Color Tunable Pigments with high NIR reflectance in Terbium doped

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Color Tunable Pigments with high NIR reflectance in Terbium doped Cerate systems for Sustainable Energy Saving Applications Athira K.V Raj, P. Prabhakar Rao, and Sreena Thankom Sreedharan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00737 • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 21, 2019

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ACS Sustainable Chemistry & Engineering

Color Tunable Pigments with high NIR reflectance in Terbium doped Cerate systems for Sustainable Energy Saving Applications Athira K.V. Raj, P. Prabhakar Rao* and T. S Sreena Materials Science and Technology Division, CSIR-National Institute for Interdisciplinary Science and Technology (NIIST), Trivandrum – 695 019, India ___________________________________________________________________________ Abstract Color tunable pigments from yellow to red hues were developed in Tb doped A2CeO4 (A = Sr and Ba) system by the conventional ceramic route. The developed pigments were analysed for their phase purity, chemical oxidation state, elemental analysis, morphology and optical properties by various advanced techniques. The Ba substitution in Sr2-xCe0.6Tb0.4O4 gently shifts the absorption edge to higher wavelengths producing bright yellows to reddish orange colors whereas, the Tb substitution in Ba2Ce1-xTbxO4 allows abrupt shift in the absorption edge to longer wavelengths leading to intense red hues. The chemical oxidation state analysis indicates the modifications of Tb3+ concentrations in both the environments due to metal to metal charge transfer transitions induced by lattice expansion. Typically the compositions SrBaCe0.6Tb0.4O4 (yellow; b* = 75.36, R = 91%) and Ba2Ce0.4Tb0.6O4 (red; a* = 30.09, R = 89%) exhibit brilliant coloristic and reflectance properties. The applicability studies revealed good coloring performance in the polymer matrix and on concrete slab with high solar reflectance. Further, the pigments are proven to be weather resistant in acid/alkali/moisture atmospheres with good thermal stability. These color characteristics with high solar reflectance of sustainable and eco-friendly compositions make them promising colorants for exterior coating formulations to mitigate the cooling energy consumption. Keywords: Terbium; Cerium; Solar reflectance; Yellow and Red pigments _____________________________________________________________ * Corresponding author. Tel.: + 91 471 2515311; Fax: + 91 471 2491712 Email ID: [email protected] (P. PrabhakarRao)

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INTRODUCTION Designing of sustainable products and processes that minimise the production and consumption of harmful materials is in great demand in the emerging world. The ability to “smarter” has to be of the way to reduce the energy consumption in global warming. Inorganic pigments play as sustainable products in a smarter way that the colorants with high solar reflectance can reduce the heat build up of the concrete buildings in the urban dwellings besides their aesthetic appearance. Cool pigments reflect back the solar energy that makes the substrate cool and reduces energy transfer by radiation. Recently, the pigment industry is focused on the synthesis of “smarter pigments” (nontoxic cool pigments) that helps in reducing the air conditioning energy consumptions.1 In view of this, eco-friendly inorganic pigments have been extensively studied for their NIR and solar reflectance properties.2-4 Especially, yellow and red inorganic pigments are in great demand in the pigment industry due to its wide applicability. Bright yellow pigments are mostly needed as a replacement for the traditionally used Cd and Pb based pigments within the exterior coating markets. The Color Pigment Manufacturers Association, Inc. recorded that the hematite, rutile and spinel structures of complex inorganic compounds give good red or reddish-brown hues.5 Hence there is a necessity in developing more thermally stable red/brown pigments which are in agreement with the technological and environmental requirements. Several studies were reported on new environmentally friendly inorganic colorants more particularly red and yellow hues having greater NIR reflectance,6 - 10 but it is very difficult to obtain these two colors within the same system by tuning the band gap energy. In the present study, we focused on the tuning of color from yellow to red by adjusting the cations, which are potential alternatives to the commercial toxic colorants. In 1998, Danielson et al. reported a new efficient rare earth based optical material: Sr2CeO4 which crystallises into an orthorhombic structure having a space group Pbam and


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was widely used in a variety of optoelectronic/displaying applications because of their high thermal and chemical stability and distinctive optical properties.11,12 The Sr2CeO4 consists of one- dimensional chain of edge sharing CeO6 octahedron separated by Sr atoms.13 Eu3+ doped Sr2CeO4 has been extensively studied for red phosphor materials.14 And, also the environmentally friendly composition of strontium cerate offers a possible candidate as a sustainable inorganic pigment. This point of view our group developed terbium doped Sr2CeO4 by the conventional high temperature ceramic route and able to produce bright yellow hue (b* = 70.30) having greater NIR reflectance as energy saving products. This invention heralded a novel oxide based system which allows to develop many inorganic colorants by compositional modifications. In this regard, by suitable cation introduction into the system, the tuning of color can be produced. In the base compound Sr2CeO4 having orthorhombic type structure, a small addition of terbium substitution in cerium site produces intense yellow colors having the same structure with different space group and replacing some or all strontium with barium gives red colors having a tetragonal structure by changing some of the terbium oxidation states from 4+ to 3+. In this context we synthesized the compounds having the general formulae: Sr2xBaxCe0.6Tb0.4O4

