Synthesis and characterisation of Ba(Zn1−xCox)2Si2O7 (0 ≤ x ≤ 0.50) for blue-violet inorganic pigments

Ba(Zn1−xCox)2Si2O7 (0 ≤ x ≤ 0.50) solid solutions were synthesized as novel blue-violet inorganic pigments by a conventional solid-state reaction method. The crystal structure, optical properties, and colour of the pigments were characterized. All the pigments were obtained in a single-phase form. The pigments strongly absorbed visible light at wavelengths from 550 to 650 nm, corresponding to the range of green to orange light. This optical absorption was caused by the d–d transition of the tetrahedrally coordinated Co2+ (4A2(F) → 4T1(P)), which was the origin of the blue-violet colour of the pigments. The most intense colour was obtained for Ba(Zn0.85Co0.15)2Si2O7, where a* = +52.2 and b* = −65.5 in the CIE (Commission Internationale de l'Éclairage) L*a*b* system. These absolute values were significantly larger than those of commercial violet pigments such as Co3(PO4)2 (a* = +33 and b* = −32) and NH4MnP2O7 (a* = +39 and b* = −21). Therefore, the Ba(Zn0.85Co0.15)2Si2O7 pigment could be a novel blue-violet inorganic pigment.


Introduction
Cobalt (Co) and its compounds have been applied in alloys, paints, catalysts, cements and ceramic pigments. [1][2][3][4][5] In particular, the Co 2+ ion is oen employed as a colouring source for blue and violet pigments. There are a number of reports on blue pigments using Co 2+ ions, such as Co 2 SiO 4 olivine, 6 (Co, Zn) 2 SiO 4 willemite, 7 CoAl 2 O 4 spinel 8 and Co 2 SnO 4 . 9 The colouring performance of these pigments mainly depends on the coordination number around the Co 2+ ion, which is very important for the appearance of the bluish colour. However, the use of a large amount of Co increases the cost of the material, which becomes expensive because of its rarity.
Accordingly, selection of an appropriate host lattice, minimization of the Co content and strong colouring performance are necessary, when Co is applied to colour pigments. Although many studies on the reduction of the amount of Co in the pigments have been reported, [10][11][12][13][14] almost of them are on the blue pigments and there are only a few reports on the violet pigments. 13,15,16 Generally, the colour of inorganic pigments is mainly affected by the crystal eld, generated by the ions surrounding the chromophore. 17 Cobalt violet (Co 3 (PO 4 ) 2 ) and manganese violet (NH 4 MnP 2 O 7 ) has been well known as current commercial violet pigments. The structure of Co 3 (PO 4 ) 2 is formed by distorted trigonal bipyramids CoO 5 , fairly regular CoO 6 octahedra and almost regular PO 4 tetrahedra. 13 Generally, cobalt ion can take various coordination numbers and represent various colours in phosphates, In Co 3 (PO 4 ) 2 $8H 2 O, Co 2+ has octahedral 6 coordination to show reddish violet colour, while in KCoPO 4 it has tetrahedral 4 coordination to evince blue-violet colour. On the other hand, NH 4 MnP 2 O 7 presents violet colour due to the d-d transition of the octahedral coordinated Mn 3+ . 18 However, the vividness of the Co 3 (PO 4 ) 2 and the NH 4 MnP 2 O 7 pigments is insufficient, that is, their absolute values of a* and b* in the CIE L*a*b* system are not so large. In this system, the parameter L* indicate the brightness or darkness of a colour on relation to a neutral grey scale, while the parameters a* (the red-green axis) and b* (the yellow-blue axis) express the colour qualitatively. In addition, the thermal resistance of NH 4 MnP 2 O 7 is not enough, because it is decomposed around 340 C. 18 Some violet pigments without Co have been proposed recently, 19-21 but their colours are not much different from those of the existing commercial violet pigments. Organic pigments are inferior to inorganic pigments in heat resistance and weather resistance. Thus, it is signicant to synthesize a novel violet inorganic pigment in which the colour property is improved.
Because of this situation, we focused on barium zinc silicate BaZn 2 Si 2 O 7 as a host lattice of the novel violet pigment. This compound has a layered structure composed of [ZnO 4 ] 2À tetrahedra connected at each corner to [SiO 4 ] tetrahedra. Each [SiO 4 ] tetrahedron is connected over three corners to one [SiO 4 ] and two [ZnO 4 ] 2À tetrahedra, and the forth corner is a nonbridging oxygen atom. The Ba 2+ ions are located in between the zinc silicate layers. 22,23 As a related compound, Ba(M, Ni) 2 Si 2 O 7 (M ¼ Zn or Mg) pigments have been ever reported, but they exhibit red and purplish red colours due to the d-d transition of the tetrahedral coordinated Ni 2+ . 24 In this study, Ba(Zn 1Àx Co x ) 2 Si 2 O 7 (0 # x # 0.50) pigments were synthesized by a conventional solid-state reaction method, and the colour properties of the pigments were investigated as novel blue-violet inorganic pigments.

