Ce and Fe doped gahnite: Cost effective solar reflective pigment for cool coating applications

Journal Pre-proof Ce and Fe doped gahnite: Cost effective solar reflective pigment for cool coating applications V. Elakkiya, Shanmugam Sumathi PII:

S0925-8388(19)34420-2

DOI:

https://doi.org/10.1016/j.jallcom.2019.153174

Reference:

JALCOM 153174

To appear in:

Journal of Alloys and Compounds

Received Date: 5 August 2019 Revised Date:

1 November 2019

Accepted Date: 24 November 2019

Please cite this article as: V. Elakkiya, S. Sumathi, Ce and Fe doped gahnite: Cost effective solar reflective pigment for cool coating applications, Journal of Alloys and Compounds (2019), doi: https:// doi.org/10.1016/j.jallcom.2019.153174. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Ce and Fe doped gahnite: Cost effective solar reflective pigment for cool coating applications V. Elakkiya and Shanmugam Sumathi* Department of Chemistry, School of advanced sciences, VIT, Vellore-632014. *Corresponding author: [email protected]

Graphical abstract

Ce and Fe doped gahnite: Cost effective solar reflective pigment for cool coating applications V. Elakkiya and Shanmugam Sumathi* Department of Chemistry, School of advanced sciences, VIT, Vellore-632014. *Corresponding author: [email protected] Abstract: In the current investigation, a new series of Ce doped gahnites of the formula ZnAl2xCexO4

(x = 0 – 0.04), Fe doped gahnites of the formula ZnAl2-yFeyO4 (y = 0 – 0.5) and Ce

and Fe co-doped gahnites of the formula ZnAl2-x-yCexFeyO4 (x = 0 – 0.04; y = 0 – 0.5) with high near infrared (NIR) reflectance were fabricated to mitigate the heat island effect. All the designed pigments were characterized by powder X-ray diffraction (PXRD), Fourier transformed infra red spectroscopy (FT-IR), Ultraviolet-Diffuse reflectance spectroscopy (UV-DRS). Scanning Electron Microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Transmission Electron Microscopy (TEM), CIE-L*a*b* colour values. Band gap of the white gahnite of the formula ZnAl2O4 (Eg = 3.8 eV) shifted to 2.70 eV and 2.15 eV for the pigments ZnAl1.96Ce0.04O4 and ZnAl1.5Fe0.5O4 respectively. Ce doped gahnites broadened the absorption centre from 250 nm to 350 nm in the UV spectrum and resulted in yellow colour. Whereas Fe doping introduced additional electronic transition between 440 nm and 490 nm and exhibited reddish

yellow

colour.

NIR

reflectance

of

ZnAl1.96Ce0.04O4, ZnAl1.5Fe0.5O4 and

ZnAl1.78Fe0.2Ce0.02O4 was found to be 86%, 65%, 80% respectively between 700 and 2500 nm regions. Good thermal and chemical stability along with high NIR reflectance made the prepared pigments as a potential candidate to design cool roofing materials. Key words: Gahnite; Citrate sol-gel method; NIR reflectance; Tauc plot; Ce and Fe doped ZnAl2O4 1 Introduction Hasty population growth, urbanization, draining natural resource are some of the important root causes for global warming which consequences “heat island effect”. Due to this effect the surface temperature of the building is increased to a number of degrees higher and hence the demand for energy to cool such buildings are increased rapidly [1-3]. However heat island effect is also the reason for the intensification of heat waves around the cities and 1

