Preparation and Chromaticity Control of CoTiO3/NiTiO3 Co-Coated TiO2 Composite Pigments

In this study, home-made amorphous TiO2 microspheres with good mono-dispersity and large numbers of mesopores on the surface were used as substrates. The intermediate microspheres were obtained by adding Co/Ni sources with different Co/Ni molar ratios in a water bath and making them react by water bath heating. By calcining the intermediate microspheres deposited on the TiO2 ones, a core–shell structured spherical CoTiO3/NiTiO3 inorganic composite pigment was prepared. The synthesized pigments were characterized by X-ray diffraction (XRD), Raman spectroscopy, field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectrometry (EDS), laser particle size (LPS) analysis and colorimetry. The results show that when the calcination temperature is 800 °C and the Co/Ni molar ratio is 0.5:0.5, the pigments consist of a TiO2 core and outer ilmenite CoTiO3/NiTiO3 shell. Moreover, the surface of the product microspheres is smooth, and the particles are of regular sphericity with a uniform particle size of about 1.8 μm. The colorimetric analysis from the samples calcined at 800 °C shows color changes from yellow-green to dark green as the Co/Ni molar ratio increases (0.1:0.9 to 0.9:0.1). A Co/Ni molar ratio that is too high or too low results in the formation of by-products such as Co3O4 or NiO, respectively, which adhere to the product surface and affect the chromaticity of the product. This work has enabled the chromatic modulation of yellow-green inorganic pigments, providing a solution for the preparation of spherical inorganic pigments that are more suitable for industrial inkjet printing.


Introduction
Owing to excellent weather resistance, thermal stability and high hiding power, inorganic pigments are widely used in ceramics, high temperature materials, inks, coating materials and plastics [1][2][3][4][5]. However, most inorganic pigments contain toxic elements such as heavy metals, making inorganic pigments not only costly, but also have adverse effects on human health and the environment, which severely restricts their development [6][7][8]. Therefore, the pigment industry necessitates the development of new inorganic pigments or the improvement of existing technology to eliminate or greatly reduce the harmful effects of toxic elements on human health in conventional inorganic pigments [9][10][11][12][13][14][15].
The precursor TiO 2 microspheres used in this study were homemade using a lowtemperature modified Stöber method, which was previously reported by our group [38]. In this method, aqueous KCl and tetrabutyl titanate were added to ethanol and reacted at a low temperature of −10 • C for 5 h. The precursor was then collected by centrifugation, washed three times with alcohol and once with water, and subsequently freeze-dried, resulting in amorphous TiO 2 microspheres.

Synthesis of NiTiO 3 /CoTiO 3 Co-Coated TiO 2 Composite Pigments
The preparation process of the composite pigment powders is shown in Figure 1. The precursor TiO2 microspheres used in this study were homemade using a lowtemperature modified Stöber method, which was previously reported by our group [38]. In this method, aqueous KCl and tetrabutyl titanate were added to ethanol and reacted at a low temperature of −10 °C for 5 h. The precursor was then collected by centrifugation, washed three times with alcohol and once with water, and subsequently freeze-dried, resulting in amorphous TiO2 microspheres.

Synthesis of NiTiO3/CoTiO3 Co-Coated TiO2 Composite Pigments
The preparation process of the composite pigment powders is shown in Figure 1. The TiO2 microspheres prepared using the above method (0.3 g) were first ultrasonically dispersed in 100 mL of deionized water, and then Co(NO3)2·6H2O, NiCl2·6H2O and 12 g urea were added under magnetic stirring for 10 min. The molar ratios of Co/Ni/TiO2 were 0.1:0.9:1, 0.3:0.7:1, 0.5:0.5:1, 0.7:0.3:1, and 0.9:0.1:1. The above suspensions were subsequently heated in a water bath at 80 °C under magnetic stirring for 8 h to obtain intermediate microspheres according to the reaction equations: The precipitate was then collected by centrifugation, washed three times with water, and dried at 50 °C for 12 h. The dried sample was calcined at 800 °C for 3 h. The corresponding reaction equations are as follows:

