Department of Chemistry,School of Advanced Sciences,VIT,Vellore,632014,India

Keywords:Nickel aluminate Sol-gel Green pigment Citric acid Chemical resistance

ABSTRACT Novel environment friendly inorganic nano green pigments,Ni1-xCuxAl2O4(x=0,0.02,0.04,0.06,0.08,0.1)were successfully synthesised by simple,cost effective sol-gel method using citric acid as a gelling agent.Synthesised nano pigments were characterised by powder XRD,FT-IR,UV-DRS,SEM-EDX and TEM.Distribution of elements such as Ni,Cu,Al and O for the pigment Ni0.98Cu0.02Al2O4was authenticated by elemental mapping analysis.The colour parameters were studied using CIE-LAB parameters.It is evident from the DRS measurement that the band gap energy of NiAl2O4(3.11 eV)has been massively diminished to 2.63 eV when x=0.02,unexpectedly changed the colour of the pigment from cyan to green.Whilst x=0.1 the pigment colour has turned into grey and the corresponding band gap condensed into 2.17 eV.Effect of mineralisers like NaF,CaF2,NH4H2PO4and Li2CO3on the colour of Ni0.98Cu0.02Al2O4was investigated.

1.Introduction

Colour plays a very imperative role in our day-to-day life.Without the influence of colour,the materials which we use every day becomes contemptible.Colours sway our sensation and commonly bump up the manner by which we enjoy our ambience.The sources of these colours leap from both natural and synthetic.It is evident that the artists used ultramarine blue,Prussian blue,Chrome green,Malachite green and Naples yellow from the prehistoric times which are sourced from the Mother Nature[1].As the chemistry becomes wiser,there was a budding significance to synthesise high-quality industrial pigment which is highly stable to chemical and thermal restraint than the natural ones[2].The advantage of blue and green inorganic solids is colourstable,environmentally benevolent and lucrative has provoked interest in the midst of researchers.

Amongst all the colours,green is the colour most commonly associated with environment,strength,youth,spring,optimism and envy[1]henceforth ceramics,plastics,paints,building materials,and glass coatings are broad and wide in green colour.Chrome oxide green(Cr2O3),cobalt green(CoO·ZnO)and green spinel of the formula Co2TiO4are some of the conventional industrial pigments.Pyrophosphate-based green pigment BaCr2(P2O7)2with 90% NIR reflectance was recently reported by Zheng Xu Tao et al.[3].Cobalt chromate-CoCr2O4and the green Victoria garnet-Ca3Cr2Si3O12are some of the other green pigments which are now limited due to the presence of toxic metals such as Cr and Co and also due to the undesirable effects on the human body and the surroundings.

In recent times,more attention is converged on the synthesis of rareearth based compounds to conquer this rigorous environmental crisis.Toshiyuki Masui et al.,reported yttrium doped rare earth cuprate as an environmental friendly green pigment following conventional ceramic method[4].Y2BaCuO5as an NIR reflective green pigment was reported by Sheetu Jose et al.,with 61%reflectance at 1100 nm following nanoemulsion method[5].Green complex pigment with TiO2@CoTiO3shell structure was developed by Jian Zou and Wei Zhang[6].Yet,the expenditure for the rare earth compounds utilised in the synthesis of the pigments is pretty high and hence not economically feasible.

Nano mixed metal oxides grasp the attention of the researchers due to their potential applications in the areas such as catalysis,sensors,semiconductors and ceramics[7].In particular mixed metal oxide pigments offer exceptional resistance to acid and base and virtually to all organic solvents.Wet chemical methodologies such as coprecipitation[8],sol-gel[9],PEG assisted sol-gel[10],hydrothermal[11],Nanoemulsion method[5]and combustion method[12-14]were adopted for the synthesis of these metal oxides.Amongst the various wet chemical methodologies,sol-gel method yields nanoparticles with explicit surface area and elevated homogeneity at quite lower temperature.Integrated network of metal ion citrate(gel)was formed by the hydrolysis and polycondensation reaction between citric acid and metal nitrate solution due to the tri-ligand nature of the citric acid.The gel was further calcined,headed the formation of metal oxide nano powders[15,16].

