Table 3. Prior works for the photocatalytic degradation of 4-nitrophenol by ZnO nanoparticles under various conditions.

Entry	Author(s)/year	ZnO catalyst	Synthesis method	Initial concentration	Catalyst dosage	Irradiation source	Irradiation time	Degradation%
1	Khatamian et al. (2012)105	ZnO/HZSM-5	Polyacrylamide pyrolysis method	20 ppm	250 mg/50 mL	Ambient visible light	75 min	100%
2	Khatamian, & Alaji (2012)106	ZnO/HZSM-5	Impregnation method	20 ppm	500 mg/100 mL	30 W UV-C lamp	90 min	91%
3	Sugiyama et al. (2012)107	Bulk ZnO	Commercially available	30 ppm	5000 mg/1000 mL	Solar light + UV light	190 min	100%
4	Zhang et al. (2012)108	TiO2/ZnO/Au nanofibers	Electrospinning + in situ reduction	10 ppm	100 mg L−1	UV light	40 min	96%
5	Khatamian et al. (2012)96	Ln-doped ZnO (La, Nd, Sm)	Polymer pyrolysis method	10 ppm	100 mg/100 mL	UV lamp (30 W, UV-C)	195 min	83.43%
6	B. Divband et al. (2013)109	Ag/ZnO	Polyacrylamide-gel method	10 ppm	100 mg/50 mL	UV light	180 min	100%
		Ag/ZnO	Photoreduction method	10 ppm	100 mg/50 mL	UV light	180 min	92%
		Ag/ZnO	Chemical reduction method	10 ppm	100 mg/50 mL	UV light	180 min	86%
7	Ansari et al. (2013)110	Modified-ZnO	Biogenic synthesis using EAB	5 ppm	2 mg/20 mL	Visible light (λ > 500 nm)	6 h	100%
8	Nezamzadeh-Ejhieh et al. (2014)111	ZnO/nano-clinoptilolite zeolite	Ion exchange and calcination	10 ppm	250 mg L−1	Hg-lamp (75 W, 253 nm)	180 min	100%
9	Aslam et al. (2014)112	V2O5–ZnO composites	Co-precipitation	30 ppm	100 mg/150 mL	Sunlight (1000 × 102 lux)	150 min	60%
10	Hameed et al. (2014)97	W6+ impregnated ZnO	Wet impregnation	50 ppm	1 g L−1	Sunlight	120 min	99%
11	Kale et al. (2015)113	ZnO–graphene composites	Hydrazine-assisted hydrothermal method	8.35 ppm	1 g L−1	UV irradiation	60 min	95%
12	Qamar et al. (2015)114	CuO@ZnO core–shell catalysts	Wet impregnation	30 ppm	150 mg/150 mL	Sunlight	180 min	99%
13	Rabbani et al. (2016)115	TCPP/ZnFe2O4@ZnO	Hydrothermal method followed by sol–gel and immobilization of TCPP	50 ppm	100 mg/50 mL	5 W LED visible light	180 min	67%
14	Verma et al. (2017)98	ZnO-reduced graphene oxide (ZGS)	Solvothermal method with ammonia-modified graphene oxide	10 ppm	100 mg/50 mL	125 W high-pressure Hg lamp	240 min	96%
15	Qin et al. (2017)116	Fe3O4@SiO2@ZnO	Microwave irradiation with (DEG) as solvent	30 ppm	400 mg L−1	300 W high-pressure Hg lamp	120 min	90.5%
16	Mou et al. (2018)117	Ag/ZnO nanosheets assemblies	Solution method	10 ppm	30 mg/100 mL	Simulated sunlight (300 W Xe lamp)	25 min	100%
17	Khairy et al. (2020)118	ZnO/CNT, ZnO/GO	Ultrasonic irradiation/hydrothermal	10 ppm	1750 mg L−1	Visible light (160 W), ultrasonic (60 W, 20 kHz)	15 min	98.3%
18	Kadam et al. (2021)119	Biogenic ZnO nanoparticles	Biological synthesis using endophytic fungi	100 ppm	100 mg L−1	UV lamp (75 W, 365 nm)	480 min	84%
19	Chakraborty et al. (2021)120	Ag2O–ZnO	Microwave-assisted synthesis	20 ppm	400 mg L−1	UV-LED (25 W, 365 nm)	60 min	99%
20	Kumar et al. (2021)121	ZnO/g-C3N4	Co-precipitation	10 ppm	50 mg/50 mL	Visible light (250 W)	180 min	90%
21	Wang et al. (2021)122	ZnO NPs	Solvothermal method	20 ppm	1500 mg L−1	25 W UV lamp	180 min	56.2%
22	Usman et al. (2023)123	N, S-codoped ZnO	Co-precipitation	4 ppm	200 mg/100 mL	500 W high-pressure Hg lamp	100 min	99.3%
23	Current work	ZnO@henna	Green synthesis	50 ppm	25 mg/25 mL solution	Blue LED (30 W)	120 min	93%