Characterization Techniques
The structural properties of the products were characterized by X-ray diffraction (XRD) using an X-ray diffractometer (PANalytical, Empyrean, Almelo, Netherlands) with Cu Kα radiation (λ = 0.154 nm) and a 2θ scan range of 20°-75° at a rate of 2°/min. Raman spectra were recorded on a XploRA PLUS spectrometer (HORIBA, Lille, France) using a 532 nm wavelength laser source.
The morphologies and elemental ratio of the samples were determined by a fieldemission scanning electron microscope (FE-SEM, SU8010, Hitachi, Tokyo, Japan) operating at 5 kV with an energy dispersive X-ray spectrometer (EDS, X-Max N , OXFORD, Oxford, Britain). TEM and HRTEM images were obtained using a transmission electron microscope (TEM, JEM-2100, JEOL, Tokyo, Japan) with an acceleration voltage of 200 kV.
A laser particle size (LPS) analyzer (Malvern MS 2000, Malvern, Britain) was employed to characterize the size distribution of the powders. The photographs of the synthesized composite pigments were obtained directly with a Canon m50 camera. The color The TiO 2 microspheres prepared using the above method (0.3 g) were first ultrasonically dispersed in 100 mL of deionized water, and then Co(NO 3 ) 2 ·6H 2 O, NiCl 2 ·6H 2 O and 12 g urea were added under magnetic stirring for 10 min. The molar ratios of Co/Ni/TiO 2 were 0.1:0.9:1, 0.3:0.7:1, 0.5:0.5:1, 0.7:0.3:1, and 0.9:0.1:1. The above suspensions were subsequently heated in a water bath at 80 • C under magnetic stirring for 8 h to obtain intermediate microspheres according to the reaction equations: The precipitate was then collected by centrifugation, washed three times with water, and dried at 50 • C for 12 h. The dried sample was calcined at 800 • C for 3 h. The corresponding reaction equations are as follows: NiO + TiO 2 → NiTiO 3 ,

Characterization Techniques
The structural properties of the products were characterized by X-ray diffraction (XRD) using an X-ray diffractometer (PANalytical, Empyrean, Almelo, Netherlands) with Cu Kα radiation (λ = 0.154 nm) and a 2θ scan range of 20 • -75 • at a rate of 2 • /min. Raman spectra were recorded on a XploRA PLUS spectrometer (HORIBA, Lille, France) using a 532 nm wavelength laser source.
The morphologies and elemental ratio of the samples were determined by a fieldemission scanning electron microscope (FE-SEM, SU8010, Hitachi, Tokyo, Japan) operating at 5 kV with an energy dispersive X-ray spectrometer (EDS, X-Max N , OXFORD, Oxford, Britain). TEM and HRTEM images were obtained using a transmission electron microscope (TEM, JEM-2100, JEOL, Tokyo, Japan) with an acceleration voltage of 200 kV.
A laser particle size (LPS) analyzer (Malvern MS 2000, Malvern, Britain) was employed to characterize the size distribution of the powders. The photographs of the synthesized composite pigments were obtained directly with a Canon m50 camera. The color tones of the pigments were evaluated by measuring L*, a*, and b* parameters with a colorimeter CR-10Plus (Konica Minolta, Tokyo, Japan), in which L* indicates the color lightness (from 0 (black) to 100 (white)), a* denotes the red/green intensity (negative and positive values correspond to green and red colors, respectively), and b* represents the yellow/blue intensity (negative and positive values correspond to blue and yellow colors, respectively). Figure 2 shows the XRD patterns of the products after calcination at 800 • C for samples with different Co/Ni molar ratios. As shown in Figure 2a, when the Co/Ni molar ratio is 0.1:0.9, the products mainly consist of CoTiO 3 and/or NiTiO 3 (the diffraction peaks of CoTiO 3 and NiTiO 3 are very similar in position), NiO and anatase TiO 2 .

