Synthesis and Characterization of Calcium Copper Titanate (CaCu3Ti4O12) Powder as a Brown-Coloured Inorganic Pigment with High Infra-Red Reflectance

To mitigate the Urban Heat Island (UHI) effect there is a stringent need to develop non-toxic and low cost Infra-red (IR) reflective pigments. Therefore, in present work the Calcium Copper Titanate (CaCu3Ti4O12) abbreviated as CCTO was prepared at different temperatures 800 °C, 850 °C, 875 °C, 900 °C, 925 °C, and 950 °C. X-ray diffraction (XRD), diffuse reflectance measurement in the Ultra Violet (UV), Visible (Vis) (295 nm to 700 nm), and Near Infra-Red (NIR) region (700 nm to 2500 nm), optical band gap and colour were measured to establish its use as IR reflective pigment. The chemical stability and cooling performance were also investigated. The XRD investigations showed the single-phase CCTO with cubic symmetry can be obtained by the calcination of reactants at an optimum temperature of 925 °C. The Diffuse reflectance data, within the heat-producing NIR band (700 nm to 1100 nm) varied from 46.23% to 53.34%. The optical band gap was observed to vary from 3.30 eV to 2.03 eV. No significant color degradation was observed upon treatment with 10% H2SO4 and 10% HNO3. Cooling performance assessment showed a significant difference of ∼5 °C between coated and uncoated red brick surfaces.

Urbanization is one of the characteristics of a city.Massive urbanization has been seen during the last few decades. 1 It has induced a global change in the Earth's environment.Global warming is a major issue and challenge. 2One of the main characteristics of urbanization is the dense cluster of residential housing and commercial buildings that continue to grow.As a consequence, the alteration of the natural environment gives support to the formation of Urban Heat Islands (UHI). 3The situation becomes grimmer when the concrete buildings in that area absorb and retain the heat generated from solar radiation energy. 3The roofs, walls, and streets become hot outdoor and indoor temperature increases.The residents feel discomfort and artificial cooling equipment like air coolers, air conditioners and refrigerators become operative, electricity demand gets increased significantly.Several techniques to prevent buildings from the heat of solar radiation have been proposed. 4,5Among them, Infrared reflective coating has been adopted widely.Coating of Infrared reflective cool pigments on roofs, and exterior walls can prevent indoor heat penetration.A recent study conducted by Macintyre H.L et al. has shown that cool roofs could reduce the local ambient temperature of the daytime as well as of the nighttime.20% to 70% of energy can be saved by this strategy. 6The 65% solar radiation reflecting roofs can reduce the peak cooling demand by up to 11%-27% in air-conditioned buildings.Non-air-conditioned buildings brought down the maximum temperature by 1.2 °C-3.7 °C. 7Numerous reports on the synthesis of Infrared reflective cool pigments have been found in the literature.][10][11] These ceramic materials are classified as inorganic pigments. 12TiO 2 has been reported as the best white-coloured inorganic pigment with >90% reflectance in the NIR band.Other than white-coloured inorganic pigments mostly chromium, cadmium and rare Earth elements based has been used as a coating material on exterior walls, roofs and surfaces (that are frequently exposed to the Sun) to reduce the indoor temperature.They are likely to be toxic and costly.However, non-toxic, cost-effective, and environmentfriendly pigments are still in their development phase.A significant amount of work has been reported on yellow pigment.A pale yellow-colored Bi 4 Ti 3 O 12 (BTO) pigment has been reported by Paraman Meenakshi et al. that showed IR reflectivity of about 90%.They achieved a bandgap in the range of 2.74 to 3.05 eV by varying the calcination temperature. 13Aijun Han et al. have reported brilliant yellow-colored Fe-doped La 2 Mo 2 O 9 pigment with near-infrared reflectance of 71%-93%. 14  with approximately 80% near-infrared reflectance, they observed that the color of the pigment varies from orange to yellow and bandgap varies from 2.04 eV to 2.11 eV with the increasing amount of Mg 2+ doping. 15Several brown inorganic pigments have been developed by many researchers 10,16,17 18 Some other reports with higher NIR reflectance of around 60% have been published but the colour of the pigment was reddish brown instead of brown. 19,20Moreover, the existing reports on NIR reflecting cool pigments mostly considered the wavelength range from 700 nm to 2500 nm for computing the NIR reflectance percentage.It is more relevant to consider the heat-producing wavelengths in the Near Infra-red region i.e. 700 nm to 1100 nm for the cool pigments. 21,22CTO was first reported in the year 1967. 23It was reported to have an unusually giant dielectric constant (∼10 5 ), and its cubic structure (Im-3) remains unchanged for the high temperatures >300 °C therefore, since then it is popular among researchers. 24,25here are only a few reports exist indicating its use as an inorganic pigment. 15,26Most of the brown pigments reported constitute costly rare Earth elements and may cause an adverse effect on health. 27hereas no health-related adverse issues have been reported for the oxides of Ca, Cu and Ti.Therefore, considering them safe and low costing, in this work, we have prepared an Inorganic CaCu 3 Ti 4 O 12 pigment powder and studied its UV/Vis/NIR reflectance properties for its application as a heat-reflecting pigment; Dielectric properties; brown color index by CIE L*a*b* color measurement; chemical stability by acid test and cooling performance.

