Effect of La₃₊ on evolution of TiO₂ coating layers on lamellar sericite and their pigmentary properties

REN Min(任 敏), YIN Heng-bo(殷恒波), LU Zhang-zhun(卢章准), GE Chen(葛 晨),

WANG Ai-li(王爱丽), YU Long-bao(喻龙宝), JIANG Ting-shun(姜廷顺)

Faculty of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China

Received 31 July 2008; accepted 21 November 2008

Abstract: TiO₂-coated sericite powders were prepared by the chemical deposition method starting from lamellar sericite and TiCl₄ in the presence of La³⁺. After calcination at 900 ℃ for 1 h, the resultant TiO₂ nanoparticles on the sericite surfaces exist in anatase phase. The light scattering indexes of the TiO₂-coated lamellar sericite powders are dozens of times higher than those of the naked lamellar sericite powders, varying with different contents of La³⁺. The presence of La³⁺ in the deposition solution is beneficial to the formation of the small-sized anatase TiO₂ nanoparticles. The presence of La³⁺ also favors the formation of the dense and uniform island-like TiO₂ coating layers in a large range of the mass ratio of TiO₂ to sericite from 5% to 20%. XPS analysis shows that when La³⁺ is absent in the reaction solution, TiO₂ coating layers anchor on the sericite surface via Ti—O—Si and Ti—O—Al bonds. The presence of La³⁺ causes the formation of Si—O—La and Al—O—La bonds on the sericite surface and Ti—O—La bond on the surface of TiO₂ coating layers.

Key words: lamellar sericite; TiO₂ coating; lanthanum

1 Introduction

Mica is a general name for a group of complex hydrous potassium-aluminum silicate minerals, such as sericite, biotite, lepidolite, muscovite, phlogopite, and vermiculite, with a lamellar structure and a high aspect ratio (ratio of diameter to thickness). Conventionally, mica is widely used as a filler for paint and plastics[1] and a substrate for preparation of mica-titania pearlescent pigment[2]. Recently, flat mica has been found as a facile substrate for preparation of conducting composites[3-4], colored pigments[5-8], indicators of halogen activities [9], glass-ceramics[10-11], radioactive metal ion adsorbent[12], and shielding materials of electromagnetic interference[13].

Coating of powdered materials improves their physicochemical performances, such as dispersibility, lasting quality, surface activity, and mechanical property [14]. The performance of the coating layers is usually affected by the combination manner between coating layer and substrate and the property of coating material. Modifying the properties of substrate and coating material is a practical route to improve the performance of resultant coating layers. In the production of commercial mica-titania pearlescent pigment by coating anatase TiO₂ on mica, muscovite as one type of mica has been widely used as a flat substrate[15]. But lamellar sericite with atomic flat surface as a subspecies of muscovite, occurring in large amounts in natural deposits, is seldom investigated.

Our present work aimed at the evolution of TiO₂ coating layers on the surface of lamellar sericite. The effects of La³⁺ present in the coating process on the morphology of TiO₂ coating layers were investigated. The resultant samples were investigated by X-ray photoelectron spectroscopy, X-ray diffractometry, and scanning electron microscopy. The pigmentary properties, such as yellowness, brightness, and relative light scattering index, of the resultant samples, were determined.

2 Experimental

2.1 Materials

Lamellar sericite, K_0.5-1(Al, Fe, Mg)₂(SiAl)₄O₁₀-(OH)₂ (30 μm), was supplied by Chuzhou Grea Minerals Co., Ltd., China. Titanium tetrachloride (TiCl₄, 98%), lanthanum(Ⅲ) nitrate hexahydrate (La(NO₃)₃·6H₂O), concentrated hydrochloric acid (37%), and sodium hydroxide, all in analytical grade, were purchased from Shanghai Chemical Reagent Co., Ltd., China, and used as received. Distilled water was used throughout the experiments.

