Near-infrared reflectance and thermal insulating performance of Mo-doped Bi2WO6 with 3D hierarchical flower-like structure as novel ceramics pigment

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Near-infrared reflectance and thermal insulating performance of Mo-doped Bi2WO6 with 3D hierarchical flower-like structure as novel ceramics pigment Mengmeng Tian, Lingyun Yao, Aijun Han∗, Xufei Zhu, Chenlu Chen, Mingquan Ye, Xin Chen School of Chemical Engineering, Nanjing University of Science & Technology, Nanjing, 210094, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Mo-doped Bi2WO6 NIR solar reflectance Ceramic pigments Thermal conductivity Carrier concentration

The pigments with the formula of Bi2W1−xMoxO6 (x = 0.0, 0.1, 0.2, 0.3, 0.4) are synthesized by one step hydrothermal method. The pigments are characterized with XRD, FE-SEM, XPS, UV/VIS/NIR spectrometer, thermal conductivity and TG-DTA. The results reveal that the samples are of orthorhombic phase with no impurity phase existing and three-dimensional hierarchical flower-like architectures with diameter ranging from 2 to 3 μm. The color of the doped samples progressively transfers from pale yellow to yellow. The band gap decreases from 2.82 eV to 2.42 eV, which is attributed to the charge transfer excitation changes from Bi 6s → O 2p to the transition of Mo 3d → O 2p. As compared with the conventional pigments with similar color, the Modoped Bi2WO6 pigments perform better on the near-infrared solar reflectivity and the highest near-infrared solar reflectance is as high as 80.80%. The distinction of the NIR solar reflectance values among the pigments is majorly attributed to the different oxygen concentration induced by the reaction formula: Bi2Mo1-xWxO6 → δ Bi2Mo1-xW6+ x-δ(W6+·2e) δO6-δ + 2 O2 ↑.The thermal conductivity of some Mo-doped Bi2WO6 pigments is apparently lower than its pigment counterpart owning to the 3D hierarchical flower-like structure. The pigments are also possessing eminent thermal and chemical stability. The Mo- doped Bi2WO6 samples not only have high near-infrared reflectance with beautiful color but also have comparatively benign thermal insulating performance, so that they are promising to be served as novel amiable ceramic pigments to manipulate the thermal property of ceramic materials.

1. Introduction

phenomenon in conduction and valence bands [12]. Meanwhile, due to the high melting point, high wear resistance, oxidation resistance, wide sources of materials and unique biological and chemical properties, ceramic materials have a wide application in many fields. Thermal insulating ceramic materials can be applied in building coatings to reduce the heat accumulation on the building surface and thus cut down the utilization rate of the temperature regulating equipment [13–15]. The ceramic materials incorporated with NIR reflectance pigment can both have the merits of beautiful color and higher thermal insulating ability. Bismuth tungstate (Bi2WO6), one of the simplest aurivillius phase oxides, has a layered structure with the perovskite layer (WO4)2− lying between (Bi2O2)2+ layers [16]. It has attracted a great deal of attentions due to its excellent physical and chemical properties such as photocatalytic behavior, ferroelectric piezoelectricity and nonlinear dielectric susceptibility [17,18]. So far, the hydrothermal route is still the most commonly used technique to synthesize bismuth tungstate [18–20]. Meanwhile, Bi2WO6 have a variety morphologies, in which

The urbanization area over the global had expanded from 0.23% in 1992 to 0.53% in 2013, and it is undoubtedly that the urban heat island had become deteriorated due to the rapid urbanization [1,2]. With the exacerbation of the heat island effect, extra refrigeration energy consumption shots up [3]. The NIR radiation accounts for 52% of the energy in the solar radiance spectrum [4–6]. Thereupon, taking advantage of pigments with high near-infrared reflectance (NIR) and good thermal and chemical stabilities to replace the traditional non-environmental paints is one of the most efficient methods to ease the heat island effect through cutting down the metropolitan heat build-ups [7–9]. Incorporating high NIR colorants without toxicity into white pigments can resolve the light pollution problem of the white pigments as well as meet human aesthetic taste [10,11]. Doping transition metal ions into inorganic oxide pigment is facilitated to tune the color-rendering of materials by manipulating the energy gaps and delocalization