X-ray Diffraction
orimeter CR-10Plus (Konica Minolta, Tokyo, Japan), in which L* indicates the color lightness (from 0 (black) to 100 (white)), a* denotes the red/green intensity (negative and positive values correspond to green and red colors, respectively), and b* represents the yellow/blue intensity (negative and positive values correspond to blue and yellow colors, respectively). Figure 2 shows the XRD patterns of the products after calcination at 800 °C for samples with different Co/Ni molar ratios. As shown in Figure 2a, when the Co/Ni molar ratio is 0.1:0.9, the products mainly consist of CoTiO3 and/or NiTiO3 (the diffraction peaks of CoTiO3 and NiTiO3 are very similar in position), NiO and anatase TiO2. The presence of NiO diffraction peaks indicates that the reaction between NiO and TiO2 is not complete, and there is a surplus of NiO. This might be because the CoTiO3 and NiTiO3 produced by the reaction of the Co and Ni sources with TiO2, respectively, are coated on the surface of the TiO2 microspheres, preventing the further reaction of NiO with TiO2 and resulting in a surplus of NiO. As the amount of Co source increases, the NiO peak gradually disappears. When the Co/Ni molar ratio is increased to 0.9:0.1, as shown in Figure 2e, the diffraction pattern suggests the presence of Co3O4 phase in the product, which could similarly be due to the excess of Co source and the pre-formed CoTiO3 and NiTiO3 on the surface of the microspheres hindering the further reaction of CoO with TiO2. Further oxidation of the remaining CoO at high temperature results in the formation of Co3O4.

X-Ray Diffraction
In addition, the results in Figure 2 show that the crystalline forms of TiO2 in the product are also influenced by the Co/Ni ratio. The home-made precursor TiO2 microspheres are amorphous [38], and when the Co/Ni ratio was low, the precursor TiO2 was converted to anatase phase in addition to reacting with Co and Ni sources; when the Co/Ni ratio was high, both anatase and rutile phases of TiO2 could be found in the products. The reason for this phenomenon could be that the presence of a Co source is more favorable to the formation of rutile phase TiO2, which also affects the brightness values of the products. The presence of NiO diffraction peaks indicates that the reaction between NiO and TiO 2 is not complete, and there is a surplus of NiO. This might be because the CoTiO 3 and NiTiO 3 produced by the reaction of the Co and Ni sources with TiO 2 , respectively, are coated on the surface of the TiO 2 microspheres, preventing the further reaction of NiO with TiO 2 and resulting in a surplus of NiO. As the amount of Co source increases, the NiO peak gradually disappears. When the Co/Ni molar ratio is increased to 0.9:0.1, as shown in Figure 2e, the diffraction pattern suggests the presence of Co 3 O 4 phase in the product, which could similarly be due to the excess of Co source and the pre-formed CoTiO 3 and NiTiO 3 on the surface of the microspheres hindering the further reaction of CoO with TiO 2 . Further oxidation of the remaining CoO at high temperature results in the formation of Co 3 O 4 .
In addition, the results in Figure 2 show that the crystalline forms of TiO 2 in the product are also influenced by the Co/Ni ratio. The home-made precursor TiO 2 microspheres are amorphous [38], and when the Co/Ni ratio was low, the precursor TiO 2 was converted to anatase phase in addition to reacting with Co and Ni sources; when the Co/Ni ratio was high, both anatase and rutile phases of TiO 2 could be found in the products. The reason for this phenomenon could be that the presence of a Co source is more favorable to the formation of rutile phase TiO 2 , which also affects the brightness values of the products.

