silica double-layer surface coating of TiO2 pigment

silica double-layer surface coating of TiO2 pigment

Colloids and Surfaces A: Physicochem. Eng. Aspects 407 (2012) 77–84 Contents lists available at SciVerse ScienceDirect Colloids and Surfaces A: Phys...

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Colloids and Surfaces A: Physicochem. Eng. Aspects 407 (2012) 77–84

Contents lists available at SciVerse ScienceDirect

Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

Hydrous alumina/silica double-layer surface coating of TiO2 pigment Jie Li a,b , Yahui Liu a,b , Yong Wang a,b,c , Weijing Wang a,b , Dong Wang a,b , Tao Qi a,b,∗ a

National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China c Department of Pharmaceutical and Chemical Engineering, Hebei University of Science and Technology, Shi Jiazhang, 050031, PR China b

h i g h l i g h t s

g r a p h i c a l

 Hydrous alumina/silica layer by layer surface coating of TiO2 pigment was prepared.  There is a chemical interaction between the hydrous alumina layer and silica layer.  Possible structure of coated layers were investigated by computer simulation.  The coated Al2 O3 ·H2 O on Si O layer is more stable than the coated AlO(OH) on it.

The structure of (a) 4AlO(OH)/4SiO2 /12TiO2 ; (b) Al2 O3 ·H2 O/4SiO2 /12TiO2 ; (c) 2H+ /4AlO(OH)/4SiO2 / 12TiO2 ; (d) 2H+ /2Al2 O3 ·H2 O/4SiO2 /12TiO2 .

a r t i c l e

a b s t r a c t

i n f o

Article history: Received 28 February 2012 Received in revised form 14 May 2012 Accepted 17 May 2012 Available online 26 May 2012 Keywords: Surface coating Double-layer Hydrous alumina (−1 1 1) facets TiO2 pigment

a b s t r a c t

The hydrous alumina/silica double-layer surface coating of TiO2 pigment was prepared by precipitation method under two different conditions. High-resolution transmission electron microscopy (HRTEM), Energy dispersive spectrometer (EDS), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectrum (FTIR) as well as ␨-potential analysis were used to characterize the morphology, structure and surface electrokinetic behavior, respectively. The results show that hydrous alumina is continuously coated on the surface of silica-coated TiO2 particle with a compact layer at 60 ◦ C, pH 4.0. There is a chemical interaction between the hydrous alumina layer and silica layer. An Al O Si bond was formed and a thin film of aluminosilicate was existed. A flocculent hydrous alumina layer was formed at 60 ◦ C, pH 8.0. The XPS spectra of O1s show that the peak of 531.2 eV is assigned to AlO(OH) for the loose and flocculent morphology. The theoretical calculations reveal that the coated Al2 O3 ·H2 O on Si O layer combined with (−1 1 1) lattice plant of rutile TiO2 is more stable than the coated AlO(OH) and the discrepancy of total energy between them is about −1.303 eV, i.e., the sample obtained at 60 ◦ C, pH 4.0 has a more stable thermodynamic property. In addition, the adsorption of H+ ion on AlO(OH) surface is easier than that on Al2 O3 ·H2 O surface, as the discrepancy of total energy between them decrease to −0.721 eV. © 2012 Elsevier B.V. All rights reserved.

1. Introduction TiO2 is used extensively as a pigment in paint, ink, plastic and fiber environments. A major problem in its use with organic resins has been the occurrence of “chalking” due to the photochemical

∗ Corresponding author. Tel.: +86 10 62631710; fax: +86 10 62631710. E-mail address: [email protected] (T. Qi). 0927-7757/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2012.05.010

oxidation of the resin [1]. As it is known, TiO2 is photoactive when it is exposed to sunlight, especially to ultraviolet radiation because of the oxygen defects existing on the particle surface [2]. In order to overcome these deficiencies of the pigment, while not interfering with its desired optical properties, the surface layers of other inorganic hydrous oxides are applied to the TiO2 particles [3,4]. In recent years, surface coating of TiO2 with inorganic hydrous oxides has been widely investigated and developed. Alumina and silica have been used most often. Other inorganic oxides used

