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Chemistry

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

Published: August 18, 2023
doi:
10.3791/65412
, , , ,
1Materials Science, Engineering, and Commercialization Program,Texas State University, 2Department of Chemistry and Biochemistry,Texas State University, 3Westlake Highschool

Summary

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.

Abstract

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's structure is examined. All reaction conditions studied result in aggregates of layered α-Ni(OH)2 nanosheets. The reaction temperature and time strongly influence the structure of the material and product yield. Synthesizing α-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.

Introduction

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: β-Ni(OH)2 and α-Ni(OH)211. β-Ni(OH)2 adopts a brucite-type Mg(OH)2 crystal structure, while α-Ni(OH)2 is a turbostratically layered form of β-Ni(OH)2 intercalated with residual anions and water molecules from the chemical synthesis4. Within α-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 α-Ni(OH)2 affect the average Ni oxidation state13 and influence the electrochemical performance of α-Ni(OH)2 (relative to β-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 identities and concentrations of the nickel (II) salt and 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 β-Ni(OH)2, but α-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 α-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 ↔ [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 α-Ni(OH)2 phase provides improved electrochemical capacity compared with the β-Ni(OH)2 phase13, and synthetic methods to synthesize α-Ni(OH)2 are of particular interest. α-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 α-Ni(OH)227. Water content in an ethylene glycol-water solution has been shown to impact the morphology of microwave-assisted synthesis of α-Ni(OH)2 nanosheets24. The reaction yield of α-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 α-Ni(OH)2 nanoflowers using a precursor solution of EtOH/H2O, nickel nitrate, and urea found that temperature (in the range of 80-120 °C) was not a critical factor, provided the reaction is conducted above the urea hydrolysis temperature (60 °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 °C, the material contained both α-Ni(OH)2 and β-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 α-Ni(OH)2 and α-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 α-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 α-Ni(OH)2 was also used as a precursor for obtaining β-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 α-Ni(OH)2 nanosheets, and the Fe substitution ratio alters the structure and magnetization32. However, a step-by-step procedure for microwave synthesis of α-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 has not been previously reported.

This protocol establishes procedures for high-throughput microwave synthesis of α-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 α-Ni(OH)2 nanosheets.

Protocol

NOTE: The schematic overview of the microwave synthesis process is presented in Figure 1.

1. Microwave synthesis of α-Ni(OH)2 nanosheets

  1. Preparation of precursor solution
    1. Prepare the precursor solution by mixing 15 mL of ultrapure water (≥18 MΩ-cm) and 105 mL of ethylene glycol. Add 5.0 g of Ni(NO3)2 · 6 H2O and 4.1 g of urea to the solution and cover.
    2. 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.
  2. Microwave reaction of the precursor solution
    1. 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.
    2. Program the microwave reactor to heat to the reaction temperature using the setting as fast as possible (to 120 or 180 °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.
    3. After the reaction is complete, vent the reaction chamber with compressed air until the solution temperature reaches 55 °C. Each stage of the reaction (heating, holding, and cooling) is performed under magnetic stirring at 600 rpm.
  3. Centrifugation and washing the microwave-reaction precipitate.
    1. 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 and then decant the supernatant.
    2. Add 25 mL of ultrapure water to resuspend the nanosheets. Centrifuge using the same conditions and then decant the supernatant.
    3. Repeat the washing, centrifuging, and decanting steps a total of five times using water, and then three times using ethanol.
      ​NOTE: Isopropyl alcohol can also be used in place of ethanol.
  4. Measuring the pH before and after the microwave reaction
    1. Measure the pH of the precursor solution before starting the microwave reaction and measure the pH of the supernatant immediately after the first centrifugation.
  5. Drying the sample
    1. 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 °C for 21 h under ambient atmosphere.
      ​NOTE: The drying time and conditions can influence the relative intensities and 2θ° values of (XRD) peaks, as described in the Representative Results.