(x = 0.25, 0.50, 0.75, 1.0, 1.25, 1.50 and 2.0) and Ba2Ce1-xTbxO4 (x = 0,

0.20, 0.40, 0.60, 0.80 and 1.0) by the conventional high temperature ceramic route in order to modify the color from yellow to red. The oxide color is a function of the amount of barium/terbium content in the system and can be produced very intense yellow as well as red hues higher than that of the commercially available pigments. The phase purity, particle size and morphology, coloristic properties and application studies of the developed pigments were analyzed by powder X-ray diffractometer (XRD), scanning electron microscope (SEM) with energy dispersive spectrometry (EDS), Particle size analyser, X-ray photoelectron spectroscopy, UV-vis NIR Spectroscopy and CIE 1976 L*a*b* color scales. Some of these



interesting pigmentary properties and their applicability studies in the polymer matrix and on cement concrete are described in the present paper. EXPERIMENTAL SECTION Materials and Methods. The terbium doped strontium/barium cerate-based pigments with the general formulae, Sr2-xBaxCe0.6Tb0.4O4 (x = 0.25, 0.50, 0.75, 1.0, 1.25, 1.50 and 2.0) and Ba2Ce1-xTbxO4 (x = 0, 0.20, 0,40, 0.60, 0.80 and 1.0) were prepared by the conventional high temperature ceramic route. SrCO3, BaCO3, CeO2 and Tb4O7 (Sigma-Aldrich, 99.99% purity) raw materials were taken in the required stoichiometric ratio and made homogeneous mixture by wet mixing in an agate mortar in acetone medium and then the product was dehydrated by keeping in a hot air oven at 100°C. The above process of (mixing and drying) was repeated thrice to get a homogenous product. The product was then heat treated in platinum crucibles in an electrically heated furnace at 1250°C (Sr2-xBaxCe0.6Tb0.4O4) and 1300°C (Ba2Ce1xTbxO4)

for 6h. The furnace temperature is programmed initially with a heating rate of

10°C/min up to 1000°C followed by a heating rate of 5°C/min to attain the respective temperatures. Characterization. The phase purity and crystalline nature of the prepared pigment powders were checked by means of a powder X-ray diffraction (XRD) method with a Ni-filtered CuK radiation of PANalytical Make X’pert Pro diffractometer operating at a power of 45 kV and 30 mA. The diffraction scan was done from 10 to 90° 2 range at a step size of 0.016°. The electronic nature of the pigment samples was characterised using X-ray photoelectron Spectroscopy (XPS), PHI 5000 Versa probe II with Al Kα monochromatic source. Morphological observation of the pigment powders was carried out by means of a scanning electron microscope of a JEOL Make model JSM-5600 LV operating at an acceleration voltage of 15 kV. Energy dispersive X-ray Spectroscopy (EDS) elemental analysis and


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mapping were done using a Silicon Drift Detector–X-MaxN attached to the SEM of Carl Zeiss Make EVO18 Model. AZtec Energy EDS Microanalysis software was used for the EDS elemental mapping. The particle size distribution of the selected samples was analysed using Anton Paar Litesizer 500 Particle Analyzer after ultra sonicating the samples in water medium. The diffuse reflectance spectra of the pigments were recorded using a UV-vis NIR Spectrophotometer of Shimadzu Make, Model UV-3600 using polytetrafluoroethylene (PTFE) as a standard with an illuminant D65, 10° complementary observer and measuring geometry d/8° as measurement conditions. The band gap (Eg) of the developed pigment samples was determined using tauc plot method.15 The NIR reflectance was measured in the range 700 - 2500 nm at a interval of 5 nm. The NIR solar reflectance of the pigment samples in the wavelength range 700 - 2500 nm was estimated from ASTM Standard G173-03.16 It is 2500

calculated using the equation 𝑅 =

∫750 𝑟(𝜆) 𝑖(𝜆)𝑑𝜆 2500

∫750 𝑖(𝜆)𝑑𝜆

in the wavelength 700-2500 nm range

where r(λ) is the spectral reflectance obtained from the experiment and i(λ) is the standard solar spectrum (Wm-2 mm-1). The color coordinates of the pigments were obtained by UVPC Color

Analysis

Personal

Spectroscopy

Software

V3

attached

to

the

UV-3600

spectrophotometer. The CIE1976 L*a*b* colorimetric method was employed to evaluate the color coordinates of the pigments in accordance with the Commission Internationale de l’Eclairage (CIE). According to this system, L* is the color lightness (0 for black, 100 for white), a* is the green (-) / red (+) axis, and b* is the blue (-) / yellow (+). The parameter chroma is expressed as C*= [(a*)2 + (b*)2]1/2 which represents the saturation of the color. The hue angle, h˚ is given in degrees and ranges from 0˚ to 360˚ and is determined using the formula h˚= tan-1(b*/a*). The thermal stability of the typical pigments: SrBaCe0.6Tb0.4O4, Ba2Ce0.4Tb0.6O4 was verified from 50 - 1000°C temperature range using SII Nanotechnology Inc., thermo gravimetric/differential thermal analysis (TG/DTA) 6200 in an air atmosphere at a heating rate of 10°C/min. The chemical stability of the typical pigments was performed



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using acid/alkali solutions and water. A small amount of the selected pigments: SrBaCe0.6Tb0.4O4, Ba2Ce0.4Tb0.6O4 having good coloring and reflectance performance was checked with 5% acid HCl, HNO3, 5% alkali NaOH, and H2O and immersed for 1h with continuous mixing by a magnetic stirrer. The soaked pigment powder was filtered, then washed with deionized water, and subsequently dried, and estimated the weight. The CIE color coordinates were evaluated, and the color difference (ΔE*ab) was determined using the following equation. ∗ 𝛥𝐸𝑎𝑏 = (𝛥𝐿 ∗ )2 + (𝛥𝑎 ∗ )2 + (𝛥𝑏 ∗ )2

(1)

Application Studies. In order to check the capability of the coloring and reflectance performance, the selected yellow and red colorants were introduced into a polymer substrate like Poly(methyl methacrylate) (PMMA), which is a well known weather resistant polymeric material with high gloss and hardness and widely employed for cold extrusion of several ceramic oxides like alumina and zirconia. PMMA (90 wt%) was made into a viscous solution by heating in a conventional electric coil heater and then 10 wt% of the pigment powder was slowly incorporated with continuous mixing and finally made into a thick paste. Then the prepared paste was put into a mould and compacted into a cylindrical disc. Also, these selected pigments were used to prepare coatings on a concrete cement slab with and without TiO2 base coat to investigate the coloring and reflectance properties. These selected pigments were made into fine powder and then ultrasonicated (Vibronics, 250W, India) for 10 min to make the total dispersion of the pigment particles in acrylic acid and polyurethane as a binder. The prepared viscous solution was coated on the concrete cement slab and was further dried in an air oven at 150 °C.


