Journal of Nanoparticle Research

, Volume 12, Issue 1, pp 135–142 | Cite as

Photoactivity passivation of TiO2 nanoparticles using molecular layer deposited (MLD) polymer films

Research Paper

Abstract

Pigment-grade anatase TiO2 particles (160 nm) were passivated using ultra-thin insulating films deposited by molecular layer deposition (MLD). Trimethylaluminum (TMA) and ethylene glycol (E.G) were used as aluminum alkoxide (alucone) precursors in the temperature range of 100–160 °C. The growth rate varied from 0.5 nm/cycle at 100 °C to 0.35 nm/cycle at 160 °C. Methylene blue oxidation tests indicated that the photoactivity of pigment-grade TiO2 particles was quenched after 20 cycles of alucone MLD film, which was comparable to 70 cycles of Al2O3 film deposited by atomic layer deposition (ALD). Alucone films would decompose in the presence of water at room temperature and would form a more stable composite containing aluminum, which decreased the passivation effect on the photoactivity of TiO2 particles.

Keywords

TiO2 Photoactivity Molecular layer deposition (MLD) Aluminum alkoxide (alucone) Pigments 

Introduction

Titanium dioxide (TiO2) is one of the most widely used white pigments in industrial applications, primarily due to its high refractive index, tinting strength, chemical inertness, thermal stability, non-toxicity, and inexpensive manufacture (Allen et al. 1992; Chow et al. 1991; Allen et al. 1991). The use of TiO2 pigments in industry often involves the incorporation of the pigment into polymeric materials, such as paints and polyolefins. However, TiO2 is a well-known UV-activated oxidation catalyst, which degrades the polymer surrounding the pigment, through the so-called “chalking” process (Allen et al. 1991; Kemp and McIntyre 2006a). Surface passivation is required for pigment-grade TiO2 particles used in commercial applications in order to quench the photocatalytic activity of the substrate material. Approaches to reduce the photoactivity of TiO2 particles following UV excitation include (1) introducing small amounts of dopant ions at the surface, or into the lattice of the TiO2 (Kemp and McIntyre 2006a, b), which has been investigated extensively using a range of transition metal ions, such as Cr or Mn ions, and (2) coating them with films, such as Al2O3 or SiO2 (Kemp and McIntyre 2006a, b; Hakim et al. 2007; King et al. 2008).

Very few methods exist to deposit conformal ultra-thin films on particles. Traditional methods for particle surface coating, including liquid-based processes (i.e., sol–gel) or gas-based processes (i.e., chemical vapor deposition), cannot coat a pin hole-free, conformal and ultra-thin layer on the particle surface, which reduces the quality of surface passivation. Atomic layer deposition (ALD) is a nanocoating process that is an ideal method for such thin film deposition (Wank et al. 2004a, b; Ferguson et al. 2002, 2004; Hakim et al. 2005; Liang et al. 2007a, b, 2008a; King et al. 2007). Ultra-thin Al2O3 and SiO2 films have been deposited on TiO2 particles by ALD for photoactivity passivation (Hakim et al. 2007; King et al. 2008). Coating ultra-thin polymer films on particle surfaces can not only reduce the photoactivity of TiO2 particles, but can also help to disperse and fix TiO2 particles in the polymer matrix. The molecular layer deposition (MLD) method (Yoshimura et al. 1991, 2007; Du and George 2007; Dameron et al. 2008), which is similar to ALD, allows conformal molecular level control over the deposition of polymer films. This control during MLD is achieved by introducing the reactants individually, separated by purge steps, in a sequential manner and carrying out self-limiting surface reactions that occur during each step on the substrate surface. In addition, MLD can deposit hybrid polymer films using suitable precursors, such as trimethylaluminum (TMA) and ethylene glycol (E.G) for aluminum alkoxide (alucone) hybrid polymer (Dameron et al. 2008). This vapor-phase method, which operates under reduced pressure and does not require solvents or catalysts, is a useful and promising technique for the fabrication of ultra-thin polymeric layers.

Our overall research goal is to quench the photoactivity of pigment-grade TiO2 particles via ultra-thin film coatings. Toward this goal, the main objective of this study is to demonstrate the feasibility of surface passivation by coating polymer films on the surface of primary pigment-grade TiO2 particles. As a proof of concept, ultra-thin alucone films were deposited on the surface of TiO2 particles by MLD in a scalable fluidized bed reactor. The photoactivity of the coated particles was tested by methylene blue oxidation.

