Novel environment friendly inorganic red pigments based on Bi₄V₂O₁₁

Abstract

Novel environment friendly inorganic red pigments, (Bi_1−x−yZr_xAl_y)₄V₂O_11+δ (0 ≤ x ≤ 0.15; 0 ≤ y ≤ 0.10), were successfully synthesized by a conventional solid state reaction method, and their colour properties were characterized. The colour of the obtained pigments was dependent on the calcination condition and the sample composition, and the highest red hue was obtained for the (Bi_0.92Zr_0.07Al_0.01)₄V₂O_11.34 pigment calcined at 800 °C for 10 h in a flow of pure O₂. The a* value corresponding to the red chromaticity of this pigment was +41.9, which was significantly greater than that of the commercial iron oxide pigment (a* = +28.9). Bi₄V₂O₁₁ is considered to be a nontoxic compound, and the other components (Zr and Al) are also safe elements. Therefore, the present pigment could be an attractive candidate as a novel environment friendly inorganic red pigment.

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Introduction

Inorganic pigments are used for a wide variety of products such as paints, inks, plastics, ceramic tiles, and porcelains.¹ Among the inorganic pigments with various colours, red ones are especially in demand due to their high visibility. Several red pigments such as cadmium red (CdS·CdSe), mercuric sulfide red (HgS), and lead oxide red (Pb₃O₄ or PbO·Pb₂O₃) have been popularly used as industrial inorganic red materials for a long period of time, because these compounds can exhibit a brilliant red colour. However, the use of these materials has been restricted because they contain toxic elements (Cd, Se, Hg, and Pb), which are harmful to human health and the environment. On the other hand, iron oxide red (Fe₂O₃) are still widely used for many applications, since this material is environmental friendly. Unfortunately, its colour is insufficient compared with those of the conventional toxic ones. Although a number of inorganic compounds have been reported as environmental friendly inorganic red pigments, the redness of these pigments still cannot exceed that of the iron oxide red.^2–10 Perovskite-type oxynitrides, Ca_(1−x)La_xTaO_(2−x)N_(1+x), have also been reported as nontoxic red pigments, but toxic and flammable ammonia gas is required for the synthesis process.¹¹ In addition, these oxynitride compounds are decomposed to oxides at 420 °C and higher temperatures,¹¹ accompanying with generation of toxic nitrogen oxides.

In our previous studies, bismuth oxide (Bi₂O₃) was chosen as a base material because this compound has already been reported to be nontoxic and insoluble.¹² Although Bi₂O₃ is a pale yellow compound, its colour was altered to be reddish by the control of the lattice size and the band structure by doping Er³⁺, Y³⁺, and Fe³⁺ ions into the Bi₂O₃ lattice.^13,14 Among them, a ((Bi_0.72Er_0.04Y_0.24)_0.80Fe_0.20)₂O₃ pigment exhibited the most brilliant red hue (a* = +33.1 in the CIE L*a*b* system).¹⁴ The redness value of this pigment surpassed that of a commercially available Fe₂O₃ pigment (a* = +28.9), but the difference was small. Accordingly, it is highly required to synthesize new red pigments with more reddish hue.

Because of this situation, we focused on bismuth vanadate (Bi₄V₂O₁₁) with orthorhombic structure (space group: Aba2)^15,16 as a new base material. The structure of the orthorhombic Bi₄V₂O₁₁ consists of Bi₂O₂ layers interleaved with vanadium oxide sheets, which are composed of edge-shared VO₄ and VO₆ units. However, half of the VO₆ units contain an oxide anion deficiency, and accordingly, the VO₄ layer acts as VO_3.5.¹⁶ The colouring mechanism of the Bi₄V₂O₁₁ is based on a charge transfer transition from a hybrid Bi_6s–O_2p orbital to V_3d orbital in the band structure. Therefore, it is expected that the colour of the Bi₄V₂O₁₁ can be tuned by introducing other elements into the lattice to control the lattice size, because the extent of the orbital hybridization in the Bi_6s–O_2p valence band should depend on the interionic distance between Bi³⁺ and O²⁻.

In our previous works, it has been elucidated that lattice shrinkage positively affects the colour properties by modulating the width of valence band, whereas, an excess amount of oxide anion vacancies gives a negative effect on the colour properties.^17–19 In accordance with these facts, in the present study, oxide anion deficiencies in the Bi₄V₂O₁₁ lattice were filled by the calcination in a flow of pure oxygen, accompanying with the oxidation state change of a part of bismuth ions from Bi³⁺ to Bi⁵⁺. Furthermore, smaller Zr⁴⁺ (ionic radius: 0.072 nm)²⁰ and/or Al³⁺ (ionic radius: 0.0535 nm)²⁰ ions were doped into the Bi³⁺ (ionic radius: 0.103 nm)²⁰ and Bi⁵⁺ (ionic radius: 0.076 nm)²⁰ sites of the lattice. In this study, therefore, novel (Bi_1−x−yZr_xAl_y)₄V₂O_11+δ (0 ≤ x ≤ 0.15; 0 ≤ y ≤ 0.10) red pigments were synthesized as advanced environmental friendly red materials. The colour properties of the samples were characterized, and the doping effects of Zr⁴⁺ and Al³⁺ were revealed.

