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YIn0.9Mn0.1O3–ZnO nano-pigment exhibiting intense blue color with impressive solar reflectance
Accepted Manuscript YIn0.9Mn0.1O3-ZnO nano-pigment exhibiting intense blue color with impressive solar reflectance Sheethu Jose, Anaswara Jayaprakash, Sourav Laha, S. Natarajan, K.G. Nishanth, M.L.P. Reddy PII:
S0143-7208(15)00361-7
DOI:
10.1016/j.dyepig.2015.09.014
Reference:
DYPI 4925
To appear in:
Dyes and Pigments
Received Date: 16 June 2015 Revised Date:
9 September 2015
Accepted Date: 10 September 2015
Please cite this article as: Jose S, Jayaprakash A, Laha S, Natarajan S, Nishanth KG, Reddy MLP, YIn0.9Mn0.1O3-ZnO nano-pigment exhibiting intense blue color with impressive solar reflectance, Dyes and Pigments (2015), doi: 10.1016/j.dyepig.2015.09.014. 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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Highlights • A blue colored nano-pigment was synthesized by a sol-gel combustion method.
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• The nano-pigment exhibits superior blue hue and high solar reflectance.
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• The potential utility of the nano-pigment as a ‘Cool Pigment’ was demonstrated.
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YIn0.9Mn0.1O3-ZnO nano-pigment exhibiting intense blue color with impressive solar reflectance L. P. Reddya,b* a
Academy
of
Scientific
and
Innovative
research
Thiruvananthapuram, India b
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Sheethu Jose,a,b Anaswara Jayaprakash,b Sourav Laha,c S. Natarajan,c K. G. Nishanth,b and M.
(AcSIR),
CSIR-NIIST
Campus,
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Materials Science and Technology Division, CSIR-National Institute for Interdisciplinary
Science and Technology (NIIST), Thiruvananthapuram 695 019, Kerala, India c
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012,
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India
ABSTRACT
The current study reports on the synthesis and characterization of a new inorganic nano-
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pigment with an intense blue color and high solar radiation reflective properties (70%). The nano-pigment YIn0.9Mn0.1O3-ZnO was synthesized by a sol-gel combustion method and characterized with the aid of X-Ray diffraction, Raman spectroscopy, Magnetic susceptibility,
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Transmission electron microscopy, UV-vis-NIR diffuse reflectance spectroscopy and CIE1976 L*a*b* color measurements. The Rietveld refinement of the XRD patterns of the
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developed nano-pigment disclosed the existence of YIn0.9Mn0.1O3 and ZnO in a 1:1 ratio with hexagonal crystal structures. For comparison, YIn0.9Mn0.1O3 was also synthesized by the solgel combustion route and its optical properties compared with that of YIn0.9Mn0.1O3-ZnO. It is interesting to note that the developed YIn0.9Mn0.1O3-ZnO nano-pigment exhibits superior blue hue (b* = -40.55) and solar reflectance (R* = 70%) values as compared to the YIn0.9Mn0.1O3 nano-pigment (b* = -22.28, R* = 50%). Most importantly, the potential utility of the nano-
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pigment as a “Cool Pigment” was demonstrated by coating onto roofing materials like aluminium roofing sheets.
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Keywords: Blue nano-pigment/coatings, Solar reflectance, Sol-gel combustion method *Corresponding author. Tel.: +91471 2515360; Fax: +91471 2491712
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E-mail address: [email protected] (M. L. P. Reddy)
Introduction
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Nowadays much effort has been made to minimize the use of toxic cobalt aluminate (CoAl2O4) in pigment formulations and also to produce cost-effective inorganic blue pigments [1-6]. With the exception of the turquoise blue pigment of vanadium-zircon, known traditional blue inorganic pigments are generally based on the cobalt ion incorporated into different
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crystalline structures namely, cobalt aluminate (CoAl2O4), cobalt orthosilicate (Co2SiO4), cobalt stannate (Co2SnO4) and zinc silicate with Co2+ [(CoZn)2SiO4] [3-9]. Therefore, the quest for intense blue inorganic pigments which are non-toxic, earth-abundant and durable are
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some of the important issues that remain as a challenge in the inorganic pigment industry. Recently, Smith et al. have shown that intense bright blue colors occur in most of the
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YIn1-xMnxO3 solid-state solutions. The Mn3+ ions substituted into the indium site exhibits an intense blue color because of an allowed d-d transition [10]. In later studies, Ocana and coworkers have developed Mn3+ doped YInO3 blue pigments at much lower temperatures (1000°C) than required by the traditional solid state method (1400°C) [11]. The developed procedure is based on the pyrolysis at 600°C of aerosols generated from aqueous solutions of Y, In, and Mn nitrates followed by an annealing treatment at 1000°C. The technological performance of these YIn1-xMnxO3 blue pigments has also been evaluated by testing their 3
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efficiency for coloring of ceramic glazes. Kim and co-workers have disclosed the synthesis and structural characterization of the blue chromophore Sr2(MgMn)Ge2O7, which has a melilite-type structure [12]. The blue color of the pigment has been attributed to the d-d
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electronic transition in Mn3+ (d4), which has trigonal bipyramidal coordination [12]. A series of new blue inorganic oxide materials LaGaGe2O7 with the introduction of Mn3+ in the trigonal bipyramidal site has also been reported [13]. However, many of these known blue
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inorganic pigments have not been explored for their solar reflective properties [12,13].
In this paper, YIn0.9Mn0.1O3 blue nano-pigments have been synthesized in the presence
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and absence of ZnO nanoparticles through sol-gel combustion route at 850 °C using citric acid as a fuel. The developed pigments were well characterized and their optical properties were investigated. Superior blue hue and high solar radiation reflectivity has been achieved in the presence of ZnO nanoparticles as compared to YIn0.9Mn0.1O3 blue nano-pigment.
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Furthermore, the optical properties of the designed blue nano-pigment YIn0.9Mn0.1O3-ZnO is found to be significantly better than those of the commercially available CoAl2O4. The coloring performance of the designed blue nano-pigment has been evaluated by coating onto
Experimental section
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2.
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roofing materials such as aluminium sheets.
2.1. Materials
The chemical reagents Y2O3 (Sigma-Aldrich, 99.8%), In2O3 (Sigma-Aldrich, 99.99%)
manganese(III) acetate (Sigma Aldrich, 97%), ZnO (Acros Organics, 99.999%) and citric acid (Sigma-Aldrich, 99.5%) are analytical grade and used without further purification.
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2.2. Preparation of YIn0.9Mn0.1O3-ZnO and YIn0.9Mn0.1O3 blue pigments The YIn0.9Mn0.1O3-ZnO pigment sample was synthesized by a sol-gel combustion method.
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Stoichiometric amounts of yttrium oxide (1.66 mmol), manganese(III) acetate (0.33 mmol), indium oxide (1.49 mmol) and zinc oxide (3.33 mmol) were dissolved in 4.0 M nitric acid (100 mL) with constant stirring and heated on a hot plate. To this solution, citric acid was
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added. The molar ratio of citric acid to metal was maintained as 3.5:1. The resultant pale yellow solution was then evaporated on a hot plate to obtain a dried gel. The gel was then
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heated at 250°C for 2 h to decompose the nitrates. The optimized calcination temperature required for the formation of the typical pigment sample YIn0.9Mn0.1O3–ZnO was ascertained by calcining the dried gel at various temperatures ranging from 600°C to 900°C for 2h in an air atmosphere. The typical blue color of the sample was observed only when the dried gel was calcined at 850°C for 2h. The calcination of the powder samples were carried out in a
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Libratherm high-temperature electrical furnace by heating the samples in platinum crucibles. The heating of the furnace was programmed to increase the temperature at 5 °C/min. The
and ZnO.
