An intense purple chromophore based on Co2+ in distorted tetrahedral coordination

Accepted Manuscript An intense purple chromophore based on Co

2+

in distorted tetrahedral coordination

P.K. Thejus, Biplab Koley, K.G. Nishanth PII:

S0143-7208(18)30031-7

DOI:

10.1016/j.dyepig.2018.05.054

Reference:

DYPI 6783

To appear in:

Dyes and Pigments

Received Date: 4 January 2018 Revised Date:

11 May 2018

Accepted Date: 24 May 2018

Please cite this article as: Thejus PK, Koley B, Nishanth KG, An intense purple chromophore 2+ based on Co in distorted tetrahedral coordination, Dyes and Pigments (2018), doi: 10.1016/ j.dyepig.2018.05.054. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.

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Graphical Abstract

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An intense purple chromophore based on Co2+ in

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distorted tetrahedral coordination

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P. K. Thejus, a Biplab Koley, b and K. G. Nishanth a* a

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Materials Science and Technology Division, CSIR-National Institute for Interdisciplinary

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Science and Technology (NIIST), Thiruvananthapuram 695019, India.

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Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur 721302, India

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E-mail address: [email protected]

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ABSTRACT

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*Corresponding author. Tel.: +91471 2515508; Fax: +91471 2491712

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An intense purple, cost-effective inorganic pigment LiZn1-xCoxPO4 (0.1 ≤ x ≤ 0.8) was

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synthesized by solid-state ceramic route and their structural, optical and chromatic properties

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were studied employing Powder X-ray Diffractometer and UV-Vis-NIR diffuse reflectance

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spectrophotometer techniques. Origin of an exotic intense purple colour of these pigments was

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investigated, by means of crystallographic as well as optical spectral studies. Structural

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refinement showed that chromophore of the pigment CoO4, formed with highly distorted

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geometry due to significantly shorter Co-O bonds. Structural changes attributed strongly on the

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spectral features leading to an excellent purple colour which is unusual for CoO4 tetrahedral

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coordination. The hue of the purple colour can be controlled by tuning the Co content. We

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accomplished an impressive NIR Solar reflectance (R* = 68%) and good chemical stability. The

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colour delivering performance of these purple pigments have also been evaluated by pigment

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incorporation in polymer matrix, where we were able to retain its excellent colour. Moreover,

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present work showed that these developed pigments can be a potential candidate for cool

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pigment by coating onto roofing materials like aluminum roofing sheets and concrete block.

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Keywords: Solid-state method, Cobalt doped LiZnPO4, Co2+ tetrahedral chromophore, Purple

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pigment, NIR Solar reflectance.

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Abbreviations

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NIR, Near infrared; TBP, Trigonal bipyramidal; PTFE, Polytetrafluoroethylene; PMMA,

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Polymethylmethacrylate.

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1. Introduction

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Inorganic pigments fascinated mankind from prehistoric time onwards, imparting

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aesthetic colours. Ancient pigments Egyptian blue (CaCuSi4O10), Chinese blue (BaCuSi4O10) and

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Chinese purple (BaCuSi2O6) have received tremendous importance in their idealistic and

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materialistic esteem way back in 221 BC to 220 AD [1-2]. However, the only purple pigment

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among them BaCuSi2O6 had serious stability and durability issues, which restricted its

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consumption [1]. Recently, there is a growing interest for purple colour in pigment industry

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because of its uniqueness, royalty and sophistication [2]. However, producing high quality

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intense purple colour, which is a combination of blue and red stain in a single-phase material, is

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a massive challenge. Consequently, the numbers of reports were comparably low. Recently,

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Tamilarasan et.al. reported a couple of purple pigments based on YGa1-xMnxO3 and spiroffite

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structure Zn1-xCoxTe3O8 [5-6]. Although these pigments have shown an excellent colour, the

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presence of expensive elements like Gallium and Tellurium is a great concern. Similarly,

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Gorodylova et.al. and Gu et.al. developed Zirconium polyphosphate derived CoZr4(PO4)6 and

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Co-KZr2(PO4)3 purple pigments respectively, which contain expensive Zr metal [7-8]. The whole

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scenario illustrates the lack of a cost-effective intense purple pigment.

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It is evident from the literature reports that Mn3+ and Co2+ are well-known purple

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chromophores for inorganic pigments. Among which Co substituted materials dominated in

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numbers [3]. Co2B2O5, Co2SiO4, Co3(PO4)2, LiCoPO4 violet pigments and CoAl2O4 blue are

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some of the well-known candidates belongs to the cobaltous pigments [9-12]. Oxidation state,

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co-ordination number and co-ordination geometry of Co chromophore play the key role behind

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their violet to blue hue. Emphasizing on Co2+ chromophore, it can occupy different geometry

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such as tetrahedral, trigonal bipyramidal (TBP), square pyramidal and octahedral, accordingly

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colour of the material also differs. Literature reports reveal that tetrahedral Co2+ geometry leads

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to bright blue colour in (Co, Zn)Al2O4, CaAl12-2xTixCoxO19 and Co-doped Zn2SiO4 whereas it

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shows a green colour in Co2+ doped ZnO [13-20].Moreover, an intense purple colouration have

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been also recently observed in Ba(Zn,Co)2Si2O7 [21]. TBP geometry gives blue colour in LiZn1-

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xCoxBO3

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colour in Co3(PO4)2, Co2(OH)AsO4, (Zn,Co)MoO4 compounds and green colour in Co3V4(PO4)6