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as the coating materials include zirconium, tin, zinc, cerium and boron oxides [5–7]. On the market at present, many types of TiO2 products are coated with alumina and silica by precipitation or co-precipitation methods, presenting good properties of pigment, especially in durability and dispersity. The precipitation of silica on titanium dioxide surfaces and the interactions between silica and rutile TiO2 surface were investigated by King et al. [1]. The film-coating process of hydrated alumina on TiO2 particles and the coating mechanism of it were investigated by Wu et al. [3]. However, most of such researches are aimed at single layer coated directly on TiO2 particles. And in general production, there always two or even more layers coating on the TiO2 particle. Few reports have been found on the study of interactions between the coating layers, which is of great importance to understand the coating process and mechanism of layer by layer coating. In addition, it can be a helpful guidance for designing and preparing the TiO2 products according to the requirements. In this paper, two different types of alumina/silica coated TiO2 samples were prepared by precipitation method through double-layer coating. The interactions between hydrous alumina layer and hydrous silica layers have been also studied. Highresolution transmission electron microscopy (HRTEM), Energy dispersive spectrometer (EDS), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectrum (FTIR) and ␨-potential analysis were used to characterize the morphology, surface structure and surface electrokinetic behavior of the two samples, respectively. Furthermore, to better identify which binding mode is predominant under the real conditions, the possible structure and stability of coated layers have been also investigated and described through computer simulation and geometric optimization. This work would be of certain help to design and prepare the TiO2 pigment products to satisfy the demands in different application.

2. Experiment 2.1. Materials Commercial TiO2 obtained from the sulfate process was rutile (JCPDS 01-086-0148), comprised of ellipsoidal and some columnar particles with an average diameter of about 300 nm, the phase pattern and morphology are presented in Figs. 1 and 2. All chemicals used in the experiments were of analytical reagents (AR) grade. Na2 SiO3 solution, Al2 (SO4 )3 solution and NaAlO2 solution were used as the precipitating agents. 20% H2 SO4 and 20% NaOH were used to adjust the pH value.

5000

R

R Rutile

Intensity a.u.

4000 3000 R

2000

R R

1000

R R

R R

R R

0 20

40

60

2Theta(deg) Fig. 1. XRD pattern of the TiO2 .

80

Fig. 2. SEM image of the TiO2 .

2.2. Sample preparation The pigmentary titanium oxide was milled wetly and then dispersed in de-ionized water at a concentration of 300 g/l to form a TiO2 suspension. Coating experiments were carried out in a cylindrical reactor of 19-cm diameter, equipped with thermometer and pH electrode. The slurry was stirred strongly and adjusted to pH 9.5 by titrating alkaline dispersant. The slurry temperature is firstly kept at 85 ◦ C, while Na2 SiO3 solution was added drop-wise into the reactor, and dilute H2 SO4 solution was used to maintain the pH value simultaneously. The ratio of SiO2 to TiO2 was 2.0% and aging for 1 h. Sample 1, adjust the pH of the above slurry to 4.0 and cooled it to 60 ◦ C, then added Al2 (SO4 )3 solution into the slurry with the dilute NaOH to keep the pH value simultaneously. Sample 2, adjust the pH of the slurry to 8.5 at 60 ◦ C and then added NaAlO2 solution into it, and the dilute H2 SO4 was used to maintain the pH value through the process. Both Sample 1 and Sample 2 were aging for 2 h, after that, the pH value of the TiO2 suspension was adjusted to 7.0 with further stirring for 20 min. The coated TiO2 particles were filtered, washed and dried at 105 ◦ C for 24 h. 2.3. Characterization The phase of the TiO2 was analyzed by XRD (X’Pert PRO MPD, PANalytical, Netherlands) recorded on a diffractometer (using Cu Ka radiation) operation at 40 kV/30 mA. The morphological analysis of the particles was performed by SEM (JEOL-JSM-6700F) and HRTEM (JEM-2100 UHR, Japan). The chemical components of coating surface were determined by EDS (JEM-2100 UHR, Japan). The surface chemical compositions of the particles and the chemical bond action at the interface of coating layers were measured by XPS (Thermo ESCALAB 250). The interfacial structure is also checked by FTIR (Spectrum GX). The ␨-potential of the particles was measured by Zetasizer R2000 (Malvern), the temperature was kept at 25 ◦ C. The particle dispersion was enhanced by an ultrasonic treatment. Dilute solutions of H2 SO4 and NaOH were used for pH adjustment. The color properties were described in terms of CIE-L*a*b* 1976 color scales. The value of CIE-L* denotes the degree of lightness and darkness of the color in relation to the scale extending from white (L* = 100) to black (L* = 0), and the values of CIE-a* and CIE-b* represent the scale extending from green (−a*) to red (+a*) axis and from blue (−b*) to yellow (+b*) axis, respectively. The tint strength of the particles was examined according to GB5211.16-2007, which was equivalent to ISO 787/17-2002.