Experimental

The (Bi_1−x−yZr_xAl_y)₄V₂O_11+δ (0 ≤ x ≤ 0.15; 0 ≤ y ≤ 0.10) pigments were synthesized using a conventional solid state reaction method. Bi₂O₃, ZrO₂, Al₂O₃, and V₂O₅ powders were mixed in a stoichiometric amount in an agate mortar. After mechanically mixed with a planetary-type ball-milling apparatus (PULVERISETTE 7, Fritsch GmbH), the homogenous mixture was calcined at 800 °C for 10 h in a flow of air or pure oxygen. Before characterization, all samples were gently ground in an agate mortar.

The crystal structures of the samples were identified by X-ray powder diffraction (XRD; Rigaku SmartLab) using Cu-Kα radiation (40 kV, 30 mA). The compositions of the samples were analyzed by X-ray fluorescence spectrometer (XRF, Rigaku Supermini200), and it was confirmed that they were in good agreement with the theoretical values. For some representative samples, quantitative analysis of oxygen was performed with an Oxygen/Nitrogen/Hydrogen Analyser (EMGA-930, HORIBA, Japan). X-ray photoelectron spectroscopy (XPS; ULVAC PHI5000 Versa Probe II) was performed at room temperature with Al-Kα radiation (1486.6 eV) without Ar⁺ ion etching. The effect of charging on the binding energies was corrected with respect to the C 1s peak at 284.6 eV. Particle morphology and size distribution were examined using scanning electron microscopy (SEM; Shimadzu, SS-550). The size distribution and average particle size were estimated by measuring the diameters of 200 particles from the SEM micrographs.

Bi L_III XANES (X-ray absorption near-edge structure) spectra of the samples were measured in the transmission mode using the BL-11 beam line at SAGA Light Source (SAGA-LS). The synchrotron radiation was monochromized with a Si (111) double-crystal monochromator. The I₀ and I ionization chambers were filled with N₂ 85%–Ar 15% mixture gas and pure Ar gas, respectively. XANES spectrum of α-Bi₂O₃ was also measured as a standard material.

Optical reflectance spectra of the samples were measured using a UV-vis spectrometer (Shimadzu UV-2600) with barium sulfate as a reference. The bandgap energies of the samples were determined from the absorption edge of the absorbance spectra represented by the Kubelka–Munk function, f(R) = (1 − R)²/2R, where R is reflectance.^21,22 The colour properties of the samples were evaluated in terms of CIE L*a*b*CH° system with a chromometer (Konica-Minolta CR-300). In this system, the parameter L* represents the brightness or darkness of a colour relative to a neutral grey scale, while the parameters a* (the red-green axis) and b* (the yellow-blue axis) qualitatively express the colour. Chroma parameter (C) represents the colour saturation of the pigments and is calculated according to the following formula: C = [(a*)² + (b*)²]^1/2. The parameter H° ranges from 0 to 360°, and is calculated with the formula, H° = tan⁻¹(b*/a*) (H° = 0, for pure red). To evaluate the humidity resistance of the samples, a representative sample was heated at 100 °C for 48 h in a flow of water vapour saturated air atmosphere at 50 °C, and the colour property was evaluated after this treatment using the chromometer.

Results and discussion

Bi₄V₂O₁₁

Fig. 1 shows the XRD patterns of the Bi₄V₂O₁₁ samples, which were synthesized in a flow of air and pure O₂. Hereafter, the former and the latter samples are denoted as Bi₄V₂O₁₁ (air) and Bi₄V₂O₁₁ (O₂), respectively. The orthorhombic Bi₄V₂O₁₁ structure was obtained in a single-phase form for both samples. A weak peak at around 2θ = 24° was assigned to the superlattice structure of the orthorhombic Bi₄V₂O₁₁.¹⁶ The lattice parameters (a, b, c, and V) calculated from the diffraction angle in the XRD patterns are listed in Table 1. The lattice parameters (a, b, c, and V) of Bi₄V₂O₁₁ (O₂) were smaller than those of Bi₄V₂O₁₁ (air). These results suggest that pentavalent Bi⁵⁺ ions were generated in the lattice of Bi₄V₂O₁₁ (O₂) by oxidization of trivalent Bi³⁺ ions, because the ionic radius of Bi⁵⁺ (0.076 nm)²⁰ is smaller than that of Bi³⁺ (0.103 nm).²⁰

Fig. 1 XRD patterns of the Bi₄V₂O₁₁ samples.

Table 1 The orthorhombic lattice parameters (a, b, c, and V) of the samples

Samples	a/nm	b/nm	c/nm	V/nm^3
Bi4V2O11 (O2)	0.5604	1.5257	0.5519	0.4719
Bi4V2O11 (air)	0.5606	1.5263	0.5528	0.4729

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The valence states of the chemical species on near the surface of the two samples were identified by XPS analysis. Fig. 2 illustrates the XPS of the Bi 4f, V 2p, and O 1s core-levels for the samples. The peak shape and binding energies were obviously depended on the calcination atmosphere. In the case of the Bi 4f spectrum of Bi₄V₂O₁₁ (air), the 4f_5/2 and 4f_7/2 peaks located at 164.1 eV and 158.8 eV can be assigned to those of trivalent Bi³⁺.^23,24 For Bi₄V₂O₁₁ (O₂), however, the additional shoulder peaks were observed at 165.8 eV and 160.5 eV corresponding to those of pentavalent Bi⁵⁺.²⁴

Fig. 2 XPS of the Bi 4f (a), V 2p and O 1s (b) core-levels for the samples.

