Enhanced Near Infrared Reflectance with Brilliant Yellow Hues in

Research Article pubs.acs.org/journal/ascecg

Enhanced Near Infrared Reflectance with Brilliant Yellow Hues in Scheelite Type Solid Solutions, (LiLaZn)1/3MoO4−BiVO4 for Energy Saving Products T. R. Aju Thara, P. Prabhakar Rao,* S. Divya, Athira K. V. Raj, and T. S. Sreena Materials Science and Technology Division, CSIR−National Institute for Interdisciplinary Science and Technology (NIIST), Trivandrum−695 019, India S Supporting Information *

ABSTRACT: Enhanced near infrared (NIR) solar reflectance with interesting yellow hues in a new series of scheelite type solid solutions, [(LiLaZn)x/3Bi1−x][MoxV1−x]O4 (x = 0, 0.1, 0.2, 0.3, 0.4) were synthesized via conventional solid state reaction (SSR) method and planetary ball milling assisted solid state reaction (PBM) method. The structural, morphology, and reflectance (absorption) properties and coloring performance of the prepared compositions were analyzed by various advanced techniques. The solid solutions undergo a phase transformation from a monoclinic to a tetragonal phase. The compounds exhibit strong absorption in the UV and blue regions of the visible spectrum displaying high NIR reflecting intense yellow shades ranging from reddish to greenish. The yellow hue and NIR reflectance is enhanced by the morphological modifications through PBM method. Typically, the pigment [(LiLaZn)0.099Bi0.7][Mo0.3V0.7]O4 displayed intense yellow color (b* = 86.63) with NIR reflectance of 95% much better values than the commercial sicopal yellow. The applicability studies of these pigments on concrete cement block and metal sheet imparts good coloring performance with high NIR solar reflectance. Chemical and light resistance tests indicate their durability in the extreme weathering conditions. Thus, the prepared compositions consisting of less toxic elements demonstrate sustainable use of the present pigments in exterior surface coating applications as energy saving products. KEYWORDS: Yellow pigment, Morphology, NIR reflectance, Cool colorants

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INTRODUCTION The sustainable development of human society will depend on how to solve the urgent resource and environmental issues. Environmental safety and the energy crisis are two major global problems.1 As requirements increase in the world’s warmer regions, global energy utilization for air conditioning is continuing to rise significantly and could have a major impact on climate change. Considering the performance of buildings, lesser surface temperatures reduce the heat penetrating into the building and thereby decrease the cooling loads while making a more comfortable interior thermal environment. Roofing materials with higher solar reflectance (the ability to reflect sunlight) and higher thermal emittance (the ability to radiate heat) stay cool in the sun. Thus, they reduce the cooling energy load in air-conditioned buildings and increase the resident comfort in unconditioned buildings.2 Conventional near infrared (NIR) reflecting pigment coatings reduce the heat buildup and minimize the use of cooling power systems in buildings, automobiles, etc., which contributes to energy savings, cost effectiveness, and environmental security. The particle size of a pigment can influence their colors, which favorably affect its reflectance properties.3 Moreover; pigments © 2017 American Chemical Society

with small particle size possess high surface areas making them useful for a variety of applications including coatings. For yellow pigments, most, but not all, of the toxicity issues are associated with heavy or toxic metals such as cobalt, cadmium, chromium, lead, etc. Over time, these metals poison the body and many of them are known or suspected to be carcinogens. There are numerous literature works on new high NIR solar reflecting and environmentally friendly yellow pigments.4−9 Throughout history, various yellow ceramic pigments have been used: yellow of vanadium-zirconia, tin vanadium yellow, cadmium yellows, lead antimoniate (PbSbO3), etc.10 Molybdate based yellow pigments get wide attention today due to their nontoxic behavior. The pigments like Y 2 Ce 2−x Mo xO 7+δ, Y6−xSixMoO12+δ, Sm6−xZrxMoO12+δ, etc., are good ecofriendly alternatives to toxic yellow pigments having better color strength compared to other inorganic pigments based on rare earth molybdenum oxides reported earlier.11−14 In comparison with the commercially existing colorants, the color performance Received: February 15, 2017 Revised: March 31, 2017 Published: April 18, 2017 5118

