Insight into the Effect of Counterions on the Chromatic Properties of Cr-Doped Rutile TiO2-Based Pigments

Rutile TiO2 pigments codoped with chromophore ion Cr3+ and various charge-balancing ions (i.e., counterions species of Sb, Nb, W and Mo) were prepared by a solid-phase reaction method. The effects of the counterions and calcination temperatures on the phase structure, color-rendering and spectroscopic properties, microstructure, and stability of the synthesized pigments were investigated in detail. The results showed that the introduction of 5–10% counterions improved the solubility of Cr3+ in the TiO2 lattice to form the single-phase rutile pigments calcined at 1100 °C for 2 h. The 10% Cr-doped pigment showed a dark brown color. Depending on the content and type of counterions, the color of the codoped pigments was tailored from yellow to reddish or yellowish-orange to black with different brightness and hue. The influence mechanism of counterions was ascribed to the lattice distortion and variation in the charge balance condition. It was found that the addition of Sb, Nb, or Mo resulted in a remarkable improvement in the NIR reflectance of pigments. The grain growth was inhibited with the codoping of Cr/Sb and Cr/Nb to achieve the nano-sized pigments. In addition, the prepared pigments exhibited good acid and alkali corrosion resistance as well as excellent stability and coloring performance in transparent ceramic glazes.


Introduction
Inorganic pigments are widely used as important colorants for various applications such as coatings, glasses, ceramics, plastics, paints, inks, enamels, and construction materials due to their outstanding thermal stability and high chemical stability as well as excellent coloring ability compared to organic counterparts [1][2][3][4][5][6]. In recent years, with the strong regulation of environmental protection departments and increasing concerns regarding environmental issues, the demand for environmentally friendly non-toxic pigments has increased. However, most of today's brightly colored and widely available inorganic pigments contain toxic heavy metal elements as the main component (e.g., CdS/CdSe, PbCrO 4 , Sb 2 O 5 ·2PbO) and their applications are severely restricted [7]. Therefore, the development of eco-friendly and cost-effective inorganic pigments has become a popular research topic in the area of pigments [8][9][10].
Titania (TiO 2 ) is an abundant and cheap compound of polymorphic forms including anatase, rutile, and brookite. Among them, rutile TiO 2 has a tetragonal crystal structure composed of the [TiO 6 ] octahedra, which are connected to each other by ribs to form a relatively stable chain extending along the c-axis, and the chains are linked by the common corner tops of the octahedra [11,12]. Therefore, the rutile TiO 2 possesses a thermodynamically stable crystalline structure, which makes it an excellent material of oxide pigments

Characterization Techniques
The crystalline structure of the pigment powders was characterized by X-ray diffraction (XRD) using Cu-Kα (λ = 0.15418 nm) radiation with a D8 Advance diffractometer operated at 40 kV for 30 mA. The XRD data were collected from a 5 to 80 • 2θ range with a step size of 0.02 • . The pigment precursors were characterized using a simultaneous TG-DSC thermal analyzer (STA449C, Netzsch-Gertebau GmbH, Selb, Germany), heating from room temperature at a rate of 10 • C/min to 1100 • C. The UV-Vis diffuse reflectance spectra (200-900 nm) and near-infrared reflectance spectra (780-2500 nm) of the pigment samples were measured by a UV-Vis-NIR spectrophotometer (Lambda 950, Perkin-Elmer, New York, NY, USA). BaSO 4 was used as the reference substance, and the scanning interval was 2 nm. The bandgap of the sample was calculated according to the UV-Visible spectrum. The specific calculation process is as follows: according to the UV-Visible reflectance R of the samples, E = 1240/λ is used as the horizontal coordinate. F(r) is calculated by the Kubelka-Munk formula: F(r) = (1 − R) 2 /2R, and (F(r)*E) 2 is used as the vertical coordinate. The graph is extrapolated as a tangent, and the intercept of the tangent on the horizontal axis is the bandgap (E g ) [21,33,34]. The colorimetric parameters of the pigments and colored glaze in the CIELab system were measured using an Automatic Whiteness meter (WSD-3C, Beijing, China), in which L* is the Lightness axis (black (0)/white (100)), a* is the green (−)/red (+) axis and b* is the blue (−)/yellow (+) axis. The color saturation C* is defined as C* = [(a*) 2 + (b*) 2 ] 1/2 . The microstructure of the samples was analyzed by a field emission scanning electron microscopy (FE-SEM, SU-8010, Hitachi, Tokyo, Japan) operated at 5 kV. To evaluate the chemical stability of the pigments, the samples synthesized by calcination at 1100 • C for 2 h were soaked in 5 wt% H 2 SO 4 , HCl, and NaOH aqueous solution for 60 min, respectively. After washing and drying, the acid and alkali corrosion resistance of samples was evaluated by comparing the color difference before and after treatment.

