Bismuth and Vanadium-Substituted Yttrium Phosphates for Cool Coating Applications

Abstract

Luminescent yttrium phosphate is engineered into an environmentally benign near infrared (NIR) reflective yellow pigment by the substitution of bismuth and vanadium metals in the host lattice. A series of YP_(1–x)V_xO₄ (x = 0, 0.05, 0.1, 0.15, 0.2, and 0.4), Y_(1–y)Bi_yPO₄ (y = 0.1 and 0.3), and Y_(1–y)Bi_yP_(1–x)V_xO₄ (x = y = 0.2, 0.4, and 0.6) were prepared by the precipitation method. Secondary phase was noticed at x = 0.2 and y = 0.2 while substituting vanadium and bismuth, respectively, due to high ionic radii of the dopant ions. Co substitution of vanadium and bismuth in the YPO₄ lattice enhanced both NIR reflectance and yellow color of all the fabricated materials. XPS spectra proved the presence of trivalent bismuth and pentavalent vanadium in Y_0.4Bi_0.6P_0.4V_0.6O₄. Due to the substitution effect, a more defined morphology was noticed, which enhanced the scattering co-efficient of the fabricated materials; hence, the NIR reflectance of the materials was increased from 68% (YPO₄) to 83% (Y_0.4Bi_0.6P_0.4V_0.6O₄). Chemical and thermal stability test of Y_0.4Bi_0.6P_0.4V_0.6O₄ confirmed the color and strength of the designed pigment. With good yellow hue (b* = +56.06), high NIR solar reflectance (R* = 83%), and good stability, Y_0.4Bi_0.6P_0.4V_0.6O₄ can act as an environmentally benign cool yellow pigment.

1. Introduction

The urban heat island (UHI) effect is one of the most serious discomforts experienced by the people who live in cities. Due to this effect, the interior of the built up area experiences more heat than its external surroundings. The UHI effect is generally caused due to the absorption of solar radiation by the buildings and other architectures in the urban areas during the long day time, and no dissipation of the absorbed energy in the night time.¹ This phenomenon significantly increases the difference in temperature between urban and rural areas (the radiation is more rapid due to natural greeneries). In order to reduce the high interior temperature, use of air conditioners are preferred which consumes around 40% of electricity.² Heat absorption is quite high when the buildings are made of asphalt sheets, cement, concretes, and metallic structures.³ Since approximately 53% of the solar radiation consists of the near-infrared region (NIR), it is obvious that building infrastructures coated with NIR-reflective materials could effectively reduce the interior temperature, in turn keeping the building cool. Hence, these NIR-reflective materials are coined as cool coating materials.⁴

Neoteric investigators are focused on the synthesis of NIR reflective materials based on rare earth materials as host/dopant compound; hence one could increase the reflectivity of the material while maintaining the aesthetic color.⁵ Sky blue color of SrCuSi₄O₁₀ was modified into dark blue by doping the Eu³⁺ ion into Sr²⁺ and the NIR reflectance of the fabricated materials increased significantly by the substitution of 20% dopant in the host system.⁶ (Y_1–xR_x)₂Cu₂O₅ (R = Gd³⁺, Sc³⁺, Yb³⁺ Lu³⁺ and Tm³⁺) green pigments⁷ were synthesized by the solid state reaction method and proved to be environmentally benevolent pigments compared to toxic Cr and Co green pigments. Pale yellow-colored Y₆Mo₆O₁₂ [Reflectance (R*) = 92%] is converted into dark yellow-colored Si_xY_6–xMo₆O₁₂ with R* = 98% and dark brown color Pr_xY_6–xMo₆O₁₂ with R* = 60%.⁸ Fe³⁺-substituted La₂Mo₂O₉ nanocrystalline material with brilliant yellow color was developed with 92% reflectance, using the poly acryl amide method.⁹ Red Li₃InB₂O₆¹⁰ was prepared using the solid-state reaction method to obstruct the infrared radiations. The incorporation of V⁵⁺ (3d⁰) in the pentavalent or the tetravalent site has been a promising strategy in recent years to obtain a yellow hue from white materials. In particular, vanadium in the pentavalent state (V₂O₅) is yellow due to the charge transfer transition from O_2p (II) to V_3d (V).¹¹ V-ZrO₂ is a well-known inorganic yellow pigment prepared by various conventional and non-conventional methods, evidencing the incorporation of V into ZrO₂ that induces the yellow hue in the resultant pigment material.¹² Gopal et al. reported the abnormal perception in the optical property of V⁵⁺-substituted BPO₄. Open framework BPO₄ (white) was converted into yellow phosphovanadate by substituting the VO₄ site into the PO₄ site.¹³ BiVO₄ was also proved as a promising yellow pigment in the industrial sector. Compiling all the preceding results, substitution or doping of bismuth and vanadium ions could be one of the iconic ideas to develop eco-friendly yellow pigments.