DOI: 10.1021/acssuschemeng.7b00485 ACS Sustainable Chem. Eng. 2017, 5, 5118−5126

Research Article

ACS Sustainable Chemistry & Engineering

In the planetary ball milling assisted solid-state reaction (PBM) method, a stoichiometric mixture of Li2CO3 (99.998% purity, SigmaAldrich), La2O3 (99.9% purity, Acros Organics), ZnO (99.0% purity Sigma-Aldrich), MoO3 (99.99% purity, Acros Organics), Bi2O3 (99.999% purity, Sigma-Aldrich), V2O5 (99.9% purity, Acros Organics) was weighed in the required stoichiometric ratio. Then the mixture was placed in silica containers along with silica balls of 10 mm diameter as grinding media (balls to powder mass ratio = 10:1). Dry mechanical milling was carried out in a Fritsch Pulverisette-6 planetary ball mill about 20 h by using a rotating disc speed of 250 rpm. Calcination conditions applied as that of the SSR route. The coating of the pigment over a concrete cement block and metal sheet has been accomplished by a two-step process. First, the concrete cement block and metal sheet surface is precoated with TiO2, a white pigment possessing high reflectance in both visible and NIR regions. Next, the particular pigment is applied to the precoated substrate material. For this, the typical pigment is ultrasonicated for 10 min to ascertain the complete dispersion of the pigment particles in an acrylic−acralyn emulsion. The ratio of pigment to binder is taken as 1:1 by weight. The obtained viscous solution is coated over the concrete cement block and metal sheet, and then, these are allowed to dry in air. Characterizations. The crystalline structure of the calcined powders are analyzed by recording the X-ray powder diffraction pattern (XRD) using a PAN alytical X’pert Pro diffractometer having Ni-filtered Cu Kα radiation with an X-ray tube operating at 40 kV, 30 mA. Data were collected from a 10 to 90° 2θ range with a step size of 0.016°. The morphology of the powder samples were performed by means of scanning electron microscopy (SEM) using a JEOL JSM5600 LV SEM instrument operated at 15 kV. Energy dispersive X-ray spectroscopy (EDS) analysis and elemental mapping of the samples were analyzed using silicon drift detector X-MaxN attached with a Carl Zeiss EVO SEM. EDS elemental mapping were conducted using Aztec Energy EDS Microanalysis software. Particle size analysis of the powder samples is carried out by means of Beckman Coulter LS 13 320 Particle Size Analyzer. For this, the pigment powder is dispersed in the distilled water then sonicated at the speed of 20 rpm. The absorbance and reflectance spectra of the samples were carried out with a UV−vis-NIR spectrophotometer (Shimadzu, UV-3600) using barium sulfate as a reference. Optical measurements were performed in the 220−2500 nm wavelength range with a step size of 2 nm. The measurement conditions were as follows: an illuminant D65, 10° complementary observer, and measuring geometry d/8°. As a crystalline semiconductor, the optical absorption near the band edge follows the formula27

of these class pigments are poor. The interests in yellow pigments with bismuth vanadate have grown considerably due to its rich yellow color strength.15 These set of circumstances lead us to focus on bismuth vanadate (BiVO4). Synthetically prepared BiVO4 pigment has a brilliant greenish yellow color. Due to its nontoxic nature and photochromic properties,16−19 it is considered to be a promising alternative to toxic lead chromate and cadmium sulfide pigments in the automobile and paint industry. The valence band (VB) of BiVO4 is composed primarily of O 2p states, with Bi 6p states contributing to the bottom and V 3d to the middle of the valence band.20 Molecular orbital theory suggests that the substitution of dopants into BiVO4 can either decrease or increase the band gap, depending on the preferred substitutional site and energetically favored crystal structure. BiVO4 has been conventionally obtained by a solid-state reaction method that produces large irregular BiVO4 crystals due to its rapid crystal growth. By controlling the morphologies of BiVO4 structures, such as sizes and shapes we can improve its properties.19 Based on these concepts earlier we have successfully synthesized brilliant yellow pigments Li0.10La0.10Bi0.8Mo0.2V0.8O4 for energy saving applications7 with NIR reflectance of 91% and b* 81.86. The synthetic methods play an important role in the chemical and physical properties of metal oxides. Numerous synthesis methods have gained attention for the preparation of fine pigment powders namely sol−gel, coprecipitation, hydrothermal, citrate gel, evaporation to dryness, and other gel combustion methods.21−25 So for further improvement of the NIR reflectance and yellow hue, we adopt some morphological modifications. To avoid the band gap altering due to particle size drop, Zn was introduced to the lattice. In this work, BiVO4 was synthesized by the conventional solid-state reaction (SSR) method and the planetary ball milling assisted solid-state reaction (PBM) method. Our work suggests that the pigment as well as the reflective performance of (LiLaZn)1/3MoO4 doped BiVO4 is greatly dependent on the structure and the morphology. The new title pigment consists of less toxic elements such as La, Zn, Mo, Bi, and V,26 and their derived compositions are reported as ecofriendly pigments which make them suitable to use for these applications.15,25 Further, they perform interesting NIR solar reflectance as well as good yellow hues and can be used in coating applications and making valuable benefits.