Experimental
Material synthesis.-Inthe beginning, high purity (99.9%) laboratory grade chemicals, Calcium carbonate (CaCO 3 ), Titanium oxide (TiO 2 ) (Merck) and Copper oxide (CuO) (Loba chemie) powders were obtained and dried to release any absorbed moisture in z E-mail: mail.vishal.pd@gmail.coma hot drying oven for 24 h.at the temperature 200 °C.After cooling at RT, immediately weighed according to the chemical formula ratio CaCu 3 Ti 4 O 12 (CCTO) and transferred into agate mortar.Thereafter wet grinding with methanol was carried out for 4 h, followed by dry grinding for another 2 h.A small portion of the mixed chemicals was then transferred into alumina crucibles and calcined to complete the reaction for the formation of the desired compound, one by one at temperatures 800 °C, 850 °C, 875 °C, 900 °C, 925 °C, and 950 °C for 2 h each.The heating rate was 5 °C to obtain the final specified temperature.The samples prepared were assigned code names based on the preparation temperatures S800, S850, S875, S900, S925 and S950.The sample S925 was used to make pallets and sintering at the temperature of 1050 °C according to the procedure mentioned in the Ref. 28.

Characterization
Powder diffraction data was collected at room temperature (RT), using the instrument PANalytical PW340 X-ray diffractometer (Cu Kα radiation).Before that, the synthesized powder samples were further grounded for 1 h, and data was collected from a 2θ angle of 20°-70°at the step size of 0.02°and the time of 0.3 s per step.The Rietveld refinement was carried out using Fullprof software.The SEM images were taken with the help of the instrument CARL ZEISS (Special Edition EVO 18).Diffuse UV/Vis/NIR reflectance was measured using Agilent Cary 5000 UV-vis-NIR spectrophotometer fitted with the integrating sphere accessory and speclaron as a reference (200 nm-2500 nm); optical band gap was calculated according to Kubelka-Munk theory; 29 color data of the prepared powder samples were collected according to CIE L*a*b* color indexing, using CIE D65 standard illuminant and observer at 10°fi tted in the spectrophotometer.Dielectric constant variation with respect to the temperature (20 °C to 475 °C) and frequency (10 2 Hz to 10 6 Hz) was recorded with the instrument Keysight's E4490B impedance analyzer and the customized heating furnace manufactured by M/s Marine India, New Delhi.

