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

We developed a protocol for rapid microwave assisted hydrothermal synthesis of nickel hydroxide nano sheets. Our work investigates how the microwave reaction temperature and time affect the structure and product yield. Nano structuring or expressing the material at nanoscale dimensions significantly influences the materials properties.

What we and others have shown is that when we work at the nanoscopic level, the structure of nickel hydroxide is incredibly sensitive to the reaction conditions used to synthesize the material. Our work provides a step-by-step procedure for the microwave synthesis of alpha and nickel hydroxide. We evaluate how varying reaction, temperature and time affects the crystalline structure, surface area porosity, and local environment of inner layer anions, which has not been previously reported.

The reaction protocol is rapid and highly adaptable to produce different metal hydroxide and oxide materials, including nickel oxide and cobalt hydroxide, and also allows for the incorporation of metal substituents such as aluminum or iron into the structure. Nickel hydroxide has many uses for energy storage, catalysis, sensors, and other applications. Our study advances the understanding of how different synthesis conditions can be used to control structure, which provides a pathway to develop improved materials.

Begin preparing the precursor solution by mixing 105 milliliters of ethylene glycol and 15 milliliters of ultrapure water. Then add 4.1 grams of uria and add 5.0 grams of nickel nitrate hexahydrate and cover the solution. Sonicate the precursor solution for 30 minutes in a bath ator filled with ice and water at 40 kilohertz frequency and full power without pulse.

Transfer 20 milliliters of the sonicated precursor solution into a microwave reaction vial fitted with a PTFE stir bar. Seal the reaction vessel using a locking lid with a PTFE liner. Place the vial inside the microwave reactor and set the reactor program to as fast as possible, which applies maximum power until the desired temperature is achieved.

Thereafter, apply variable power to maintain the reaction temperature for 13 to 30 minutes. Upon completion of the reaction, vent the reaction chamber with compressed air until the solution temperature reaches 55 degrees Celsius. Transfer the post reaction solution to 50 milliliter centrifuge tubes.

Centrifuge the tubes at room temperature for four minutes and decant the supernatant. Resuspend the resulting nickel hydroxide nano sheets in 25 milliliters of ultrapure water and centrifuge once more as demonstrated previously. After decanting the supernatant, cover the centrifuge tubes with a tissue or paper towel, serving as a porous cover to minimize potential contamination.

Finally, dry the nano sheets under an ambient atmosphere in a sample oven at 70 degrees Celsius for 21 hours. At 120 degrees Celsius, the maximum reaction pressure generated was nine to 11.5 pounds per square inch or PSI. Whereas at 180 degrees Celsius, it increased to 138 PSI.

Increasing the reaction temperature from 120 to 180 degrees Celsius also changed the post reaction supernatant color from green to blue. In C2 photographs of the reaction at 180 degrees Celsius showed a murky green color when the reaction terminated, but it changed to murky blue as the reaction cooled. After centrifugation, washing and drying, all the reactions irrespective of time and temperature produced a green powder.

The pH of the supernatant of the 180 degree Celsius reaction was much higher relative to the 120 degree Celsius reaction supernatants. The 180 degree Celsius reaction also recorded a much higher yield than the reactions at 120 degrees Celsius. To characterize the morphology and composition of the microwave synthesized nickel hydroxide nano sheets 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 nano sheets, add 25 milligrams of nickel hydroxide nano sheets 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 two theta of five 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 silicon ATR crystal and obtained 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 nano sheet aggregates. However, increasing the reaction temperature from 120 to 180 degrees Celsius increased the lateral sheet dimensions of individual nano sheets 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 nano sheets. The nano sheets had BET surface areas ranging from 61 to 85 square meters per gram, average pore diameter of 21 to 35 angstroms and cumulative pore 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 nano sheets showed characteristic nick 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.