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αhν = A(hν − Eg )n /2

(1)

where α, ν, Eg and A are absorption coefficient, light frequency, band gap, and a constant, respectively. n depends on the characteristics of the transition in a semiconductor, i.e. direct transition (n = 1) or indirect transition (n = 4). The energy gap of the samples were calculated from the Tauc plots of the (αhν)2 versus photon energy (hν). The intercept of the tangent to the x-axis gives a good approximation of the band gap energy. The color coordinates were determined by coupling analytical software (UVPC Color Analysis Personal Spectroscopy Software V3, Shimadzu) to the UV-3600 spectrophotometer. The color of the pigments was evaluated according to The Commission Internationale del’ Eclairage (CIE) through L*a*b* 1976 color scales (CIE-LAB 1976 color scales). In this system, L* is the lightness axis (L* is zero for black and 100 for white), a* is the green (−)/ red (+) axis, and b* is the blue (−)/yellow (+) axis. The parameter C* (chroma) represents saturation of the color and is defined as C* = ((a*)2 + (b*)2)1/2 and h° represents the hue angle. The hue angle, h° is expressed in degrees and ranges from 0° to 360° and is calculated using the formula h° = tan−1(b*/a*). The infrared reflectance of the powdered pigment samples was measured with a UV−vis-NIR spectrophotometer (Shimadzu, UV-3600 with an integrating sphere attachment) using polytetrafluoroethylene (PTFE) as a reference in the 700−2500 nm

EXPERIMENTAL SECTION

Materials and Methods. The pigments of the formula [(LiLaZn)x/3Bi1−x][MoxV1−x]O4 (x = 0, 0.1, 0.2, 0.3, 0.4) were prepared by the conventional solid state reaction (SSR) route. Li2CO3 (99.998% purity, Sigma-Aldrich), La2O3 (99.9% purity, Acros Organics), ZnO (99.0% purity Sigma-Aldrich), Bi2O3 (99.999% purity, Sigma-Aldrich), MoO3 (99.99% purity, Acros Organics), and V2O5 (99.9% purity, Acros Organics) were weighed in the required stoichiometric ratio and then were wet mixed in an agate mortar using acetone as the wetting reagent. For comparison, BiVO4 samples were also prepared. The mixed product was dried in an air oven at 100 °C for 1 h. This process of mixing and drying was repeated thrice to obtain a homogeneous product. The obtained mixture calcined at in a platinum crucible in an air atmosphere furnace. The furnace was programmed by increasing the temperature initially at 10 °C per minute up to 400−500 °C, and then, the heating rate was decreased to 5 °C per minute up to the desired temperature (600 °C). The samples were soaked at 600 °C for 6 h. The calcined compounds were ground into fine powder for carrying out further characterization. 5119


Research Article

ACS Sustainable Chemistry & Engineering wavelength range with a step size of 5 nm. The IR solar reflectance was calculated in accordance with the ASTM standard number G173-03.28 The IR solar reflectance is expressed as the integral of the percent reflectance times the solar irradiance divided by the integral of thesolar irradiance when integrated over the 700−2500 nm range as shown in the formula, 2500

R=

∫700 r(λ)i(λ) dλ 2500

∫700 i(λ) dλ

(2)

where r(λ) is the spectral reflectance obtained from the experiment and i(λ) is the standard solar spectrum (W m−2 nm−1) obtained from the standard. The NIR solar reflectance spectra were determined from ASTM Standard G173-03.28 Chemical resistance tests using acid/base solutions and water were performed. The typical pigment with best color was treated with 5% HCl, HNO3, NaOH, or H2O and soaked for 1 h with continuous stirring using a magnetic stirrer. The pigment powder was then filtered, washed with deionized water, dried, and weighed. The light resistance test was carried out by exposing the pigment powder to natural sun light. The difference in color of the pigment after exposure to sunlight for 70 h was examined using the color parameters. The CIE * ) was color coordinates were measured, and the color difference (ΔEab calculated from the following equation.