2. Material characterization and analysis

  1. Characterizing the morphology and composition using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS)
    1. 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.
    2. 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 °C.
    3. 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.
  2. Analyzing the surface area and porosity using nitrogen physisorption porosimetry
    1. 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 °C for 16 h before analysis.
    2. Transfer the sample tube from the degassing port to the analysis port to collect nitrogen (N2) isotherms.
    3. Analyze the N2 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.
    4. 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 methodologies.33 The specific analysis software package used to perform BJH analysis is included in the Table of Materials.
  3. Structural analysis using powder X-ray diffraction (XRD)
    1. Fill the sample well of a zero-background powder XRD holder with Ni(OH)2, ensuring the powder surface is flat.
    2. Collect powder X-ray diffractograms using a CuKα radiation source between 5°-80° 2θ using a 0.01-step increment.
    3. Analyze the d-spacing using Bragg's law,
      nλ = 2d sinθ,
      where n is an integer, λ is the wavelength of the X-rays, d is the d-spacing, and θ is the angle between the incident rays and the sample.
    4. Analyze the crystallite domain size, D, using the Scherrer equation,
      Equation 1
      where Ks is the Scherrer constant (a Scherrer constant of 0.92 was used for the analysis), λ is the wavelength of the X-rays, β2θ is the integral breadth of the diffraction peak, and θ is the Bragg angle (in radians). For analysis, β2θ was taken as the full width at half maximum (fwhm) and multiplied by a constant of 0.939434.
  4. Characterizing the material using attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR)
    1. Equip the attenuated total reflectance (ATR) attachment to the Fourier transform infrared (FTIR) spectrometer.
    2. Press a small amount of Ni(OH)2 powder between two glass slides to create a small pellet.
    3. 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.

Representative Results

Influence of reaction temperature and time on the synthesis of α-Ni(OH)2
Before the reaction, the precursor solution [Ni(NO3)2 · 6 H2O, urea, ethylene glycol, and water] is a transparent green color with a pH of 4.41 ± 0.10 (Figure 2A and Table 1). The temperature of the microwave reaction (either 120 °C or 180 °C) influences the in situ reaction pressure and color of the solution (Figure 2B–G and Figure 3). For the 120 °C reaction, the microwave radiation heats the precursor solution to a temperature of 120 °C in less than 1 min 30 s. The microwave reactor holds the temperature at 120 °C for 13 to 30 min under variable microwave power, and then the vessel requires 3 min to cool to 55 °C (Figure 3A). Once the temperature is applied, the 120 °C reaction generates a modest amount of pressure, achieving a maximum reaction pressure of 9-11.5 psi. The solution pH rises from 4.41 ± 0.10 to 6.75 ± 0.04 after 13 min at 120 °C, and rises to 7.03 ± 0.04 after 30 min at 120 °C. Centrifugation separates the precipitated powder from the green supernatant (Figure 2B–F). The subsequent washing and drying yield a green powder (Figure 2H) with a yield of 62 ± 12 mg for the 13 min reaction time and a yield of 131 ± 24 mg for the 30 min reaction time at 120 °C (Table 1).

Increasing the reaction temperature from 120 °C to 180 °C results in significant pressure accumulation (Figure 3A vs. 3B), changes to the post-reaction supernatant color (Figure 2B vs. 2D and 2E vs. 2G), and increases the reaction yield relative to 120 °C reactions at both the 13 and 30 min reaction times (Table 1). Using a reaction temperature of 180 °C, the reaction achieves a maximum reaction pressure of 138 psi, coinciding with the termination of the reaction (Figure 3A). To determine the relative contributions of the different components to the pressure, the pressure generated using the original precursor solution to the pressure generated using a solution of water, ethylene glycol, and urea and a solution of water and ethylene glycol are compared (Supplementary Figure 1). From the comparison of the pressure generated from each solution using a reaction temperature of 180 °C (Supplementary Figure 1), solutions containing urea result in higher pressures. The solution of water and ethylene glycol contributes a steady 50 psi throughout the reaction; the solution of water, ethylene glycol, and urea has a similar pressure profile as the solution of nickel nitrate, water, ethylene glycol, and urea (seen in red and blue in Supplementary Figure 1). The additional pressure generated at 180 °C within the urea-containing solutions is attributed to the decomposition of urea27 into gas-phase CO2 and NH3 (as discussed in the following section) with vapor-phase H2O contributing to the overall pressure.