2.2 Preparation of TiO₂-coated sericite in the presence of La³⁺

Hundred grams of lamellar sericite powders were put into 800 mL of distilled water and stirred at 85 ℃. The pH value of the sericite suspension was adjusted to 2 by adding HCl (0.5 mol/L) aqueous solution. A given amount of La(NO₃)₃ (0.1 mol/L) aqueous solution was added dropwise into the sericite suspension while stirring at 85 ℃ for 0.5 h. Then the prescribed amounts of TiCl₄ (1.5 mol/L) and NaOH (2.0 mol/L) aqueous solutions were added into the suspension with two constant flow rate pumps. The feeding time was fixed at 2 h. The pH value of the reaction solution was kept at 2 during the coating process by adjusting the flow rate of the NaOH aqueous solution. After feeding, the resultant suspension was aged at pH 2 and 85 ℃ for 2 h. The precipitate was filtrated and washed with distilled water until the conductivity of the filtrate was less than 10 mS/m. The washed precipitate was dried in an electric oven at 100 ℃ for 8 h and calcined in a furnace at 900 ℃ for 1 h. The as-prepared samples were kept in a desiccator for characterization.

To illustrate the effect of La³⁺ on the evolution of TiO₂ coating layers, TiO₂-coated sericite samples were also prepared in the same procedures as described above in the absence of La³⁺.

2.3 Characterization

X-ray diffraction(XRD) analysis was performed on a Phillips diffractometer using Cu K_α radiation with a scanning rate of 2 (?)/min. The morphologies of the as-prepared TiO₂-coated sericite powders were analyzed on a scanning electron microscope (JSM 7001F) operating at 10 kV. X-ray photoelectron spectroscopy (XPS) analysis was carried out on an ESCALAB 250 (Thermal Electron Corp.) spectrometer equipped with  Al K_α X-ray source, operating at 150 W. For all the samples, the spectra of C 1s, Si 2p, Si 2s, O 1s, Al 2p,   La 3d_5/2, and Ti 2p_3/2 were recorded. The binding energies were referenced to the C 1s binding energy at 284.5 eV. The yellowness, brightness, and the relative scattering index of the as-prepared TiO₂-coated sericite powders were analyzed on a spectrophotometer (CM-2500d), Minolta Co. LTD, XL-30, and D65 illuminant.

3 Results and discussion

3.1 XRD analysis

The powder X-ray diffraction patterns of the naked sericite and TiO₂-coated sericite powders are shown in Figs.1-3.

Fig.1 XRD diffraction patterns of naked lamellar sericite (sample a) and TiO₂-coated sericite powder samples prepared with mass ratios of TiO₂ to sericite of 1% (b), 5% (c), 10% (d), and 20% (e), respectively

Fig.2 XRD diffraction patterns of naked lamellar sericite (sample a) and TiO₂-coated sericite powder samples prepared in the presence of La³⁺ in reaction solution with mass ratio of La³⁺ to sericite of 1% and mass ratios of TiO₂ to sericite of 0 (a1), 1% (b1), 5% (c1), 10% (d1), and 20% (e1), respectively

Fig.3 XRD diffraction patterns of naked lamellar sericite (sample a) and TiO₂-coated sericite powder samples prepared in the presence of La³⁺ in reaction solution with mass ratio of La³⁺ to sericite of 5% and mass ratios of TiO₂ to sericite of 0(a3), 1%(b3), 5%(c3), 10%(d3), and 20%(e3), respectively

The XRD peaks appearing at 2θ=26.8?, 27.8?, 36.06?, and 45.57? are ascribed to sericite. For the samples prepared in the presence of La³⁺ in the reaction solution with mass ratios of La³⁺ to sericite of 1% and 5%, respectively, there is no any lanthanum species found by XRD analysis, meaning that La³⁺ is well dispersed in the samples. For the TiO₂-coated sericite powders prepared in the absence or presence of La³⁺, the XRD peaks appearing at 2θ=25.28?, 37.80?, 48.05?, and55.06? ascribed to anatase TiO₂ (PDF #21-1272) are observed when the mass ratio of TiO₂ to sericite is up to 10%. The intensity of the XRD peaks of the anatase TiO₂ increases with further increasing the mass ratio of TiO₂ to sericite to 20%. Therefore, it can be concluded that the anatase TiO₂ is formed on the sericite surface and the anatase TiO₂ loading increases with the increase in mass ratio of TiO₂ to sericite. Although the presence of La³⁺ has no obvious effect on crystal phase transformation of TiO₂, the presence of La³⁺ significantly affects the morphology of TiO₂ coating layers on lamellar sericite surfaces as certified by SEM analysis.
 