Experimental methods

Alucone MLD on TiO2 particles

TMA and E.G were used as reactants for alucone MLD at a reaction temperature range of 100–160 °C. The MLD half-reactions between TMA and E.G are listed as follows (Dameron et al. 2008):
$$ \begin{gathered} {\text{A: }}{-}{\text{OH* + Al}}\left( {{\text{CH}}_{ 3} } \right)_{ 3} \to - {\text{OAl}}\left( {{\text{CH}}_{ 3} } \right)^{*}_{ 2} {\text{ + CH}}_{ 4} \hfill \\ {\text{B: }}{-} {\text{AlCH}}^{*}_{ 3} {\text{ + OHCH}}_{ 2} {\text{CH}}_{ 2} {\text{OH }} \to - {\text{AlOCH}}_{ 2} {\text{CH}}_{ 2} {\text{OH* + CH}}_{ 4} \hfill \\ \end{gathered} $$
Here the asterisks designate the surface species. A fluidized bed reactor was used to deposit alucone films on TiO2 particles, as shown in Fig. 1. The fluidized bed system consists of a reactor column, a vibration generation system, a gas flow control system, and a data acquisition and control system with LabView®, which has been described in detail previously (Liang et al. 2007a, b). The reaction system was operated at reduced pressures and utilized mechanical vibration to help overcome part of the interparticle forces and improve the quality of fluidization. High purity N2 was used as the purge gas to remove any byproducts formed during the reaction and the unreacted precursor.
Fig. 1

Schematic diagram of molecular layer deposition-fluidized bed reactor

For a typical run, ~10 g of pigment-grade TiO2 particles (160 nm anatase, Millennium Chemicals) were loaded into the reactor. During the MLD reaction, TMA (97%, Sigma Aldrich) was fed through the distributor of the reactor based on the driving force of its room-temperature vapor pressure. The room-temperature vapor pressure of E.G (anhydrous, 99.8%, Sigma Aldrich) is very low and is much lower than that required for the fluidization of reasonable bed masses. Heating the E.G liquid can raise its vapor pressure. A bubbler was applied to dilute the heated E.G vapor stream and allowed for vapor delivery to the reactor in a controllable fashion and prevented the overdose of the precursor. The bubbler inlet was controlled using a mass flow controller (MKS Instruments) to allow a calibrated amount of N2 to be bubbled through the precursor reservoir. In this case, a flow rate of 4 sccm of N2 was sufficient to deliver E.G, which was pre-heated to 80 °C, to the reactor. A typical coating cycle used the following sequence: TMA dose, N2 purge, evacuation; E.G dose, N2 purge, evacuation.

The coated samples were analyzed using a JEOL 2010F 200 kV Schottky field emission transmission electron microscope coupled with an Oxford detector unit for elemental analysis. The surface area of the particles before and after coating was obtained using a Quantachrome Autosorb-1.

Photoactivity test

Methylene blue (C16H18N3ClS, MB) is the probe molecule in this study for evaluating the passivation effect of the alucone films. For a typical test, 0.1 g of sample particles was dispersed in 100-mL MB aqueous solution having a concentration of 10 ppm. The solution was kept in the dark under stirring to measure the adsorption of MB into each sample. MB concentration in the solution was found to be constant after 60 min on all the samples prepared. Therefore, 60 min was long enough to establish an adsorption/desorption equilibrium condition. For UV irradiation, one UV lamp (Mineralogical Research Co.) of 100 W was used at the distance of 10 cm from the solution. The strength of UV light with a wavelength of 360 nm at the position of the solution surface was measured to be ~10 mW/cm2 using an IL1400A Radiometer/Photometer (International Light). The solution was continuously stirred during UV irradiation. Concentration of MB in the solution was measured as a function of irradiation time of UV rays. At the given time intervals, analytical samples were taken from the suspension and passed through a 0.2-μm Millipore filter to remove the particles before analysis. The determination of MB concentration (concentration-dependent absorbance) was carried out using 2 mL of solution, which was sampled from absorbance change at the wavelength of 664 nm with a Perkin Elmer Lambda 35 UV/VIS/NIR spectrometer.