Lanthanide-based orthophosphates (LnPO₄) are recently materialized as an important class of inorganic substances, whose applications are widespread in various fields such as laser host materials, luminescent and phosphor materials, upconversion of photons, and bio-imaging.¹⁴ These unique phosphates exhibit a variety of polymorphic forms, namely, monoclinic monazite, tetragonal xenotime, tetragonal zircon, monoclinic churchite, and hexagonal rhabdophane.¹⁵ The formation of the foregoing crystal structures are decided by the synthesis methodology and the nature of the metal cation.¹⁶ Some of the important rare earth phosphates that fit into the abovementioned profile are CePO₄, PrPO₄, YPO₄, LuPO₄, LaPO₄, GaPO₄, and EuPO₄. The physical, chemical, and optical properties of these materials were engineered by introducing a suitable substituent in the host lattice; hence the desired properties were achieved. Among various rare earth phosphates, yttrium phosphate (YPO₄) has versatile applications due to its sturdy nature and high chemical stability.¹⁷ It exists in two distinctive crystal structures, namely, hexagonal (D₂ symmetry) and tetragonal (D_2d symmetry).¹⁸ The tetragonal YPO₄ coexists with other elements in the mineral called xenotime,¹⁹ which belongs to the space group of I4₁/amd, wherein the Y³⁺ ion follows D_2d symmetry. The structure of YPO₄ is formed by chains of corner-shared dodecahedron YO₈ and tetrahedron PO₄.²⁰

YPO₄ is considered as one of the best host material to prepare luminescent compounds, since they acquire high quantum efficiency and because of the existence of explicit absorption and emission spectral regions.²¹ High absorption and high vacuum stability during the radiation process in the ultraviolet (UV) domain make them as an efficient source of red light in normal and plasma display process.²² Eu³⁺-doped YPO₄ was reported to have the aforementioned quality; hence the possibility of using the material as a red light-emissive source was proposed by Chanchan and Singh.²¹ Due to its high thermal expansion coefficient, it was explored as green barrier coatings to protect silicon ceramics. Ce³⁺ and Tb³⁺ co-doped yttrium phosphate was reported as potent glass ceramic material for WLED application due its adorable luminescent property on doping of a rare earth cation into yttrium lattice.²³ Dy³⁺/YPO₄ and Sm³⁺:YPO₄ co-doped with Bi³⁺ was reported as luminescent material with better emission intensity.^24,25

As it is widely accepted that the optical, magnetic, and other physical/chemical properties are greatly influenced by the preparation methodology, it is necessary to carry out a detailed research so as to select a suitable method of synthesis to achieve the desired properties (exclusive size, shape, particle distribution, and crystallinity) on the resultant material.²⁶ Water-borne methods are much preferred over organic solvent-based methods due their non-toxic nature. A lot of research is being undertaken to synthesize micro- and nano-structured YPO₄ with desired properties, from the traditional solid state reaction route to versatile wet chemical routes, including precipitation method, hydrothermal method, solvo-thermal methods, and ion-exchange reactions.^27,28 Among these methods, the chemical co-precipitation method has attracted much attention due to high production and purity of the desired product, use of less or no organic solvents, good reproducibility of the product, and low production cost. In a typical precipitation method, expected products are precipitated as their hydroxides using appropriate precipitating agents such as ammonium hydroxide, NaOH, and tetra methyl ammonium hydroxide from the metal precursors, following the calcination process. Hence, in the current research, the precipitation method is preferred.²⁹

Though YPO₄-based materials are studied for their luminescent property, it was not explored as a NIR-reflective pigment. To prove its application in another interesting field, herein, we investigated the NIR-reflective properties and color properties of YPO₄-, bismuth-, and vanadium-substituted YPO₄ synthesized by a simple precipitation method and demonstrated its excellence in cool coating applications.

2. Materials and Methods

2.1. Materials

Yttrium oxide (99%), di-ammonium hydrogen phosphate (98%), aqueous ammonia (NH₄OH), bismuth nitrate penta hydrate (99%), and ammonium meta vanadate (98%) are used for the synthesis. All the chemicals were purchased from S.D. Fine Chemicals Private Limited, India, and used without further purification.