Raman Spectroscopy
Because the diffraction peaks of CoTiO 3 and NiTiO 3 are very similar in position, making it difficult to distinguish between the two by X-ray diffraction, the structural properties and phase changes of the samples were further investigated by Raman spectroscopy. Figure 3a-c show the Raman spectra of the products after calcination at 800 • C for samples with Co/Ni molar ratios of 0.1:0.9, 0.5:0.5 and 0.9:0.1, respectively. Seven peaks from P1 to P7 can be observed in Figure 3a. A review of the literature shows that the characteristic peaks of anatase TiO 2 are around 637.7, 515 and 396 cm −1 [39], corresponding to the three peaks at P4, P5 and P6 in Figure 3a. The characteristic peaks of NiTiO 3 are around 238, 284, 338 and 705 cm −1 , corresponding to the peaks at P1, P2, P3 and P7 in Figure 3a [40]. This indicates that when the Co/Ni molar ratio is 0.1:0.9, the product is dominated by NiTiO 3 . When the Co/Ni molar ratio is increased to 0.9:0.1, peaks P1, P2, P3 and P7 shift to P1 , P2 , P3 and P7 in Figure 3c, which correspond to those of CoTiO 3 (the characteristic peaks of CoTiO 3 are around 237, 266, 335 and 695 cm −1 [9]). It is worth mentioning that, in Figure 3a, the peak near 460 cm −1 corresponds to the characteristic peak of NiTiO 3 [40], which can be explained by the fact that when the Co/Ni molar ratio is very low, and the phase in the product is dominated by NiTiO 3 . As the Co/Ni molar ratio increases, the peak shifts to near 445 cm −1 in Figure 3c, a position that corresponds to the characteristic peak of the rutile phase TiO 2 [41]. This is in agreement with the XRD results in Figure 2, where the rutile phase appears in the product when the Co/Ni molar ratio is high, as seen in Figure 2e. The change in the position of the peaks between Figure 3a,c indicates that the product changes from being NiTiO 3 dominated to CoTiO 3 dominated as the Co/Ni molar ratio increases.
ing it difficult to distinguish between the two by X-ray diffraction, the structural properties and phase changes of the samples were further investigated by Raman spectroscopy. Figure 3a-c show the Raman spectra of the products after calcination at 800 °C for samples with Co/Ni molar ratios of 0.1:0.9, 0.5:0.5 and 0.9:0.1, respectively. Seven peaks from P1 to P7 can be observed in Figure 3a. A review of the literature shows that the characteristic peaks of anatase TiO2 are around 637.7, 515 and 396 cm −1 [39], corresponding to the three peaks at P4, P5 and P6 in Figure 3a. The characteristic peaks of NiTiO3 are around 238, 284, 338 and 705 cm −1 , corresponding to the peaks at P1, P2, P3 and P7 in Figure 3a [40]. This indicates that when the Co/Ni molar ratio is 0.1:0.9, the product is dominated by NiTiO3. When the Co/Ni molar ratio is increased to 0.9:0.1, peaks P1, P2, P3 and P7 shift to P1′, P2′, P3′ and P7′ in Figure 3c, which correspond to those of CoTiO3 (the characteristic peaks of CoTiO3 are around 237, 266, 335 and 695 cm −1 [9]). It is worth mentioning that, in Figure 3a, the peak near 460 cm −1 corresponds to the characteristic peak of NiTiO3 [40], which can be explained by the fact that when the Co/Ni molar ratio is very low, and the phase in the product is dominated by NiTiO3. As the Co/Ni molar ratio increases, the peak shifts to near 445 cm −1 in Figure 3c, a position that corresponds to the characteristic peak of the rutile phase TiO2 [41]. This is in agreement with the XRD results in Figure 2, where the rutile phase appears in the product when the Co/Ni molar ratio is high, as seen in Figure 2e. The change in the position of the peaks between Figure 3a,c indicates that the product changes from being NiTiO3 dominated to CoTiO3 dominated as the Co/Ni molar ratio increases.  Figure 4 shows FE-SEM images of the precursor TiO2 microspheres, the intermediate microspheres at a Co/Ni molar ratio of 0.5:0.5 and the resulting composite pigment powder after calcination at 800 °C. The surface of the precursor TiO2 microspheres is mesoporous, the sphericity of the microspheres is regular and the particle size is relatively uniform, as can be seen in Figure 4(a1,a2). The detailed microscopic and structure characterization of the precursor TiO2 microspheres has already been reported by us [38]. Figure  4(b1,b2) shows many flakes attached to the surface of the intermediate microspheres, which are probably the reaction products of Co and Ni salts with urea during the water  Figure 4 shows FE-SEM images of the precursor TiO 2 microspheres, the intermediate microspheres at a Co/Ni molar ratio of 0.5:0.5 and the resulting composite pigment powder after calcination at 800 • C. The surface of the precursor TiO 2 microspheres is mesoporous, the sphericity of the microspheres is regular and the particle size is relatively uniform, as can be seen in Figure 4(a1,a2). The detailed microscopic and structure characterization of the precursor TiO 2 microspheres has already been reported by us [38]. Figure 4(b1,b2) shows many flakes attached to the surface of the intermediate microspheres, which are probably the reaction products of Co and Ni salts with urea during the water bath heating process (see Equations (1) and (2)). As the precursor TiO 2 is a porous structured microsphere, and TiO 2 tends to adsorb hydroxyl groups, this makes it easier for Co(OH) 2 and Ni(OH) 2 to be generated and attached to the TiO 2 surface during the water bath process. Figure 4(c1,c2) shows that, after calcination, the flakes attached to the surface of the synthesized microspheres have disappeared due to the solid phase reaction between the oxides generated by the decomposition of Co(OH) 2 and Ni(OH) 2 at high temperatures and TiO 2 , which yields CoTiO 3 and NiTiO 3 on the surface of the synthesized microspheres (see Equations (3) and (4)), which agrees with the results of X-ray diffraction and Raman spectroscopy. For further proof, EDS analysis of the red boxed area in Figure 4(c2) was carried out to determine the elements of the product, and the results are shown in Figure 5. The spectrum confirms the presence of Ti, Ni, Co and O elements in the product, indicating that CoTiO 3 and NiTiO 3 were indeed generated on the surface of the synthesized microspheres. The estimated atomic ratios of these elements are close to the nominal ones. In contrast to the results of Zou et al. [9,20], the pigment particles prepared in this work have better mono-dispersity and a more homogeneous particle size while ensuring the purity of the sample, making them more suitable for industrial inkjet printing.

