Nonaqueous sol-gel synthesis of anatase nanoparticles and their

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Nonaqueous sol-gel synthesis of anatase nanoparticles and their electrophoretic deposition in porous alumina Cedric Frantz, Alessandro Lauria, Cristina Vicente Manzano, Carlos Guerra-Nuñez, Markus Niederberger, Cédric Storrer, Johann Michler, and Laetitia Vérnique Philippe Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02103 • Publication Date (Web): 19 Sep 2017 Downloaded from http://pubs.acs.org on September 20, 2017

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Nonaqueous sol-gel synthesis of anatase nanoparticles and their electrophoretic deposition in porous alumina Cédric Frantza*, Alessandro Lauriab, Cristina V. Manzanoa, Carlos Guerra-Nuñeza, Markus Niederbergerb, Cédric Storrerc, Johann Michlera, Laetitia Philippea Laboratory for Mechanics of Materials and Nanostructures, Empa, Swiss Federal Laboratories

a.

for Materials Science and Technology, Feuerwerkerstrasse 39, CH-3602 Thun, Switzerland Laboratory for Multifunctional Materials, Department of Materials, ETH Zürich, Vladimir-

b.

Prelog-Weg 5, 8093 Zürich, Switzerland. c.

Coloral, Rue de Beauregard 24, 2000 Neuchâtel, Switzerland

Abstract Titanium dioxide (TiO2) nanoparticles were synthesized by nonaqueous sol-gel route using titanium tetrachloride and benzyl alcohol as the solvent. The obtained 4 nm size anatase nanocrystals were readily dispersible in various polar solvents allowing for simple preparation of colloidal dispersions in water, isopropyl alcohol, dimethyl sulfoxide, and ethanol. Results showed that dispersed nanoparticles have acidic properties and exhibit positive zeta-potential which is suitable for their deposition by cathodic electrophoresis. Aluminum substrates were anodized in phosphoric acid in order to produce porous anodic oxide layers with pores ranging from 160 to 320 nm. The resulting nanopores were then filled with TiO2 nanoparticles by electrophoretic deposition. The influence of the solvent, the electric field, and the morphological characteristics of the alumina layer (i.e. barrier layer, porosity) were studied.

Introduction Titanium dioxide (or titania) has demonstrated its high potential for several applications in various fields thanks to its exceptional catalytic and optical properties, chemical stability, and biocompatibility. In the recent years, it has gained particular interest for photo-electrocatalysis (e.g. water splitting, dye-sensitized solar cells, depollution),1, 2 smart coatings (e.g. self-cleaning surfaces, coatings for medical applications),3, 4 and energy storage applications (e.g. supercapacitors, Li-ions batteries).5-7 Titania can occur in three stable polymorphs under ambient conditions: rutile (tetragonal, 2 coordinated Ti), anatase (tetragonal, 4 coordinated Ti), and brookite (orthorhombic, 8 coordinated Ti). Multiscale micro/nanostructured TiO2 materials promises improvements for most of the aforementioned applications since they are related to the active surface area; i.e. enhancement of catalytic reaction rate, increase in the double layer capacity, increase of the charge/discharge rate of Li-ion batteries, tuning of the optical reflectivity and absorbance. The electrophoretic deposition of TiO2 nanoparticles onto patterned electrodes is a very interesting approach since it allows for accurate control over a range of characteristics such as geometric features, 8

particle size, crystallinity, and composition. It also enables the formation of nanocomposite coatings by 9-11

electrophoretic co-deposition from mixed colloidal solutions.

However, to be successful, it is important to

control the packing of nanoparticles for it will affect the inner porosity, the material density and the mechanical, electrical and optical properties. Several research groups already considered electrophoretic deposition as a promising method for decorating 12, 13

nanostructured substrates with nanoparticles; studied substrates include carbon fibers and nanotubes, 14, 15

nanotubes,

16

diblock copolymers,

17, 18

ion-track etched polymer membranes,

TiO2

and porous anodic aluminum

oxide (AAO).19-23 In fact, several additional properties are expected to be inferred to these templated structures,

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e.g. optical, catalytic, mechanical, electrochemical properties. Previous studies demonstrated the influence of 23

various parameters on incorporation of oxide nanoparticles within porous alumina. Wang et al.

