Conformal Multilayer Photocatalytic Thin Films on Fine Particles by

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Conformal Multilayer Photocatalytic Thin Films on Fine Particles by Atmospheric Pressure Fluidized Bed Chemical Vapor Deposition Sajjad Habibzadeh, Oleg Zabeida, Alberto Argoitia, Robert Sargent, Jolanta Ewa Klemberg-Sapieha, Jamal Chaouki, and Ludvik Martinu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00756 • Publication Date (Web): 11 Jul 2018 Downloaded from http://pubs.acs.org on July 14, 2018

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Industrial & Engineering Chemistry Research

Conformal Multilayer Photocatalytic Thin Films on Fine Particles by Atmospheric Pressure Fluidized Bed Chemical Vapor Deposition Sajjad Habibzadeh1,3,4*, Oleg Zabeida1, Alberto Argoitia2, Robert Sargent2, Jolanta KlembergSapieha1, Jamal Chaouki3, and Ludvik Martinu1*

1

Department of Engineering Physics, Polytechnique Montreal, Montreal (Quebec) Canada 2

3

Viavi Solutions Inc, Santa Rosa, CA, United States

Department of Chemical Engineering, Polytechnique Montreal, Montreal (Quebec) Canada 4

Department of Chemical Engineering, Amirkabir University of Technology (Tehran Polytechnic), Tehran, Iran

Abstract Conformal multilayer TiO2/SiO2/TiO2 coatings were deposited on the surface of ~27 µm spherical soda lime glass (SLG) particles using fluidized bed chemical vapor deposition (FB CVD) at atmospheric pressure. Cost-effective precursors of titanium and silicon chlorides together with water were employed to deposit titania and silica films at 300°C and room temperature, respectively. Focused ion beam cross-sectional transmission electron microscopy, scanning

electron

microscopy,

energy

dispersive

spectroscopy,

X-ray

photoelectron

spectroscopy, Brunauer-Emmett-Teller surface area analysis, and X-ray diffraction were used to characterize the multilayer-coated particles. The results revealed a pin-hole free and uniform multilayer of anatase TiO2 and amorphous SiO2 with a thickness of ~110 and 20 nm. Moreover, the photodegradation performance of the coated particles was examined by the degradation of methylene blue as the model reaction. It was found that a multilayer thin film of titania and silica 1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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can effectively prevent sodium ion diffusion from the SLG microsphere substrates, thus improving the photo-catalytic performance of such system.

Keywords: FB CVD, Titanium dioxide; Silica; Multilayer coatings; Surface modification; Photocatalysis

2 ACS Paragon Plus Environment

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1. Introduction Surface properties of fine particles can be tuned through single- and/or multi-layer deposition of films or coatings, without altering their bulk properties, of particular interest in different fields such as catalysis, energy production, microelectronics, optoelectronics, etc. 1-4. In particular, multilayer deposition on particles offers a broad range of nanostructured thin films suitable for multiple applications, including photocatalysis 7, 8

5, 6

, advanced energy storage systems

, drug delivery 9, 10, etc. Coating of powders can be applied by either wet chemical processes,

such as sol-gel and impregnation 11, 12, or by dry techniques (i.e., the use of a reactive gas phase), including pyrolysis and chemical vapor deposition (CVD)

13

; however, most attention in the

literature has been paid to the deposition on fine particles using wet chemical processes 14. This includes synthesis of core-shell particles or layer-by-layer (LbL) assembly, leading to the construction of thin-film-based nano-architectures

15

. Nonetheless, liquid phase methods have

been found to be rather expensive because of precursor/solution waste, and further separation and drying steps prior to particle usage. These methods typically yield only a small quantity of material, which hinders its practicality in manufacturing. Therefore, gas-phase coating techniques, such as CVD and atomic layer deposition (ALD) onto particles have been extensively developed

16-18

. In a typical CVD process the substrate is exposed to one or more

gaseous precursors that react on a surface to produce desired films. Nevertheless, highly conformal deposition may be a problem when fine particles are considered as substrates. This limitation is given by the fact that exposing the entire surface of particles to the reactive gas phase is rather complicated and not easily addressed. In such case, a technique which meets the requirement of a full gas–solid contact for individual particles, i.e., fluidization, can be employed. Such technique is also able to be associated with the gas–solid reactions that are often



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used in the context of various CVD processes. Therefore, fluidized bed chemical vapor deposition (FB CVD) is expected to offer efficient and cost-effective particle treatment processes 17

. In addition, FB ALD processes have been shown to produce a conformal layer on the surface

of fine particles; however, one should be aware of its limited application for coatings thicker than about 50 nm due to a low deposition rate 19. The fluidization behavior of fine particles with respect to their physical, chemical and mechanical properties has been a subject of numerous investigations for several decades

20-23

.