Materials and methods
The Ba(Zn 1Àx Co x ) 2 Si 2 O 7 (0 # x # 0.50) pigments were synthesized by a conventional solid-state reaction method. BaCO 3 (Kishida Chemical, Japan), ZnO (Kishida Chemical, Japan), SiO 2 (Wako Pure Chemical, Japan) and Co 3 O 4 (Wako Pure Chemical, Japan) were used as starting materials. The raw materials were mixed in a stoichiometric amount in an agate mortar. The mixture was calcined in an alumina boat at 1250 C for 6 h. The samples were ground in an agate mortar before characterisation.

Characterisation
The composition of the samples was conrmed by X-ray uorescence spectroscopy (XRF; Rigaku, ZSX Primus). The crystal structures of the samples were identied by X-ray powder diffraction (XRD; Rigaku, Ultima IV) with Cu-Ka radiation (40 kV, 40 mA). The sampling width and the scan speed were 0.02 and 6.0 min À1 , respectively. The lattice parameters and volumes were calculated from the XRD peak angles, which were rened using a-Al 2 O 3 as a standard and using the CellCalc Ver. 2.20 soware. The size and morphology of the Ba(Zn 0.85 Co 0.15 ) 2 Si 2 O 7 particles were observed by using scanning electron microscopy (SEM; JEOL, JSM-6701F). Gold was sputtered before observation to avoid the charge-up of the samples. The purity of the samples was analysed by energy dispersive X-ray analysis (EDX; Oxford Instruments, INCA Energy). An X-ray photoelectron spectrum (XPS; ULVAC-PHI, PHI5000 VersaProve II) of the Ba(Zn 0.85 -Co 0.15 ) 2 Si 2 O 7 pigment was measured using Mg-Ka radiation to investigate the oxidation state of the Co ion.
The optical reectance of the Ba(Zn 1Àx Co x ) 2 Si 2 O 7 (0 # x # 0.50) samples were measured using a UV-Vis spectrometer (Shimadzu, UV-2550) with barium sulphate as a reference. The colour properties of the samples were estimated in terms of the CIE L*a*b*Ch system using a calorimeter (Konica-Minolta, CR-300). This colour measurement was made for powder samples. In the case of the blue-violet pigment, positive a* and negative b* values are desirable. Chroma parameter (C) represents the colour saturation of the pigments and is calculated according to the following formula: C ¼ [(a*) 2 + (b*) 2 ] 1/2 . The parameter h ranges from 0 to 360 (300 # h # 330 means blue-violet), and is calculated with the formula, h ¼ tan À1 (b*/a*).

Results and discussion
X-ray uorescence analysis (XRF) The XRF analysis data of the samples were listed in Table 1. All compositions were almost in good agreement with those of the nominal stoichiometric ones.
X-ray powder diffraction (XRD) Fig. 1 shows the XRD patterns of the Ba(Zn 1Àx Co x ) 2 Si 2 O 7 (0 # x # 0.50) pigments. All samples were obtained in a single-phase form and the diffraction patterns were well indexed to that of the monoclinic BaZn 2 Si 2 O 7 structure whose space group was C2/c. 22,23,25 Fig. 3. It was conrmed that Ba, Zn, Si, Co and O were present and non-impurities were observed without Au (chargeup preventer). The X-ray dot mapping analysis results is depicted in Fig. 4, indicating that the component elements were uniformly distributed in the particle.

