Exploration of Methylene Blue Degradation over ZnO Nanorods Mechanism using Scavenging Reagents.

Introduction

Recently, semiconductor nanomaterials have been the focal point of various research aspects due to their attractive and fascinating optical and electrical properties. Such interesting qualities may mainly be attributed to their size reduction¹. In particular, ZnO nanostructures with a large bandgap ( > 3.35 eV), great excitonic energy (60 meV) and are mechanically and thermally stable at ambient conditions. These excellent properties have drawn substantial interest for applications such as electronic and optoelectronic devices and lasers^2,3 chemical sensing ⁴, bio-sensing⁵, biological markers⁶, dye-sensitized solar⁷ and electrochemical cells⁸. Different routes are adopted for ZnO nanostructures preparation, including chemical vapor deposition⁹, pyrolysis^{10, 11}, high-temperature decomposition¹², hydrothermal¹³, sol-gel^{14, 15} and precipitation^16-18. Precipitation at low temperature is an inexpensive, scalable facile method for large types of ZnO nanostructures fabrication. Precipitation procedures have been successfully employed in prior works to synthesize various ZnO assemblies¹⁹.

Whether anionic and cationic, industrial dyes are extensively employed in textile; leather processing²⁰. Being toxic, carcinogenic and mutagenic to aquatic biological systems, these dyes should be treated before discarding in the runoffs²¹. Still, many types of azo dyes including MB are quite persistent and challenging to degradation methods²². Consequently, their interesting photocatalytic and catalytic detoxification have drawn substantial concern in eco-friendly remediation²³. The photocatalytic demineralization of these hazardous organic dyes is an ecologically preferred approach, as no additional chemicals are needed, and thus, no pollution by-products are produced²⁴. During the photodegradation process, where incident radiation with enough energy shines on a semiconductor, it excites electrons from the valence to the conduction band, generating an electron deficiency or hole (h⁺). Oxygen molecules in the aqueous medium react with the electrons to breed strong reducing O₂^.- ions while the adsorbed water molecules and hydroxide ions react with the (h⁺) to generate the strong oxidizing •OH radicals, that are considered the primary radicals for the photocatalytic degradation²⁵.

In the present work, ZnO nanorods were synthesized through a precipitation process. The nanostructure produced was having 15 nm size, 3.229 eV bandgap energy, and 10.13 m².g^-1 surface area. The photocatalyst’s ability to decolorize organic dyes was tested using the MB, where the process fitted well with the first order kinetics with a rate constant of 5.9 x 10^-3min^-1.

Experimental Part

Nanoparticles preparation

The method reported by Kumar et al.²⁶ was followed where an aqueous solution of  1 molar zinc sulfate (200 mL) was added to 2 molar sodium hydroxide solution(200 mL) dropwise under strong stirring for 12 hrs. The colored precipitate obtained was filtered after several times washing (10 times) with deionized water. The precipitate was firstly dried for 2 hrs then calcined at 300°C for 2 hrs.

Characterization of the nanoparticles

The crystalline structure of the powders was investigated by X-ray powder diffraction (XRD) using Bruker high-resolution diffractometer equipped with Cu-Ka radiation (1.5418 Å), operating at 40 kV and 40 mA. Morphological images were recorded by field emission scanning electron microscopy (FE-SEM) using high-resolution Jeol JSM 7600F and transmission electron microscopy (TEM) using JEOL, JEM-2100. The Brunauer – Emmett – Teller (BET) specific surface area was assessed via N2 adsorption – desorption isotherms by employed ASAP 2020 Micromeritics device. The optical properties were determined by means of diffuse reflectance spectroscopy (DRS) using JASECO V-770 spectrophotometer in the wavelength range 300-800 nm. Vibration and bending modes of the all samples were documented by means of Fourier Transform Infrared (FTIR) spectra (JASCO FI-IR 460 spectrometer) in the range 400 – 4000 cm⁻¹.

Photodegradation using Methylene Blue

For the degradation MB as a typical contaminant dye, 10mg/L was used to inspect the ZnO photocatalytic activity. Prior to commencing the experiment, 0.050g of ZnO were dispersed in 150 mL of  MB solution and stirred in the dark for 60 min to reach equilibrium. Afterwards, the photocatalytic evolution was monitored under 265 nm ultraviolet illumination. Throughout the photocatalytic course, (6 mL) of the solution was withdrawn at various times interludes, then centrifuged to clear the solution and then its absorbance was examined between 200 and 700 nm using UV–vis spectrophotometer. The photocatalytic degradation percentage was computed the equation²⁷:       

Knowing that C_o and C_t are the initial concentration (10 mg/L) and time t MB concentration, respectively.

