Characteristics and properties of surface coated nano-TiO₂

GAO Jia-cheng(高家诚), ZOU Jian(邹 健), TAN Xiao-wei(谭小伟), WANG Yong(王 勇)

College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China

Received 10 February 2006; accepted 18 July 2006

Abstract: Nano-TiO₂ was coated with A1₂O₃, SiO₂ and silane coupling agent by chemical liquid deposition. The coating was characterized by Raman spectroscopy, XRD, TEM and FT-IR. The coating content and anti-ultraviolet capacity of nano-TiO₂ were measured by XRF and UV-vis spectrometer. The results show that dense coatings containing 5% Al₂O₃ or SiO₂ can be obtained by mixing slurry at pH 10, adding coating reagent and neutralization reagent into the slurry for 60 min at 85-95 ℃, and finally aging for 120 min. Noncrystal SiO₂ was coated on the surface of nano-TiO₂ to form silica gel polymer with a Ti—O—Si bond, while aluminum compound exists in the form of AlOOH and part Al(OH)₃. The integrated dense film can shield photocatalysis effectively. The inorganic coating film can increase the wettability for xylene and stability in water. The surface modification of nano-TiO₂ will not impair its ability for anti-ultraviolet radiation, and more short band ultraviolet radiation can be absorbed. In addition, the optimal coating amount of silane coupling agent should be less than 3% and the best wettability for xylene can be reached when the amount is 1.2%.

Key words: nano-TiO₂; surface modification; coating; weatherability; dispersing

1 Introduction

Nano-TiO₂, as a new inorganic material, has many excellent properties，such as nontoxic, stable in chemistry and high efficient photocatalytic effect[1,2]. As fine particles, it is easy to agglomerate in practical use, especially difficult to disperse in organic solvent[3]. And because of its strong photocatalytic effect, especially under ultraviolet radiation it will lead painting to powder and lower weatherability[4]. Surface modification of nano-TiO₂ material has been reported from a lot of researches. They focused mainly on the coating process and the development of different coating materials while a little on surface coating characteristics and properties; also the mechanism for film coating was not discussed in detail. The effective characterization and better properties after surface modification are key problem also. In this work, nano-TiO₂ is coated by inorganic oxide to close its high activity, improve the weatherability and keep high lightness[5], and then is modified by organics to disperse in organic solvent stably[6].

2 Experimental

The experimental materials included 6% nano-rutile slurry, sodium silicate coating agent (100 g/L), partial sodium aluminate (100 g/L), aluminum sulphate (100 g/L), hydrophile silane coupling agent, hexad partial sodium phosphate (100 g/L) dispersant, xylene and self-made desalt water. The regulators for pH were sulphuric acid, hydrochloric acid, sodium hydroxide with different concentrations.

Slurry was adjusted to suitable pH value about 8-11 and mixed efficiently at room temperature. Then it was neutralized to constant pH value about 7. The coating temperature is 85-95 ℃. After sampling at different time, vacuum filtrating, cleaning and drying were done. Excessive silane coupling agent was added at different time for organic coating, to fix on the surface of nano-TiO₂ uniformly. The coated powders were washed several times and then roasted at 105 ℃ for 24 h.

The uncoated and coated titania powders were analyzed quantitatively by X-ray fluorescence spectrum. The organic coating sample was calcined in mufflefurnace at 850 ℃ to measure the mass loss. The coating amount can be obtained from it. Phase analysis was carried out on Japan D/MAX-1200 full-automation X-ray diffraction and infrared radiation spectrophoto- meter. Various slurry materials and nano-TiO₂ samples before and after coating were measured by Raman analyzer at Peking University. The dispersing stability and wetting angle of each sample were also measured.

3 Results and discussion

3.1 Coating layer characteristics

The XRD spectra of uncoated and coated nano-TiO₂ are shown in Fig.1. It can be seen that the uncoated sample is in rutile state. The average grain size of particle is 57 nm, calculated by Scherrer formula[7]. The XRD spectra of uncoated and coated nano-TiO₂ are similar, and the coated samples do not contain the diffraction peaks of other oxide crystal. Fig.2 shows the XRD spectra of coated specimen calcined at 1 300 ℃ for 1 h. Apart from rutile TiO₂, there exist Al₂O₃ in Fig.2(a) and SiO₂ in Fig.2(b). There are both Al₂O₃ and SiO₂ phases in specimen coated with Al₂O₃ and SiO₂ (Fig.2(c)). These results indicate that the coatings are noncrystal before calcining, while become crystal A1₂O₃ and SiO₂ after being calcined at 1 300 ℃.