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[24-28]. Square pyramidally occupied Co2+ in grey coloured SrZn1-xCoxP2O7 was discussed by

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A. El Jazouli et al [29]. The octahedral Co2+ in Co2SiO4 and Zn-Co2SiO4 olivine exhibit violet

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colour, while it gives rise to green colour in Co-doped MgTi2O5 pseudobrookite (karrooite)

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and LiMg1-xCoxBO3 [22-23]. A combination of octahedral and TBP Co2+ gives violet

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[18,30-32]. Thus, from the careful analysis of geometry dependent colour-versatility of Co2+

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chromophore we were decided to precede it as the ideal choice for purple colour. We have selected a monophosphate system with general formula ABPO4, where, A

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denotes monovalent and B denotes divalent cations. A promising candidate among ABPO4

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system is LiZnPO4, which is rather a low-cost ceramic powder, has four polymorphs, (i)

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phenakite form (ii) α-LiZnPO4 (iii) δ1-LiZnPO4 (iv) CR1-LiZnPO4 [33-34]. α-LiZnPO4 is a white

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ceramic material, structurally well-studied moiety and explored in the field of phosphor light

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emitting diodes, electrochemical studies, corrosion inhibitor etc [35-38]. α-LiZnPO4 form a

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monoclinic crystal system with Cc space group, with a structure consist of LiO4, ZnO4 distorted

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tetrahedra linked to a PO4 tetrahedra. Taking advantage of the ZnO4 tetrahedra, we have worked

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on to extend the application of α-LiZnPO4 in the field of pigments.

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2. Experimental section

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2.1.

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A conventional solid-state synthetic route was selected for the synthesis of Co doped

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pigments having general formula LiZn1-xCoxPO4 (0 ≤ x ≤ 0.8). High purity Li2CO3 (Sigma-

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Aldrich, 99.9%), ZnO (Sigma-Aldrich, 99.9%), NH4H2PO4 (Sigma-Aldrich ≥ 98%), CoCO3

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reagents (Aldrich, 99.99%) were used as received without further purification.

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Materials and methods

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Stoichiometric proportions of precursors were mixed in ethanol wetting medium for 2hrs and

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calcined in a muffle furnace. Calcination temperature was confirmed by the Thermogravimetry

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and Differential thermogravimetry (TG-DTG) of the precursor mixture, which was carried out in

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the temperature range 50–1000 °C using an SII Nanotechnology Inc., TG-DTA 6200 in nitrogen 4

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atmosphere at a heating rate of 20 °C/min. Calcination was performed in two steps with a

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grinding in between, where the first step at 500 °C for 3hrs enable to remove H2O and volatile

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gases (NH3, CO2) and the second heating for 5 hrs at 900 °C gave rise to pigment formation.

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2.2.

Preparation of roof coatings and Hiding strength test

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Composition showing best chromatic properties and reflectance was chosen for NIR

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reflective coating studies on building materials like concrete slab and aluminum sheet. The

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pigment emulsion was coated on a TiO2 base coating. Pigment emulsion consists of a dispersion

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(1:1 weight ratio) of pigment and acrylic binder (50% pigment volume concentration) by

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ultrasonication.

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Hiding strength of the pigment has been evaluated. Prepared pigment emulsion using

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commercially available alkyd resin binder, having pigment to binder ratio 1:4, coated on a black

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as well as white chart. CIE 1976 L*a*b* colourimetric method was employed to determine

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hiding power of the pigment by comparing colour co-ordinate values [39].

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2.3.

Characterization techniques

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Obtained pigments were characterized by Powder X-ray Diffraction (PXRD), Scanning

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Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) instrumental

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techniques. PXRD pattern of the pigments was recorded using Philips X’pert Pro diffractometer,

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Ni–filtered Cu-Kα (λ = 0.154060 nm) radiation. Data were collected by step scanning over a 2θ

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range from 10 - 75° with a step size of 0.03° and 20 s counting time at each step. PXRD data for

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Rietveld refinement of the structure were collected by means of a Bruker D8 Advance

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diffractometer, using a Cu Kα radiation (α1 = 1.54056 Å, α2 = 1.54439 Å) in the 2θ range 10 -

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80° with a step size of 0.02° and 30 s counting time at each step, employing the program JANA

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2006 (see experimental conditions for data collection in Table 1). Morphology analysis was done by Scanning Electron Microscope (SEM) JEOL JSM-5600.

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EDS analysis was conducted using silicon drift detector X-MaxN attached with a Carl Zeiss

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EVO SEM apparatus. UV-Vis-NIR Spectrophotometer (Shimadzu UV-3600 with an integrating

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sphere attachment, ISR-2200) was used for the optical property study of pigment samples with

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barium sulfate as reference for UV-Vis range (300-700 nm) and polytetrafluoroethylene (PTFE)

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for NIR range (700-2500 nm). The measurement conditions were as follows: an illuminant D65,

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10° complementary observer and measuring geometry d/8°. The colour coordinates

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measurements were done with analytical software (UVPC Colour Analysis Personal

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Spectroscopy Software V3, Shimadzu) coupled with the UV-3600 spectrophotometer. The CIE

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1976 L*a*b* colourimetric method was used, as recommended by the Commission

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Internationale del’Eclairage (CIE). In this method, L* is the colour lightness (L* = 0 for black

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and L* = 100 for white), a* is the green (-)/red (+) axis, and b*is the blue (-)/yellow (+) axis. The

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parameter C* (chroma) represents saturation of the colour and is defined as C* =

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the hue angle, h° is expressed in degrees and ranges from 0 to 360° and is

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calculated by using the formula h° = tan-1(b*/a*). For each colourimetric parameter of a sample,

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measurements were made in triplicate, and an average value was taken as the result. Typically,

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for a given sample, the standard deviation of the measured CIE-L*a*b* value is less than 0.10,

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and the relative standard deviation < 1%, indicating that the measurement error can be ignored.