In contrast to the green-colored supernatant following the 120 °C microwave reaction (Figure 2E,F), the supernatant obtained following the 180 °C reaction is blue (Figure 2G). In situ photographs of the reaction show the blue coloration after the reaction has cooled (Figure 3C), and the solution undergoes a gradual color change between the end of microwave heating (box #2 in Figure 3B) and the end of the cooling step (box #3 in Figure 3B). Before microwave-induced heating of the solution to 180 °C, the nickel salt gives the solution a transparent green color (Figure 3C, corresponding to block box #1 in Figure 3B). The solution is murky pale green when the reaction terminates (Figure 3C, block box #2 in Figure 3B), but as the reaction is cooled and pressure decreases, the solution changes color from murky green to blue (Figure 3C, block box #3 in Figure 3B). The supernatant of the 180 °C reaction has a pH of 8.91 ± 0.03, which is much higher relative to the 120 °C supernatant (pH of 6.75 ± 0.04 for 13 min reaction time), and the higher pH may be related to higher levels of urea decomposition. Centrifugation, washing, and drying of the 180 °C reaction for 13 min results in a green powder (no hints of blue color with the powder were observed) with a yield of 202 ± 4 mg, which is much higher than the yields of the 120 °C reactions (Table 1).

Effect of reaction time and temperature on the morphology, composition, and porosity of α-Ni(OH)2
Scanning electron micrographs (SEM) reveal the synthesized Ni(OH)2 materials are composed of aggregates (~1-5 µm in diameter) of ultrathin nanosheets that are randomly interwoven (Figure 4). From SEM images, the reaction temperature influences the relative directional growth of individual nanosheets within the overall aggregate. For the 180 °C reactions (Figure 4D-F), an individual nanosheet within the aggregate appears to have longer lateral dimensions of sheets relative to those of the 120 °C reactions (Figure 4A-C and 4G-L). The comparison of SEM images of materials synthesized for 13 min at 120 °C (Figure 4A) to 30 min at 120 °C (Figure 4G) shows that increasing the reaction time from 13 to 30 min at 120 °C increases the size of nucleated nanostructure aggregates from ~3 µm to ~5 µm. High-resolution transmission electron microscopy imaging of similar materials showed the nanosheets consist of multiple crystallites, rather than a nanosheet being a single crystal32. The analysis of materials produced from a variation of this synthesis route also showed that the nanosheets are 2-12 nm thick and composed of organized stacks of individual (001) layers12.

Energy dispersive X-Ray spectroscopy (EDS) shows a uniform distribution of nickel, oxygen, carbon, and nitrogen within all the synthesized nanosheet materials (Figure 5). Carbon and nitrogen incorporated into the structure arise from residual compounds from the reaction precursors (e.g., nitrates, urea, and ethylene glycol) and derivatives4,12,35, and the presence of these compounds within the structure is supported by FTIR analysis, as described below.

From nitrogen physisorption analysis, the microwave-synthesized Ni(OH)2 nanosheets have BET surface areas ranging from 61-85 m2·g-1, average pore volumes of 21-35 Å, and cumulative pore volumes of 0.426-0.630 cm3·g-1 (Table 1). Using IUPAC nomenclature for isotherm type and pore widths33, the materials made using this protocol all exhibit Type IV isotherms, and the pore size distribution plots show that the majority of the free volume is in the mesopore (pore width 2-50 nm) and macropore (pore width >50 nm) ranges (Supplementary Figure 2). From these measurements, the surface areas of the materials prepared at different reaction temperatures and times are within the experimental error of each other. The material synthesized at 180 °C for 13 min has a smaller pore diameter and pore volume than the material synthesized at 120 °C for 13 min, indicating that reaction temperature affects the material's porosity.