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RESULTS AND DISCUSSION Effect of Ba Incorporation on the Color Tunable Properties of Sr2Ce0.6Tb0.4O4 Yellow Pigments Powder X-ray Diffraction Studies. The powder X-ray diffraction patterns of Sr2xBaxCe0.6Tb0.4O4 (x

= 0.25, 0.50, 0,75, 1.0, 1.25, 1.50 and 2.0) samples are shown in Figure 1.

The strong and narrow diffraction lines in the patterns indicate good crystallinity of the powders. All the compositions are isostructural with the cubic orthorhombic crystal system. As can be confirmed from the XRD patterns, the diffraction lines of the compounds (x = 0.25 to 1.25) matches well with the JCPDS file number 01-089-5546, forming an orthorhombic phase having the space group Pbam. Further, the compositions with x = 1.50, 1.75 and 2.0 crystallises into the orthorhombic phase having the space group Pmcn, which is indexed in accordance with the JCPDS card number 01-071-6749. This confirms that progressive substitution of Ba2+ brings a structural transition in the system. Minor phase of SrTb2O4 can be detected from x = 0.25 substitution. From the previously reported literature, it is shown that phase pure Sr2CeO4 is difficult to synthesise even though different methods adopted.17, 18 The cell parameters and volume obtained for various dopant concentrations are listed in Table S1 in the Supporting Information. The substitution of Ba2+ (0.142 nm) for Sr2+ (0.126 nm) introduces a small distortion in the lattice and the cell volume increases generally with increasing dopant concentration which signifies the formation of a effective solid solution by the substitution of Ba2+. In addition, the trend of the Bragg reflection in the 2θ range 27-32° (Figure S1) is also assessed which shows a regular shift towards the left in the Ba substituted Sr2-xCe0.6Tb0.4O4. This also evidences the effective substitution of Ba in the lattice. X-ray Photoelectron Spectroscopic Studies. X-ray photoelectron spectra of the Sr2xBaxCe0.6Tb0.4O4

(x = 0.25, 0.50, 0.75, 1.0, 1.25, 1.50 and 2.0) pigments were carried out and



the wide survey scan in the energy range 0–1300 eV are given in Figure S2. The XPS spectra were charge compensated using C 1s signal at 284.6 eV. A series of peaks from Sr 2d, Ba 2d, Ce 3d, Tb 3d, C 1s and O 1s are clearly seen. The XPS of Ce 3d and Tb 3d core levels spectra for the synthesised samples are shown in Figure 2. It was found that the different valence states of the cerium and terbium species (+3 and +4) are present in the system. The broad Ce 3d and Tb 3d peaks are deconvoluted for correct assessment and provides the confirmation of the composition having mixed chemical states present in the composition (Figure S3 and S4). The XPS results confirm that Ce and Tb are in a mixed valence state; Ce: 3+ (880.4, 885.5, 898.81 and 904.07 ± 0.7 eV), and 4+ (882.7, 888.96, 898.2, 901.30, 907 and 916.7 ± 0.7 eV),19 Tb: 3+ (1239.09 and 1274.94 ± 0.7 eV) and Tb: 4+ (1242.59 and 1276.28 ± 0.7 eV)20 corresponding to the 3d5/2 and 3d3/2 spin-orbit doublets. The peak areas of Ce3+ / Tb3+ were employed to obtain the amount of Ce3+ / Tb3+ concentration in the lattice.21, 22 The Ce3+ / Tb3+ concentration was calculated from the relative molar ratio of Ce3+ / Tb3+ to the (Ce3+ + Ce4+ / Tb3+ + Tb4+) and the obtained values are given in Table 1. These results confirmed that the mixed valence states of the cerium and terbium species (+3 and +4) are present in the composition and Ce4+ is predominantly seen in the composition. When Ba2+ was substituted into Sr2Ce0.6Tb0.4O4, the Ce 3d XPS indicates that the peaks of Ce4+ 3d3/2, and Ce4+ 3d5/2 become more stronger indicating the decrease of Ce3+ concentration with the increase of Ba substitution in the sample. While the peaks of Tb4+ 3d3/2 and Tb4+ 3d5/2 of Tb 3d XPS decreasing and Tb3+ 3d3/2 and Tb3+ 3d5/2 peaks increasing evidence the increase of Tb3+ concentration with Ba2+ substitution in the sample. This change in terbium oxidation states leads to the tuning of the band gap, in turn changes the color from yellow to red by Ba2+ substitution. Morphological Studies. The preparation process condition and the heat treatment affect the morphological features and size of the particle which can be analysed using Scanning


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electron microscopy (SEM).23,24 Figure 3 shows the typical SEM micrographs of Sr2xBaxCe0.6Tb0.4O4

