characterization and analysis of &#945;-ni(oh)2 nanosheets

Microwave Synthesis and Characterization of Nickel Hydroxide Nanosheets

Nickel hydroxide, Ni(OH)2, is used for numerous applications including nickel-zinc and nickel-metal hydride batteries1,2,3,4, fuel cells4, water electrolyzers4,5,6,7,8,9, supercapacitors4, photocatalysts4, anion exchangers10, and many other analytical, electrochemical, and sensor applications4,5. Ni(OH)2 has two predominant crystal structures: &#946;-Ni(OH)2 and &#945;-Ni(OH)211. &#946;-Ni(OH)2 adopts a brucite-type Mg(OH)2 crystal structure, while &#945;-Ni(OH)2 is a turbostratically layered form of &#946;-Ni(OH)2 intercalated with residual anions and water molecules from the chemical synthesis4. Within &#945;-Ni(OH)2, the intercalated molecules are not within fixed crystallographic positions but have a degree of orientational freedom, and also function as an interlayer glue stabilizing the Ni(OH)2 layers4,12. The interlayer anions of &#945;-Ni(OH)2 affect the average Ni oxidation state13 and influence the electrochemical performance of &#945;-Ni(OH)2 (relative to &#946;-Ni(OH)2) toward battery2,13,14,15, capacitor16, and water-electrolysis applications17,18.
Ni(OH)2 can be synthesized by chemical precipitation, electrochemical precipitation, sol-gel synthesis, or hydrothermal/solvothermal synthesis4. Chemical precipitation and hydrothermal synthesis routes are widely utilized in the production of Ni(OH)2, and different synthetic conditions alter the morphology, crystal structure, and electrochemical performance. The chemical precipitation of Ni(OH)2 involves adding a highly basic solution to an aqueous nickel (II) salt solution. The phase and crystallinity of the precipitate are determined by the temperature and&#160;identities&#160;and concentrations of the nickel (II) salt and&#160;basic solution used4.
Hydrothermal synthesis of Ni(OH)2 involves heating an aqueous solution of precursor nickel (II) salt in a pressurized reaction vial, allowing the reaction to proceed at higher temperatures than ordinarily allowed under ambient pressure4. Hydrothermal reaction conditions typically favor &#946;-Ni(OH)2, but &#945;-Ni(OH)2 can be synthesized by (i) using an intercalation agent, (ii) using a non-aqueous solution (solvothermal synthesis), (iii) lowering the reaction temperature, or (iv) including urea in the reaction, resulting in ammonia-intercalated &#945;-Ni(OH)24. The hydrothermal synthesis of Ni(OH)2 from nickel salts occurs via a two-step process that involves a hydrolysis reaction (equation 1) followed by an olation condensation reaction (equation 2).19
[Ni(H2O)N]2+ + hH2O &#8596; [Ni(OH)h(H2O) N-h](2-h)++ hH3O+ (1)
Ni-OH + Ni-OH2 Ni-OH-Ni + H2O (2)
Microwave chemistry has been used for the one-pot synthesis of a wide variety of nanostructured materials and is based on the ability of a specific molecule or material to convert microwave energy into heat20. In conventional hydrothermal reactions, the reaction is initiated by the direct absorption of heat through the reactor. In contrast, within microwave-assisted hydrothermal reactions, the heating mechanisms are dipolar polarization of the solvent oscillating in a microwave field and ionic conduction generating localized molecular friction20. Microwave chemistry can increase the reaction kinetics, selectivity, and yield of chemical reactions20, making it of significant interest for a scalable, industrially viable method to synthesize Ni(OH)2.
For alkaline battery cathodes, the &#945;-Ni(OH)2 phase provides improved electrochemical capacity compared with the &#946;-Ni(OH)2 phase13, and synthetic methods to synthesize &#945;-Ni(OH)2 are of particular interest. &#945;-Ni(OH)2 has been synthesized by a variety of microwave-assisted methods, which include microwave-assisted reflux21,22, microwave-assisted hydrothermal techniques23,24, and microwave-assisted base-catalyzed precipitation25. The inclusion of urea within the reaction solution significantly influences the reaction yield26, mechanism26,27, morphology, and crystal structure27. The microwave-assisted decomposition of urea was determined to be a critical component for obtaining &#945;-Ni(OH)227. Water content in an ethylene glycol-water solution has been shown to impact the morphology of microwave-assisted synthesis of &#945;-Ni(OH)2 nanosheets24. The reaction yield of &#945;-Ni(OH)2, when synthesized by a microwave-assisted hydrothermal route using an aqueous nickel nitrate and urea solution, was found to depend on the solution pH26. A prior study of microwave synthesized &#945;-Ni(OH)2 nanoflowers using a precursor solution of EtOH/H2O, nickel nitrate, and urea found that temperature (in the range of 80-120 &#176;C) was not a critical factor, provided the reaction is conducted above the urea hydrolysis temperature (60 &#176;C)27. A recent paper that studied the microwave synthesis of Ni(OH)2 using a precursor solution of nickel acetate tetrahydrate, urea, and water found that at a temperature of 150 &#176;C, the material contained both &#945;-Ni(OH)2 and &#946;-Ni(OH)2 phases, which indicates that temperature can be a critical parameter in the synthesis of Ni(OH)228.
Microwave-assisted hydrothermal synthesis can be used to produce high-surface area &#945;-Ni(OH)2 and &#945;-Co(OH)2 by using a precursor solution composed of metal nitrates and urea dissolved in an ethylene glycol/H2O solution12,29,30,31. Metal-substituted &#945;-Ni(OH)2 cathode materials for alkaline Ni-Zn batteries were synthesized using a scaled-up synthesis designed for a large-format microwave reactor12. Microwave-synthesized &#945;-Ni(OH)2 was also used as a precursor for obtaining &#946;-Ni(OH)2 nanosheets12, nickel-iridium nanoframes for oxygen evolution reaction (OER) electrocatalysts29, and bifunctional oxygen electrocatalysts for fuel cells and water electrolyzers30. This microwave reaction route has also been modified to synthesize Co(OH)2 as a precursor for cobalt-iridium nanoframes for acidic OER electrocatalysts31 and bifunctional electrocatalysts30. Microwave-assisted synthesis was also used to produce Fe-substituted &#945;-Ni(OH)2 nanosheets, and the Fe substitution ratio alters the structure and magnetization32. However, a step-by-step procedure for microwave synthesis of &#945;-Ni(OH)2 and the evaluation of how varying reaction time and temperature within a water-ethylene glycol solution affects the crystalline structure, surface area, and porosity, and local environment of interlayer anions within the material&#160;has not been previously reported.
This protocol establishes procedures for high-throughput microwave synthesis of &#945;-Ni(OH)2 nanosheets using a rapid and scalable technique. The effect of reaction temperature and time were varied and evaluated using in situ reaction monitoring, scanning electron microscopy, energy dispersive X-ray spectroscopy, nitrogen porosimetry, powder X-ray diffraction (XRD), and Fourier transform infrared spectroscopy to understand the effects of synthetic variables on reaction yield, morphology, crystal structure, pore size, and local coordination environment of &#945;-Ni(OH)2 nanosheets.