Results and Discussion
The combined DSC-TG thermal analysis and XRD measurements were conducted to study the physical and chemical changes of the pigment precursor during the heat treatment. Figure 1a shows the DSC-TG curves of the Ti 0.8 Cr 0.1 Sb 0.1 O 2 pigment precursors heated up from room temperature to 1100 • C. The total mass loss was about 15%, of which the vast majority (14.5%) occurred at around 600 • C, as shown in the TG curve. Accordingly, a sharp endothermic peak appeared near 608 • C in the DSC curve, which was mainly caused by the decomposition and dehydration of metatitanic acid (hydrated TiO 2 ). Exothermic peaks also appeared in the DSC curve near 804 • C and 1042 • C, but no noticeable mass loss could be observed in the corresponding TG curve. Therefore, the two exothermic peaks were caused by the generation of the anatase TiO 2 crystal phase and the phase transformation of anatase to rutile, respectively, which was similar to the reported results in the literature [4]. The transformation process of the anatase to rutile phase is exothermic, and the transition temperature generally ranges from 400 to 1200 • C, depending on the raw material properties, doped ions, synthesis method, and heat treatment system, etc. [25]. Figure 1b,c present the XRD patterns of Ti 0.8 Cr 0.1 Sb 0.1 O 2 pigments calcined at 950-1200 • C for 2 h. It can be seen that the powder calcined at 950 • C showed the main crystalline phase of rutile TiO 2 (PDF#: 21-1276) while a trace Cr 2 O 3 phase also existed, suggesting that the solid-phase reaction was not fully completed. When the temperature was increased up to 1000-1200 • C, only a single rutile TiO 2 phase was obtained without any impurity, which indicates that Cr 3+ and Sb 5+ cations completely dissolved in the TiO 2 lattice [23]. Note that the introduced Sb 3+ by Sb 2 O 3 would be oxidized to Sb 5+ in the calcination process [4,35,36]. The XRD results showed that the temperature for the formation of the rutile phase was lower than the exothermic peak temperature (about 1042 • C) generated by the phase transformation of anatase to rutile, as shown Figure 1a. This may be due to the temperature lag caused by the rapid temperature rise during the thermal analysis. In addition, the intensity of the diffraction peaks increased gradually with increasing temperature, indicating that the crystallization of the synthesized pigment was promoted. Combined with the color-rendering properties shown in Figure 1d, the optimal calcination temperature can be determined as 1100 • C, at which a bright yellow Ti 0.8 Cr 0.1 Sb 0.1 O 2 pigment with the highest yellowness value (b*) and thus more vivid color can be synthesized.  To investigate the effect of different high-valence counterions on the Cr-doped TiO2based pigments, Ti0.9-xCr0.1MxO2 (M = Sb, Nb, Mo and W; x = 0, 0.05 and 0.10) powders were synthesized at 1100 °C for 2 h and characterized in terms of phase composition, chromatic performance, and spectroscopies. Figure 2 shows the XRD patterns of obtained pigments doped with different counterions at 5% and 10% (in molar ratio). As can be seen from Figure 2a, for the sample with a nominal composition of Ti0.9Cr0.1O2, an impurity of chromium-titanium oxide (Ti0.78Cr0.12O1.74) was formed in addition to the rutile phase. With the codoping of 5% counterions (Sb, Nb, W, or Mo ions), all the characteristic diffraction peaks of the obtained pigment powders could be well indexed to the rutile TiO2 structure (PDF#21-1276) [20], which indicates that both the counterions and Cr 3+ had entirely incorporated into the TiO2 lattice to form a solid solution. The XRD analysis showed that the  Figure 1d shows the CIELab chromaticity parameters of Ti 0.8 Cr 0.1 Sb 0.1 O 2 pigments calcined at 950-1200 • C for 2 h. It can be seen that the calcination temperature had an obvious influence on the chromatic performance of Ti 0.8 Cr 0.1 Sb 0.1 O 2 pigments. With the increase in temperature, the L* value (lightness) presented a decreasing trend. In particular, when the temperature was ≥1150 • C, a significant decrease in the L* value could be observed, which might be due to the reduction in Ti 4+ to a lower valence at higher