hence increases the human anxiety [4-6]. Hence, several technologies were adopted to mitigate the heat island effect and to reduce the energy consumption by air conditioners. One such option is the development of cool roofing and cool pavement materials. Since the solar heat energy is distributed 5% in the ultra-violet region, 43% in the visible region and 52% in the near infrared region, a material with high reflectance in the near infrared region could serve as a cool pigment and use of these pigments in paints, polymers, tiles, ceramics and automobiles possibly will reduce the energy needed for air-conditioning [7, 8]. Stability towards the chemical and thermal environment makes the inorganic pigment superior over the organic pigment though the former produces duller hues [9]. Titanium dioxide is an inorganic white pigment known for its high solar reflectance (approximately 87%), yet customer’s preference on aesthetic colours and the glare problems created by light colours demand the choice of dark coatings [10]. On the other hand, most of these coloured inorganic pigments (chromium green, cobalt blue, cadmium yellow, nickel titanate yellow and lead chromate yellow) encompass toxic metal ions and hence their consumption is being restricted [11]. Hence development of environmental friendly inorganic pigments is highly essential [12]. Recently lanthanide ions based pigments are reported as environmentally benign alternative for commercial toxic pigment with pronounced reflectance in the near infrared (NIR) region. Pr doped TiCeO4 (red) which is environmentally benign alternative for the CdS based toxic pigments [13]. Pr4+ doped Y6MoO12 (red) with NIR reflectance (R*) of 66% [14], Fe3+ doped Y2Ce2O7 (brown) with R* value 80% [15] are investigated. (LiLa)1/2MoO4 substituted BiVO4 with 85% reflectance [16], Mo doped cerium gadolinium oxide (R* = 91%) [9] are some of the environmentally benign rare earth based cool pigments. Doping of Ce, Pr or Nd into CoAl2O4 increased the R* by 10% [17]. ZnAl2O4 (gahnite) is a spinel type, multifunctional material with a wide band gap energy of approximately 3.8 eV, which has been extensively used in various applications such as pigment [18], photo catalysis, semiconductor, optoelectronics [19]. In addition, numerous literature reported on the incorporation of rare earth metal ions (Eu3+, Dy3+, Ce3+, Tb3+) into the ZnAl2O4 lattice which resulted in the enhancement of luminescent intensity of the host material. Hence they can be used in display technologies [20, 21, 22]. Ce3+ doped ZnAl2O4 as a photo-catalyst is reported by Sumathi and Kavi priya [23]. Fe3+ doped ZnAl2O4 is investigated in detail for its photo luminescent property [24]. Since, ZnAl2O4 based pigments are highly stable towards chemical, thermal environment along with higher NIR reflectance they could serve as potential candidate for high temperature ceramic and glazing applications.

2

Diana Visinescu et al. reported Co doped ZnAl2O4 as a nano blue pigment [25], its NIR reflectance was further explored by Robert Ianos et al.[26] CoxZnAl2-xO4 was reported as a pink ceramic pigment, Cr3+ doped ZnAl2O4 pink pigment was prepared hydrothermally by Miron and Grozescu [27]. Above mentioned reports focused mainly on the synthesis of blue and pink coloured pigments based on ZnAl2O4 with pronounced near infrared (NIR) reflectance whereas very less attention was given to other coloured ZnAl2O4 pigments. Typical green and brown colour pigments of the formula Zn1-xMxAl2O4 (M = Ni, Mn) was reported by Buvaneswari et al. [28]. A series of Mn/N codoped ZnAl2O4 pigments were reported by Yang et al. [29] in which white ZnAl2O4 was changed into flesh colour as the Mn/N doping increases, but the NIR reflectance was decreased from 85% (ZnAl2O4) to 48% (Mn/N co doped ZnAl2O4). While many of the commercial yellow pigments are made of toxic heavy metal cations, recently researchers are focusing on the alternative strategies to prepare environmentally benign yellow pigments. This stimulates our attention to explore ZnAl2O4 based yellow pigment. Since many reports evidences Ce3+ can enhance near infrared reflectance and Fe3+ can enhance the yellow hue [30], herein a detailed investigation is carried out to study the effect of individual doping of Fe3+, Ce3+ into ZnAl2O4 lattice and the effect of co-doping Fe3+, Ce3+ into ZnAl2O4 lattice on the colour and NIR solar reflectance of the prepared materials.

2 Materials and Method 2.1 Materials Zinc nitrate hexahydrate [Zn(NO3)2·6H2O (99% pure)], aluminium nitrate nono hydrate [Al(NO3)3·9H2O (98 % pure)], citric acid monohydrate [C6H8O7·H2O (99.7 % pure)], Cerium

nitrate

hexahydrate

[Ce(NO3)3.6H2O

(99

%

pure)],

iron

nono

hydrate

[Fe(NO3)3.9H2O (99.99% pure)] were purchased from S. D Fine chemicals and used without further purification.

2.2 Method To synthesise the desired pigments simple citrate sol-gel methodology was adopted. In a typical procedure to synthesize undoped gahnite, Zn(NO3)2, Al(NO3)3 and citric acid were used in the molar ratio 1 : 2 : 3. Initially zinc nitrate and citric acid were dissolved in 50 ml of distilled water, magnetically stirred and heated at 60⁰C for half an hour. Later aluminium nitrate solution (dissolved in 50 ml of water) was added to that, and stirred with heating for 2 3

h to acquire a highly gelatinous solution. This solution was kept in hot air oven at 120⁰C for 12 h to obtain a fluffy mass. This powder was ground, calcined at various temperatures ranging from 300 to 700⁰C for 4 h. To synthesize ZnAl2-xCexO4 (x = 0.01- 0.04), Zn1−yFeyAl2O4 (y = 0.1- 0.5) and ZnAl2-x-yCexFeyO4 (x = 0.01, 0.02, 0.03, 0.04; y = 0.1, 0.2, 0.3, 0.4, 0.5) iron nitrate and cerium nitrate were added along with aluminium nitrate [31]. Chemical formula of all the synthesised materials along with code is given in Table 1.