Trapping of Reactive Oxygen Species (ROS)

To study the photocatalytic degradation mechanism of MB over the surface of ZnO nanoparticles, scavenging reagents were used to determine the responsible reactive oxygen species by using isopropyl alcohol (IPA) for scavenging hydroxyl radicals (HO^-.)^27,28, p-benzoquinone (PBQ) for scavenging superoxide radicals (O₂^-.)^27,28, disodium ethylenediaminetetra acetic acid (Na₂EDTA) for scavenging photogenerated holes (h⁺)²⁹, and dimethyl sulfoxide for scavenging photogenerated electrons (e^–)²⁹. The decrease in photocatalytic activity, produced by scavenging reagent, would signify the reactive oxygen species responsible for the photodegradation. (50.0 mmol/L) concentrations were used except for PBQ, which its optimum concentration was 1.0 mmol/L for 10.0 mg/L of MB²⁹.

Results and discussion

XRD analysis

The XRD pattern of the fabricated NPs (Fig. 1) displays an archetypal XRD configuration of ZnO crystal pattern as inveterated by 2Ө ≈ 31.58, 34.24, 36.10, 47.32, 56.38, 62.64, 67.82 and 68.98ᵒ that serially correspond to the (1 0 0), (0 02), (1 01), (1 0 2), (11 0), (1 03), (2 0 0), (112 and, (201) Miller indices indexed to the hexagonal wurtzite ZnO³⁰ and in agreement with the pdf #36-1451 card for ZnO³¹. The purity of the prepared sample is confirmed by the absence of any peak due to impurity. The sharp peaks displayed in the pattern designates the synthesis of a highly crystalline material in the nanometer range.

Figure 1: XRD patterns of ZnONPs Click here to View figure

The  crystallite sizes (D) of the ZnO NPs was computed according to Scherrer’s formula³²:

The terms λ , Ө and β are respectively assigned to the Cu Kα line (1.5406 Aᵒ), the XRD diffraction angle and th (FWHM). The crystallite size of ZnO was determined using the most intense peak and was found to be 15.32 nm. The lattice parameters and the spacing between planes as designed as a,c and d respectively were calculated using equations 3 – 5 ^33-35

Where Ө₁₀₀ and Ө₀₀₂  correspond the Miller indices (100) and (002) correspondingly, the a and c values are inconsistent with the ZnO (JCPDS) card³⁶, connoting the synthesis of the nano-size ZnO. Moreover the d-spacing calculated via the theoretical formula (4) and Bragg’s law (5) are almost identical (Table 1). The length of the Zn–O bond was obtained from the formula (6)

Here (µ) is a factor that describes the magnitude of the atom displacement from its neighbor alongside the c axis, as articulated by (7):

The obtained Zn – O bond value (Table.1) is around the previously reported 1.9767 A° value^37,38. The strain-induced broadening in nanopowder due to crystal imperfection and distortion was calculated using the formula³⁹:

The micro-strain (ε_z) along the c-axis was estimated using the expression⁴⁰

The terms c and c₀ (5.2066 Å) stand for the computed typical lattice parameters in that order. For the ZnO, the micro-strain is equal to 0.51 %which is tensile strain type that may stimulate the crystallite’s orientation along the z-axis⁴¹.

Table 1: The ZnO NPs data obtained from the XRD analysis

Parameter	β	2Ө101	D(nm)	a (Aᵒ)	c(Aᵒ)	c/a	d (Aᵒ)	d*(Aᵒ)	u	Zn – O(Aᵒ)	ε
Values	0.5481 ±0.0003	36.10 ±1.44	15.32 ±0.75	3.2687 ±0.163	5.2335 ±0.210	1.6011	2.4861 ±0.124	2.4903 ±0.125	0.3800 ±0.011	1.9889 ±0.080	0.0025 ±0.0001

 

SEM analysis

The sample’s SEM micrograph displayed in Fig.(2a)shows a large hierarchical porous flower-like structure consisting of several flake-like structures under low magnification. At medium magnification (Fig. 2b), several scattered rod-like nano-structures were covering the flakes’ surface. The detailed micrograph Fig. 2c exhibits high aspect ratio (8.5) nanorods. The (EDS) spectrum (Fig. 2d), snapped from a dense NPs region divulges strong peaks assigned to Zn, O atoms, whereas weaker signal form S discernible as well. The existence of the S element may have come from the sulfate in the zinc precursor.