temperatures, generating free electrons and consequently resulting in enhanced absorption of visible light [37]. The a* value (redness) of pigments increased with increasing temperature, while the b* value (yellowness) increased first and then decreased. At 1100 • C, the highest b* value of 46.64 was achieved due to the full incorporation of Cr 3+ and Sb 5+ into the TiO 2 lattice, which was much higher than those (b* < 40) of the Cr/Sb codoped pigments synthesized via the precipitation or hydrothermal methods [23]. Accordingly, the obtained pigment exhibited the highest C* value (color saturation) of 52.99. The chromaticity coordinates of the samples calcined at different temperatures are shown in Figure 1e, which are consistent with the results in Figure 1d. The above results show that the doped TiO 2 -based pigment synthesized at 1100 • C for 2 h possessed the best chromogenic properties with high yellowness value and color saturation.
To investigate the effect of different high-valence counterions on the Cr-doped TiO 2based pigments, Ti 0.9−x Cr 0.1 M x O 2 (M = Sb, Nb, Mo and W; x = 0, 0.05 and 0.10) powders were synthesized at 1100 • C for 2 h and characterized in terms of phase composition, chromatic performance, and spectroscopies. Figure 2 shows the XRD patterns of obtained pigments doped with different counterions at 5% and 10% (in molar ratio). As can be seen from Figure 2a, for the sample with a nominal composition of Ti 0.9 Cr 0.1 O 2 , an impurity of chromium-titanium oxide (Ti 0.78 Cr 0.12 O 1.74 ) was formed in addition to the rutile phase. With the codoping of 5% counterions (Sb, Nb, W, or Mo ions), all the characteristic diffraction peaks of the obtained pigment powders could be well indexed to the rutile TiO 2 structure (PDF#21-1276) [20], which indicates that both the counterions and Cr 3+ had entirely incorporated into the TiO 2 lattice to form a solid solution. The XRD analysis showed that the introduction of counterions could help to improve the solubility of Cr 3+ in the rutile lattice, which may be due to, on one hand, lattice distortion caused by the radius difference of codoped ions improving the capability of the rutile structure to accommodate Cr 3+ ; on the other hand, the counterions with higher valence could reduce the charge imbalance of TiO 2 caused by lower valence Cr 3+ and keep the whole system electrically neutral [38]. As shown in Figure 2b, when the doping level of counterions was increased to 10%, the Sb, Nb, or Mo-codoped Ti 0.9 Cr 0.1 O 2 maintained a single rutile structure phase, while for the W-doped sample, a minor impurity of WO 3 was also formed, indicating that the solubility of W in the TiO 2 lattice was lower than that of other counterions. This phenomenon was similar to that of the Ti 0.9 Cr 0.1 O 2 pigment, but the introduction of 10% high valent W 6+ would lead to surplus positive charges, so not all of the W 6+ could be incorporated into the rutile lattice. Figure 3 shows the values of CIELab chromaticity parameters of Ti 0.9−x Cr 0.1 M x O 2 (M = Sb, Nb, Mo, and W; x = 0, 0.05 and 0.10) pigments. As can be seen from Figure 3a, the Ti 0.9 Cr 0.1 O 2 pigment had a dark brown color with low a* and b* values. After codoping with 5% Sb, W, or Mo, respectively, the color rendering performance of the obtained pigments varied greatly. Among them, the Sb-codoped pigment showed a typical yellow color with relatively high L* and b* values and low a* value, while the W-codoped one presented a reddish-orange with a reduced L* value and close a* and b* values. With the codoping of 5% Mo, a black pigment was obtained with the lowest L*, a*, and b* values. However, when codoped with Nb, the obtained pigment showed a dark brown color, which was similar to that of Ti 0.9 Cr 0.1 O 2 without the codoping of counterions. As shown in Figure 3b, when the codoping amount of Sb was increased to 10%, a brighter yellow color could be achieved with obviously increased L*, a*, b* values and color saturation (C*). Surprisingly, for the Ti 0.8 Cr 0.1 Nb 0.1 O 2 pigment, all the chromatic parameters enhanced markedly and a bright yellowish-orange color was obtained. However, the chromatic parameters of the Ti 0.9−x Cr 0.1 W x O 2 