2.3 Chemical stability test In a typical procedure 0.5 g of the pigment was immersed in 2% NaOH, 2% H2SO4, 2% HNO3, and 2% HCl. After 30 minutes of stirring, the samples were washed with distilled water filtered and dried. The colour stability of the pigments were analysed through UV-DRS spectrophotometer.

2.4 Characterization techniques All the synthesised samples were subjected to PXRD (Powder X-ray Diffraction) analysis using BRUKER D8 advanced X-ray diffractometer in which Ni filter was used with Cu-Kα radiation (λ = 1.54056 Ǻ) and the operation was done at 45 kV and 30 mA. Collected data were compared with standard data. JASCO FT-IR 4100 was used to obtain the FT-IR spectra of fine powders of synthesised materials. Optical properties such as absorbance and NIR reflectance were derived using UV-DRS spectrophotometer using PTFE (Poly Tetra Fluoro Ethylene) as a reference with a step size of 5 nm and the wavelength range between 200 to 2500 nm. Band gap energy of all the samples were estimated using Tauc relationship with the reflectance data obtained from UV-DRS measurement. In the Tauc plot the photon energy value (hν) is on the abscissa whereas the value (αhν)n is on the ordinate. Here α = absorption coefficient and n = 2 and ½ for indirect and direct transition respectively. KM (Kubelka - Munk) function [15] was used to calculate the absorption coefficient which can be defined as, F(R) = α/S = (1- R) 2/2R,

---------------------------------- (1)

Where R= Reflectance of the sample/Reflectance of the reference, S = Scattering coefficient and F(R) = Kubelka - Munk function. Hence, Tauc relationship [32] is given as F(R)hν = A (hν-Eg)n,

4

---------------------------------- (2)

Extrapolation of the linear region of this plots to (F(R)hν)n = 0 reveal the band gap of the material. Sample surface morphology was observed using ZESIS EVO18 scanning electron microscopy instrument. The size of the particles were analysed using Malvern Zeta sizer (U.K) using Helium – Neon laser at 633 nm. Transmission Electron Microscope (FEITecnai, G2 20 Twin) was used to analyse the size and shape of the particles. The NIR solar reflectance (R*) of the synthesised samples were analysed within the wavelength range 700 nm to 2500 nm following the equation [33],

∗



=

.

. .

-------------------------------- (3)

Where i(λ) is the solar spectral irradiance (W/m2.nm) obtained from ASTM G173-03 and r(λ) is the spectral reflectance obtained from the experiment. Colour values of the obtained pigments were analysed through L*a*b* coordinates following the recommendation given by Commission Internationale de l’Eclairage (CIE) using D65 standard illuminant with 10⁰ observer angle. Where L* indicate the lightness or darkness (L* = 100 for white colour and L* = 0 for black), a* represent the greenness (-a*) or redness (+a*) and b* denotes the blueness (-b*) or yellowness (+b*) of the axis. The hue angle (h⁰) of the pigment is calculated

with the formula ℎ = formula, C = √

3

∗

+

∗

∗| ∗

and the colour saturation (C) is calculated using the

.

Results and Discussion

3.1 XRD analysis X-ray diffraction technique (XRD) is used to investigate the crystal structure of ZnAl2O4 (ZA), Ce doped ZnAl2O4 (ZC), Fe doped ZnAl2O4 (ZF) and Fe and Ce co-doped ZnAl2O4 (ZFC) (Figures 1, 2, 3 and 4). All the samples after calcinations shows comparable XRD patterns, which could be readily indexed as the cubic spinel phase ZnAl2O4 with Fd3m space group, which is finely consistent with the standard JCPDS file no. 96-101-1002. No impurity peaks were observed and also individual doping of Fe3+, Ce3+ and co-doping of Fe3+ and Ce3+ into ZnAl2O4 evidence that Fe3+ and Ce3+ do not dominate in the formation of pure single phase gahnite structure. In the case of Fe3+ doping, after ZnAl0.95Fe0.5O4 (x > 0.5) the synthesis was not continued due to the change in color of the materials (brown). Considering Ce3+ doped ZnAl2O4, CeO2 peak was observed as a secondary phase when 0.05 M of Ce3+ was