Figure 2: SEM images at different magnifications (a, b and c) and EDS of ZnO NPs (d) Click here to View figure

FTIR bonding analysis

FTIR spectra of the ZnO have been recorded to inspect the nature of bonds in the prepared materials, in the range 400–4000 cm⁻¹ (Fig. 3). The peaks at 3462 cm⁻¹ and 1647 cm⁻¹ are respectively due to O–H stretching and O–H bending vibrations of water molecule⁴². This is due to water molecules’ existence on the nanostructures’ surface. The band positioned at 2376 cm⁻¹may indicates CO₂ molecules in the air⁴³. The peak at 418 cm⁻¹ corresponds to Zn-O symmetric bending vibration, and the peak at 604 cm⁻¹ is due to the weak vibration of Zn-O ⁴⁴. The other peak at 785 cm⁻¹ attributed to Zn-S symmetric bending ⁴⁵, confirming the EDS finding.

Figure 3: FTIR spectrum of ZnO NPs Click here to View figure

Nitrogen adsorption isotherms

The isotherm profile delivers evidence about surface consistency, and it could be generally characterized as displayed in Fig. 4; the nitrogen adsorption-desorption isotherm for the ZnO sample is type II, as categorized by IUPAC and Brunauer-Emmet-Teller (BET). The isotherm shows a type H₃ hysteresis loop, distinctive of accumulated particles with mesoporous solids and free monolayer-multilayer adsorption. This hysteresis at P/P₀range 0.5and 1 is attributed to condensation in the mesopores related to the inter-particle cavities of the ZnO structure⁴⁶. The point, at the start of the linear central part of the chart, shows the step where the monolayer coverage is completed and multilayer adsorption starts⁴⁷. The specific surface of 10.13 m².g^-1estimatedapplying the BET conventional method is a characteristic of a material with a crystallized material^{48, 49}. 10.05 cm³/g and 18.25 nm, are the pore volume and pore diameter, respectively. The particle size distribution (Fig. 4 inset) reveals that the average around 30 nm, which is consistent with the isotherm analysis data.

Figure 4: Adsorption isotherms and pore size distribution. Click here to View figure

Optical properties and bandgap determination

To examine the optical properties of the ZnO NPs its UV–visible spectrum was verified and plotted in Fig.5. The graph exhibits a sharp absorption edge rising at 400 nm, which is shifted to a higher wavelength relative to the standard ZnO peak⁵⁰, ⁵¹. The diffuse reflectance spectroscopy (DRS)analysis and (αhv)² versus hv plots were carried out probe the band energy of ZnO NPs (Fig. 5 inset), which was found to be 3.229 eV as estimated applying theTauc equation⁵²:

Here h,ν, α, and E_g are Planck’s constant, frequency, absorption coefficient and bandgap energy. A is a constant, and n represents the electron transition type (for directly allowed transitions, n = 1/2). The E_g for the NPs (3.229eV) is less than the standard 3.37 eV value for ZnO⁵³.

Figure 5: UV-Vis absorbance and Tauc plots (inset)of ZnO NPs. Click here to View figure

Methylene Blue (MB) photodegradation

The UV photodegradation of 10 mg/L Methylene Blue (MB) dye solution was done to monitor the photocatalytic actions of the ZnO NPs. 30 mg of nanomaterials were sonicated in 100 ml of MB solution. Prior to commencing photocatalysis, the solution was stirred for 40 min in the dark to achieve equilibrium. Then, the photocatalytic evolution was probed under ultraviolet irradiation at 𝜆 =265 nm. Throughout the photocatalysis course, 5 mL of the solution was pipetted at different times intervals, centrifuged to eliminate suspended nanoparticles, and its absorbance was examined between 500 and 800nm using UV–vis spectrophotometer. The proportion of the photocatalytic degradation was assessed using the relation²⁷:         

Where A_o and A_t are the initial and time t MB absorbance, respectively.

The photocatalytic action of the ZnO nanostructures was scrutinized by photodegrading the MB organic contaminant dye. The process was observed by determining the MB absorbance at λ_max = 662 ⁵⁴ and different UV radiation time intervals (Fig. 6). A gradual decrease in absorbance is evidence of enhanced catalytic efficiency of the ZnO (Fig. 6). The data reveals a catalytic competence by the ZnO photocatalyst (Fig. 7 a), which is higher than that shown by BaTiO₃ nanoparticles that showed comparable results but with a lower MB concentration ⁵⁴. The result is in general similar to many previously reported findings for Ag/ZnO ⁵⁵ZnO/NiFe2O4 ⁵⁶ and SrFe12O19 ⁵⁷ for the degradation of MB. The MB photocatalytic degradation kinetics is demonstrated in Fig. 7 b, where the data was plotted according to the pseudo-first kinetics model:

The graph demonstrates pseudo-first-order kinetics as reflected by (R²) value (0.97)⁵⁸. The rate constant calculated from the slope is equal to k = 5.9 x 10^-3min^-1, t_1/2 = 117 min and E_a = 12.72 kJ. Comparable results showing the consistency of MB photocatalytic degradation with the first order kinetic model were testified using different nanomaterials^{56, 57, 59-61}.