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XRD pattern of the calcined powder indicates the presence of 1:1 mixture of YIn0.9Mn0.1O3
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A similar procedure has been followed for the preparation of YIn0.9Mn0.1O3 powders. Stoichiometric quantities of yttrium oxide (1.66 mmol), manganese(III) acetate (0.33 mmol) and indium oxide (1.49 mmol) were dissolved in 4.0 M nitric acid (100 mL) with constant stirring and heating on a hot plate. Citric acid was then added to the above metal nitrate solution (The molar ratio of citric acid to metal was maintained as 1.67:1). The resulting pale yellow solution was evaporated at 80 °C on a water bath to obtain a dried gel. The dried gel was then heated at 250 °C to decompose the nitrates. The resultant powders were then 5
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calcined at 850°C for 2h to obtain the pigment powders. Examination of the product by powder XRD pattern revealed that heating the dried gel at 850 °C for 2h resulted in a single
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phase of YIn0.9Mn0.1O3 product. 2.3. Characterization techniques
Thermogravimetry and differential thermal analysis (TG-DTA) was carried out in the
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temperature range of 50-1000°C using an SII Nanotechnology Inc., TG/DTA 6200 in an air atmosphere at a heating rate of 20°C/min. The powder X-ray diffraction pattern of the
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synthesized pigment powder was recorded using the diffractometer (Philips X’pert Pro) with Ni–filtered Cu Kα (λ = 0.154060 nm) radiation. Data were collected by step scanning over a 2θ range from 10 - 70˚ with a step size of 0.08˚ and 5 s counting time at each step. Powder Xray diffraction (PXRD) data for structure refinement were collected by means of a
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PANalytical X’pert Diffractometer operated at 40 kV and 30 mA using Ni-filtered Cu – Kα radiation in the 2θ range 10 – 120o with a step size of 0.02o and step duration of 50s, Rietveld refinement of the crystal structure was carried out with the program GSAS [14]. The XRD
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patterns were typically refined for lattice parameters, atomic coordinates, scale, factors, background (Fourier polynomial background function), pseudo-Voigt (U, V, W and X) and
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isothermal temperature factors (Uiso). A Perkin-Elmer Spectrum One FT-IR spectrometer was used to obtain the IR spectral data (neat KBr). Raman spectroscopic measurements on powder samples were performed on a WITec Raman microscope (Witec Inc. Germany, alpha 300R) with a laser beam directed to the sample through 20x objective and a Peltier cooled CCD detector. Samples were excited with a 633 nm excitation wavelength laser, and Stokes shifted Raman spectrum were collected in the range of 400 to 4000 cm−1 with 1 cm−1 resolution. Prior to every measurement, a 6
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calibration with a silicon standard (Raman peak centered at 520 cm−1) was performed. WITec Project Plus (v 2.1) software package was used for data evaluation. Magnetic measurements were carried out as a function of temperature using PPMS
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(Quantum Design, Dynacool) in an applied field of 500 Oe. The measurement was performed in the temperature range 5 K to 300 K. Dynamic Light Scattering experiment was conducted using Malvern Instruments Zeta sizer nano Zis Model No: ZEN 3600 to determine the particle
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size of the nano-pigment. Transmission electron microscopy (TEM) [JEOL 2010 (300 kV)] was used to understand the morphology and particle size of the synthesized samples.
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The optical properties of the powdered pigment samples were measured (380-780 nm) with a UV-vis Spectrophotometer (Shimadzu UV-3600 with an integrating sphere attachment, ISR3100) using barium sulfate as a reference. The measurement conditions were as follows: an illuminant D65, 10˚ complementary observer and measuring geometry d/8˚. The color
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coordinates were determined by coupling with analytical software (UVPC Color Analysis Personal Spectroscopy Software V3, Shimadzu) to the UV-3600 spectrophotometer. The CIE 1976 L*a*b* colorimetric method was used, as recommended by the Commission
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Internationale de I’Eclairage (CIE). In this method, L* is the color lightness (L* = 0 for black and L* = 100 for white), a* is the green (-)/red (+) axis, and b* is the blue (-)/yellow (+) axis.
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The parameter C* (chroma) represents the saturation of the color and is defined as C* = (a*)2 + (b*) 2 . The hue angle, h˚ is expressed in degrees and ranges from 0 to 360˚
b* and is calculated by using the formula h˚ = tan−1 . For each colorimetric parameter of a a * sample, measurements were made in triplicate and an average value was taken as the result. Typically, for a given sample, the standard deviation of the measured CIE–L*a*b* values is
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less than 0.10, and the relative standard deviation ≤ 1%, indicating that the measurement error can be ignored. The near–infrared reflectance spectrum of the powdered pigment samples and the aluminium roofing sheet coatings were recorded with a UV–vis–NIR spectrophotometer UV-3600
with
an
integrating
sphere
attachment,
ISR
3100)
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(Shimadzu,
using
polytetrafluoroethylene (PTFE) as a reference. Optical measurements were carried out in the 700-2500 nm range. The solar reflectance (R*) in the wavelength range from 700-2500 nm
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was calculated according to ASTM standard number E891–87 as reported elsewhere [15-20]. The NIR solar reflectance or the fraction of solar radiation incident at wavelengths between
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700 and 2500 nm that is reflected by a surface is the irradiance-weighted average of its spectral reflectance, r(λ), and can be determined using the relationship, 2500
∫ r (λ ) i (λ ) d ( λ )
R* =
700 2500
∫ i (λ ) d (λ )
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700
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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2.4. Preparation of aluminium roofing sheet coatings The synthesized pigment powder was applied as paint on aluminium roofing sheets as per the standard method described elsewhere [18-22]. The paint formulation was prepared by dispersing the known quantities of the blue nano-pigment (2 g) and acrylic binder (2 g) in 4.0 mL of acrylic-acralyn emulsion (Acralyn-R M/s Asian acrylates, Mumbai, India) and ultrasonicated (Vibronics, 250 W, India) for 10 min to ensure the complete dispersion of pigment particles. The pigment to binder ratio was maintained at 1:1 (by weight). The
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resultant viscous paint formulation was coated onto the surface of aluminium roofing sheets and was allowed to dry in air. The thickness of the pigment coating on aluminium roofing sheet surface was measured by employing LEICA DMRX optical microscope.
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3. Results and discussion 3.1. FT-IR and thermogravimetric analyses
Fig. 1 shows the typical FT-IR spectra of the synthesized citrate gel as well as calcined
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pigment powders (850°C). The broad band observed in the spectrum of citrate gel at 3400 cmcan be assigned to O–H stretching frequencies of the intermolecular hydrogen-bonded water
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[23,24]. Further, the FT-IR spectrum of the citrate gel evidenced intense absorption bands characteristic of the carboxylate groups at 1380 and 1560 cm-1, which are assigned to the symmetric νs (C=O) and asymmetric νas (C=O) vibrations, respectively [24]. The difference between the asymmetric and symmetric stretching vibration modes falls in the range 180 cm-1,
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which indicates that carboxylate groups are coordinated to metal ions in either bridging or chelating modes [25,26]. The band at 1378 cm-1 observed can be assigned to vibrations of free nitrate ion [11,24]. However, this band is overlapped with the citrate ion absorption bands.
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The absence of the broad absorption band at 3400 cm-1 in the FT-IR spectrum of calcined pigment powder indicates the absence of moisture in the pigment powder. Further, the
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absence of absorption bands corresponding to citrate and nitrate ions clearly indicates the decomposition of nitrate and citrate ions after the calcination of the gel at 850°C. As shown in Fig. 2, there are weight loss processes at 80-130°C, 180-290°C, 350-500°C
and 500-800°C. The observed endothermic peak in the DTA curve between 80 and 135°C with respective weight loss for the citrate gel indicates the removal of absorbed water molecules [23]. The exothermic peak in the DTA curve at 180-290°C was related to the 9
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nitrate decomposition process [11]. The other strong exothermic process noted at 400°C corresponds to the citric acid decomposition [27], which the liberation of combustion gases and dissipation of the heat reduces the sinterization of particles [27]. The thermogravimetric
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analysis showed that the major part of the weight loss that is associated with the decomposition and oxidation of citrate precursors was completed at 600°C [27]. However, when the thermal treatment is performed at 800°C, the color of the YIn0.9Mn0.1O3-ZnO
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powder was dark grey due to the presence of residual carbon. On the other hand, blue color was noted for the pigment sample when the gel calcined at 850°C. The thermal analysis
pigment sample at 850°C. 3.2.
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results of the citrate gel are found to be in good agreement with the FT-IR data of the calcined
Powder X-Ray diffraction analysis
The optimized calcination temperature required for the formation of the typical pigment
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sample YIn0.9Mn0.1O3–ZnO was ascertained by calcining the dried gel at various temperatures ranging from 600°C to 900°C for 2h in an air atmosphere. The typical blue color of the sample was observed only when the dried gel was calcined at 850°C for 2h. Fig. 3 displays the
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PXRD profiles of YIn0.9Mn0.1O3 and YIn0.9Mn0.1O3–ZnO calcined at 850°C/2h. The PXRD
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pattern of YIn0.9Mn0.1O3 indicates the formation of single phase hexagonal YInO3 structure [28]. This is in good agreement with the earlier reports, where YIn0.9Mn0.1O3 calcined at 1400°C exhibits d4 cation in trigonal bipyramidal sites of hexagonal YInO3 [10]. It is important to note that the calcination temperature (850°C) used in the preparation of the YIn0.9Mn0.1O3-ZnO compound by the sol-gel combustion route in the current study is found to be significantly lower than that involved in the traditional ceramic procedure using oxide precursors (1400°C) [10]. On the contrary, the PXRD pattern of YIn0.9Mn0.1O3–ZnO exhibits 10
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a mixture of hexagonal YInO3 phase and ZnO [28, 29]. Rietveld refinement was carried out on the PXRD data of YIn0.9Mn0.1O3–ZnO on both the phases (Fig. 4). The refinement reveals that YIn0.9Mn0.1O3–ZnO is almost 1:1 molar mixture of two constituents, YIn0.9Mn0.1O3 and
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ZnO. In hexagonal (space group P63cm) YIn0.9Mn0.1O3, In/Mn ions are trigonal bipyramidally coordinated by oxide ions (Fig. 5 (left) and Table 1). The three equatorial In/Mn–O bonds are almost equal and relatively shorter compared to the two axial bonds. Each In/MnO5 trigonal
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bipyramid (TBP) shares corners with three neighbouring In/MnO5 TBPs along the equatorial directions (crystallographic ab-plane) to form a two dimensional layer of In/MnO5 TBPs. The
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layers of In/MnO5 TBPs are stacked alternately along the crystallographic c-direction with the YO7 polyhedral layers (not shown in the figure for clarity). The ZnO crystallizes in hexagonal wurtzite (space group P63mc) structure where zinc ions are tetrahedrally coordinated by oxide ions (Fig. 5 (right) and Table 1) and vice versa. These tetrahedra share all the four corners
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with neighbouring tetrahedra to build alternating layers of tetrahedra along the crystallographic c-direction.