∗

Corresponding author. E-mail address: [email protected] (A. Han).

https://doi.org/10.1016/j.ceramint.2020.02.019 Received 29 November 2019; Received in revised form 14 January 2020; Accepted 2 February 2020 0272-8842/ © 2020 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Mengmeng Tian, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2020.02.019

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including flower-like [21], core-shell structure [22] and cubic structure [23]. However, the three-dimensional Bi2WO6 with specific hierarchical structures and high ordered characteristic has not been studied as NIR reflective pigments yet. Furthermore, the flower-like Bi2WO6 is more worthy to be studied owing to its narrow band gap (2.6–2.8 eV), large surface areas, abundant meso-pores with ordered open pore frameworks, and simultaneously possessing the properties of both micron and nano materials [18,24]. The incorporation of the transition ion Mo6+ into the Bi2WO6 lattice is favorable to manages the color and the NIR absorption ability by introduce a novel charge transfer excitation form. Similarly, though the photocatalytic performance of Mo doped Bi2WO6 material has been studied extensively, yet the NIR reflectance and the chrominance properties studies are still rare in current literatures. In this work, a series of novel Bi2Mo1-xWxO6 (x = 0.0, 0.1, 0.2, 0.3, 0.4) pigments were projected to be synthesized by hydrothermal method. The crystal structure, morphology, chromaticity, NIR reflective properties, thermal conductivity and the thermal stability of the pigments would be characterized and analyzed. Finally, the possibility of the Mo-doped Bi2WO6 pigments as a new a NIR reflective and ceramic thermal insulating pigment to replace the convention pigments with similar color would be analyzed.

700–2500 nm region can be computed by the formula below:

R∗ =

2500 ∫700 r (λ) i (λ)d(λ) 2500 ∫700 i (λ)d(λ)

(2)

where r(λ) denotes the spectral reflectance (W m−2) obtained from the UV/VIS/NIR spectrometer and i(λ) denotes the solar spectral irradiance (W m−2 nm−1) obtained from ASTM standard G173-03 [25,26]. The original data (X, Y, Z) of the chromatic coordinate for the samples were collected by a colorimeter (Konica-Minolta, CM-2500d) with an integrating sphere attachment. On the basis of the 1931 CIE Standard Source C (illuminant C (0.3101, 0.3162)), the chromatic coordinates (x, y) can be obtained by means of the equations below:

x=

X X+Y+Z

(3)

y=

Y X+Y+Z

(4)

X=

∫ x‾ (λ) s (λ) dλ

(5)

Y=

∫ y‾ (λ) s (λ) dλ

(6)

2. Experiments

 Z = z‾ (λ ) s (λ ) dλ

2.1. Sample preparation

where x‾ (λ ) ,   y‾ (λ ) and ‾z (λ ) denote the 1931 CIE x, y, and z color matching functions, respectively. And the s(λ) denotes the spectrum of a light source. The thermal conductivity data of the samples were collected by a Hot Disk TPS 2500S (Göteborg, Sweden) thermal conductivity testing device. A spiral wound nickel probe is embedded in the interval between two identical samples; the probe is used as a heat source as well as a temperature sensor [27]. The collected data error is kept within 5% [28]. Tests were carried out in a TFO-1 Forced Air Oven (Cascade Tek, USA), which is able to offer a testing surroundings of steady temperature. Three measurements were taken at 25 °C for each sample. The high thermal resistance of the samples was characterized by simultaneous thermo gravimetry and differential thermal analysis (TGDTA) (HCT-4) under nitrogen atmosphere in the temperature range of 25–1000 °C at a heating rate of 5 °C/min.