Fig.1 XRD spectra of uncoated and coated nano-TiO₂

In order to investigate the surface modification effect on nano-TiO₂, the film morphology of samples before and after treatment was observed by TEM.  Fig.3(a) shows the TEM morphology of unmodified nano-TiO₂, Figs.3(b)-(d) show the TEM pictures of nano-TiO₂ particles coated with Al₂O₃ in acidic condition, with Al₂O₃ in alkaline condition and coated with Al₂O₃ and SiO₂, respectively. Compared with Figs.3(a) and (b), the dispersing degree of samples shown in Figs.3(c) and (d) is greatly improved. And there are no spherical alumina particles in Figs.3(b) and (c).

Fig.2 XRD spectra of coated specimen calcined at 1 300 ℃: (a) Coated with Al₂O₃; (b) Coated with SiO₂; (c) Coated with Al₂O₃/SiO₂

The IR spectra of coated specimens are shown in Fig.4. The absorption peak at 1 096 cm^-1 is caused by flex vibration of Si—OH group, the one at 891 cm^-1 by Si—O—Si and Ti—O—Si bond and the one at 3 416 cm^-l by flex vibration of —OH polymerization, which is in superposition with the absorption peak of crystal water in Fig.4(a). Combined with XRD analysis, it is shown that the coating film of nano-TiO₂ coated with SiO₂ is amorphous silicon-oxygen polymerization substance. It is also shown that the coating film of nano-TiO₂ coated with Al₂O₃ exists in the form of AlOOH and amorphous Al(OH)₃ (Fig.4(b)). The IR spectrum of the specimen coated with Al₂O₃ and then modified with organic substance is shown in Fig.4(c). Compared Fig.4(b) with Fig.4(c), it is shown that the —OH flex vibration peaks of AlOOH at 2 986 cm^-1, 2 934 cm^-l , 2 852 cm^-1 are reduced to two peaks (a strong and a weak) after organic modification, and the relative intensity is strengthened because of CH₂— and CH₃—groups. The shape also varies at 1 388 cm^-1 and a new absorption peak occurs at 1 265 cm^-1 after modification. The intensity of absorp- tion peak at l 096 cm^-1 and l 055 cm^-1 is strengthened after modification, especial for that at 886 cm^-1. The absorption peak at 800 cm^-1 occurs after organic modification, which is caused by the vibration absorption of Si—O bond in silane coupling agent. Therefore nano-TiO₂ coated with silane coupling agent has organic silane coatings.

Fig.3 TEM morphologies of samples coated under different conditions: (a) Uncoated; (b) Coated with Al₂O₃ in acidic condition; (c) Coated with Al­₂O₃ in alkaline condition; (d) Complexly coated with Al₂O₃ and SiO₂

Fig.4 IR spectra of coated Al₂O₃ specimen

Fig.5 shows Raman spectra of nano-TiO₂ specimens coated and uncoated with Al₂O₃. In Fig.5(a), there are two strong characteristic peaks of rutile between 250-500 cm^-1. While in Fig.5(b), two characteristic peaks of alumina stronger than rutile peak in intensity exist between l 000-1 250 cm^-1. This indicates that the surface of nano-TiO₂ is coated with alumina.

The relative content of oxides on the nano-TiO₂ power is shown in Table 1. The samples of series 1 mean for complex coating film coated first with SiO₂ and then with Al₂O₃ in proportion of l:2, l:l, 3:2. It can be seen from Table 1 that the result is similar to the charge quantity. The series 2 represents samples coated with SiO₂, where sample 2-1 is held for 30 min, with less half amount of coating material than sample 2-2, since coating agent can not deposit on nano-TiO₂ surface completely in short holding time. The series 3 means samples coated with Al₂O₃, where sample 3-1 is coated in acid condition and sample 3-2 in alkali conditions, both with 5% charge.

Fig.5 Raman spectra of nano-TiO₂ powder specimen uncoated(a) and coated with alumina(b)

Table 1 Result of XRF analysis on partial specimen

The excessive quantity of 50% polymer is used and the remainder is removed with high speed centrifugal machine in the trial. Therefore, it can be regarded that coating agent absorbs tightly on the surface. The coating rates in Table 2 present that coating agents can not be removed by centrifugal process[8].