(x = 0.25, 0.50, 0.75, 1.0, 1.25, 1.50 and 2.0) pigments. The particles are

seen with some amount of aggregation with a size range of 1–4 μm. The particle size distribution of the selected compositions Sr2-xBaxCe0.6Tb0.4O4 (x = 0.25, 1.0 and 2.0) is presented in Figure S5 and the mean particle size is in the range 2.28 to 3.16 µm. This data also shows similar trend of increasing particle size with substitution of Ba supplementing the SEM morphology. The elemental composition was verified by energy dispersive spectrometer (EDS) analysis attached with SEM. Figure S6 shows the EDS elemental analysis of the SrBaCe0.6Tb0.4O4 sample and confirms the occurrence of all the elements in the desired stoichiometric ratio. Elemental X-ray dot mapping analysis of the typical SrBaCe0.6Tb0.4O4 pigment is given in Figure 4. This mapping reveals the uniform distribution of all the elements in the sample. UV-visible Studies. The absorption spectra of the Sr2-xBaxCe0.6Tb0.4O4 (x = 0.25, 0.50, 0.75, 1.0, 1.25, 1.50 and 2.0) pigments in the UV–vis–NIR region are shown in Figure 5. The absorption spectra of the pigments exhibits a broad absorption band attributed to the charge transfer transition from the O(2p) orbitals in the valence band to the conduction band originated from the Ce(5d) orbitals and the secondary Ba(2p) orbitals. Strong visible blue absorption in the wavelength below 490 nm is observed and by the Ba2+ substitution the absorption edge is gradually red shifted to the higher wavelength side. The band gap energies (Eg) of the prepared samples are determined by tauc plot method from the absorption edge of the spectra and the trend of the band gap variation is presented in Table 2. Accordingly, the developed pigments exhibit the fine tuning of colors from yellow to red by gradual shift of the band gap from 2.45 eV to 2.08 eV. From the XPS analysis, the increase in the Ce4+ concentration coupled with an increase of Tb3+ concentration is observed with Ba2+ substitution. This variation suggests that metal to metal charge transfer transition is taking



place in this system. Cerium is easily getting oxidised to Ce4+ by charge transferring to terbium enabling it to reduce to Tb3+. The gradual reduction in the band gap can be described by the occupation of the 4f electron energy levels of the Tb3+ above the O2p valance band causing a reduction in the band gap. As observed, the band gap reduces gradually with the increase of Tb3+ concentration from 2.45 to 2.08 eV.25 Hence, the Ba2+ substitution allows fine tuning the color from yellow to red hues by tuning the band gap energy. Color Analysis. The coloristic properties of the pigments can be expressed quantitatively in terms of color co-ordinates by using the CIE 1976 color coordinate system. The CIE 1976 chromaticity coordinates of the Ba2+ doped Sr2Ce0.6Tb0.4O4 powdered pigment samples are listed in Table 2. This shows that the Ba2+ substitution increases the b* value from 64.81 (x = 0.25) to 75.36 (x = 1.0) i.e the intense yellow hue of the pigment sample is enhanced and further increase in Ba2+ substitution reduces the b* to 48.60. Simultaneously, the increase in doping concentration leads to an increase in red hue of the pigment that is clear from the increasing color coordinate values of the red component a* i.e from 8.26 (x = 0.25) to 28.92 (x = 2.0). The hue angle (h°) of the developed pigments decreases gradually from 82.73 to 61.96. The hue angles (h°) of the developed pigment samples, Sr2-xBaxCe0.6Tb0.4O4 (x = 0.25, 0.50, 0.75 and 1.0) lies in the yellow region of the cylindrical color space (h °= 70°−105° for yellow) and Sr2-xBaxCe0.6Tb0.4O4 (x = 1.25, 1.50 and 2.0) in the orange-red region of the cylindrical color space (h° = 35-70 for orange-red). Among these pigments, SrBaCe0.6Tb0.4O4 exhibited the strong yellow hue component b* = 75.36 which is higher than that of the terbium doped Sr2CeO4 yellow pigments17 and comparable with the commercial sicopal yellow pigment (b* = 76.9, C* = 78.7).26 However, in this study only lower amounts of terbium is needed to get reasonable color characteristics, which makes them very costeffective for the commercial applications. Photographs of the developed pigments are shown in Figure 6. From these values/photographs, it was clear that the substitution of Ba2+ in the


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Sr2+ site of Sr2Ce0.6Tb0.4O4 yellow pigment leads to fine tuning the color from yellow to red hues by tuning the band gap energy. NIR Reflectance Studies. Non white formulations which absorbs more solar energy in the visible region will be less reflective which is main responsible for heat buildup, whereas a non white coating that absorbs less will be more reflective in the near-infrared (NIR) region of the electromagnetic spectrum which cools the inside of the buildings/automotives enabling reduction in the energy consumption.27 In order to investigate the reflectance properties of the developed pigments, the NIR reflectance and solar reflectance spectra of the Sr2xBaxCe0.6Tb0.4O4

(x = 0.25, 0.50, 0.75, 1.0, 1.25, 1.50 and 2.0) pigments in the range 200 to

2500 nm by using PTFE as reference standard are shown in Figure 7a–b and the values are given in Table 2. Successive doping of Ba2+ into Sr2Ce0.6Tb0.4O4 enhances the NIR and solar reflectance up to 91% and 89% for x = 1.0 and a further increase in the Ba2+ concentration reduces the reflectance value to some extent. Moreover, all the synthesized pigments possess a very high reflectance value which makes them as interesting cool colorants to decrease heat buildup. The Influence of Tb doping on the Color Performance of Ba2CeO4 as High NIR Reflecting Red Pigments Powder X-ray Diffraction Studies. The powder XRD patterns of the Ba2Ce1-xTbxO4 (x = 0, 0.20, 0.40, 0.60, 0.80 and 1.0) pigments by the conventional ceramic route are given in Figure 8. The sharp and intense diffraction lines in the patterns confirm the good crystallinity of the powders. It was found that the gradual doping of Tb4+ brings a structural transition from the orthorhombic to the tetragonal form i.e the compositions x = 0 to x = 0.4 crystallizes into the pure orthorhombic phase with a space group Pmcn, and all the diffraction lines can be indexed in accordance with the JCPDS File No. 01-071-6749. Mixed phase formation