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* = ΔEab

(ΔL*)2 + (Δa*)2 + (Δb*)2

(3)

RESULTS AND DISCUSSION X-ray Diffraction Analysis. The XRD patterns of [(LiLaZn)x/3Bi1−x][MoxV1−x]O4 (x = 0, 0.1, 0.2, 0.3, 0.4) pigment powders synthesized via conventional SSR and PBM methods are shown in Figure 1. The compositions x = 0 and 0.1 crystallize with a pure and partial monoclinic phase with the space group I2/b, and all the peaks are indexed according to the JCPDS database number 01-075-1866. Partial phase transformation occurred for x = 0.1 substitution from pure m-BiVO4 to a mixture of m-BiVO4 and t-BiVO4. This leads to an expansion in unit cell volume, increase in compressive lattice strain, conduction band edge uplift, and band gap widening.29 The compositions with x = 0.2, 0.3, and 0.4 form the tetragonal phase, which can be indexed as per the JCPDS card number 01075-2481 with the space group I41/a. This shows that progressive doping of Li+, La3+, Zn2+, and Mo6+ brings a structural transition from the monoclinic scheelite to the tetragonal scheelite type.30−32 The splitting of the peaks at 18.5°, 35°, and 45° of 2θ is characteristic of the scheelite monoclinic phase. The intense and sharp peaks in the diffraction patterns confirm the crystalline nature of the powders. This reveals that all the samples are homogeneous, and the doping of Li+, La3+, Zn2+, and Mo6+ form solid solution in BiVO4. Minor peaks of V4O9 can be seen from x = 0.2 onward. In BiVO4, the V site bounded with four oxygen atoms forms a VO4 tetrahedron and Bi site bounded with eight oxygen atoms forms a BiO8 dodecahedron. The transition metal tetrahedral arrangements determine the structural difference of monoclinic and tetragonal scheelite forms. The interlinked transition metal tetrahedra were observed in tetragonal scheelite form, whereas they are isolated in the monoclinic scheelite form.33 The ionic radii of Bi3+ is 0.117 nm (CN = 8) and V5+ is 0.035 nm (CN = 4). The doping of Li+ (0.092 nm), La3+(0.116 nm), and Zn2+(0.090 nm) into Bi3+ and Mo6+ (0.041 nm)34 into V5+ results in small distortions in the VO4 tetrahedron and BiO8 dodecahedron of the lattice due to the difference in ionic radius. The lattice volume increases with

Figure 1. Powder X-ray diffraction patterns of [(LiLaZn)x/3Bi1−x][MoxV1−x]O4 (x = 0, 0.1, 0.2, 0.3, 0.4) pigments synthesized by SSR and PBM methods.

increasing dopant concentration from x = 0 to 0.4 which signifies the formation of solid solution. The crystallite size calculated from the Debye−Scherrer formula is D = 0.9λ /β cos θ

(4)

where D is the crystallite size, λ is the wavelength of X-ray used, β and θ are the half width of the X-ray diffraction lines and half diffraction angle 2θ. The instrumental broadening was rectified using silicon as the external standard. The crystallitesize of BiVO4 is found to be 63.84 nm, 36.5 nm in SSR and PBM routes, respectively. In SSR route, the addition of dopants did notaffect much change in the crystallite size. On doping with Li+, La3+, Zn2+, and Mo6+, an increasein crystallite size up to 69.81 for x = 0.3 and then decrease to 63.83 for x = 0.4 is observed in the PBM route. The lattice parameters and crystallite size obtained at various dopant concentrations in both of these methods are shown in Tables S1 and S2 in the Supporting Information. Morphological Studies. The synthesis conditions and treatment process affect the morphological feature of the 5120


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Figure 2. Scanning electron micrographs of [(LiLaZn)x/3Bi1−x][MoxV1−x]O4 (x = 0, 0.1, 0.2, 0.3, 0.4) pigments synthesized by SSR method.

