Abu ul Hassan Sarwar Rana

Microwave-assisted Facile and Ultrafast Growth of ZnO Nanostructures and Proposition of Alternative Microwave-assisted Methods to Address Growth Stoppage.

The time constraint in the growth of ZnO nanostructures when using a hydrothermal method is of paramount importance in contemporary research, where a long fabrication time rots the very essence of the research on ZnO nanostructures. In this study, we present the facile and ultrafast growth of ZnO nanostructures in a domestic microwave oven within a pressurized environment in just a few minutes. This method is preferred for the conventional solution-based method because of the ultrafast supersaturation of zinc salts and the fabrication of high-quality nanostructures. The study of the effect of seed layer density, growth time, and the solution’s molar concentration on the morphology, alignment, density, and aspect ratio of ZnO nanorods (ZNRs) is explored. It is found in a microwave-assisted direct growth method that ~5 mins is the optimum time beyond which homogeneous nucleation supersedes heterogeneous nucleation, which results in the growth stoppage of ZNRs. To deal with this issue, we propound different methods such as microwave-assisted solution-replacement, preheating, and PEI-based growth methods, where growth stoppage is addressed and ZNRs with a high aspect ratio can be grown. Furthermore, high-quality ZnO nanoflowers and ZnO nanowalls are fabricated via ammonium hydroxide treatment in a very short time.

Growth Method-Dependent and Defect Density-Oriented Structural, Optical, Conductive, and Physical Properties of Solution-Grown ZnO Nanostructures

It is time for industry to pay a serious heed to the application and quality-dependent research on the most important solution growth methods for ZnO, namely, aqueous chemical growth (ACG) and microwave-assisted growth (MAG) methods. This study proffers a critical analysis on how the defect density and formation behavior of ZnO nanostructures (ZNSs) are growth method-dependent. Both antithetical and facile methods are exploited to control the ZnO defect density and the growth mechanism. In this context, the growth of ZnO nanorods (ZNRs), nanoflowers, and nanotubes (ZNTs) are considered. The aforementioned growth methods directly stimulate the nanostructure crystal growth and, depending upon the defect density, ZNSs show different trends in structural, optical, etching, and conductive properties. The defect density of MAG ZNRs is the least because of an ample amount of thermal energy catered by high-power microwaves to the atoms to grow on appropriate crystallographic planes, which is not the case in faulty convective ACG ZNSs. Defect-centric etching of ZNRs into ZNTs is also probed and methodological constraints are proposed. ZNS optical properties are different in the visible region, which are quite peculiar, but outstanding for ZNRs. Hall effect measurements illustrate incongruent conductive trends in both samples.

Least Squares Neural Network-Based Wireless E-Nose System Using an SnO2 Sensor Array

Over the last few decades, the development of the electronic nose (E-nose) for detection and quantification of dangerous and odorless gases, such as methane (CH4) and carbon monoxide (CO), using an array of SnO2 gas sensors has attracted considerable attention. This paper addresses sensor cross sensitivity by developing a classifier and estimator using an artificial neural network (ANN) and least squares regression (LSR), respectively. Initially, the ANN was implemented using a feedforward pattern recognition algorithm to learn the collective behavior of an array as the signature of a particular gas. In the second phase, the classified gas was quantified by minimizing the mean square error using LSR. The combined approach produced 98.7% recognition probability, with 95.5 and 94.4% estimated gas concentration accuracies for CH4 and CO, respectively. The classifier and estimator parameters were deployed in a remote microcontroller for the actualization of a wireless E-nose system.

NH4OH Treatment for an Optimum Morphological Trade-off to Hydrothermal Ga-Doped n-ZnO/p-Si Heterostructure Characteristics