X-ray photoelectron spectrum (XPS)
The XPS of the Ba(Zn 0.85 Co 0.15 ) 2 Si 2 O 7 pigment is shown in Fig. 5. This spectrum was deconvoluted into three components, considering the spin-orbit doublets. The intense peaks at 795.3 eV and 780.2 eV were attributed to the Ba 2+ 3d 3/2 and 3d 5/2 congurations, respectively. 28,29 Although the small peaks observed at 793.7 eV and 778.6 eV were assigned to the Co 3+ 2p 3/2 and 2p 1/2 lines, more intense peaks were also detected at 796.3 eV and 781.0 eV, corresponding to those of Co 2+ . 29

Chromatic properties
The L*a*b*Ch colour coordinate data for the Ba(Zn 1Àx Co x ) 2 -Si 2 O 7 (0 # x # 0.50) pigments are summarized in Table 3. They were compared using powder samples. It is obvious that the a* and b* values became signicantly positive and negative, respectively, by the introduction of Co 2+ in the host BaZn 2 Si 2 O 7 lattice. As mentioned in the previous section, the Ba(Zn 1Àx -Co x ) 2 Si 2 O 7 pigments absorbed the green-orange lights but reected the complementary blue and red lights. This is the reason for that positive a* and negative b* values were obtained in these pigments. The photographs of the Ba(Zn 1Àx Co x ) 2 Si 2 O 7 (0 # x # 0.50) samples are shown in Fig. 7. The colour of the Ba(Zn 1Àx Co x ) 2 Si 2 O 7 (0 # x # 0.50) pigments gradually changed from white to dark blue-violet as the Co 2+ concentration increased. Among the samples synthesized in this study, the largest absolute values in the colour coordinate data were obtained for Ba(Zn 0.85 Co 0.15 ) 2 Si 2 O 7 (a* ¼ +52.2 and b* ¼ À65.5).
They were compared with those of the commercially available Co 3 (PO 4 ) 2 and NH 4 MnP 2 O 7 pigments in Table 4. It is notable that the absolute values of a* and b* for the Ba(Zn 0.85 -Co 0.15 ) 2 Si 2 O 7 were signicantly larger than those for the commercial violet pigments.

Chemical stability tests
The chemical stability of the Ba(Zn 0.85 Co 0.15 ) 2 Si 2 O 7 pigment was also evaluated using a powder sample. The pigment was soaked into 4% acetic acid and 4% ammonium bicarbonate. Aer leaving them at room temperature for 2 h, the pigments were washed with deionized water and ethanol, and then dried at room temperature. The colour of the pigment aer the leaching test was evaluated using the calorimeter. As seen in Table 5, the colour of the present Ba(Zn 0.85 Co 0.15 ) 2 Si 2 O 7 pigment was almost unchanged.

Conclusions
Ba(Zn 1Àx Co x ) 2 Si 2 O 7 (0 # x # 0.50) solid solutions were successfully synthesized as novel blue-violet inorganic pigments. The samples strongly absorbed the visible light from 550 to 650 nm (green to orange), which was originated by the d-d transition of tetrahedrally coordinated Co 2+ . Ba(Zn 0.85 Co 0.15 ) 2 Si 2 O 7 showed the most intense colour among the samples, and the L*a*b*Ch parameters were L* ¼ 28.6, a* ¼ +52.2, b* ¼ À65.5, C ¼ 66.3, and h ¼ 308.6. The absolute values of a* and b* of Ba(Zn 0.85 Co 0.15 ) 2 Si 2 O 7 were signicantly larger than those of the commercial Co 3 (PO 4 ) 2 (a* ¼ +33 and b* ¼ À32) and NH 4 MnP 2 O 7 (a* ¼ +39 and b* ¼ À21) pigments. Furthermore, the Ba(Zn 0.85 Co 0.15 ) 2 Si 2 O 7 pigment has excellent chemical resistance and thermal stability. These results indicate that Ba(Zn 0.85 Co 0.15 ) 2 Si 2 O 7 could serve as an effective alternative to the conventional blue-violet inorganic pigments.

Conflicts of interest
There are no conicts to declare.    This journal is © The Royal Society of Chemistry 2018