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Fig. 3. (a) (−1 1 1) surface slab is cleaved from rutile TiO2 with the (3 × 3 × 3) unit cell, optimized at the DFT level; (b) the adsorption of Si4 O8 layer on the cleaved surface; (c) the adsorption of hydrous alumina layer on Si4 O8 surface. The periodic rectangular supercell is circumscribed with black frame.

2.4. Theoretical calculation All density functional theory (DFT) were performed using the D mol3 code [8,9] with the generalized gradient approximation (GGA) functional of Perdew–Wang exchange and correlation functional (PW91) [10,11]. To take into account any relativistic effects, the effective core pseudopotentials (ECP) were chosen [12]. The rutile TiO2 , obtained from the database of material studio, was selected the (−1 1 1) lattice plane and then separated by a vac˚ When this geometry was optimized, the Si O uum region of 20 A. layer was covered on its surface. This combined structure was proposed to explain silica layer coating on rutile TiO2 surface. The bulk rutile TiO2 unit cell and its silica adsorbed model are depicted in Fig. 3(a) and (b), respectively. After optimization, the periodic model of Si4 O8 –Ti12 O24 is fixed with space group of P1, unit cell ˚ c = 8.788 A, ˚ ˛ = ˇ =  = 90◦ . ˚ b = 6.497 A, parameters: a = 32.00 A,

3. Results and discussion 3.1. Morphology of coated TiO2 particles In the first step of coating process, sodium silicate solution and dilute sulfuric acid were added slowly and uniformly to the slurry. The mono-silica acid is firstly formed and polymerized to (HO)3 SiOSi(OH)3 in an alkaline environment coated on the rutile [13]. Fig. 4 shows the HRTEM image of the hydrous alumina coated TiO2 particles. It can be seen from Fig. 4(a) (Sample 1) that a thin film is continuously coated on the TiO2 particle surface. The thickness of the coating layer is about 3–5 nm (Fig. 4(b)). While in Fig. 4(c) and (d) (Sample 2), a loose and flocculent structure of the coating layer can be seen on the TiO2 particle surface. It can be deduced that the different morphology of the two samples is depended on the different coating pH value of hydrous alumina precipitation. The sedimentation speed of OH Al species is higher than the directed growth speed at a lower pH (pH 4.0), a continuous and amorphous

film coating can be produced because of the random condensation of OH Al species and OH groups on the particle surface. When the pH value is higher at 8.5, the directed growth speed is higher than the sedimentation speed of the OH Al species, a loose floccules coating is produced because of the directed condensation [14]. 3.2. Surface composition of TiO2 particles The surface composition of TiO2 particles can be qualitatively determined by energy dispersion spectrum (EDS), as shown in Fig. 5. The presence of Al and Si are identified in both Sample 1 and Sample 2, which is demonstrated that the hydrous alumina and silicon are coated on the particle surface. The relative surface compositions of the two samples and uncoated TiO2 particle expressed quantitatively as atom percentage, derived from the XPS intensities, are presented in Table 1. It can be seen from Table 1 that the uncoated TiO2 particles contain 4.5 at% Al, which is involved in the materials as doping agent for producing titanium dioxide, while the coated TiO2 particles (Sample 1 and Sample 2) contain 15.84 at% and 15.75 at% Al, respectively. Si component is examined on the surface of the both two samples. The contents of Ti are obviously decreased from 22.88 at% of uncoated TiO2 particle to 2.61 at% and 5.23 at% of the two samples, respectively. Furthermore, Ti component of Sample 1 is less than that of Sample 2. It is possible that the hydrous alumina is coated on TiO2 particle as a continuous and compact film in Sample 1, while the hydrous alumina film is coated on TiO2 particle as a loose and flocculent structure in Sample 2. Sodium component has also been Table 1 The surface composition of samples from XPS analysis (at.%). Sample