5

doped into ZnAl2O4, thus further increase in the Ce3+ content was not carried out. Lattice parameter of all the designed pigments were calculated using the formula [34],

a=d

ℎ +

+

------------------

(4)

Debye Scherrer approximation [35] was used to calculate the average crystalline size of the particles following the equation,

!= 3.1.1

".#

$% &'

---------------------

(5)

Effect of Fe doping Figure 1 and Table S1 shows the XRD patterns and lattice parameter of ZnAl2-yFeyO4

(y = 0, 0.1, 0.3, 0.5). As the doping concentration increases from 0 to 0.5, 2θ values 31.18⁰ and 36.89⁰ are shifted to lower angle and the peak broadening was observed linearly. This trait also mentioned in figure 1(b) and the corresponding lattice parameter value increased linearly (Fig. 5). Acquired tendency corroborates a piece of evidence that shift in the diffraction peak position is inversely related to the lattice parameter [24]. To better understand the obtained trend, ionic radii of the dopant ion and the nature of the dopant ion replaced are considered. Since the size of Fe3+ [0.49 Ǻ (tetrahedral) / 0.65 Ǻ (octahedral)] is higher than that of the Al3+ [0.39 Ǻ (tetrahedral) / 0.54 Ǻ (octahedral)] [36], doping of bigger Fe3+ into smaller Al3+ produces strain in the crystal lattice hence the particle size decreased (table 1) whereas the lattice constant increased [37]. The outcome is in paramount with Vegard’s law [38]. ZnAl2O4 normally forms a normal spinel where tetrahedral positions are occupied by Zn2+ ions and the octahedral positions are occupied by Al3+ but in some cases it also forms inverse spinel structure depends on the preparation methodology [39]. It is expected that, to compensate the charge neutrality Al3+ should be replaced by Fe3+ hence the later ions also enters into octahedral site forms a normal spinel. Pathak et al. reported Fe doped ZnAl2O4, in which Fe3+ ion enters into Zn2+ lattice formed inverse spinel. Since the size of Zn2+ [(0.60 Ǻ (tetrahedral)/ 0.74 Ǻ (octahedral)] is bigger than Fe3+ the lattice constant value was decreased and the shifting of high intense peak towards higher theta angle was also observed [24]. But, in the present work, linear increase in lattice constant and shifting of high intense peak towards lower theta angle evidence the formation of normal spinel phase [40].

6

(a)

(b)

Figure 1 XRD pattern of a) ZnAl2-yFeyO4 [y = 0.1, 0.3, 0.5] and b) Expanded pattern

3.1.2

Effect of Ce doping XRD patterns and the lattice parameter of ZnAl2-xCexO4 (x = 0, 0.01, 0.02, 0.03, 0.04,

0.05) are divulged in figure 2 and table S1. As Ce3+ increases from 0.01 to 0.04 high intense peak at 31.18⁰ and 36.89⁰ of ZnAl2O4 was altered to slight lower degree (Fig. 1b) [41] and accordingly widened peaks were observed (Here due to the very low doping of Ce3+, very low difference in 2θ angle is observed). Since Ce3+ is bigger in size (1.03 Ǻ) compared to that Al3+ [23] one could expect compressed strain in ZnAl2O4 lattice resulting in reduced average particle size. It is evidenced from figure 5, a linear increase in lattice parameter is observed with decrease in particle size.

7

Figure 2 XRD pattern of a) ZnAl2-xCexO4 [ x = 0.01, 0.02, 0.03, 0.04] and b) Expanded pattern

3.1.3

Effect of co-doping Ce and Fe into Al3+ of ZnAl2O4 Figure 3, 4 and table S1 shows the XRD pattern and lattice parameter value of ZnAl2-x-

yCexFeyO4

(x = 0.01, 0.04; y = 0.1, 0.2, 0.3, 0.5). (XRD pattern of x = 0.02, 0.03 is given in

Figure S2 and S3). From the corresponding expanded XRD patterns (Figure 3b, 4b) we inferred that tunnelling of high intense peak towards lower angle is observed in all the composition ensuring the formation of normal spinel. From figure 5 we could infer that doping of individual cations (Ce3+ and Fe3+) into Al3+ results linear increase in lattice parameter and decrease in particle size. Such linear change was not observed while co-doping Ce3+ and Fe3+ into Al3+ lattice which could be due to the synergic effect.