On the other hand, in the cases of V 2p and O 1s for Bi₄V₂O₁₁ (air), the 2p_1/2 and 2p_3/2 peaks located at 524.1 eV and 516.4 eV can be attributed to those of pentavalent V⁵⁺,²⁵ and the 1s peak at 529.7 eV can be assigned to the lattice oxygen.²⁵ In the spectrum of Bi₄V₂O₁₁ (O₂), however, some additional peaks appeared: a shoulder peak located at 531.3 eV (O 1s′) can be attributed to an excess amount of oxygen species introduced into the lattice,²⁵ and a shoulder peak located at 517.9 eV (V⁵⁺ (2p_3/2)′) is considered to be attributable to the excessive oxygen species.

From the above results, it is revealed that partial oxidization of Bi³⁺ to Bi⁵⁺ occurred, due to the calcination of Bi₄V₂O₁₁ in a flow of pure O₂. As a result, oxide anions were introduced by the charge compensation mechanism into the oxide anion deficiencies, which originally exist in the Bi₄V₂O₁₁ lattice. Accordingly, it is precise to express Bi₄V₂O₁₁ (air) and Bi₄V₂O₁₁ (O₂) as Bi₄V₂O₁₁ and Bi₄V₂O_11+δ′, respectively.

Fig. 3 depicts the UV-vis diffuse reflectance spectra for Bi₄V₂O₁₁ and Bi₄V₂O_11+δ′. Between the two samples, Bi₄V₂O_11+δ′ reflected the red light (605–780 nm) more effectively. Table 2 summarizes the CIE L*a*b* colour coordinate data and bandgap energies (E_g) for the samples. The a* value corresponding to red chromaticity for Bi₄V₂O_11+δ′ (a* = +37.1) was by about 10 points higher than that of Bi₄V₂O₁₁ (a* = +27.3).

Fig. 3 UV-vis diffuse reflectance spectra for Bi₄V₂O₁₁ and Bi₄V₂O_11+δ′.

Table 2 CIE L*a*b* colour coordinate data and bandgap energies (E_g) for the samples

Samples	L*	a*	b*	Eg/eV
Bi4V2O11+δ′	47.3	+37.1	+36.6	2.20
Bi4V2O11	43.5	+27.3	+29.3	2.15

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As already mentioned in the XPS analysis, pentavalent Bi⁵⁺ ions were partially generated in the Bi₄V₂O_11+δ′ sample, and the amount of the oxide anion deficiencies was decreased. Consequently, the impurity energy levels originated in these oxide anion deficiencies in the band structure were partially eliminated from the Bi₄V₂O_11+δ′ sample. Therefore, the reflection in the red light increased for Bi₄V₂O_11+δ′, leading to the significant increase in the redness (a*), although the bandgap energy of this sample was slightly larger than that of Bi₄V₂O₁₁, as seen in Table 2. Based on these results, all samples were synthesized in a flow of pure O₂ in the following sections.

(Bi_1−xZr_x)₄V₂O_11+δ1 (0 ≤ x ≤ 0.15)

XRD patterns of the (Bi_1−xZr_x)₄V₂O_11+δ1 (0 ≤ x ≤ 0.15) samples are shown in Fig. 4. For the (Bi_1−xZr_x)₄V₂O_11+δ1 (0 ≤ x ≤ 0.08) samples, the orthorhombic Bi₄V₂O₁₁ structure was obtained in a single phase form, and there were no diffraction peaks of impurity phases. On the other hand, a monoclinic BiVO₄ structure was observed as a secondary phase for the (Bi_1−xZr_x)₄V₂O_11+δ1 (0.10 ≤ x ≤ 0.15) samples. Table 3 lists the orthorhombic Bi₄V₂O₁₁ lattice parameters (a, b, c, and V) for the (Bi_1−xZr_x)₄V₂O_11+δ1 (0 ≤ x ≤ 0.15) samples. The lattice parameters (a, b, c, and V) linearly decreased with increasing Zr⁴⁺ content, in the single-phase region (0 ≤ x ≤ 0.08), because the ionic radius of Zr⁴⁺ (0.072 nm)²⁰ is smaller than those of Bi⁵⁺ (0.076 nm)²⁰ and Bi³⁺ (0.103 nm).²⁰ However, the lattice parameters (a, b, c, and V) became approximately constant in the two-phase region (0.10 ≤ x ≤ 0.15). These results indicated the formation of solid solutions with a single-phase orthorhombic Bi₄V₂O₁₁ structure, and the solubility limit of Zr⁴⁺ ions was about 8 mol% in the (Bi_1−xZr_x)₄V₂O_11+δ1 system.

Fig. 4 XRD patterns of the (Bi_1−xZr_x)₄V₂O_11+δ1 (0 ≤ x ≤ 0.15) samples.