material. Figure 2 shows the scanning electron micrographs of [(LiLaZn)x/3Bi1−x][MoxV1−x]O4 (x = 0, 0.1, 0.2, 0.3, 0.4) pigments synthesized using the SSR route. The microstructure explains the crystalline nature of the particles. Thus, addition of (LiLaZn)1/3MoO4 causes a reduction in the particle size of BiVO4. The particles are agglomerated and have sharp edges. There is a wide distribution of particle sizes of [(LiLaZn)x/3Bi1−x][MoxV1−x]O4 (x = 0, 0.1, 0.2, 0.3, 0.4) solid solutions with an average size of 1−8 μm prepared by the SSR method. The SEM micrographs obtained by the PBM method are shown in Figure 3. Ball milling helps to maintain uniform particle size and shape. The particles are slightly agglomerated. SEM analysis shows that the morphology is almost spherical and the average size of sample increases as the concentration of BiVO4 increases. The particle sizes of all the compositions synthesized by PBM method vary from 0.7 to 1.5 μm. This variation in the morphology with respect to synthesis method has been found to improve the color and reflective properties of the pigments which has been discussed in the later part of the text. The EDS was used to further determine the chemical composition of the as-obtained pigments. EDS spectra (Figure S 1 o f t h e S u p po r ti n g In f o r m a tio n) o f a ty p ica l [(LiLaZn)0.099Bi0.7][Mo0.3V0.7]O4 sample synthesized by two methods shows the presence of Bi, V, La, Zn, Mo and O elements, with close approximation to the calculated value. The Li element is not detected due to going beyond the detection range of the instrument. X-ray mapping analysis of the classic

Figure 3. Scanning electron micrographs of [(LiLaZn)x/3Bi1−x][MoxV1−x]O4 (x = 0, 0.1, 0.2, 0.3, 0.4) pigments synthesized by PBM method.

[(LiLaZn)0.099Bi0.7][Mo0.3V0.7]O4 pigment synthesized by PBM method shown in Figure 4 also shows that the elements are 5121


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Figure 4. X-ray dot mapping of [(LiLaZn)0.099Bi0.7][Mo0.3V0.7]O4 pigment synthesized by PBM method.

routes show mean particle diameters of 1.04 and 0.98 μm, respectively. UV−visible Studies. The absorption spectra and diffuse reflectance spectra of the [(LiLaZn)x/3Bi1−x][MoxV1−x]O4 (x = 0, 0.1, 0.2, 0.3, 0.4) pigment samples in the visible region, synthesized by SSR and PBM methods, are shown in Figure 5. A strong absorption below 485 nm in the UV−vis reflectance spectrum of Li+, La3+, Zn2+, and Mo6+ free BiVO4 sample is due to the charge-transfer transition of hybrid orbitals of Bi 6s and O 2p to V 3d orbitals. The yellow hue of BiVO4 is due to the absorption in the blue region, since blue is a complementary color to yellow. Absorption of successive doping of additives on BiVO4 is blue-shifted. The diffuse reflectance spectra reveals the influence of Li+, La3+, Zn2+, and Mo6+ doping on the optical properties of BiVO4 based pigments. The band gap energies of BiVO4 are calculated as 2.37 and 2.44 eV in SSR and PBM routes respectively from Tauc plots. This is due to the decreasing particle size. When particle size decreases, the number of overlapping of orbitals or energy level decreases and the width of the band gets narrower. This will cause an increase in energy gap between the valence band and the conduction band. Doping with (LiLaZn)1/3MoO4 into BiVO4 alters the O 2p valence band and increases the bandgap energy by modification of the Bi 6s−O 2p hybrid orbital. The addition of Mo 4d, La 5d, and Zn 3d orbitals above the V 3d orbitals results in widening of the conduction band. This leads to reduced interaction between O 2p and V 3d orbitals. This in turn increases the band gap. With gradual increasing, the concentration of dopants increases the band gap energy. Color Analysis. The CIE 1976 color coordinates of the (LiLaZn)1/3MoO4 doped BiVO4 powdered pigment samples

uniformly distributed within the matrix. SEM EDS analysis confirms the close agreement between the stoichiometric and the actual composition. Thus, the structural and morphological analysis confirms that La3+, Zn2+, and Mo6+ have been effectively inserted into the BiVO4 lattice. Particle Size Analysis. Mean particle diameter of all samples synthesized by both SSR and PBM routes and calcined at 600 °C are shown in Table 1. The particle size agrees Table 1. Mean Particle Size of [(LiLaZn)x/3Bi1−x][MoxV1−x]O4 (x = 0, 0.1, 0.2, 0.3, 0.4) Pigments Synthesized by SSR and PBM Methods mean particle diameter (μm) sample

