Table 3.

Summary of the gas sensing properties of 1-D ZnO nanostructures for different gases.

Gas Tested	Morphology	Size		Detection Range	Detection Temperature (°C)	Optimum Working Temperature (°C)	Response			Response Time	Recovery Time	Reference
		Diameter	Length				Sensitivity	Concentration	Temperature (°C)
Ethanol	Nanowire	25 ± 5	-	1–200 ppm	300	–	32 A	100 ppm	300	-	-	[9]
	Nanowire	80 nm	1 μm	50–1,500 ppm	180–300	–	43 D	100 ppm	300	-	-	[47]
	Nanorod (flowerlike)	150 nm	Few micron	0.5–1,000 ppm	300	–	14.6 A	100 ppm	300	-	-	[53]
	Nanorod (bushlike)	15 nm	1 μm	1–1,000 ppm	300	–	29.7 A	100 ppm	300	-	-	[57]
	Nanorod (vertically aligned)	50 nm	500 nm	1–100 ppm	300	–	100 A	100 ppm	300	-	-	[7]
H2S	Nanorod	70–110 nm	0.2–1.3 μm	0.005–10 ppm	25–400	25–200	1.7 A	0.05 ppm	25	-	-	[5]
	Hierarchical dendrite	60–800 nm	-	10–500 ppm	30	–	17.3 A	100 ppm	30	15–20 s	30–50 s	[20]
H2	Nanorod (single)	-	-	1–1,000 ppm	RT	–	4% C	200 ppm	RT	30–40 s	50–90 s	[11]
	Nanowire	10–30 nm	50–250 nm	100–1,000 ppm	RT	–	3 A	200 ppm	RT	-	–	[59]
	Nanobelt	10 nm (thickness)	50 nm (width)	0.06–1%	150–450	385	14.3 C	1%	385	48 s	336 s	[22]
	Nanorod (Pd coated)	30–150 nm	2–10 μm	10–500 ppm	RT-200	–	4.2% E	500 ppm	-	-	<20 s	[23]
NO2	Nanowire	80–120 nm	10–20 μm	0.5–20 ppm	225	–	>95 B	20 ppm	250	24 s	12 s	[54]
	Nanorods(vertically aligned)	50 nm	500 nm	10 ppb–10 ppm	150–400		824% F	100 ppb	250	4.5 min	4 min	[12]
	Nanobelt	10 nm (thickness)	50 nm (width)	0.51–1.06 ppm	150–450	350	0.81 D	8.5 ppm	350	180 s	268 s	[22]
	Fibre-mats	100–300	–	0.1–0.5	20–150	20	>100 D	0.04	20	Order of minutes	Order of minutes	[21]
	Cauliflower	20–30				100	-		-
Propane	Nanobelt	10 nm (thickness)	50 nm (width)	0.25–1%	150–450	370	0.17 C	1%	370	72 s	252 s	[21]
HCHO (Methanal)	Nanorod	20–40 nm	100 nm	50–1,000 ppm	100–425	300	11.8 A	100 ppm	300	3 s	9 s	[18]
		40–60 nm	200 nm				9 A			4 s	11 s
C6H4(CH3)2(Xylene)	Nanorod	20–40 nm	100 nm	50–1,000 ppm	100–425	150	9.6 A	100 ppm	150	6 s	12 s	[18]
		40–60 nm	200 nm				6 A			7 s	20 s
CO	Nanowire	50–125 nm	1.1–5.4 μm	500 ppm	320	–	57% F	500 ppm	320	–	-	[60]

Note: A: S = R_a/R_g, B: S = R_g/R_a, C: S = ΔR/R_g, D: S = ΔR/R_a, E: S = (ΔR/R_g) × 100% and F: S = (ΔR/R_a) × 100%.