Table 3 The orthorhombic lattice parameters (a, b, c, and V) of the samples

Samples	a/nm	b/nm	c/nm	V/nm^3
Bi4V2O11+δ′	0.5604	1.5257	0.5519	0.4719
(Bi0.95Zr0.05)4V2O11+δ1a	0.5585	1.5244	0.5510	0.4690
(Bi0.92Zr0.08)4V2O11+δ1b	0.5580	1.5240	0.5508	0.4686
(Bi0.90Zr0.10)4V2O11+δ1c	0.5581	1.5238	0.5506	0.4684
(Bi0.85Zr0.15)4V2O11+δ1d	0.5582	1.5239	0.5507	0.4685

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Fig. 5 shows the UV-vis diffuse reflectance spectra for the (Bi_1−xZr_x)₄V₂O_11+δ1 (0 ≤ x ≤ 0.15) samples. To realize a red pigment, it is important to satisfy both high reflection in the red light region (605–780 nm) and effective absorption (i.e., low reflection) in the green light region (490–560 nm), because green and red are the complementary colours. For all samples, strong reflection in the red light region (605–780 nm) and effective absorption in the green light region (490–560 nm) were observed, and, therefore, these samples are red. Among the single-phase samples of (Bi_1−xZr_x)₄V₂O_11+δ1 (0 ≤ x ≤ 0.08), (Bi_0.92Zr_0.08)₄V₂O_11+δ1b reflected the red light most effectively. In the two-phase region (0.10 ≤ x ≤ 0.15), the green light absorption decreased, although further increase in the red light reflection was observed.

Fig. 5 UV-vis diffuse reflectance spectra of the (Bi_1−xZr_x)₄V₂O_11+δ1 (0 ≤ x ≤ 0.15) samples.

The CIE L*a*b* colour coordinate data and bandgap energies (E_g) for the (Bi_1−xZr_x)₄V₂O_11+δ1 (0 ≤ x ≤ 0.15) samples are listed in Table 4. The a* value, which corresponds to red chromaticity in the positive direction, increased with increasing the content of Zr⁴⁺ ions in the single phase region (0 ≤ x ≤ 0.08), and, the (Bi_0.92Zr_0.08)₄V₂O_11+δ1b sample exhibited the highest a* value at +40.5. However, the redness (a*) decreased in the two-phase region (0.10 ≤ x ≤ 0.15), due to the formation of the impurity monoclinic BiVO₄ phase, which exhibits yellow hue.^17–19

Table 4 The CIE L*a*b* colour coordinate data and bandgap energies (E_g) of the (Bi_1−xZr_x)₄V₂O_11+δ1 (0 ≤ x ≤ 0.15) samples

Samples	L*	a*	b*	Eg/eV
Bi4V2O11+δ′	47.3	+37.1	+36.6	2.20
(Bi0.95Zr0.05)4V2O11+δ1a	49.4	+39.7	+40.6	2.19
(Bi0.92Zr0.08)4V2O11+δ1b	49.6	+40.5	+41.1	2.18
(Bi0.90Zr0.10)4V2O11+δ1c	50.1	+40.4	+42.5	2.19
(Bi0.85Zr0.15)4V2O11+δ1d	52.1	+38.9	+44.5	2.33

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(Bi_1−yAl_y)₄V₂O_11+δ3 (0 ≤ y ≤ 0.10)

Fig. 6 depicts the XRD patterns of the (Bi_1−yAl_y)₄V₂O_11+δ3 (0 ≤ y ≤ 0.10) samples. For the (Bi_1−yAl_y)₄V₂O_11+δ3 (0 ≤ y ≤ 0.05) samples, the orthorhombic Bi₄V₂O₁₁ phase was observed and there were no extra lines due to other compounds or impurities. In contrast, for the samples in which the Al³⁺ content was more than 5 mol%, the monoclinic BiVO₄ structure was observed as an impurity phase, in addition to the main orthorhombic Bi₄V₂O₁₁ phase. The orthorhombic Bi₄V₂O₁₁ lattice parameters (a, b, c, and V) for the (Bi_1−yAl_y)₄V₂O_11+δ3 (0 ≤ y ≤ 0.10) samples are tabulated in Table 5. The lattice parameters (a, b, c, and V) proportionally decreased with increasing the amount of Al³⁺ in the single-phase region (0 ≤ y ≤ 0.05), because the ionic radius of Al³⁺ (0.0535 nm)²⁰ is smaller than those of Bi⁵⁺ (0.076 nm)²⁰ and Bi³⁺ (0.103 nm).²⁰ However, the lattice parameters (a, b, c, and V) became nearly constant in the two-phase region (0.07 ≤ y ≤ 0.10). From these results, it is elucidated that solid solutions of the orthorhombic Bi₄V₂O₁₁ structure were successfully formed for the (Bi_1−yAl_y)₄V₂O_11+δ3 (0.03 ≤ y ≤ 0.05) samples, and the solubility limit of Al³⁺ ions was around 5 mol% in (Bi_1−yAl_y)₄V₂O_11+δ3. Furthermore, it is considered that the Al³⁺ ions were doped not into the trivalent Bi³⁺ site, but dominantly into the pentavalent Bi⁵⁺ site, because the ionic radius of Al³⁺ (0.0535 nm)²⁰ is closer to that of Bi⁵⁺ (0.076 nm)²⁰ than that of Bi³⁺ (0.103 nm).²⁰

Fig. 6 XRD patterns of the (Bi_1−yAl_y)₄V₂O_11+δ3 (0 ≤ y ≤ 0.10) samples.