pigment decreased obviously with W content incrementing from 5% to 10%, and consequently the color darkened. The increase in Mo content had little effect on the color performance of the obtained pigment since only a minor decrease in L*, a*, b* parameters was observed in Figure 3. The variation of color rendering performance for the obtained rutile pigments can mostly be ascribed to the alteration of charge balance and degree of lattice distortion caused by the counterions with different valences, radii, and contents. For the 10% Sb 5+ or Nb 5+ codoped samples, the electric neutrality of crystal structure can be achieved, preventing the generation of electronic defects, and consequently, greatly enhanced yellowness (b* value) could be obtained. However, for the introduced counterion of Mo 6+ by MoO 3 , it would be easily subjected to a reduction to lower valent Mo 3+/4+/5+ [19,39,40], which would result in the occurrence of electronic defects and thus high visible light absorption rate, yielding a black-toned rutile pigment. Regarding the redox stable W 6+ , the electric neutrality was attained at the doping level of 5%, and thus the further increase in W 6+ content to 10% led to the surplus positive charge and even residual second phase WO 3 , as shown in Figure 2b, resulting in greatly reduced chromatic parameters and darkening of the pigment. The influence of different counterions is further clarified below according to the UV-Visible spectroscopy results shown in Figure 4a. In conclusion, the obtained results show that multi-colored rutile TiO 2 -based pigments can be achieved by the rational design of incorporated counterions and their content to regulate the structure distortion of [TiO 6 ] octahedrons and the degree of charge imbalance. other hand, the counterions with higher valence could reduce the charge imbalance of TiO2 caused by lower valence Cr 3+ and keep the whole system electrically neutral [38]. As shown in Figure 2b, when the doping level of counterions was increased to 10%, the Sb, Nb, or Mo-codoped Ti0.9Cr0.1O2 maintained a single rutile structure phase, while for the W-doped sample, a minor impurity of WO3 was also formed, indicating that the solubility of W in the TiO2 lattice was lower than that of other counterions. This phenomenon was similar to that of the Ti0.9Cr0.1O2 pigment, but the introduction of 10% high valent W 6+ would lead to surplus positive charges, so not all of the W 6+ could be incorporated into the rutile lattice.  Figure 3 shows the values of CIELab chromaticity parameters of Ti0.9-xCr0.1MxO2 (M = Sb, Nb, Mo, and W; x = 0, 0.05 and 0.10) pigments. As can be seen from Figure 3a, the Ti0.9Cr0.1O2 pigment had a dark brown color with low a* and b* values. After codoping with 5% Sb, W, or Mo, respectively, the color rendering performance of the obtained pigments varied greatly. Among them, the Sb-codoped pigment showed a typical yellow color with relatively high L* and b* values and low a* value, while the W-codoped one thus the further increase in W 6+ content to 10% led to the surplus positive charge and even residual second phase WO3, as shown in Figure 2b, resulting in greatly reduced chromatic parameters and darkening of the pigment. The influence of different counterions is further clarified below according to the UV-Visible spectroscopy results shown in Figure 4a. In conclusion, the obtained results show that multi-colored rutile TiO2-based pigments can be achieved by the rational design of incorporated counterions and their content to regulate the structure distortion of [TiO6] octahedrons and the degree of charge imbalance. To further elucidate the effect of different counterions on the color-rendering properties of series Ti0.8Cr0.1M0.1O2 (M = Sb, Nb, W, Mo) pigments, the UV-Vis diffuse reflectance properties were also investigated, as shown in Figure 4a. It can be observed that the pigment doped with only 10% Cr showed a very low reflectance from yellow to orange to red bands with the wavelength in the range of 550-780 nm, which could well justify its dark brown color. The codoping of Sb and Nb resulted in a significant enhancement of reflectance in the same wavelength range. In particular, the Sb-codoped