2.2. Method of Synthesis

YP_(1–x)V_xO₄ (x = 0, 0.05, 0.1, 0.15, 0.2, and 0.4), Y_(1–y)Bi_yPO₄ (y = 0.1 and 0.3), and Y_(1–x)Bi_xP_(1–y)V_yO₄ (x = y = 0.2, 0.4, and 0.6) were prepared following the precipitation method. Details of the prepared samples and the corresponding sample are given in Table 1. To synthesize yttrium phosphate, 1 M of yttrium oxide was dissolved 1:1 HNO₃. To this, 1 M of di-ammonium hydrogen phosphate was added drop by drop, after which the pH of the resulting solution was adjusted to 4. Once the precipitation is completed, it was washed with distilled water, filtered, and dried in an oven at 100 °C. The dried precursor powder was ground well and subjected to calcinations at 300, 500, and 900 °C. To synthesize bismuth-substituted yttrium phosphate, bismuth nitrate solution was added along with yttrium nitrate solution and the abovementioned process was repeated. To synthesize vanadium-substituted materials, yellow ammonium meta vanadate solution which was pre-dissolved using 1:1 HNO₃ was added after adding ammonium phosphate solution.

Table 1. Prepared Samples and Its Acronyms.

acronym	composition
YP	YPO4
YV05	YP0.95V0.05O4
YV1	YP0.90V0.10O4
YV15	YP0.85V0.15O4
YV2	YP0.8V0.2O4
YV4	YP0.6V0.4O4
YB1	Y0.9Bi0.1PO4
YB3	Y0.7Bi0.3PO4
YB2V2	Y0.9Bi0.2P0.8V0.2O4
YB4V4	Y0.6Bi0.4P0.6V0.4O4
YB6V6	Y0.4Bi0.6P0.4V0.6O4

2.3. Chemical Stability Test and Thermal Stability Test

Concerning the stability of the pigment toward aggressive chemicals, 100 mg of specific pigment was soaked in 2% HNO₃, 2% H₂SO₄, 2% NaOH, and 2% HCl individually and stirred for an hour. Treated pigments were strained using filter paper, rinsed using distilled water, and air dried, and their weight was re-examined. Thermal stability of the particular pigment was studied by subjecting it to high calcination temperatures from 900 to 1100 °C for 6 h.

2.4. Characterization Techniques

The powder X-ray diffraction (PXRD) technique was employed to identify the phase formation of synthesized materials. Structural analysis was accomplished using the BRUKER D8 Advanced instrument via a Cu K alpha radiation with 1.54056 Å handling wavelength. The lattice parameter of the synthesized materials was calculated using powder X software. AlO₄, PO₄, and Al–O stretching and bending vibrations and the functional groups present in the fabricated pigment components were recognized with the Fourier transform infrared spectroscopy (FT-IR) technique in the 400 and 4000 cm^–1 range with the JASCO FT-IR instrument. The modification in the morphology due to substitution and the presence of elements were established using the ZESIS EVO18 scanning electron microscopy (SEM) instrument. HR-TEM (FEI-TECHNAI, G2-20 TWIN) with the operating voltage of 200 kV was employed to analyze the detailed morphology of the selected pigment particle, and the selected area diffraction pattern (SAED) was also employed with the same instrument. Absorbance and diffuse reflectance spectra were recorded using the JASCO-V670 UV-DRS spectrophotometer in the 200 to 2500 nm operating range with the step size of 1 nm. The color coordinates such as L* = lightness or darkness of the pigment (0–100), a* value representing the greenness (−a*) or redness (+a*), and b* values representing the yellowness (+b*) and blueness (−b*) of any synthesized material were calculated according to the recommendation by Commission Internationale de l’Eclairage (CIE) using D 65 illuminant through a 10° viewer angle. The amount of saturation is calculated with the term Chroma (C), which can be calculated by . The hue in the cylindrical color space extends from 0 to 360°, which could be found by the formula . NIR reflectance (R*) of the fabricated materials was found via the following formula

At this juncture, the reflectance acquired from the experimental data is noted as r(λ) and the solar irradiance is termed as i(λ) with the unit of (W/m²·nm).

3. Results and Discussion

3.1. TGA/DTA Analysis

Simultaneous TGA/DTA was carried out to identify the thermal properties of YPO₄. Two stages of weight losses were observed from thermogravimetric analysis, as shown in Figure 1. According to Figure 1, a total weight loss of 11.5% was observed after thermal treatment from ambient temperature to 1200 °C. Initial weight loss of about 8.7% was noticed from 100 to 400 °C, and further increase in temperature till 800 °C brought a slight weight loss of around 2.8%,³⁰ after which no significant weight loss was observed, which showed that the system is stable above 800 °C due to the loss of lattice water. The DTA curve showed an endothermic peak below 100 °C, which was attributed to the disappearance of water molecules and nitrates.