Figure 6: MB absorbance at different time intervals under UV light irradiation. Click here to View figure

Figure 7: % degradation at different time intervals (a) and  kinetics of MB Photo degradation (b). Click here to View figure

Blocking of Reactive Oxygen Species (ROS)

The data in Table 2 presents that using isopropanol, as a scavenging reagent for hydroxyl radical did not affect the photodegradation efficiency, regardless of ZnO nanostructure photocatalyst identity. This observation implied that hydroxyl radical has no role in the photodegradation process of MB. However, the use of  para-benzoquinone, for superoxide radical, the photodegradation efficiency was falling down, , from 75% (in the absence of all scavenging reagent) to 45%. This result showed that superoxide radical’s minor role in the photodegradation of MB. However, the use of disodium ethylenediaminetetraacetic acid for the photogenerated holes, caused a remarkable reduction in photodegradation efficiency 75% to 16% over ZnO. This observation demonstrates the importance of holes direct interaction with MB on ZnO nanorods’ surface in photocatalytic degradation.  The role of electrons photogenerated was also examined by using dimethylsulfoxide (DMSO). It was found that they did not dramatically affect photodegradation, which was lowered from 75% to 54% over ZnO. This result indicated that MB photodegradation did not depend on photogenerated electrons over ZnO.  Based on scavenging reagents experiments, we conclude that the valence band is where photodegradation of MB on ZnO occurs.  

Table 2: Effects of scavenging reagents on MB photocatalytic degradation over ZnO photocatalyst

Scavenging reagent	None	IPA	DMSO	PBQ	Na2EDTA
MB photocatalytic degradation	75%	75%	54%	45%	16%

 

Photocatalyst Recycling

We examined the recyclability of ZnO photocatalysts for reuse in the industrial sector. We found that the photodegradation efficiency decreased from 73% (first cycle) to ~68% (second cycle) to 42% (third cycle) over ZnO nanorods (Table 3). This decrease in photodegradation efficiency with the reused photocatalyst could be due to photodegradation products’ accumulation on the photocatalyst surface⁶². 

Table 3: ZnO photocatalyst recovering for MB photo degradation.

Cycle number	1	2	3
MB photo degradation	73%	68%	42%

 

Mechanism of Photocatalysis

Ultraviolet light irradiation cause electrons excitation offering them enough energy to transit from the valence band (VB) to the conduction band (CB) of ZnO nanoparticles, opening positive holes (h⁺) in the VB ( ZnO →^UV  e__CB– + h__VB⁺)⁶³. The strong oxidative species O₂^−•generated from the electrons and O₂ reaction (O₂+ e⁻ →O₂^−•) associate with H⁺ from solution to generate the peroxide H₂O₂ (O₂+ 2 H⁺+e_CB⁻ →H₂O₂)⁶⁴. The producedH₂O₂ then goes through chain reactions with electron to the avail the active •OH radicals (H₂O₂ + e⁻ → OH⁻ +^•OH). Similarly ^•OH can be created via the h⁺/ surface adsorbed H₂O reactions²¹. The degradation process proceeds by the consecutive attacks on the organic dye by •OH radicals (R + ^•OH → R^•’+    H₂O) or h⁺ (R + h⁺ → R^•+ → products) ⁶⁵. Fig. 8 is a diagrammatic exemplification for the proposed photo degradation mechanism.

Figure 8: Elucidation of MB photo degradation. Click here to View table

Conclusion

 ZnO NPs of 15 nm size of archetypal wurtzite structure and cell parameters were efficaciously prepared through a precipitation approach. The SEM images exposed flower-like structures NPs, which were further found to compose primarily of Zn and Oelements as supported by the EDS analysis. Moreover, the establishment of Zn – O bonding was exhibited by the FTIR vibrational frequency assignments. According to the optical examination, the synthesized nanomaterials showed 3.229 eV bandgap. The reaction was pseudo-first-order kinetics, and the degradation mechanism was proposed using scavenging reagents. They were suggesting that the photodegradation of MB on ZnO occurs on its valence band. Also, the recyclability of the photocatalysts was studied. This photocatalyst application for MB degradation marks it an excellent candidate for pollutant dyes detoxification.

