Table 1.

Summary of methods of obtaining zinc oxide.

Method	Precursors	Synthesis conditions	Properties and applications	References
Mechanochemical process	ZnCl2, Na2CO3, NaCl	calcination: 2 h, 600 °C	hexagonal structure; particles diameter: 21–25 nm	[30]
		400–800 °C	hexagonal structure; particles diameter: 18–35 nm	[31]
		400 °C	regular shape of particles; diameter ~27 nm, BET: 47 m2/g	[32]
		0.5 h	particles diameter: 27–56 nm	[33]
		300–450 °C	particles diameter: ~51 nm, BET: 23 m2/g	[34]
Precipitation process	Zn(CH3COO)2, and KOH as a water solutions	temperature of process: 20–80 °C; drying: 120 °C	particles diameter: 160–500 nm, BET: 4–16 m2/g	[35]
	Zn(CH3COO)2, (NH4)2CO3, PEG10000 as a water solutions	drying: 12 h, 100 °C; calcination: 3 h, 450 °C	zincite structure; spherical particles (D ~ 30 nm); application: as a photocatlyst in photocatalytic degradation	[36]
	Zn(NO3)2	calcination: 2 h, 600 °C; aging: 240 h, 320 °C	wurtize structure; particles diameter: 50 nm; application: as a gas sensor	[37]
	Zn(NO3)2, NaOH	synthesis: 2 h; drying: 2 h, 100 °C	particles of spherical size of around 40 nm	[38]
	ZnSO4, NH4HCO3, ethanol	drying: overnight, 100 °C; calcination: 300–500 °C	wurtize structure; crystallite size 9–20 nm; particle size D: ~12 nm, BET: 30–74 m2/g	[39]
	Zn(CH3COO)2, NH3 aq.	precipitation temperature: 85 °C; drying: 10 h, 60 °C	hexagonal structure, shape of rods, flower-like particles: L: 150 nm, D: 200 nm	[40]
	ZnSO4, NH4OH, NH4HCO3	reaction: 30 min, 60 °C; drying: 12 h, 100 °C; calcination: 2 h, 400 °C	hexagonal structure, flake-like morphology (D: 0.1–1 μm, L: 60 nm)	[41]
	microsized ZnO powder, NH4HCO3	reaction: ~2 h, 25 °C; drying: 80 °C; calcination:1 h, 350 °C	hexagonal wurtize structure; flower-like and rod-like shape (D: 15–25 nm, BET: 50–70 m2/g)	[42]
	Zn(CH3COO)2, NaOH	reaction: 30 min, 75 °C; drying: overnight, room temperature	hexagonal structure; flower shape (L: >800 nm); application: antimicrobial activity	[43]
Precipitation in the presence of surfactants	ZnCl2, NH4OH, CTAB	aging: 96 h, ambient temperature, calcination: 2 h, 500 °C	zincite structure; particles diameter: 54–60 nm, BET = ~17 m2/g	[44]
	Zn(NO3)2, NaOH, SDS, TEA (triethanolamine)	precipitation: 50–55 min, 101 °C	wurtize structure, shape of rod-like (L: 3.6 μm, D: 400–500 nm) shape of nut-like and rice-like, size: 1.2–1.5 μm	[45]
Sol-gel	Zn(CH3COO)2, oxalic acid, ethanol and methanol	reaction temperature: 60 °C; drying: 24 h, 80 °C; calcination: 500 °C	zincite structure; aggregate particles: ~100 nm; shape of rod; particles L: ~500 nm, D: ~100 nm; BET: 53 m2/g; application: decontamination of sarin (neuro-toxic agent)	[46]
	Zn(CH3COO)2, oxalic acid (C2H2O4), ethanol	reaction: 50 °C, 60 min; dried of gel: 80 °C, 20 h; calcined: under flowing air for 4 h at 650 °C	hexagonal wurtize structure; uniform, spherically shaped of particles	[47]
	zinc 2-ethylhexanoate, TMAH ((CH3)4NOH), ethanol and 2-propanol	reaction: room temperature; drying: 60 °C	cylinder-shaped crystallites, D: 25–30 nm; L: 35–45 nm	[48]
	Zn(CH3COO)2, diethanolamine, ethanol	reaction: room temperature; annealed of sol: 2 h, 500 °C	hexagonal wurtize structure; particles: nanotubes of 70 nm	[49]
Solvothermal hydrothermal and microwave techniques	ZnCl2, NaOH	reaction: 5–10 h, 100–220 °C in teflon-lined autoclave	particles morphology: bullet-like (100–200 nm), rod-like (100–200 nm), sheet (50–200 nm), polyhedron (200–400 nm), crushed stone-like (50–200 nm)	[50]
	Zn(CH3COO)2, NaOH, HMTA (hexamethylenetetraamine)	reaction: 5–10 h, 100–200 °C; HMTA concentration: 0–200 ppm	spherical shape; particles diameter: 55–110 nm	[51]
	Zn(CH3COO)2, Zn(NO3)2, LiOH, KOH, NH4OH	reaction: 10–48 h, 120–250 °C	hexagonal (wurtize) structure, size of microcrystallites: 100 nm–20 μm	[52]
	Zn(CH3COO)2, NH3, zinc 2-ethylhexanoate, TMAH, ethanol, 2-propanol	time of autoclaving: 15 min, 2–72 h; final pH: 7–10	particles with irregular ends and holes; aggregates consist particles of 20–60 nm, BET: 0.49–6.02 m2/g	[53]