Figure 1.

TGA/DTA analysis of YPO₄.

3.2. PXRD Analysis

The PXRD technique was employed to identify the crystal structure, phase purity, lattice details, and the average crystallite size of the synthesized materials. Figure 2 shows the PXRD pattern of YPO₄. Peaks corresponding to 2θ angle observed are well harmonized with the standard structure (JCPDS. no. 74-2429) with the space group of I4₁/amd,³¹ and the lattice values are found to be a = b = 6.913, c = 6.174 with the average crystallite size of 34 nm. Table 1 shows the prepared pigments and its acronyms.

Figure 2.

PXRD pattern of YPO₄.

3.2.1. Effect of Vanadium Substitution on the Structure of YPO₄

Figure 3a shows the diffraction pattern of YP_1–xV_xO₄, x = 0.05, 0.1, 0.15, and 0.2 prepared by the chemical precipitation method. One could affirm the formation of the tetragonal structure after the substitution of V⁵⁺ into P⁵⁺ lattice with the shift in the diffraction peak to lower theta angle (Figure 3b). The shift is obvious due to the substitution of higher ionic radii (0.36 Å) V⁵⁺ with lower ionic radii (0.17 Å) P⁵⁺ having the co-ordination number of 4. Due to this substitution, lattice strain in the crystal structure occurs owing to inadequate ionic radii of P⁵⁺, which could not accommodate bigger V⁵⁺ in its lattice and possibly shifted the high intense peak in the direction of the lower theta angle.³² Further addition of V⁵⁺ (x = 0.4) leads to the formation of secondary phase V₂O₅ in the YPO₄ structure (Figure 4a). Comparable finding was discerned by Wujczyk et al.²⁰ In the abovementioned research, a series of luminescent materials were formed by doping Er³⁺ and Tm³⁺ co-doped with Yb³⁺ in YP_1–xV_xO₄ (x = 0–1.0). They concluded that lattice strain and shift in diffraction peaks toward the lower angle is due to V⁵⁺ replacement and not due to the other rare earth ions present.

Figure 3.

(a) YP_1–xV_xO₄, x = 0.05, 0.1, 0.15, and 0.2. (b) Expanded view.

Figure 4.

PXRD pattern of (a) YP_1–xV_xO₄ (x = 0, 0.2, and 0.4). (b) Y_1–xBi_xPO₄ (x = 0, 0.1, and 0.2).

3.2.2. Effect of Bismuth Substitution on the Structure of YPO₄

Figure 4b depicts the PXRD spectra of Y_1–yBi_yPO₄ (y = 0, 0.1, and 0.2). When y = 0.1, that is, Y_0.9Bi_0.1PO₄, we could observe the formation of the tetragonal crystal structure without the formation of secondary phase. However, at y = 0.2, Y_0.8Bi_0.2O₄, bismuth phosphate as BiPO₄ appeared as a secondary phase. Emergence of the secondary phase was noticed due to the substitution of larger Bi³⁺ (ionic radii of bismuth = 1.03 Å) into smaller Y³⁺ (ionic radii of yttrium = 0.9 Å) lattice, which leads to lattice expansion.³³ Higher loading of bismuth ion resulted in concoction of YPO₄ and BiPO₄ phases. Lower and higher substitution of bismuth into YPO₄ lattice did not result in the expected color; therefore, synthesis of this series was not continued. Table 2 shows 2 theta and relative peak intensity (R.P.I) values of YP_1–xV_xO₄, x = 0, 0.05, 0.1, 0.15, and 0.2) and Y_0.9Bi_0.1PO_4.

Table 2. 2 Theta and R.P.I Values of YP_1–xV_xO₄, x = 0, 0.05, 0.1, 0.15, and 0.2) and Y_0.9Bi_0.1PO₄.