Nickel aluminate is a spinel group pigment,whose general formula is AB2O4and the space group is Fd3m.It is a partially inverse spinel,where the divalent(Ni2+)and trivalent cations(Al3+)are randomly arranged both in tetrahedral and octahedral sites[17].Blue coloured NiAl2O4was reported by Hun et al.,using reverse micelle processing[18].Nano Mg1-xNixAl2O4was prepared by polymeric combustion technique[17],and the low-temperature combustion technique[19].Here both the methodologies gave unique colour in which the foregoing method yielded pale green and the posterior method yielded cyan colour.Ni doped ZnAl2O4was prepared by Diana Visinescu et al.,[20].However,there are very few reports on the synthesis of green pigment without the influence of Co,Cr and the huge expenditure rare earth elements[4-6].Here,we made an attempt to prepare NiAl2O4based cost effective green pigment using copper as a dopant and its band gap engineering was performed for the first time.Its chemical resistance was studied using 10% H2SO4and 10% NaOH and the thermal resistance was examined by calcining the sample up to 1200 °C.

2.Experimental

2.1.Materials

The chemicals used were nickel nitrate[Ni(NO3)2·6H2O 98%S.d.fine],aluminium nitrate[Al(NO3)3·9H2O 98% S.d.fine],copper nitrate[Cu(NO3)2·6H2O 99%,S.d.fine]and citric acid[C6H8O7,97%,S.d.fine].

2.2.Synthesis

The desired green pigment of the composition Ni1-xCuxAl2O4(x=0.02,0.04,0.06,0.08 and 0.1)were prepared by simple costeffective sol-gel method(Fig.1).In a typical synthesis of un-doped nickel aluminate,1.1631 g of Ni(NO3)2·6H2O and 3.001 g of Al(NO3)3·9H2O was dissolved in 100 ml of double distilled water and magnetically stirred at 60°C for 1 h with 2.3 g of citric acid to obtain the sol which was further heated at 80 °C for 2 h.The obtained gel was kept at 120°C for 12 h.Finally,the obtained precursor powder was calcined at 800°C for 3 h in a muffle furnace.In the copper doped analogue Cu(NO3)2·6H2O was added along with Ni(NO3)2·6H2O[21].

2.3.Characterisation

The crystalline phase of the synthesised pigments was characterised by X-ray powder diffraction(XRD-BRUKER D8 Advanced)technique using a Ni-filtered Cu-Kαradiation(λ=0.154056 nm)operated at 45 kV and 30 mA.The data were collected in the range of 2θ 10°-70°.Fourier Transformed Infrared spectra(FT-IR)were acquired in the range of 400-4000 cm-1from JASCO FT-IR 4100 instrument using KBr pellet press.XPS measurement has been made using a custom built ambient pressure XPS system from Prevac and equipped with VG Scienta mono-chromator(MX650).UV-Visible-diffuse reflectance spectroscopy(JASCO-V670 spectrophotometer)was used to analyse the optical properties and colour parameters.Surface morphology and EDX of the synthesised compounds were analysed using ZESIS EVO18 SEM instrument.The colour characteristics of the materials were assessed through L*a*b* coordinates according to CIE(Commission Internationale de l'Eclairage)in which L*denotes the degree of lightness and darkness of the colour in relation to the scale extending from white(L*=100)to black(L*=0),a*denotes the scale extending from green(-a*)to red(+a*)axis,and b*denotes the scale extending from blue(-b*)to yellow(+b*)axis[22].

The samples were tested for their chemical resistance by dipping 0.1 g of the powder samples in 10% H2SO4and 10% NaOH solutions.Pre-weighed samples were magnetically stirred in either an acidic or alkaline solution for half an hour at room temperature.The resultant powder was filtered,washed with water,dried and reweighed[12]and analysed using UV-Visible-diffuse reflectance spectroscopy and X-ray diffraction technique.

Fig.1.Schematic representation of formation of NiAl2O4by sol-gel method.

3.Results and Discussion

3.1.Powder XRD analysis

The X-ray diffractogram of Ni1-xCuxAl2O4(x=0,0.02,0.04,0.06,0.08 and 0.1)along with the standard pattern is shown in Fig.2.The peaks at 2θ values 19.12°,31.37°,37.12°,45.03°,56.07°,59.72°,and 65.62°correspond to the(111),(220),(311),(400),(422),(511)and(440)planes of NiAl2O4nanoparticle indicating the presence of single phase cubic structure according to the JCPDS card No.10-0339,which belongs to Fd3m space group.The unit cell parameter is calculated using the formula[23],

Fig.2.XRD pattern of Ni1-xCuxAl2O4(x=0,0.02,0.04,0.06,0.08 and 0.1).