Acknowledgement

The authors wish to express thanks to Imam Mohamed Ibn Saud Islamic University, where this work has been done.

Conflicts of Interest

The authors certify that they have NO affiliations with or involvement in any organization or entity with any financial interest or non-financial interest in the subject matter or materials discussed in this manuscript.

References

1. Iqbal, J., Abbasi, B.A., Mahmood, T., Kanwal, S., Ahmad, R., Ashraf, M., 2019. Plantextract mediated green approach for the synthesis of ZnONPs: characterization and evaluation of cytotoxic, antimicrobial and antioxidant potentials. J. Mol. Struct. 1189, 315–327. https://doi.org/10.1016/j.molstruc.2019.04.060
   CrossRef
2. R. Bomila, S. Suresh, and• S. Srinivasan, Synthesis, characterization and comparative studies of dual doped ZnO nanoparticles for photocatalytic applications, Journal of Materials Science: Materials in Electronics (2019) 30:582–592.
   CrossRef
3. Khan, M.M., Saadah, N.H., Khan, M.E., Harunsani, M.H., Tan, A.L., Cho, M.H., 2019. Potentials of Costus woodsonii leaf extract in producing narrow band gap ZnO nanoparticles. Mater. Sci. Semicond. Process. 91, 194–200. https://doi.org/ 10.1016/j.mssp.2018.11.030.
   CrossRef
4. Huang, J., Wu,Y., Gu,C., Zhai, M., Yu,K., Yang, M., andLiu, J., Large-scale synthesis of flowerlike ZnO nanostructure by a simple chemical solution route and its gas-sensing property. Sensors and Actuators B: Chemical, 2010, 146(1),206-212.
   CrossRef
5. Xia, C., Wang, N., Lidong, L., and Lin, G. , Synthesis and characterization of waxberry-like microstructures ZnO for biosensors. Sensors and Actuators B: Chemical, 2008, 129(1), 268-273.
   CrossRef
6. Bruchez, M., Moronne, M., Gin,P.,Weiss, S., and Alivisatos, P.,Semiconductor nanocrystals as fluorescent biological labels. science,1998, 281(5385), 2013-2016.
   CrossRef
7. Lee, D., Bae,WK., Park, I., Yoon, DY., and Lee, S. , Transparent electrode with ZnO nanoparticles in tandem organic solar cells. Solar Energy Materials and Solar Cells, 2011, 95(1), 365-368.
   CrossRef
8. Marina Bandeira a,b , Marcelo Giovanela b , Mariana Roesch-Ely b , Declan M. Devine a , Janaina da Silva Crespo, Green synthesis of zinc oxide nanoparticles: A review of the synthesis methodology and mechanism of formation, Sustainable Chemistry and Pharmacy, 15 (2020), 100223.
   CrossRef
9. Muccillo, E., S. Tadokoro, and R. Muccillo, Physical characteristics and sintering behavior of MgO-doped ZrO 2 nanoparticles. Journal of Nanoparticle Research, 2004, 6(2), 301-305.
   CrossRef
10. Yu, W., X. Li, and X. Gao, Catalytic synthesis and structural characteristics of high-quality tetrapod-like ZnO nanocrystals by a modified vapor transport process. Crystal growth & design, 2005, 5(1), 151-155.
    CrossRef
11. Roshitha, S.S., Mithra, V., Saravanan, V., Sadasivam, S.K., Gnanadesigan, M., 2019. Photocatalytic degradation of methylene blue and safranin dyes using chitosan zinc oxide nano-beads with Musa X paradisiaca L. pseudo stem. Bioresour. Technol. Rep. 5, 339–342. https://doi.org/10.1016/j.biteb.2018.08.004.
    CrossRef
12. Liewhiran, C., S. Seraphin, and S. Phanichphant, Synthesis of nano-sized ZnO powders by thermal decomposition of zinc acetate using Broussonetia papyrifera (L.) Vent pulp as a dispersant. Current Applied Physics, 2006. 6(3), 499-502.
    CrossRef
13. Liu, B. and H.C. Zeng, Hydrothermal synthesis of ZnO nanorods in the diameter regime of 50 nm. Journal of the American Chemical Society, 2003. 125(15), 4430-4431.
    CrossRef
14. s. Sandeep Arya, Prerna Mahajan, Sarika Mahajan, Ajit Khosla, Ram Datt, Vinay Gupta, Sheng-Joue Young, and Sai Kiran Orugant, Review—Influence of Processing Parameters to ControlMorphology and Optical Properties of Sol-Gel Synthesized ZnONanoparticle, ECS Journal of Solid State Science and Technology, 202110023002