Considering the relative difference between particle and gas densities, fine particles with a size of 20 µm < dp < 30 µm lie in the A/C transitional region based on the Geldart’s classification of powders

24, 25

. However, it should be noted that such classification can only be addressed when

the physical properties of powders are referred at ambient conditions. Namely, such particles can exhibit the fluidization behavior of either C- or A-powder depending on the experimental conditions (i.e., gas viscosity, gravity, gas adsorption, temperature, etc.); in particular, an elevated temperature causes an increase in the Hamaker constant, which in turn, leads to an increase of the universal van der Waals force of attraction between the particles. This consequently raises the inter-particle attractive force (IPF)-to-particle weight ratio (i.e., granular Bonding number)

26

. As a result, the fluidization behavior of a group A powder may change to

the one of a cohesive powder, i.e., typical Geldart group C powder, as the temperature would increase. Therefore, fluidization of fine particles at elevated temperatures is liable to clustering either by agglomeration (reversible) or aggregation (irreversible) processes, leading to heterogeneous fluidization. In response to the progress and challenges described above, in the present study we explore the FB CVD technique for the fabrication of multilayer TiO2/SiO2 systems onto soda lime fine 4 ACS Paragon Plus Environment

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particles in a FB CVD reactor under atmospheric pressure. Such multilayer coated particles were employed as the photocatalysts in order to mineralize the organic contaminants in water under UV irradiation. However, nanosized TiO2 cannot efficiently be recovered from treated water, which hinder its extensive application. Therefore, nanocrystalline TiO2 immobilized on supporting materials such as activated carbon, glass and alumina have been used to immobilize photocatalysts in photocatalytic reactors for wastewater decontamination

27, 28

. However, apart

from a lower cost of SLG, one should consider the superior potential of SLG towards the transmition of UV light, offering the benefits rather than using activated carbon or alumina as the substrates. This would also provide increased photocatalytic activity since passing UV light would effectively increase the surface area available for UV light to interact with the photocatalyst. To the best of our knowledge, this is the first time when multilayer coatings on fine particles by FB CVD has been considered. However, this work complements few other studies on multilayer coatings of radioactive particles (dp ~500 µm) as high-temperature reactor fuel elements using CVD spouted bed reactors 29, 30. In addition, we demonstrate, for the first time, deposition of SiO2 onto the particle surface at room temperature using a low-cost precursor, namely silicon tetrachloride (SiCl4). In the past, CVD of SiO2 films has been performed at various temperatures; this consists of quite high temperature of 1200 K using dichlorosilane (SiH2Cl2) as precursor

31

, medium temperatures of

900 to 1000 K using tetraethyl orthosilicate (TEOS) decomposition temperatures of 500 to 700 K using SiCl4 and SiH4

33

32

, and relatively low

. Few research studies used gas-phase

hydrolysis of SiCl4 with or without contribution of a catalyst to perform atomic layer deposition of SiO2 on wafer substrates at near-room temperature under reduced pressure 34-38. 5 ACS Paragon Plus Environment


2. Experimental section 2.1

Materials In order to fabricate TiO2 and SiO2 films, TiCl4, SiCl4 (Sigma Aldrich) and deionized

water were used as reactants, while soda lime glass (SLG) microspheres (MO-SCI corporation, USA) were employed as substrates. The morphology of the powder, which was characterized by scanning electron microscopy (SEM), can be observed in Figure. 1a. The mean size of the particles was about D50 ~ 27 µm (see Figure. 1b). Loose bulk density of the powder was 1360 kg/m3, and the skeletal density of the primary particles was measured at 2470 kg/m3 by a pycnometer. In addition, the Brunauer-Emmett-Teller (BET) specific surface area of the powder was estimated to be about 0.07 m2/g, indicating a non-porous microstructure of the substrate particles.

Frequency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.1

1

10

100

Size (µ µm)

Figure 1. a) SEM image, and b) particle size distribution of bare SLG substrate particles.