Table 5 The orthorhombic lattice parameters (a, b, c, and V) of the samples

Samples	a/nm	b/nm	c/nm	V/nm^3
Bi4V2O11+δ′	0.5604	1.5257	0.5519	0.4719
(Bi0.97Al0.03)4V2O11+δ3a	0.5586	1.5229	0.5507	0.4681
(Bi0.95Al0.05)4V2O11+δ3b	0.5582	1.5217	0.5505	0.4678
(Bi0.93Al0.07)4V2O11+δ3c	0.5583	1.5219	0.5503	0.4679
(Bi0.90Al0.10)4V2O11+δ3d	0.5584	1.5214	0.5504	0.4677

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Fig. 7 illustrates the UV-vis diffuse reflectance spectra of the (Bi_1−yAl_y)₄V₂O_11+δ3 (0 ≤ y ≤ 0.10) samples. For (Bi_1−yAl_y)₄V₂O_11+δ3 (0 ≤ y ≤ 0.05), which were obtained in the single orthorhombic Bi₄V₂O₁₁ phase, the absorption in the green light region (490–560 nm) increased with increasing the content of Al³⁺. Among them, (Bi_0.97Al_0.03)₄V₂O_11+δ3a reflected the red light (605–780 nm) most effectively, although the absorption in the green light region (490–560 nm) of this sample was slightly weaker than that of (Bi_0.95Al_0.05)₄V₂O_11+δ3b.

Fig. 7 UV-vis diffuse reflectance spectra of the (Bi_1−yAl_y)₄V₂O_11+δ3 (0 ≤ y ≤ 0.10) samples.

The CIE L*a*b* colour coordinate data and bandgap energies (E_g) for the (Bi_1−yAl_y)₄V₂O_11+δ3 (0 ≤ y ≤ 0.10) samples are listed in Table 6. The a* value corresponding to the red chromaticity of these pigments depended on their compositions, and the highest redness was obtained for (Bi_0.97Al_0.03)₄V₂O_11+δ3a (a* = +41.6), as in the UV-vis diffuse reflectance measurement.

Table 6 The CIE L*a*b* colour coordinate data and bandgap energies (E_g) of the samples

Samples	L*	a*	b*	Eg/eV
Bi4V2O11+δ′	47.3	+37.1	+36.6	2.20
(Bi0.97Al0.03)4V2O11+δ3a	47.7	+41.6	+38.0	2.17
(Bi0.95Al0.05)4V2O11+δ3b	47.8	+40.6	+36.8	2.19
(Bi0.93Al0.07)4V2O11+δ3c	52.5	+40.2	+38.8	2.20
(Bi0.90Al0.10)4V2O11+δ3d	53.0	+36.9	+43.4	2.21

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(Bi_1−x−yZr_xAl_y)₄V₂O_11+δ2 (0.01 ≤ x ≤ 0.07; 0.01 ≤ y ≤ 0.07; x + y = 0.08)

As mentioned in the previous sections, dissolution of Zr⁴⁺ or Al³⁺ into the Bi₄V₂O_11+δ′ lattice is effective to increase the redness of the resulting pigments. In this section, therefore, Zr⁴⁺ and Al³⁺ were co-doped into the Bi₄V₂O_11+δ′ lattice and the composition was optimized. In this regard, the total amount of Zr⁴⁺ and Al³⁺ was kept constant at 8 mol%, because an excess amount of the dopants had negative effect on the red hue of the pigments by the formation of yellowish monoclinic BiVO₄ phase as an impurity.

XRD patterns of the (Bi_1−x−yZr_xAl_y)₄V₂O_11+δ2 (0.01 ≤ x ≤ 0.07; 0.01 ≤ y ≤ 0.07; x + y = 0.08) samples are shown in Fig. 8. The orthorhombic Bi₄V₂O₁₁ structure was obtained in a single-phase form for all samples. As shown in Table 7, the lattice parameters (a, b, c, and V) of the (Bi_1−x−yZr_xAl_y)₄V₂O_11+δ2 (0.01 ≤ x ≤ 0.07; 0.01 ≤ y ≤ 0.07; x + y = 0.08) samples became smaller than those of the non-doped Bi₄V₂O_11+δ′ sample. These results indicate the formation of solid solutions with the orthorhombic Bi₄V₂O₁₁ structure for all samples.

Fig. 8 XRD patterns of the (Bi_1−x−yZr_xAl_y)₄V₂O_11+δ2 (0.01 ≤ x ≤ 0.07; 0.01 ≤ y ≤ 0.07; x + y = 0.08) samples.