3.2 Properties of coated nano-TiO₂

The ultraviolet radiation transmittivity of sample is shown in Fig.6, where samples 1-5 represent the unmodified, SiO₂ and Al₂O₃ complexly coated, SiO₂ coated, Al₂O₃ coated, and A1₂O₃ and SiO₂ coated respectively. The ultraviolet radiation transmittivity of all samples at 260-400 nm is nearly zero, while at 200-260 nm it is different, especial for sample l the transmittivity is 14% and for others the transmittivity is 7%. In fact only the middle-length wave band of ultraviolet radiation from sun to earth (290- 400 nm) is harmful to human. So the wave band of ultraviolet radiation can be well shielded and the requirement of anti-ultraviolet radiation can be met by use of coated samples.

Table 2 Mass loss of specimen coated with polymer after being calcined at 850 ℃

Fig.6 Ultraviolet transmittivity of specimens

The settlement experiment result is shown in Table 3. For uncoated samples, the complete sedimentation only need 20 min, which means poor dispersing stability, and for coated samples there is a little sedimentation in 5 months, which means good dispersing stability in distilled water. For Al₂O₃ and SiO₂ complexly coated samples, however, the slurry concentration is gradually increased from the top to bottom. In fact there is no evident boundary and the liquid in upper layer is colloid of milkiness transparence.

Table 3 Result of sedimentation experiment for various specimens

The dispersing stability of modified nano-TiO₂ in distilled water is improved greatly. According to DLVO theory, the stability of colloid is determined by the relative size of repulsion potential energy and attraction potential energy of colloid particle[9]. The attraction potential energy bears a relation with surface property of particle and repulsion potential energy with electrical potential of nano-TiO₂ surface. At different pH values, the electrical potential is decided by particle surface structure, for example, the equipotential (=0) of hydrated A1₂O₃ responds to pH 12, hydrated SiO₂ to pH 2, nano-TiO₂ to pH 6 and general rutile titania to pH 3.6[10].

The surface structure of particle may be altered by surface coating, for example, the surface of normal rutile TiO₂ coated with 0.5% hydrated SiO₂ is consistent with hydrated SiO₂[11]. The surface structure of samples coated with Al₂O₃ is similar to hydrated Al₂O₃ and the equipotential is at pH 12, while the pH value of distilled water is close to neutrality far from equipotential, so the higher relative electrical potential and the larger repulsion force is, the better the dispersing stability is. After coated with SiO₂, the equipotential of hydrated SiO₂ is at pH 2, far from pH 7, so there is good dispersing stability also. And the pH value of uncoated nano-TiO₂ is 6-7, which is similar to the pH value of distilled water. As a result, the repulsion force between particles is small and large attraction force lead particles to agglomerate together.

Through varying the proportion of A1₂O₃/SiO₂ on surface coating particle, different electric properties are obtained, such as, positive electricity at A1₂O₃/SiO₂＞1 and negative at Al₂O₃/SiO₂＜1. The final slurry delaminates quickly at A1₂O₃/SiO₂ about 1, but it is difficult to delaminate at A1₂O₃/SiO₂ about 0.47 or 1.5 in this trial.

The h²—t plots of water and TiO₂ coated and uncoated sample are shown in Fig.7. Based on Washbum formula, that is h²=(γcosθ?σ?t)/2η_L (defining R=γcosθ), factors such as R₁=7.29×10^-4; R₂=2.45×10^-4; R₃= 6.86×10^-4; R₄=3.43×10^-4; i.e. R₁＞R₃＞R₄＞R₂, γ constant; θ₂＞θ₄＞θ₃＞θ₁ are obtained. So the hydrophilicity of nano-TiO₂ will be altered through surface modification.

The h²—t plots of xylene and TiO₂ inorganic- organic coated and uncoated are shown in Fig.8. It can be seen that the sample coated with SiO₂ has the maximum slope, which corresponds to the smallest R and θ, i.e. best lipophilicity. Coating with A1₂O₃ and complex coating samples that modified by organic make the lipophilicity of nano-TiO₂ reduce, among them the lowest lipophilicity is for samples coated with A1₂O₃ and modified by organic.

The R curves of the amount of organic modifiers are shown in Fig.9. It is shown that first, R is evidently increased with coating amount increasing and the largest R occurs at 1.2% coating amount. Then it decreases with coating amount increasing and keeps constant at 3% coating amount.