YP		YV05		YV1		YV15		YV2		YB1
2θ	aR.P.I	2θ	aR.P.I	2θ	aR.P.I	2θ	aR.P.I	2θ	aR.P.I	2θ	aR.P.I
19.751	973	19.723	969	19.719	978	19.711	945	19.698	986	19.715	792
25.932	3465	25.927	2920	25.910	2858	25.865	2789	25.804	2788	25.807	2756
32.633	385	32.629	450	32.620	378	32.608	365	32.596	340	32.760	333
35.275	1757	35.268	1749	35.256	1787	35.289	1723	35.265	1748	35.065	984
37.138	583	37.136	580	37.128	575	37.121	563	37.117	598	36.913	612
39.867	488	39.765	476	39.667	497	39.546	465	39.446	472	39.804	447
42.323	790	42.312	795	42.309	778	42.324	765	42.319	758	42.237	588
47.103	537	47.97	528	47.94	549	47.925	556	47.893	543	47.086	497
50.294	530	50.281	514	50.276	528	50.286	535	50.283	546	50.178	497
51.911	1291	51.891	1285	51.880	1274	51.876	1269	51.860	1245	51.845	734
53.385	576	53.378	586	53.369	592	53.360	567	53.367	583	53.365	576
60.348	375	60.343	363	60.339	397	60.326	384	60.321	384	59.923	340
65.544	392	65.539	373	65.520	365	65.513	386	65.497	395	65.512	269
67.999	392	67.986	385	67.964	374	67.986	358	67.976	376	67.839	400
74.281	441	74.276	435	74.276	467	74.275	456	74.263	472	74.042	400

^aRelative peak intensity.

3.2.3. Effect of Simultaneous Substitution of Both V⁵⁺ and Bi³⁺ into YPO₄ Lattice

Figure 5 shows the PXRD spectra of Y_1–xBi_xP_1–yV_yO₄ (x = y = 0, 0.2, 0.4, and 0.6). Co-substitution of bismuth and vanadium in the YPO₄ lattice was proposed to improve its color properties. At x = y = 0.2, one could not notice any secondary phase formation. Higher substitution leads to the formation of mixed phases of YPO₄ and BiPO₄. Absence of V₂O₅ as the secondary phase is due to the possibility of iso-structural formation of YPO₄ and YVO₄.²⁹

Figure 5.

PXRD pattern of Y_1–xBi_xP_1–yV_yO₄ (x = y = 0, 0.2, 0.4, and 0.6).

3.3. Investigation on the FT-IR and Raman Spectra of Synthesized Materials

The interaction between the metal ions present (Y, V, P, Bi, and O) in the fabricated materials with oxide ion (O^2–) is identified by decoding the FT-IR spectra of the materials YPO₄, YP_0.8V_0.2O₄, Y_0.9Bi_0.1PO₄, and Y_0.8Bi_0.2P_0.8V_0.2O₄, as illustrated in Figure 6. In all the portrayed samples, two different significant vibrations were noticed at 637 and 518 cm^–1 corresponding to (PO₄)^3– phosphate moiety, due to distorted cockeyed vibrations. A peak at 987 cm^–1 is due to P–O stretching vibrations.³⁴⁻³⁶ An additional vibration at 835 and 800 cm^–1 was noticed for YV2 and YB2V2 due to the presence of VO₄^3–. A similar observation was noticed when Eu³⁺ and V⁵⁺ were doped into YPO₄ lattice.³⁴ The presence of metal oxide vibrations such as Bi–O and Y–O is noticed between 400 and 450 cm^–1.³⁷ The abovementioned results emphasized the formation of yttrium phosphate-, vanadium-, and bismuth-doped yttrium phosphates.

Figure 6.

FT-IR spectra of YPO₄, YP_0.8V_0.2O₄, Y_0.9Bi_0.1PO₄, Y_0.8Bi_0.2P_0.8V_0.2O₄.

Raman spectral analysis of Y_0.8Bi_0.2P_0.8V_0.2O₄ was carried out to identify the necessary vibration in the material, and the spectrum is depicted in Figure 7. As shown in Figure 7, the spectrum consists of various Raman vibrations that correspond to VO₄, PO₄, and Bi–O vibrations. The Raman shifts observed at 1011 cm^–1 refer to asymmetric stretching vibrations of the PO₄ group. The peaks at 501 cm^–1 corresponding to bending mode of the PO₄ group and the band at 196 cm^–1 could be due to O–Bi–O bending mode of vibrations.^38,39 VO₄ stretching vibrations are observed at 841 cm^–1.⁴⁰ Significant vibrations indicate the presence of metal oxide bonding, phosphate, and vanadate moiety in the YB2V2 material.

Figure 7.

Raman spectrum of Y_0.8Bi_0.2P_0.8V_0.2O₄.