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3.3. Structural and morphological analysis
Raman spectroscopic studies were performed on YIn0.9Mn0.1O3-ZnO nano-powders and
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the results are depicted in Fig. 6. The Raman spectrum of the biphasic sample is assumed to be the signature of the hexagonal phase [30,31]. The band occurs at around 364 cm-1, which can be attributed to the (Fg + Ag, where Ag is the Raman active symmetric stretching vibration and Fg is the Raman active triply degenerate symmetric stretching vibration) mode of the (YIn)O6 octahedra [30]. The highest band at 612 cm-1 may be assigned to (YIn)-O stretch [31]. The sharp peak observed in the Raman spectrum at 450 cm-1 clearly indicates the E2
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mode of ZnO nanoparticles. The position of this peak is practically the same as in the bulk material [31].
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The magnetic susceptibility measurements were carried out to confirm the presence of Mn3+ in the YIn0.9Mn0.1O3-ZnO sample. Fig. 7 illustrates the temperature dependence of χ and inverse χ per Mn for YIn0.9Mn0.1O3-ZnO. It is clear from the results that the susceptibility
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increases in the low-temperature region, indicating the spin alignment. Further, this curve obeys the Curie-Weiss law, χ = C/(T-θ), in the measured temperature region. The theoretical
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effective Bohr magneton, µeff, was calculated from the equation µeff = 2[S(S + 1)]1/2 to be 5.92, 4.90, and 3.87 for S=5/2 (d5), 4/2 (d4), and 3/2 (d3), respectively [32]. Thus the experimental µeff value (4.80 BM) for YIn0.9Mn0.1O3-ZnO is close to 4.90, indicating Mn3+ 3d4 (high spin) in the present designed composite material.
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The TEM and High resolution transmission electron microscopic (HRTEM) images of YIn0.9Mn0.1O3-ZnO (Fig. 8(a) and 8(b)) indicate the presence of well dispersed and homogeneous nanoparticles. As shown in Fig. 8(a) the developed YIn0.9Mn0.1O3-ZnO pigment
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powders are granular in nature and present a homogeneous nanoscale particle size. Excellent dispersion is observed with a narrow grain sizes with the distribution in the range of 80-150
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nm. The HRTEM image clearly shows ordered fringes corresponding to the crystalline planes. The spacing of the lattice fringes equals 2.0 Å which corresponds to a lattice constant of c ≈ 12.17 Å assigned to the (006) plane, in agreement with the XRD data of YIn0.9Mn0.1O3. The selected area electron diffraction (SAED) pattern presented in Fig. 8(c) also pointed out a relatively high crystallinity degree of the sample. The energy dispersive spectroscopy (EDS) analyses (Fig. 8(d)) of YIn0.9Mn0.1O3-ZnO showed a high purity and suitable stoichiometry of
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the investigated powder. The particle size of the pigment sample is found to be 180 nm from dynamic light scattering (DLS) analysis (Fig. 9).
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3.4. Diffuse reflectance and chromatic properties Fig. 10 shows the optical absorption spectra of the YIn0.9Mn0.1O3 and YIn0.9Mn0.1O3ZnO developed pigment powders. A strong and narrow absorption in the red-green region of
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the visible spectrum, centered at 1.9 eV has been observed in the case of YIn0.9Mn0.1O3 powders at a low doping concentration of Mn3+ (5.0 mol%). Then the absorption decreases
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between 2.5-3 eV. The residual absorption noted in the blue region (2.5-3 eV) of the spectrum clearly indicates the poor blue hue of the synthesized YIn0.9Mn0.1O3 compound (Fig. 11) with a band gap of 2.63 eV. On the other hand, the presence of ZnO particles dramatically decreases the absorption of YIn0.9Mn0.1O3-ZnO at 1.9 eV. Further, the absence of absorption
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between 2.5-3.0 eV attests the intense blue color of the compound YIn0.9Mn0.1O3-ZnO (Fig. 11). The band gap of the blue nano-pigment sample is found to be 2.74 eV. The blue colored nano-pigments YIn0.9Mn0.1O3 and YIn0.9Mn0.1O3-ZnO samples have a strong absorption band
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at 1.9 eV, which can be ascribed to an allowed transition from 3dx2-y2,xy to 3dz2 in D3h symmetry of Mn3+ [32, 33]. A weak transition noted at 2.2 eV originates from 3dxz,yz to 3dz2 is forbidden.
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Further, both of the samples show a charge transfer transition in the region 3.5-5.5 eV from O2p to 3dz2 [10].
For better understanding of the optical properties of the developed pigment powders, the
diffuse reflectance spectra of YIn0.9Mn0.1O3 and YIn0.9Mn0.1O3-ZnO have been measured and the results are depicted in Fig. 12. The results demonstrated that the optical reflectance of YIn0.9Mn0.1O3 in the presence of ZnO powders significantly enhances in the region 400-500
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nm. The chromatic properties of the developed pigment particles have been evaluated based on CIE 1976 color coordinates values and are summarized in Table 2. The presence of ZnO in YIn0.9Mn0.1O3 lattice results in an increase in the magnitude of the b* value from -22.28 to -
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40.55 as compared to YIn0.9Mn0.1O3 powders, which illustrates the remarkable enhancement in the intensity of the blueness of the pigment sample. As a consequence, the C* value which represents the richness of the blue hue, intensifies about two-fold. Further, the lightness of the
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pigment has also been improved significantly (from L* = 39.71 to 49.94). The observed hue angle values of these pigments lie in the blue region of the cylindrical color space (ho for blue
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= 190-280). It is interesting to note that the color coordinates of the typical pigment sample YIn0.9Mn0.1O3-ZnO (L* = 49.94, a* = -0.88, b* = -40.55) are found to be appreciably higher than the traditional blue pigments such as CoAl2O4 (L* = 44.80, a* = 2.10, b* = -32.70) [11] and currently developed pigment YIn0.9Mn0.1O3 (L* = 39.71, a* = -5.78, b* = -22.28). Infrared reflectance and solar reflectance of the blue nano- pigments
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3.5.
Fig. 13 depicts the IR reflectance and the corresponding solar reflectance spectra (inset
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of Fig. 13) of YIn0.9Mn0.1O3 and YIn0.9Mn0.1O3-ZnO powdered pigment samples. The average reflectance of the intense blue inorganic pigment based on YIn0.9Mn0.1O3-ZnO in the IR
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region (80%) is found to be much higher than that of UV-vis region (16%). Further, the average IR reflectance in the presence of ZnO sample is found to be 12% greater than that of currently designed YIn0.9Mn0.1O3 blue nano-pigment sample (68%). The observed high reflectance may be due to the presence of ZnO nanoparticles, which is known to have high IR reflectance (90%) [35]. The solar reflectance of the YIn0.9Mn0.1O3-ZnO pigment sample (R*= 70%) is also found to be notably higher than that of ZnO free pigment sample (R*= 50%).
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Thus, the present results clearly demonstrate the potential utility of the designed intense blue inorganic pigment powders as “Cool pigments” for various surface coating applications.
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3.6. IR solar reflectance analysis of nano-pigment coatings on aluminium roofing sheets
The IR reflectance and the corresponding solar reflectance spectra of the blue nanopigment sample YIn0.9Mn0.1O3-ZnO coated on the aluminium roofing sheet are shown in Figs.
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14 and 15, respectively. The corresponding photograph of the typical coating is displayed in Fig. 16. The CIE 1976 color coordinates and average reflectance in UV-vis and IR region of
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the blue nano-pigment coatings are given in Table 3. The IR reflectance as well as the corresponding solar reflectance of the blue nano-pigment YIn0.9Mn0.1O3-ZnO coated on the aluminium roofing sheet (R at 1100 nm = 84%, R* = 67%) is found to be higher than that of bare aluminium roofing sheet (R at 1100 nm = 41%, R* = 38%). These findings reveal the
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potential application of the currently developed blue inorganic pigments as “Cool pigments” for building roofing materials.
3.7. Thermal and chemical stability studies of the designed blue pigment powders
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The thermal stability (TG-DTA analysis) of the developed blue nano-pigment YIn0.9Mn0.1O3-ZnO after calcination at 850 °C for 2 h was examined in the temperature range
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50-1000 °C in an air atmosphere and the results are given in Fig. 17. The results clearly demonstrate that there is negligible weight loss and phase transition up to 1000 °C. Thus, it can be concluded from the above study that the designed intense blue pigment is found to be thermally stable.
The chemical stability of the typical pigment sample YIn0.9Mn0.1O3-ZnO was also investigated. 2.0 g of the pigment sample was treated with H2O/aqueous solution having
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different pH values (pH = 3.0 and pH = 13.0) and soaked for 30 min with constant stirring on a magnetic stirrer. The resultant pigment was then filtered, washed with water, dried, and weighed. Negligible weight loss of pigment was noticed at all the conditions tested. The color
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coordinates of the pigments were measured after acid/alkali and water treatment and the total * of the pigments are found to be negligible as evident from the data color difference, ∆Eab
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* * reported in Table 4. The industrially acceptable limits of ∆Eab ≤1 are as follows: when ∆Eab
unit indicates that the color change is almost indistinguishable from the original color,
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* ≤ 2 units are considered to be good [36]. The above studies highlight that the whereas, ∆Eab
designed blue nano-pigment is chemically and thermally stable.