Impact of reaction time and temperature on the structure of α-Ni(OH)2
The XRD patterns of all three microwave-synthesized samples show characteristic peaks of α-Ni(OH)2. Several diffraction peaks are observed in the range of 11-12°, 23-24°, 33°, 36°, and 59° 2θ, corresponding to the (001), (002), (110), (111), and (300) planes of α-Ni(OH)2, respectively (Figure 6A)12. The peak positions observed within the X-ray diffractogram of the material synthesized at 120 °C for 13 min match those of a hydrated α-Ni(OH)2 structure (ICDD card no. 00-038-0715). For the 120 °C reaction, as the synthesis time increases from 13 to 30 min, the position of the (001) reflection shifts to a lower 2θ value (Figure 6B), enlarging the interlayer gallery height from 7.85 to 7.94 Å. Increasing the synthesis time from 13 to 30 min at 120 °C does not significantly influence the crystallite domain size in the (001) or (110) directions beyond experimental error (results summarized in Table 2).

In addition to the effects of reaction time, increasing the microwave reaction temperature from 120 °C to 180 °C also induces changes to the α-Ni(OH)2 crystal structure. At elevated temperatures, the (001) diffraction plane shifts to a higher 2θ value (Figure 6B), shortening the interlayer gallery height from 7.85 to 7.36 Å and resulting in a narrower (002) peak, indicating a higher degree of order within the interlayer region (Figure 6A). The (001) diffraction plane of α-Ni(OH)2 synthesized at 180 °C occurs at a position between that of a hydrated α-Ni(OH)2 (ICDD card no. 00-038-0715) and a nitrated α-Ni(OH)2 (ICDD card no. 00-022-0752), and therefore the structure is consistent with a hydrated/nitrated α-Ni(OH)2 (Figure 6B). As prior work indicates that the peak position of the (001) reflections in α-Ni(OH)2 is dependent on drying conditions36, the same drying conditions (70 °C, 21 h, ambient atmosphere) were applied to the samples to avoid the potential effect of drying conditions on the (001) peak position. For comparison, the effect of other drying conditions was also evaluated. Drying conditions of 16 h under ambient atmosphere or vacuum resulted in (001) d-spacings within the experimental error of our standard drying conditions of 70 °C for 21 h under ambient atmosphere (Supplementary Figure 3B). Using a longer drying time of 24 h under ambient atmosphere results in a (001) d-spacing that is slightly beyond experimental error; however, the shifts in d-spacing of the (001) reflection from using different reaction conditions (Table 2) are beyond the experimental error of different drying conditions (Supplementary Figure 3B).

The nanosheet morphology results in significantly different sizes of crystalline domains composed of (001) and (110) planes, which are orthogonal planes within the α-Ni(OH)2 crystal structure (Figure 6C). The (001) planes arise from ordering of Ni(OH)2 layers, while the (110) planes result from the ordering of atoms within the plane of the nanosheet. For the α-Ni(OH)2 material synthesized at 120 °C, the crystallite domain sizes of 4.5 nm (001) and 12.9 nm (110) are consistent with SEM images that show larger lateral dimensions of the sheets relative to the thickness of the sheets (Figure 4). Comparing the α-Ni(OH)2 synthesized at 120 °C and 180 °C for 13 min, the material synthesized at 180 °C has larger domain sizes of 6.6 nm (001) and 15.2 nm (110) relative to the values obtained at 120 °C (Table 2), which is consistent with SEM microscopy that shows larger and flatter nanosheets within the aggregate relative to the 120 °C materials (Figure 4). The material synthesized at a higher temperature has a larger domain size, which is consistent with the smaller pore diameter and pore volume from nitrogen physisorption analysis (Table 1).

The ATR-FTIR spectra of the microwave-synthesized nanosheets in the 400-4,000 cm-1 region (Figure 7A and Table 2) show a Ni-O lattice mode35 between 400-800 cm-1, modes from ligands and structural molecules35 between 800-2,000 cm-1, cyanate bands31 between 2,000 and 2,500 cm-1, and α-OH lattice modes35 between 3,500 and 3,800 cm-1. Included in the supplementary figures are expanded regions of the Ni-O lattice modes (Supplementary Figure 4A), cyanate modes (Supplementary Figure 4B), and the α-OH lattice modes (Supplementary Figure 4C). Experimental wavenumbers for materials prepared during different reaction conditions and peak assignments from prior studies are included in Supplementary Table 1. Within the region-labeled ligand and structural molecules of the FTIR spectra (Figure 6B), all samples show two distinct nitrate vibrational modes, a bound nitrate, ν3(NO3–), and a free nitrate, ν3(free NO3–), a commonality among α-Ni(OH)2 synthesized from nickel nitrates solutions12,35. All three samples show vibrational stretching modes arising from urea-derived cyanates, νs(C-O-CN)/νs(OCN–)12,31 and bending modes from free water, δs(H-O-H)35. The ν(C-O) mode is attributed to carbonates within the α-Ni(OH)2 material31. Increasing the reaction time from 13 min to 30 min at 120 °C results in decreasing the relative intensity of the ν(C-O) mode, which supports that longer reaction times influence the incorporation of carbonates within the material, affecting the interlayer region4.