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2.2

Fluidized bed chemical vapor deposition SLG particles were coated in a fluidized bed reactor using chemical vapor deposition

schematically shown in Figure 2. The reactor was made of a 35 mm diameter, 300 mm long quartz column equipped with a 20 µm pore size porous stainless-steel disc installed at the bottom and used as the gas distributor. A porous stainless-steel filter element (10-µm pore size) was used at the outlet of the reactor column to assure that no powder particles reach the scrubber. High-purity nitrogen was used as fluidizing gas as well as a carrier gas for the precursor delivery. Flow rates were controlled by mass flow controllers, (Model 1179A, MKS Instruments). Pressure transducers, (Model Omega PX277) were placed above the distributor plate and at the outlet of the fluidization column to measure the pressure drop across the bed of powder and to determine minimum fluidization velocity.



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5

P

o o oo

6

Ventilation

Water supply TiCl4 8

9

1

SiCl4 3

Fluidizing gas N2 supply

2

4

10 7

5

P

Precursor

Figure 2. Schematic diagram of fluidized bed chemical vapor deposition system: (1) fluidization column, (2) band heaters, (3) water diffuser, (4) thermocouple, (5) differential pressure transducers, (6) scrubber, (7) metal salt precursor diffuser, (8) mass flow controllers, (9) valves, (10) porous disc distributor.

The deposition conditions for single- and multilayer coatings of titanium dioxide and silicon dioxide on the SLG particles are summarized in Table 1. The reaction zone was heated by a set of band heaters. The reaction temperature was set at 300°C for TiO2 deposition, while SiO2 was deposited at room temperature (RT) (see Table 1). The gas feed lines were kept at ~ 65°C using heating tapes. The TiCl4 precursor and water containers were heated up to 50°C and 80°C, respectively, whereas the container with SiCl4 was kept at room temperature.


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Table 1. Fabrication conditions of multilayer deposition onto SLG particles. Sample

Bed temperature, °C

TiCl4, ml/min

Carrier (N2) gas flow SiCl4, ml/min

H2O, ml/min

TiO2/SLG

300

50

0

50

SiO2/TiO2/SLG

~20

0

25

50

TiO2/ SiO2/TiO2/SLG

300

50

0

50

2.3

Fluidization quality assessment and optimization The fluidization behavior of the powder was measured in a bed with a static bed height

(H0) of about 3.5–4.5 cm (H0/D = 1 – 1.3), where D is the inner diameter of the fluidized bed column. For a typical run, 20 g of particles were loaded into the reactor. Upon starting the fluidization test, the particle bed was first loosened with a gas flow at 2.0 cm/s for 10 min. To measure the minimum fluidization velocity, the N2 velocity was first increased to a high value and then reduced gradually. When decreasing the gas velocity, Ug, the pressure drop, ∆P, between the top and the bottom of the reactor, and the bed height, H, were simultaneously measured. In this study, the normalized pressure drop ∆P/∆P0 was used, which is defined as the ratio of the measured pressure drop across the whole bed to the normal pressure caused by the particle weight, ∆P0 = mg/A, where m denotes the weight of solids in the bed and A stands for the cross-sectional area of the fluidized bed column. When the entire bed is fluidized, the normalized pressure drop should reach unity and remain stable thereafter even if the gas velocity further increases. 2.4

Coating Characterization The cross-sections of coated particles were prepared by microtome sectioning technique

and then studied by transmission electron microscopy (JEOL, JSM7600F). The uniformity and



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composition of the deposited titania and silica films were assessed by X-ray photoelectron spectroscopy (XPS) using the VG Escalab MK-II instrument, by means of non-monochromated AlK∝ radiation with an energy resolution of 0.8 eV. X-ray diffraction (XRD) measurements were performed via a powder x-ray diffractometer (Bruker D8 Discover). The specific surface area of the particles before and after coating was calculated by the BET method from the N2 adsorption isotherms obtained at 77 K. The measurements were carried out by ASAP 2020 physisorption analyzer. 2.5