Stability test of alucone MLD films in water

TiO2 particles with different thicknesses of alucone MLD films were soaked in deionized H2O at room temperature. The MLD coating was carried out at 100 °C, and the particles were coated with 5, 10, 15, and 20 cycles, respectively. In the test, for every sample, 0.5 g of particles was put in a 30-mL vial and soaked in 5-mL deionized H2O. After given times, particles were filtered and vacuum dried. Both the aluminum concentration in the filtrate and the aluminum concentration on the particles were measured. Several drops of nitric acid were added to the filtrate to make sure that the aluminum was not precipitating out of the samples, and the aluminum concentration in the filtrate was directly measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES) using an Applied Research Laboratories ICP-AES 3410+. The filtered particles were dried, digested with NaOH solution for 24 h at 95 °C, and then measured by ICP-AES.

Results and discussion

Conformal coating of primary nanoparticles

The alucone-coated TiO2 particles were analyzed by TEM. A representative TEM image of the coated TiO2 particles after 20 cycles deposited at 160 °C is shown in Fig. 2. The alucone films appeared to be very uniform and smooth. The thickness of the alucone film was ~7 nm, which represented a growth rate of ~0.35 nm/cycle at this experimental condition. The composition of the alucone films has been confirmed by energy dispersive spectroscopy (EDS) (data not shown). The growth rates of alucone MLD increased at lower substrate temperatures, with a rate of 0.5 nm/cycle at 100 °C based on TEM analysis. The alucone MLD is thought to begin with hydroxyl groups on the substrate surface. Native hydroxyl groups are present on titania particle surfaces, and can react with TMA to initiate the MLD film growth during the first coating cycle. The temperature-dependent film growth rate suggested that the residence time of the reactants on the surface was important in determining the film growth rate. The reaction rate decreased with decreasing residence time of the precursors on particle surfaces at higher temperatures.
Fig. 2

Transmission electron micrograph of 160-nm anatase titania particle after 20 cycles of alucone MLD coating at 160 °C

The surface area of the coated particles after 20 cycles deposited at 100 °C was 8.51 ± 0.31 m2/g, while the surface area of the uncoated particles was 9.47 ± 0.24 m2/g. A slight decrease in surface area suggested that a small degree of permanent aggregation occurred, which indicated that the majority of titania particles functionalized by the alucone MLD films were coated on primary particles, and a high degree of aggregation did not occur.

Passivation effect of alucone MLD films

The photocatalytic performance of the TiO2 particles with different thicknesses of alucone MLD coatings deposited at 100 °C is shown in Fig. 3. For the uncoated TiO2 particles, the relative concentration of MB in the solution had an exponential relation with the irradiation time, as reported in the literature (Inagaki et al. 2004), and the C/C0 was almost zero after 90 min. In contrast, the alucone-coated TiO2 particles showed mitigated photoactivity. For example, for the particles of alucone coating with 2 cycles (~1 nm thick), with 5 cycles (~2.5 nm thick), and with 10 cycles (~5 nm thick), the C/C0 ratio was 0.42, 0.68, and 0.80 after 190 min of UV radiation, respectively. And, for the particles with 20 cycles (~10 nm thick) of alucone coating, no measurable activity over the same length of time was observed. These results demonstrated the effectiveness of the alucone coatings to prevent photocatalytic activity on the surface of TiO2 nanoparticles.
Fig. 3

Methylene blue concentration in photocatalytic experiments as a function of UV irradiation time. TiO2 particles were coated with different thicknesses of alucone MLD films at 100 °C

For comparison, TiO2 particles were also coated with different thicknesses of Al2O3 ALD films by alternating reactions of TMA and H2O at 177 °C. The growth rate of Al2O3 ALD was ~0.12 nm/cycle at this reaction condition (Hakim et al. 2005; Liang et al. 2008b). The passivation effect of Al2O3 ALD films for TiO2 particles with different thicknesses of Al2O3 films is shown in Fig. 4. The Al2O3-coated TiO2 particles also showed mitigated photoactivity. For the particles of Al2O3 coating with 10 cycles (~1.2 nm thick), with 30 cycles (~3.6 nm thick) and with 50 cycles (~6 nm thick), the C/C0 ratio was 0.55, 0.91, and 0.95, respectively, after 190 min of UV radiation. And, for the particles with 70 cycles (~8.4 nm thick) of Al2O3 coating, no measurable activity over the same length of time was observed. These results demonstrated the effectiveness of the Al2O3 coatings to prevent photocatalytic activity on the surface of TiO2 nanoparticles. 20 cycles of alucone MLD films was comparable to 70 cycles of Al2O3 ALD films for photoactivity passivation.
Fig. 4