A series of Mo-doped Bi2WO6 3D hierarchical microspheres powders were prepared via a template-free hydrothermal method. The synthetic raw materials include Bi(NO3)3·5H2O, Na2WO4·2H2O, Na2MoO4·2H2O and acetic acid glacial. All chemicals were analytical grade without further purification. In a typical process, 2 mmol of Bi(NO3)3 ·5H2O were dissolved into 30 mL acetic acid glacial; after stirred magnetically for 40 min, formed solution A. Next, definite stoichiometric amounts of Na2WO4·2H2O and Na2MoO4·2H2O were dissolved by deionized water, termed solution B. Solution B was added, dropwise, into solution A and stirred for another 40 min, termed solution C. Subsequently, solution C was transferred into a 100 mL Teflon vessel, sealed in autoclave and heated at 180 °C for 20 h. After that, the autoclave was cooled to room temperature naturally and obtained the prophase product. After separated by vacuum filtration, washed by deionized water and ethanol, dried at 60 °C for 12 h of the prophase products, the ultimate Mo6+doped Bi2WO6 products were obtained.

(7)

3. Results and analysis 3.1. XRD analysis

2.2. Characterization techniques The crystallinity and phase purity of the samples were characterized by XRD and the experiment data were dealt with by Jade 6.0 software. Fig.1a shows that all diffraction patterns of the Mo-doped Bi2WO6 samples are well matched with the standard pattern of Bi2WO6 with orthorhombic phase and B2ab (41) space group (PDF # 73–2020). It is worth noting that as shown in Fig.1a, the higher of the Mo6+ doping concentration, the weaker the diffraction peaks are, which implies that the introduction of Mo6+ into Bi2WO6 structure is detrimental to the crystallization of Bi2WO6. There are no impurities peaks are observed in the sample patterns, indicating a high purity of the synthetic samples. In the patterns of the samples, differentiate from standard peaks, the intensity of the diffraction peak (131) is only twice times than the intensity of (200), implying a special anisotropic growth of the Mo6+doped Bi2WO6 crystal [29]. In Fig. 1b, there appears a red shift of the (131) diffraction peak as the Mo6+ doping concentration increases (2θ shifts from 28.349° to 28.248°). According to the Bragg’s law, the red shift of the diffraction peak should be attributed to the expansion of the interplanar spacing caused by the substitution of Mo6+ ions with smaller radius (134.2pm) for the W6+ ( radius 137pm). Thus, it is confirmed by the XRD analysis that the Mo6+ has successfully enter into the Bi2WO6 structure.

The crystalline structures of the samples were analyzed by powder X-ray powder diffraction instrument (λ = 0.154060 nm) (XRD, Bruker, D8 ADVANCE) equipped with Ni-filtered Cu Kα radiation. Data with 2θ ranging from 10 to 80° were collected at scan rate of 2°/min with a step size of 0.05° and 5 s counting time at each step. The morphologies of samples were characterized by field-emission scanning electron microscopy (FESEM, HITACHI, S-4800Π FE-SEM) with an acceleration voltage of 15 kV. The X-ray photoelectron spectroscopy (XPS, PHIQUANTERA II SXM) was used to determine the chemical composition of the testing samples. The optical reflectance of the samples was measured via using a UV/ VIS/NIR spectrometer (PerkinElmer, Lambda 950 with an integrating sphere attachment) in the 300–2500 nm region, with barium sulfate served as reference. The UV-VIS absorption spectrum of the sample is acquired from the corresponding region of NIR reflectance (R) according to the Kubelka-Munk function:

F(R) = (1 − R)2 /2R

(1)

where R symbolizes the reflectance obtained from the UV/VIS/NIR spectrometer experiment. The NIR solar reflectance (R*) in the 2

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Fig. 1. (a)The XRD patterns of Bi2MoxW1-xO6 (x = 0, 0.1, 0.2, 0.3, 0.4); (b) the partial enlarged XRD patterns of Bi2MoxW1-xO6 samples.

3d and O 1s states exists in the patterns, indicating the existence of O, Bi, Mo and W elements in the samples. The high-resolution orbits scan of Bi, Mo and W elements are exhibited in Fig. 3b–d. In Fig. 3b, the binding energy peaks at 157.86 eV and 163.20 eV are corresponding to the Bi 4f 7/2 and Bi 4f5/2, respectively. In Fig. 3c, the peaks at 34.16 eV and 36.33 eV are assigned to W 4f7/2 and W 4f5/2, which indicates the existence of the W ions in WO6 octahedron [30,31]. In Fig. 3d, peaks located at binding energies of 231.02 eV and 234.38 eV are corresponding to Mo 3d doublets, 3d5/2 and 3d3/2, respectively [32,33].