Increasing the reaction temperature from 120 °C to 180 °C alters the frequencies and relative intensities of the cyanate, nitrate, hydroxyl, and water vibrational modes (Figure 7B). Comparing the materials at 120 °C and 180 °C for 13 min, at the higher reaction temperature of 180 °C, the frequency of the δ(α-OH) mode shifts to a higher wavenumber (green highlighted region of Figure 7B), indicating a change in the local potential energy environment of -OH coordinated to the Ni-center. Reaction temperature also changes the relative intensities of cyanate, nitrate, and free water modes. The comparison of spectra of samples heated to 180 °C and 120 °C shows that relative to the ν3(NO3–) mode (grey highlighted region of Figure 7B), the intensities of the ν(C-O-CN) mode (red inset of Figure 7B) and the δ(H-O-H, free) mode (blue inset of Figure 7B) are lower within the 180 °C material compared with the 120 °C material. In addition, the relative intensity of the nitrate modes, ν3(NO3–) and ν3(NO3–, free), compared to the δ(H-O-H, free) mode is higher at elevated reaction temperatures. The increase in the relative intensity of the nitrate modes compared to the δ(H-O-H, free) mode at elevated reaction temperatures supports the XRD analysis that the increasing reaction temperature from 120 to 180 °C results in the material being expressed as a hydrated-nitrated α-Ni(OH)2. The peak shape of the cyanate mode that occurs between 2,000 and 2,500 cm-1 also changes with an elevation in reaction temperature (Supplementary Figure 4B), where there appears to be two bands in the samples. In the cyanate mode region, the sample heated to 180 °C has a different relative intensity of the higher frequency peak compared to that within the 120 °C samples.

The observed changes in frequency and relative intensity indicate a reaction temperature and time change in the local potential energy environment of these moieties, and additional analysis is needed to further establish frequency-structure correlations of these vibrational modes within these materials.

Figure 1
Figure 1: Schematic representation of α-Ni(OH)2 nanosheet synthesis. The process used 20 mL aliquots of a stock solution (Ni(NO)3 · 6 H2O, urea, ethylene glycol, and H2O) microwaved under variable reaction times (13 or 30 min) and temperatures (120 or 180 °C) producing α-Ni(OH)2 nanosheets. Please click here to view a larger version of this figure.

Figure 2
Figure 2: Pictures of the microwave reaction solution composed of nickel nitrate, urea, ethylene glycol, and water. (A) Before microwave radiation; after microwave radiation for (B) 13 min at 120 °C, (C) 30 min at 120 °C, and (D) 13 min at 180 °C. Pictures of the samples after the first centrifugation [which separates Ni(OH)2 from any unreacted nickel nitrate, urea, ethylene glycol, and water]: (E) 13 min at 120 °C, (F) 30 min at 120 °C, and (G) 13 min at 180 °C. (H) The washed and dried powders of materials synthesized using 13 min at 120 °C, 30 min at 120 °C, and 13 min at 180 °C. Please click here to view a larger version of this figure.

Figure 3
Figure 3: Time, temperature, and pressure profiles of the microwave reactions of solutions composed of nickel nitrate, urea, ethylene glycol, and water. The influence of reaction time on the pressure of the microwave synthesized Ni(OH)2 at (A) 120 °C for 13 and 30 min and (B) 180 °C for 13 min. (C) In situ reaction photographs of the reaction at 180 °C. Pink insets 1-3 in (B) correspond to the in situ reaction photographs of the reaction in (C). Please click here to view a larger version of this figure.