8

(a)

(b)

Figure 3 XRD pattern of a) ZnAl2-x-yCexFeyO4 [x = 0.01; y = 0.1, 0.2, 0.3, 0.5] and b) Expanded pattern (a)

(b)

Figure 4 XRD pattern of a) ZnAl2-x-yCexFeyO4 [x = 0.04; y = 0.1, 0.2, 0.3, 0.5] and b) Expanded pattern 9

Table 1 Chemical formula of synthesised materials and corresponding codes

Composition ZnAl2O4

Code ZA

Composition Code ZnAl1.89Fe0.1Ce0.01O4 ZF1C1

Composition ZnAl1.87Fe0.1Ce0.03O4

Code ZF1C3

ZnAl1.9Fe0.1O4

ZF1

ZnAl1.79Fe0.2Ce0.01O4

ZF2C1

ZnAl1.77Fe0.2Ce0.03O4

ZF2C3

ZnAl1.7Fe0.3O4

ZF3

ZnAl1.69Fe0.3Ce0.01O4

ZF3C1

ZnAl1.67Fe0.3Ce0.03O4

ZF3C3

ZnAl1.5Fe0.5O4

ZF5

ZnAl1.49Fe0.5Ce0.01O4

ZF5C1

ZnAl1.47Fe0.5Ce0.03O4

ZF5C3

ZnAl1.99Ce0.01O4

ZC1

ZnAl1.88Fe0.1Ce0.02O4

ZF1C2

ZnAl1.86Fe0.1Ce0.04O4

ZF1C4

ZnAl1.98Ce0.02O4

ZC2

ZnAl1.78Fe0.2Ce0.02O4

ZF2C2

ZnAl1.76Fe0.2Ce0.04O4

ZF2C4

ZnAl1.97Ce0.03O4

ZC3

ZnAl1.68Fe0.3Ce0.02O4

ZF3C2

ZnAl1.66Fe0.3Ce0.04O4

ZF3C4

ZnAl1.96Ce0.04O4

ZC4

ZnAl1.48Fe0.5Ce0.02O4

ZF5C2

ZnAl1.46Fe0.5Ce0.04O4

ZF5C4

3.2 FT-IR analysis Figure 6 depicts the FT-IR spectra of ZA, ZC2, ZC4, ZF3, ZF2C1, ZF2C2 and ZF5C4. It is well known that FT-IR analysis is a suitable technique to study about the chemical interactions (bonding) between any two atoms [42]. In general spinel oxides are identified by their featured absorption peaks observed between 900 and 400 cm-1 [43]. ZA shows three prominent peaks at 643 cm-1, 541 cm-1 and 478 cm-1. These frequencies are corresponds to symmetric stretching and bending vibration modes of Al-O [44] and stretching vibration of Zn-O [45] respectively. In the doped aluminate the frequencies are shifted to slight higher or lower angle which indicates the successful doping of Fe3+ and Al3+ into ZnAl2O4 lattice.

10

Figure 5 Lattice parameter and average crystallite size of synthesised pigment

11

Figure 6 FT-IR Spectra of ZA, ZC2, ZC4, ZF3, ZF2C1, ZF2C2 and ZF5C4

12

Figure 7 XPS spectra of ZF2C2

13

3.3. XPS analysis In order to confirm the chemical states of all the elements present in the particular material XPS analysis is being performed. Figure 7 depicts the XPS spectrum of ZF2C2. A complete survey scan was performed that confirmed the existence of Zn 2p, Al 2p, Fe 2p, Ce 3d and O 1s peaks. Binding energies obtained from the de-convoluted spectra were found to be in good agreement with the values mentioned in literature. From the figure, we could found the Zn 2p core peaks consists of two spin-orbit doublets Zn 2p3/2 (1024.36 and 1025.43 eV) and Zn 2p1/2 (1047.42 and 1048.23 eV) indicates the presence of Zn2+ ions [46]. Significant peaks at 72.10 and 72.82 eV corresponds to Al 2p3/2 and Al 2p1/2 which confirms the presence of Al3+ ions [47]. Fe 2p core level XPS-spectrum illustrates Fe 2p3/2 (710.45 and 712.31eV) and Fe 2p1/2 (724.65 eV) and a shake-up satellite peak at 717.65 eV validate the presence of Fe3+ ions [48]. As stated in the literature, 5 spin orbit doublets together makes a complete cerium 3d spectrum, Ce 3d3/2 (assigned as u), Ce 3d1/2 (assigned as v). These could be differentiated depending upon the chemical oxidation state of cerium. Hence u/v (900.80/881.97 eV), u”/v” (907.59/889.30 eV), u”’/v”’ (916.07/896.09 eV) are consigned to cerium in +4 oxidation state (Ce4+), whereas u⁰/v⁰ (898.19/879.47 eV) and u’/v’ (903.62/884.90 eV) are corresponds to cerium in +3 oxidation state (Ce3+) [49]. Hence, in ZF2C2 cerium exist in both +3 and +4 oxidation state. O 1s core peak spectrum evidence the presence of chemisorbed oxygen at 531.36 eV, Zn-O bond at 529.05 eV and Zn-Fe-O lattice oxygen at 530.14 eV [50].