Results and Discussion
X-Ray diffraction.-Thecrystalline structure of any material plays a major role in deciding its physical properties.The end product of a reaction may contain single or multiple intermediate compounds.It can be confirmed with the help of X-ray diffraction data. 30Therefore, X-ray diffraction was carried out on the prepared CCTO powder samples S800, S850, S875, S900, S925 and S950.The obtained diffraction patterns are shown in Fig. 1.For the samples S925 and S950 single phase was observed, and all the major diffraction peaks corresponding to miller indices (hkl) values (022), (031), ( 222), (132), (040) and (242) were found matching with crystal planes of cubic symmetry (spacegroup-Im-3) CaCu 3 Ti 4 O 12 , ICSD reference code 96-153-2159. 31Their peak intensities were observed to increase with increasing calcination temperature which may be an attribute of increasing crystallite size. 32 1.Which implies the applied thermal energy and duration was not enough to complete the formation of the desired compound. 32etveld analysis.-Rietveldanalysis on the XRD pattern of the sample S925 was carried out with the help of fullprof software suit 33 to ensure the observed presence of CCTO and also to investigate the crystallographic parameters.The obtained refinement profile is shown in Fig. 2 and obtained parameters are given in Table I.
From Fig. 2 and Table I, a small difference in the observed and the calculated XRD patterns along with the low value of R-factors has been obtained.Suggesting the end product is single-phase CCTO with cubic symmetry and the obtained crystallographic parameters are reliable.SEM image analysis.-Surfaceimages of the samples can be very helpful in determining the grain size and morphology.Thus, the properties related to grain size can be determined.Therefore, the SEM images were captured for samples S925 and S950.As shown in Figs.3a and 3b their average grain size measured with ImageJ software was ∼199 nm and ∼269 nm respectively.The increased grain size may be attributed due to the comparatively higher temperature induced the higher rate of diffusion among the powder  Total solar reflectance.-Thetotal solar reflectance (200 nm--2500 nm) of the prepared samples is shown in Fig. 4. Significant absorption has been seen in the UV-vis range, with two distinct peaks around 450 nm and 675 nm.The manifestation of these peaks is explored in the next section.In the NIR range high reflectance was observed.
UV-vis reflectance.-Toensure that the prepared material can be employed in stated applications UV-vis reflectance studies are a must.Therefore, the studies were carried out on each of the prepared powder samples.Figure 5 shows the diffuse reflectance of the prepared CaCu 3 Ti 4 O 12 powder samples in the UV-vis range (200 nm-750 nm).A broad reflectance for the wavelengths in the range ∼350 nm to 750 nm for the sample S800 was observed.It may be an attribute of the grey colour.The colour of the sample as seen in Fig. 6 is evidence.Moreover, according to the theory of colurs grey color is the result of mixing black and white colored substances. 34he XRD observations shown in Fig. 1 revealed the presence of unreacted black-coloured CuO, white-coloured Ca 2 Ti 2 O 6 , and Ca 4 Ti 3 O 10 , therefore, the resultant powder appeared grey.
From Fig. 5 it has been observed that, as the reaction temperature was increased the broad reflectance range has been shifted to the higher range ∼600 nm to 750 nm.Physically it can be interpreted as the brown colour of the samples as depicted in Figs.6b to 6f.This may have been attributed due to the transformation of reacting species into the brown-coloured CCTO.As a consequence of this, the lower reflectance of blue colour in comparison with red colour was there.However, around the wavelengths, ∼450 nm and ∼675 nm minimum in the reflectance has been observed.This may be attributed due to the hybridization of Cu 3d orbital and O 2p orbital with Ti 3d orbital which may have resulted in an unoccupied Cu 3d band.Upon excitation, with the wavelength of ∼450 nm (possessing comparatively higher energy) may have caused the direct transition of electrons from Cu 3d orbital to O 2p orbital.Whereas for lower energy ∼675 nm, a transition from the valance band to the unoccupied Cu 3d band may have occurred. 35,36ergy band gap.-According to Kubelka-Munk theory, the Kubelka-Munk function (F(R)) of a material having intermediate band gap energy (semiconductor) is directly proportional to the absorption coefficient of the same.Therefore, the Tauc equation Therefore, it can be transformed into,   To calculate the band gap (Eg), the diffuse reflection spectra of each prepared sample were recorded as F(R), versus wavelength. 37he band gap values were then calculated as per the procedure shown in Fig. 7 and the obtained values are given in Table II.A variation in band gap values was observed.This was attributed due to the degenerated valance bands of chemicals due to the formation of the compounds including the intermediates. 38R reflectance.-Ofthe total Sun's energy that reaches the Earth approximately 50% occurs in the visible region (400 nm--700 nm) and the other 50% occurs in the non-visible IR region (700 nm-2500 nm).
The heating of an object under the illumination of Sunlight is a direct effect of NIR radiations (700 nm-1100 nm). 21,22Therefore, it is important to measure the diffuse NIR reflectance of the cool pigments in the heat-producing NIR range (700 nm-1100 nm). Figure 8a shows diffuse reflectance measured for the samples within the 700 nm-1100 nm and Fig. 8b the other part of the NIR range (1100 nm-2500 nm).From both the figures and Table III, it has been observed that among all samples, the powder sample S925 was most reflective in the heating-producing NIR range as well as in the other part of the NIR range, with an average reflectance of 53% and 90% respectively.For the whole NIR range (700 nm-2500 nm) it showed almost 82% average reflectance.Which is quite higher in comparison with earlier reported pigments. 16,17Therefore, it could be a good material for NIR reflecting coatings and is expected to reduce the temperature of exterior walls, roofs, and surfaces that frequently come under solar irradiation and add an aesthetic appearance.Dielectric properties.-It is well known that with the increase in temperature, material may undergo several physical changes including a change in the crystal structure. 39Whereupon the applications when material experiences high temperature the dielectric properties may be useful to determine the temperature of crystal structure change called the phase transition temperature (T p ) 40 and possibly the working temperature range.The sample S925, which showed the highest NIR reflectance, was therefore selected for the dielectric constant measurements.The measurements were carried out at different temperatures from 20 °C to 475 °C for frequencies  ranging from 10 2 Hz to 10 6 Hz.The obtained results for higher frequencies ⩾10 kHz are shown in Fig. 9 at lower frequencies <10 kHz the values were very extremely high so they are omitted in Fig. 9 for keeping consistency in the graph.It has been observed that the dielectric constant value (∼1400 at 1 MHz) and the crystal structure were stable up to the temperature of 200 °C there after an anomalously high value of the dielectric constant (∼5000 at 1 MHz) and the crystal structure was changed.The phase transition temperature at which the highest value of the dielectric constant was attained was 369 °C.The obtained results are in agreement with the previous studies carried out on CCTO. 24,25lor measurement.-Thecolour of an object is a qualitative property.But its quantitative measurement is more important while representing it.Therefore, the colour of the synthesized samples has been measured according to the procedure mentioned elsewhere. 41able IV shows the colour indexing results.It was found that the samples prepared by calcination at temperatures >800 °C resemble the brown colour.
Chemical stability.-Thechemical stability test of the synthesized inorganic CCTO pigment against 10% H 2 SO 4 and 10% HNO 3 was carried out. 1 gm of the CCTO powder sample S925 was weighed and stirred first with 10 ml of 10% H 2 SO 4 for 1 hr.at room temperature.The resulting pigment was filtered with the Whatman 42 filter paper and washed with Milli-Q (Type-1) water and dried.The same procedure was followed with the 10% HNO 3 .Filtered, washed and dried powders were then subjected to color measurement.The results obtained are shown in the Table V.No significant change in the colour coordinates was observed.
Performance measurement.-Toestablish the use of pigment as a heat-reflecting pigment paint, the heat insulation characteristics were measured.For this, a paint was prepared by mixing 5 gm of the prepared pigment (S925) with 20 ml of commercially available