3.2. SEM and XPS analysis The morphology and particle sizes of the Mo6+-doped Bi2WO6 samples were investigated by SEM. Fig. 2a displays a typical panoramic image of pure Bi2WO6. As can be seen from a low magnification SEM image of Fig. 2a and its build-in figure, the sample consists of a large quantity of homogeneous and flower-like architectures ranging from 2 to 3 μm in diameter. An amplified SEM image (Fig. 2b) of an individual microsphere indicates the three dimensional microspheres are assembled by two-dimensional nano-sheets with thickness of 15–30 nm (as seen in the inset of Fig. 2b). In addition, the nano-sheets are smooth and possess a uniform thickness along the growth direction. As seen from Fig. 2c–2f, the 3D hierarchical microspheres still can be obtained when Mo6+ is introduced into the Bi2Wo6 lattice. It is observed that with the addition of Mo6+, the uniformity of the samples decreases and slight of nano-sheets are not assembled into the flower-like microspheres. However, the thickness of the nano-sheets and the diameter of the flower-like microspheres basically do not change. This implies that the doping of Mo6+ only has slight impact on the micro-morphologies of Bi2WO6 lattice and all the prepared samples appear the structure of 3D flower-like microspheres. The X-ray photoelectron spectroscopy was used to determine the chemical composition and elements state of the samples. The peak of C 1s is served as a reference, the binding energy of which is fixed at 284.6 eV. As shown in Fig. 3a, the major peaks for the Bi 4f, W 4f, Mo

3.3. Chromatic properties analysis The UV–Vis diffuse reflectance spectra of the Mo-doped Bi2WO6 3D hierarchical microspheres are shown in Fig. 4a, and the absorption spectra obtained from the corresponding UV–Vis diffuse reflectance spectra on the basis of equation (1) are displayed in Fig. 4b. As seen from Fig. 4a, the UV–Vis reflectance of pure Bi2WO6 is as high as 80%. The light absorption of Bi2WO6 in 300–500 nm region can be assigned to the transition from the hybrid orbital of O 2p and Bi 6s to the W 5d orbital [21]. Because of the high reflectance to the visible light, the color of the Bi2WO6 powder appears to be white. Due to the introduction of the Mo6+ into Bi2WO6 lattice, the absorption edge progressively moves to higher wavelength region (from 440 to 512 nm) as the doping concentration gradually increase. The band gap energies of the pigments are evaluated from the plots of [(F(R∝)hv]2 versus the absorbed

Fig. 2. The SEM images of Bi2MoxW1-xO6 (a, b: x = 0.0; c, d: x=0.2; e, f: x=0.4) samples. 3

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Fig. 3. The XPS spectrum of Bi2W0.8Mo0.2O6: (a) Survey scan; (b)Bi 4f; (c)W 4f; (d)Mo 3d.

1931 CIE Standard Source C (illuminant C (0.3101, 0.3162)), are shown in Fig. 7. It can be seen from Fig. 7 that the samples gradually close to the yellow area as the Mo content increases, which is consistent with the color transition of the sample from Fig. 6. The transition of the sample color is ascribed to the introduction of Mo6+ into the Bi2WO6 crystal lattice, leading to the decline of the band gap energy. The decreased band gap energy leads to absorption edge of the pigments shift to higher wavelength. And then the absorption to the blue light is increased, causing the samples reflect more yellow light (the complementary light of blue light), thus the yellow color of the pigments gradually enhance as the Mo6+ concentration increase. Thereupon, all the UV–Vis absorption spectra, optical photographs and the chromatic coordinates of the prepared samples point that the Mo-doped Bi2WO6 samples appear a deepening yellow color accompanied with the increase of the doping content.