3.4. X-ray Photoelectron Spectroscopy Analysis

To appreciate the chemical state of all the metal cations present in the prepared sample, XPS was carried out. Figure 8 depicts the XPS spectra of Y_0.4Bi_0.6P_0.4V_0.6O₄ (as a representation) synthesized by the chemical precipitation method. Standard C 1s binding energy was compared with C 1s energy obtained by the experiment, and the results are used to calibrate of binding energy of elements present in the system. The high resolution peak are fitted to show Y 3d, P 2p, V 2p, O 1s, and Bi 4f states present in the material. The valence energy state of metal ions is identified based on their binding energy locations. The Y 3d state was recognized by the presence of peaks at 157.17 and 158.88 eV corresponding to 3d_5/2 and 3d_3/2.⁴¹ Phosphorous in its +5 oxidation state was identified by the existence of a peak at 133.32 eV, suggesting the presence of the P 2p energy level of the cation. From the figure, one could confirm the presence of the V 2p state by observing the binding energy at 517 eV corresponding to the V 2p_1/2 level and 524.53 eV corresponding to the V 2p_3/2 level. Parhi and Manivannan⁴² reported the existence of the V 2p_1/2 energy level at 518.4 eV in YVO_4. A shift in the peak position to a lower energy level is noticed in the current investigation, which could be due to the substitution effect. The deconvoluted spectrum of oxygen also presented in the same illustration. Here, the O 1s core level is fitted by the presence of three binding energies centered at 531.64, 530.57, and 529.61 eV corresponding to the surface hydroxyl group, oxygen in the surface lattice, and the metal–oxygen bond present in the material.⁴³ Significant spin orbit doublets, namely, Bi 4f_5/2 and Bi 4f_7/2 were centered at 158.71 and 164.12 eV, which confirmed the presence of bismuth in its +3 oxidation state. These doublet peaks are further deconvoluted, resulting in four different binding energies found at 158.81, 158.29, 163.76, and 164.29 eV.⁴⁴

Figure 8.

XPS spectra of Y_0.4Bi_0.6P_0.4V_0.6O₄. (a) Y 3d, (b) P 2p, (c) V 2p and O 1s, (d) Bi 4f.

3.5. SEM, TEM, and EDAX Analyses

SEM micrographs of YPO₄, YP_0.8V_0.2O_4, Y_0.9Bi_0.1PO_4, and Y_0.4Bi_0.6P_0.4V_0.6O₄ are illustrated in Figure 9. The images clearly show that the particles are highly aggregated due to the rapid growth of particles during the precipitation reaction. This process of aggregation of nanoparticles begins at the stage of pre-nuclei formation after the dissolution of precursor materials. The nanoparticles are formed after the formation of a new boundary between the nuclei due to the free energy supplied to them. However, in some cases, there is a possibility of formation of pre-nucleation clusters, which leads to aggregation of small particles. Such aggregated particles are very stable and difficult to separate during mechanical grinding.⁴⁵ No significant change in the morphology was identified due to the introduction of vanadium and bismuth into tetragonal YPO₄, whereas co-substitution of both vanadium and bismuth in the YPO₄ lattice induced the change in morphology as a well-defined rectangular particle, which is clearly seen in Figure 9. The change in morphology could be due to the substitution effect.⁴⁶ The elemental composition obtained from EDAX analysis of selected elements is tabulated in Table 3, which confirmed the presence of Y, P, V, Al, Bi, and O in the currently reported materials. In order to arrive at a better conclusion about the morphology of the fabricated materials, TEM analysis was carried out. Figure 10 shows the TEM and EDAX images of Y_0.4Bi_0.6P_0.4V_0.6O₄. Rectangular-shaped particles with particle size ranging between 85 and 100 nm are obtained, which are in good agreement with the results obtained by PXRD. Dark spots from the SAED pattern indicates the crystalline nature of the sample.

Figure 9.

SEM images of (a) YPO₄, (b) YP_0.8V_0.2O₄, (c) Y_0.9Bi_0.1PO₄, and (d) Y_0.4Bi_0.6P_0.4V_0.6O₄ pigments.

Table 3. Elemental Composition of the Selected Materials Obtained from EDAX Analysis.

composition	element	weight %	atomic %
Y0.9Bi0.1PO4	Y	17.23	3.53
	Bi	3.81	0.33
	P	3.13	1.84
	O	83.44	94.96
	total	100
Y0.4Bi0.6P0.4V0.6O4	Y	14.28	3.47
	P	1.58	1.10
	Bi	12.14	1.26
	V	3.40	1.44
	O	68.60	92.73
	total	100

Figure 10.

TEM images and EDAX spectrum of the Y_0.4Bi_0.6P_0.4V_0.6O₄ pigment.