4.
Conclusions
An intense blue nano-pigment (particle size :180 nm) with impressive NIR reflectance (79% derived from 1:1 mixtures of YIn0.9Mn0.1O3 and ZnO by a sol-gel
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at 1100 nm) was
combustion method at relatively lower temperatures (850 °C). Most importantly, the
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developed blue nano-pigment powder sample exhibits excellent solar reflectance (R* = 67%), when coated onto an aluminium roofing sheet. Thus, the developed blue nano-pigment
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powder has potential applications as “Cool materials” used for building roofing materials with energy saving performance. Further, the designed pigment powder not only exhibit a rich blue color (b*= -40.55: R* = 70%) and high solar reflectance properties as compared to the existing commercial blue based on CoAl2O4 (b*= -32.70: R* = 29%) as well as the currently developed Mn3+ doped YInO3 nano-pigments (b*= -22.28: R* = 50%).
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Acknowledgments: The authors acknowledge financial support from the Council of Scientific and Industrial Research (CSIR Network Project SURE CSC-0132), New Delhi. One
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Research Fellowship and AcSIR for Ph. D registration.
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of the authors Sheethu Jose also acknowledges CSIR, New Delhi for the award of Senior
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2007;36:15-30. [2] Hedayati HR, Sabbagh Alvani AA, Sameie H, Salimi R, Moosakhani S, Tabatabaee F, et al. Synthesis and characterization of Co1−xZnxCr2−yAlyO4 as a near-infrared reflective
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color tunable nano-pigment. Dyes Pigment 2015;113:588-95.
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interstitial oxygen. J Mater Chem C 2013;1:5843-8. [13] Saraswathy D, Rao PP, Sameera S, James V, Raj AKV. Monoclinic LaGa1-xMnxGe2O7: a
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Cr2O3–TiO2–Al2O3–V2O5 green pigment. Ceram Int 2011;37:543-8.
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[18] Giable G, Vishnu VS, Reddy MLP. The synthesis, characterization and optical properties of silicon and praseodymium doped Y6MoO12 compounds: Environmentally benign inorganic pigments with high NIR reflectance. Dyes Pigment 2011;88:109-15. [19] Jose S, Prakash A, Laha S, Natarajan S, Reddy ML. Green colored nano-pigments
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derived from Y2BaCuO5: NIR reflective coatings. Dyes Pigment 2014;107:118-26. [20] Vishnu VS, Reddy MLP. Near-infrared reflecting inorganic pigments based on molybdenum and praseodymium doped yttrium cerate: Synthesis, characterization and
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optical properties. Sol Energy Mater Sol Cells 2011;95:2685-92.
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[22] Jose S, Reddy MLP. Lanthanum–strontium copper silicates as intense blue inorganic pigments with high near-infrared reflectance. Dyes Pigment 2013;98:540-6.
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[23] Vivekanandhan S, Venkateswarlu M, Satyanarayana N. Synthesis and characterization of nanocrystalline LiNi0.5Co0.5VO4 powders by citric acid assisted sol-gel combustion process. J Alloys Compd 2008;462:328-34.
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[26] Ramya AR, Sharma D, Natarajan S, Reddy MLP. Highly luminescent and thermally coordination
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[27] Anjos dos PNM, Pereira EC, Gobato YG. Study of the structure and optical properties of rare-earth doped aluminate particles prepared by an amorphous citrate sol-gel process. J Alloys Compd 2005;391:277-83.
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[28] Pistorious CWFT, Kruger GJ. Stability and structure of noncentrosymmetric hexagonal
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LnInO3 (Ln = Eu, Gd, Tb, Dy, Ho, Y). J Inorg Nucl Chem 1976;38:1471-5. [29] Abrahams SC, Bernstein JL. Remeasurement of the structure of hexagonal ZnO. Acta Crystallogr 1969;B25:1233-6.
[30] Shukla R, Gupta S, Grover V, Natarajan V, Tyagi A. The role of reaction conditions in the polymorphic control of Eu(3+) doped YInO3: structure and size sensitive luminescence. Dalton trans 2015:10628-35.
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[31] N. Romcevic RK, M. Romcevic, B. Hadzic,I. Kuryliszyn-Kudelska, W. Dobrowolski,U. Narkiewicz and D. Sibera. Raman Scattering from ZnO(Fe) Nanoparticles. Acta physica
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polonica A 2008;114:1323-8. [32] Hiroshi M, Arthur WS, Subramanian MA. New Oxides Showing an Intense Blue Color Based on Mn3+ in Trigonal-Bipyramidal Coordination. Inorg Chem 2011;50:10-2.
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[33] Tamilarasan S, Debajit S, Reddy MLP, Natarajan S, Gopalakrishnan J. YGa1−xMnxO3: A
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novel purple inorganic pigment. RSC Advances 2013;3:3199-202.
[34] Sri Parasara R, Kalarical Janardhanan S, Balachandran Unni N. Rare earth doped cobalt aluminate blue as an environmentally benign colorant. JAdv Ceram 2013;1:301-9. [35] Kiomarsipour N, Shoja Razavi R. Hydrothermal synthesis of ZnO nanopigments with
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high UV absorption and vis/NIR reflectance. Ceram Int 2014;40:11261-8. [36] Buxbaum G, Pfaff G. Industrial Inorganic Pigments. Wiley-VCH, Weinheim, Germany
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2002.
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Figure captions Fig. 1. FT-IR spectra of YIn0.9Mn0.1O3-ZnO-citrate gel and calcined pigment powder at 850°C.
of TG-DTA curves. Fig. 3. PXRD patterns of blue pigments calcined at 850°C/2h.
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Fig. 2. TG-DTA curves of the YIn0.9Mn0.1O3-ZnO-citrate gel. The inset shows enlarged view
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Fig. 4. Rietveld refinement of YIn0.9Mn0.1O3-ZnO from PXRD data. Observed (o), calculated (redline) and difference (bottom blue line) profiles are shown. The red and black vertical bars
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indicate positions of the Bragg reflections of ZnO and YIn0.9Mn0.1O3, respectively. Fig. 5. Crystal structure of YIn0.9Mn0.1O3 (left) and ZnO (right).
Fig. 6. Raman spectrum of synthesized blue pigment YIn0.9Mn0.1O3-ZnO. Fig. 7. The temperature dependence of χ(black) and χ-1 (blue) of YIn0.9Mn0.1O3-ZnO.
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Fig. 8. (a) TEM image, (b) HRTEM image (inset shows the corresponding SAED pattern) (c) diffraction pattern and (d) Energy dispersive spectrum of YIn0.9Mn0.1O3-ZnO synthesized pigment powder.
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Fig. 9. Particle size distribution of YIn0.9Mn0.1O3-ZnO. Fig. 10. Optical absorption spectra of the synthesized blue pigments.
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Fig. 11. Photographs of the synthesized blue nano-pigments a) YIn0.9Mn0.1O3 and b) YIn0.9Mn0.1O3-ZnO calcined at 850°C/2h. Fig. 12. UV-vis diffuse reflectance spectra of the synthesized blue pigments. Fig. 13. IR reflectance spectra of the blue pigment powders. The inset shows the corresponding solar reflectance spectra. Fig. 14. IR reflectance spectra of aluminium roofing sheet coatings.
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Fig. 15. Solar reflectance spectra of aluminium roofing sheet coatings. Fig. 16. Photographs of the aluminium roofing sheet coatings.
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Fig. 17. Thermogram of the YIn0.9Mn0.1O3-ZnO recorded in an air atmosphere.