found that the

electrophoretic deposition of ZnO nanoparticles in AAO nanochannels was faster at high pH because of its effect on the zeta-potential. Complementarily, they showed that faster deposition was obtained under higher electric field but resulted in the formation of nanotubules. Similar conclusions were attained by Fori et al.22,

24

who

successfully performed the electrophoretic deposition on the barrier layer of anodized aluminum using SiO2 colloidal dispersion in isopropyl alcohol (IPA). They emphasized that higher electric field and dispersion 21

conductivity improves the impregnation and flocculation of the nanoparticles in the pores. Xu et al.

performed

electrophoretic deposition of copper phthalocyanine from chloroform in porous AAO that was still supported on the aluminum substrate. However, they applied a progressive anodization technique in order to thin down the barrier layer thereby allowing the electrophoretic deposition to occur at lower potential. Template-assisted electrophoretic deposition was previously used for growing TiO2 nanowires but was always performed into 17, 19, 20

barrier-free AAO membranes.

Despite the numerous related articles, no systematic study describing the

simultaneous effects of the AAO characteristics, the electric field, and the role of the solvent is published. The electrophoretic deposition of TiO2 nanocrystalline films has been intensively studied and was already achieved 25, 26

in various solvents (e.g. different alcohols,

12

acetylacetone,

27

acetone,

28

and water ). Producing stable colloidal

dispersions in suitable solvents is a prerequisite for successfully achieving electrophoretic deposition and it usually involves a good control of the nanoparticle surface chemistry through functionalization and/or adjustment of the electrolyte pH. Many strategies have been developed for the synthesis of TiO2 nanoparticles, including flame spray 29, 30

pyrolysis, hydrothermal, solvothermal, and sol-gel methods.

While aqueous sol-gel is suitable for obtaining

submicron or nanosized particles at low temperature, non-aqueous sol-gel processes allow for more accurate control over a variety of parameters such as size, shape, composition, and crystallinity of colloidal 31-33

nanomaterials.

34

For example, Kotsokechagia et al.

reported a nonaqueous sol-gel strategy leading to the

formation of anatase nanocrystals with well-defined diameter. These nanoparticles could be easily functionalized by highly polar groups in order to tune their surface charge and ensure their stability in colloidal aqueous solutions. While this work aims at filling nanoporous AAO with TiO2 nanoparticles using electrophoretic deposition, which appeared to be the most suitable technique for obtaining satisfying filling of the nanopores, this research also provides a systematic study and describes how the colloidal dispersion properties, the applied electric field, and the AAO characteristics (i.e. barrier layer and porosity) affect the formation of TiO2 nanowires inside nanopores. TiO2 nanocrystals were synthesized by nonaqueous sol-gel process and characterized in terms of crystal structure and surface chemistry, the latter rendering them readily dispersible in various solvents such as water, ethanol, dimethyl sulfoxide (DMSO), and, at a lesser extent, in IPA. The obtained colloidal dispersions were characterized and compared with respect to electrophoretic deposition results. These results offer new inputs in templateassisted electrophoretic deposition and opportunities for many applications such as functional or decorative coatings.

Experimental details Preparation of colloidal dispersions Titanium dioxide nanoparticles were synthesized in a multigram scale by a nonaqueous sol-gel process, slightly 34

modified from previous reports.

In a typical procedure, 7 ml of titanium tetrachloride (TiCl4, Aldrich) were slowly

added to 40 ml of ethanol (absolute, Aldrich); during this step a vigorous development of HCl occurs. The pale green solution was cooled to room temperature and added dropwise to 120 mL of benzyl alcohol (puriss. p.a. > 99.0 % - Aldrich) under vigorous stirring. The obtained orange clear solution was heated under reflux at 60 °C; after aging for 48 h the mixture turned yellow and the reaction was quenched by the addition of excess diethyl ether, leading to the flocculation of a white product. The resulting suspension was centrifuged, and the precipitate was thoroughly washed with diethyl ether. The obtained wet precipitates were dispersed in the minimum amount of water sufficient to obtain a clear colloidal solution, which remained stable for several weeks. The concentration


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of TiO2 was found to be 252 mg/ml, by measuring the mass of the inorganic phase obtained after drying and calcining at 900 °C a known volume of solution, in order to remove the organic portion of the material. These 3

solutions showed a density of 1,22 g/cm , corresponding to a concentration of 20,6 %(w/w). The water stable dispersions could therefore be diluted to the desired concentration of 15 or 0,15 mg/ml by the simple addition of water or other polar solvents. The reaction yield was estimated to be 60 ± 10 %. Anodization of aluminium 35

Anodic aluminum oxide (AAO) films were fabricated using a two-step anodization process.