	trimethylamine N-oxide, 4-picoline N-oxide, HCl, toluene, ethylenediamine (EDA), N,N,N’,N’-tetramethylethylenediamine (TMEDA)	reaction: 24–100 h, 180 °C	wurtize structure; particles morphology: nanorods (40–185 nm), nanoparticles (24–60 nm)	[54]
Solvothermal hydrothermal and microwave techniques	Zn(CH3COO)2, Zn(NO3)2, ethanol, imidazolium tetrafluoroborate ionic liquid	reaction: 150–180 °C; drying: 80 °C in vacuum oven; calcinations: 500 °C	hexagonal (wurtize) structure, hollow microspheres (2–5 μm) consisted nano-sized particles and contained channels (10 nm); hollow microspheres consisted of nanorods (~20 nm); flower-like microspheres (2.5 μm)	[55]
	zinc acetylacetonate, methoxy-ethoxy- and n-butoxyethanol, zinc oximate	precursor concentration: 2.5–10 wt%; microwave heating: 800 W, 4 min; drying: 75 °C in air	zincite structure; average crystallite size: 9–31 nm; particles diameter: 40–200 nm; BET: 10–70 m2/g	[56]
	Zn(NO3)2, deionized water, HMT (hexamethylenetetramine)	microwave heating: 2 min, 90 °C; drying: 2 h, 60 °C	hexagonal wurtize structure, nanorod and nanowire shape (L: ~0.7 μm, D: ~280 nm); application: electronic and optoelectronic devices	[57]
Emulsion	Zn(NO3)2, surfactant (ABS, Tween-80 and 40, C21H38BrN)	reaction: 25 °C, pH~8; drying: 24 h, 80 °C; calcination: 2 h, 600 °C	grain size: cationic surfactants (40–50 nm), nonionic surfactants (20–50 nm), anonic surfactants (~20 nm)	[37]
	Zn(C17H33COO)2, NaOH, decane, water, ethanol	reaction: 2 h, room temperature or 90 °C	particles morphology: irregular particles aggregates (2–10 μm); needle-shaped (L: 200–600 nm, T: 90–150 nm); nearly spherical and hexagonal (D: 100–230 nm); spherical and pseudospherical aggregates (D: 150 nm)	[58]
	Zn(CH3COO)2, heptanes, Span-80, NH4OH	reaction: 1 h; aging: 2.5 h; drying: in rotary evaporator; calcination: 2 h, 700–1000 °C	hexagonal structure; spherical shape; particles diameter: 0.05–0.15 μm	[59]
	Zn(CH3COO)2, NaOH and KOH, cyclohexane, non-ionic surfactants	reaction: ambient temperature; drying: 24 h, 120 °C	hexagonal structure; particles morphology: solids (164–955 nm, BET: 8 m2/g), ellipsoids (459–2670 nm, BET: 10.6 m2/g), rods (396–825 nm, BET: 12 m2/g), flakes (220–712 nm, BET: 20 m2/g); crystallites size: 32–77 nm; application: as a photocatalyst	[60]
Microemulsion	Zn(NO3)2, NaOH, heptane, hexanol, Triton X-100, PEG400	reaction: 15 h, 140 °C; drying: 60 °C	hexagonal (wurtize) structure; particles morphology: needle (L: 150–200 nm, D: ~55 nm), nanocolumns (L: 80–100 nm, D: 50-80 nm), spherical (~45 nm)	[61]
Microemulsion	Zn(NO3)2, oxalic acid, isooctane, benzene, ethanol, diethyl ether, chloroform, acetone, methanol, Aerosol OT	reaction: 1 h; calcination: 3 h, 300 °C	equivalent spherical diameter: 11.7–12.9 nm, BET: 82–91 m2/g; grain size: 11–13 μm	[62]
	Zn(CH3COO)2, Aerosol OT, glycerol, C20H37NaO7S, n-heptane, NaOH, methanol, chloroform	reaction: 24 h, 60–70 °C; drying: 1 h, 100 °C; calcination: 3 h, 300–500 °C	hexagonal wurtize structure, spherical shape (15–24 nm), rods shape (L: 66–72 nm, D: 21–28 nm)	[63]
	ZnCl2, Zn(CH3COO)2, heptane, BTME (1,2-trimethoxysilyl)ethane, TMOS (tetramethoxysilane), methanol, Aerosol OT, NaOH	reaction: 2–3 h, room temperature or 40 °C; drying: under vacuum overnight; calcinations: 24 h, 700 °C	hexagonal structure, uniformly dispersed small particles, size of particles ~10 nm	[64]
Other method	Zn(CH3COO)2	thermal decomposition: 350–800 °C	uniform size of particles 20–30 nm	[65]
	Zn(NO3)2, deionized water, HMT (hexamethylenetetramine)	ultrasonic irradiation: 30 min, 80 °C; drying: 2 h, 60 °C	hexagonal wurtize structure, nanorod and nanowire shape (L: ~1 μm, D: ~160 nm); application: electronic and optoelectronic devices	[57]
	micron scale zinc metal powder	feed rate: 1 g/min; plasma power: 1 kW; O2 flow rate: 2.5 lpm; N2 flow rate: 12.5 lpm; reaction: 900 °C	nanowires shape (L: 1–30 μm, D: 5–50 nm) application: as hydrodesulfurization catalyst	[66]
	diethylzinc (DEZ), oxygen	helium as a carrier gas	wurtize structure; average particle size: 9 nm	[67]

Note: BET—surface area calculated based on BET equation; D—particles diameter; L—particles length.