where a,the unit cell parameter,d is the interplanar distance and h,k,and l indicate the miller indices.The average crystalline size of pure nickel aluminate and the copper doped nickel aluminate are calculated using the Debye-Scherrer's formula[23],

where D,the average crystalline size(nm),λ,the wavelength of X-ray beam(0.154 nm),θ is the Bragg's angle and β,the full width at half maximum(FWHM)are calculated and given in Table 1.We noticed the gradual increase in the lattice parameter and decrease in the average crystalline size of nickel aluminate as the doping of copper ion increases.Otera Arean et al.,[24]prepared copper doped nickel aluminate solidsolutions by solid state reaction method in which the complete conversion of spinel phase took place only after 300 h heating at 1223 K.According to their observation the lattice parameter of nickel aluminate increases as the copper doping increases from 0 to 1.The reason could be due to the larger ionic radii of Cu2+(Cutet=57 pm,Cuoct=73 pm)than Ni2+(Nitet=55 pm,Nioct=69 pm)[25].

Table 1 Lattice parameter and average crystalline size of Ni1-xCuxAl2O4(x=0,0.02,0.04,0.06,0.08 and 0.1)

3.2.Fourier transformed infrared(FT-IR)analysis

The FT-IR spectra(Fig.3)of the synthesised pigments of composition Ni1-xCuxAl2O4(x=0,0.02,0.04,0.06,0.08 and 0.1)exhibited bands at 682 cm-1,505 cm-1and 442 cm-1indicating the formation of spinel phase structure and are associated with thevibration modes,which are located in both tetrahedral and octahedral environments[17,26,27].vibration modes are also held in the same region[28],which is aroused due to Cu2+substitution for Ni2+in NiAl2O4.This trait bears out the partial inverse spinel structure of nickel aluminate.

Fig.3.FT-IR spectra of Ni1-xCuxAl2O4(x=0,0.02,0.04,0.06,0.08 and 0.1).

3.3.Morphological studies

3.3.1.Scanning Electron Microscopy(SEM)and EDX analysis of synthesised pigments

Fig.4.Scanning electron microscopy images(left)and Energy dispersive X-ray analysis(right)of a)NiAl2O4b)Ni0.98Cu0.02Al2O4c)Ni0.96Cu0.04Al2O4d)Ni0.94Cu0.06Al2O4e)Ni0.92Cu0.08Al2O4f)Ni0.9Cu0.1Al2O4.

Fig.4 shows the scanning electron microscopy images and EDX spectra of Ni1-xCuxAl2O4(x=0,0.02,0.04,0.06,0.08 and 0.1).Irregular morphology was perceived in all the cases with serious aggregation of small particles.This occurrence could be due to the bonding of the adjacent particles through hydrogen bonding of water,followed by appropriate capillary action created in the course of drying the precursors[29].However,when x=0.1(higher concentration of copper)regular morphology with rice like particles was obtained.Energy dispersive X-ray analysis further confirms the presence of elements such as Ni,Cu,Al and O.From the elemental mapping images(Fig.5)we noticed that all the elements are consistently distributed and the formation of corresponding pigment composition is evident,which is in good agreement with the X-ray analysis.

3.3.2.TEM analysis of Ni0.98Cu0.02Al2O4

In view of the fact that highly agglomerated particles are noticed in the SEM images and the aggregation is in micrometre range to further emphasis the morphology,transmission electron microscopy analysis was carried out.Fig.6 illustrates the bright-field image of Ni0.98Cu0.02Al2O4at different magnifications.Even in 10 nm range more number of particles aggregated together to form cluster of particles with average particle size less than(15±2)nm.On the other hand,nonuniform grain sizes are noticed in the sample.Fig.6(f)shows the SAED pattern Ni0.98Cu0.02Al2O4with a ring pattern suggesting the crystalline spinel structure.This result is in accordance with the XRD results(Fig.2).

3.3.3.X-ray Photo Electron Spectroscopy analysis(XPS)

XPS analysis of Ni0.98Cu0.02Al2O4was performed to investigate the valence state of the elements and chemical composition and the results are shown in Fig.7.The peaks at 877.6 eV and 859.4 eV are due to Ni 2p1/2and Ni 2p3/2respectively[30].The corresponding satellite peaks observed at 884.3 eV and 867.7 eV were consistent with Ni 2p1/2and Ni 2p3/2respectively[30].The two signals detected in the region of 953.4 and 933.3 eV are related to Cu 2p1/2and Cu 2p3/2respectively,and the equivalent satellite peaks that are observed at 960.7 eV and 940.3 eV confirmed the presence of Cu2+[31].Al3+binding energies that were found at 72.9 eV and 73.4 eV correspond to Al 2p3/2and Al 2p1/2[32,33]and the 1s peak at 531.5 eV and 535.1 eV can be assigned to lattice oxygen and hydroxyl group of water respectively[34].XPS data clearly shows the presence of Ni2+,Cu2+,and Al3+in the synthesised compound.