Figure 4
Figure 4: Scanning electron micrographs of microwave-synthesized α-Ni(OH)2 nanosheets at different magnifications. (A–C) 13 min at 120 °C, (D–F) 13 min at 180 °C, and (G–L) 30 min at 120 °C. Please click here to view a larger version of this figure.

Figure 5
Figure 5: Energy dispersive X-ray spectroscopic elemental mapping of nickel (Ni), oxygen (O), carbon (C), and nitrogen (N) within the microwave-synthesized α-Ni(OH)2 nanosheets. (A–E) 13 min at 120 °C, (F–J) 13 min at 180 °C, and (K–O) 30 min at 120 °C. Please click here to view a larger version of this figure.

Figure 6
Figure 6: X-ray diffraction pattern of microwave-synthesized α-Ni(OH)2 nanosheets prepared under different reaction conditions (13 min at 120 °C, 13 min at 180°C, and 30 min at 120 °C). (A) Powder XRD pattern between 5°-80° 2 regions. (B) An expanded region of the diffractogram in the 10-14° 2 regions showing the (001) plane of α-Ni(OH)2. (C) Comparison of a model α-Ni(OH)2 crystal structure to the microwave-synthesized α-Ni(OH)2 nanosheets created using a crystal structure software37. Please click here to view a larger version of this figure.

Figure 7
Figure 7: Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) spectra of microwave-synthesized α-Ni(OH)2 nanosheets. The nanosheets were prepared under different reaction conditions (13 min at 120 °C, 13 min at 180 °C, and 30 min at 120 °C) and analyzed by ATR-FTIR in the (A) 400-4,000 cm-1 region, and (B) expanded view in the 800-2,000 cm-1 region; peak assignments are shown, and details of peak assignments are provided in the text. Please click here to view a larger version of this figure.

Reaction Conditions Elemental composition  determined by  EDS Nitrogen Physisorption 
Reaction temperature (°C) Reaction time (minutes) pH before reaction pH after reaction Yield (mg) Atomic % Ni Atomic % O BET  Surface area (m2 • g-1)  Pore  Diameter (Å) Pore volume (cm3  • g-1)
120 °C 13 4.41 ± 0.10  6.75 ± 0.04  62 ± 12 21 ± 2 68 ± 4 79 ± 19 35 ± 6 0.630 ± 0.093 
180 °C 13 4.41 ± 0.10  8.91 ± 0.03  202 ±  4 21 ± 1 67 ± 4 85 ± 10 21 ± 2 0.497 ± 0.085
120 °C 30 4.41 ± 0.10  7.03 ± 0.04  131 ± 24  16 ± 4 67 ± 4 61 ± 21 21 ± 14 0.426 ± 0.115

Table 1: Physiochemical characteristics of microwave-synthesized Ni(OH)2. The characteristics were measured at different temperatures (120 °C and 180 °C) and reaction times (13 min and 30 min); pH, yield, elemental composition from energy dispersive X-ray spectroscopy (EDS), and nitrogen porosimetry data; details are provided in the text.

Microwave Reaction Conditions X-ray diffraction Infrared Spectroscopy
(001) d-spacing (Å) Crystallite domain size (nm) Wavenumber (cm-1)
<001> <110> ν(Ni-O)  δ(α-OH) ν3(NO3–) νs(OCN–)
13 min  at 120 °C 7.85 ± 0.17 4.5  ± 1.1  12.9  ± 1.3 617 1487 1289 2183
13 min  at 180 °C 7.36  ± 0.03 6.6 ± 0.5 15.2 ± 0.6 620 1493 1291 2207
30 min at 120 °C 7.94  ±  0.02  5.2 ± 0.6  12.0  ± 1.7  620 1498 1294 2197

Table 2: Structural analysis of microwave-synthesized α-Ni(OH)2 nanosheets. Structural analysis of nanosheets prepared under different reaction conditions (13 min at 120 °C, 13 min at 180 °C, and 30 min at 120 °C ) obtained from powder XRD and Fourier Transform Infrared spectroscopy. Details are provided in the text.

