S.W.K. and C.P.R. gratefully acknowledge support from the Office of Naval Research Navy Undersea Research Program (Grant No. N00014-21-1-2072). S.W.K. acknowledges support from the Naval Research Enterprise Internship Program. C.P.R and C.M. acknowledge support from the National Science Foundation Partnerships for Research and Education in Materials (PREM) Center for Intelligent Materials Assembly, Award No. 2122041, for analysis of the reaction conditions.

NOTE: The schematic overview of the microwave synthesis process is presented in Figure 1.
1. Microwave synthesis of &#945;-Ni(OH)2 nanosheets
<ol>
	<li>Preparation of precursor solution
	<ol>
		<li>Prepare the precursor solution by mixing 15 mL of ultrapure water (&#8805;18 M&#937;-cm) and 105 mL of ethylene glycol. Add 5.0 g of Ni(NO3)2&#160;&#183; 6&#160;H2O and 4.1 g of urea to the solution and cover.</li>
		<li>Place the precursor solution in an ice and water-filled bath sonicator (40 kHz frequency) and sonicate at full power (no pulse) for 30 min.</li>
	</ol>
	</li>
	<li>Microwave reaction of the precursor solution
	<ol>
		<li>Transfer 20 mL of the precursor solution into a microwave-reaction vial with a polytetrafluoroethylene (PTFE) stir bar and seal the reaction vessel with a locking lid with a PTFE liner.</li>
		<li>Program the microwave reactor to heat to the reaction temperature using the setting as fast as possible (to 120 or 180 &#176;C) and hold at that temperature for 13-30 min. 
		NOTE: Heating as fast as possible is a microwave setting that applies maximum microwave power until the desired temperature is achieved; apply variable power thereafter to maintain the reaction temperature.</li>
		<li>After the reaction is complete, vent the reaction chamber with compressed air until the solution temperature reaches 55 &#176;C. Each stage of the reaction (heating, holding, and cooling) is performed under magnetic stirring at 600 rpm.</li>
	</ol>
	</li>
	<li>Centrifugation and washing the microwave-reaction precipitate.
	<ol>
		<li>Transfer the post-reaction solution to 50 mL centrifuge tubes. Centrifuge the post-reaction solution at 6,000 rpm/6,198 rcf for 4 min at room temperature&#160;and then decant the supernatant.</li>
		<li>Add 25 mL of ultrapure water to resuspend the nanosheets. Centrifuge using the same conditions and then decant the supernatant.</li>
		<li>Repeat the washing, centrifuging, and decanting steps a total of five times using water, and then three times using ethanol. 
		&#8203;NOTE: Isopropyl alcohol can also be used in place of ethanol.</li>
	</ol>
	</li>
	<li>Measuring the pH before and after the microwave reaction
	<ol>
		<li>Measure the pH of the precursor solution before starting the microwave reaction and measure the pH of the supernatant immediately after the first centrifugation.</li>
	</ol>
	</li>
	<li>Drying the sample
	<ol>
		<li>Cover the centrifuge tubes with a tissue or paper towel to act as a porous cover to reduce potential contamination and dry them in a sample oven at 70 &#176;C for 21 h under ambient atmosphere. 
		&#8203;NOTE: The drying time and conditions can influence the relative intensities and 2&#952;&#176; values of (XRD) peaks, as described in the Representative Results.</li>
	</ol>
	</li>
</ol>
2. Material characterization and analysis
<ol>
	<li>Characterizing the morphology and composition using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS)
	<ol>