occurred for x = 0.4 substitutions at the phase boundary of orthorhombic to tetragonal transformation. Further, the compositions with x = 0.6 to x =1.0 crystallise into tetragonal phase, that can be referenced as per the JCPDS File No. 01-073-9083 having the space group I4/mcm. In orthorhombic Ba2CeO4, the Ce site surrounded with six oxygen atoms forms a CeO6 octahedron and Ba site surrounded with twelve oxygen atoms forms a BaO12 polyhedron. The amount of rare earth element determines the structure of the synthesized compounds. The doping of Tb4+ (0.076 nm; CN = 6) in the Ce4+ site (0.087 nm; CN = 6) gives a small distortion in CeO6 octahedron of the lattice due to mismatch in the ionic radius. The lattice volume decreases with the substitution of Tb (x = 0 - 1.0).28 Further, the XRD diffraction peak in the 2θ range 27-32° indicates a right shift on substitution of Tb in the Ba2CeO4 system (Figure S7) confirming the effective solid solution. The variation of cell parameters and the volume obtained at different dopant concentrations are shown in Table S2. X-ray Photoelectron Spectroscopic Studies. X-ray photoelectron spectra of the Ba2Ce1xTbxO4

(x = 0, 0.20, 0.40, 0.60, 0.80 and 1.0) developed pigments were carried out and the

wide survey scan in the range 0–1100 eV are presented in Figure S8. The XPS of Ce 3d and Tb 3d core levels spectra for the synthesised samples are shown in Figure 9. The Ce XPS spectrum clearly evidences three pairs of spin–orbit doublets, which indicates that Ce(IV) dominates in our system. The deconvolution of the Ce 3d and Tb 3d photoelectron peaks were done for the quantitative measurement of Ce3+ / Tb3+ concentrations in the system and the values are given in Table 3. It was found that the concentration of Ce3+ reduces with the increase in dopant concentration and comparatively less with that of the Sr2CeO4 systems. As the terbium concentration increases, Tb3+ is predominantly present in the developed system. Morphological Studies. Figure 10 gives the typical SEM photographs of Ba2Ce1-xTbxO4 (x = 0, 0.20, 0.40, 0.60, 0.80 and 1.0) pigments. The particle size increases with the dopant


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concentration with a size range of 1-5 μm and the particles are observed with some amount of aggregation. Particle size distribution of the typical compositions Ba2Ce1-xTbxO4 (x = 0, 0.40 and 1.0) is depicted in Figure S9 and the mean particle size range varies from 2.18 to 4.69 µm. This increasing trend of particle size also supports the morphological observations through SEM. Figure S10 shows EDS spectra of typical Ba2Ce0.6Tb0.4O4 sample which confirms the occurence of all the expected elements (Ba, Ce, Tb and O) in the sample. Elemental X-ray dot mapping analysis of this pigment is shown in Figure 11 which confirms the uniform distribution of elements in the sample. UV-visible Studies. UV–vis absorption spectra of the developed pigments: Ba2Ce1-xTbxO4 (x = 0, 0.20, 0.40, 0.60, 0.80 and 1.0) are shown in Figure 12. Ba2CeO4 have a strong absorption band around 410 nm which can be attributed to the charge-transfer transition of O2- – Ce4+. By Tb substitution, the absorption edge is drastically red shifted to the higher wavelength side i.e the outer shell 4f electrons of terbium introduce an additional electronic energy level between the O2- valence band and Ce4+ conduction band and red shift in the absorption edge (decrease in the band gap from 3.16 eV to 1.86 eV) is observed and the change of band gap with increase of terbium substitution is given in Table 4. It was noticed that there is a reducing trend of band gap up to x = 0.60, further there is a slight increase. Similar results have also been observed in the terbium doped Sr2CeO4 systems which may be due to the structural transition.17 Color Analysis. The chromatic properties of the Ba2Ce1-xTbxO4 (x = 0, 0.20, 0.40, 0.60, 0.80 and 1.0) developed pigments were assessed by CIE 1976 L* a* b* color coordinates values (Table 4). Doping of terbium in Ce4+ site shows in the reduction of L* values and a gradual increase in the a* values from 19.03 (x = 0.20) to 32.47 (x = 1. 0). The chromaticity coordinates values of the typical pigment powder (Ba2TbO4) are notably higher than that of the commercially available Fe2O3 pigment (L* = 38.9, a* = 28.9, b* = 25.3).27 The values of



hue angle divulges that the terbium doped Ba2CeO4 pigments falls in the reddish brown region of the cylindrical color space (h° = 0-35 for red and 35-70 for orange). The photographs of the developed pigments are given in Figure 13. NIR Reflectance Studies. NIR reflectance and solar reflectance of the developed pigments are given in Figure 14 a-b. It was seen that the undoped Ba2CeO4 sample exhibits NIR reflectance (R) and NIR solar reflectance (R’) of about 87% and 85% respectively. With the substitution of Tb, the reflectance of the pigment slightly increases and all the pigments possess high NIR and solar reflectance that are greater than those of the reported red pigments.29 The percentage of NIR reflectance and NIR solar reflectance values are given in Table 4. These values suggest that the developed pigments hardly absorb the NIR solar energy.23 Hence, the developed pigments having high solar reflectance has the potential to replace the conventional inorganic red pigments as a way forward. Charge transfer from cerium to terbium ions. The color tunability from yellow to red in the present system has been assessed through the quantification of the oxidation states of cerium and terbium through XPS analysis. Figure 15 shows the variation of the band gap and Tb3+ concentration with barium/terbium substitution in the system. As can been revealed the band gap reduces with increase of Tb3+ concentration. In view of this, the modifications of Tb3+ concentration is analyzed in relation to the Ce4+ concentration in the system. From the assessment of the concentration quantification, Ce3+ concentration decreases with the increase of Tb3+ concentration in both the cases. This can be explained as follows: Cerium is structurally stabilized in octahedral coordination with 4+ oxidation state and terbium most stable state is 3+. On substitution of Barium, it is observed that the lattice expansion is taking place resulting in an increase in the volume allowing more polarizability of the ions. As cerium redox potential (1.72 V) is relatively less than that of terbium (3.1 V), thus, Ce is getting more stabilized in 4+ state in the system. In the process, Ce3+ = Ce4+ + e-, Tb4+ + e =