Conclusions
In this work, the CaCu 3 Ti 4 O 12 (CCTO) was synthesized and studied as an inorganic brown pigment.The synthesis was accomplished via conventional low-cost and simple solid-state reaction.The Optimum temperature to form the highest NIR reflecting browncoloured CCTO pigment powder was 925 °C.The prepared powder showed very high (82.45%)NIR reflectance for the wavelengths in the range 700 nm to 2500 nm and for the heat-producing NIR range of 700 nm to 1100 nm, it showed 53.34% reflectance.Which is very high compared to the earlier reported brown-coloured pigments. 17,26 change in phase was observed up to the temperature of 200 °C.Good resistance to the acids (H 2 SO 4 , HNO 3 ) with no major change in colour was observed.Performance measurements showed a huge temperature difference of approximately 5 °C between a coated and uncoated piece of red brick.Therefore, the CaCu 3 Ti 4 O 12 powder synthesized at 925 °C has a broad spectrum of applications.

Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Figure 2 .
Figure 2. Rietveld refinement performed on the XRD pattern of the sample S925 (black and red are the observed and the calculated patterns respectively; blue is the difference; green is the Bragg positions).

Figure 3 .
Figure 3. SEM image of the samples S925 (a) and S950 (b) at the same magnification of 102.12 K.

Figure 4 .
Figure 4. Total solar reflectance of the prepared samples in the wavelength range 200 nm to 2500 nm.

Figure 7 .
Figure 7. Band gap measurement for the prepared samples.

Figure 9 .
Figure 9.The dielectric constant measurement for the sample S925 at different frequencies 10 kHz, 100 kHz, 1 MHz.

Figure 10 .
Figure 10.Setup to measure the performance of the prepared paint using the sample S925; uncoated brick (a); coated brick (b); temperature at each reading (c).
Xu, Z et al. have reported Mg 2+ -doped CaCu 3 Ti 4 O 12 but all of them show comparatively low NIR reflectance.Recently G Monrós et al. have reported the rare Earth element-based brown pigment with 55% NIR reflectance.

Table II .
Band gap values calculated for the prepared samples.

Table III .
The average percentage of NIR reflectance in different regions, for the prepared samples.

Table IV .
CIE L*a*b* color index and average reflectance in NIR band.