Discussion

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 α-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 pressure gauge, a very small amount of pressure accumulation occurs during both 120 °C reactions, but increasing the reaction temperature from 120 °C to 180 °C generates substantial reaction pressure. Urea decomposes into NH3 and CO2 (equation 3) and then further reacts, generating CO32- and OH– (equations 4 and 5), and NH4+ and OCN– (equation 6)27, with the continual release of OH– driving the hydrolysis and condensation reactions that result in the growth of the Ni(OH)2 structure27.

H2NCONH2(s) + H2O(l) Equation 2 2 NH3(g) +CO2(g) (3)
H2O(l) + CO2(g) → CO32(aq)+ 2 H+(aq) (4)
NH3(g) + H2O(l) → NH4+(aq) + OH–(aq) (5)
H2NCONH2(s) + H2O(l) → OCN– (aq) + NH4+(aq)+ H2O(l) (6)

The increase in pressure generated during the reaction conducted at 180 °C is attributed to the gas generated from the decomposition of urea. The 180 °C reaction also generates higher pH levels resulting from urea-mediated alkalinization (rise in pH) of the solution. The increase in pH and higher product yield of the 180 °C reaction results from the increased rate of urea decomposition, which may drive the nickel nitrate hydrolysis and condensation reaction at a faster rate. This analysis is consistent with prior work, which reported the synthesis of α-Ni(OH)2 in urea and H2O and found that the reaction yield depends on pH evolution26.

The reaction pressure and higher pH obtained at the higher reaction temperature may also influence the preferred lateral growth direction of the nanosheets, as observed from SEM images (Figure 4), with 120 °C synthesized nanosheets organizing more randomly relative to the more planar organization of the 180 °C nanosheets. Upon termination of the 180 °C reaction, the solution color changes from murky green to blue as the pressure is released and may be related to a reaction of ammonia with unreacted Ni in the solution, turning the solution blue. The green-to-blue color change may be due to NH3 reacting with any remaining unreacted Ni2+ and forming a blue solution that may contain an NH3-coordinated Ni complex39. However, further analysis is needed to identify the specific speciation of the blue coloration.

A prior study by the Suib group reported that, in the range of 80-120 °C (above the urea-hydrolysis temperature of 60 °C), the temperature is not a critical factor in the microwave-assisted, urea-mediated synthesis of α-Ni(OH)2 nanoflowers27. In this study, higher temperatures (120-180 °C) influence the crystalline structure, local structure, and porosity of α-Ni(OH)2. This microwave synthesis solution differs from theirs in the selection of solvent; this reaction uses a mixed ethylene glycol/H2O, whereas Suib's group used ethanol/H2O as the reaction medium27. The BET surface areas of α-Ni(OH)2 nanosheets synthesized using this protocol (61-85 m2·g-1) are larger compared to Ni(OH)2 synthesized from NiCl2 (9.2 m2·g-1)22, but lower than the microwave-assisted reflux of nickel nitrate in ethanol (173 m2·g-1)21.

When synthesized in urea and H2O, the α-Ni(OH)2 crystallite growth is reported to be anisotropic, increasing in the (001) direction [with the (110) growth remaining constant] with reaction time and leveling off with the depletion of Ni2+ in solution26. In this study, a greater change in crystallite size in the (001) and (110) directions is observed at elevated temperatures compared to crystallite size changes induced by a longer reaction time. The interlayer region of metal-substituted α-Ni(OH)2 has been shown to change the d-spacing and interlayer ordering based on electrostatics and the population of molecules in the interlayer12. Similarly, this work shows changes in the d-spacing and interlayer ordering, but at elevated reaction temperatures and longer reaction times, rather than by using metal dopants. XRD, FTIR, and SEM-EDS analyses indicate that using a reaction temperature of 120 °C, the α-Ni(OH)2 material's structure is a hydrated form of α-Ni(OH)2, with an interlayer region containing predominantly free nitrates, free water molecules, and other residual molecules from the starting chemicals. A representation of the α-Ni(OH)2 crystal structure with interlayer water and ions is shown in Figure 6C, where the interlayer water and ions are not within fixed crystallographic positions in the unit cell but have some freedom to rotate and translate in the ab-plane4.

