Table 6.

Representative methods from the literature for sol–gel preparation of Ni(OH)₂ materials. a-PEO, amorphous polyethylene oxide; CTAB, cetyltrimethyl ammonium bromide; DEA, diethanolamine; EDA, 1,2-ethanediamine; FTO, fluorine-doped tin oxide; OAc, [O₂CCH₃]⁻ (acetate); OEt, [OCH₂CH₃]⁻ (ethoxide); PVA, polyvinyl alcohol; SDS, sodium dodecylsulfate.

Ni(II) salt	solvent(s) and reagents	method	product phase	comments	refs
NiCl2	OC(NH2)2	stir reactants at room temperature for 2 h	α-Ni(OH)2	product contains some β-Ni(OH)2	[148]
	SDS	heat at 80°C for 6 h		yields nanoparticle (approx. 250 nm) xerogel
	H2O	centrifuge to obtain product		product is mesoporous (dpore=3.6 nm)
		dry overnight at 100°C
NiCl2	NaOEt	stir NiCl2, NaOEt and HOEt at 70–80°C for 3–5 h	α-Ni(OH)2	dried with supercritical CO2 to yield an aerogel	[146]
	HOEt	add aqueous NH4OH to obtain a sol
	NH4OH	leave overnight to obtain a hydrogel
	H2O
NiCl2	ethylene glycol	stir NiCl2 in ethylene glycol and n-butanol 30 min	α-Ni(OH)2	film contains NiO	[21]
	n-butanol	add HOAc and reflux at 70°C for 3 h		yields nanoparticle (d∼30 nm) xerogel
	HOAc	add H2O and a-PEO/DEA then reflux for 1 h
	H2O	cool to room temperature to obtain sol
	a-PEO	dip-coat onto SnO2 glass
	DEA	dry at 100°C for 10–30 min
NiCl2	EtOH	stir NiCl2 with water and EtOH	β-Ni(OH)2	stacking fault disorder	[149]
	N2H4	stir N2H4 and NaOH in water		yields clusters of nanoparticles (d∼65 nm)
	NaOH	add second solution to first dropwise until pH≈10
		centrifuge the sol to obtain a gel (powder)
Ni(NO3)2	H2O	stir Ni(NO3) and NH4OH, in water	β-Ni(OH)2	Some stacking fault disorder	[150]
	NH4OH	stir 1 h at 70°C		yields nanoparticle (d = 50–80 nm) xerogel
		centrifuge, rinse and dry at 80°C for 12 h
Ni(NO3)2	H2O	stir Ni(NO3), NH4OH and PEG-400 in water	β-Ni(OH)2	stacking fault disorder	[150]
	NH4OH	stir 1 h at 70°C		yields nanoflakes (d=20–60 nm) and needles (d=10–20 nm, l=100–200 nm)
	PEG-400	centrifuge, rinse and dry at 80°C for 12 h
Ni(NO3)2	H2O	stir Ni(NO3), NH4OH and Tween-80 in water	β-Ni(OH)2	stacking fault and hydration disorder	[150]
	NH4OH	stir 1 h at 70°C		low crystallinity
	Tween-80	centrifuge, rinse and dry at 80°C for 12 h		yields needles (d∼10 nm, l≈100 nm)
Ni(NO3)2	OC(NH2)2H2O	stir Ni(NO3)2 and urea in water	α-Ni(OH)2	very low crystallinity	[151]
	H2O	add maleic acid and stir 10 min to obtain sol		yields nanoparticle (d∼500 nm) xerogel
	maleic acid	heat at 100°C for 24 h		product is mesoporous (dpore=16 nm)
		centrifuge to obtain product
Ni(NO3)2	NaOH	stir Ni(NO3)2, NaOH and EDA in water	β-Ni(OH)2	yields hollow, ‘dandelion-like’ microspheres	[116]
	H2O	Dry at 100°C for 0–3 h		(d∼5 μm) composed of nanosheets
	EDA			hydration disorder
NiSO4	H2O	combine NiSO4 and NaOH solutions	β-Ni(OH)2	stacking fault disorder	[152]
	NaOH	centrifuge the sol to obtain a gel and dry at 80°C		yields nanoparticle (d∼10 nm) xerogel
NiSO4	H2O	stir all reagents except KOH in water	β-Ni(OH)2	high surface area, mesoporous xerogel	[153]
	sodium citrate	add KOH solution dropwise
	NaOAc	centrifuge, rinse and dry at 80°C
	KOH
NiSO4	CTAB	dissolve NiSO4 in CTAB, butanol, cyclohexane	α/β-Ni(OH)2	mostly, the α-phase materials is produced	[154]
	n-butanol	add NaOH solution dropwise		product contains SO${\hspace{0.17em}}_4^{2-}$
	cyclohexane	transfer to ice water
	NaOH	centrifuge, rinse and dry
NiSO4	H2O	combine Ni(II) and LiOH solutions	α-Ni(OH)2	film contains SO${\hspace{0.17em}}_4^{2-}$ and CO${\hspace{0.17em}}_3^{2-}$ foreign anions	[155]
	LiOH	wash precipitate with water
	HOAc	acidify with acetic acid
	formamide	stir with formamide, glycerol, PVA
	glycerol	wet substrate with Teloksid A-30
	PVA	dip-coat onto Pt, CdTe or Si substrate
	Teloksid A-30
Ni(OAc)2	H2O	stir Ni(OAc)2 and water for 2 days	β-Ni(OH)2	film may contain OAc− foreign anions	[156]
		dip-coat onto Ni foil substrate		hydration disorder
				film was dried to yield NiO xerogels
Ni(OAc)2	glycerine	stir all reagents except water	unknown	clear gelatinous mass	[143]
	KOH	let stand several days
	HOEt	dilute with water then partially dry
	H2O
Ni(OAc)2	glycerine	stir reagents	α-Ni(OH)2	Ref. also prepared Ni1−xCox(OH)2, 0≤x≤0.5	[157]
	KOH	spin- or dip-coat onto FTO glass
	n-butanol
Ni(OAc)2	glycerine	spread sol onto glass, quartz, FTO- or Pt-coated Si substrate α-Ni(OH)2	α-Ni(OH)2	yields nanoparticles (d<10 nm)	[158]
	KOH	dry under vacuum then remove glycerine at 240°C
	n-butanol