Table 7 The orthorhombic lattice parameters (a, b, c, and V) of the samples

Samples	a/nm	b/nm	c/nm	V/nm^3
Bi4V2O11+δ′	0.5604	1.5257	0.5519	0.4719
(Bi0.92Zr0.07Al0.01)4V2O11+δ2a	0.5587	1.5227	0.5506	0.4684
(Bi0.92Zr0.04Al0.04)4V2O11+δ2b	0.5586	1.5222	0.5505	0.4681
(Bi0.92Zr0.01Al0.07)4V2O11+δ2c	0.5585	1.5220	0.5503	0.4678

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Fig. 9 depicts the UV-vis diffuse reflectance spectra of the (Bi_1−x−yZr_xAl_y)₄V₂O_11+δ2 (0.01 ≤ x ≤ 0.07; 0.01 ≤ y ≤ 0.07; x + y = 0.08) samples. Both the reflection in the red light (605–780 nm) and the absorption in the green light (490–560 nm) were dependent on their compositions. Among these pigments, the (Bi_0.92Zr_0.07Al_0.01)₄V₂O_11+δ2a pigment reflected the red light (605–780 nm) most effectively.

Fig. 9 UV-vis diffuse reflectance spectra of the (Bi_1−x−yZr_xAl_y)₄V₂O_11+δ2 (0.01 ≤ x ≤ 0.07; 0.01 ≤ y ≤ 0.07; x + y = 0.08) samples.

Table 8 summarizes the CIE L*a*b*CH° colour coordinate data and bandgap energies (E_g) for the (Bi_1−x−yZr_xAl_y)₄V₂O_11+δ2 (0.01 ≤ x ≤ 0.07; 0.01 ≤ y ≤ 0.07; x + y = 0.08) samples. The a* value (red chromaticity) depended on their compositions, and the highest red hue was obtained for (Bi_0.92Zr_0.07Al_0.01)₄V₂O_11+δ2a and (Bi_0.92Zr_0.04Al_0.04)₄V₂O_11+δ2b, where a* = +41.9 for both samples. Furthermore, the hue angle (H°) corresponding to colour purity, for (Bi_0.92Zr_0.07Al_0.01)₄V₂O_11+δ2a (H° = 39.0) was smaller than that for (Bi_0.92Zr_0.04Al_0.04)₄V₂O_11+δ2b (H° = 40.0). Therefore, (Bi_0.92Zr_0.07Al_0.01)₄V₂O_11+δ2a produced the deepest red hue among these pigments.

Table 8 The CIE L*a*b*CH° colour coordinate data and bandgap energies (E_g) of the samples

Samples	L*	a*	b*	C	H°	Eg/eV
Bi4V2O11+δ′	47.3	+37.1	+36.6	52.1	44.6	2.20
(Bi0.92Zr0.07Al0.01)4V2O11+δ2a	49.6	+41.9	+34.0	53.9	39.0	2.18
(Bi0.92Zr0.04Al0.04)4V2O11+δ2b	49.7	+41.9	+35.2	54.7	40.0	2.18
(Bi0.92Zr0.01Al0.07)4V2O11+δ2c	51.2	+38.9	+43.2	58.1	48.0	2.19

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To investigate chemical states of bismuth species in the bulk for the samples, Bi L_III XANES spectra of non-doped Bi₄V₂O_11+δ′, (Bi_0.92Zr_0.08)₄V₂O_11+δ1b, and (Bi_0.97Al_0.03)₄V₂O_11+δ3a were measured as shown in Fig. 10. Bi L_III XANES spectra of α-Bi₂O₃ were also collected as the standard of trivalent Bi³⁺. In the spectra of non-doped Bi₄V₂O_11+δ′, (Bi_0.92Zr_0.08)₄V₂O_11+δ1b, and (Bi_0.97Al_0.03)₄V₂O_11+δ3a, a peak assigned to pentavalent Bi⁵⁺ ions was observed at 13 440 eV.²⁶ On the contrary, this peak was absent in the spectrum of α-Bi₂O₃, in which bismuth ions exist only in trivalent state. In addition, the absorption edge shifted to the higher energy side for these pigments, compared to that of α-Bi₂O₃. These results elucidate the existence of pentavalent Bi⁵⁺ ions in the bulk of non-doped Bi₄V₂O_11+δ′, (Bi_0.92Zr_0.08)₄V₂O_11+δ1b, and (Bi_0.97Al_0.03)₄V₂O_11+δ3a.

Fig. 10 Bi L_III XANES spectra of non-doped Bi₄V₂O_11+δ′, (Bi_0.92Zr_0.08)₄V₂O_11+δ1b, and (Bi_0.97Al_0.03)₄V₂O_11+δ3a.