Photocatalytic activity test by oxidation of methylene blue Oxidation of methylene blue (C16H18N3ClS) was performed in order to assess the

photocatalytic performance of the coated particles. The aqueous slurry, prepared with the catalysts and methylene blue (MB) (10 ppm) was kept in dark for 30 min to ensure the saturation of adsorbed MB on the catalysts. Agitation was provided by a magnetic stirrer (250 rpm). A 20 W UV lamp placed 10 cm above the reaction slurry was used as the UV radiation source. The concentration of photocatalysts in the reaction solution (100 ml beaker) was set by about 1 gL-1 for all the runs. The solution was subjected to the UV irradiation while continuously stirred. At certain time intervals, the concentration of MB in the solution was measured as a function of UV irradiation time. Determination of the MB concentration was carried out using 2 mL of solution, which was sampled from absorbance change at a wavelength of 664 nm using a Perkin-Elmer Lambda 1050 spectrometer. 3. Results and discussions 3.1

Fluidization characteristics A fluidization process can be characterized by measuring the bed pressure drop and the

corresponding bed expansion of a particular powder. This consequently results in finding an 10 ACS Paragon Plus Environment

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appropriate operating conditions to conduct the deposition process. Figure 3a shows the bed pressure drop and expansion as a function of superficial gas velocity at room temperature. The incipient or minimum fluidization velocity (Umf), determined by finding the turning point on the ∆P vs. Ug curve, corresponds to about 0.22 cm/s (see Figure 3a). In addition, fluidization index (FI)

39

, i.e., the ratio of calculated minimum fluidization velocity (Umf,cal ~ 0.15, estimated by

available correlation 40) to the observed minimum fluidization velocity (Umf,obs) determined from the pressure drop curve, was found to be about 0.7. This value of FI assigns the powder employed in this study to the Geldart A/C group 39. The fluidization behaviour of SLG particles at 300°C is shown in Figure 3b. To implement the above described test, first the particles were fully fluidized at room temperature (fully fluidized processes) and then the whole bed was heated up to 300°C. It can be seen that the value of Umf decreases to about 0.09 cm/s by increasing the temperature up to 300°C, which is in line with its theoretical value of ~0.11 cm/s

40

. The influence of temperature on the Umf was

discussed in the literature according to the relative importance of IPFs and the hydrodynamic forces (HDFs) on the flow behaviour of the particles

41

. A change of the Umf at an elevated

temperature can be owing to the change of fluidizing gas density and viscosity when HDFs dominate the interaction of the fluid and the particles 42. In particular, as the flow of gas around the particles adopted in this study is laminar, the fluid-particle interaction force is dominated by the gas viscosity, which increases by raising the temperature, leading to a decrease of Umf

43

.

Moreover, the larger bed expansion at a higher temperature of 300oC (see Figure 3b) can be attributed to the increase of the drag force exerted by the fluidizing gas. It should be noted that 35 times the Umf was selected as the fluidizing gas velocity.



1

1.3

0.8

0.6

H/H0

∆P/∆P0

1.2

0.4 1.1

(a)

0.2

0

1

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

Ug, cm/s 1

1.8

0.8

1.6

0.6 1.4

H/H0

∆P/∆P0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.4 1.2

0.2

(b)

0

1 0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

Ug, cm/s Figure 3. Pressure drop across the fluidized bed, and the bed expansion versus superficial gas velocity for 20 g of SLG powder at a) ambient, and b) 300oC. Here, first full fluidization of the powder was established, then the bed was subjected to heating.

3.2 Multilayer coating deposition Deposition of TiO2 and SiO2 using the TiCl4 and SiCl4 precursors and water follows the CVD reaction of 44-46: MCl + 4H O → M OH + 4HCl, M OH → MO + 2H O, where M = Ti, Si 12 ACS Paragon Plus Environment

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The reaction mechanism encompasses hydrolysis and pyrolysis. The hydrolysis reaction of TiCl4 and water results in the formation of Ti(OH)4 at a lower temperature, playing a dominant role in the TiO2 coating process

46

. This is due to the fact that a lower activation energy is

required to deposit TiO2 from Ti(OH)4 rather than TiCl4. Lowering the deposition temperature, in turn, facilitates the fluidization process by altering the nature of resulting agglomerations. Besides, operating the FB CVD process at relatively low temperature might prohibit the formation of titanium oxide fine particles resulting from the homogenous gas-phase reaction at higher temperatures, i.e., > 500oC 47, 48. Thermodynamic analysis based on the Gibbs free energy showed that the hydrolysis of SiCl4 is feasible at any temperature in the gas phase