3.4.Optical and colour properties

UV-Visible spectra of Ni1-xCuxAl2O4(x=0,0.02,0.04,0.06,0.08 and 0.1)pigments are shown in Fig.8.The spectra show four absorption bands in the range 230-344 nm,350-400 nm,586-669 nm and 900-1250 nm.From the literature,we originated that the absorption bands at 230-344 nm and 350-400 nm are due to the charge transfer between metal and oxygen[35];the band between 586 and 669 nm is assigned to3T1(F)→3T1(P)which corresponds to tetrahedrally co-ordinated Ni2+[36],the band at 900-1250 nm is due to3A2g→3T2g(F)transition,attributed to octahedrally coordinated Ni2+[37].

But in the copper doped NiAl2O4new bands appear at 430-580 nm,a trivial absorption between 720 and 780 nm along with a wide band between 900 and 1250 nm in which the foregoing wavelength is coupled with the charge transfer that took place between oxygen and copper positioned in octahedral and tetrahedral sites and the posterior two wavelengths are associated with the d-d transition of copper atoms residing in octahedral and tetrahedral positions respectively[12,35,37].The spectra show a high number of bands due to the concurrent occurrence of nickel and copper in the octahedral and tetrahedral positions.

The frequency in which the light energy is absorbed brings forth the colour of the material which in turn changes the band gap,resulting from the excitation of electrons from anionic band to the cationic band[27].Tauc plot(Fig.9)is used to calculate the optical band gap of the material.At this point,the optical band gap was found using the following equation[19].

Fig.5.Elemental mapping of a)NiAl2O4b)Ni0.98Cu0.02Al2O4c)Ni0.9Cu0.1Al2O4.

αhv=A（hv-Eg）1/2

where,α—coefficient of absorption,h—planks constant,ν—frequency of light,Eg—optical absorption edge energy and A—proportionality constant.The band-gap energy is calculated from the extrapolation of the plot of(αhν)2vs photon energy(hν)intercept of the line on the abscissa(αhν)2=0 giving the value of optical absorption edge energy.Here,the band gap energy(3.11 eV)of NiAl2O4is related to the anionic valence band consisting of filled O2pand the cationic conduction band derived from the M2+ions.Gouda et al.,reported the band gap of NiAl2O4to be 3.4 eV[38]which competes with our observation.Doping of copper ions in NiAl2O4introduces an additional electronic level between O2-valence band and M2+conduction band,and a massive reduction in the band gap is observed from 3.11 eV to 2.63 eV.The shrink in the band gap energy perhaps correlated to the electronic structure of the Cu2+dopant,which provokes a defect level in the band gap of the host lattice[21].The deviation of band gap with increasing copper doping is tabulated in Table 2.Thus Cu2+substitution in NiAl2O4leads to a bathochromic shift in the wavelength of absorbance.The outcome is the change in colour from cyan to green due to the doping of copper into NiAl2O4pigment lattice.

Fig.6.TEM images of Ni0.98Cu0.02Al2O4at various magnifications(a)100 nm(b)50 nm(c)20 nm(d)10 nm(e)5 nm and(f)SAED pattern.

Fig.7.XPS spectra of a)Ni 2p b)Cu 2p c)Al 2p d)O 1s of Ni0.98Cu0.02Al2O4.

The deviation in CIE L*a*b*colorimetric value for the nickel aluminate and copper doped nickel aluminate is given in Table 2.Negative value of a*and b*in the case of NiAl2O4shows that it is a mixture of green and blue resulting in cyan colour.As the copper doping increases,the negative b*value decreases from-9.22 to-0.59 and-0.01 and the relative lightness values(L*)also fall from 71.86 to 56.27 and 63.17 when x=0.02,0.04 respectively resulting in green tone in Ni1.98Cu0.02Al2O4and Ni1.96Cu0.04Al2O4compositions.Whereas when x=0.06 and 0.08 slight increase in positive b*values is observed(1.67 and 1.09)which indicates slight yellow tinge in the compound.When x=0.1 the b*value becomes 3.9,and a*value is changed into-0.41 which resulted in grey colour(Fig.10).

Fig.8.UV-DRS Spectra of Ni1-xCuxAl2O4(x=0,0.02,0.04,0.06,0.08 and 0.1).