Fig. 11 illustrates XPS of (Bi_0.92Zr_0.08)₄V₂O_11+δ1b, (Bi_0.97Al_0.03)₄V₂O_11+δ3a, and (Bi_0.92Zr_0.07Al_0.01)₄V₂O_11+δ2a. The spectrum of non-doped Bi₄V₂O_11+δ′ is also depicted as a reference. The proportion of pentavalent Bi⁵⁺ in total bismuth ions (Bi⁵⁺/(Bi⁵⁺ + Bi³⁺)) for these pigments was estimated from the area of corresponding peak in the spectra, and the results are tabulated in Table 9. The proportion of Bi⁵⁺ was 35% for (Bi_0.92Zr_0.08)₄V₂O_11+δ1b, while it was 26% for non-doped Bi₄V₂O_11+δ′. This result suggests that the amount of Bi⁵⁺ was increased by the Zr⁴⁺ doping into the bismuth sites of the Bi₄V₂O_11+δ′ lattice, and thereby, the oxide anion deficiencies were decreased in (Bi_0.92Zr_0.08)₄V₂O_11+δ1b, because of the reason already mentioned above. In the case of (Bi_0.97Al_0.03)₄V₂O_11+δ3a, the proportion of Bi⁵⁺ was 21%, and this value was smaller than that for non-doped Bi₄V₂O_11+δ′ (26%). The binding energies of Bi 4f_7/2 (159.6 eV) and Bi 4f_5/2 (164.9 eV) attributable to those of pentavalent Bi⁵⁺ were observed at lower energies than those of non-doped Bi₄V₂O_11+δ′: Bi 4f_7/2 (160.5 eV) and Bi 4f_5/2 (165.8 eV). These results indicate that significant lattice shrinkage was occurred in the (Bi_0.97Al_0.03)₄V₂O_11+δ3a lattice to induce significant electronic repulsion in the 4f orbital, even the 4f electrons are well shielded from the surroundings by the outer 5d and 6s orbitals. As a result, the conduction band composed of the V_3d orbital was also enlarged by the lattice shrinking to increase the crystal field splitting. In fact, the bandgap energy for this pigment was the smallest among the three samples. Furthermore, in the case of the (Bi_0.92Zr_0.07Al_0.01)₄V₂O_11+δ2a pigment, both of the increase in the proportion of Bi⁵⁺ and the decrease in the binding energies (Bi 4f_7/2 and Bi 4f_5/2) were confirmed.

Fig. 11 XPS of (Bi_0.92Zr_0.08)₄V₂O_11+δ1b, (Bi_0.97Al_0.03)₄V₂O_11+δ3a, and (Bi_0.92Zr_0.07Al_0.01)₄V₂O_11+δ2a.

Table 9 The proportion of pentavalent Bi⁵⁺ in total bismuth ions (Bi⁵⁺/(Bi⁵⁺ + Bi³⁺)) for the samples

Samples	Proportion of Bi^5+/%
Bi4V2O11+δ′	26
(Bi0.92Zr0.08)4V2O11+δ1b	35
(Bi0.97Al0.03)4V2O11+δ3a	21
(Bi0.92Zr0.07Al0.01)4V2O11+δ2a	30

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To determine the oxygen content in the representative samples, quantitative analysis of oxygen was performed for Bi₄V₂O₁₁, Bi₄V₂O_11+δ′, (Bi_0.92Zr_0.08)₄V₂O_11+δ1b, (Bi_0.97Al_0.03)₄V₂O_11+δ3a, and (Bi_0.92Zr_0.07Al_0.01)₄V₂O_11+δ2a samples, and the results are listed in Table 10. The oxygen amounts in the samples calcined in a flow of pure O₂ were higher than that of Bi₄V₂O₁₁ calcined in a flow of air. Taking account of the results in Fig. 11 and Table 9 into consideration, it was found that the oxide ion content monotonically increased with the proportion of Bi⁵⁺.

Table 10 The analysed compositions of the representative samples

Samples	Analysed compositions
Bi4V2O11	Bi4V2O11.00
Bi4V2O11+δ′	Bi4V2O11.17
(Bi0.92Zr0.08)4V2O11+δ1b	(Bi0.92Zr0.08)4V2O11.41
(Bi0.97Al0.03)4V2O11+δ3a	(Bi0.97Al0.03)4V2O11.14
(Bi0.92Zr0.07Al0.01)4V2O11+δ2a	(Bi0.92Zr0.07Al0.01)4V2O11.34

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Fig. 12 shows SEM images and size distributions of the Bi₄V₂O_11.17, (Bi_0.92Zr_0.08)₄V₂O_11.41, (Bi_0.97Al_0.03)₄V₂O_11.14, and (Bi_0.92Zr_0.07Al_0.01)₄V₂O_11.34 samples. Particles with indefinite shapes were observed for all samples, and the mean particle sizes of the Bi₄V₂O_11.17, (Bi_0.92Zr_0.08)₄V₂O_11.41, (Bi_0.97Al_0.03)₄V₂O_11.14, and (Bi_0.92Zr_0.07Al_0.01)₄V₂O_11.34 samples were 7.52, 7.20, 7.58, and 7.29 µm, respectively. There is no evident change in the mean particle sizes, size distributions, and morphologies of these samples. These results indicate that the change in the colour properties of these samples was caused by the doping elements, and the contribution from the particle size and morphology can be eliminated.

Fig. 12 SEM images and size distributions of the (a) Bi₄V₂O_11.17, (b) (Bi_0.92Zr_0.08)₄V₂O_11.41, (c) (Bi_0.97Al_0.03)₄V₂O_11.14, and (d) (Bi_0.92Zr_0.07Al_0.01)₄V₂O_11.34 pigments.

From the results mentioned above, the doping effect of Zr⁴⁺ and Al³⁺ on the red hue of the pigments can be concluded as follows.