Fig.7 h²—t plots of water and TiO₂ samples

Fig.8 h²—t plots of xylene and TiO₂ modified with silane coupling agent

Fig.9 Effect of amount of silane coupling agent on nano-TiO₂ wetting capacity

TiO₂ has good hydrophilicity since hydroxyl formed through water polarization. The hydrophilicity of Al₂O₃/ SiO₂ is poor because of its weak polarization. Therefore, it affects the paint absorption of modified nano-TiO₂ surface. Obviously the kind and amount of coating agent will affect the hydrophilicity of nano-TiO₂. Large polarization hydroxyl is reduced by coated hydrated SiO₂ with weak polarization. In Fig.9, the wettability change with coating content has an extremum. The wettability is mainly determined by surface radicle, similar with inorganic surface modification. Along with organic content increment, hydrophilia —OH is gradually replaced by paint absorption function group in silane coupling agent. OH disappears at 1.2% coating content, having the best paint absorption. In fact, organic coating on inorganic surface is really a chemical absorption that forms organic film. It can be divided into two stages[12], i.e. single molecule absorption for good hydrophilicity and surface colloid for opposite. Single molecule absorption occurs when the content of silane coupling agent in coating is below 1.2% and complete single molecule film is formed at 1.2% content, i.e. nano-TiO₂ coated completely with silane coupling agent, as the best silane coupling agent. However it is opposite paint absorption with colloid increment, by which single molecule layer is replaced at 3%. This is a reason why stable paint absorption is at over 3% silane coupling agent.

3.3 Influence factors of coating

The pH value will affect the dispersion of slurry and the coating process. In this trial, nucleus coating is suitable for acidity and film coating for alkali conditions at same A1₂O₃ content. There is the better coating film at pH 8-10 for samples coated with SiO₂.

Mixing slurry sufficiently can get the high dispersion, supersaturating and uniform pH of coating material, so as to form a full uniformity film. For organic film, sufficient stirring is needed to disperse particles and form a fully uniform organic film on nano-TiO₂.

Dispersing time means stirring time before coating. Short time stirring will induce insufficient dispersion for slurry, which leads more particles group to be coated together without nano-particle. If it is too short, it will make the pH value non-uniform. Neutralizing time should be long enough. If it is too short, it will make coating film not coat on nano-TiO₂ surface but increase the supersaturation of slurry, which finally forms nucleus film, such as 1 h neutralizing for coated SiO₂. Sufficient holding time will make coating material completely coat on the surface of particle and form a dense film, for example, a dense film has been gained by holding coated SiO₂ sample for 3 h.

The coating temperature will greatly affect the property of slurry. If it is too low, the diffusion rate of coating material is relatively slow, which is easy to form an abnormal coating. If the temperature is too high, such as above boiling point, water in slurry will easily vaporize and the concentration greatly varies, which is difficult to form the film. General coating temperature should not be lower than 60 ℃, only organic coating forms covalent bond when heating for dehydration.

The pH and concentration may be controlled in a small range by using the process of first dripping coating agent and then adding neutralizing agent. Such as in coated SiO₂ process it will get dense film to keep over pH 9. Contrarily it will make pH or coating agent non-uniform at local part by using the process of adding coating material and neutralizing agent at the same time. Concentration of neutralizing agent should be about 1%. If it is too high, it will make the pH or coating agent locally non-uniform and obtain a poor coating.

4 Conclusions

1) A dense film containing 5% SiO₂ or A1₂O₃ can be obtained by sufficiently stirring the slurry at pH 10, neutralizing for 60 min at 85-95 ℃ and aging for 120 min.

2) The uniform film on nano-TiO₂ coated with SiO₂ has a microstructure of internal Si—O—Si bond, Si—OH group on the surface and Ti—O—Si bond combining with nano-TiO₂. For Al₂O₃ coating, it is AlOOH and Al(OH)₃.

3) The growth of karyotheca is superior to new nucleus formation. The longer the aging time is, the denser the coating is. Nucleus coating is easily obtained in acid slurry with insufficient mixing. Complete dense coating film has good shielding ability for nano -TiO₂.

4) When modifying with silane coupling agent, the surface active —OH on nano-TiO₂ is a key. The content of silane coupling agent can reach up to 7%. The best wettability is at 1.2% silane coupling agent and the invariableness wettability is at 3%.

5) The dispersing stability and wettability of nano-TiO₂ are improved efficiently through the surface modification. The surface coating does not reduce the absorption of ultraviolet radiation for nano-TiO₂.

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(Edited by YUAN Sai-qian)

Foundation item: Project(2002126) supported by the Chongqing Science Committee, China

Corresponding author: GAO Jia-cheng; Tel: +86-23-65102821; E-mail: gaojch@cqu.edu.cn