₄ powder started after 5 min milling time and finished after 30 min milling time at a higher speed (1000 rpm). The mechanical treatment up to 300 min did not lead to phase and structure change of ZnWO₄. The product obtained contained nanoparticles with a size of about 50 nm. The photocatalytic activity of the ZnWO₄ powders obtained was investigated by degradation of a model aqueous solution of Malachite Green (MG) upon UV-light irradiation.     

The synthesis of novel ZnO microcrystals through a simple solvothermal method and their optical properties

ZnO microcrystals with novel structures have been synthesized by a solvothermal method that is facile, low-cost and environment-friendly. Zn(NO₃)₂·6H₂O is the only precursor and absolute ethanol is the solvent. By controlling the reaction time, temperature and molarity of zinc nitrate, ZnO entities with the shape of flower, nut, hexagon-pillar, popcorn, brush and sphere can be synthesized in high selectivity. The ZnO micronuts (length ∼8 μm and width ∼5 μm) are uniform in morphology, displaying an open gap on the surface that divides the body into two. The investigation on the optical properties of the ZnO microcrystals reveals that all the ZnO samples exhibit an excitonic absorption edge around 376 nm, and compared to bulk ZnO, there is a modest red shift of ∼6 nm that can be ascribed to size effect as well as the unique morphologies of the ZnO microcrystals.     

Thixoforming of AA 2017 aluminum alloy composites

A comparative evaluation has been carried out on the microstructure of aluminum based SiC and Al₂O₃ particle reinforced composites produced by semi-solid direct squeeze forming of composite powder at temperatures of 635-645 °C. The study is focused on the distribution of the reinforcement and the intermetallic phases, the porosity content, the microstructure of the matrix phase, the interfacial state and mechanical properties. The particle size of the reinforcements, the time of the high-energy ball milling procedure for the fabrication of composite powder and the semi-solid forming temperature had a strong influence on the quality of sample in terms of distribution of reinforcement and interfacial interaction. Ball milling improves the interface formation between reinforcement and matrix and influences the remelting behaviour. Increasing ball milling time and decreasing semi-solid forming temperature with isothermal holding time resulted in relatively homogenous microstructures and in a reduced amount of interaction between SiC and metal matrix. Best results were obtained for 5 vol.% SiC_p composites after 3 h ball milling, semi-solid formed at 635 °C and held for 10 min.     

Facile hydrothermal synthesis of CeO2 nanosheets with high reactive exposure surface

CeO₂ nanosheets with (1 1 0) dominated surface were synthesized for the first time by a facile one-step hydrothermal method without the assistance of any surfactant or template. The role of NH₃·H₂O on tailoring the morphology of CeO₂ nanocrystals was investigated. It was found the NH₃·H₂O not only serves as the precipitant, but also acts as structural direction agent in the formation of (1 1 0)-dominated CeO₂ nanosheets. Raman and XRD spectra showed that the sample has a cubic fluorite structure. Compared with bulk CeO₂ materials, the prepared CeO₂ nanosheets exhibit an obvious blue-shift in UV absorbance. The increase of the direct band gap energy of the obtained sample exceeds 8%. This method provides an environmentally friendly way for preparing CeO₂ nanostructures and tailoring their morphology. It may also be extended to the synthesis of other nanomaterials.