Methylene blue concentration in photocatalytic experiments as a function of UV irradiation time. TiO2 particles were coated with different thicknesses of alumina ALD films at 177 °C

It is demonstrated that the encapsulation of the undoped TiO2 particle with an ultra-thin layer leads to a reduction in photoactivity of the pigment. TiO2 photocatalytic oxidation occurs when the high-energy photogenerated holes oxidize the surface hydroxyl ions of TiO2, [Ti4+–OH], forming free radicals \( \hbox{H}_{2} \hbox{O} + \hbox{O}_{2} \xrightarrow{{{\rm TiO}_{2} , {\rm hv}}} \bullet \hbox{OH} + \hbox{HO}_{2} \bullet \) (Luo and Gao 1992). The coating on the pigment particle surface is behaving as a trap recombination center, which prevents the surface reactions of electrons/holes with adsorbed species, by promotion of recombination reactions, \( 2 \bullet \hbox{OH} \xrightarrow{{}} \hbox{H}_{2} \hbox{O} + 1/2 \hbox{O}_{2} \)(Allen et al. 1991; Kemp and McIntyre 2006b; Allen et al. 1993).

Al2O3 and SiO2 ALD films have been reported for photoactivity mitigation of TiO2 nanoparticles (Hakim et al. 2007; King et al. 2008). Though Al2O3 and SiO2 ALD films could effectively mitigate the photoactivity of TiO2, they modified the TiO2 substrate particle color to be bluish, which is not favorable for white-pigment applications. The mechanism of this color change for ALD coating is not clear. However, with alucone MLD coatings, this bluish color change was significantly reduced. For example, with 5 cycles of alucone MLD coating fabricated at 100 °C, the color of substrate particles was almost the same as that of uncoated particles. Also, the bluish color effect decreased when the alucone MLD films were coated at lower temperatures. For example, there was almost no color change for TiO2 particles with 20 cycles of alucone MLD coating deposited at 80 °C. Another advantage of an alucone MLD coating, compared to an Al2O3 or SiO2 ALD coating, is the much higher film growth rate. For example, typically the growth rate of Al2O3 ALD at 177 °C is ~0.12 nm/cycle, and the growth rate of an alucone MLD film is 0.5 nm/cycle at 100 °C and 0.35 nm/cycle at 160 °C.

Stability of alucone MLD films in water

Alucone MLD-coated TiO2 particles were soaked in deionized H2O for 1 week and then dried, which is called the aging process here. The passivation effect of the aged alucone films was tested by MB oxidation. The photocatalytic performance of the TiO2 particles with different thicknesses of aged alucone MLD coatings is shown in Fig. 5. The TiO2 particles with aged alucone films showed mitigated photoactivity, but not as efficient as those alucone films without the aging process. For example, for the particles with 15 cycles of alucone coating, C/C0 was 0.24 after 190 min of UV radiation. For the particles with 20 cycles of alucone coating, C/C0 was 0.35 after 190 min of UV radiation. This result demonstrated that alucone films would degrade or shrink in H2O.
Fig. 5

Methylene blue concentration in photocatalytic experiments as a function of UV irradiation time. TiO2 particles with different thicknesses of alucone MLD films deposited at 100 °C were aged in deionized H2O at room temperature for 1 week

In order to test the stability of alucone MLD films in H2O, ICP-AES was used to measure the solubility of Al in alucone films in water. TiO2 particles with 20 cycles of alucone coating deposited at 100 °C were put into vials containing deionized water at room temperature and allowed to digest for different periods of time. It is expected that the dissolved amount of alucone films will be reflected by the aluminum content in H2O. The Al concentration in water versus time is shown in Fig. 6. For comparison, the solubility of Al in an Al2O3 film on pigment-grade TiO2 particles in H2O is also shown in the figure. The results indicated that the Al2O3 ALD film would dissolve slightly in H2O. In contrast, Al in alucone films did not dissolve in H2O, and it could remain on the particle surface, though alucone films would degrade and the structures of the films could be changed.
Fig. 6