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Table 1 Crystallographic data for YIn0.9Mn0.1O3-ZnO. YIn0.9Mn0.1O3 - phase Atom
Site
X
Y
Z
Uiso(Å2)
Y1
2a
0.0
0.0
0.266(1)
0.002(1)
1
Y2
4b
0.333
0.667
0.234(1)
0.028(1)
1
In/Mn
6c
0.334(1)
0.0
0.002(1)
0.021(1)
0.90(3)/0.10(3)
O1
6c
0.324(5)
0.0
0.171(1)
O2
6c
0.632(3)
0.0
0.330(3)
O3
2a
0.0
0.0
O4
4b
0.333
0.667
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Occupancy
1
0.072(5)
1
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0.002(2)
0.485(5)
0.286(1)
1
0.032(2)
0.011(4)
1
space group P63cm : a = 6.262(1) Å, c = 12.172(1) Å
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Bond lengths: Y1 – O1 = 2.331(1) x 3; Y1 – O2 = 2.434(1) x 3; Y1 – O3 = 2.685(1); Y2 – O1 = 2.255(1) x 3; Y2 – O2 = 2.305(1) x 3; Y2 – O4 = 2.464(1); In/Mn – O1 = 2.080(1); In/Mn – O2 = 2.077(1); In/Mn – O3 = 2.098(1); In/Mn – O4 = 2.120(1); In/Mn – O4 = 2.121(1)
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ZnO - phase Site
X
Y
Z
Uiso(Å2)
Occupancy
Zn
2b
0.333
0.667
0.265(1)
0.032(1)
1
O
2b
0.333
0.667
0.382(2)
0.022(4)
1
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Atom
space group P63mc : a = 3.240(1) Å, c = 5.188(1) Å Bond lengths: Zn – O = 1.966(1); Zn – O = 1.973(1) x 2: Zn – O = 1.973(2) Reliability Factors: Rp = 2.15%, Rwp = 2.76%, RF2 = 8.76%, χ2 = 2.66
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Table 2 Color coordinates, band gap values, IR reflectance and solar reflectance of blue pigments. Color coordinates L*
a*
Band
b*
C*
ho
IR
Solar
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Pigment
Gap
reflectance
reflectance,
(eV)
at 1100 nm
R* (%)
39.71
-5.78 -22.28
23.21
255
2.63
63
50
YIn0.9Mn0.1O3-ZnO
49.94
-0.88 -40.55
40.56
269
2.74
79
70
*CoAl2O4[11, 34]
44.80
2.10
-
-
-
-
29
-
-
-
-32.70
-3.90 -15.62
16.10
-
*YIn0.9Mn0.1O3 [11]
0.50
-
-
49.50
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-29.20
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*- reported blue nano-pigments
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YIn0.9Mn0.1O3
-
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Table 3 Solar reflectance and color coordinates of YIn0.9Mn0.1O3-ZnO pigment synthesized by sol-gel
Parameter
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combustion method coated on aluminium roofing sheets. Al sheet
coatings with blue pigment alone
Average reflectance in UV-vis region (%)
14
66
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Average reflectance in IR region (%)
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41
Thickness (µm)
IR reflectance at 1100 nm (%)
84
Solar reflectance (R*) (%)
67
L*
44.84
a*
4.78
C*
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b*
-42.16 276
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Table 4
tests. Color coordinates
a
Aqueous solution condition a*
b*
Synthesized pigment powder
49.94
-0.88
-40.55
H2O
49.90
-0.87
Aqueous solution of pH = 3
49.92
Aqueous solution of pH = 13
49.99
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0.05
-0.93
-40.49
0.11
-0.91
–40.60
0.17
* ∆Eab = [(∆L*)2+(∆a*)2+(∆b*)2]½
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* ∆Eab
-40.52
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L*
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The color coordinates of the YIn0.9Mn0.1O3-ZnO powder pigments after chemical resistance
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S0143-7208(15)00361-7
DOI:
10.1016/j.dyepig.2015.09.014
Reference:
DYPI 4925
To appear in:
Dyes and Pigments
Received Date: 16 June 2015 Revised Date:
9 September 2015
Accepted Date: 10 September 2015
Please cite this article as: Jose S, Jayaprakash A, Laha S, Natarajan S, Nishanth KG, Reddy MLP, YIn0.9Mn0.1O3-ZnO nano-pigment exhibiting intense blue color with impressive solar reflectance, Dyes and Pigments (2015), doi: 10.1016/j.dyepig.2015.09.014. 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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Highlights • A blue colored nano-pigment was synthesized by a sol-gel combustion method.
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• The nano-pigment exhibits superior blue hue and high solar reflectance.
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• The potential utility of the nano-pigment as a ‘Cool Pigment’ was demonstrated.
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YIn0.9Mn0.1O3-ZnO nano-pigment exhibiting intense blue color with impressive solar reflectance L. P. Reddya,b* a
Academy
of
Scientific
and
Innovative
research
Thiruvananthapuram, India b
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Sheethu Jose,a,b Anaswara Jayaprakash,b Sourav Laha,c S. Natarajan,c K. G. Nishanth,b and M.
(AcSIR),
CSIR-NIIST
Campus,
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Materials Science and Technology Division, CSIR-National Institute for Interdisciplinary
Science and Technology (NIIST), Thiruvananthapuram 695 019, Kerala, India c
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012,
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India
ABSTRACT
The current study reports on the synthesis and characterization of a new inorganic nano-
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pigment with an intense blue color and high solar radiation reflective properties (70%). The nano-pigment YIn0.9Mn0.1O3-ZnO was synthesized by a sol-gel combustion method and characterized with the aid of X-Ray diffraction, Raman spectroscopy, Magnetic susceptibility,
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Transmission electron microscopy, UV-vis-NIR diffuse reflectance spectroscopy and CIE1976 L*a*b* color measurements. The Rietveld refinement of the XRD patterns of the
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developed nano-pigment disclosed the existence of YIn0.9Mn0.1O3 and ZnO in a 1:1 ratio with hexagonal crystal structures. For comparison, YIn0.9Mn0.1O3 was also synthesized by the solgel combustion route and its optical properties compared with that of YIn0.9Mn0.1O3-ZnO. It is interesting to note that the developed YIn0.9Mn0.1O3-ZnO nano-pigment exhibits superior blue hue (b* = -40.55) and solar reflectance (R* = 70%) values as compared to the YIn0.9Mn0.1O3 nano-pigment (b* = -22.28, R* = 50%). Most importantly, the potential utility of the nano-
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pigment as a “Cool Pigment” was demonstrated by coating onto roofing materials like aluminium roofing sheets.
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Keywords: Blue nano-pigment/coatings, Solar reflectance, Sol-gel combustion method *Corresponding author. Tel.: +91471 2515360; Fax: +91471 2491712
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E-mail address: [email protected] (M. L. P. Reddy)
Introduction
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Nowadays much effort has been made to minimize the use of toxic cobalt aluminate (CoAl2O4) in pigment formulations and also to produce cost-effective inorganic blue pigments [1-6]. With the exception of the turquoise blue pigment of vanadium-zircon, known traditional blue inorganic pigments are generally based on the cobalt ion incorporated into different
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crystalline structures namely, cobalt aluminate (CoAl2O4), cobalt orthosilicate (Co2SiO4), cobalt stannate (Co2SnO4) and zinc silicate with Co2+ [(CoZn)2SiO4] [3-9]. Therefore, the quest for intense blue inorganic pigments which are non-toxic, earth-abundant and durable are
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some of the important issues that remain as a challenge in the inorganic pigment industry. Recently, Smith et al. have shown that intense bright blue colors occur in most of the
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YIn1-xMnxO3 solid-state solutions. The Mn3+ ions substituted into the indium site exhibits an intense blue color because of an allowed d-d transition [10]. In later studies, Ocana and coworkers have developed Mn3+ doped YInO3 blue pigments at much lower temperatures (1000°C) than required by the traditional solid state method (1400°C) [11]. The developed procedure is based on the pyrolysis at 600°C of aerosols generated from aqueous solutions of Y, In, and Mn nitrates followed by an annealing treatment at 1000°C. The technological performance of these YIn1-xMnxO3 blue pigments has also been evaluated by testing their 3
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efficiency for coloring of ceramic glazes. Kim and co-workers have disclosed the synthesis and structural characterization of the blue chromophore Sr2(MgMn)Ge2O7, which has a melilite-type structure [12]. The blue color of the pigment has been attributed to the d-d
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electronic transition in Mn3+ (d4), which has trigonal bipyramidal coordination [12]. A series of new blue inorganic oxide materials LaGaGe2O7 with the introduction of Mn3+ in the trigonal bipyramidal site has also been reported [13]. However, many of these known blue
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inorganic pigments have not been explored for their solar reflective properties [12,13].
In this paper, YIn0.9Mn0.1O3 blue nano-pigments have been synthesized in the presence
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and absence of ZnO nanoparticles through sol-gel combustion route at 850 °C using citric acid as a fuel. The developed pigments were well characterized and their optical properties were investigated. Superior blue hue and high solar radiation reflectivity has been achieved in the presence of ZnO nanoparticles as compared to YIn0.9Mn0.1O3 blue nano-pigment.
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Furthermore, the optical properties of the designed blue nano-pigment YIn0.9Mn0.1O3-ZnO is found to be significantly better than those of the commercially available CoAl2O4. The coloring performance of the designed blue nano-pigment has been evaluated by coating onto
Experimental section
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2.
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roofing materials such as aluminium sheets.
2.1. Materials
The chemical reagents Y2O3 (Sigma-Aldrich, 99.8%), In2O3 (Sigma-Aldrich, 99.99%)
manganese(III) acetate (Sigma Aldrich, 97%), ZnO (Acros Organics, 99.999%) and citric acid (Sigma-Aldrich, 99.5%) are analytical grade and used without further purification.
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2.2. Preparation of YIn0.9Mn0.1O3-ZnO and YIn0.9Mn0.1O3 blue pigments The YIn0.9Mn0.1O3-ZnO pigment sample was synthesized by a sol-gel combustion method.
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Stoichiometric amounts of yttrium oxide (1.66 mmol), manganese(III) acetate (0.33 mmol), indium oxide (1.49 mmol) and zinc oxide (3.33 mmol) were dissolved in 4.0 M nitric acid (100 mL) with constant stirring and heated on a hot plate. To this solution, citric acid was
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added. The molar ratio of citric acid to metal was maintained as 3.5:1. The resultant pale yellow solution was then evaporated on a hot plate to obtain a dried gel. The gel was then
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heated at 250°C for 2 h to decompose the nitrates. The optimized calcination temperature required for the formation of the typical pigment sample YIn0.9Mn0.1O3–ZnO was ascertained by calcining the dried gel at various temperatures ranging from 600°C to 900°C for 2h in an air atmosphere. The typical blue color of the sample was observed only when the dried gel was calcined at 850°C for 2h. The calcination of the powder samples were carried out in a
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Libratherm high-temperature electrical furnace by heating the samples in platinum crucibles. The heating of the furnace was programmed to increase the temperature at 5 °C/min. The
and ZnO.