3.4. Particle size and Morphological analysis (SEM and TEM) The size of the particles are analysed by dynamic light scattering technique (DLS) and the result obtained for the samples ZC4 and ZF2C2 are shown in figure 8. From the figure, the mean particle size of ZC4, ZF2C2 was found to be 0.578 μm and 0.894 μm respectively. Since the particles were dispersed in water medium during the analysis aggregation of particles are highly possible [51]. Scanning electron microscopic (SEM) images of some selected pigments (ZA, ZC4, ZF3, ZF1C1, ZF2C2 and ZF1C4) are shown in fig. 9. The images obtained from SEM analysis visibly illustrate that the particles were agglomerated. Though the obtained results from SEM and DLS suggesting microcrystalline nature of the pigments, they could be an aggregate of more number of small crystallites [52]. To further study about the size and shape of the pigment particles, Transmission Electron Microscopy (TEM) analysis was carried out for the selected pigment ZnAl1.78Ce0.02Fe0.2O4 (Fig.10). TEM 14

images show irregular distribution of particles with crystallite sizes ranges from 20 to 40 nm. Hence it is apparent that a single microcrystalline particle comprise of more number of small crystallites in the nano regime. The results obtained from TEM analysis are in good agreement with the XRD result. Appearance of microcrystalline particles in the SEM could be due to the serious aggregation of small particles through hydrogen bonding during the process of gelling and drying of the initial precursors [53]. Dark spot obtained from SAED pattern shows the crystalline nature of the material.

Figure 8 DLS Spectra of ZnAl1.96Ce0.04O4 (ZC4) and ZnAl1.78Ce0.02Fe0.2O4 (ZF2C2)

15

a

b

c

d

e

f

Figure 9 SEM images of a) ZnAl2O4 b) ZnAl1.96Ce0.04O4 c) ZnAl1.7Fe0.3O4 d) ZnAl1.89Fe0.1Ce0.01O4 e) ZnAl1.87Fe0.2Ce0.02O4 and f) ZnAl1.86Fe0.1Ce0.04O4

16

Figure 10 TEM images and SAED pattern of ZnAl1.78Ce0.02Fe0.2O4 3.5. Optical properties 3.5.1. UV-Vis Diffuse reflectance and colour properties The colour origin of the developed pigment was further investigated by means of diffuse reflectance spectra of ZnAl2O4, ZnAl2-xCexO4 (x = 0.01, 0.02, 0.03, 0.04), ZnAl2yFeyO4 (y

= 0.1, 0.3, 0.5), ZnAl2-x-yCexFeyO4 (x = 0.01, 0.02, 0.03, 0.04; y = 0.1, 0.2, 0.3, 0.5)

and the information is illustrated in figure 11. To emphasize the relation between absorbance, reflectance and transmittance, a comparative spectra for ZF1C2 is given in figure 12. The spectra suggest that the peak of transmittance and reflectance coincides and are inversely proportional to the absorbance of the material. In the case of ZnAl2-xCexO4 (x = 0, 0.01, 0.02, 0.03, 0.04), white gahnite show a weak absorbance band between 273 and 290 nm, which is 17

shifted to higher wavelength at the instance when Ce3+ introduced into gahnite structure. Peaks at 290 and 380 nm was observed when x = 0.01 in ZnAl2-xCexO4, the intensity of the peak increased with increase in the doping concentration from x = 0.01 to 0.04 along with widening of the absorption edge which could be due to the additional electronic level created from O 2p to Ce 4f [9] hence shortened the band gap (Fig. S4) of the material from 3.72 eV (x = 0) to 2.70 eV (x = 0.04). Oxygen 2p electrons forms a valence band and nd/(n+1)s electrons forms a conduction band in the formation of band structure in any metal oxide semiconductor [52]. In the case of cubic ZnAl2O4 the band structure is derived from O 2p and Zn 3d electrons and the band gap energy was found to be 3.72 eV which is par with the reported literature [54]. Introduction of Fe3+ into ZnAl2O4 (ZnAl2-yFeyO4 (y = 0.1, 0.3, 0.5)) introduced additional absorption centre between 440 and 490 nm, consequences batho chromic shift of wavelength significantly condense the band gap from 3.72 eV to 2.15 eV (Fig. S4). Similar trend was observed by Weiwei et al. [55] when Fe3+ is introduced into Co0.5Mg0.5Al2O4 structure. Formation of an inter band between the conduction band and valence band is also feasible in the case of Fe doped material. This trait could be the reason for drop in the band gap of iron doped materials [56]. Co-doping of Fe3+ and Ce3+ into ZnAl2O4 structure consequences widening of the absorbance edge between 280 nm and 400 nm with the introduction of additional electronic transition between 450 and 500 nm accordingly the band gap of the synthesised pigment decreased [57]. The designed pigments absorb in the purple region hence the complementary yellow colour is observed.