Solubility of aluminum in an alucone MLD film (deposited at 100 °C, total Al concentration of 26,987 ppm) and an alumina ALD film (deposited at 177 °C, total Al concentration of 65,952 ppm) in water at room temperature

Further testing was done by soaking TiO2 particles having different thicknesses of alucone MLD films in deionized H2O at room temperature for 1 week. The particles were coated with 5, 10, 15, and 20 cycles of alucone films. The filtrates were tested by ICP-AES for aluminum concentration in H2O, as shown in Fig. 7, based on which, the relative amount of aluminum dissolved in H2O can be easily estimated. The results indicate that only trace amounts of aluminum (<0.03 wt.%) were dissolved in H2O, which is consistent with the results shown in Fig. 6.
Fig. 7

Aluminum concentration in H2O and the percentage of aluminum dissolved in H2O. Alucone MLD-coated (deposited at 100 °C) titania particles of 0.5 g were soaked in deionized H2O at room temperature for 1 week. Aluminum concentration on titania for 5, 10, 15, and 20 cycles of alucone MLD coating was 10,461, 17,579, 26,216, and 34,323 ppm, respectively

The aluminum concentration on filtered particles was also measured by ICP-AES, as shown in Fig. 8. It is shown that there is almost no difference between the aluminum concentrations on particles before and after the aging process. The small difference could be caused by the systematic error of the ICP-AES, since these two groups of samples were not analyzed at the same time. These results indicated that the alucone films would decompose slowly in the presence of water, but Al in the film would not dissolve in H2O; a more stable composite containing Al could form, but it could only then be dissolved in a strong basic solution.
Fig. 8

Aluminum concentration on alucone-coated titania particles before and after the aging process versus the number of coating cycles

Although the alucone MLD reaction was very robust, the resulting alucone MLD films were found to be unstable in water. Recent studies also showed that H2O may facilitate the chemical transformation of the alucone MLD films, which may cause the film to shrink (Dameron et al. 2008). The film shrinkage would also be accompanied by chemical changes in the alucone films that were consistent with either dehydration or dehydrogenation reactions. This study further indicated that alucone films would decompose in water and Al would remain on the particle surface to form a more stable composite, since there were only trace amounts of Al dissolved in H2O. During this aging process, bubbles would form, which could result in a porous structure of the aged alucone films on the TiO2 particle surfaces, and the porous structure of the films could reduce the passivation effect of the aged alucone films.

Conclusions

Coating ultra-thin polymer films on TiO2 particle surfaces can not only reduce the photoactivity of TiO2 particles, but also help to potentially disperse and embed TiO2 particles in the polymer matrix. A new class of organic–inorganic hybrid polymer films was coated on pigment-grade 160-nm TiO2 particles by MLD. The coated alucone MLD films appeared to be very uniform and smooth. The growth rate varied from 0.5 nm/cycle at 100 °C to 0.35 nm/cycle at 160 °C. Methylene blue oxidation tests showed that alucone MLD films played an important role in controlling the photoactivity of the pigments. Alucone MLD films were not stable in H2O, which may facilitate the chemical transformation of the alucone films. Modifying the basic chemistry of alucone MLD using other bifunctional monomers should allow forming more stable alucone polymer. Combination of different ALD and MLD surface chemistries can provide another solution to solve the stability of alucone films, such as providing an external ultra-thin SiO2 ALD film to prevent H2O diffusion into the alucone films (Dameron et al. 2008). Particle MLD is a viable passivation method for pigment-grade TiO2 particles.

Notes

Acknowledgments

The authors thank Dr. Peng Li (University of New Mexico) and Fredrick G. Luiszer (University of Colorado) for the TEM analysis and the ICP-AES analysis, respectively.

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Copyright information

© Springer Science+Business Media B.V. 2009

Authors and Affiliations

  1. 1.Department of Chemical and Biological EngineeringUniversity of ColoradoBoulderUSA

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