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XRD pattern of the calcined powder indicates the presence of 1:1 mixture of YIn0.9Mn0.1O3
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A similar procedure has been followed for the preparation of YIn0.9Mn0.1O3 powders. Stoichiometric quantities of yttrium oxide (1.66 mmol), manganese(III) acetate (0.33 mmol) and indium oxide (1.49 mmol) were dissolved in 4.0 M nitric acid (100 mL) with constant stirring and heating on a hot plate. Citric acid was then added to the above metal nitrate solution (The molar ratio of citric acid to metal was maintained as 1.67:1). The resulting pale yellow solution was evaporated at 80 °C on a water bath to obtain a dried gel. The dried gel was then heated at 250 °C to decompose the nitrates. The resultant powders were then 5
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calcined at 850°C for 2h to obtain the pigment powders. Examination of the product by powder XRD pattern revealed that heating the dried gel at 850 °C for 2h resulted in a single
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phase of YIn0.9Mn0.1O3 product. 2.3. Characterization techniques
Thermogravimetry and differential thermal analysis (TG-DTA) was carried out in the
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temperature range of 50-1000°C using an SII Nanotechnology Inc., TG/DTA 6200 in an air atmosphere at a heating rate of 20°C/min. The powder X-ray diffraction pattern of the
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synthesized pigment powder was recorded using the diffractometer (Philips X’pert Pro) with Ni–filtered Cu Kα (λ = 0.154060 nm) radiation. Data were collected by step scanning over a 2θ range from 10 - 70˚ with a step size of 0.08˚ and 5 s counting time at each step. Powder Xray diffraction (PXRD) data for structure refinement were collected by means of a
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PANalytical X’pert Diffractometer operated at 40 kV and 30 mA using Ni-filtered Cu – Kα radiation in the 2θ range 10 – 120o with a step size of 0.02o and step duration of 50s, Rietveld refinement of the crystal structure was carried out with the program GSAS [14]. The XRD
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patterns were typically refined for lattice parameters, atomic coordinates, scale, factors, background (Fourier polynomial background function), pseudo-Voigt (U, V, W and X) and
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isothermal temperature factors (Uiso). A Perkin-Elmer Spectrum One FT-IR spectrometer was used to obtain the IR spectral data (neat KBr). Raman spectroscopic measurements on powder samples were performed on a WITec Raman microscope (Witec Inc. Germany, alpha 300R) with a laser beam directed to the sample through 20x objective and a Peltier cooled CCD detector. Samples were excited with a 633 nm excitation wavelength laser, and Stokes shifted Raman spectrum were collected in the range of 400 to 4000 cm−1 with 1 cm−1 resolution. Prior to every measurement, a 6
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calibration with a silicon standard (Raman peak centered at 520 cm−1) was performed. WITec Project Plus (v 2.1) software package was used for data evaluation. Magnetic measurements were carried out as a function of temperature using PPMS
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(Quantum Design, Dynacool) in an applied field of 500 Oe. The measurement was performed in the temperature range 5 K to 300 K. Dynamic Light Scattering experiment was conducted using Malvern Instruments Zeta sizer nano Zis Model No: ZEN 3600 to determine the particle
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size of the nano-pigment. Transmission electron microscopy (TEM) [JEOL 2010 (300 kV)] was used to understand the morphology and particle size of the synthesized samples.
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The optical properties of the powdered pigment samples were measured (380-780 nm) with a UV-vis Spectrophotometer (Shimadzu UV-3600 with an integrating sphere attachment, ISR3100) using barium sulfate as a reference. The measurement conditions were as follows: an illuminant D65, 10˚ complementary observer and measuring geometry d/8˚. The color
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coordinates were determined by coupling with analytical software (UVPC Color Analysis Personal Spectroscopy Software V3, Shimadzu) to the UV-3600 spectrophotometer. The CIE 1976 L*a*b* colorimetric method was used, as recommended by the Commission
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Internationale de I’Eclairage (CIE). In this method, L* is the color lightness (L* = 0 for black and L* = 100 for white), a* is the green (-)/red (+) axis, and b* is the blue (-)/yellow (+) axis.
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The parameter C* (chroma) represents the saturation of the color and is defined as C* = (a*)2 + (b*) 2 . The hue angle, h˚ is expressed in degrees and ranges from 0 to 360˚
b* and is calculated by using the formula h˚ = tan−1 . For each colorimetric parameter of a a * sample, measurements were made in triplicate and an average value was taken as the result. Typically, for a given sample, the standard deviation of the measured CIE–L*a*b* values is
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less than 0.10, and the relative standard deviation ≤ 1%, indicating that the measurement error can be ignored. The near–infrared reflectance spectrum of the powdered pigment samples and the aluminium roofing sheet coatings were recorded with a UV–vis–NIR spectrophotometer UV-3600
with
an
integrating
sphere
attachment,
ISR
3100)
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(Shimadzu,
using
polytetrafluoroethylene (PTFE) as a reference. Optical measurements were carried out in the 700-2500 nm range. The solar reflectance (R*) in the wavelength range from 700-2500 nm
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was calculated according to ASTM standard number E891–87 as reported elsewhere [15-20]. The NIR solar reflectance or the fraction of solar radiation incident at wavelengths between
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700 and 2500 nm that is reflected by a surface is the irradiance-weighted average of its spectral reflectance, r(λ), and can be determined using the relationship, 2500
∫ r (λ ) i (λ ) d ( λ )
R* =
700 2500
∫ i (λ ) d (λ )
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700
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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2.4. Preparation of aluminium roofing sheet coatings The synthesized pigment powder was applied as paint on aluminium roofing sheets as per the standard method described elsewhere [18-22]. The paint formulation was prepared by dispersing the known quantities of the blue nano-pigment (2 g) and acrylic binder (2 g) in 4.0 mL of acrylic-acralyn emulsion (Acralyn-R M/s Asian acrylates, Mumbai, India) and ultrasonicated (Vibronics, 250 W, India) for 10 min to ensure the complete dispersion of pigment particles. The pigment to binder ratio was maintained at 1:1 (by weight). The
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resultant viscous paint formulation was coated onto the surface of aluminium roofing sheets and was allowed to dry in air. The thickness of the pigment coating on aluminium roofing sheet surface was measured by employing LEICA DMRX optical microscope.
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3. Results and discussion 3.1. FT-IR and thermogravimetric analyses
Fig. 1 shows the typical FT-IR spectra of the synthesized citrate gel as well as calcined
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pigment powders (850°C). The broad band observed in the spectrum of citrate gel at 3400 cmcan be assigned to O–H stretching frequencies of the intermolecular hydrogen-bonded water
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[23,24]. Further, the FT-IR spectrum of the citrate gel evidenced intense absorption bands characteristic of the carboxylate groups at 1380 and 1560 cm-1, which are assigned to the symmetric νs (C=O) and asymmetric νas (C=O) vibrations, respectively [24]. The difference between the asymmetric and symmetric stretching vibration modes falls in the range 180 cm-1,
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which indicates that carboxylate groups are coordinated to metal ions in either bridging or chelating modes [25,26]. The band at 1378 cm-1 observed can be assigned to vibrations of free nitrate ion [11,24]. However, this band is overlapped with the citrate ion absorption bands.
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The absence of the broad absorption band at 3400 cm-1 in the FT-IR spectrum of calcined pigment powder indicates the absence of moisture in the pigment powder. Further, the
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absence of absorption bands corresponding to citrate and nitrate ions clearly indicates the decomposition of nitrate and citrate ions after the calcination of the gel at 850°C. As shown in Fig. 2, there are weight loss processes at 80-130°C, 180-290°C, 350-500°C
and 500-800°C. The observed endothermic peak in the DTA curve between 80 and 135°C with respective weight loss for the citrate gel indicates the removal of absorbed water molecules [23]. The exothermic peak in the DTA curve at 180-290°C was related to the 9
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nitrate decomposition process [11]. The other strong exothermic process noted at 400°C corresponds to the citric acid decomposition [27], which the liberation of combustion gases and dissipation of the heat reduces the sinterization of particles [27]. The thermogravimetric
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analysis showed that the major part of the weight loss that is associated with the decomposition and oxidation of citrate precursors was completed at 600°C [27]. However, when the thermal treatment is performed at 800°C, the color of the YIn0.9Mn0.1O3-ZnO
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powder was dark grey due to the presence of residual carbon. On the other hand, blue color was noted for the pigment sample when the gel calcined at 850°C. The thermal analysis
pigment sample at 850°C. 3.2.