37

. However, Kochubei

38

experimentally

found certain temperature ranges in which the hydrolysis reaction can be applied. Namely, this study confirmed that the low-temperature hydrolysis takes place in the range of 20-100°C, where the reaction orders are 1 and 2 with respect to SiCl4 and H2O, respectively. In addition, the reaction rate drops to zero by increasing the temperature up to 100°C at which the hydrolysis reaction does not show further progress. However, the first-order hydrolysis reaction for both SiCl4 and H2O recommences at 470 °C. In the range of 470-800°C, SiCl4 hydrolysis is a regular bimolecular reaction with a positive activation energy of 121.9 kJ/mol

38

. However, the SLG

particles used in this work cannot tolerate such high temperatures, 49, thus the deposition of SiO2 layer was performed at room temperature. In order to demonstrate a possibility to achieve a good quality of multilayer coatings, we performed a series of consecutive depositions summarized in Table 1. Focused ion beam (FIB) cross-sectional TEM image of the coated particle is shown in Figure 4a. It clearly displays a conformal TiO2/SiO2/TiO2 multilayer system deposited on the surface of primary particles at 300°C, 20°C and 300°C, respectively. The deposition process was conducted during 3 hrs for each layer. Figure 5b demonstrates the average film thicknesses of TiO2 and SiO2 to be about 110 nm and 20 nm, corresponding to the growth rates of ~ 37 nm/hr and ~ 7 nm/hr, respectively. It should be noted that the difference of measured TiO2 thicknesses of the first and third layers (see Figure 4b) are not statistically significant (p > 0.05). In addition, one can compare the resulting growth rates of titania and silica by the FB CVD technique with the ones obtained by FB ALD. Namely, TiO2 growth rate of ~ 0.4 Å/cycle on ZrO2 particles were obtained by 13 ACS Paragon Plus Environment


employing the same reactants at 327°C via the FB ALD technique under reduced pressure

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45

.

However, SiO2 FB ALD on BN particles using SiCl4 and H2O at a higher temperature of 427°C and under reduced pressure resulted in a growth rate of 0.9 Å/cycle 50.

(a) nd

2

TiO2 Polymer layer

SiO2

Au coating st

1 TiO2

Soda lime

Figure 4. a) Focused ion beam (FIB) cross-sectional TEM image, and b) the average film thicknesses of the TiO2/SiO2/TiO2 system on the surface of SLG fine particles. The gold and polymer layers are added during the preparation of the TEM sample.

The uniformity and composition of the deposited multilayer films were further tested by XPS. Figure 5 shows the elemental composition of each deposited layer on the SLG particles. The peak for silica photoelectron line from the substrate was completely attenuated in the first layer (TiO2 film), indicating a pin-hole free and uniform coverage of the particle substrate. Similar trends were also observed for the second and third deposited layers (i.e., SiO2 and TiO2, respectively) in which titania and silica from the layer underneath the top layer were not detected, respectively.


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(a) O1s

Na1s

Ti2p Ti 3p Ti 3s Cl 2p

C1s

(b)

Si 2p Si 2s

(c)

Figure 5. Broad scan XPS spectrum of a) TiO2-coated, b) SiO2/TiO2-coated, and c) TiO2/SiO2/TiO2coated soda lime fine particles.

Details of the XPS elemental composition of coated particles are listed in Table 2. The coverage factor of titanium, i.e., Ti/(Ti+Si), indicates the content of titanium in each deposited layer. In addition, sodium which is found in the first layer, resulting from its diffusion from substrate at 300°C, diminishes in the second and third layers. This underlies the significance of multilayer coatings when the presence of certain elements is detrimental for a desired application. For instance, sodium from SLG substrates are considered as a suppressing agent for the photocatalytic activity of titanium oxide 51-53.



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Table 2. XPS Elemental composition of deposited layers. Batch number

Si%

Cl%

C%

Ti%

O%

Na%

TiO2 coverage factor

Soda lime 325 (SLG)

16.6

-

15.9

-

56.9

8.4

0

TiO2 /SLG

-

6.7

27.4

14.6

39.5

11.8

1

SiO2/TiO2/SLG

22.0

0.4

13.6

-

63.4

0.6

0

TiO2/SiO2/TiO2/SLG

-

0.8

27.7

18.9

52.1

0.5

1

A high-resolution XPS spectrum showing the Ti 2p region on the surface of the TiO2/SiO2/TiO2-coated SLG particles is shown in Figure 6a. Considering the different oxidation states of titanium, the Ti 2p peak can be fitted to several contributions, each encompassing a doublet of 2p3/2 and 2p1/2 peaks. The main doublet composed of two symmetric peaks appears at 458.3±0.2 eV and 464±0.1 eV, corresponding to the Ti4+ oxidation state in TiO212. Figure 6b shows Si 2p photoelectron response on the surface of SiO2/TiO2-coated particles. The corresponding resolved (deconvoluted) spectrum resulted in a perfectly-fitted peak at 103.35 eV, assigned to the binding energy of SiO2 54.