light energy and which is shown in Fig. 5. As shown in Fig. 5, the band gap of the pigment decreases from 2.82 eV to 2.42 eV gradually after Mo6+ replaces W6+ progressively. There are mainly two factors affecting the band gap of the Bi2 MoxW1-xO6 powders [29]: (i) the delocalization degree of excitation energy due to the lattice distortion caused by doping with Mo ions (ii) the degree of Mo 4d and W 5d orbitals being involved in the conduction band of Bi2 MoxW1-xO6. The presence of Mo6+ in Bi2WO6 does not modify the position of the valence band edge of Bi2WO6; instead, it introduces new energy levels of the Mo ions into the band gap of Bi2WO6. Therefore, it is the introduction of Mo ions into Bi2WO6 lattice that narrows the band gap of the Bi2WxMo1-xO6 [33], the replacement of the Mo6+ for W6+ making the charge transfer excitation from Bi 6s to O 2p turns to the transition from Mo 3d to O 2p. It is noteworthy that the initiate of the diffuse reflection spectra with Mo substitution are not linearly shifted. That is to say, the change in band gap energy is not necessarily proportional to the doping amount of molybdenum ions. The optical photographs of the Bi2W1-xMoxO6 samples are shown in Fig. 6. It can be detected by human eyes from Fig. 6 that the color of the doping samples changes from pale yellow to yellow as the doping concentration increases. The chromatic coordinates (x, y) of the Bi2W1xMoxO6 (x = 0, 0.1, 0.2, 0.3, 0.4) pigments, obtained in line with the

3.4. NIR reflectance analysis The NIR reflectance spectra and the corresponding NIR reflectance energy distribution image of the Bi2W1-xMoxO6 (x = 0, 0.1, 0.2, 0.3, 0.4) pigments are given in Fig. 8. The pristine Bi2WO6 exhibits a relatively lower NIR reflectance around 67–80% in 700–2500 nm region.

Fig. 4. UV–Vis diffuse reflectance spectra and absorption spectra of Bi2MoxW1-xO6 (x = 0, 0.1, 0.2, 0.3, 0.4) powdered pigments. 4

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Fig. 5. The Kubelka-Munk transformed reflectance spectra of Bi2W1-xMoxO6 (x = 0, 0.1, 0.2, 0.3, 0.4) powdered pigments.

With the addition of Mo6+, the NIR reflectivity of the samples increases markedly in the range of 700–2500 nm. Especially, the NIR reflectance of the Bi2Mo0.3W0.7O6 increases by more than 14% in the range of 700–2500 nm compared to pure Bi2WO6. The built-in figure in Fig. 8 shows the NIR reflectance energy distribution of the pigments in 700–2500 nm region. It clearly demonstrates that the reflectance energy of NIR radiation is mainly concentrated in the shortwave NIR range (700–1100 nm). The NIR solar reflectance (R*) values of the doped samples together with the conventional pigments with the same color are reckoned on the basis of equation (2) and are listed in Table 1. The Mo6+-doped Bi2WO6 pigments exhibit high NIR solar reflectance (ranging from 72.75% to 80.80%) in 700–2500 nm region and which is definitely higher than the NIR solar reflectance of conventional pigment counterpart. Thus, the comparatively high solar reflectance makes the Mo6+-doped Bi2WO6 potential pigments to be served as novel “cool pigments” with better energy saving capability. Meanwhile, the difference of the NIR solar reflectance values among the Mo-doped Bi2WO6 pigments can be originated from the disparity of the carrier concentration [4,33]. According to the classical Drude theory [34,35]:

Fig. 7. Chromaticity coordinates of Bi2MoxW1-xO6 (x = 0, 0.1, 0.2, 0.3, 0.4) samples depicted according to the 1931 CIE Standard Source C.

Fig. 6. Photographs of Bi2W1-xMoxO6 (x = 0, 0.1, 0.2, 0.3, 0.4) powdered pigments. 5

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Table 2 Thermal conductivity and NIR solar reflectance (R*) values of the Bi2W1xMoxO6 (x = 0.2, 0.3, 0.4) and the conventional pigment colored ceramic samples. Sample