Ultrapure (99.999 %)

Al foils (Advent Research Materials, England) were first cleaned by sonication in acetone, water, isopropanol, and ethanol. Al foils were electropolished in an ethanol:perchloric acid solution (3:1) under a constant potential of 20 V for 4 minutes in order to reduce the roughness before the anodization process. The anodization was performed in 36

an aqueous solution of methanol and phosphoric acid under 180 V at -4 °C.

The first anodization time was 3 h

and the first anodic layer was removed by chemical etching in an aqueous solution of chromic oxide (1.8 wt. % Sigma Aldrich) and phosphoric acid (7 wt. %, 85 % Sigma Aldrich) at room temperature. The second anodization was carried out using the same electrolyte and applied potential as the first anodization for 3 h. In order to study the effect of the barrier layer and the porosity of the templates on the filling ratio, two different post-treatments were performed in order to reduce the barrier layer thickness. An isotropic chemical etching (chem. etching) in phosphoric acid (5 wt. %, 85 % Sigma Aldrich) at 35 °C for 60 min led to an enlargement in the pore diameter and a reduction of the barrier layer from 210 nm to 55 nm. Alternative to the previous process, an electrochemical thinning of the barrier layer (EC BL thinning) was performed. Directly after the second anodization step, the barrier layer was reduced by exponentially decreasing the applied potential from 180 V to 47 V in 3 h. After this decrease, 47 V was applied for 30 min in order to ensure the homogeneity of the barrier layer thickness. This latter process allowed for obtaining a 55 nm thick barrier layer without modifying the pore diameter. Electrophoretic deposition Anodized aluminum substrates were used as cathodes in two-electrode electrochemical setups. The anode was a platinum disc which was placed parallel to the cathode at 1.5 cm distance. A power supply SM300-5 (Delta Elektronika) was used to apply potentials from 30 V to 300 V. A resistor of 1 kΩ was connected in series. The voltage across the resistor was measured in order to record the current with higher resolution. The voltage drop across the resistor was always lower than 0.5 % of the applied voltage and was therefore negligible. After the electrophoretic deposition process, the samples were rinsed in a mixture of ethanol and acetone (50 %v/v) in order to minimize cracking. Acetone was chosen for its low surface tension whereas ethanol was added in order to decrease the evaporation rate. Characterization techniques X-ray diffraction (XRD) measurements were performed in reflection mode (Cu Kα radiation at 45 kV and 40 mA) either on an XPertPro diffractometer or on an Empyrean diffractometer (equipped with a high temperature chamber model HTK 1200 from Anton Paar), both from PANalytical (The Netherlands). Dynamic light scattering (DLS) and zeta potential were determined on diluted solutions (typical range 1-0.01 mg/ml) on a Zetasizer NS instrument (Malvern, UK) in backscattering mode (scattering angle 173°) at a temperature of 25 °C. Measurements operated on particle dispersions in DMSO were made in a Malvern ZEN1002 dip cell. Thermogravimetric analysis (TGA) was performed in an aluminum oxide crucible (70 µL) with a Mettler Toledo TGA/SDTA851e instrument at a –1

heating rate of 10 °C·min

from room temperature to 1100 °C under ambient atmosphere. Transmission Electron

Microscope (TEM) images were recorded on a Philips FEI Tecnai F30 microscope operated at 300 kV on samples prepared by depositing either 10 µl of aqueous particles dispersions or powders dried right after the washing with ether onto carbon coated Cu grids. Attenuated total reflectance (ATR) measurements were performed on a Bruker -1

Alpha FT-IR Spectrometer equipped with diamond ATR optics, operated with 128 scans and 2 cm resolution. The samples were grinded in a mortar and dried prior to the measurement. Raman spectra were obtained using an upright confocal Raman spectrometer (NT-MDT NTEGRA) operated with a solid state laser with a wavelength of 532 nm. A 50× objective lens with a numerical aperture of 0.75 was used. Spectra were recorded at dry conditions with a spectral resolution of 2.7 cm

−1

and a laser power below 100 µW. The acquisition time was set to 30 s.


