Preparation and characterisation of nickel oxide and nickel oxide codoped with magnesium and zinc

The present work focuses on the formulation and characterisation of undoped NiO and NiO codoped with Mg and Zn. The samples were characterized by x-ray diffractometer and ultraviolet-visible spectrophotometer. For the detection of elements present in the samples, energy dispersive x-ray analysis was employed. Two sets of codoped samples were prepared varying the amount of Mg and Zn. A set of codoped samples was prepared keeping the quantity of Mg at a fixed value and allowed the quantity of Zn to fluctuate. The second set of codoped samples was prepared keeping the extent of Zn at a fixed value and allowed the quantity of Mg to fluctuate. The bandwidth of undoped and codoped materials was computed raising Tauc plot. Crystallite size was computed following Scherrer formula.


Introduction
The significance of undoped, doped and codoped metal oxides in various disciplines like medical field, photocatalysis, electronics and spintronics is illustrated by the copious number of research publications in various national and international reputed journals [1][2][3][4][5][6][7].Zinc oxide codoped with silicon and manganese was investigated for its photocatalytic activity in the reduction of hazardous chromium [8].Zinc oxide and zinc oxide doped with manganese prepared under different conditions were studied for the application in photocatalysis [9].Foxtail millet husk was used in the preparation of silica-silver composite with antimicrobial and free radicle scavenging activities [10].Zinc oxide codoped with nickel and magnesium was reported along with the influence of the quantity of of the codopants on the bandwidth of zinc oxide [11].There is a report on another photocatalyst based on silica incorporated magnesium-zinc oxide [12].A report is available on electrochemical sensing based on mixed metal oxide [13].A microcidal, non-cytotoxic and free radical scavenging composite was reported based on opaline silica [14].Zinc oxide doped with magnesium was studied for its structural, optical, and photocatalytic properties [15].A detailed report is available on the investigation of synthesis, structural and optical properties of zinc oxide doped with manganese [16].Magnesium oxide codoped with nickel and cobalt was investigated for its electrochemical biosensor applications 1291 (2023) 012021 IOP Publishing doi:10.1088/1757-899X/1291/1/012021 2 [17].A review available in public domain is more than enough to illustrate the significance of nickel oxide in different areas [18].Effect of annealing on the properties of nickel oxide thin film was investigated and reported [19].A study on the magnetic properties of nickel oxide nanoparticles was reported in addition to the study on thermal conductivity [20,21].Present work aims at the formulation and characterisation of undoped NiO and NiO codoped with Mg and Zn.Two sets of codoped samples were prepared varying the amount of Mg and Zn.While preparing first set of NiO codoped with Mg and Zn, masses of NiSO4.6H2O and ZnSO4.7H2O were kept constant, while mass of the other dopant, MgSO4.7H2O was varied.The second set of NiO codoped with Mg and Zn was prepared keeping masses of NiSO4.6H2O and MgSO4.7H2Oconstant, while mass of the dopant, ZnSO4.7H2O was varied.Tauc plot was employed for computing bandwidth.

Experimental 2.1 Chemicals used in the work
Chemicals used include NiSO4.6H2O,NaOH, ZnSO4.7H2O and MgSO4.7H2O.For preparing solutions and washing of products, double deionised water was used.

Formulation of Nickel oxide
Weighed out exactly 4 g of NiSO4.6H2Ointo 250 ml beaker and dissolved in 100 ml of deionised water.It was kept for agitating over magnetic stirrer at normal temperature.While stirring, added dropwise, solution of 2.0679 g NaOH in 50 ml of deionised H2O.The amount of NaOH was determined based on the balanced chemical equation representing the reaction.After addition, continued stirring for 10 minutes.The precipitate of Ni(OH)2 formed was then sorted out of G4, splashed with deionised H2O till washings freed from Ni +2 , SO4 −2 and NaOH.Chemical test was conducted to check presence or absence of these kinds in the washings.After washing with deionised water, the precipitate was ultimately bathed with acetone then dried for 10 minutes at 60 0 C in an oven.Dried Ni(OH)2 was transferred into silica crucible, then kept in a muffle furnace at normal temperature.Raised temperature of furnace to 450 0 C from normal temperature then stayed at 450 0 C for 2 hours.Following 2 hours, switched off furnace, then permitted to cool to room temperature.Once cooling to normal temperature, silica crucible containing NiO was taken out.The material, NiO was powdered and used for further study.