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Tb3+ and the overall Ce3+ + Tb4+ = Ce4+ + Tb3+. This oxidation state modifications clearly shows that there is a charge transfer from Cerium to Terbium resulting variation in the oxidation states. On terbium substitution in Ba2Ce1-xTbxO4, similar results are observed except relatively increase in the Tb3+ concentration. The band gap variation is accounted for the amount of Tb3+ concentration in the system. As known the lanthanides have 4f shell electrons energy levels lie above the valance band. By introducing this energy level between the valance band and conduction band causes a decrease in the band gap. Accordingly, the occupied 4f8 band in the terbium lies above the O2p band and reduces the band gap. The band gap variation is pronounced with the Tb3+ concentration producing tunable colors from yellow to red with either barium or terbium substitution in the present system. Acid/Alkali resistance and Thermal stability studies. The chemical stability in acid and alkaline media is essential in order to utilise the pigments for potential applications and these pigments are to be stable in the substrate due to the accumulation of acidic pollutants in the atmosphere that leads to the tarnishing of the substrates. Here, we have taken the typical pigments that exhibited better color and reflectance properties; SrBaCe0.6Tb0.4O4 yellow and Ba2Ce0.4Tb0.6O4 red pigment. The selected pigments were soaked in 5% HCl, HNO3, NaOH and H2O solution. The chromaticity coordinates of the soaked pigment samples are evaluated and compared with the untreated original powder. The total color difference (ΔE*ab) was determined and listed in Table S3 in the supporting information. The low values of ΔE*ab indicate that the designed pigments are chemically stable in the acid/alkali/H2O media. Thermal stability of the typical pigments was investigated by thermo-gravimetric analysis in the temperature range 50 - 1000˚C and the results clearly show that there is an trivial weight loss (SrBaCe0.6Tb0.4O4 = 0.19%; Ba2Ce0.4Tb0.6O4 = 0.27%) of the pigment in the investigated temperature range (Figure. S11 and S12 in the supporting information).



Application Studies. To evaluate the usefulness of the designed pigments for energy saving applications, their applicability was checked on plastic substrate and building roofing material like concrete cement. For the coloration of plastics, 10 wt% SrBaCe0.6Tb0.4O4 yellow and Ba2Ce0.4Tb0.6O4 red pigment powder was dispersed in Poly(methyl methacrylate) (PMMA) and compacted into a cylindrical disc and is displayed in Figure 16. The color strength of the PMMA substrate will depend on the amount of the pigment used. The color coordinates of the polymer plastic were tested at different spots on the surface using portable spectrophotometer Miniscan EZ4000S (Hunter Lab USA) and the obtained L*a*b* values are very close revealing the homogenous distribution of the pigment particles in the polymer matrix. Hence, the designed pigments are potential to be used in the coloration of plastic materials. Pigment coatings which absorb less solar radiation other than the visible region can provide high solar reflectance along with pleasing color than that of conventional roofing materials and to keep objects cooler than they would be. To analyse the applicability of the designed pigments as cool colorants for reducing the heat build-up, the typical pigments were coated on the concrete cement with and without TiO2 base coat. The thickness of the coatings are estimated by observing the cross section using scanning electron microscope and found to be around 50 - 60 µm. The NIR solar reflectance spectra of the SrBaCe0.6Tb0.4O4 yellow and Ba2Ce0.4Tb0.6O4 red pigment coated are shown in Figure 17. The photographs of the coated concrete materials are shown in Figure 18 and the obtained reflectance percentage values are given in Table 5. It is worth to note that the NIR reflectance and the corresponding solar reflectance of both yellow and red pigment coated substrates are higher than that of the bare concrete. Thus these non-white coatings (yellow and red coatings) can decrease the surface temperature of the building roofs and lead to electrical energy savings.


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CONCLUSIONS A new series of cerium based inorganic pigments with the formulae: Sr2-xBaxCe0.6Tb0.4O4 (x = 0.25, 0.50, 0.75, 1.0, 1.25, 1.50 and 2.0) and Ba2Ce1-xTbxO4 (x = 0, 0.20, 0.40, 0.60, 0.80 and 1.0) have been successfully developed via the conventional ceramic route. The substitution of Ba2+ in Sr2Ce0.6Tb0.4O4 changes the color from bright yellow to red, whereas the substitution of Tb in Ba2Ce1-xTbxO4 produces intense red hues. Terbium oxidation state plays the key role in the band gap tuning for producing various colors in the system. The pigments SrBaCe0.6Tb0.4O4 (yellow; L* = 77.34, a* = 18.09, b* = 75.36, R = 91%) and Ba2Ce0.4Tb0.6O4 (red; L* = 45.76, a* = 30.09, b* = 39.53, R = 89%) exhibits brilliant coloristic and reflectance properties. All the pigments are thermally and chemically stable, furthermore, the application studies reveal that the synthesized pigments are potential candidates for cool roof as well as surface coating applications. SUPPORTING INFORMATION Table S1-S3: Lattices parameters and volume, Color co-ordinates data after chemical resistance tests. Figures S1-S12: XRD peak shift, XPS Spectra, Particle size distribution, EDS analysis, TGDTA analysis ACKNOWLEDGMENTS The authors would like to acknowledge the International Centre for Diffraction Data, USA and Council of Scientific and Industrial Research, New Delhi, Government of India, for the research facility and financial support.