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Scanning Electron Microscope (SEM) images were taken using a Hitachi FE-SEM S4800 operated at 1.5 kV and 10 µA. For cross section observations, a notch was made in the aluminum substrate by using a sharp scalpel then the sample was cleaved mechanically. Image analyses were performed using XnView (XnSoft) and ImageJ (National Institutes of Health) softwares in order to determine the pore size distribution, the porosity, and the pore density. The determinations of uncertainties are detailed in the “electronic supporting information” section. The mass of sample was measured using a high precision balance (0.01 mg). The samples were first measured before electrophoretic deposition, directly after deposition, then after removal of particles deposited on the surface in order to assess the mass present in the pores. This thick overgrowth was first removed by using a thin sharp razor blade then the AAO surface was cleaned by using a clean polishing satin fabric which was rinsed and soaked with deionized water. Details about the calculation of uncertainties are given in the “electronic supporting information” section.

Results and discussion Nonaqueous sol-gel and colloidal dispersions The TiO2 nanoparticles were produced by a nonaqueous sol-gel method with slight modification of a previously 34

published protocol. 37, 38

alcohol,

In contrast to the chemical pathway involving the direct reaction of TiCl4 with benzyl

the here applied method implies the additional use of ethanol reacting with the chloride precursor

prior to the reaction of the mixture with benzyl alcohol. As previously highlighted by Garnweitner and Grote,39 this methodology enables several advantages, for instance in terms of experimental benefits. Indeed, the exchange between TiCl4 and ethanol leads to a substantial warming of the mixture, the appearance of a yellow color, and to HCl evolution, yet in a much less violent reaction with respect of benzyl alcohol, reducing the issues on a potential scale-up of the process. In this step, the exchange of chlorine with ethoxy groups leads to the formation of HCl according to the following reaction: + → ( ) +

Equation 1

It has been suggested that this exchange is not always complete (x < 4, in Equation 1), as suggested by the 39

presence of Cl2Ti(EtO)2 complexes in the ethanol solution.

After the addition of benzyl alcohol, several processes

are initiated. The formation of TiO2 nanocrystals proceeds likely through both alkyl halide elimination and ether elimination mechanisms, occurring simultaneously. This stage is expected to form, as result of the Ti-O-Ti network growth, several alkyl chlorides (benzyl chloride, ethyl chloride) and organic ethers, among which diethyl ether, dibenzyl ether, and benzyl ethyl ether. The surface chemistry of TiO2 particles arising from this reaction has not been fully identified, and consequently, the stabilization mechanism in water is not yet fully understood. As a matter of facts the particles readily disperse in deionized water, even after being dried. Interestingly, the obtained dispersions show a quite acidic pH (pH ~ 1 for NPs concentration higher than 100 mg/ml), which increases when the dispersions are diluted (pH = 4.5 for NPs concentration of 0.1 mg/ml). This fact suggests that the employment of hydrous benzyl alcohol and the use of chloride precursors confer to the nanocrystals a peculiar surface chemistry, likely including acidic species, which could play a crucial role in the stabilization of the particles dispersions. The XRD pattern (Figure 1a) shows only the presence of the anatase polymorph. The Scherrer equation was applied to the (101) and (200) peaks and gave an average size of 3.6±0.1 nm for both reflections. This value is further confirmed by TEM observations (Figure 1b-e). In fact, low resolution images display well dispersed nanoparticles with sizes below 5 nm, consistent with XRD results (Figure 1b). Representative high resolution images are shown in Figure 1c-e, revealing round-shaped, but faceted anatase nanocrystals. The same particles morphology was observed when either dry powders or diluted dispersions of nanocrystals (0,15 mg/ml) were placed over the TEM grid (Figure 1c and e, respectively). However, the particle size distribution is slightly broader, due to the occurrence of a minority of larger crystals like that shown in Figure 1d, although crystals larger than 10 nm were not observed (see also Figure S2 in Supporting Information).