Conventional

x = 0.2

x = 0.3

x = 0.4

Thermal conductivity (W/mk) R*

1.1600 76.12%

1.1475 82.25%

1.0895 81.61%

0.70685 86.86%

3.5. Thermal conductivity analysis The thermal conductivity characterization is carried out to detect the thermal conductivity of the pigmented ceramic samples prepared by compacting the powder mixture into a cylinder with a thickness of 5 mm at 20 kpa, in which each powder mixture is comprised of the silica, aluminium oxide and the testing pigment (conventional pigment, Bi2W1-xMoxO6 with x = 0.2, 0.3, 0.4). The mass fraction of the silica, aluminium oxide and the pigment is 73%, 17% and 10%, respectively, which is a general proportion ratio of the main components of ceramic materials. Besides, slight of phenol formaldehyde resin is also added into the powder mixture as adhesive. The thermal conductivities of the samples are measured using a Hot Disk TPS 2500S thermal conductivity testing device at 25 °C. The results of the thermal conductivity values of the pigments ceramic samples are displayed in Table 2. From Table 2, it can be seen that the thermal conductivity values of the samples containing the Mo-doped Bi2WO6 pigments perform better than the conventional pigment sample. The thermal conductivity of the prepared pigment decreases with the growth of the Mo6+ doping concentration in the Bi2WO6. With all other affecting components in the ceramic sample are the same, the only different pigment component is the only factor that determine the thermal conductivity value. Hence, we can pick out that the Mo-doped Bi2WO6 pigments exhibit lower thermal conductivity values than the conventional pigment. The low thermal conductivity of the prepared pigment is conducive to prevent the external heat energy transfer to the interior of the materials, which is beneficial to cut down the accumulating temperature on the materials. Besides, the low thermal conductivity of the prepared pigments can be assigned to the enlarged thermal conducting distance resulting from the staggered arrangement of the two-dimensional nano-sheets assembles in the 3D hierarchical flower-like structure. The lower thermal conductivity of the prepared pigment than the conventional pigment of the similar color make the prepared pigment more suitable for adding into the ceramic materials to strengthen the thermal insulating capability of them. Besides, the NIR solar reflectance of the ceramic samples of powders was also measured and the results are displayed in Table 2. Samples added with Mo-doped Bi2WO6 pigments perform higher NIR solar reflectance than samples mixed with conventional pigment. The higher NIR solar reflectance and lower thermal conductivity of ceramic samples implies the Mo-doped Bi2WO6 pigments are possession of exceptional thermal insulation performance.

Fig. 8. (a) The NIR reflectance curves and (b) the built-in NIR reflectance energy distribution image of Bi2W1-xMoxO6 (x = 0, 0.1, 0.2, 0.3, 0.4) samples. Table 1 The NIR solar reflectance of Bi2W1-xMoxO6 (x = 0.1, 0.2, 0.3, 0.4) samples and conventional yellow pigment. Samples

Conventional pigment

x = 0.0

x = 0.1

x = 0.2

x = 0.3

x = 0.4

R*

68.8%

74.76%

72.75%

77.95%

80.80%

78.75%

⎧ω = ⎪ ⎨λ ∝ ⎪ p ⎩

Ne 2 ε0m m N

(8)

where the plasma frequency (ω) symbolizes the infrared crossover from high transmittance to high reflectivity, N denotes the charge carrier concentration, e denotes the charge of electron, ε0 denotes the permittivity of the free space, m denotes the effective mass of free electron and λp represents the plasma wavelength. It can be deduced that the λp would have a blue shift once N increases. After plasma wavelength (λp), NIR reflectance will get rise, and the NIR solar reflectance of the pigment is increased in line with the increase of the carrier concentration. On the other hand, the carrier concentration is affected by one factor in the term of the circumstance of Mo6+-doped Bi2WO6 lattices, which is the oxygen vacancy concentration. Due to the element stoichiometric deviation when Mo6+ ions were introducing into the Bi2WO6 lattices, oxygen vacancies are formed: partial oxygen ions (O2−) in Mo-doped Bi2WO6 lattices separate from the original lattices, combine to form oxygen and form oxygen vacancy. The electrons ionized by vacancy are easily captured by W6+ and become W6+·2e. The obtained electrons and W6+ are weakly bound, and the electrons are easily excited to the conduction band to form carriers. The reaction formula is as follows:

Bi2 Mo1 −x Wx O6 → Bi2 Mo1 −x W 6 + x− δ(W 6 +2e)δO6 − δ  +

3.6. Thermal and chemical stability analysis The pigments of Bi2WO6 and Bi2Mo0.2W0.8O6 were selected as testing samples and examined for their thermal stability from 25 °C to 1000 °C. The TG curves in Fig. 9 indicate that there is a negligible weight loss happen to the testing pigments after TG-DTA measurement. Also, there is no obvious endothermic and exothermic peak in DTA curve, indicating no phase transition occurred from the testing samples during the whole heating process. The steady TG-DTA curves demonstrate that the prepared pigments possess excellent thermal stability. The high temperature resistance of the pigments implying that the Bi2W1-xMoxO6 (x = 0, 0.1, 0.2, 0.3, 0.4) samples are promising to be served as ceramics pigments to regulate the ceramic color as well as keep an excellent thermal stability.