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results of the citrate gel are found to be in good agreement with the FT-IR data of the calcined
Powder X-Ray diffraction analysis
The optimized calcination temperature required for the formation of the typical pigment
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sample YIn0.9Mn0.1O3–ZnO was ascertained by calcining the dried gel at various temperatures ranging from 600°C to 900°C for 2h in an air atmosphere. The typical blue color of the sample was observed only when the dried gel was calcined at 850°C for 2h. Fig. 3 displays the
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PXRD profiles of YIn0.9Mn0.1O3 and YIn0.9Mn0.1O3–ZnO calcined at 850°C/2h. The PXRD
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pattern of YIn0.9Mn0.1O3 indicates the formation of single phase hexagonal YInO3 structure [28]. This is in good agreement with the earlier reports, where YIn0.9Mn0.1O3 calcined at 1400°C exhibits d4 cation in trigonal bipyramidal sites of hexagonal YInO3 [10]. It is important to note that the calcination temperature (850°C) used in the preparation of the YIn0.9Mn0.1O3-ZnO compound by the sol-gel combustion route in the current study is found to be significantly lower than that involved in the traditional ceramic procedure using oxide precursors (1400°C) [10]. On the contrary, the PXRD pattern of YIn0.9Mn0.1O3–ZnO exhibits 10
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a mixture of hexagonal YInO3 phase and ZnO [28, 29]. Rietveld refinement was carried out on the PXRD data of YIn0.9Mn0.1O3–ZnO on both the phases (Fig. 4). The refinement reveals that YIn0.9Mn0.1O3–ZnO is almost 1:1 molar mixture of two constituents, YIn0.9Mn0.1O3 and
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ZnO. In hexagonal (space group P63cm) YIn0.9Mn0.1O3, In/Mn ions are trigonal bipyramidally coordinated by oxide ions (Fig. 5 (left) and Table 1). The three equatorial In/Mn–O bonds are almost equal and relatively shorter compared to the two axial bonds. Each In/MnO5 trigonal
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bipyramid (TBP) shares corners with three neighbouring In/MnO5 TBPs along the equatorial directions (crystallographic ab-plane) to form a two dimensional layer of In/MnO5 TBPs. The
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layers of In/MnO5 TBPs are stacked alternately along the crystallographic c-direction with the YO7 polyhedral layers (not shown in the figure for clarity). The ZnO crystallizes in hexagonal wurtzite (space group P63mc) structure where zinc ions are tetrahedrally coordinated by oxide ions (Fig. 5 (right) and Table 1) and vice versa. These tetrahedra share all the four corners
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with neighbouring tetrahedra to build alternating layers of tetrahedra along the crystallographic c-direction.
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3.3. Structural and morphological analysis
Raman spectroscopic studies were performed on YIn0.9Mn0.1O3-ZnO nano-powders and
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the results are depicted in Fig. 6. The Raman spectrum of the biphasic sample is assumed to be the signature of the hexagonal phase [30,31]. The band occurs at around 364 cm-1, which can be attributed to the (Fg + Ag, where Ag is the Raman active symmetric stretching vibration and Fg is the Raman active triply degenerate symmetric stretching vibration) mode of the (YIn)O6 octahedra [30]. The highest band at 612 cm-1 may be assigned to (YIn)-O stretch [31]. The sharp peak observed in the Raman spectrum at 450 cm-1 clearly indicates the E2
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mode of ZnO nanoparticles. The position of this peak is practically the same as in the bulk material [31].
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The magnetic susceptibility measurements were carried out to confirm the presence of Mn3+ in the YIn0.9Mn0.1O3-ZnO sample. Fig. 7 illustrates the temperature dependence of χ and inverse χ per Mn for YIn0.9Mn0.1O3-ZnO. It is clear from the results that the susceptibility
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increases in the low-temperature region, indicating the spin alignment. Further, this curve obeys the Curie-Weiss law, χ = C/(T-θ), in the measured temperature region. The theoretical
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effective Bohr magneton, µeff, was calculated from the equation µeff = 2[S(S + 1)]1/2 to be 5.92, 4.90, and 3.87 for S=5/2 (d5), 4/2 (d4), and 3/2 (d3), respectively [32]. Thus the experimental µeff value (4.80 BM) for YIn0.9Mn0.1O3-ZnO is close to 4.90, indicating Mn3+ 3d4 (high spin) in the present designed composite material.
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The TEM and High resolution transmission electron microscopic (HRTEM) images of YIn0.9Mn0.1O3-ZnO (Fig. 8(a) and 8(b)) indicate the presence of well dispersed and homogeneous nanoparticles. As shown in Fig. 8(a) the developed YIn0.9Mn0.1O3-ZnO pigment
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powders are granular in nature and present a homogeneous nanoscale particle size. Excellent dispersion is observed with a narrow grain sizes with the distribution in the range of 80-150
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nm. The HRTEM image clearly shows ordered fringes corresponding to the crystalline planes. The spacing of the lattice fringes equals 2.0 Å which corresponds to a lattice constant of c ≈ 12.17 Å assigned to the (006) plane, in agreement with the XRD data of YIn0.9Mn0.1O3. The selected area electron diffraction (SAED) pattern presented in Fig. 8(c) also pointed out a relatively high crystallinity degree of the sample. The energy dispersive spectroscopy (EDS) analyses (Fig. 8(d)) of YIn0.9Mn0.1O3-ZnO showed a high purity and suitable stoichiometry of
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the investigated powder. The particle size of the pigment sample is found to be 180 nm from dynamic light scattering (DLS) analysis (Fig. 9).
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3.4. Diffuse reflectance and chromatic properties Fig. 10 shows the optical absorption spectra of the YIn0.9Mn0.1O3 and YIn0.9Mn0.1O3ZnO developed pigment powders. A strong and narrow absorption in the red-green region of
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the visible spectrum, centered at 1.9 eV has been observed in the case of YIn0.9Mn0.1O3 powders at a low doping concentration of Mn3+ (5.0 mol%). Then the absorption decreases
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between 2.5-3 eV. The residual absorption noted in the blue region (2.5-3 eV) of the spectrum clearly indicates the poor blue hue of the synthesized YIn0.9Mn0.1O3 compound (Fig. 11) with a band gap of 2.63 eV. On the other hand, the presence of ZnO particles dramatically decreases the absorption of YIn0.9Mn0.1O3-ZnO at 1.9 eV. Further, the absence of absorption
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between 2.5-3.0 eV attests the intense blue color of the compound YIn0.9Mn0.1O3-ZnO (Fig. 11). The band gap of the blue nano-pigment sample is found to be 2.74 eV. The blue colored nano-pigments YIn0.9Mn0.1O3 and YIn0.9Mn0.1O3-ZnO samples have a strong absorption band
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at 1.9 eV, which can be ascribed to an allowed transition from 3dx2-y2,xy to 3dz2 in D3h symmetry of Mn3+ [32, 33]. A weak transition noted at 2.2 eV originates from 3dxz,yz to 3dz2 is forbidden.
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Further, both of the samples show a charge transfer transition in the region 3.5-5.5 eV from O2p to 3dz2 [10].
For better understanding of the optical properties of the developed pigment powders, the
diffuse reflectance spectra of YIn0.9Mn0.1O3 and YIn0.9Mn0.1O3-ZnO have been measured and the results are depicted in Fig. 12. The results demonstrated that the optical reflectance of YIn0.9Mn0.1O3 in the presence of ZnO powders significantly enhances in the region 400-500
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nm. The chromatic properties of the developed pigment particles have been evaluated based on CIE 1976 color coordinates values and are summarized in Table 2. The presence of ZnO in YIn0.9Mn0.1O3 lattice results in an increase in the magnitude of the b* value from -22.28 to -
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40.55 as compared to YIn0.9Mn0.1O3 powders, which illustrates the remarkable enhancement in the intensity of the blueness of the pigment sample. As a consequence, the C* value which represents the richness of the blue hue, intensifies about two-fold. Further, the lightness of the
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pigment has also been improved significantly (from L* = 39.71 to 49.94). The observed hue angle values of these pigments lie in the blue region of the cylindrical color space (ho for blue
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= 190-280). It is interesting to note that the color coordinates of the typical pigment sample YIn0.9Mn0.1O3-ZnO (L* = 49.94, a* = -0.88, b* = -40.55) are found to be appreciably higher than the traditional blue pigments such as CoAl2O4 (L* = 44.80, a* = 2.10, b* = -32.70) [11] and currently developed pigment YIn0.9Mn0.1O3 (L* = 39.71, a* = -5.78, b* = -22.28). Infrared reflectance and solar reflectance of the blue nano- pigments
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3.5.
Fig. 13 depicts the IR reflectance and the corresponding solar reflectance spectra (inset
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of Fig. 13) of YIn0.9Mn0.1O3 and YIn0.9Mn0.1O3-ZnO powdered pigment samples. The average reflectance of the intense blue inorganic pigment based on YIn0.9Mn0.1O3-ZnO in the IR
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region (80%) is found to be much higher than that of UV-vis region (16%). Further, the average IR reflectance in the presence of ZnO sample is found to be 12% greater than that of currently designed YIn0.9Mn0.1O3 blue nano-pigment sample (68%). The observed high reflectance may be due to the presence of ZnO nanoparticles, which is known to have high IR reflectance (90%) [35]. The solar reflectance of the YIn0.9Mn0.1O3-ZnO pigment sample (R*= 70%) is also found to be notably higher than that of ZnO free pigment sample (R*= 50%).
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Thus, the present results clearly demonstrate the potential utility of the designed intense blue inorganic pigment powders as “Cool pigments” for various surface coating applications.