		<li>Prepare the samples for SEM and EDS analysis by suspending a small amount of Ni(OH)2 powder in 1 mL of ethanol using a water bath sonicator.</li>
		<li>Drop cast the Ni(OH)2/ethanol mixture on an SEM stub and evaporate the ethanol by placing the SEM stub in a sample oven at 70 &#176;C.</li>
		<li>Collect SEM micrographs and EDS spectra. Collect SEM images using an accelerating voltage of 10 kV and a current of 0.34 nA at magnifications of 6.5 kX, 25 kX, and 100 kX. Collect EDS spectra on selected regions using an accelerating voltage of 10 kV, a current of 1.4 nA, and a magnification of 25 kX.</li>
	</ol>
	</li>
	<li>Analyzing the surface area and porosity using nitrogen physisorption porosimetry
	<ol>
		<li>Prepare the samples for analysis by adding 25 mg of Ni(OH)2 into the sample tube. Perform a pre-analysis degassing and drying procedure under vacuum at 120 &#176;C for 16 h before analysis.</li>
		<li>Transfer the sample tube from the degassing port to the analysis port to collect nitrogen (N2) isotherms.</li>
		<li>Analyze the N2&#160;isotherm data using Brunauer-Emmett-Teller (BET) analysis to determine the specific surface area. Perform the BET analysis according to International Union of Pure and Applied Chemistry (IUPAC) methodologies33. The specific analysis software package used to perform the BET analysis is included in the Table of Materials.</li>
		<li>Analyze the desorption branch of the isotherm using the Barrett-Joyner-Halenda (BJH) method to obtain pore volume, pore diameter, and pore size distribution. Perform the BJH analysis according to IUPAC&#160;methodologies.33 The specific analysis software package used to perform BJH analysis is included in the Table of Materials.</li>
	</ol>
	</li>
	<li>Structural analysis using powder X-ray diffraction (XRD)
	<ol>
		<li>Fill the sample well of a zero-background powder XRD holder with Ni(OH)2, ensuring the powder surface is flat.</li>
		<li>Collect powder X-ray diffractograms using a CuK&#945;&#160;radiation source between 5&#176;-80&#176; 2&#952; using a 0.01-step increment.</li>
		<li>Analyze the d-spacing using Bragg&#39;s law, 
		n&#955; = 2d sin&#952;, 
		where n is an integer,&#160;&#955; is the wavelength of the X-rays, d is the d-spacing, and &#952;&#160;is the angle between the incident rays and the sample.</li>
		<li>Analyze the crystallite domain size, D,&#160;using the Scherrer equation, 
		<img alt="Equation 1" src="/files/ftp_upload/65412/65412eq01.jpg" style="margin: auto;" /> 
		where Ks is the Scherrer constant (a Scherrer constant of 0.92 was used for the analysis), &#955; is the wavelength of the X-rays,&#160;&#946;2&#952; is the integral breadth of the diffraction peak, and&#160;&#952; is the Bragg angle (in radians). For analysis,&#160;&#946;2&#952; was taken as the full width at half maximum (fwhm) and multiplied by a constant of 0.939434.</li>
	</ol>
	</li>
	<li>Characterizing the material using attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR)
	<ol>
		<li>Equip the attenuated total reflectance (ATR) attachment to the Fourier transform infrared (FTIR) spectrometer.</li>
		<li>Press a small amount of Ni(OH)2 powder between two glass slides to create a small pellet.</li>
		<li>Place the Ni(OH)2 pellet on the silicon ATR crystal and obtain an FTIR spectrum between 400 and 4,000 cm-1. Infrared spectra represent the average of 16 individual scans with 4 cm-1 resolution.</li>
	</ol>
	</li>
</ol>