Formulation of NiO codoped with Mg and Zn
Two sets of codoped samples were prepared varying the amount of Mg and Zn.While preparing first set of NiO codoped with Mg and Zn, masses of NiSO4.6H2O and ZnSO4.7H2O were kept constant, while mass of other dopant, MgSO4.7H2O was varied.Weighed out NiSO4.6H2O, ZnSO4.7H2O and MgSO4.7H2O, as given in table 1, into a 250 ml beaker and dissolved in 100 ml of deionised water.It was kept for agitating over magnetic stirrer at normal temperature.While stirring, added dropwise, a solution of the required amount of NaOH, calculated based on the balanced chemical equation representing the reaction, in 50 ml of deionised water.After addition, continued stirring for 10 minutes.The precipitate formed was then sorted out of G4.Washed precipitate using deionised water till washing was free from Mg 2+ , SO4 −2 , Zn 2+ , Ni +2 and NaOH.Chemical test was conducted to check presence or absence of these kinds in the washings.After washing with deionised water, precipitate formed was ultimately bathed with acetone then dried for 10 minutes at 60 0 C in an oven.After drying at 60 0 C, the precipitate was transferred into silica crucible, then kept in muffle furnace at normal temperature.Raised temperature of furnace to 450 0 C from normal temperature, then remained at 450 0 C for 2 hours.Following 2 hours, switched off furnace, then permitted to cool to normal temperature.Following cooling to normal temperature, silica crucible containing NiO codoped with Mg and Zn was taken out.The doped sample was powered and used for further study.All samples as given in table 1 were prepared taking masses as mentioned in this table.The second set of NiO codoped with Mg and Zn was prepared keeping masses of NiSO4.6H2O and MgSO4.7H2Oconstant, while mass of the dopant, ZnSO4.7H2O was varied.Following the same procedure adopted for the preparation of the first set, the second set was also prepared considering the masses as given in table 2.

Characterisation of the samples -Instrumental analysis 3.1 Energy Dispersive X-ray analyses (EDX)
Elements in materials were detected with EDX mentioned as EDS or EDAX.

X-Ray Diffraction (XRD) analyses
To study details of crystals of prepared materials, powder X-ray diffraction analysis was performed.

Ultraviolet-visible spectroscopy analyses
For determining band gap of materials, solid-state ultraviolet-visible spectral analyses were made.

Elemental analyses -EDAX result
Results of elemental analysis of materials H, I, J, K, L H2, I2, J2, K2, and L2 are presented in figure 1 to figure 10.The samples are all codoped nickel oxides, codoped with Mg and Zn.The expected elements from all these samples include Ni, Zn, Mg and O.The results confirm occurrence of the four elements in the samples.The peak, in all samples, at about 0.5 keV corresponds to oxygen.There are peaks in all samples at about 1 keV, 8.5 keV and 9.5 keV, corresponding to zinc.The peak, in all samples, at about 7.5 keV corresponds to nickel.The peak, in all samples, at about 1.5 keV corresponds to magnesium.Nickel oxide belongs to cubic crystal system.However, it undergoes a distortion from cubic to rhombohedral [22][23][24].With variation of codoping percentage, we observed a shift in peak as given by figure 24 and figure 25 .As codoping percentage increases, it could be observed that the peak is shifting towards lower 2θ values.This shift in the peak, with respect to the undoped sample, is a good support for the conclusion that the codopants, Mg and Zn, entered the crystal lattice of nickel oxide, that is, codoping occurred.Since the shift in peak is exceedingly small, as one can observe from figure 24 and figure 25, it can be concluded that codoping could not distort the rhombohedral structure of nickel oxide, and so all the samples, codoped and undoped, possess rhombohedral crystal structure.The Tauc plots are presented for set 2 samples G2, H2, I2, J2, K2, and L2, figure 32 to figure 37. Band-gap energies of set 2 samples are summarised in table 4. Band gap energy of NiO, G2, is 3.06 eV.Value available in literature varies from 3.6 eV to 4 eV.Upon codoping, the value increases.As codoping percentage increases, that is, from sample G2 to sample K2, it gradually increases as shown in the table 4. But sample L2 shows a decrease.

Conclusion
Two sets of codoped samples were prepared varying the amount of Mg and Zn.A set of codoped samples was prepared keeping the extent of Zn at a fixed value and allowed the quantity of Mg to fluctuate, set 1. Another set of codoped samples was prepared keeping the quantity of Mg at a fixed value and allowed the quantity of Zn to fluctuate, set 2. The presence of the elements Ni, Zn, Mg and O in the samples was confirmed by EDAX.All samples were exposed to powder XRD analysis.XRD lines could be indexed as (003), ( 202), ( 220), (223), and (042).They correspond to rhombohedral crystal system of NiO.Both undoped and codoped samples possess rhombohedral crystal structure, and the codoping did not lead to the distortion of rhombohedral crystal lattice of NiO.With variation of codoping percentage, we observed a shift in peak.As codoping percentage increases, it could be observed that the peak is shifting towards lower 2θ values.This shift in the peak, with respect to the undoped sample, is a good support for the conclusion that the codopants, Mg and Zn, entered the crystal lattice of nickel oxide, that is, codoping occurred.Since the shift in peak is exceedingly small, it can be concluded that codoping could not distort the rhombohedral structure of nickel oxide, and so all the samples possess rhombohedral crystal structure.Band-gap energies of set 1 samples shows a decrease as codoping percentage increases, that is, from sample G to sample L. Band-gap energies of set 2 samples shows an increase as codoping percentage increases, that is, from sample G2 to sample K2.FWHM and crystallite size were also obtained.

Table 1 :
Set 1-Preparation of NiO and NiO codoped with Mg and Zn

Table 2 :
Set 2-Preparation of NiO and NiO codoped with Mg and Zn

Table 3 :
Set 1-Band gap energies in the undoped and codoped NiO

Table 4 :
Set 2-Band gap energies in the undoped and codoped NiO

Table 5 :
Set 1-Full width half maxima and crystallite size Sl.

Table 6 :
Set 2-Full width half maxima and crystallite size Sl.