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3.6. IR solar reflectance analysis of nano-pigment coatings on aluminium roofing sheets
The IR reflectance and the corresponding solar reflectance spectra of the blue nanopigment sample YIn0.9Mn0.1O3-ZnO coated on the aluminium roofing sheet are shown in Figs.
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14 and 15, respectively. The corresponding photograph of the typical coating is displayed in Fig. 16. The CIE 1976 color coordinates and average reflectance in UV-vis and IR region of
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the blue nano-pigment coatings are given in Table 3. The IR reflectance as well as the corresponding solar reflectance of the blue nano-pigment YIn0.9Mn0.1O3-ZnO coated on the aluminium roofing sheet (R at 1100 nm = 84%, R* = 67%) is found to be higher than that of bare aluminium roofing sheet (R at 1100 nm = 41%, R* = 38%). These findings reveal the
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potential application of the currently developed blue inorganic pigments as “Cool pigments” for building roofing materials.
3.7. Thermal and chemical stability studies of the designed blue pigment powders
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The thermal stability (TG-DTA analysis) of the developed blue nano-pigment YIn0.9Mn0.1O3-ZnO after calcination at 850 °C for 2 h was examined in the temperature range
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50-1000 °C in an air atmosphere and the results are given in Fig. 17. The results clearly demonstrate that there is negligible weight loss and phase transition up to 1000 °C. Thus, it can be concluded from the above study that the designed intense blue pigment is found to be thermally stable.
The chemical stability of the typical pigment sample YIn0.9Mn0.1O3-ZnO was also investigated. 2.0 g of the pigment sample was treated with H2O/aqueous solution having
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different pH values (pH = 3.0 and pH = 13.0) and soaked for 30 min with constant stirring on a magnetic stirrer. The resultant pigment was then filtered, washed with water, dried, and weighed. Negligible weight loss of pigment was noticed at all the conditions tested. The color
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coordinates of the pigments were measured after acid/alkali and water treatment and the total * of the pigments are found to be negligible as evident from the data color difference, ∆Eab
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* * reported in Table 4. The industrially acceptable limits of ∆Eab ≤1 are as follows: when ∆Eab
unit indicates that the color change is almost indistinguishable from the original color,
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* ≤ 2 units are considered to be good [36]. The above studies highlight that the whereas, ∆Eab
designed blue nano-pigment is chemically and thermally stable.
4.
Conclusions
An intense blue nano-pigment (particle size :180 nm) with impressive NIR reflectance (79% derived from 1:1 mixtures of YIn0.9Mn0.1O3 and ZnO by a sol-gel
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at 1100 nm) was
combustion method at relatively lower temperatures (850 °C). Most importantly, the
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developed blue nano-pigment powder sample exhibits excellent solar reflectance (R* = 67%), when coated onto an aluminium roofing sheet. Thus, the developed blue nano-pigment
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powder has potential applications as “Cool materials” used for building roofing materials with energy saving performance. Further, the designed pigment powder not only exhibit a rich blue color (b*= -40.55: R* = 70%) and high solar reflectance properties as compared to the existing commercial blue based on CoAl2O4 (b*= -32.70: R* = 29%) as well as the currently developed Mn3+ doped YInO3 nano-pigments (b*= -22.28: R* = 50%).
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Acknowledgments: The authors acknowledge financial support from the Council of Scientific and Industrial Research (CSIR Network Project SURE CSC-0132), New Delhi. One
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Research Fellowship and AcSIR for Ph. D registration.
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of the authors Sheethu Jose also acknowledges CSIR, New Delhi for the award of Senior
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Figure captions Fig. 1. FT-IR spectra of YIn0.9Mn0.1O3-ZnO-citrate gel and calcined pigment powder at 850°C.
of TG-DTA curves. Fig. 3. PXRD patterns of blue pigments calcined at 850°C/2h.
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Fig. 2. TG-DTA curves of the YIn0.9Mn0.1O3-ZnO-citrate gel. The inset shows enlarged view
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Fig. 4. Rietveld refinement of YIn0.9Mn0.1O3-ZnO from PXRD data. Observed (o), calculated (redline) and difference (bottom blue line) profiles are shown. The red and black vertical bars
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indicate positions of the Bragg reflections of ZnO and YIn0.9Mn0.1O3, respectively. Fig. 5. Crystal structure of YIn0.9Mn0.1O3 (left) and ZnO (right).
Fig. 6. Raman spectrum of synthesized blue pigment YIn0.9Mn0.1O3-ZnO. Fig. 7. The temperature dependence of χ(black) and χ-1 (blue) of YIn0.9Mn0.1O3-ZnO.
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Fig. 8. (a) TEM image, (b) HRTEM image (inset shows the corresponding SAED pattern) (c) diffraction pattern and (d) Energy dispersive spectrum of YIn0.9Mn0.1O3-ZnO synthesized pigment powder.
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Fig. 9. Particle size distribution of YIn0.9Mn0.1O3-ZnO. Fig. 10. Optical absorption spectra of the synthesized blue pigments.
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Fig. 11. Photographs of the synthesized blue nano-pigments a) YIn0.9Mn0.1O3 and b) YIn0.9Mn0.1O3-ZnO calcined at 850°C/2h. Fig. 12. UV-vis diffuse reflectance spectra of the synthesized blue pigments. Fig. 13. IR reflectance spectra of the blue pigment powders. The inset shows the corresponding solar reflectance spectra. Fig. 14. IR reflectance spectra of aluminium roofing sheet coatings.
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Fig. 15. Solar reflectance spectra of aluminium roofing sheet coatings. Fig. 16. Photographs of the aluminium roofing sheet coatings.
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Fig. 17. Thermogram of the YIn0.9Mn0.1O3-ZnO recorded in an air atmosphere.
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Table 1 Crystallographic data for YIn0.9Mn0.1O3-ZnO. YIn0.9Mn0.1O3 - phase Atom
Site
X
Y
Z
Uiso(Å2)
Y1
2a
0.0
0.0
0.266(1)
0.002(1)
1
Y2
4b
0.333
0.667
0.234(1)
0.028(1)
1
In/Mn
6c
0.334(1)
0.0
0.002(1)
0.021(1)
0.90(3)/0.10(3)
O1
6c
0.324(5)
0.0
0.171(1)
O2
6c
0.632(3)
0.0
0.330(3)
O3
2a
0.0
0.0
O4
4b
0.333
0.667
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Occupancy
1
0.072(5)
1
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0.002(2)
0.485(5)
0.286(1)
1
0.032(2)
0.011(4)
1
space group P63cm : a = 6.262(1) Å, c = 12.172(1) Å
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Bond lengths: Y1 – O1 = 2.331(1) x 3; Y1 – O2 = 2.434(1) x 3; Y1 – O3 = 2.685(1); Y2 – O1 = 2.255(1) x 3; Y2 – O2 = 2.305(1) x 3; Y2 – O4 = 2.464(1); In/Mn – O1 = 2.080(1); In/Mn – O2 = 2.077(1); In/Mn – O3 = 2.098(1); In/Mn – O4 = 2.120(1); In/Mn – O4 = 2.121(1)
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ZnO - phase Site
X
Y
Z
Uiso(Å2)
Occupancy
Zn
2b
0.333
0.667
0.265(1)
0.032(1)
1
O
2b
0.333
0.667
0.382(2)
0.022(4)
1
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Atom
space group P63mc : a = 3.240(1) Å, c = 5.188(1) Å Bond lengths: Zn – O = 1.966(1); Zn – O = 1.973(1) x 2: Zn – O = 1.973(2) Reliability Factors: Rp = 2.15%, Rwp = 2.76%, RF2 = 8.76%, χ2 = 2.66
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Table 2 Color coordinates, band gap values, IR reflectance and solar reflectance of blue pigments. Color coordinates L*
a*
Band
b*
C*
ho
IR
Solar
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Gap
reflectance
reflectance,
(eV)
at 1100 nm
R* (%)
39.71
-5.78 -22.28
23.21
255
2.63
63
50
YIn0.9Mn0.1O3-ZnO
49.94
-0.88 -40.55
40.56
269
2.74
79
70
*CoAl2O4[11, 34]
44.80
2.10
-
-
-
-
29
-
-
-
-32.70
-3.90 -15.62
16.10
-
*YIn0.9Mn0.1O3 [11]
0.50
-
-
49.50
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-29.20
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-
-
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Table 3 Solar reflectance and color coordinates of YIn0.9Mn0.1O3-ZnO pigment synthesized by sol-gel
Parameter
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combustion method coated on aluminium roofing sheets. Al sheet
coatings with blue pigment alone
Average reflectance in UV-vis region (%)
14
66
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Average reflectance in IR region (%)
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41
Thickness (µm)
IR reflectance at 1100 nm (%)
84
Solar reflectance (R*) (%)
67
L*
44.84
a*
4.78
C*
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-42.16 276
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Table 4
tests. Color coordinates
a
Aqueous solution condition a*
b*
Synthesized pigment powder
49.94
-0.88
-40.55
H2O
49.90
-0.87
Aqueous solution of pH = 3
49.92
Aqueous solution of pH = 13
49.99
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0.05
-0.93
-40.49
0.11
-0.91
–40.60
0.17
* ∆Eab = [(∆L*)2+(∆a*)2+(∆b*)2]½
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* ∆Eab
-40.52
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The color coordinates of the YIn0.9Mn0.1O3-ZnO powder pigments after chemical resistance
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