Microwave Synthesis and Characterization of Nickel Hydroxide Nanosheets

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The authors have no conflicts of interest.

Microwave synthesis provides a route to generate Ni(OH)2 that is significantly faster (13-30 min reaction time) relative to conventional hydrothermal methods (typical reaction times of 4.5 h)38. Using this mildly acidic microwave synthesis route to produce ultrathin &#945;-Ni(OH)2 nanosheets, it is observed that reaction time and temperature influence the reaction pH, yields, morphology, porosity, and structure of the resulting materials. Using an in situ reaction pr...

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.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>ATR-FTIR</td>
			<td>Bruker</td>
			<td></td>
			<td>Tensor II FT-IR spectrometer equipped with a Harrick Scientific SplitPea ATR micro-sampling accessory</td>
		</tr>
		<tr>
			<td>Bath sonicator</td>
			<td>Fisher Scientific</td>
			<td>15-337-409</td>
			<td>--</td>
		</tr>
		<tr>
			<td>Ethanol&#160;</td>
			<td>VWR analytical</td>
			<td>AC61509-0040</td>
			<td>200 proof</td>
		</tr>
		<tr>
			<td>Ethylene Glycol</td>
			<td>VWR analytical</td>
			<td>BDH1125-4LP</td>
			<td>99% purity</td>
		</tr>
		<tr>
			<td>Falcon Centrifuge tubes</td>
			<td>VWR analytical</td>
			<td>21008-940</td>
			<td>50 mL</td>
		</tr>
		<tr>
			<td>KimWipes</td>
			<td>VWR analytical</td>
			<td>21905-026</td>
			<td>--</td>
		</tr>
		<tr>
			<td>Lab Quest 2</td>
			<td>Vernier&#160;</td>
			<td>LABQ2</td>
			<td>--</td>
		</tr>
		<tr>
			<td>Microwave Reactor</td>
			<td>Anton Parr</td>
			<td>165741</td>
			<td>Monowave 450</td>
		</tr>
		<tr>
			<td>Ni(NO3)2 &#183; 6 H2O</td>
			<td>Ward&#39;s Science</td>
			<td>470301-856</td>
			<td>Research lab grade</td>
		</tr>
		<tr>
			<td>pH Probe</td>
			<td>Vernier&#160;</td>
			<td>PH-BTA</td>
			<td>Calibrated vs standard pH solutions (pH= 4, 7, 11)</td>
		</tr>
		<tr>
			<td>Porosemeter</td>
			<td>Micromeritics&#160;</td>
			<td>--</td>
			<td>ASAP 2020. Analysis software: Micromeritics, version 4.03</td>
		</tr>
		<tr>
			<td>Powder x-ray diffactometer</td>
			<td>Bruker</td>
			<td></td>
			<td>AXS Advanced Poweder x-ray diffractometer; d-spacing, and crystallite size analyses were performed using Highscore XRD software, and crystal structures were created using VESTA 3 software.</td>
		</tr>
		<tr>
			<td>Reaction vial</td>
			<td>Anton Parr</td>
			<td>82723</td>
			<td>30 mL G30 wideneck, 20 mL max fill capacity</td>
		</tr>
		<tr>
			<td>Reaction vial locking lid</td>
			<td>Anton Parr</td>
			<td>161724</td>
			<td>G30 Snap Cap</td>
		</tr>
		<tr>
			<td>Reaction vial PTFE septum</td>
			<td>Anton Parr</td>
			<td>161728</td>
			<td>Wideneck</td>
		</tr>
		<tr>
			<td>Scanning electron microscope</td>
			<td>FEI</td>
			<td>--</td>
			<td>Helios Nanolab 400</td>
		</tr>
		<tr>
			<td>Urea</td>
			<td>VWR analytical</td>
			<td>BDH4602-500G</td>
			<td>ACS grade</td>
		</tr>
	</tbody>
</table>

effect of microwave synthesis conditions on the structure of nickel hydroxide nanosheets

A protocol for rapid, microwave-assisted hydrothermal synthesis of nickel hydroxide nanosheets under mildly acidic conditions is presented, and the effect of reaction temperature and time on the material&#39;s structure is examined. All reaction conditions studied result in aggregates of layered &#945;-Ni(OH)2 nanosheets. The reaction temperature and time strongly influence the structure of the material and product yield. Synthesizing &#945;-Ni(OH)2 at higher temperatures increases the reaction yield, lowers the interlayer spacing, increases crystalline domain size, shifts the frequencies of interlayer anion vibrational modes, and lowers the pore diameter. Longer reaction times increase reaction yields and result in similar crystalline domain sizes. Monitoring the reaction pressure in situ shows that higher pressures are obtained at higher reaction temperatures. This microwave-assisted synthesis route provides a rapid, high-throughput, scalable process that can be applied to the synthesis and production of a variety of transition metal hydroxides used for numerous energy storage, catalysis, sensor, and other applications.

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

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

Influence of reaction temperature and time on the synthesis of &#945;-Ni(OH)2 
Before the reaction, the precursor solution [Ni(NO3)2 &#183; 6 H2O, urea, ethylene glycol, and water] is a transparent green color with a pH of 4.41 &#177; 0.10 (Figure 2A and Table 1). The temperature of the microwave reaction (either 120 &#176;C or 180 &#176;C) influences the in situ reaction pressure and color of the sol...

Watch this Scientific Journal Video about A Rapid, Microwave-Assisted Hydrothermal Synthesis Of Nickel Hydroxide Nanosheets at JoVE.com

Nickel hydroxide nanosheets are synthesized by a microwave-assisted hydrothermal reaction. This protocol demonstrates that the reaction temperature and time used for microwave synthesis affects the reaction yield, crystal structure, and local coordination environment.

A protocol for rapid, microwave-assisted hydrothermal synthesis of nickel hydroxide nanosheets under mildly acidic conditions is presented, and the effect ...

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