δ O2 ↑ 2

The oxygen vacancy concentration increases as far as the Mo6+ doping ions content increases to some extent. As the doping ion concentration continually increases, the electron trap [36] induced by the abundant oxygen coming from the grain boundary or from the pores inside the crystal particles increases, thus the Schottky barrier is formed and then the carrier mobility and the carrier concentration decreases. Thereupon, as shown in Table 1, the NIR solar reflectance presents a trend of first ascends and then descends when Mo6+ doping content keeping increase. 6

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Fig. 9. TG-DTA curves of (a) Bi2WO6 and (b) Bi2W0.8Mo0.2O6 powdered pigments.

materials possess both have high NIR solar reflectance, better heat insulating capability and excellent thermal and chemical stabilities.

Table 3 The color coordinates of the Bi2W0.8Mo0.2O6 powdered pigments after acid/ alkali resistance tests. Acid/alkali

L*

a*

b*

ΔE*

Declaration of competing interest

CH3COOH H2SO4 HNO3 NaOH

89.22 88.71 89.43 88.13

−1.83 −1.48 −1.78 −1.48

13.21 12.88 13.79 12.69

0.78 1.26 1.14 1.77

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Appendix A. Supplementary data

To evaluate the chemical stability of the synthesized samples, five Bi2W0.8Mo0.2O6 samples with same weight were soaked in 5% CH3COOH, H2SO4, HNO3 and NaOH solution respectively, then magnetic stirring for 10 min (GB/T5211.5–2008). Afterwards, the soaked samples were washed by deionized water, filtered, dried and weighed again. Negligible weight change is detected between the samples before and after soaking in the acids/alkali solutions. Table 3 lists the color coordinates and the color disparity values (△E*) of the test sample after acid/alkali treatment. It can be seen from Table 3 that the ΔE* values of Bi2W0.8Mo0.2O6 are small (ΔE* < 2.0). In general, when ΔE* ≤ 2 or 3, the color difference cannot be recognized by human eye. The small ΔE* values of the samples indicates an eminent acid/alkali resistance of the samples, In other words, the pigments possess good chemical stability. The excellent chemical stability of the Bi2W1-xMoxO6 (x = 0, 0.1, 0.2, 0.3, 0.4) samples implies that the sample has good compatibility with other components in the ceramic when it acts as a ceramic pigment.

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4. Conclusions As the doping concentration increases, the orthorhombic Bi2W1(x = 0, 0.1, 0.2, 0.3, 0.4) samples with 3D hierarchical flowerlike structure assembled by two-dimensional nano-sheets transfers from faint yellow to yellow. The change of the sample colors is attributed to the reduced band gap caused the incorporation of the Mo6+ into the Bi2WO6 lattice. The Mo-doped Bi2WO6 pigments exhibit higher NIR solar reflectance (> 72.8%) than conventional pigment (68.8%) of the similar color. The factor influencing the NIR solar reflectance of the doped pigments is affected by the carrier concentration which is determined by the oxygen vacancy concentration in Mo6+-doped Bi2WO6 lattices. What’s more, the synthetic pigments also perform better on the heat insulating capability than its nanoparticles counterpart due to its unique hierarchical flower-like structure morphology, which indicates that the prepared pigments both have higher NIR solar reflectance and heat insulating capability and that is much facilitated to reduce the accumulating temperature of the ceramic materials. Benign chemical and thermal stabilities are also found of the prepared pigments. Consequently, our results clearly demonstrates that the Bi2W1-xMoxO6 (x = 0.0, 0.1, 0.2, 0.3, 0.4) powders are potential to take place of the conventional pigments with the similar color to assist the ceramic xMoxO6

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