Shahid Khan

A two-step synthesis of nanosheet-covered fibers based on α -Fe 2 O 3 /NiO composites towards enhanced acetone sensing

A novel hierarchical heterostructures based on α-Fe2O3/NiO nanosheet-covered fibers were synthesized using a simple two-step process named the electrospinning and hydrothermal techniques. A high density of α-Fe2O3 nanosheets were uniformly and epitaxially deposited on a NiO nanofibers. The crystallinity, morphological structure and surface composition of nanostructured based on α-Fe2O3/NiO composites were investigated by XRD, SEM, TEM, EDX, XPS and BET analysis. The extremely branched α-Fe2O3/NiO nanosheet-covered fibers delivered an extremely porous atmosphere with huge specific surface area essential for superior gas sensors. Different nanostructured based on α-Fe2O3/NiO composites were also explored by adjusting the volume ratio of the precursors. The as-prepared samples based on α-Fe2O3/NiO nanocomposite sensors display apparently enhanced sensing characteristics, including higher sensing response, quick response with recovery speed and better selectivity towards acetone gas at lower operating temperature as compared to bare NiO nanofibers. The sensing response of S-2 based α-Fe2O3/NiO nanosheet-covered fibers were 18.24 to 100 ppm acetone gas at 169 °C, which was about 6.9 times higher than that of bare NiO nanofibers. The upgraded gas sensing performance of composites based on α-Fe2O3/NiO nanosheet-covered fibers might be ascribed to the exclusive morphologies with large surface area, p-n heterojunctions and the synergetic performance of α-Fe2O3 and NiO.

Study of annealing effect upon the structural and solar- selective properties of C/Ni/NiO composite coatings prepared by sol–gel method

AbstractThe Ni/C/NiO composite thin films have been prepared by one-step sol–gel method. The prepared samples were annealed at 450 °C with different holding times. When the metallic nickel nanoparticles were homogeneously dispersed in the composite, it is observed that both NiO and metallic Ni are surrounded by resident carbon. Comprehensive analysis techniques have been carried out in order to analyze the structural, optical, and solar- selective absorber properties of the composite. The main structure variations with various holding times are (i) the decrease in thickness and (ii) enhancement of metallic Ni nanoparticles size. The solar-selective absorber properties for one-layer Ni/C/NiO composite film annealed at 450 °C with 10 min of holding time show the optimal performance in the present work, exhibiting an absorbance of about 0.80 and an emittance of about 0.11, and capable of a good candidate as SSA for the low–middle temperature applications.

Carbonaceous nickel oxide composite films with homogenously dispersed metallic Ni nanoparticles by a facile sol–gel method

AbstractA composite of Ni/C/NiO thin films, in which the metallic nickel nanoparticles (MNNs) are homogeneously dispersed, has been prepared by a facile one-step sol–gel technique as a solar-selective absorber (SSA). Comprehensive analysis of the as-prepared composite films has been carried out employing various characterization techniques. The results reveal that both metallic Ni and NiO nanoparticles are surrounded by the resident carbon and the MNNs emerge from the NiO precursor reduced by the resident carbon. The size of the homogeneously dispersed MNNs increases with the annealing temperature from ~4 to ~7 nm. A HCP–FCC phase transition of MNNs was found at about 400–450 °C, while the contents of Ni and Ni2+ are almost equal at the same time. The solar-selective absorptance properties of Ni/C/NiO composite films were studied by measuring the reflectance in the UV-Vis and infrared wavelength range. The results indicate that the smaller MNNs and the thinner films attenuate the absorptance but enhance the emittance. The one-layer Ni/C/NiO composite film on copper substrate annealed at 450 °C is preferred in the present work, exhibiting an absorptance of about 0.86 and an emittance of about 0.11, and capable of a good candidate as SSA for the low–middle temperature applications.

Marangoni effect induced macro porous surface films prepared through a facile sol-gel route

Based on TiO2 as a model system, the sol-gel one step facile method is established to fabricate the macro-porous morphology films on the basis of Marangoni effect. In this proposed mechanism, the binary mixture of hydrophilic CuCl2 and lipophilic Ti-O network is used in sol to produce phase separation. A suitable evaporation rate in the gel film process leads to the macro-porous film due to Marangoni effect. It is observed that the macro-porous morphology of the film sustains during the annealing process, which suggests the creation of porous surface morphology in gel film stage rather than due to annealing. To analyze the preparation mechanism, the sol-gel process and microstructure of films are examined using TG-DTA, SEM, TEM, XRD, Raman, UV-Vis, XPS and FTIR. Furthermore, the optical-thermal properties are studied for the potential applications of such porous surface films as solar selective absorber.