3.5.Thermal and chemical stability test

Application of the pigment in various fields is based on its chemical and thermal resistance behaviour.The resistivity of the copper substituted nickel aluminate towards the acidic and basic medium was examined.The percentage of weight loss before and after treatment of acid and base was less than 2%.Colour change after the treatment is perceived to be thin upon interacting with the acid and base with increase in copper content(Fig.S1).There is no considerable change in the UV-DRS profile(Figs.11 and 12)after weathering with 10%NaOH and 10% H2SO4.As a representation,powder XRD and UV profile of Ni1.98Cu0.02Al2O4before and after treatment with acid and base is given in Fig.13(a)and(b).To ensure the thermal stability,Ni1.98Cu0.02Al2O4was heated at 1000°C and 1200°C.XRD profile after thermal treatment is given in Fig.14.

3.6.Absence of heavy metal cations

In the recent decades,wide range researchers are working on the synthesis of eco-friendly and economically viable materials with enhanced properties in all the fields.Table 3 discloses the details of green pigments available in the literature.It is clearly evident that Co and Cr are the two heavy metals present in all the existing green pigments.Very few literatures reported the green pigments without the heavy metal cations but such pigments used the rare earth cations as the alternative sources which are not economically feasible.As these heavy metals are broadly regarded as scarce and toxic,doping of inexpensive elements into the existing noxious pigment leads to a reduction in the manufacture costs,moreover diminishing the environmental scratch.However,there are no reports on the green pigments without the presence of heavy and rare earth cation.Doping of 0.02 mol·L-1Cu into the Ni2+site of NiAl2O4dramatically changed the colour of the pigment from cyan to green.This is the first attempt in the literature to synthesise a green pigment without the presence of heavy and rare earth cation.

Fig.9.Tauc's plot of Ni1-xCuxAl2O4(x=0,0.02,0.04,0.06,0.08 and 0.1).

Table 2 Colour coordinates and band gap of Ni1-xCuxAl2O4(x=0,0.02,0.04,0.06,0.08 and 0.1)

3.7.Mineraliser effect

Fig.10.Photographs of Ni1-xCuxAl2O4.

Fig.11.UV-DRS spectra of Ni1-xCuxAl2O4(x=0,0.02,0.04,0.06,0.08 and 0.1)after base treatment.

Fig.12.UV-DRS spectra of Ni1-xCuxAl2O4(x=0,0.02,0.04,0.06,0.08 and 0.1)after acid treatment.

The chemical reagent which is added in very small quantities to expedite the physico-chemical properties is termed as mineraliser[45].In order to improve the green hue of the pigment composition Ni0.98Cu0.02Al2O4,mineralisers such as NaF,CaF2,NH4H2PO4,and Li2CO3were used.The effect of various mineralisers on the colour and the corresponding CIE co-ordinates are given in Fig.15.The addition of 2% Li2CO3reduced the green tint of the pigment to some extent whereas the addition of NaF,CaF2and NH4H2PO4increased a*value(Fig.15).a*value was higher with NH4H2PO4mineralizer compared to other mineralisers.b* value increased by the addition of the mineralisers.The results evidenced that mineralizers played a crucial role in the colour properties of the compounds.Fig.16 illustrates the band gap of Ni0.98Cu0.02Al2O4after adding various mineralisers.Change in band gap of the material after adding various mineralisers could be the reason for change in colour of the material[46].

4.Conclusions

Fig.13.(a)XRD profile(b)UV-DRS profile of Ni0.98Cu0.02Al2O4before and after treatment with 10%H2SO4and 10%NaOH.

Fig.14.XRD profile of Ni0.98Cu0.02Al2O4after thermal treatment.

The existing work scrutinised the colorimetric outcome,concerning the copper doping in nickel aluminate pigment by the cost-effective sol-gel method.Very low doping(0.02)of copper on nickel aluminate yielded environmentally benevolent green pigment without the influence of high cost rare earth elements and heavy metals.Ni0.98Cu0.02Al2O4exhibited a band gap of 2.63 eV which is comparatively lower than its host lattice(NiAl2O4=3.11 eV).Chemical resistance of all the phases holds good,even though its colour became feeble as the copper content increases.Hence,the prepared pigment could be used potentially in paints and plastic coloration.

Acknowledgements

We would like to thank the VIT university management for providing all required facilities and VIT seed fund for financial assistance to carry out the experiments.

Table 3 Green pigments available in the literature

Fig.15.Colour and CIE Co-ordinates of Ni0.98Cu0.02Al2O4after addition of 2%mineraliser.

Fig.16.Band gap of Ni0.98Cu0.02Al2O4after various mineralisers.

Appendix A.Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2019.01.008.