When the reaction temperature is increased to 180 °C, a smaller (001) d-spacing, a more ordered (002) interlayer region, and a decrease in the relative intensity of the δ(HOH, free) vibrational mode are observed. During the synthesis at 180 °C, the pressure accumulation in the reaction occurs in part from the decomposition of urea, but also from the vaporization of H2O, resulting in less solution-phase H2O available to incorporate into the α-Ni(OH)2 structure. The differences in the XRD peak positions and relative intensities of the nitrate modes, ν3(NO3–) and ν3(NO3–, free), compared to the δ(H-O-H, free) mode at 120 and 180 °C support that conducting the reaction at higher temperature increases the relative concentration of nitrates within the structure; however, further analysis is needed to determine how nitrate anions interact within the structure. Urea decomposition contributes to an increased reaction pressure, driving the Ni(OH)2 hydrolysis and condensation reaction toward higher reaction yields relative to the 120 °C reactions. The relative intensity of the νs(C-O-CN) mode of the 180 °C sample decreases relative to the 120 °C sample, which indicates less of the urea decomposition product is present within the structure at higher temperatures.

This work provides a protocol for the microwave-assisted synthesis of Ni(OH)2 and shows that reaction temperature and time affect the α-Ni(OH)2 structure and yield. Using different synthesis conditions to control structure provides a pathway to develop improved materials for batteries and other applications. Limitations of this reaction include relatively large volumes of nickel-containing aqueous waste resulting from the washing/centrifuging steps of the protocol. The reaction can generate high pressures and ammonia as a byproduct and may not be suitable for open-air reactors or non-ventilated workspaces. Additionally, this protocol was evaluated in a lab-scale microwave reactor and can be modified to scale the synthesis route to the kilogram scale.

Supplementary Figure 1: Comparison of reaction temperature and reaction pressure versus time. Please click here to download this File.

Supplementary Figure 2: Analysis of microwave-synthesized αNi(OH)2 nanosheets. Please click here to download this File.

Supplementary Figure 3: Effects of drying conditions on X-ray diffraction patterns of α-Ni(OH)2. Please click here to download this File.

Supplementary Figure 4: Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectra of microwave-synthesized α-Ni(OH)2 nanosheets. Please click here to download this File.

Supplementary Table 1: FTIR analysis of microwave-synthesized α-Ni(OH)2 nanosheets. Please click here to download this File.

Disclosures

The authors have nothing to disclose.

Acknowledgements

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.

Materials

ATR-FTIR Bruker Tensor II FT-IR spectrometer equipped with a Harrick Scientific SplitPea ATR micro-sampling accessory
Bath sonicator Fisher Scientific 15-337-409 —
Ethanol  VWR analytical AC61509-0040 200 proof
Ethylene Glycol VWR analytical BDH1125-4LP 99% purity
Falcon Centrifuge tubes VWR analytical 21008-940 50 mL
KimWipes VWR analytical 21905-026 —
Lab Quest 2 Vernier  LABQ2 —
Microwave Reactor Anton Parr 165741 Monowave 450
Ni(NO3)2 · 6 H2O Ward's Science 470301-856 Research lab grade
pH Probe Vernier  PH-BTA Calibrated vs standard pH solutions (pH= 4, 7, 11)
Porosemeter Micromeritics  — ASAP 2020. Analysis software: Micromeritics, version 4.03
Powder x-ray diffactometer Bruker 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.
Reaction vial Anton Parr 82723 30 mL G30 wideneck, 20 mL max fill capacity
Reaction vial locking lid Anton Parr 161724 G30 Snap Cap
Reaction vial PTFE septum Anton Parr 161728 Wideneck
Scanning electron microscope FEI — Helios Nanolab 400
Urea VWR analytical BDH4602-500G ACS grade
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Tags

Nickel HydroxideNanosheetsMicrowave SynthesisHydrothermal SynthesisReaction ConditionsCrystalline StructureSurface AreaPorosityReaction TemperatureReaction TimeAlpha Nickel HydroxideMetal HydroxidesMetal OxidesEnergy StorageCatalysisSensors

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Kimmel, S. W., Kuykendall, V., Mough, C., Landry, A., Rhodes, C. P. Effect of Microwave Synthesis Conditions on the Structure of Nickel Hydroxide Nanosheets. J. Vis. Exp. (198), e65412, doi:10.3791/65412 (2023).
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