Page 18 of 43

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15. Sameera, S.; Rao, P. P.; Divya, S.; Athira, K. V. R. Brilliant IR reflecting yellow colorants in rare earth double molybdate substituted BiVO4 solid solutions for energy saving applications. ACS Sustainable Chem. Eng. 2015, 3(6), 1227-1233, DOI 10.1021/acssuschemeng.5b00198. 16. Standard tables for reference solar spectral irradiances: Direct normal and hemispherical on 37° tilted surface; ASTM International: West Conshohocken, PA, 2012. 17. Athira, K. V. R.; Rao, P. P.; Sameera, S.; Vineetha, J.; Divya, S. Synthesis of novel nontoxic yellow pigments: Sr2Ce1-xTbxO4. Chem. Lett. 2014, 43(7), 985-987, DOI 10.1246/cl.140172. 18. Gryb, T.; Szczeszak, A.; Rozowska, J.; Legendziewicz, J.; Lis, S. Tunable luminescence of Sr2CeO4: M2+ (M = Ca, Mg, Ba, Zn) and Sr2CeO4: Ln3+ (Ln = Eu, Dy, Tm) nanophosphors. J. Phys. Chem. C. 2012, 116(5), 3219-3226, DOI 10.1021/jp208015z. 19. Amit, K.; Suresh, B.; Karakoti, A. S.; Schulte, A.; Seal, S. Luminescence properties of europium-doped cerium oxide nanoparticles: role of oxygen vacancy and oxidation states. Langmuir. 2009, 25(18), 10998-11007, DOI 10.1021/la901298q. 20. Li, H.; Li. W.; Gu. S.; Wang, F.; Zhou, H. In-built Tb4+/Tb3+ redox centers in terbiumdoped bismuth molybdate nanograss for enhanced photocatalytic activity. Catal. Sci. Technol. 2016, 6(10), 3510-3519, DOI 10.1039/C5CY01730K. 21. Pemba‐Mabiala, J. M.; Lenzi, M.; Lenzi, J.; Lebugle, A. XPS study of mixed cerium– terbium orthophosphate catalysts. Surf Interface Anal. 1990, 15(11), 663-667, DOI: 10.1002/sia.740151105. 22. Sharma, A.; Varshney, M.; Park, J.; Ha, T. K.; Chae, K. H.; Shin, H. Bifunctional Ce1−xEuxO2 (0 ≤ x ≤ 0.3) nanoparticles for photoluminescence and photocatalyst applications: an X-ray absorption spectroscopy study. Phys. Chem. Chem. Phys. 2015, 17(44), 30065-30075, DOI 10.1039/C5CP05251C.


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23. Huang, B.; Xiao, Y.; Zhou, H.; Chen, J.; Sun, X. Synthesis and characterization of yellow pigments of Bi1.7RE0.3W0.7Mo0.3O6 (RE = Y, Yb, Gd, Lu) with high NIR reflectance.

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Page 22 of 43

Table 1. Ce3+ and Tb3+ concentration values of Sr2-xBaxCe0.6Tb0.4O4 (x = 0.25, 0.50, 0.75, 1.0, 1.25, 1.50 and 2.0) pigments. [Ce3+] %

[Tb3+]

x=0

33.92

39.76

x = 0.25

32.68

38.32

x = 0.50

32.59

35.71

x = 0.75

32.51

36.90

x = 1.0

33.34

38.33

x = 1.25

31.76

39.19

x = 1.50

30.30

43.76

x = 1.75

29.95

52.59

x = 2.0

29.38

54.32

Table 2. Color coordinates, band gap, NIR reflectance (R) and solar reflectance (R') values of the Sr2-xBaxCe0.6Tb0.4O4 (x = 0.25, 0.50, 0.75, 1.0, 1.25, 1.50 and 2.0) pigments. L*

a*

b*

x = 0.25

82.03

8.26

64.81 65.34

x = 0.50

81.57

9.69

x = 0.75

79.78

x = 1.00

Cab

h° 82.73

Eg (eV) 2.45

R (%) 89

R’ (%) 84

64.66 65.39

81.47

2.42

89

85

12.25

67.96 69.05

79.78

2.38

90

87

77.34

18.09

75.36 77.56

78.14

2.37

91

89

x = 1.25

67.58

28.20

68.14 73.75

67.51

2.35

91

89

x = 1.50

64.56

27.70

58.86 65.05

65.12

2.32

90

89

x = 1.75

65.55

27.18

58.63 64.63

64.79

2.29

88

83

x = 2.0

58.82

28.92

48.60 55.06

61.96

2.08

86

81


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Table 3. Ce3+ and Tb3+ concentration values of Ba2Ce1-xTbxO4 (x = 0, 0.20, 0.40, 0.60, 0.80 and 1.0) pigments. [Ce3+] %