15. Peng, W., Qu, SC., Cong, GW., and Wang, ZG., Structure and visible luminescence of ZnO nanoparticles. Materials science in semiconductor processing, 2006. 9(1), 156-159.
    CrossRef
16. ManjunathaRL, Usharani KVand Dhananjay Nai, Synthesis and characterization of ZnO nanoparticles: A review Journal of Pharmacognosy and Phytochemistry 2019; 8(3): 1095-1101E-ISSN:2278-4136P-
17. Li, C., Yu, Z., Fang, S., Wang,H., Gui,Y., and Xu, J.,Preparation and performance of ZnO nanoparticle aggregation with porous morphology. Journal of Alloys and Compounds,2009. 475(1), 718-722.
    CrossRef
18. Kanitta Phongarthit, Pongsaton Amornpitoksuk and Sumetha Suwanboon, Synthesis, characterization, and photocatalytic properties of ZnO nanoparticles prepared by a precipitation-calcination method using a natural alkaline solution, Mater. Res. Express (2019), 6 (4), 045501
    CrossRef
19. Sepulveda-Guzman, S., Reeja-Jayan, B., Rosa de la, E., Torres-Castro, A., Gonzalez-Gonzalez, V., and Jose-Yacaman, M., Synthesis of assembled ZnO structures by precipitation method in aqueous media. Materials Chemistry and Physics, 2009. 115(1), 172-178.
    CrossRef
20. Wang, W., Tadé,M.O., and Shao, Z., Research progress of perovskite materials in photocatalysis-and photovoltaics-related energy conversion and environmental treatment. Chemical Society Reviews, 2015. 44(15), 5371-5408.
    CrossRef
21. Wu, G. and Xing,W., Fabrication of ternary visible-light-driven semiconductor photocatalyst and its effective photocatalytic performance. Materials Technology, 2018,1-9.
22. Narayanan, K.B. and Sakthivel, N., Synthesis and characterization of nano-gold composite using Cylindrocladium floridanum and its heterogeneous catalysis in the degradation of 4-nitrophenol. Journal of hazardous materials, 2011. 189(1-2),519-525.
    CrossRef
23. Badr, Y. and Mahmoud, M., Photocatalytic degradation of methyl orange by gold silver nano-core/silica nano-shell. Journal of Physics and Chemistry of Solids, 2007, 68(3),413-419.
    CrossRef
24. Ajmal, A., Majeed, I., Malik, RN. , Idriss, H. , and Nadeem, MA. , Principles and mechanisms of photocatalytic dye degradation on TiO 2 based photocatalysts: a comparative overview. RSC Advances, 2014,4(70), 37003-37026.
    CrossRef
25. Ibhadon, A. and Fitzpatrick, P., Heterogeneous photocatalysis: recent advances and applications. Catalysts, 2013, 3(1), 189-218.
    CrossRef
26. Kumar, S.S., Venkateswarlu, P, Rao, VR., and Rao, GN., Synthesis, characterization and optical properties of zinc oxide nanoparticles. International Nano Letters, 2013, 3(1), 30.
    CrossRef
27. Meng, A., Xing,J., Li, Z., and Li, Q., Cr-doped ZnO nanoparticles: Synthesis, characterization, adsorption property, and recyclability. ACS applied materials & interfaces, 2015, 7(49), 27449-27457.
    CrossRef
28. Chang, J.S., Strunk, J., Chong, M.N., Poh, P.E. and Ocon, J.D. Multi-dimensional zinc oxide (ZnO) nanoarchitectures as efficient photocatalysts: What is the fundamental factor that determines photoactivity in ZnO, Journal of hazardous materials. 2020, 381, 120958.
    CrossRef
29. Shtarev, D., Shtareva, A., Blokh, A., Goncharova, P. and Makarevich, K. On the question of the optimal concentration of benzoquinone when it is used as a radical scavenger. Applied Physics A, 2017, 123, 602.
    CrossRef
30. Zak, A.K., Razali, R., Abd Majid, WH., and Darroudi, M., Synthesis and characterization of a narrow size distribution of zinc oxide nanoparticles. International journal of nanomedicine, 2011, 6, 1399.
    CrossRef
31. Swarthmore, P., Powder diffraction file, joint committee on powder diffraction standards. International Center for Diffraction data. Card, 1972,3-0226.