Ti

Al

Si

O

Na

Uncoated TiO2 Sample 1 Sample 2

22.88 2.76 5.36

4.50 15.77 15.73

0 6.40 5.85

72.02 74.13 72.45

– 0.94 0.61

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Fig. 4. HRTEM images of coated TiO2 particles (a, b) Sample1 and (c, d) Sample 2.

found in the two samples with a small amount, which might be due to the rest of sodium ion in the filter process. 3.3. Analysis of interfacial structure The infrared spectra of uncoated and coated TiO2 samples are shown in Fig. 6. The broad band between 3500 and 3000 cm−1 ,

centered at around 3440 cm−1 , indicated the presence of the highenergy OH stretching vibrations on the surface of titania. The absorption in 2364 cm−1 is the C O belonging to the CO2 in the air. The band around 1600 cm−1 (1631 cm−1 ) ascribed to the O H bending mode and demonstrated the presence of molecularly adsorbed water in the prepared sample [15]. It can be seen that compared with the spectrum of uncoated TiO2 (Fig. 6(a)), an absorption band 5000

1500

Ti Ka

Ti Ka

Sample 1

4000

Sample 2 1000

O Ka

Cu Ka

Counts

Counts

3000

2000

O Ka

500

Al Ka Cu La Si Ka

Al Ka Si Ka Cu La

1000

Ti Kb

Cu Kb

Cu Ka Ti Kb Cu Kb

0

0 0

200

400

600

Energy (keV)

800

1000

0

200

400

600

Energy (keV)

Fig. 5. EDS analysis of coated TiO2 particles (Sample 1 and Sample 2).

800

1000

Transimittance(a.u.)

J. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 407 (2012) 77–84

a b c

2364

1205

1065

1232

d

4000

1632

1078 1137

3440

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm ) Fig. 6. FTIR analysis of uncoated and coated TiO2 samples: (a) uncoated TiO2 ; (b) hydrous SiO2 -coated TiO2 ; (c) Sample 2; (d) Sample 1.

in 1000–1300 cm−1 occurs in the spectrum of hydrous SiO2 -coated TiO2 (Fig. 6(b)). It is attributes to the chemical action between the coated hydrous silica and the TiO2 particle surface. The Si O tetrahedron is broken, and the absorption band in 1000–1300 cm−1 is inferred by O Si asymmetric flexible vibration [16]. The band around 1078 cm−1 of Sample 2 (Fig. 6(c)) demonstrated the presence of AlO(OH) in the prepared sample. Under the effect of Al atom, the original simple band splits or the noninfrared vibration of Si O is changed into infrared vibration, resulted the vibration frequency shifts to high frequency. Therefore, this absorption band