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Optical measurements were carried out in the 700-2500 nm range. The NIR solar reflectance

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(R*) in the wavelength range from 700 to 2500 nm was calculated according to ASTM standard

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number E891-87 as reported elsewhere [40-42]. The NIR solar reflectance or the fraction of solar

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radiation incident at wavelengths between 700 and 2500 nm that is reflected by a surface is the

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irradiance-weighted average of its spectral reflectance, r(λ), and can be determined using the

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relation,

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R* =

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where r(λ) is the experimentally obtained spectral reflectance (Wm-2) and i(λ) is the solar

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spectral irradiance (Wm-2 nm-1) obtained from ASTM standard E891-87.

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3. Results and discussion

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3.1. Thermogravimetric analysis

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To use the tetrahedral geometry of Zn atom in LiZnPO4 host material, we have attempted

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to synthesize a series of pigments substituted with Co2+ at Zn2+ site having composition LiZn1-

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xCoxPO4

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shown in Fig. 1. TG curves undergoes three-step weight loss, where an initial weight loss up to

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160 °C indicates the evaporation of adsorbed water [8]. Possible overall decomposition reaction

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could be

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½ Li2CO3 + 0.9 ZnO + 0.1 CoCO3 + NH4H2PO4 → LiZn0.9Co0.1PO4 + 0.6 CO2 + NH3 + 3⁄2H2O

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Subsequent weight loss in TG observed between temperature 160-240 °C corresponds to

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evolved NH3 gas from NH4H2PO4 precursor, weight loss percentage calculated from the graph to

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be approximately, 4.47% [43]. Mass loss between 240-300 °C was due to the removal of H2O

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(0 ≤ x ≤ 0.8). Thermogravimetric analysis of the precursor mixture has been done and

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molecule and 300-610 °C due to CO2 and H2O molecule together, where H2O formation could

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arise from the reaction between 3H+ (coming from NH4H2PO4 decomposition into NH3 + H3PO4)

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and oxide anions (3⁄2O2-) coming from ZnO and carbonates decomposition (½Li2CO3 and

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CoCO3) between 300-610 °C [44-45]. Accordingly, H2O removal could be taking place at two

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different temperature ranges, occurring the second one above 300ºC, since Li2CO3 and CoCO3

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must first decompose to provide additional oxide anions. Hence, weight loss percentage

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calculation of H2O and CO2 may not be accurate. After 610 °C TG curve found stable up to 1000

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°C.

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3.2. Powder X-ray diffraction analysis

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PXRD pattern of the pigments LiZn1-xCoxPO4 (0 ≤ x ≤ 0.4) are shown in the Fig. 2 (b). A

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single phase monoclinic LiZnPO4 structure was confirmed from PXRD patterns (JCPDS 89-

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4699). On the other hand, in the compositions LiZn1-xCoxPO4 (0.6 ≤ x ≤ 0.8), a secondary phase

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LiCoPO4 (marked #) having orthorhombic crystal structure and Pnma space group (JCPDS 89-

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6192) was observed in diffraction pattern along with LiZnPO4 phase and when x = 1 LiCoPO4

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become the predominant phase, exhibited in Fig. 2 (c). The intense peak at 2θ = 36º also

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corresponds to (031) crystal plane of LiCoPO4 rather (621) peak of LiZnPO4 alone [46]

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According to the previous reports, Co2+ exist in six co-ordinate octahedral geometry in LiCoPO4

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[11]. A gradual change in the Co geometry from tetrahedral to octahedral has been identified

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from this observation in PXRD. It was later confirmed by UV-Vis absorption spectra.

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Rietveld refinement analysis was conducted for the LiZn0.9Co0.1PO4 pigment to determine

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the crystal structure, corresponding refined XRD is presented in Fig. 3 and crystallographic data

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in Table 2. The result was in good agreement with its parent compound α-LiZnPO4, where three

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tetrahedra PO4, distorted LiO4 and ZnO4, one of each kind, share one corner. In the refined

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crystal structure Zn2+/Co2+ are associated in two different crystallographic sites, forming corner

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sharing tetrahedral geometry, which are shown in Fig. 4. The current investigation of Co

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substitution at Zn site made an isotropic increase in the lattice parameters and cell volume from

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the parent system (Table 1). This is attributed to an appreciable decrease in bond length of Co-O

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with respect to Zn-O, even though, Shannon-Prewit effective ionic radii for Co2+ and Zn2+ ions in

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tetrahedral site are 58 pm and 60 pm. Calculated bond lengths and bond angles of the Zn/CoO4

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tetrahedra were tabulated in Table 3 and Table 4, respectively. It must be due to the variation in

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bond covalence occurring with this substitution. According to previous investigations, decrease

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in ionic size will increase covalency, which in turn decreases the bond distance [47]. Hence, the

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smaller Co2+ ions will eventually lead into decrease in Co-O bond length. Hence, despite of the

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smaller ionic radius of Co2+ with respect to Zn2+ in tetrahedral MO4 sites, the replacement of

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Zn2+ by Co2+ ions in LiZn1-xMxPO4 pigment compositions results in a slight increase of both cell

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parameters and cell volume [13, 47].