3.2 Morphology of TiO₂-coated sericite

The SEM image of the naked sericite shows that the sericite has a smooth surface (Fig.4(a)). When TiO₂-coated sericite powders are prepared by the deposition of TiO₂ in the absence of La³⁺ cations, the sericite powders are partially coated by the TiO₂ nano- particles with an average particle size of 26 nm as the mass ratio of TiO₂ to sericite is 1% (Fig.4(b)). While increasing the mass ratio of TiO₂ to sericite to 5%, the surfaces of the sericite powders are almost completely coated by the TiO₂ nanoparticles with an average particles size of 23 nm (Fig.4(c)). With further increasing the mass ratios of TiO₂ to sericite to 10% and 20%, the SEM images show that dense and uniform TiO₂ coating layers have been formed on sericite surfaces and the average particle sizes of TiO₂ nanoparticles are 28 and 40 nm, respectively (Figs.4(d) and (e)).

Fig.4 SEM images of naked lamellar sericite (a) and TiO₂-coated sericite samples prepared with mass ratios of TiO₂ to sericite of 1% (b), 5% (c), 10% (d), and 20% (e), respectively

Figs.5 and 6 show the SEM images of the TiO₂-coated sericite powders prepared with different TiO₂ loadings in the presence of La³⁺ with the mass ratios of La³⁺ to sericite of 1% and 5%, respectively.

Fig.5 SEM images of TiO₂-coated sericite samples after first being treated in La³⁺ solution with mass ratio of La³⁺ to sericite of 1%, and then coated by TiO₂ with mass ratios of TiO₂ to sericite of 0 (a), 1% (b), 5% (c), 10% (d) and 20% (e), respectively

While sericite powders are only treated with La³⁺, the sericite surface has the same smoothness as that of the naked sericite (Figs.5(a) and 6(a)), meaning that no lanthanum species is formed on the sericite surface.

Fig.6 SEM images of TiO₂-coated sericite samples after first being treated in La³⁺ solution with mass ratio of La³⁺ to sericite of 5%, and then coated by TiO₂ with mass ratios of TiO₂ to sericite of 0 (a), 1% (b), 5% (c), 10% (d), and 20% (e), respectively

While TiO₂-coated sericite powders are prepared with different TiO₂ loadings in the presence of La³⁺ cations with a mass ratio of La³⁺ to sericite of 1%, the SEM images (Fig.5) show that small-sized TiO₂ nanoparticles with an calculated average particle size of 14 nm are uniformly dispersed on the sericite surface as the mass ratio of TiO₂ to sericite is 1% (Fig.5(b)). When the mass ratios of TiO₂ to sericite are elevated to 5%, 10%, and 20%, the dense and uniform TiO₂ coating layers are formed on the surfaces of sericite powders and the average particle sizes of TiO₂ nanoparticles are 19, 21, and 24 nm, respectively (Figs.5(c) and (e)).

With further increasing the mass ratio of La³⁺ to sericite to 5%, the SEM images show that when the mass ratio of TiO₂ to sericite is 1%, the TiO₂ nanoparticles are uniformly dispersed on the sericite surface with an calculated average particle size of 12 nm (Fig.6(b)). With further increasing the loadings of the TiO₂ nanoparticles to 5%, 10%, and 20%, the dense and uniform TiO₂ coating layers also have been formed on the surfaces of sericite powders and the average particle sizes of the TiO₂ nanoparticles are 21, 24, and 24 nm, respectively (Figs.6(c) and (e))

The results show that the presence of La³⁺ in the reaction solution is beneficial to the formation of the small-sized TiO₂ nanoparticles and promotes the dispersibility of the TiO₂ nanoparticles on the sericite surfaces, resulting in the formation of the dense and uniform TiO₂ coating layers. The effect of La³⁺ on the formation of dense and uniform TiO₂ coating layers with small particle sizes is obvious even the mass ratio of La³⁺ to sericite is at a lower lever of 1%.