pigment exhibited the highest reflectance, which agreed well with its chromatic performance. For the Wcodoped sample, a slight increase of reflectance could also be observed in the orange to red bands. However, the codoping of Mo even led to a much lower visible reflectance (1 21%) than only Cr-doping in the whole UV-Visible band, indicating that the Cr/Mo co oped TiO2 rutile sample of the pigment could absorb most of the UV-Visible light an thus appeared black. Figure 4a also shows that all the doped rutile pigments had relative low reflectance to blue-green light, and the reflectance/absorption edge was red-shift from ca. 500 to ca. 700 nm in the order of Sb, Nb, W, and Mo. The curves of the forbidd bandgaps derived from the spectra according to the Kubelka-Munk theory are shown Figure 4b, where the values of the intersection of the tangent line and the horizontal ax are the values of the band gaps (Eg). The obtained Eg values and corresponding intrin absorption wavelength λi (Eg = 1240/λi) are listed in Table 2. It can be seen that the sing Cr-doped pigment had an Eg of 1.83 eV. The doping of Sb, Nb, and W produced an i crease in Eg to above 2.03 eV, while the introduction of Mo ions resulted in a significa reduction in Eg to 1.36 eV. These differences in bandgap can also be attributed to the d ferent degrees of crystal structure distortion and defects caused by the cation doping. F the 10% Cr, Cr/Mo, and Cr/W doped TiO2, the generation of electronic defects due charge imbalance creates additional energy levels in the band structure, which should the main reason for the significant increase in visible light absorptance. The calculat values of Eg and λi well justify the color rendering characteristics of the as-synthesiz pigments with different counterions.  As is well-known, the solar spectrum consists of 48% of UV-Visible radiation an 52% of near-infrared radiation (NIR). The demand for near-infrared radiation he shielded pigments (so-called cool pigments) has been increasing in recent years due to t urban heat island (UHI) effect [41]. Therefore, the NIR reflectance properties from 780 2500 nm in wavelength of the doped TiO2 pigments were investigated, as shown in Figu 5. It was found that the codoping of Sb, Nb, and Mo resulted in a remarkable enhanceme in the NIR reflectance of pigments, among which the Sb-codoped sample possessed t highest NIR reflectance of more than 69% in the whole NIR region. In contrast, the sing Cr-doped sample showed relatively low NIR reflectance. With the W-codoping, a slig increase in the NIR reflectance in the wavelength of 940-2500 nm could also be observe Furthermore, considering that the largest part of the NIR energy was distributed in t region with a wavelength below 900 nm, the rutile Ti0.8Cr0.1Sb0.1O2 and Ti0.8Cr0.1Nb0.1  Figure 4a. It can be observed that the pigment doped with only 10% Cr showed a very low reflectance from yellow to orange to red bands with the wavelength in the range of 550-780 nm, which could well justify its dark brown color. The codoping of Sb and Nb resulted in a significant enhancement of reflectance in the same wavelength range. In particular, the Sb-codoped pigment exhibited the highest reflectance, which agreed well with its chromatic performance. For the W-codoped sample, a slight increase of reflectance could also be observed in the orange to red bands. However, the codoping of Mo even led to a much lower visible reflectance (18-21%) than only Crdoping in the whole UV-Visible band, indicating that the Cr/Mo codoped TiO 2 rutile sample of the pigment could absorb most of the UV-Visible light and thus appeared black. Figure 4a also shows that all the doped rutile pigments had relatively low reflectance to blue-green light, and the reflectance/absorption edge was red-shifted from ca. 500 to ca. 700 nm in the order of Sb, Nb, W, and Mo. The curves of the forbidden bandgaps derived from the spectra according to the Kubelka-Munk theory are shown in Figure 4b, where the values of the intersection of the tangent line and the horizontal axis are the values of the band gaps (E g ). The obtained E g values and corresponding intrinsic absorption wavelength λ i (E g = 1240/λ i ) are