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Considering Zn2/CoO4 tetrahedra, among the four Co-O bonds two of them are unusually

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smaller, compared to CoAl2O4 system (Co-O = 1.972 Å) [47]. Resulted in far more strained

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geometry, which in turn forced to deviates its bond angle from regular 109°28’ so that the bond

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strain must be minimize and eventually formed a highly distorted tetrahedron. Zn7/CoO4

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tetrahedra were also found in distorted geometry, where, instead of two, three shorter and one

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slightly longer Co-O bonds are observed. Hence, from the refinement data, we confirmed the

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formation of heavily distorted Zn/CoO4 tetrahedra, which might have influenced the geometry of

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corner shared PO4 and LiO4 tetrahedra and ended up in an increase in unit cell volume.

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3.3. Morphology

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SEM images of phase pure pigment samples are shown in Fig. 5. The SEM image shows

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the presence of aggregated smaller particles with a few disconnected larger particles. Mean

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particle size calculated from the SEM images was found to be 3.0-4.0 µm. EDS spectrum of the

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sample LiZn0.9Co0.1PO4 shown in Fig. 6, confirms the presence of Zn, Co, P and O elements.

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Measured average value for the Zn/Co/P molar ratio effectively confirm that it is very close to

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the theoretical 0.9/0.1/1 ration in sample x = 0.1, Table 5.

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3.4. UV-Vis-NIR absorption properties

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UV-Vis-NIR absorption spectra of the synthesized phase pure pigments LiZn1-xCoxPO4

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(0.1 ≤ x ≤ 0.4) were displayed in Fig. 7 (a). Very strong absorption has observed in a broad

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spectral range of 1.80-2.70 eV with triplet band of unequal intensities at 2.03, 2.15 and 2.36 eV.

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This corresponds to a spin allowed d-d transition of tetrahedrally co-ordinated Co2+ 3d7

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electrons, as per Tanabe-Sugano diagram it is attributed to a 4A2(F) → 4T1(P) transition [9, 16,

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48-51]. 4A2(F) → 4T1(P) transition split into threefold due to Jahn-Tellar distortion of tetrahedral

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sites and, especially, strong spin-orbit (L-S Russell-Saunders) coupling effects. In addition, by

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the mixing of quadruplet and doublet states from 2G term: 2E, 2T1, 2T2 and 2A1, which will also

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lead to a considerable intensity increase of spin-forbidden transition to 2G term. First two peaks

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correspond to overlap with 2E and 2T1 states and third peak due to overlap with 2T2 and 2A1 states

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[47][52]. Moreover, enhanced splitting of spin allowed transition 4A2(F) → 4T1(P) implied

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changes in crystal field strength and increased the spin-orbit L-S coupling effects which in turn

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lead to a broadened absorption peak. Very low intense absorption bands were also located at 2.78

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and 3.06 eV, correspond to spin forbidden d-d transitions 4A2(F) → 2T1(2P) and 4A2(F) →

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T1(2H), respectively [46-47, 27]. Which was allowed only because of spin selection rule

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relaxation, by intermixing of quartet and doublet states [9]. Moreover, the increased Co doping

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from x = 0.1 to x = 0.4 resulted an increase in absorption intensity due to an increase in optical

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density. Noteworthy, the energy of the three peaks constituting this three-fold band is not shifted

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with the increase of Co2+ doping, thus indicating that the average Co-O distances are not

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changing substantially with Co doping. However, this triplet band becomes slightly broader with

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the increase of Co doping, indicating an increase of L-S coupling effects and a decrease in the

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isotropy of CoO4 tetrahedra. Further light on Co2+ co-ordination geometry was obtained from

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absorption profile in the IR region where a broad three band absorption has registered between

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0.59 and 1.29 eV, associated with 4A2(F) → 4T1(F) and 4A2(F) → 4T2(F) electronic transitions

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[11]. It is reported that, a tetrahedral CoO4 geometry results either a blue or green colour [19-20].

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However, the present study reveals that Td coordination will also lead to bright purple colour, by

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the absorption of a broad region of visible spectrum.

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A comparison of the visible absorption spectra of CoAl2O4 blue and LiZn0.9Co0.1PO4

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reveals that though the chromophore is essentially same, it is noteworthy that there is a profound

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blue shift occurred for LiZn0.9Co0.1PO4 system by 0.25 eV, shown in Fig. 7 (c). This changed the

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colour of the pigment from blue to purple. CIE 1976 colour co-ordinate measurements gave a

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better understanding of the chromatic properties, which are summarized in Table 1. Evidence for

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such an interesting observation was obtained from the crystal structure of CoAl2O4 and

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LiZn0.9Co0.1PO4. A highly distorted CoO4 tetrahedron with shorter Co-O bonds in

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LiZn0.9Co0.1PO4, brings the ligand field closer to metal orbitals such that it experiences a stronger

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ligand field and results in an increase in energy separation between e and t2 orbitals, it eventually

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leads to a blue-shifted absorption, developing an excellent purple pigment.