Figure 6. Deconvoluted XPS high resolution spectra of (a) Ti and (b) of Si on the surface of SiO2/TiO2coated particles. Dots represent the experimental spectrum and the solid line represents the corresponding curve-fitted spectrum. The deconvoluted contributions are presented by dashed lines.


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Crystalline structure of TiO2/SiO2/TiO2films was further tested using XRD. Figure 7 exhibits a representative diffractogram of the TiO2/SiO2/TiO2-coated particles in which the topmost layer of titania (third layer) was deposited at 300°C (see Table 1). The anatase crystalline phase of deposited TiO2 in accordance with JCPDS card no. 21-1272 is clearly observed without any further annealing step. It should be mentioned that the XRD pattern of the SiO2/TiO2-coated particles (not shown here) revealed an amorphous structure of the SiO2 layer (second layer, deposited at room temperature), whereas an anatase crystalline TiO2 (similar to the third deposited layer) was found for the TiO2-coated particles (first deposited layer). Therefore, the TiO2/SiO2/TiO2 multilayer films are composed of anatase crystalline titanium oxide layers and amorphous silicon oxide layer.

(101)

Intensity (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(200)

(211) (105)

20

30

40

50

(204)

60

70

80

2ϴ (o)

Figure 7. Typical XRD pattern of the TiO2/SiO2/TiO2-coated particles. The peaks are due to an anatase crystallographic structure.

Figure 8a shows an SEM micrograph of the multilayer-coated particles after the deposition (9 hours total deposition time) of the three layers. No agglomeration is seen even after such long fluidization process at low and high temperatures. This can also be confirmed by the higher BET surface area of the coated powder, ~ 0.09 m2/g, as compared to the one of uncoated 17 ACS Paragon Plus Environment


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particles, ~ 0.07 m2/g. Specifically, BET surface area measurement can determine whether the coating permanently bind the individual particles into agglomerates. If particles had been coated as agglomerates, the surface area would have drastically been decreased. Nevertheless, there is an expected alteration in the surface area of fine particles, which may stem from the change in the physical (particle size and density) and surface properties of the SLG during deposition. It is worth mentioning that the surface area of powders composed of microsphere particles can be predicted by Spredicted (m2/g) = 6×10-4 / (ρD), where ρ (g/cm3) and D (cm) are particle density and diameter, respectively. However, the contribution of SLG size and density to the corresponding surface area is negligible. Indeed, the density and size of the coated SLG particles would increase only by less than 1%. Therefore, the change of surface properties (roughness and porosity) might be responsible for the slight increase of BET surface area after multilayer coatings of the particles by FB CVD (see the SEM images in Figure 8). The EDS elemental mapping analyses of the(b) TiO2/SiO2/TiO2-coated particles are shown in (a) Figure 8 (b-d). As is observed, the coated particles mainly consist of Si (yellow), Ti (blue) and O (blue-green). In addition, TiO2 and SiO2 are well distributed in TiO2/SiO2/TiO2-coated particles.


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a

b

c

d

Figure 8. a) SEM image as well as b) “Si”, c) “Ti”, and d) “O” EDS elemental mapping of the TiO2/SiO2/TiO2-coated particles.

Photocatalytic performance of the coated particles was examined by the degradation of MB as the model reaction. We also compared the photodegradation ability of the coated particles with a commercial TiO2 nanopowder (P25-Degussa). Figure 9a shows a slight change of MB concentration during photolysis (no catalyst in the solution but under UV-light exposure), thus no photocatalytic activity. However, the first encapsulated layer of TiO2 on the SL325 are capable of 70% photodegradation of MB after 40 minutes. The presence of sodium in the first TiO2 layer (see the XPS results in Table 1), as recombination centers for electrons and holes might be responsible for retarding a complete photocatalytic degradation reaction. Karches et al. reported that a minimum film thickness of 50 µm is required to prevent sodium ion poisoning of the photocatalyst