3.3 XPS analysis

Fig.7 shows the O 1s, Al 2p, Si 2p, and Ti 2p_3/2 peaks of the naked sericite and the TiO₂-coated sericite powders prepared in the absence of La³⁺. The binding energy of O 1s of the naked sericite is 532.00 eV. When the TiO₂-coated sericite powders are prepared with the mass ratios of TiO₂ to sericite of 1% and 20%, the binding energies of O 1s are 531.53 eV and 529.63 eV, respectively. The O 1s peaks shift to a lower level of binding energy with an increase in TiO₂ loading. The   O 1s peak located at 532.00 eV should be ascribed to sericite and the O 1s peak at 529.63 eV to Ti—O bond of anatase TiO₂[16]. The binding energies of Si 2p and Al 2p of the naked sericite are 102.53 eV and 74.20 eV, respectively. When the TiO₂-coated sericite powders are prepared with the mass ratios of TiO₂ to sericite of 1% and 20%, the binding energies of the Si 2p, Al 2p, and Ti 2p_3/2 of the samples are 102.17, 74.00, 457.90 eV; and 101.86, 73.70, 458.05 eV, respectively. Increasing TiO₂ loading makes the Si 2p and Al 2p peaks shift to a lower binding energy, while the Ti 2p_3/2 peaks shift to a higher binding energy. The shifts of the O 1s, Si 2p, Al 2p, and Ti 2p_3/2 peaks reveal that the chemical states of O, Si, Al, and Ti are changed. From SEM images, it is found that TiO₂ coating layers tightly anchor on the surfaces of sericite powders. Therefore, it is reasonable to conclude that TiO₂ coating layers anchor on the sericite surface by the formation of Ti—O—Si and Ti—O—Al chemical bonds at the interface of the TiO₂ coating layer and the sericite powders.

Fig.7 XPS spectra of naked lamellar sericite (sample a) and TiO₂-coated sericite powder (samples b and e) prepared with mass ratios of TiO₂ to sericite of 1% and 20 %, respectively

The O 1s, Al 2p, Si 2s, Ti 2p_3/2, and La 3d_5/2 peaks of the naked sericite and TiO₂-coated sericite powders prepared in the presence of La³⁺ are shown in Fig.8. The binding energies of O 1s, Si 2s, Al 2p, and La 3d_5/2 of the sericite powders solely treated with La³⁺ cations are 531.83, 153.41, 74.00, and 836.53 eV, respectively. The binding energy of Si 2s of the naked sericite is 153.57 eV. By comparing the binding energy of La 3d_5/2 with that known in Ref.[17], lanthanum can be confirmed to be in the chemical state of La³⁺. The O 1s, Si 2s, and Al 2p peaks of the sericite powders solely treated with La³⁺ cations shift to lower binding energy as compared with those of the naked sericite, meaning that La³⁺ can combine on the sericite surface. The molar percentage of La³⁺ on the sericite surface is only 0.29% as certified by XPS analysis. Therefore, it is reasonable to conclude that no lanthanum compound is formed on the sericite surface in a large scale and that the La³⁺ should be chemically adsorbed on the sericite surface via Si—O—La and   Al—O—La bonds.

Fig.8 XPS spectra of naked lamellar sericite (sample a), and samples a2, b2 and e2 after first being treated in La³⁺ solution with mass ratio of La³⁺ to sericite of 3%, and then coated by TiO₂ with mass ratios of TiO₂ to sericite of 0, 1%, and 20%, respectively