FE-SEM Observations
of the synthesized microspheres have disappeared due to the solid phase reaction between the oxides generated by the decomposition of Co(OH)2 and Ni(OH)2 at high temperatures and TiO2, which yields CoTiO3 and NiTiO3 on the surface of the synthesized microspheres (see Equations (3) and (4)), which agrees with the results of X-ray diffraction and Raman spectroscopy. For further proof, EDS analysis of the red boxed area in Figure  4(c2) was carried out to determine the elements of the product, and the results are shown in Figure 5. The spectrum confirms the presence of Ti, Ni, Co and O elements in the product, indicating that CoTiO3 and NiTiO3 were indeed generated on the surface of the synthesized microspheres. The estimated atomic ratios of these elements are close to the nominal ones. In contrast to the results of Zou et al. [9,20], the pigment particles prepared in this work have better mono-dispersity and a more homogeneous particle size while ensuring the purity of the sample, making them more suitable for industrial inkjet printing.   Figure 4(c2)). Figure 6 shows the FE-SEM images of the samples with different Co/Ni molar ratios after calcination at 800 °C. As shown in Figure 6(a1,a2), at a Co/Ni molar ratio of 0.1:0.9, many particles approximately 100 μm in size are attached to the surface of the calcined product. Combined with the XRD results in Section 3.1, we can conclude that these parti- Figure 5. EDS spectrum of products calcined at 800 • C with a Co/Ni ratio of 0.5:0.5 (the analyzed area corresponds to the red box marked in Figure 4(c2)). Figure 6 shows the FE-SEM images of the samples with different Co/Ni molar ratios after calcination at 800 • C. As shown in Figure 6(a1,a2), at a Co/Ni molar ratio of 0.1:0.9, many particles approximately 100 µm in size are attached to the surface of the calcined product. Combined with the XRD results in Section 3.1, we can conclude that these particles are mainly NiO left over from the incomplete reaction with TiO 2 . From Figure 4(c1,c2) and Figure 6(b1,b2), it can be stated that: (i) the remaining particles on the surface of the synthesized microspheres gradually decrease in number as the amount of Co source increases, and (ii) when the Co/Ni molar ratio is 0.5:0.5, the surface of the product microspheres is the smoothest, and the synthesized particles have regular sphericity and are homogeneous in size. Laser particle size analysis of the sample with a Co/Ni molar ratio of 0.5:0.5 was carried out, and the results are shown in Figure 7. It can be seen that the average particle size of composite pigments is around 1.8 µm. With a further increase in the Co/Ni molar ratio at 0.7:0.3 and 0.9:0.1, Figure 6(c1,c2,d1,d2) indicate that the number of particles attached to the surface of the synthesized microspheres gradually increases again. When the Co/Ni molar ratio is 0.9:0.1, it can be seen in Figure 6(d1,d2) that, similar to the situation when the Co/Ni molar ratio is 0.1:0.9, the particles attached to the product surface are mainly Co 3 O 4 formed by further oxidation of the remaining CoO that has not reacted completely with TiO 2 .