32. Barrett, C.S., Structure of metals1943: McGraw-Hill Book Company, Inc.; New York.
33. Seetawan,U.; Jugsujinda,S. ; Seetawan,T.; Ratchasin ,A.; Euvananont,C.; Junin,C.; Thanachayanont, C.; Chainaronk,P. Effect of calcinations temperature on crystallography and nanoparticles in ZnO disk. Materials Sciences and Applications, 2011, 2(09), 1302.
    CrossRef
34. Pandiyarajan, T. and B. Karthikeyan, Cr doping induced structural, phonon and excitonic properties of ZnO nanoparticles. Journal of Nanoparticle Research, 2012, 14(1), 647.
    CrossRef
35. Pal, U., Garcia Serrano a , J.; Santiago, P.; Gang Xiong, Ucer, K.B.; Williams, R.T. Synthesis and optical properties of ZnO nanostructures with different morphologies. Optical Materials, 2006, 29(1), 65-69.
    CrossRef
36. Chung, F.H., Quantitative interpretation of X-ray diffraction patterns of mixtures. II. Adiabatic principle of X-ray diffraction analysis of mixtures. Journal of Applied Crystallography, 1974, 7(6), 526-531.
    CrossRef
37. Modwi, A.; Abbo, M.; Hassan, E; Taha, K.; Khezami, L.; Houas, A, Influnce of annelling temperature on the properties of ZnO synthesid via 2.3. dihydroxysuccinic acid using flash sol-gel method. Journal of Ovonic research, 2016. 12(2).
38. Taha, K.; M’hamed, M.; and Idriss, H. Mechanical fabrication and characterization of zinc oxide (ZnO) nanoparticles. J. Ovon. Res, 2015, 11(6), 271-276.
39. Mote, V.;Purushotham, Y. and Dole, B. Williamson-Hall analysis in estimation of lattice strain in nanometer-sized ZnO particles. Journal of Theoretical and Applied Physics, 2012, 6(1), p. 6.
    CrossRef
40. Karthika, K. and Ravichandran, K. Tuning the microstructural and magnetic properties of ZnO nanopowders through the simultaneous doping of Mn and Ni for biomedical applications. Journal of Materials Science & Technology, 2015, 31(11), 1111-1117.
    CrossRef
41. Snega, S.; Ravichandran, K.; Baneto, M.; Vijayakumar, S., Simultaneous enhancement of transparent and antibacterial properties of ZnO films by suitable F doping. Journal of Materials Science & Technology, 2015. 31(7), 759-765.
    CrossRef
42. Anbuselvan, D. and S. Muthukumaran, Defect related microstructure, optical and photoluminescence behaviour of Ni, Cu co-doped ZnO nanoparticles by co-precipitation method. Optical Materials, 2015, 42, 124-131.
    CrossRef
43. Abdelouhab, Z.A.; Djouadia, D.; Chelouchea , A.; Hammichea , L.; Touam, T., Effects of precursors and caustic bases on structural and vibrational properties of ZnO nanostructures elaborated by hydrothermal method. Solid State Sciences, 2019. 89, 93-99.
    CrossRef
44. Devi, P.G. and A.S. Velu, Structural, optical and photoluminescence properties of copper and iron doped nanoparticles prepared by co-precipitation method. Journal of Materials Science: Materials in Electronics, 2016. 27(10), 10833-10840.
    CrossRef
45. Roychowdhury, A.; Pati,S.; Kumar,S.; Das,D., Effects of magnetite nanoparticles on optical properties of zinc sulfide in fluorescent-magnetic Fe3O4/ZnS nanocomposites. Powder Technology, 2014. 254, 583-590.
    CrossRef
46. Mercera, P.;Van Ommen,J.;Doesburg,E.;Burggraa,A.Ross,J., Zirconia as a Support for Catalysts: Evolution of the Texture and Structure on Calcination in Air. Applied catalysis, 1990. 57(1),127-148.
    CrossRef
47. Sing, K.S., Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity (Recommendations 1984). Pure and applied chemistry, 1985. 57(4), 603-619.
    CrossRef
48. Azouaou, N.; Sadaoui, Z.; Djaafri, A.; Mokaddem, H., Adsorption of cadmium from aqueous solution onto untreated coffee grounds: Equilibrium, kinetics and thermodynamics. Journal of Hazardous Materials, 2010. 184(1), 126-134.
    CrossRef