[Tb3+]

x=0

32.74

--

x = 0.2

29.40

54.32

x = 0.4

29.38

55.79

x = 0.6

28.38

58.13

x = 0.8

17.99

56.20

x = 1.0

--

55.92

Table 4. Color coordinates, band gap NIR reflectance (R) and solar reflectance (R') values of the Ba2Ce1-xTbxO4 (x = 0, 0.20, 0.40, 0.60, 0.80 and 1.0) pigments. L*

a*

b*

Cab

h°

x=0

91.44

-1.12

5.71

5.82

x = 0.20

67.24

19.03

50.22

x = 0.40

58.82

28.92

48.6

x = 0.60

45.76

x = 0.80 x = 1.0

101.05

Eg (eV) 3.16

R (%) 87

R’ (%) 85

53.7

69.24

2.15

89

86

55.06

61.96

2.08

88

87

30.09

39.53 49.68

52.72

1.86

89

86

48.09

31.48

43.39 53.61

54.03

2.01

88

84

50.70

32.47

51.52 60.89

57.77

2.13

88

85



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Table 5. NIR reflectance and NIR Solar reflectance data of SrBaCe0.6Tb0.4O4 yellow and Ba2Ce0.4Tb0.6O4 red pigment coated on concrete cement. Coatings

SrBaCe0.6Tb0.4O4

Ba2Ce0.4Tb0.6O4

NIR reflectance (%)

NIR Solar reflectance (%)

NIR reflectance NIR Solar (%) reflectance (%)

Bare Concrete

35.63

33.49

35.63

33.49

Concrete + Pigment

78.34

76.25

60.10

56.45

Concrete + TiO2 + Pigment

80.38

79.08

84.27

82.89


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Figure 1

Figure 1. X-ray powder diffraction patterns of Sr2-xBaxCe0.6Tb0.4O4 (x = 0.25, 0.50, 0.75, 1.0, 1.25, 1.50 and 2.0) pigments (* indicates SrTb2O4 phase)



Figure 2

Figure 2. XPS spectra of a) Ce (3d) and b) Tb (3d) core levels of Sr2-xBaxCe0.6Tb0.4O4 (x = 0.25, 0.50, 0.75, 1.0, 1.25, 1.50 and 2.0) pigments.


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Figure 3

Figure 3. Typical SEM photographs of Sr2-xBaxCe0.6Tb0.4O4 (x = 0.25, 0.50, 0.75, 1.0, 1.25, 1.50 and 2.0) pigments.



Figure 4

Figure 4. Elemental X-ray dot mapping of SrBaCe0.6Tb0.4O4 yellow pigment.


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Figure 5

Figure 5. Absorption spectra of Sr2-xBaxCe0.6Tb0.4O4 (x = 0.25, 0.50, 0.75, 1.0, 1.25, 1.50 and 2.0) pigments.



Figure 6

Figure 6. Photographs of the Sr2-xBaxCe0.6Tb0.4O4 (x = 0.25, 0.50, 0.75, 1.0, 1.25, 1.50 and 2.0) pigments.


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Figure 7

Figure 7. NIR reflectance and NIR solar reflectance spectra of the Sr2-xBaxCe0.6Tb0.4O4 (x = 0.25, 0.50, 0.75, 1.0, 1.25, 1.50 and 2.0) pigments



Figure 8

Figure 8. X-ray powder diffraction patterns of Ba2Ce1-xTbxO4 (x = 0, 0.20, 0.40, 0.60, 0.80 and 1.0) pigments (* indicates BaTbO3 phase)


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Figure 9

Figure 9. XPS spectra of a) Ce (3d) and b) Tb (3d) core levels of Ba2Ce1-xTbxO4 (x = 0, 0.20, 0.40, 0.60, 0.80 and 1.0) pigments.



Figure 10

Figure 10. Typical SEM photographs of Ba2Ce1-xTbxO4 (x = 0, 0.20, 0.40, 0.60, 0.80 and 1.0) pigments.


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Figure 11

Figure 11. Elemental X-ray dot mapping of Ba2Ce0.4Tb0.6O4 red pigment.



Figure 12

Figure 12. Absorption spectra of Ba2Ce1-xTbxO4 (x = 0, 0.20, 0.40, 0.60, 0.80 and 1.0) pigments.


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Figure 13

Figure 13. Photographs of the Ba2Ce1-xTbxO4 (x = 0, 0.20, 0.40, 0.60, 0.80 and 1.0) pigments



Figure 14

a

b

Figure 14. a) NIR reflectance and b) NIR solar reflectance spectra of the Ba2Ce1-xTbxO4 (x = 0, 0.20, 0.40, 0.60, 0.80 and 1.0) pigments.


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Figure 15

a

b

Figure 15. Band gap and Tb3+ concentration as a function of x of the a) Sr2-xBaxCe0.6Tb0.4O4 (x = 0.25, 0.50, 0.75, 1.0, 1.25, 1.50 and 2.0) and b) Ba2Ce1-xTbxO4 (x = 0, 0.20, 0.40, 0.60, 0.80 and 1.0) pigments.



Figure 16

Figure 16. Photograph of 10 wt% SrBaCe0.6Tb0.4O4 + PMMA and Ba2Ce0.4Tb0.6O4 + PMMA. The polymer matrix exhibit uniform distribution of pigment particles in the substrate.


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Figure 17

Figure 17. NIR solar reflectance spectra of pigments a) SrBaCe0.6Tb0.4O4 yellow and b) Ba2Ce0.4Tb0.6O4 red coated concrete cement with and without TiO2 base coat (NIR reflectance in the inset)



Figure 18

Figure 18. Photographs of bare cement slab, SrBaCe0.6Tb0.4O4 yellow and Ba2Ce0.4Tb0.6O4 red pigment coated cement slab without and with TiO2 base coat.


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Color tunable pigments in sustainable eco-friendly compositions A2Ce1-xTbxO4 exhibit brilliant color characteristics with high solar reflectance as energy saving products.


Color Tunable Pigments with high NIR reflectance in Terbium doped

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