SSR

PBM

BiVO4 [(LiLaZn)0.0333Bi0.9][Mo0.1V0.9]O4 [(LiLaZn)0.0666Bi0.8][Mo0.2V0.8]O4 [(LiLaZn)0.0999Bi0.7][Mo0.3V0.7]O4 [(LiLaZn)0.1332Bi0.6][Mo0.4V0.6]O4

7.18 1.06 1.10 1.04 1.07

1.21 0.86 0.99 0.98 1.02

effectively with the results obtained from SEM. Compared to BiVO4 synthesized by SSR route, there is a noticeable difference in particle size and it discloses a mean diameter of 7.18 μm (size of 90% particles < 12.64 μm, 75% particles < 9.76, 50% particles < 6.75 μm, 25% particles < 4.35 μm, and 10% particles < 2.55 μm). As concentration increases, the difference of particle size prepared by both the methods diminish. The particle size distributions of the classic pigment [(LiLaZn)0.099Bi0.7][Mo0.3V0.7]O4 prepared by SSR and PBM 5122


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0.1, 0.2, 0.3, 0.4) pigment samples were found to be in the yellow region of the cylindrical color space (h°= 70°−105° for yellow). At the same time, the increase of doping contents leads to a loss of red hue of the pigment that is evident from the lower values of the color coordinate a* (a* changes from 12.82 to −2.32). Table 3 summarizes the CIE 1976 color coordinates of the powdered [(LiLaZn)x/3Bi1−x][MoxV1−x]O4 (x = 0, 0.1, 0.2, 0.3, 0.4) pigment samples synthesized by the PBM method. Addition of (LiLaZn)1/3MoO4 into BiVO4 (from x = 0−0.3) leads to a continuous increase in the yellow component (b* from 79.24 to 86.63) and chroma (C* from 79.74 to 86.64) values of the pigments. The hue angle (h°) also increases considerably from 83.59 to 93.71, and [(LiLaZn)0.099Bi0.7][Mo0.3V0.7]O4, with a hue angle of 90.6 which is very close to 90°, is the most satisfied yellow pigment. Also, with the increase of x, the a* value gradually shifts from 8.89 to −5.36 indicating a greenish yellow hue characteristic of BiVO4 pigments. When x reaches 0.4, L* increases to 90.74 and a* reaches −5.36; however, the b* values continue diminishing. Among these pigments [(LiLaZn)0.099Bi0.7][Mo0.3V0.7]O4 showed the most vivid yellow hue with a yellowness b* value of 86.63 which is significantly greater than those for the commercially available BiVO4 pigment sicopal yellow (b* = 76.9, C* = 78.7).15 Photographs of the developed pigments are shown in Figure 6. NIR Reflectance Studies. On the exposure to sunlight, if an object absorbs more solar energy, the heat buildup will be more, while the more reflective an object, the less it will heat. Infrared (IR) radiation in the electromagnetic spectrum is mainly responsible for heat buildup. The NIR solar reflectance spectra of powdered samples synthesized by both SSR and PBM routes in comparison with ASTM Standard G173-0328 are presented in Figure 7. This gives the reflectance in the entire 750−2500 nm region. The NIR solar reflectance of all the samples synthesized by SSR and PBM methods are shown in Tables 2 and 3, respectively. Successive doping of Li+, La3+, Zn2+, and Mo6+ into BiVO4 enhances the NIR solar reflectance up to 95% for x = 0−0.3 and further increases in the concentration reduce the value to a small degree, 94%, for the pigments synthesized by the PBM method. The NIR reflectance spectra of [(LiLaZn)x/3Bi1−x][MoxV1−x]O4 (x = 0, 0.1, 0.2, 0.3, 0.4) samples synthesized by the SSR route are given in the inset of Figure 7. It is seen that the NIR reflectance is enhanced as 88% in the 1100 nm range at x = 0.3. The inset of Figure 7 provides the NIR reflectance spectra of powdered [(LiLaZn)x/3Bi1−x][MoxV1−x]O4 (x = 0, 0.1, 0.2, 0.3, 0.4) pigment samples prepared by the PBM route. It is seen that the NIR reflectance is enhanced up to 95% in the 1100 nm range for x = 0.3. These results show that the developed pigment is competent of replacing currently used pigments. An increase in NIR reflectance is due to the improvement of pigment particle morphology and decrease in the particle size. These results