Author Spotlight: A Rapid, Microwave-Assisted Hydrothermal Synthesis Of Nickel Hydroxide Nanosheets 

Effect of Microwave Synthesis Conditions on the Structure of Nickel Hydroxide Nanosheets

Watch this Scientific Journal Video about Characterization and Analysis of α-NiOH2 Nanosheets at JoVE.com

Characterization and Analysis of &#945;-Ni(OH)2 Nanosheets  

Characterization and Analysis of &#945;-Ni(OH)2 Nanosheets

To characterize the morphology and composition of the microwave synthesized nickel hydroxide nanosheets by SEM and EDS, prepare the sample by suspending a small amount of nickel hydroxide powder in one milliliter of ethanol using a water bath sonicator. Then drop cast the nickel hydroxide and ethanol mixture on an SEM stub and evaporate the ethanol by heating the stub in a sample oven at 70 degrees Celsius before collecting the SEM micrographs and EDS spectra. In order to determine the surface area and porosity of the nanosheets, add 25 milligrams of nickel hydroxide nanosheets into the sample tube.Carry out a pre-analysis degassing and drying procedure under vacuum at 120 degrees Celsius for 16 hours. Lastly, transfer the sample tube from the degassing port to the analysis port to collect nitrogen isotherms. For structural analysis using X-ray diffraction or XRD, fill the sample well of a zero-background powder XRD holder with nickel hydroxide.Collect powder X-ray diffractograms using a copper K-alpha radiation source between a 2-theta of 2 to 80 degrees with a 0.01 step increment. Equip the attenuated total reflectance or ATR attachment to the Fourier Transform Infrared or FTIR spectrometer. Press a small amount of nickel hydroxide powder between two glass slides to create a pellet.Then place the nickel hydroxide pellet on the silicon ATR crystal and obtain an FTIR spectrum between 404, 000 reciprocal centimeters as an average of 16 individual scans with a resolution of four reciprocal centimeters. SEM revealed that the microwave synthesized nickel hydroxide was composed of randomly interwoven ultra thin nanosheet aggregates. However, increasing the reaction temperature from 120 to 180 degrees Celsius increased the lateral sheet dimensions of individual nanosheets in the aggregates.Also, increasing the reaction time from 13 to 30 minutes at 120 degrees Celsius increased the nucleated aggregate sizes from about three to five microns. EDS showed a uniform distribution of nickel, oxygen, carbon, and nitrogen within all the synthesized nanosheets. The nanosheets had BET surface areas ranging from 61 to 85 square meters per gram, average pore diameter of 21 to 35 angstroms, and cumulative poor volumes of 0.4 to 0.6 cubic centimeters per gram.The XRD patterns of all three microwave synthesized samples showed characteristic peaks of alpha nickel hydroxide. However, reaction time and temperature affected the peaks as demonstrated by the shifting of the 001 plane. The ATR-FTIR spectra of the microwave synthesized nanosheets showed characteristic nickel oxide lattice mode.Modes from the ligands and structural molecules, cyanate bands and alpha hydroxide lattice modes. Increasing the reaction temperature from 120 to 180 degrees Celsius altered the frequencies and relative intensities of the cyanate, nitrate, hydroxyl, and water vibrational modes.

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368 Views.  Texas State University. To characterize the morphology and composition of the microwave synthesized nickel hydroxide nanosheets by SEM and EDS, prepare the sample by suspending a small amount of nickel hydroxide powder in one milliliter of ethanol using a water bath sonicator. Then drop cast the nickel hydroxide and ethanol mixture on an SEM stub and evaporate the ethanol by heating the stub in a sample oven at 70 degrees Celsius before collecting the SEM micrographs and EDS spectra. In order to determine the surfac...