49. Miretzky, P., C. Munoz, and E. Cantoral-Uriza, Cd2+ adsorption on alkaline-pretreated diatomaceous earth: equilibrium and thermodynamic studies. Environmental Chemistry Letters, 2011. 9(1), 55-63.
    CrossRef
50. Karlsson, M.E., Mamie,Y.; Calamida,A.; Gardner,J.; Ström,V.; Pourrahimi,A. and Olsson,R., Synthesis of zinc oxide nanorods via the formation of sea urchin structures and their photoluminescence after heat treatment. Langmuir, 2018. 34(17), 5079-5087.
    CrossRef
51. Xu, T.; Zhang,L.; Cheng,H.; Zhu ,Y., Significantly enhanced photocatalytic performance of ZnO via graphene hybridization and the mechanism study. Applied Catalysis B: Environmental, 2011. 101(3-4), 382-387.
    CrossRef
52. Tauc, J., Optical properties and electronic structure of amorphous Ge and Si. Materials Research Bulletin, 1968. 3(1), 37-46.
    CrossRef
53. Ridha, N.J., Haji Jumali,M. ; Umar,A. and Alosfur, F. Defects-controlled ZnO nanorods with high aspect ratio for ethanol detection. Int. J. Electrochem. Sci, 2013, 8, 4583-4594.
54. Kappadan, S.;Gebreab ,T.; Thomas,S.; Kalarikkal,N.,Tetragonal BaTiO3 nanoparticles: an efficient photocatalyst for the degradation of organic pollutants. Materials Science in Semiconductor Processing, 2016. 51, 42-47.
    CrossRef
55. Rafaie, H.; Nor,R.; Azmina,M.; Ramli,N.,andMohamed, R. ,Decoration of ZnO micro-structures with Ag nanoparticles enhanced the catalytic photodegradation of methylene blue dye.J Environ Chem Eng, 2017, 5, 3963-3972.
    CrossRef
56. Adeleke,J.; Theivasanthi,T.; Thiruppathi, M.; Swaminathan,M.; Akomolafe,T.; Alabi,A.; Photocatalytic degradation of methylene blue by ZnO/NiFe2O4 nanoparticles, Applied Surface Science, 2018,455, 195-200,
    CrossRef
57. Mishra, D.D. and G. Tan, Visible photocatalytic degradation of methylene blue on magnetic SrFe12O19. Journal of Physics and Chemistry of Solids, 2018. 123, 157-161.
    CrossRef
58. Sopajaree, K.; QASIM, S.; BASAK,S. and RAJESHWAR,K. An integrated flow reactor-membrane filtration system for heterogeneous photocatalysis. Part I: Experiments and modelling of a batch-recirculated photoreactor. Journal of applied electrochemistry, 1999. 29(5), 533-539.
    CrossRef
59. Zhang, T., Oyama,T.; Aoshima ,A.; Hidaka ,H, Zhao , J.; Serpone,N., Photooxidative N-demethylation of methylene blue in aqueous TiO2 dispersions under UV irradiation. Journal of Photochemistry and Photobiology A: Chemistry, 2001. 140(2), 163-172.
    CrossRef
60. Abbaslou, R.M., V. Vosoughi, and A.K. Dalai, Comparison of nitrogen adsorption and transmission electron microscopy analyses for structural characterization of carbon nanotubes. Applied Surface Science, 2017. 419, 817-825.
    CrossRef
61. Vig, A., Gupta, A. and Pandey, O., Efficient photodegradation of methylene blue (MB) under solar radiation by ZrC nanoparticles. Advanced Powder Technology, 2018. 29(9), 2231-2242.
    CrossRef
62. Pant, A., Tanwar, R., Kaur, B. & Mandal, U.K. A magnetically recyclable photocatalyst with commendable dye degradation activity at ambient conditions. Scientific reports,2018,8, 14700.
    CrossRef
63. Wang, J., shen, H.; Dai, X.;Li,C.;Shi,W.,Yan,Y., Graphene oxide as solid-state electron mediator enhanced photocatalytic activities of GO-Ag3PO4/Bi2O3 Z-scheme photocatalyst efficiently by visible-light driven. Materials Technology, 2018. 33(6),421-432.
    CrossRef
64. Saikia, L., Bhuyan,D.; Saikia,M.; Banajit,M.; Dutta,D.; Sengupta,P., Photocatalytic performance of ZnO nanomaterials for self sensitized degradation of malachite green dye under solar light. Applied Catalysis A: General, 2015. 490, 42-49.
    CrossRef
65. Houas, A., Lachheb,H.; Ksibi,M.; Elaloui a,E.; Guillard,C.; Herrmann,J., Photocatalytic degradation pathway of methylene blue in water. Applied Catalysis B: Environmental, 2001. 31(2), 145-157.
    CrossRef

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