81

in 1078 cm−1 and 1232 cm−1 are inferred by Al O vibration. Compared the infrared spectra of Sample 2 (Fig. 6(c)) to Sample 1 (Fig. 6(d)), the absorption in 1232 cm−1 is disappeared. A strong and broad band around 1100 cm−1 (centred at 1137 cm−1 ) in the spectrum of Sample 1 (Fig. 6(d)) assigned to the asymmetric Al O Si stretch [17], which may be inferred by the influence of O Al vibration when compact hydrous Al2 O3 film has been formed. The high-resolution XPS spectra of the O1s peaks of uncoated TiO2 and coated TiO2 particles are shown in Fig. 7. The O1s peaks of uncoated TiO2 particles reveal two oxygen band peaks for Ti O and OH at 529.6 and 530.5 eV, respectively (Fig. 7(a)). The 532.7 eV peak found in Fig. 7(b) is identified of the binding energy of Si O band [18,19]. While for Sample 1, three peaks of oxygen band at 530.3, 531.8 and 532.8 eV are found (Fig. 7(c)), comparing with standard spectra, and it is known that the three peaks are corresponded to Ti O, Al O Si and OH bonds, respectively. The stronger peak of 531.8 eV is attributable to the coating of hydrous Al2 O3 coated on the particle surface. It is deduced that the 531.8 eV peak of O 1s in Sample 1 resulted from a chemical shift of 532.7 eV peak of O 1s in SiO2 -coated TiO2 . It can be inferred that Al is combined onto the surface of TiO2 particle, forming an Al O Si bond, which can be also identified as the Al2 OSiO4 formula. Because the electronegativity of Al(1.61) is weaker than that of Si(1.90), O 1s peak has a chemical shift of −0.9 eV. Fig. 7(d) shows the O 1s spectra of Sample 2. Three peaks of O 1s band can be deduced at 529.7, 531.2 and 532.1 eV, respectively. According to the standard spectra, the three peaks can be corresponded to Ti O, AlO(OH) as formula and Si O bonds, respectively. The peak of 532.1 eV is broader than that of hydrous SiO2 coated TiO2 particle and there is a decrease (about −0.7 eV) in the binding energy of Si O bond after hydrous alumina coating, which can be ascribed to the formation of Si O Al bond. The peak of 531.2 eV is assigned to AlO(OH), which is confirmed to the HRTEM (Fig. 4(d)) for the loose and flocculent morphology.

Fig. 7. O 1s XPS spectra of uncoated and coated TiO2 particle: (a) uncoated TiO2 particle; (b) silica-coated TiO2 particle; (c) Sample 1; (d) Sample 2.

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Al 2p 3/2 Al2OSiO4

C/S

(74.8 eV)

Sample 1 AlO(OH) (74.2 eV)

Sample 2

85

80

75

70

65

B.E.(eV) Fig. 8. Al 2p 3/2 XPS spectra of coated TiO2 particle.

Therefore, the coating process of hydrous alumina film on SiO2 coated TiO2 is an absorption and precipitation process, which is a physical and chemical change process. In addition, a new thin layer of aluminosilicate is formed between the hydrous alumina film and hydrous silica film. Al 2p spectra of Sample 1 and Sample 2 are shown in Fig. 8. The binding energy of Al 2p peak of Sample 1 is 74.8 eV, which is characterized as Al2 OSiO4 according to the standard spectra. While, the binding energy of Al 2p peak of Sample 2 is 74.2 eV, which is assigned to AlO(OH). It is also an evidence for deducing that Al O Si bond was formed and a thin film of aluminosilicate was existed. 3.4. Electro-kinetic behavior and coating process The ␨-potential plays an important role in the particle-coating process. For small particles in liquid, there is no satisfactory technique to determine surface charge of the particles. For measured the potential at the slipping or shear plane, zeta potential is a very important parameter for colloids or nano particles in suspension. Its value is closely related to suspension stability and particle surface morphology. Fig. 9 shows the ␨-potential of uncoated TiO2 , pure Si(OH)4 , pure Al(OH)3 and coated samples. It can be seen that the isoelectric point (IEP) of the uncoated TiO2 particle is about 4.6. After coated with

uncoated TiO2 60

SiO2-coated TiO2 Si(OH)4

-potential (mV)

40

Sample 2 Sample 1 Al(OH)3

20 0 -20 -40 -60 0

1

2

3

4

5

6

7

8

9

10

pH value Fig. 9. The ␨-potential of pure substances and coated samples.