Doping effect of Zr⁴⁺. Zr⁴⁺ doping into the bismuth sites in the Bi₄V₂O_11.17 lattice positively affects the colour property. The oxide anion deficiencies were occupied by the charge compensation mechanism, leading to the elimination of the impurity energy levels between the valence and the conduction bands in the band structure. However, the introduction of an excess amount of Zr⁴⁺ ions induces the formation of yellowish monoclinic BiVO₄ as an impurity phase to decrease the red hue.

Doping effect of Al³⁺. Dissolution of Al³⁺ reduces the bandgap energy of Bi₄V₂O_11.17 by broadening the width of V_3d conduction band, although it increases the amount of the oxide anion deficiencies (oxide anion vacancies). Accordingly, both positive and negative effects are produced simultaneously. It is considered that the positive effect is dominant when a small amount of Al³⁺ is doped into the lattice, but adversely the negative effect will become prominent when the amount of Al³⁺ is excessive.

Consequently, the synergetic effects were observed when both Zr⁴⁺ and Al³⁺ were doped into the Bi₄V₂O_11.17 lattice. As a result, the band structure was optimized at the composition of

Table 11 summarizes the CIE L*a*b*CH° colour coordinate data and bandgap energies (E_g) of the (Bi_0.92Zr_0.07Al_0.01)₄V₂O_11.34 pigment, the commercial cadmium selenide red (Holbein works, Ltd, PG002), commercial mercuric sulfide red (Holbein works, Ltd, PG009), and commercial iron oxide (Morishita Bengara Kogyo Co. Ltd, MR-320A) pigments. Although the redness value (a*) and the colour saturation parameter (C) for the (Bi_0.92Zr_0.07Al_0.01)₄V₂O_11.34 pigment (a* = +41.9, C = 53.9) fell short of those for the toxic cadmium selenide red (a* = +63.7, C = 84.6) and mercuric sulfide red (a* = +56.5, C = 69.5) pigments, they were significantly higher than those for the environmental friendly iron oxide pigment (a* = +28.9, C = 38.4). In addition, the brightness value (L*) of this pigment (L* = 49.6) was also larger than that of the iron oxide pigment (L* = 38.9), thus, the (Bi_0.92Zr_0.07Al_0.01)₄V₂O_11.34 pigment produced a light red hue.

Table 11 The CIE L*a*b*CH° colour coordinate data and bandgap energies (E_g) of the pigments

Pigments	L*	a*	b*	C	H°	Eg/eV
(Bi0.92Zr0.07Al0.01)4V2O11.34	49.6	+41.9	+34.0	53.9	39.0	2.18
Commercial cadmium selenide red (CdS·CdSe)	51.9	+63.7	+55.8	84.6	41.2	2.11
Commercial mercuric sulfide red (HgS)	52.0	+56.5	+40.5	69.5	35.6	2.13
Commercial iron oxide (Fe2O3)	38.9	+28.9	+25.3	38.4	41.2	2.21

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Furthermore, the (Bi_0.92Zr_0.07Al_0.01)₄V₂O_11.34 sample was heated at 100 °C for 48 h in a flow of the moist air, and the colour coordinate data were measured after this treatment to evaluate the humidity resistance. The colour coordinate data were measured and the colour difference (ΔE) was calculated from the data before and after the test using the following equation:

ΔE = [(L^*_after − L^*_before)² + (a^*_after − a^*_before)² + (b^*_after − b^*_before)²]^1/2

A small ΔE indicates that the colour of the pigment is stable. From the result listed in Table 12, it is evident that the present (Bi_0.92Zr_0.07Al_0.01)₄V₂O_11.34 pigment possesses high humidity resistance.

Table 12 The CIE L*a*b* colour difference (ΔE) of the (Bi_0.92Zr_0.07Al_0.01)₄V₂O_11.34 samples before and after the humidity resistance test

Samples	L*	a*	b*	ΔE
Before the test	49.6	+41.9	+34.0	—
After the test	49.9	+41.4	+37.6	3.6

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Conclusions

Novel environmental friendly red pigments, (Bi_1−x−yZr_xAl_y)₄V₂O_11+δ (0 ≤ x ≤ 0.15; 0 ≤ y ≤ 0.10), were successfully synthesized by the conventional solid state reaction method. The colour of these pigments depended on the calcination condition and the composition, and the most vivid red hue was obtained for the (Bi_0.92Zr_0.07Al_0.01)₄V₂O_11.34 pigment calcined at 800 °C for 10 h in a flow of pure O₂, which has CIE L*a*b* colour parameters of L* = 49.6, a* = +41.9, and b* = +34.0. The a* value, which corresponds to the redness, was significantly higher than that of a commercial iron oxide pigment (a* = +28.9). Since Bi₄V₂O₁₁ is considered to be a nontoxic compound, and Zr and Al are also safe elements, the present pigment should be an effective alternative for the conventional iron oxide red pigment.

Acknowledgements

The authors wish to thank Mr Akihiro Hirano and Mr Satoshi Yoshida, HORIBA, Ltd for the oxygen/nitrogen/hydrogen analysis. This work was supported by the Development of Alternative Technology for Hazardous Chemical Substances and Development of Novel Environment- and Human-friendly Inorganic Pigments for Three Primary Colors (FY2010-2014) programs of the New Energy and Industrial Technology Development Organization (NEDO) and the Ministry of Economy, Trade and Industry, Japan (METI).

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