55

. Nevertheless, our XPS results demonstrate that only about 20 nm of SiO2 19 ACS Paragon Plus Environment


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can suppress the diffusion of sodium from the substrate. This can be justified when the photocatalytic performance of TiO2/SiO2/TiO2-coated particles are evaluated (Figure 9a). It is seen that the latter coated particles reveal a comparable photodegradation performance with the commercial P25 nanoparticles. This could be counted as a significant achievement considering the quite smaller surface area of multilayer thin films of titania and silica, ca. 500X, as compared to the commercial P25 nanoparticles. This makes coated SLG more favorable to TiO2 nanoparticles (with a large specific surface area) that are used in slurry reactors as photocatalysts in the waste water treatment. Although these nanoparticles show a slightly superior photocatalytic activity, it is problematic and costly to separate the catalyst from the treated water. Immobilization of titanium oxide on solid support substrates such as the SLG particles used here can help to overcome this issue. It should be noted that the photocatalytic performance of the bare SLG particles (not shown here), indicated no degradation, rendering SLG an inactive material for the MB photodecompsition. The photocatalytic decomposition of MB follows pseudo-first order kinetics with the reaction kinetic model of ln (C/C0) = -kt. C and C0 stand for actual and initial concentrations, respectively, while k represents a photodecomposition reaction rate constant. Figure 9b presents the fitted photocatalytic kinetic model with the corresponding experimental data for the commercial TiO2 nanopowder and coated particles. The photodegradation reaction rate constants are estimated about k = 0.064 min-1 and k = 0.06 min-1 for the commercial TiO2 and TiO2/SiO2/TiO2-coated particles, respectively. This also confirms their approximate reaction rate observed in Figure 9a.


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1.2

Dark

UV-light

1

C/C0

0.8 UV (photolysis)

0.6

Ti/SL 325

(a)

Si/Ti/SL 325

0.4

Ti/Si/Ti/SL 325 P25

0.2

0

-40

-30

-20

-10 0 10 20 Irridiation Time (min)

30

40

50

0.5 0 -0.5

Ln (C/C0)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-1 -1.5

Ti/SL 325

(b)

Si/Ti/SL 325

-2

Ti/Si/Ti/SL 325 -2.5

P25

-3 0

10

20

30

40

50

Irradiation time (min)

Figure 9. a) MB concentration as a function of UV irradiation time in the photocatalytic activity test of the coated particles, b) photodegradation reaction rate constants of the commercial TiO2 and of the coated particles. The individual samples are indicated in Table 1.

4. Conclusions A multilayer film of TiO2/SiO2/TiO2 was deposited on the surface of the fine SLG particles by the FB CVD technique. The optimal values of the gas velocity for different temperatures of the bed were obtained from the differential pressure measurements. Titanium dioxide was 21 ACS Paragon Plus Environment


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deposited at 300°C, whereas the silicon dioxide deposition was implemented at room temperature. FIB cross-sectional TEM images showed a thickness of about 110 nm and about 20 nm for titanium and silicon dioxide deposited layers, respectively. XPS results confirmed pinhole free and uniform multilayer coatings on the surface of silica microspheres. Moreover, the anatase crystalline and amorphous structures of TiO2 and SiO2, respectively, were detected by XRD of the multilayer-coated particles. Uniform distribution of Ti and Si were also observed by EDS elemental mapping analysis. In addition, it was found that the multilayer coatings developed in this study can be effectively employed as a photocatalyst in waste water treatments.

Acknowledgements The authors wish to thank NSERC and Viavi Solutions for financial support through the NSERC Multisectorial Industrial Research Chair in Coatings and Surface Engineering, Mr. Francis Turcot and Mr. Sébastien Chenard for technical support, and Dr. Thomas Schmitt for performing the XRD measurements.

Supporting Information Supporting information contains full XPS spectra from different layers (SLG, TiO2/SLG, SiO2/TiO2/SLG, TiO2/SiO2/TiO2/SLG, Figure S1).

Conflict of Interest Statement The authors declare no conflict of interest.


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AUTHOR INFORMATION Corresponding Authors *E-mails: [email protected]; [email protected]



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For Table of Contents Only

Metal oxide multilayer coating on particles

Polymer layer

nd

2

TiO2

Si O

Au coating

st

1

Ti

Substrate


Conformal Multilayer Photocatalytic Thin Films on Fine Particles by

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