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Further Co doping (> 0.4 atoms per formula unit, apfu) derived a dual phase compound,

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where a progressive blue shift in the UV-Vis absorption edge from 1.80 to 1.87 eV has observed

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leading to a colour change from purple to violet exhibited in Fig. 7 (b). A predominant

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absorption registered at 2.15 eV with a shoulder peak at 2.36 eV and apart from a drop in overall

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absorption intensity, existing peak at 2.03 eV gradually diminished with simultaneous emerge of

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new absorption peaks at 2.53 and 1.57 eV. At a high Co2+ concentration (> 0.4apfu) absorption

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profile had a close resemblance with CoO6 spectral pattern and Tanabe-Sugano diagram

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confirmation for the respective absorption peaks are 4T1g(F) →4T1g(P) for 2.15eV, 2.36 and 2.53

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eV likewise 4T1g(F) →4A2g(F) for 1.57 eV and at low energy IR region absorption peak centered

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at 0.79 eV, 4T1g(F) →4T2g(F) [11, 27]. The data confirms that Co concentration reaches its limit

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at 0.4 apfu and afterward forming a secondary phase, where the chromophore has octahedral

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geometry (CoO6). From the knowledge of secondary phase from XRD results we have compared

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LiCoPO4 UV-Vis spectra with LiZn1-xCoxPO4 (0.6 ≤ x ≤ 0.8) and found a very close match,

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hence further confirmed the secondary phase.

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3.5. Chromatic and Reflectance properties

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CIE 1976 colour co-ordinate measurements are summarized in Table 6. It is interesting to

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note that high b* (-52.32) and a* value (+25.25) with an impressive lightness L* (49.74) were

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obtained for even a very low concentration of Co substitution. Hence, LiZn0.9Co0.1PO4 pigment

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presents an intense purple colour, which is found to be better than some reported purple pigments

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(Table 7), though it is not the best, depicted in Fig. 8. C* value > 50, represent the richness of

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purple colour and hue angle lie in between blue and red region, gave further confirmation.

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Enhancement in the colour intensity with slight increase in redness a* value resulted for LiZn1-

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xCoxPO4,

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x = 0.2 and 0.4 compositions, but lightness found to be at lower side. Therefore, we 12

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opted LiZn0.9Co0.1PO4 for further surface coating applications. Added Co concentration did not

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have any significant impact on the improvement in purple colour, rather it changed to a violet

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hue, due to the secondary phase formation as evident from the XRD data (Fig. 2c). A sudden

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decline in b* indicates this colour change, which is well identified from photographs displayed in

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Fig. 8.

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NIR solar reflectance spectra of purple pigment powder, is demonstrated in Fig. 9. 46%

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NIR reflectance observed at typical hot region 1100 nm and an average IR reflectance 54% and

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NIR solar reflectance 68% (700-2500 nm), for LiZn0.9Co0.1PO4 composition found to be an

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exceptional achievement among cobalt containing pigments. Because of strong absorption due to

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the electronic transitions 4A2(F) → 4T1(F) and 4A2(F) → 4T2(F) in the NIR region (700-2500 nm)

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cobalt compounds show comparatively lower total solar reflectance. Well known blue pigment

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CoAl2O4 had very low R* value 29%, similarly Cobalt chromite blue (Pigment Blue 36) shows

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only 30% R* [53-54]. 57.9% solar reflectance was reported for Co0.5Zn0.5Cr2O4 compound [52].

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Among which LiZn0.9Co0.1PO4 pigment stands alone with an incredible 68% R*. Considering the

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low concentration (3.5 atomic wt.%) of Co2+ ion and low-cost parent compound, this must be an

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encouraging candidate among purple ‘cool roof’ pigments.

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Hiding strength has been evaluated where, variation in lightness (∆L* = 7.31) between

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coating on white as well as black surfaces, together with colour co-ordinates ∆a* = 35.30 and

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∆b* = 46.89 figure out a moderate hiding strength of the pigment, Table 8. A better hiding

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strength could be achieved by improving the uniformity of particles.

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3.6. Chemical stability

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Chemical stability of the synthesized pigments was ensured, to take over as a surface

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coating material. Investigation on chemical stability was done in aqueous (pH = 7), acid (HCl,

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HNO3 and H2SO4 of pH 3.0-3.2) and base (NaOH pH = 10.52) medium as well. Pre-weighed

284

LiZn0.9Co0.1PO4 pigment was allowed to treat with respective medium for 30 min, with

285

continuous stirring. Subsequently, the pigment samples were filtered, washed, dried and weighed

286

which resulted only a negligible weight loss. Further colour co-ordinate measurements are

287

tabulated in Table 9. Total colour difference value determined to be well within the industrial

288

limit, ∆E*ab ≤ 1 unit suggesting the developed LiZn0.9Co0.1PO4 purple pigment does not undergo

289

colour fading by the attack of acids or base, hence ensured the chemical stability [55-57].

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3.7. Application studies

291

In order to understand the performance of the developed pigment coatings NIR solar

292

reflectance spectra was taken, depicted in Fig.10. Pigment coated sample exhibited a significant

293

improvement in NIR solar reflectance compared to bare Al sheet and concrete surfaces,

294

enumerated in Table 10. R* above 50% particularly reveal that LiZn0.9Co0.1PO4 pigment coatings

295

can reduce the heat buildup appreciably during summer season. Moreover, colour execution of

296

the material also fairly good, maintaining the b* value above -43 for both coatings, which

297

establish the colour deliverability of the pigment.