3.6. UV-DRS Analysis

Figure 11 shows the UV-DRS spectra of YPO₄, YP_1–xV_xO₄ (x = 0.05, 0.1, 0.15, 0.2, and 0.4), and Y_1–xBi_xP_1–yV_yO₄ (x = y = 0, 0.2, 0.4, and 0.6) prepared using the precipitation method. As mentioned in another study, YPO₄ shows a weak absorption peak between 230 and 330 nm due to charge transfer transition between the divalent oxygen anion (O₂^2–) and the pentavalent phosphorus cation (P⁵⁺),⁴⁷ and the color of material was found to be white. An additional transition between 350 and 470 nm was perceived because of the increase in vanadium content and due to the charge transfer between V⁵⁺–O^2– rather than d–d transition, since V⁵⁺ had a d⁰ electronic configuration.⁴⁸ As a result, yellow tinge was developed for the engineered materials. Bismuth and vanadium co-substituted materials slowly shifted the absorption maximum to higher wavelength, and it reached to 450 nm while substituting 60% of bismuth and vanadium in the YPO₄ lattice. Since 400–450 nm is a violet region, its complementary yellow color is observed. Here, introduction of vanadium in the phosphorous lattice produces VO₄^3– transition. Along with this VO₄^3– transition, a hybrid orbital transition is also possible from the mixture of Bi 6s and O 2p orbitals to V 3d orbitals.⁴⁹ The abovementioned phenomenon is responsible for the intense yellow color of the co-doped materials.

Figure 11.

UV-DRS spectrum of YPO₄, YP_1–xV_xO₄ (x = 0.05, 0.1, 0.15, 0.2, and 0.4), and Y_1–xBi_xP_1–yV_yO₄ (x = y = 0, 0.2, 0.4, and 0.6).

The digital images of vanadium-substituted YPO₄ and bismuth and vanadium co-substituted YPO₄ are presented in Figures 12 and 13, respectively, and their corresponding color values are tabulated in Table 4. When x = 0, a negative b* (−1.29) value was obtained, confirming their white color. Substitution of bismuth into yttrium lattice does not significantly affect the color; hence they are white, which is further confirmed by its corresponding color values. Substitution of 0.2 M of vanadium slightly increased the yellow tinge of material to +15.71. 0.4 M of vanadium in the P⁵⁺ increased the yellowness of the material to +26.6 with a slight decrease in lightness of the material. Here, the amount of V⁵⁺ content was doubled from 0.2 to 0.4 M so as to increase the yellow hue of the pigment. Further addition of V⁵⁺ in the YPO₄ structure did not increase the b* value. Simultaneous addition of both bismuth and vanadium in the YPO₄ steadily increased the yellowness value from +26.16 to +56.06 (Table 4). The hue angle represents the color of the system in terms of angular position in the cylindrical color space. It is observed from the literature, if the hue angle was found to be between 70 and 105°, the material will exhibit yellow color. All the optically engineered materials in the current study are having the hue angle between 70 and 90°, which confirmed the yellow color of the pigments.

Figure 12.

Digital images of YPO₄, YP_1–xV_xO₄ (x = 0.2 and 0.4), and Y_1–xBi_xPO₄ (x = 0.1, and 0.3).

Figure 13.

Digital images of Y_1–xBi_xP_1–yV_yO₄ (x = y = 0, 0.2, 0.4, and 0.6).

Table 4. Color Properties and NIR Solar Reflectance of Synthesized Compounds.

	color co-ordinates
compounds	L	a*	b*	C*	h°	NIR solar reflectance R* (%)
YPO4	95.09	0.56	–1.29	1.41	26.53	68
Y0.9Bi0.1PO4	96.23	2.00	1.26	1.72	32.21	81
Y0.7Bi0.3PO4	94.78	1.78	1.90	1.89	46.86	65
YP0.8V0.2O4	95.97	–1.02	5.71	5.23	77.96	78
YP0.6V0.4O4	89.36	–2.86	25.62	26.16	87.48	64
Y0.8Bi0.2P0.8V0.2O4	85	4.20	30.14	31.54	82.06	72
Y0.6Bi0.4P0.6V0.4O4	91.35	–5.81	30.55	35.76	87.68	82
Y0.4Bi0.6P0.4V0.6O4	81.36	1.87	56.06	60.25	88.08	83