With increasing the mass ratio of TiO₂ to sericite from 1% to 20%, the binding energies of O 1s, Si 2s,   Al 2p, Ti 2p_3/2, and La 3d_5/2 of the TiO₂-coated sericite powders prepared in the presence of La³⁺ cations are 531.55, 153.24, 74.00, 458.20, 836.30 eV; 530.05, 153.14, 73.98, 458.78, 835.07 eV, respectively. The binding energy of Al 2p keeps constant in the presence of La³⁺, certifying that the Al sites on the sericite surfaces are dominantly occupied by La³⁺ before the deposition of TiO₂. The binding energies of O 1s and Si 2s of the TiO₂-coated sericite powders shift to lower value, while the binding energy of Ti 2p_3/2 shifts to higher value, with increasing TiO₂ loading, conforming that in the presence of La³⁺, TiO₂ coating layers anchor on the sericite surfaces mainly via Ti—O—Si bond. On the other hand, the binding energies of Ti 2p_3/2 of the TiO₂-coated sericite powders prepared in the presence of La³⁺ are higher than those prepared in the absence of La³⁺. The binding energy of La 3d_5/2 is markedly decreased by 1.23 eV as increasing the mass ratio of TiO₂ to sericite from 1% to 20%. Therefore, it can be concluded that La³⁺ also reacts with TiO₂ on the surfaces of the resultant TiO₂ nanoparticles through Ti—O—La bonding. Furthermore, XPS analysis shows that the molar percentage of La³⁺ in the TiO₂-coated sericite prepared with a TiO₂ loading of 20% is 0.4%, which is higher than that of the sericite solely treated with La³⁺, revealing the existence of the interaction between La³⁺ and the surfaces of the TiO₂ coating layers.

3.4 Pigmentary properties of TiO₂-coated sericite

Table 1 shows that when TiO₂ coating layers are formed on the sericite surface, the yellowness of the TiO₂-coated sericite powders is increased while the lightness is slightly decreased as compared with that of the naked sericite. In the substrate, sericite (K_0.5-1(Al, Fe, Mg)₂(SiAl)₄O₁₀(OH)₂), iron ions coexist with potassium, magnesium, and aluminum ions to balance the negative charges of lamellar sericite[18]. Therefore, when sericite powders are deposited by TiO₂ at a lower pH value of 2 and then calcined at 900 ℃, iron-containing compounds probably form and move to the surface of the TiO₂-coated sericite, resulting in the increase of the yellowness and the decrease of the lightness.

Table 1 Color schemes of TiO₂-coated sericite samples (in system CIE)

The light scattering properties of the TiO₂-coated sericite powders are listed in Table 2. The light scattering indexes of the TiO₂-coated sericite powders are increased with the increasing of TiO₂ loading, which could be explained as being due to the increase of anatase TiO₂ coverage on sericite surface. Furthermore, the TiO₂-coated sericite powders prepared in the presence of La³⁺ cations have higher light scattering indexes than those prepared in the absence of La³⁺, which should be due to the evolution of dense and uniform TiO₂ coating layers with small-sized TiO₂ nanoparticles in the presence of La³⁺.

Table 2 Light scattering performances of TiO₂-coated sericite samples

4 Conclusions

1) In the absence of La³⁺ in the reaction solution, the dense and uniform TiO₂ coating layers form on the sericite surface by the direct deposition of TiO₂ when the mass ratio of TiO₂ to sericite is up to 10%. The TiO₂ coating layers anchor on the sericite surface via       Ti—O—Si and Ti—O—Al bonding. The light scattering indexes of the TiO₂-coated sericite powders are greater than those of the naked sericite powders.

2) While La³⁺ is present in the reaction solution, small-sized TiO₂ nanoparticles form since the formation of Ti—O—La bonds on TiO₂ surfaces inhibits TiO₂ crystal growth. La³⁺ anchors on the sericite surface by the formation of La—O—Al and La—O—Si bonds and the aluminum sites on the sericite surface are dominantly occupied by La³⁺. When sericite is pretreated with La³⁺, TiO₂ anchors on the sericite surface via Ti—O—Si  bonds, giving a higher dispersibility of TiO₂ nanoparticles, subsequently, constructing dense and uniform TiO₂ coating layers. The light scattering indexes of the TiO₂-coated sericite powders prepared in the presence of La³⁺ are higher than those prepared in the absence of La³⁺.

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Foundation item: Project(2003406) supported by the Chinese Education Department; Project(BG2006025) supported by Bureau of Science and Technology of Jiangsu Province, China

Corresponding author: YIN Heng-bo; Tel: +86-511-88787591; E-mail: yin@ujs.edu.cn

DOI: 10.1016/S1003-6326(08)60324-6

(Edited by YANG Hua)