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Colour distribution ability in a polymer substrate was analyzed by incorporating 10% of

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pigment sample in PMMA matrix. A uniform distribution of the purple colour has been observed

300

from the prepared polymer disc and CIE 1976 colour co-ordinate measurements L* = 24.23±0.1,

301

a* = +23.49±0.5 and b* = -39.08±0.5 carried out on different points of disc were substantiated 14

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the observation. Overall results deliver the excellent applicability of the developed purple

303

pigment, as a ‘cool pigment’.

4. Conclusion

305

A cost-effective novel inorganic pigment with brilliant purple colour has been developed

306

from a white LiZnPO4 parent compound by Co2+ substitution. Origin of unusual purple colour

307

was explained by crystallographic structural refinement analysis, wherein the chromophore CoO4

308

tetrahedra exist in a highly distorted geometry because of the shorter Co-O bonds formed due to

309

the difference in covalence to that of Zn-O bond. Shorter Co-O bonds increased the energy gap

310

by creating stronger ligand field on metal atom thereby motivated a blue shift in UV-Vis spectra

311

leading to a purple colour pigment. Secondary phase formation for high Co concentration

312

resulted purple to violet colour change, where the UV-Vis-NIR absorption spectra concluded a

313

gradual co-ordination geometry change for the chromophore from tetrahedral to octahedral.

314

Composition with very low Co2+ concentration (3.5 atomic wt%) exhibited intense purple colour,

315

a* = +25.25, b* = -52.32, thereby able to manage the expense to its minimum and shown

316

impressive NIR solar reflectance (R*) of 68%, which lead to surface coating application study on

317

concrete block, Al roofing sheet. The excellent coating study result, maintaining an R* of >50%

318

and b* of around -45 revealed the potential of developed pigments to put forward as an excellent

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‘cool pigment’.

320

Acknowledgements

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Financial support from Science and Engineering Research Board (SERB), DST,

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Government of India, through Early Career Research Award (ECR/2016/000098). We thank Dr.

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A. S. Prakash, CSIR-CECRI Chennai unit for slow scan XRD measurements and Mr Peer

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Mohamed, CSIR-NIIST, Thiruvananthapuram for TGA measurements.

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References

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MLP. YIn0.9Mn0.1O3-ZnO nano-pigment exhibiting intense blue color with impressive

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Figure captions

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Fig. 1. TG curve of LiZn0.9Co0.1PO4 precursor mixture.

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Fig. 2. (a) Comparison of PXRD patterns of LiZn0.9Co0.1PO4 pigment with LiZnPO4 system,

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calcined at 900°C (b) PXRD patterns of LiZn1-xCoxPO4 (0 ≤ x ≤ 0.4) pigment and (c) LiZn1-

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xCoxPO4

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Fig.3. Rietveld refinement of the structure of LiZn0.9Co0.1PO4 from powder XRD data using

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JANA2006 software.

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Fig. 4. Distorted geometry of (a) Zn2/CoO4 (b) Zn7/CoO4.

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Fig. 5. SEM micrographs of (a) LiZn0.9Co0.1PO4 (b) LiZn0.8Co0.2PO4 (c) LiZn0.6Co0.4PO4.

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Fig. 6. EDS analysis of LiZn0.9Co0.1PO4.

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Fig. 7. UV-Vis Absorption spectra of synthesized (a) LiZn1-xCoxPO4 (0.1 ≤ x ≤ 0.4),

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(b) LiZn1-xCoxPO4 (0.6 ≤ x ≤ 1), (c) comparison of LiZn0.9Co0.1PO4 and CoAl2O4 spectra, (d)

484

comparison of x = 0.2 and x = 0.8 absorption spectra.

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Fig. 8. Photograph of synthesized LiZn1-xCoxPO4 (0.1 ≤ x ≤ 0.8) pigments.

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Fig. 9. NIR Solar reflectance (R*) spectra of LiZn1-xCoxPO4 (0.1 ≤ x ≤ 0.8) pigments.

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Fig. 10. Comparison of NIR solar reflectance (R*) of (a) bare and pigment coated concrete

488

block, (b) bare and pigment coated Al sheet.

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(0.6 ≤ x ≤ 1) (# denotes LiCoPO4 phase).

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LiZn0.9Co0.1PO4

Symmetry

Monoclinic

Monoclinic

Space group

Cc

Cc

Unit cell parameters

a = 17.250 Å, b = 9.767 Å, c = 17.106 Å, α = γ = 90°, β = 110.9°

a = 17.2751 Å, b = 9.7688 Å, c = 17.116 Å, α = γ = 90°, β = 110.902°

Volume

2691.8 Å3

2698.362 Å3

Z

32

Dc

3.30 gcm-3

Radiation

λ (Mo Kα) = 0.7107 Å

Cu Kα1 = 1.54056 Å

Measuring range

10° ≤ 2θ ≤ 80°

10° ≤ 2θ ≤ 80°

Rp factor

2.89%

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LiZnPO4 [33]

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3.2944 gcm-3

7.38%

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Table 1 Rietveld data of LiZn0.9Co0.1PO4 and experimental conditions for data collection.