18

Figure 11 UV-DRS spectra of the synthesised pigments

Figure 12 Comparative spectra of absorbance, transmittance and reflectance of ZF1C2

19

Table S2 symbolize the colour values and the band gap of the groomed pigments. According to CIE LAB colour measurements positive b* denotes yellowness value as mentioned in section 2.4. ZnAl2O4 is well known wide band gap semiconductor material which appear as white in colour with b* value of -1.29. Very low doping of cerium (0.04) into ZnAl2O4 changes its colour from white to yellow (b* = 15.76) and the experimental hue angle (h⁰ = 76.13) of the fabricated product are headed in the yellow regime (h⁰ = 70 - 105⁰) of the cylindrical colour space. b* value increased from -1.29 to 26.60 as the Fe3+ doping increases from x = 0 to x = 0.5 in the ZnAl2-yFeyO4 system. Since the individual doping of Ce3+ and Fe3+ enhanced the b* value of ZnAl2O4, to further enhance the b* value, co-doping of Ce and Fe into Al3+ lattice of gahnite structure was attempted with various compositions of Fe3+ ranges from x = 0.1 to 0.5 and Ce3+ from x = 0.01 to 0.04 were fabricated. Their yellowness (b*) values and hue angle (h⁰) were demonstrated in table S2. All the co-doped pigments were shown the b* values from 7 to 27 with good yellow hues (h⁰) (table S2). Fine tuning of the band gap of all the synthesised materials resulted in range of colours from mild yellow to brown. Figure 13 shows the colour of ZnAl2O4, ZnAl1.96Ce0.04O4 and ZnAl1.78Ce0.02Fe0.2O4.

Figure 13 Digital photograph of selected pigments

20

3.5.2. NIR Reflectance properties As discussed in the introduction section a pigment is considered as cool pigment only when it has superior reflectance in the near infrared region (NIR). To emphasize the coolpigment ability, the solar reflectance of the designed pigments were calculated and the results are shown in figure 14 and table S2. Exploration of the NIR reflectance of the fabricated yellow pigments discloses fascinating results and they are comparable with that of the already existing reports (Table 2). The reflectance may possibly entrust on the particle size and its distribution. K-M theory states that increased scattering coefficient (S) can be attained by decreasing the particle size of the pigment [58]. Average solar reflectance of Ce doped ZnAl2O4 pigments was improved from 80% to 86% compared to that of its host material. Whereas Fe3+ doping decreases the solar reflectance from 80% (x = 0) to 65% (x = 0.5). Average NIR solar reflectance of Fe3+ and Ce3+ co-doped ZnAl2O4 did not fall below 70% which shows its ability to serve as a cool pigment. Table 2 depicts the recent explorations of various yellow pigments with its average near infrared reflectance. This is the first report on ZnAl2O4 based yellow pigments which are having comparatively higher b* value than the previous report by Yang et al. [29]. They reported Mn/N doped ZnAl2O4 as a NIR reflective pigment with maximum b* value of 16.43 and h⁰ of 60.41⁰. Their report exemplify that doping of Mn and co-doping of Mn/N into ZnAl2O4 structure reduced the NIR reflectance to a greater extend i.e., from 90.58 % to 30.62 % as well as 85.26 % to 48.53 % for Mn doping and Mn/N doping respectively. But in the current work ZnAl1.96Ce0.04O4 and ZnAl1.78Ce0.02Fe0.2O4 shows pronounced reflectance (86 % and 80% respectively). Hence the fabricated pigments have prominent application in the preparation of NIR reflective cool materials. Figure 15 depicts the trend in NIR solar reflectance, colour and band gap followed by each pigment due to the doping of cerium and iron respectively into ZnAl2O4 lattice. These traits suggest that cerium doping into ZA increases both yellow hue and NIR solar reflectance with the reduction of band gap. On the other hand, substitution of Fe3+ increased the yellow hue but reduced the reflectance in the NIR region. However, reduction in band gap is noticed. As a consequence of doping of various molar concentrations of Fe3+ and Ce3+ into ZA to attain proper colour (yellow hue) and high solar reflectance, ZF2C2 was found to be good candidate with comparative yellow hue (b* = 21) and high NIR solar reflectance (R* = 80%).