11

hydrous silica, the IEP of SiO2 -coated TiO2 particle changed to 1.7, which is similar to IEP of pure Si(OH)4 species. After coating of hydrous alumina at different pH value, the electro-kinetic behavior of Sample 1 is very similar as pure Al(OH)3 species according to the change of pH value, with the IEP roughly the same as about 7.2. It is indicated that the particle surface of Sample 1 is completely coated by hydrous alumina. While, the electro-kinetic behavior of Sample 2 has some difference with Sample 1, i.e., the ␨-potential value of Sample 2 at pH 7 is about 24 mV, whereas that of Sample 1 at pH 7 is about 2 mV, which enables the dispersion of hydrous alumina coated TiO2 particle at pH 8.5 to be much easier than that of coated TiO2 particle at pH 4.0. When the coating pH value is 4.0, the hydrous SiO2 -coated TiO2 particles would carry a negative charge due to many OH on its surface [19,20]. When the alumina sulfate is added in the solution, it would hydrolyze to Al(OH)2+ . According to DLVO theory [21], the positive Al(OH)2+ will be easily attracted to the TiO2 particle surface by OH, and then coating process begins. When other conditions are constant, lower supersaturation is beneficial to the heterogeneous nucleation of alumina on the surface of the TiO2 particles, which is layer coating. It is a physical and chemical change process [22]. 3.5. Stable conformation in different pH conditions To better identify experimentally which binding mode is predominant under realistic adsorption conditions, the possible existing forms of hydrous alumina were investigated. It is well known that the hydrous alumina compounds are often complex and not always accurately defined because of the difficulties associated with analysis of the numerous species that exist in such solution. Here, the first principle density functional theory (DFT) to optimize the structure of hydrous alumina adsorbed on SiO2 coated (−1 1 1) lattice plane was used. Comparing with the (−1 1 1) lattice plane ˚ which means the Al-Al distance is 3.23 A˚ at parameter b of 6.46 A, least, the distance of Al Al is about 2.9–3.0 A˚ in Al (OH)2 Al clusters and 3.2–3.9 A˚ in Al O Al cluster. Therefore, the structure of Al(OH)3 and Al2 O(OH)4 can be excluded. The results of theoretical calculations show that the adsorption of AlO(OH) or Al2 O3 ·H2 O can obtain the optimized results, as those structures quite match the dimension of silica surface. In addition, the results of theoretical calculations also confirm that the coated structure of Al2 O3 ·H2 O is more stable than that of AlO(OH), which is consistent with the chemical shift of Al 2p 3/2 (depicted in Fig. 8), i.e., the element of Al in Sample 1 locates in a more stable environment, and needs more energy to excite the core-level electron. The large discrepancy of total energy between the coated structure a and b, depicted in Fig. 8, is about −1.303 eV. Therefore, the transformation from AlO(OH) to Al2 O3 ·H2 O is thermodynamically prohibited. The pH value affects the surface natures and determinates the final existing forms of hydrous alumina. The ␨-potential as an important surface nature is widely used to character particlecoating materials. At pH 7, the ␨-potential values of Sample 2 and Sample 1 are about 24 mV and 2 mV, respectively. Under this condition, more ions like H+ would adsorb on the surface of Sample 2 than that of Sample 1. Therefore, the adsorption of H+ on the surface of [Al2 O3 ·H2 O]2 and [AlO(OH)]4 are considered in DFT calculations to simulate that of Sample 1 and Sample 2. It confirms that the adsorption of H+ ion occurring on [AlO(OH)]4 surface (structure c, in Fig. 10) is easier than that on [Al2 O3 ·H2 O]2 surface (structure d, in Fig. 10), as the discrepancy of total energy between structure c and d is only −0.721 eV after H+ adsorbed on O active sites. The adsorbed H+ ion can strongly affect the bond length and bond angle of Al O Al units, and some key geometric parameters are listed in Table 2. It is a stabilized area of Al1–Al4, because of the fixed Al and O with Si O layer. The position of O active sites can

J. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 407 (2012) 77–84

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Fig. 10. The structure of (a) 4AlO(OH)/4SiO2 /12TiO2 ; (b) Al2 O3 ·H2 O/4SiO2 /12TiO2 ; (c) 2H+ /4AlO(OH)/4SiO2 /12TiO2 ; (d) 2H+ /2Al2 O3 ·H2 O/4SiO2 /12TiO2 . Table 2 Optimized geometric parameters of hydrous alumina on Si O surface combined with rutile TiO2 (−1 1 1) lattice plane. Compounds