Previous studies on Ga-doped ZnO nanorods (GZRs) have failed to address the change in GZR morphology with increased doping concentration. The morphology-change affects the GZR surface-to-volume ratio and the real essence of doping is not exploited for heterostructure optoelectronic characteristics. We present NH4OH treatment to provide an optimum morphological trade-off to n-GZR/p-Si heterostructure characteristics. The GZRs were grown via one of the most eminent and facile hydrothermal method with an increase in Ga concentration from 1% to 5%. The supplementary OH− ion concentration was effectively controlled by the addition of an optimum amount of NH4OH to synchronize GZR aspect and surface-to-volume ratio. Hence, the probed results show only the effects of Ga-doping, rather than the changed morphology, on the optoelectronic characteristics of n-GZR/p-Si heterostructures. The doped nanostructures were characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, photoluminescence, Hall-effect measurement, and Keithley 2410 measurement systems. GZRs had identical morphology and dimensions with a typical wurtzite phase. As the GZR carrier concentration increased, the PL response showed a blue shift because of Burstein-Moss effect. Also, the heterostructure current levels increased linearly with doping concentration. We believe that the presented GZRs with optimized morphology have great potential for field-effect transistors, light-emitting diodes, ultraviolet sensors, and laser diodes.

Recent Advances in Metal Chalcogenides (MX; X = S, Se) Nanostructures for Electrochemical Supercapacitor Applications: A Brief Review

Supercapacitors (SCs) have received a great deal of attention and play an important role for future self-powered devices, mainly owing to their higher power density. Among all types of electrical energy storage devices, electrochemical supercapacitors are considered to be the most promising because of their superior performance characteristics, including short charging time, high power density, safety, easy fabrication procedures, and long operational life. An SC consists of two foremost components, namely electrode materials, and electrolyte. The selection of appropriate electrode materials with rational nanostructured designs has resulted in improved electrochemical properties for high performance and has reduced the cost of SCs. In this review, we mainly spotlight the non-metallic oxide, especially metal chalcogenides (MX; X = S, Se) based nanostructured electrode materials for electrochemical SCs. Different non-metallic oxide materials are highlighted in various categories, such as transition metal sulfides and selenides materials. Finally, the designing strategy and future improvements on metal chalcogenide materials for the application of electrochemical SCs are also discussed.

NH4OH-Oriented and pH-Dependent Growth of ZnO Nanostructures via Microwave-Assisted Growth Method

We proffer NH4OH-oriented and pH-dependent growth of ZnO nanostructures via a microwaveassisted growth method. The fabrication of ZnO nanorods (ZNRs), nanoflowers (ZNFs), nanostars (ZNSs), and nanotetrapods (ZNTs) is presented. NH4OH was used as a mineralizer to change the solution pH for nanostructure growth, where temperature and other variables were fixed. Because of an efficient heat transfer and facile growth of nanostructures, a domestic microwave oven was used to facilitate the nanostructure growth in the span of just 10-15 min. The results showed that the growth of ZnO nanostructures was dependent upon the number of growth units and ZnO nuclei present in the solution, which ultimately depend upon the pH of the solution. At the outset, without the addition of NH4OH, the pH of the solution was ~6.8 and the ZNRs were formed in the solution or on a seeded substrate which persisted in the pH range of ~6.8-9. An abrupt change in the shapes and the types of the nanostructures was observed when the pH was boosted beyond 10. A transition from ZNRs to ZNFs was observed at pH 10 and ZNFs were formed at pH 11. The solution gave birth to ZNSs and ZNTs when the pH was further raised to 12 and 13, respectively.

High-Speed Growth of ZnO Nanorods in Preheating Condition Using Microwave-Assisted Growth Method

In this study, we present the microwave-assisted growth (MAG) of ZnO nanorods (ZNRs) using a preheating hydrothermal method under tailored preheating and postheating growth conditions. The perimeters such as solution concentration, preheating time, and postheating time, were changed to optimize ZNR growth and the growth was carried out in a domestic 850 watt microwave oven. Preheated solution was utilized as an accelerator to increase the aspect ratio of the ZNRs and reduce the fabrication time. Because of a long fabrication time and limited length in the conventional MAG method, preheating condition was used for efficient growth of nanorods through homogeneous nucleation in the solution and then heterogeneous nucleation of the formed ZNRs on seeded substrate during postheating process. The nanostructures were characterized with scanning electron microscopy to look at the morphology and dimensions. Dimensions of ZNRs kept on increasing as the molar concentration went higher. Preheating time highly affected the morphology, dimensions, and aspect ratio of ZNRs and postheating time not only ensured the stability of ZNRs with substrate due to heterogeneous nucleation process but also influenced the morphology of ZNRs.