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Table 2 Crystallographic data for LiZn0.9Co0.1PO4

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Occupancy 1 0.6/0.4 1 1 1 1 0.6/0.4 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

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z 0.333(2) 0.328(2) 0.328(3) 0.084(2) 0.577(3) 0.582(2) 0.834(3) 0.071(2) 0.263(6) 0.270(5) 0.022(6) 0.520(5) 0.269(6) 0.263(5) 0.519(5) 0.010(5) 0.352(11) 0.273(8) 0.190(11) 0.278(10) 0.315(11) 0.338(10) 0.244(10) 0.192(10) 0.105(10) 0.433(11) 0.040(9) 0.003(10) 0.457(10) 0.606(9) 0.502(10) 0.028(9) 0.185(10) 0.351(8) 0.259(10) 0.344(10) 0.694(11) 0.243(10) 0.342(11) 0.296(11) 0.552(11) 0.430(10) -0.009(10) 0.086(10)

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y 0.210(4) 0.458(5) 0.176(4) 0.204(4) 0.050(4) 0.285(4) 0.081(4) 0.483(4) 0.441(8) 0.445(8) 0.192(9) 0.255(8) 0.207(9) 0.661(8) 0.050(8) 0.439(9) 0.035(17) 0.311(16) 0.023(14) 0.335(17) 0.014(15) 0.526(16) 0.321(17) 0.552(17) 0.291(16) 0.227(16) 0.140(16) 0.094(17) 0.217(14) 0.157(15) 0.177(16) 0.544(16) 0.276(18) 0.316(15) 0.079(18) 0.246(15) 0.212(16) 0.558(19) 0.793(16) 0.577(17) 0.203(17) 0.007(16) 0.090(15) 0.031(15)

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x 0.479(2) 0.232(3) -0.001(3) 0.266(3) 0.009(2) 0.292(2) 0.264(2) 0.040(2) 0.567(5) 0.037(4) 0.080(5) 0.098(5) 0.287(5) 0.328(6) 0.353(5) 0.320(5) 0.101(10) 0.693(8) 0.063(8) 0.516(9) 0.468(9) 0.128(9) 0.029(9) 0.058(10) 0.041(10) 0.579(9) 0.149(10) 0.010(11) 0.038(8) 0.122(9) 0.170(8) 0.123(9) 0.288(9) 0.200(9) 0.279(10) 0.380(11) 0.305(10) 0.239(9) 0.331(9) 0.401(10) 0.391(10) 0.276(10) 0.409(9) 0.341(9)

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atom Zn(1) Zn(2)/Co Zn(3) Zn(4) Zn(5) Zn(6) Zn(7)/Co Zn(8) P(1) P(2) P(3) P(4) P(5) P(6) P(7) P(8) O(11) O(12) O(13) O(14) O(21) O(22) O(23) O(24) O(31) O(32) O(33) O(34) O(41) O(42) O(43) O(44) O(51) O(52) O(53) O(54) O(61) O(62) O(63) O(64) O(71) O(72) O(73) O(74)

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0.407(9) 0.358(9) 0.242(9) 0.320(9) 0.63(3) 0.12(2) 0.36(2) 0.46(4) 0.13(3) 0.15(3) 0.38(3) 0.38(3)

0.373(15) 0.515(13) 0.328(16) 0.457(17) 0.14(4) 0.27(4) 0.16(4) 0.81(5) 0.53(5) 0.06(5) 0.36(4) 0.00(4)

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0.028(9) 0.094(9) -0.003(9) 0.440(10) 0.22(3) 0.23(2) 0.41(3) 0.64(4) 0.46(3) 0.41(3) 0.19(3) 0.70(3)

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1 1 1 1 1 1 1 1 1 1 1 1

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O(81) O(82) O(83) O(84) Li(1) Li(2) Li(3) Li(4) Li(5) Li(6) Li(7) Li(8)

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Table 3 Comparison in bond length (in Å) parent ZnO4 and Zn/CoO4 tetrahedra. LiZn0.9Co0.1PO4

Zn2

Zn2

dZn2-O22=1.961(5)

dZn2-O84=1.98(14)

dZn2-O18=1.970 (4)

dZn2-O62=1.79 (19)

dZn2-O32=1.943(8)

dZn2-O52=1.59 (16)

dZn2-O6=1.956 (5) Zn7

SC

dZn2-O22=1.98 (18) Zn7

dZn7-O63=1.67 (16)

TE D

dZn7-O19=1.964(5)

dZn7-O12=1.67(14)

dZn7-O26=1.930(6)

dZn7-O72=1.78 (19)

dZn7-O2=1.956(5)

dZn7-O53=2.10(19)

EP

dZn7-O23=1.945 (5)

530

AC C

529

RI PT

LiZnPO4

M AN U

528

531

532

28

ACCEPTED MANUSCRIPT

533

Table 4 Bond angle of Zn/CoO4 tetrahedra. Bond angle (in degree)

Zn7

θO84-Zn2-O64 = 122.602

θO72-Zn7-O53 = 101.758

θO84-Zn2-O52 = 88.319

θO72-Zn7-O63 = 115.385

θO84-Zn2-O22 = 109.000

θO72-Zn7-O12 = 133.761

M AN U

SC

RI PT

Zn2

536

537

538

θO63-Zn7-O53 = 111.600

θO52-Zn2-O22 = 82.254

θO53-Zn7-O12 = 108.153

TE D

θO64-Zn2-O22 = 102.328

EP

535

θO63-Zn7-O12 = 85.310

AC C

534

θO64-Zn2-O52 = 143.595

539

540

29

ACCEPTED MANUSCRIPT

Table 5 EDS analysis of LiZn0.9Co0.1PO4 pigment

Element

Weight %

OK

68.34

PK

14.98

Co K

1.56

Zn M

15.11

Total

100.00

SC

542

M AN U

543

544

545

550

551

EP

549

AC C

548

TE D

546

547

RI PT

541

552

553

30

ACCEPTED MANUSCRIPT

554

Table 6 Colour co-ordinates and reflectance measurement of the synthesized purple pigments.