Al1–Al2

(a) 4AlO(OH)/4SiO2 /12TiO2 (b) 2Al2 O3 ·H2 O/4SiO2 /12TiO2 (c) 2H+ /4AlO(OH)/4SiO2 /12TiO2 (d) 2H+ /2Al2 O3 ·H2 O/4SiO2 /12TiO2

Al1–Al3

Al1–Al4

lAl1–Al2

 Al1–O–Al2

lAl–O

lO–H /lO· · ·H

lAl1–Al3

 Al1–O–Al3

lAl–O

lO–H /lO· · ·H

lAl1–Al4

 Al1–O–Al4

lAl–O

3.25 3.37 3.36 3.33

140.21 138.39 127.07 137.63

1.72–1.74 1.78–1.83 1.87–1.88 1.76–1.81

–/– –/1.64 0.99/– –/1.60

2.99 3.17 2.98 3.28

102.22–102.49 123.04 99.06–103.30 115.44

1.79–2.03 1.77–1.84 1.83–2.08 1.87–2.00

0.98–0.98 –/1.84 0.97–0.98 0.99/1.99

3.93 3.67 3.88 3.59

154.70 149.39 158.95 153.96

1.98–2.06 1.85–1.96 1.97–1.98 1.80–1.88

Bond length (l) in Å and bond angle () in degree. The number behind Al is labeled in Fig. 8.

be estimated from their chemical environment – they should isolate from Si (see Fig. 10(a) and (b), the area of Al1–Al2) or far from the adjacent H+ which is chemically combined water of crystallization of Al2 O3 ·H2 O, as shown in Fig. 10(b). When the H+ ions strongly adsorbed on the surface of structure a and b, they increase the length of their combined Al O from 1.72–1.74 A˚ to 1.87–1.88 A˚ (see Table 2(a) and (c) with the area of Al1–Al2) and 1.77–1.84 A˚ to 1.87–2.00 A˚ (see Table 2(b) and (d) with the area of Al1–Al3), respectively.

Sample 2 are increased than that of Sample 1, from 97.65 to 98.27 and 96.57 to 97.02, respectively. The tint strength of Sample 1 is slightly higher than that of the uncoated TiO2 , while the Sample 2 presents a better tint strength of 105. The related mechanism can be proposed that the loose floccules coating structure (Section 3.4) promote the dispersity of particles, which can enhance the light scattering power.

4. 4 Conclusions 3.6. The pigmentary property of coated TiO2 particles Table 3 shows the pigmentary performance of the uncoated TiO2 and the coated samples. The brightness and the whiteness of Table 3 Pigmentary properties of uncoated and coated TiO2 particle. Samples

L*

a*

b*

Wh

Tint strength/%

TiO2 Sample 1 Sample 2

97.74 97.65 98.27

−0.53 −0.43 −0.50

1.62 1.54 1.64

96.35 96.57 97.02

97 98 105

Two samples of hydrous alumina/silica surface coated TiO2 particles were prepared by precipitation method through double-layer coating. The results show that the outer layer of hydrous alumina is presented in different morphology and surface structure of the two samples, i.e., one is a compact and continuous film obtained at 60 ◦ C, pH 4.0, while the other one is a loose and flocculent film obtained at 60 ◦ C, pH 8.5. Under the effect of Al atom, the original simple band splits or the non-infrared vibration of Si O is changed into infrared vibration, resulted the vibration frequency shifts to high frequency. Al oxide is combined onto the surface of silica-coated TiO2 particle through

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chemical bond Al O Si. Under the interactions of the two coating layers, an aluminosilicate thin film is formed between them. DFT calculations have confirmed that the sample obtained at pH 4.0 has a more stable thermodynamic property than the sample obtained at pH 8.5, which is well agreement with XPS data. Furthermore, the stability of H+ adsorbed on AlO(OH) surface is larger than that on Al2 O3 ·H2 O surface, and it can well explain why the samples obtained under different pH conditions exhibit different ␨-potential values. Acknowledgments The authors gratefully acknowledge supports from the Major Program of the National Natural Science Foundation of China (Grant No. 51090380), National Science Foundation for Distinguished Young Scholars of China (Grant No. 51125018), National Natural Science Foundation of China (Grant Nos. 51104139, 51004091, 21006115). References

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