Figure 5. UV visible reflection spectra of [(LiLaZn)x/3Bi1−x][MoxV1−x]O4 (x = 0, 0.1, 0.2, 0.3, 0.4) pigments synthesized by SSR and PBM methods.

prepared by SSR method are shown in Table 2. This shows that the simultaneous doping of Li+, La3+, Zn2+, for Bi3+ and Mo6+ for V5+ in BiVO4 increases the b* value from 62.85 to 80.41. Hence the yellowness of the pigment samples is enhanced and further increase of dopants reduces the b* to 72.04. The chroma (C*) ranges from 64.14 to 80.48 for x = 0−0.3, and it diminishes at x = 0.4 as 72.08. The hue angle (h°) also increases considerably from 78.47 to 91.86. The hue angles (h°) of the powdered [(LiLaZn)x/3Bi1−x][MoxV1−x]O4 (x = 0,

Table 2. L*, a*, b* Color Coordinates, Band Gap Energy, and NIR Solar Reflectance for the [(LiLaZn)x/3Bi1−x][MoxV1−x]O4 (x = 0, 0.1, 0.2, 0.3, 0.4) Pigments Synthesized by the SSR Method sample

L*

a*

b*

C*

h0

Eg (eV)

R* %

BiVO4 [(LiLaZn)0.0333Bi0.9][Mo0.1V0.9]O4 [(LiLaZn)0.0666Bi0.8][Mo0.2V0.8]O4 [(LiLaZn)0.0999Bi0.7][Mo0.3V0.7]O4 [(LiLaZn)0.1332Bi0.6][Mo0.4V0.6]O4

75.24 78.6 80.45 84.66 81.49

12.82 11.19 6.88 3.37 −2.32

62.85 75.01 76.13 80.41 72.04

64.14 75.84 76.44 80.48 72.08

78.47 81.50 84.83 87.59 91.86

2.38 2.39 2.43 2.47 2.52

68 83 82 88 87

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Table 3. L*, a*, b* Color Coordinates, Band Gap Energy, and NIR Solar Reflectance for the [(LiLaZn)x/3Bi1−x][MoxV1−x]O4 (x = 0, 0.1, 0.2, 0.3, 0.4) Pigments Synthesized by the PBM Method sample

L*

a*

b*

C*

h0

Eg (eV)

R* %

BiVO4 [(LiLaZn)0.0333Bi0.9][Mo0.1V0.9]O4 [(LiLaZn)0.0666Bi0.8][Mo0.2V0.8]O4 ((LiLaZn)0.0999Bi0.7][Mo0.3V0.7]O4 [(LiLaZn)0.1332Bi0.6][Mo0.4V0.6]O4

83.32 85.11 86.98 90.08 90.74

8.89 4.87 4.70 −0.89 −5.36

79.24 85.20 85.03 86.63 83.04

79.74 85.34 85.16 86.64 83.21

83.59 86.72 86.82 90.60 93.71

2.44 2.47 2.51 2.53 2.54

91 89 92 95 94

demonstrate that the developed yellow colored pigments are promising candidates as cool colorants to reduce heat buildup. Application Studies. Pigments that absorb wavelengths other than the visible region can have attractive effects on coating properties. The main aim of IR-reflective coatings is to keep objects cooler than they would be. Here, we selected [(LiLaZn)0.099Bi0.7][Mo0.3V0.7]O4 pigment prepared by the PBM method to analyze the appropriateness of colored IRreflective coatings on concrete and metal sheet. The thickness of all the paint films estimated using Bruker’s DektakXT stylus profiler was found to be in the 60−65 μm range. The NIR solar reflectance and the corresponding NIR reflectance spectra (shown inset of Figures 8 and 9) of the yellow pigment sample

Figure 6. Photographs of [(LiLaZn)x/3Bi1−x][MoxV1−x]O4 (x = 0, 0.1, 0.2, 0.3, 0.4) pigments synthesized by SSR and PBM methods.