listed in Table 2. It can be seen that the single Cr-doped pigment had an E g of 1.83 eV. The doping of Sb, Nb, and W produced an increase in E g to above 2.03 eV, while the introduction of Mo ions resulted in a significant reduction in E g to 1.36 eV. These differences in bandgap can also be attributed to the different degrees of crystal structure distortion and defects caused by the cation doping. For the 10% Cr, Cr/Mo, and Cr/W doped TiO 2 , the generation of electronic defects due to charge imbalance creates additional energy levels in the band structure, which should be the main reason for the significant increase in visible light absorptance. The calculated values of E g and λ i well justify the color rendering characteristics of the as-synthesized pigments with different counterions. As is well-known, the solar spectrum consists of 48% of UV-Visible radiation and 52% of near-infrared radiation (NIR). The demand for near-infrared radiation heat-shielded pigments (so-called cool pigments) has been increasing in recent years due to the urban heat island (UHI) effect [41]. Therefore, the NIR reflectance properties from 780 to 2500 nm in wavelength of the doped TiO 2 pigments were investigated, as shown in Figure 5. It was found that the codoping of Sb, Nb, and Mo resulted in a remarkable enhancement in the NIR reflectance of pigments, among which the Sb-codoped sample possessed the highest NIR reflectance of more than 69% in the whole NIR region. In contrast, the single Cr-doped sample showed relatively low NIR reflectance. With the W-codoping, a slight increase in the NIR reflectance in the wavelength of 940-2500 nm could also be observed. Furthermore, considering that the largest part of the NIR energy was distributed in the region with a wavelength below 900 nm, the rutile Ti 0. 8  pigments are preferred for energy-saving applications such as architectural coatings, vehicle paints, exterior wall tiles, and tile glazes.  Figure 6 illustrates the SEM images of the prepared Ti0.9Cr0.1O2 and Ti0.8Cr0.1M0.1O2 (M = Sb, Nb, W, Mo) pigments calcined at 1100 °C for 2 h. As seen in Figure 6a, the single Crdoped pigment powder was composed of irregular and polyhedral grains with sizes mainly distributed from 500 to 1000 nm. Although the powders mostly maintained a characteristic polyhedral structure of the tetragonal crystal system at different degrees of integrity, their microscopic morphology changed significantly after the codoping of different counterions. As shown in Figure 6b, the 10% Sb-codoping contributed to grains with a significantly reduced size of 100-300 nm and good dispersion. A similar phenomenon could also be observed for the Nb-codoped powder, except that its particle size of 250-500 nm was a little larger than that of the Sb-codoped sample (Figure 6c). However, for   Figure 6a, the single Crdoped pigment powder was composed of irregular and polyhedral grains with sizes mainly distributed from 500 to 1000 nm. Although the powders mostly maintained a characteristic polyhedral structure of the tetragonal crystal system at different degrees of integrity, their microscopic morphology changed significantly after the codoping of different counterions. As shown in Figure 6b, the 10% Sb-codoping contributed to grains with a significantly reduced size of 100-300 nm and good dispersion. A similar phenomenon could also be observed for the Nb-codoped powder, except that its particle size of 250-500 nm was a little larger than that of the Sb-codoped sample (Figure 6c). However, for the W and Mo codoped pigments, their maximum particle size was close to that of the single Cr-doped samples. As shown in the XRD pattern of Figure 2b, there should be a small amount of residual WO 3 particles for the W-codoped samples in Figure 6d. However, due to the similar crystallization habits, it is difficult to distinguish the orthorhombic WO 3 particles from the rutile particles in the SEM image. The grain growth behavior of the pigment powders during the heat-treatment process is understandable by considering the effect of doping such as the formation of defects and distortion of the crystalline lattice. The obtained results indicate that the surface diffusion