TEM Images
The microstructures of the obtained composite pigments with a Co/Ni molar ratio of 0.5:0.5 were further characterized by TEM, and the images are shown in Figure 8. The as-prepared pigments' diameter was mainly around 1.8 µm (Figure 8a), which is consistent with the FE-SEM and laser particle size analysis results shown in Figure 4(c1,c2) and Figure 7, respectively. The composite pigment powder has a typical core-shell structure with a relatively smoothed out layer that is approximately 80 nm thick (Figure 8a). The high-resolution TEM (HRTEM) image of the particle edge provides information on the phase of the shell of the composite pigment. As shown in Figure 8b, the outer layer exhibits clear lattice fringes, which denotes a crystalline structure. The inter-planar spacing was calculated to be around 0.27 nm, which is consistent with the distance between two (104) crystal planes of ilmenite CoTiO 3 and NiTiO 3 [9,37]. By combining XRD and Raman results, we can conclude that when the Co/Ni ratio is 0.5:0.5, the prepared composite pigment particles consist of an outer CoTiO 3 /NiTiO 3 shell and inner TiO 2 core.

TEM Images
The microstructures of the obtained composite pigments with a Co/Ni molar ratio of 0.5:0.5 were further characterized by TEM, and the images are shown in Figure 8. The asprepared pigments' diameter was mainly around 1.8 μm (Figure 8a), which is consistent with the FE-SEM and laser particle size analysis results shown in Figures 4(c1,c2) and 7, respectively. The composite pigment powder has a typical core-shell structure with a relatively smoothed out layer that is approximately 80 nm thick (Figure 8a). The high-resolution TEM (HRTEM) image of the particle edge provides information on the phase of the shell of the composite pigment. As shown in Figure 8b, the outer layer exhibits clear lattice fringes, which denotes a crystalline structure. The inter-planar spacing was calculated to be around 0.27 nm, which is consistent with the distance between two (104) crystal planes of ilmenite CoTiO3 and NiTiO3 [9,37]. By combining XRD and Raman results, we can conclude that when the Co/Ni ratio is 0.5:0.5, the prepared composite pigment particles consist of an outer CoTiO3/NiTiO3 shell and inner TiO2 core.  Figure 9 shows photographs of the composite pigment powders obtained after calcination at 800 °C for samples with different Co/Ni molar ratios. Figure 10 depicts the values  Figure 9 shows photographs of the composite pigment powders obtained after calcination at 800 • C for samples with different Co/Ni molar ratios. Figure 10 depicts the values of the colorimetric parameters of the corresponding composite pigment powders shown in Figure 9. As can be seen from Figure 9, the color of the calcined product changes from yellow-green to dark green as the Co/Ni molar ratio increases, and this trend is further confirmed by the values of the colorimetric parameters ( Figure 10).  As seen from the results in Figure 10, the luminosity values (L*) of the calcined products decrease from 60.1 to 42.4 as the Co content increases. This might be due to the solid phase reaction of bright TiO2 with NiO and CoO to generate green CoTiO3 and yellow NiTiO3, respectively, which act as a shell covering the surface of TiO2, resulting in a decrease in the luminosity values (L*) of the calcined products. Meanwhile, as the Co content increases, the excess black Co3O4 phase in the product shades the surface of the spherical powder, which leads to a decrease in the luminosity values (L*) of the calcined products [42].