Figure 8. NIR solar reflectance spectra of bare and pigment coated concrete cement block with and without TiO2 base coat. (inset) NIR reflectance.

coated on the concrete slab along with bare concrete slab as well as metal sheet surface with and without TiO2 base coat are shown in Figures 8 and 9. The bare concrete cement slab possesses NIR reflectance of 37% which can be raised by 88% and 80% using pigment coating with TiO2 base coat and without base coat, respectively. The pigment coatings on both concrete block and metal sheet find very good NIR reflectance and can be used in buildings, automobiles, etc. The CIE 1976 color coordinates and reflectance in the NIR region of the yellow pigment coatings are presented in Table 4, and the photographs of the coating samples shown in Figure 10. Chemical Resistance Studies of the Pigment. Rapid growth of industries leads to the accumulation of acidic pollutants in the atmosphere. These stick to the ground and other surfaces such as vehicles, houses, buildings, etc., which leads to the tarnishing of the coating surfaces. So it is necessary

Figure 7. NIR solar reflectance spectra of [(LiLaZn)x/3Bi1−x][MoxV1−x]O4 (x = 0, 0.1, 0.2, 0.3, 0.4) pigments synthesized by SSR and PBM method. 5124


Research Article


Figure 9. NIR solar reflectance spectra of pigment coated metal sheets with and without TiO2 base coat (inset) NIR reflectance.

Table 4. IR Solar Reflectance and Color Coordinates of [(LiLaZn)0.099Bi0.7][Mo0.3V0.7]O4 Pigments Synthesized by the PBM Method Coated over Concrete Cement Blocks and Metal Sheets coatings

L*

a*

b*

C*

h0

R%

concrete + pigment concrete + TiO2+ pigment metal sheet + pigment metal sheet + TiO2 + pigment

83.55 86.14

−6.5 −2.33

80.72 87.02

80.98 87.06

90.96 91.49

80 88

81.47 85.40

−1.78 −1.58

78.45 80.18

78.47 80.19

91.31 91.14

70 77

Figure 10. Photographs of pigment coated concrete cement block and metal sheet.

morphology and reduction in particle size resulting in significant improvements of NIR reflectance. Typically the pigment [(LiLaZn)0.099Bi0.7][Mo0.3V0.7]O4 displayed intense yellow color (b* = 86.63) with near-infrared reflectance of 95% much better values than the commercial sicopal yellow. These colorants demonstrated good coloring performance with high solar reflectance in cement blocks and on metal sheets. These compositions containing less toxic elements can be used as sustainable pigments in surface coating applications as energy saving products.

for conducting chemical resistance test. The acid/alkali/water resistance of the typical [(LiLaZn)0.099Bi0.7][Mo0.3V0.7]O4 pigment synthesized by the PBM method was carried out in 5% HCl, HNO3, NaOH, or H2O. There is no noticeable weight loss of the pigment for the acid, alkali, and water test. The color coordinates of the resulting tested sample measured and compared with the unprocessed samples. From this, the total color difference (ΔEab * ) was calculated and tabulated in Table S3 in the Supporting Information. The small values of ΔE*ab reveals that the pigments are chemically stable in the acid/ alkali/H2O. The light resistance of the developed pigment powder also conducted by exposing it to the natural sunlight for 70 h which gives an ΔE*ab value of 0.78. This also shows the high durability of the pigment. The acceptable limits of color difference are as follows: when ΔEab * ≤ 1 unit, the color change is almost indistinguishable from the original color, whereas ΔE*ab ≤ 5 units is considered very good.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00485. Tables S1−S3 and Figure S1 as mentioned in the text (PDF)

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AUTHOR INFORMATION

Corresponding Author

CONCLUSIONS New ecofriendly yellow inorganic pigments based on BiVO4 having high NIR solar reflectance have been successfully designed. Doping of Li+, La3+, Zn2+, and Mo6+ into the trivalent and pentavalent site of the BiVO4 system causes changes in the absorption properties and also improved its optical and reflective properties. The influence of morphology on the optical and reflective properties of pigment powders is studied on pigments created by the conventional solid-state reaction and planetary ball milling assisted solid-state reaction methods. The pigments synthesized through PBM yielded uniform

*Tel.: + 91 471 2515311. Fax: + 91 471 2491712. E-mail: [email protected] (P.P.R.). ORCID

P. Prabhakar Rao: 0000-0003-3204-4748 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the Council of Scientific and Industrial Research (CSIR Project MLP-0012), 5125


Research Article


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New Delhi, Government of India, and the International Centre for Diffraction Data (ICDD), Pennsylvania, USA, for financial support.

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Enhanced Near Infrared Reflectance with Brilliant Yellow Hues in

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