barrier of grains was increased, and hence the grain growth was hindered with the addition of 10% Sb and Nb. It is also noteworthy that polyhedral particles with a smooth surface and relatively uniform size were formed when Mo was codoped (Figure 6e), which may be related to the generation of a little liquid phase during the powder synthesis process due to the relatively low melting point of MoO 3 . For the oxide pigments, the particle morphology and size of the powders have an important influence on their performance for various applications. For instance, when used as the ink pigments, the well-dispersed superfine particles derived from the Cr/Sb and Cr/Nb doping could help to shorten the grinding time and improve the fluidity of the ink, thereby improving the color performance and reducing the processing cost. The synthesized Ti0.9Cr0.1O2 and Ti0.8Cr0.1M0.1O2 (M = Sb, Nb, W, and Mo) pigments were also treated with 5 wt% H2SO4, HCl, and NaOH aqueous solution for 60 min, respectively, in order to evaluate their chemical stability. No obvious mass loss was found for all the tested samples, suggesting that the obtained pigments had good resistance to acid and alkali corrosion. Table 3 presents the CIELab chromatic parameters of pigments before and after acid and alkali corrosion tests. For comparison, the calculated chromatic aberration indexes (ΔE*) were also included. It is evident that the chromaticity parameter L*, a*, b* values did not change much after the 5 wt% acid and alkali corrosion tests, and all the ΔE* values were less than 2.44, indicating that the synthesized pigments had excellent stability [42][43][44][45]. This is mainly due to the excellent chemical stability of fully crystallized rutile TiO2 subjected to high-temperature calcination. The doped Cr 3+ and counterions were not easily leached out by dilute acid and alkali solutions after they solidified into the rutile lattice, which is beneficial in improving the stability of pigments in applications of paint and plastic coloring. Table 3. Comparison of chromatic parameters for the Cr-doped TiO2 pigments with and without counterions before and after the acid and alkali corrosion tests. were also treated with 5 wt% H 2 SO 4 , HCl, and NaOH aqueous solution for 60 min, respectively, in order to evaluate their chemical stability. No obvious mass loss was found for all the tested samples, suggesting that the obtained pigments had good resistance to acid and alkali corrosion. Table 3 presents the CIELab chromatic parameters of pigments before and after acid and alkali corrosion tests. For comparison, the calculated chromatic aberration indexes (∆E*) were also included. It is evident that the chromaticity parameter L*, a*, b* values did not change much after the 5 wt% acid and alkali corrosion tests, and all the ∆E* values were less than 2.44, indicating that the synthesized pigments had excellent stability [42][43][44][45]. This is mainly due to the excellent chemical stability of fully crystallized rutile TiO 2 subjected to high-temperature calcination. The doped Cr 3+ and counterions were not easily leached out by dilute acid and alkali solutions after they solidified into the rutile lattice, which is beneficial in improving the stability of pigments in applications of paint and plastic coloring. It is well-known that the pigments applied for ceramic or enamel glaze coloring and glass decoration should have excellent color-rendering properties and high-temperature stability in the glaze or glass melts [46]. In this work, the prepared pigments (5% of the total weight of the glaze) were applied in a transparent glaze to investigate the coloring performance and hiding power as well as high-temperature chemical stability. Figure 7 shows the surface photos of the colored glazes prepared by sintering at 1000 • C for 20 min, and their corresponding colorimetric parameters are shown in Table 4. As can be seen, all the colored glazes exhibited sufficient hiding power to cover the ceramic bodies, which can be attributed to the strong opacifying ability of rutile TiO 2 with high refractive index, and their color tones were similar to those of the respective pigments. In addition, the yellowness (b* value) was increased to some extent due to the synergistic effect of the glaze and pigment. These results show that the prepared pigments have good coloring power and stability in high-temperature glass fluxes, and have good application potential in ceramic, enamel, and glass decorations. It is well-known that the pigments applied for ceramic or enamel glaze coloring and glass decoration should have excellent color-rendering properties and high-temperature stability in the glaze or glass melts [46]. In this work, the prepared pigments (5% of the total weight of the glaze) were applied in a transparent glaze to investigate the coloring performance and hiding power as well as high-temperature chemical stability. Figure 7 shows the surface photos of the colored glazes prepared by sintering at 1000 °C for 20 min, and their corresponding colorimetric parameters are shown in Table 4. As can be seen, all the colored glazes exhibited sufficient hiding power to cover the ceramic bodies, which can be attributed to the strong opacifying ability of rutile TiO2 with high refractive index, and their color tones were similar to those of the respective pigments. In addition, the yellowness (b* value) was increased to some extent due to the synergistic effect of the glaze and pigment. These results show that the prepared pigments have good coloring power and stability in high-temperature glass fluxes, and have good application potential in ceramic, enamel, and glass decorations.

Conclusions
The rutile TiO 2 -based pigments codoped with chromophore ion of Cr 3+ and various charge-balancing ions of Sb, Nb, W, and Mo were prepared by a low-cost solid-state reaction method. The introduction of counterions has significant effects on the phase composition, color-rendering, and spectroscopic properties. For the 10% Cr single-doped pigment, an obvious second-phase impurity of Ti 0.78 Cr 0.12 O 1.74 was formed in addition to the rutile phase calcined at 1100 • C for 2 h. With the codoping of 5% and 10% counterion species (Sb, Nb, W, or Mo), all the obtained pigments possessed the main crystalline phase of the rutile structure, indicating that the addition of counterions effectively enhanced the solubility of Cr 3+ in the TiO 2 lattice. The pigment doped with only 10% Cr showed a dark brown color. With the codoping of 5% Sb, the yellow color was obtained, and much higher yellowness and chroma could be achieved with a further increase to 10%, while the addition of 10% Nb led to a yellowish-orange. The color of the rutile pigment can also be tailored to be reddish-orange and dark reddish-brown, respectively, at the W-doping level of 5% and 10%. However, both Mo-doped pigments appeared to have a similar black color. The influence of counterions on the chromatic performance of obtained rutile pigments could be attributed to the lattice distortion and variation in the charge balance condition depending on the radius and valence of codoped cations, which can change the crystal band structure and thus the visible light absorption/reflection properties. The addition of Sb, Nb, and Mo enhanced the NIR reflectance of pigments significantly in the whole NIR region. Furthermore, the inhibition of grain growth and better dispersion could also be achieved by the codoping of Sb or Nb, resulting in the nano-sized pigment powders. All the prepared rutile pigments exhibited good chemical stability subjected to the corrosion of dilute acid and alkali solutions as well as excellent color-rendering properties in the ceramic glaze. The obtained results show the effectiveness of counterions in tailoring the color-rendering performance of rutile pigments. Based on this, it can be predicted that the performance of rutile pigments with different hues can be further optimized by rational design of the chromophore ion-counterion pairs and their proportions. In the same way, the development of high NIR reflectance pigments as cool pigments is also worthy of research in the near future, especially in the wavelength of 750-1350 nm. Thus, the present research contributes to a good foundation for the development of eco-friendly pigments with the required color for diverse industrial applications.