3.7. NIR Reflectance Analysis

Figures 14 and 15 show the NIR reflectance of YP_1–xV_xO₄ (x = 0, 0.2, and 0.4) and Y_1–xBi_xP_1–yV_yO₄ (x = y = 0, 0.2, 0.4, and 0.6). The NIR solar reflectance of white YPO₄ in the 800–2500 nm NIR region was found to be 68%. Significant increase in the NIR reflectance was observed after the addition of 0.2 M of V⁵⁺ ion in the YPO₄ structure. Nearly 10% increase in the reflectance was noticed; hence, the NIR reflectance increased from 68 to 78%. Further addition of V⁵⁺, that is, 0.4 M addition of vanadium content significantly reduced the NIR reflectance from 68 to 64%. Concurrent addition of both bismuth and vanadium in the YPO₄ structure increased the NIR reflectance of the hybrid material gradually and it reached to 82% for Y_0.4Bi_0.6P_0.4V_0.6O₄. The reason for the increase in reflectance of material is explained due to the morphology change that occurred during co-substitution of both bismuth and vanadium in the YPO₄ structure. According to the Kubelka–Munk (KM) theory, the mean particle size (d) is inversely related to the scattering coefficient (S). Hence, if the mean particle size is less (d < 1 μm), the scattering efficiency of the material increases, which in turn increases the reflectance.⁵⁰ In the current study, the co-substituted materials have the least mean particle size of 98 nm (YB6V6), 83 nm (YB4V4), 112 nm (YB2V2) compared to vanadium-substituted YPO₄ materials. According to KM theory, more regular particles with a definite morphology are also important for high NIR reflectance. Rectangular-shaped particles with more regular distribution could be one of the important reasons for high reflectance of YB6V6. The abovementioned result was supported by Tao et al., who stated that the high reflectance could be due to the uniformed spherical particles and low particle size.⁵¹

Figure 14.

NIR solar reflectance spectrum of YP_1–xV_xO₄ (x = 0, 0.2, and 0.4); reflectance spectra are provided in the inset.

Figure 15.

NIR solar reflectance spectrum of Y_1–xBi_xP_1–yV_yO₄ (x = y = 0, 0.2, 0.4, and 0.6).

3.8. Chemical and Thermal Stability Test

Table 5 shows the CIE Lab color values of Y_0.4Bi_0.6P_0.4V_0.6O₄ after it was subjected to chemical and thermal treatment. From the results, one could observe very less change in the b* value after treating with 2% acid and base medium and a negligible amount of weight loss, indicating that they are chemically stable and can be used as metallic coating materials. To check the suitability of the material for the application in ceramic and glaze substances, it was calcined to higher temperatures from 900 to 1000 °C and 1200 °C. As the temperature increases, we could observe that the b* value increases from 56.06 to 70.64, which shows that the yellowness value increases as the temperature increases.

Table 5. Color Values of Y_0.4Bi_0.6P_0.4V_0.6O₄ after Chemical and Thermal Treatment.

	color parameters
2% acid or alkali	L*	a*	b*
	81.36	1.87	56.06
NaOH	82.40	1.73	55.93
HNO3	83.98	1.69	54.82
HCl	81.28	1.85	55.92
H2SO4	81.40	1.89	56.04
Thermal Stability
900 °C	81.36	1.87	56.06
1000 °C	78.46	1.12	63.49
1200 °C	50.65	0.65	70.64

4. Conclusions

Tetragonal YPO₄-, vanadium-, and bismuth-substituted YPO₄ and Bi and V co-substituted YPO₄ are prepared by a simple precipitation method and authenticated by PXRD, FTIR, and Raman spectroscopy. From XRD, we could notice that 0.4 M of vanadium substitution into the YPO₄ structure formed a secondary phase structure V₂O₅, whereas the color of the material changed from white to yellow. Substitution of bismuth into the YPO₄ structure does not increase the b* value, which proved that the bismuth ion does not play a significant role in the formation of yellow color of the pigment, while influence of vanadium in the development of yellow color is noticed. On the other hand, co-substitution of both vanadium and bismuth in the YPO₄ lattice enhanced the b* value and NIR reflectance. Y_0.4Bi_0.6P_0.4V_0.6O₄ showed 83% NIR solar reflectance, and its b* value was also found to be higher (56.06). The reason for higher reflectance could be due to the formation of more uniform particles, which was confirmed by TEM images. The fabricated material was found to be stable while treating with 2% H₂SO₄. Higher thermal treatments (subjected to calcination at 1000 and 1200 °C) increased the b* value of YB6V6 to 63.49 and 70.64. Hence, with higher b* and NIR solar reflectance, Y_0.4Bi_0.6P_0.4V_0.6O could serve as an environmentally benign cool yellow pigment.

The authors declare no competing financial interest.