Colorimetric Analysis
The yellow values (b*) show a similar trend, decreasing from 18.3 to 1.7 as the amount of Ni source decreases, resulting in a decrease in the quantity of yellow NiTiO3 formed by  As seen from the results in Figure 10, the luminosity values (L*) of the calcined products decrease from 60.1 to 42.4 as the Co content increases. This might be due to the solid phase reaction of bright TiO2 with NiO and CoO to generate green CoTiO3 and yellow NiTiO3, respectively, which act as a shell covering the surface of TiO2, resulting in a decrease in the luminosity values (L*) of the calcined products. Meanwhile, as the Co content increases, the excess black Co3O4 phase in the product shades the surface of the spherical powder, which leads to a decrease in the luminosity values (L*) of the calcined products [42].
The yellow values (b*) show a similar trend, decreasing from 18.3 to 1.7 as the amount of Ni source decreases, resulting in a decrease in the quantity of yellow NiTiO3 formed by the solid phase reaction with TiO2.
The green values (−a*) tend to increase and then decrease with the increase in Co As seen from the results in Figure 10, the luminosity values (L*) of the calcined products decrease from 60.1 to 42.4 as the Co content increases. This might be due to the solid phase reaction of bright TiO 2 with NiO and CoO to generate green CoTiO 3 and yellow NiTiO 3, respectively, which act as a shell covering the surface of TiO 2 , resulting in a decrease in the luminosity values (L*) of the calcined products. Meanwhile, as the Co content increases, the excess black Co 3 O 4 phase in the product shades the surface of the spherical powder, which leads to a decrease in the luminosity values (L*) of the calcined products [42].
The yellow values (b*) show a similar trend, decreasing from 18.3 to 1.7 as the amount of Ni source decreases, resulting in a decrease in the quantity of yellow NiTiO 3 formed by the solid phase reaction with TiO 2 .
The green values (−a*) tend to increase and then decrease with the increase in Co content. The green values reach the maximum value of 16.6 when the Co/Ni molar ratio is 0.5:0.5, and then decrease. The reason for the initial increase in the green values is the increase in Co content, which causes TiO 2 to react with more CoO in the solid phase to form more CoTiO 3 , which is green in color. As the Co content increases further, excess black Co 3 O 4 phase is generated, which shades the CoTiO 3 and leads to a decrease in the green values of the calcined products. The variation in the green values with the Co/Ni molar ratio for the samples prepared in this work follows a similar tendency to that observed by Zou et al. [9] with an increasing and then decreasing trend.
The colorimetric analysis shows that the change in phase composition leads to a difference in the color properties of the products, and the experimental results show that the chromaticity properties of the pigments can be modulated by changing the molar ratio of Co/Ni.

Conclusions
In this study, CoTiO 3 /NiTiO 3 co-coated TiO 2 core-shell structure composite pigment particles were prepared by wet chemical synthesis and high-temperature calcination using as substrates amorphous TiO 2 microspheres with good mono-dispersity and a large number of mesopores on the surface. When the calcination temperature is 800 • C and the Co/Ni molar ratio is 0.5:0.5, the obtained product consists of a TiO 2 core and outer ilmenite CoTiO 3 /NiTiO 3 shell, and no impurities such as Co 3 O 4 or NiO are found. The Raman spectroscopy technique has allowed us to identify the phases NiTiO 3 and CoTiO 3 . The surface of the microspheres is smooth, and the particles are of regular sphericity, with a uniform particle size of about 1.8 µm. As the Co content increases, the luminosity (L*) and yellow (b*) values of the calcined products decrease (L* value: from 60.1 to 42.4, and b* value: from 18.3 to 1.7), and the green values (−a*) show increasing and then decreasing trends (−a* value: from 11.6 to 16.6 and then to 7.0). At the same time, the color of the product changes from yellow-green to dark green. A Co/Ni molar ratio that is too high or too low results in the formation of by-products such as Co 3 O 4 or NiO, respectively, which adhere to the product's surface and affect the chromaticity of the product. This work has enabled the chromatic modulation of yellow-green inorganic pigments, providing a solution for the preparation of spherical inorganic pigments that are more suitable for industrial inkjet printing.

Conflicts of Interest:
The authors declare no conflict of interest.