21

Figure 14 NIR solar reflectance spectra of the synthesized compounds

22

Figure 15 Band gap values, % NIR solar reflectance (R* value) and Colour co-ordinates of the synthesized compounds

23

Table 2 Comparison of yellowness values (b*) and NIR reflectance (R*)

Pigment

NIR reflectance (R*) %

Yellowness value (b*)

CeO2

85

31.8

Reference

[59] Bi doped CeO2

85

49.02

V doped BiPO4

88

36.31

[60]

Y doped Bi2Ce2O7

91

51.48

[61]

Bi4Ti3O12

95

22.66

[62]

Ni2P2O7

50

46.2

[63]

40

16.43

48

12.25

-

70.30

[16]

86

15.76

Present work

65

26.60

Present work

79

20.94

Present work

Mn doped ZnAl2O4 Mn/N doped ZnAl2O4 Praseodymium yellow Ce doped ZnAl2O4 Fe doped ZnAl2O4 Fe and Ce co doped ZnAl2O4

3.6.

[29]

Thermal stability and chemical stability Ceramic industry uses natural and synthetic pigments as colouring agents for glazing

applications such as floor tiles and wall wares, thus such pigments are mainly made of oxide materials that are highly stable towards high temperature (glaze firing temperature). Hence to investigate the thermal stability of the prepared pigments ZnAl1.96Ce0.04O4 and ZnAl1.78Ce0.02Fe0.2O4 were chosen, heated at 900⁰C and 1100⁰C. PXRD pattern (Fig S1) of the above samples after each calcination temperature shows that the peak intensity increased along with the introduction of CeO2 peak at 900⁰C and 1100⁰C. 24

Acid/alkaline resistance is one of the key characters of a good quality pigment which extend the scope of a pigment application in various fields. ZnAl1.96Ce0.04O4 and ZnAl1.78Ce0.02Fe0.2O4 were tested for its chemical resistance. After weathering with various acid and base ZnAl1.96Ce0.04O4 does not show any colour change which was confirmed by colour values obtained from UV-DRS spectrophotometer (Table S2).Whereas colour stability of ZnAl1.78Ce0.02Fe0.2O4 was decreased upon treating with acid and base

4. Conclusion In summary, novel gahnite based yellow pigments were synthesised for the first time by the introduction of Ce3+ and Fe3+ in Al3+ lattice. Doping of cerium and iron in zinc aluminate increased the lattice parameter and decreased the particle size. Decrease in band gap was observed due to the doping of trivalent ions in zinc aluminate. Red shift in absorption edge along with increased reflectance in the NIR region (86%) was noticed by the introduction of Ce3+ in the lattice whereas Fe3+ introduction increased the b* (26.60) compared to Ce3+ (15.76) but reduced the NIR reflectance. Ce3+ and Fe3+ co-doped zinc aluminate ZnAl1.78Fe0.2Ce0.02O4 showed better reflectance (80%) with comparable b* (20.94). Thermal stability of the compounds was found to be high. Hence our investigation could give a good lead to make novel non-toxic yellow pigment based on gahnite (ZnAl2O4). Our future investigation will be based on developing ZnAl2O4 based yellow pigment with vivid yellow hues by suitably doping various divalent and trivalent metal ions in the place of Zn2+ and Al3+ lattice.

25

Acknowledgment The authors would like to thank VIT, Vellore for providing lab facilities. One of the author V. Elakkiya would like to thank Ms. Sheetu jose, Central University of Kerala for NIR solar reflectance interpretation.

26

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Ce and Fe doped gahnite: Cost effective solar reflective pigment for cool coating applications V. Elakkiya and Shanmugam Sumathi* Department of Chemistry, School of advanced sciences, VIT, Vellore-632014. *Corresponding author: [email protected] Highlights Gahnite (ZnAl2O4) based yellow pigment were prepared for the first time. Very low concentration of Ce changes the white ZnAl2O4 into yellow. Addition of various mineralisers changes the colour values to considerable extent. Prepared pigments show good resistant towards chemical and thermal environment.