NIR Solar Sample

L*

a*

b*

C*

ho

49.74

25.25

-52.32

58.10

295.77

68.04

X = 0.2

39.47

28.11

-52.81

59.83

298.03

54.34

X = 0.4

35.75

31.16

-52.29

60.87

X = 0.6

41.40

20.65

-36.75

42.16

X = 0.8

38.33

29.04

-31.89

43.14

559

560

561

299.34

M AN U

34.01

312.32

29.58

TE D

39.62

EP

558

300.79

AC C

557

SC

X = 0.1

555

556

RI PT

Reflectance R*(%)

562

563

31

ACCEPTED MANUSCRIPT

Table 7 Comparison of colour co-ordinates of LiZn0.9Co0.1PO4 with reported samples.

L*

a*

b*

YGa0.95Mn0.05O3

47.45

25.01

-30.01 [5]

LiCoPO4

41.00

34.50

Ba(Zn0.85Co0.15)2Si2O7

28.60

52.20

Ba(Zn0.95Co0.05)2Si2O7

43.80

27.90

Zn0.8Co0.2MoO4

52.20

12.70

Zn0.9Co0.1MoO4

47.03

22.93

-62.72 [28]

CoAl2O4

44.80

2.10

-32.70 [36]

YIn0.9Mn0.1O3

39.71

-5.78

-22.28 [56]

LiZn0.9Co0.1PO4

49.74

25.25

-52.32 [Present study]

565 566 567

572 573 574 575 576 577

-49.50 [11]

-65.50 [21] -46.60 [21] -45.70 [27]

SC

EP

571

AC C

570

TE D

568 569

RI PT

Pigment

M AN U

564

578 579 580 581

32

ACCEPTED MANUSCRIPT

582

Table 8 Colour co-ordinates involved in hiding strength experiment

583

a*

Covering power on white surface

35.46

41.04

Pigment coated black surface

28.15

5.74

M AN U

584 585 586 587 588

593 594 595 596 597 598

EP

592

AC C

591

TE D

589 590

b*

RI PT

L*

599 600 601 602

33

-62.71

-15.82

SC

Sample

ACCEPTED MANUSCRIPT

606

607

608

609

b*

C*

Pigment

-

49.74

25.25

-52.32

Water

7.00

49.70

25.22

-52.08

HCl

3.05

49.48

24.99

-52.02

HNO3

3.17

49.68

25.11

-51.89

H2SO4

3.18

49.55

NaOH

10.52

49.38

# ∆E*ab

RI PT

a*

58.10

-

57.86

0.24

57.71

0.47

57.64

0.45

M AN U

SC

L*

25.09

-52.01

57.74

0.39

25.20

-52.15

57.91

0.40

NB #: ∆E*ab = [(∆L*)2 +(∆a*)2 +(∆b*)2]1/2

TE D

605

pH

EP

604

Table 9 Color co-ordinates of LiZn0.9Co0.1PO4 pigment after chemical treatment test.

AC C

603

610

611

34

ACCEPTED MANUSCRIPT

612

Table 10 Colour-co-ordinates and the NIR solar reflectance measurements of the pigment

613

coating on different roofing materials.

NIR Solar L*

a*

b*

C*

h

RI PT

Sample

o

Reflectance R*(%)

25.25

-52.32

58.10

Al sheet

54.15

16.66

-43.04

46.15

291.16

50.98 (44.87)

Concrete block

52.94

18.67

-45.48

49.16

292.33

53.34 (43.72)

NB : R* of respective bare surface is given in bracket.

615

AC C

EP

TE D

616

617

295.77

35

68.04

SC

49.74

M AN U

614

LiZn0.9Co0.1PO4

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

618

622

623

624

EP

621

AC C

620

TE D

619

36

627

628

629

EP

626

AC C

625

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

630

631

37

634

635

636

637

EP

633

AC C

632

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

638

639

38

ACCEPTED MANUSCRIPT

640

641

(a)

(b)

RI PT

642

643

SC

644

M AN U

645

646

647

651

652

653

EP

650

AC C

649

TE D

648

654

655

656

39

ACCEPTED MANUSCRIPT

657

(b)

(a)

659

1 µm

660

M AN U

662

663

1 µm

664

669

670

EP

668

AC C

667

TE D

665

666

1 µm

SC

(c)) )

661

RI PT

658

40

RI PT

ACCEPTED MANUSCRIPT

SC

671

M AN U

672

673

674

678

679

680

EP

677

AC C

676

TE D

675

681

682

41

685

686

687

EP

684

AC C

683

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

688

42

RI PT

ACCEPTED MANUSCRIPT

689

SC

690

691

M AN U

692

693

694

698

EP

697

AC C

696

TE D

695

43

701

702

703

704

EP

700

AC C

699

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

705

44

SC

RI PT

ACCEPTED MANUSCRIPT

M AN U

706

707

708

712

713

714

715

EP

711

AC C

710

TE D

709

716

717

45

ACCEPTED MANUSCRIPT Highlights An intense purple coloured pigment was synthesized by simple solid-state method.

•

Highly distorted tetrahedral Co2+ chromophore geometry.

•

High NIR Solar reflectance achieved for very low dopant concentration.

•

Application studies reveal the potential utility of the pigment as a ‘cool pigment’

AC C

EP

TE D

M AN U

SC

RI PT

•