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Figure 1: XRD pattern of dried TiO2 nanoparticles (a). The reference pattern for anatase [ICDD 00-021-1272] is also shown for comparison (bars). TEM analysis of dried TiO 2 nanoparticles; (b) bright field, and (c-e) high resolution images, low magnification image showing singly dispersed particles (b); high resolution images of dry powders (c), and water dispersion (e) of nanocrystals with sizes mostly between 3 and 4 nm, although few larger crystals with diameter up to 10 nm can be occasionally observed (d).

For the preparation of colloidal solutions, the obtained wet precipitates were first dispersed in water and the resulting clear dispersion remained stable for several weeks, without showing settling of particles, for concentrations up to 252 mg/ml. Figure 2 shows optical images and absorption spectra of aqueous dispersions charged with 252, 15.0, and 0.15 mg/ml of anatase nanoparticles. The solutions look yellowish when light comes from behind whereas the tint turns blueish when the solutions are illuminated from the observed side. Two different phenomena may contribute to this appearance, namely scattering and absorption. On the one hand, considering the high concentration and the high refractive index of titania (nTio2 > 2,58 in the visible range) compared to that of the used solvents (n < 1,50), light scattering can play a big role in the observed optical properties of solutions. This is, as expected, more evident at higher concentration. On the other hand, additional absorption bands may arise due to the near UV bandgap broadening or to oxygen vacancies, likely present in a higher amount in very small crystals. Although the red shift of the absorption edge caused by oxygen deprivation was reported to be able to decrease the band gap energy down to the red portion of the visible spectrum (i.e. less than 2 eV),40 only a broadening of the edge tail was observed, extending to the blue region (between 350 and 450 nm), and spectrally responsible for the samples yellow tint, which did not prevent a good transparency also at the highest concentration (see Figure S3 in “Supporting Information”).


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Figure 2: Optical images of aqueous stable dispersions of 252, 15, and 0.15 mg/ml of TiO2 nanoparticles (left), and their corresponding inline absorption spectra.

FTIR spectra (Figure 3) were recorded on particles dried right after the washing with ether and after re-dispersion in water. The idea was to reveal compositional information which could potentially indicate any special surface coating. Organic residuals revealed by low intensity peaks in the 1500-1200 cm-1 region, similar to those obtained by T. Kotsokechagia et al.,34 are only visible before addition of water. As the reaction took place in the presence of benzyl alcohol, ethanol and ethers (reaction products), the absorptions at 1495, 1454, and 1206 cm

-1

are

consistent with the benzyl ring, to CH3 or benzyl vibration, and to C-O stretching, respectively. This observation suggests that the nanoparticles are initially coordinated by ethyl or benzyl groups as also suggested by 39

Garnweitner and Grote.

After the addition of water, these species may dissociate from the surface and partially

contribute to the increase of proton activity which is observed after dispersing the nanoparticles (Table 1). Nevertheless, the acidic pH may also result from traces of HCl, formed during the initial reaction of Ti chloride, which was bound to the nanoparticle surface, similarly to what was observed by De Roo et al.41 for HfCl4. Alternatively, chloride ions at the nanoparticle surface could also arise from the incomplete condensation of titanium alkoxy chlorides. Interestingly, it also appears that water was already present at the nanoparticle surface after washing in ether; yet it is worth mentioning that the powders could be highly hygroscopic. These observations are also supported by TGA analysis (see Figure S4 in “supporting information”). The 10 % weight loss observed at temperatures up to 120 °C is likely due to the desorption of volatile species such as water, ethanol or diethyl ether. In the range 120-300 °C, a further weight loss of 8 % could be ascribed to the desorption of less volatile aromatic species like benzyl chloride, benzyl alcohol, and benzyl ethers. The weight loss at higher temperatures may arise from organic residuals which are more tightly bound to the nanoparticle surface, and represent less than 5 % of the initial mass. The presence of ethanol during the reaction as well as the tiny amount of water (i.e. originating from the hydrous benzyl alcohol), together with Cl ions at the surface of nanocrystals, might be responsible for the high dispersibility in water at concentrations up to more than 20 %w/w.


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Figure 3: FTIR spectra collected on particles dried right after the washing with ether and after dispersion in water.

The water dispersions were then diluted to the desired concentration (i.e. 0.15 mg/ml) by adding water, isopropyl alcohol (IPA), dimethyl sulfoxide (DMSO), or ethanol. However, colloidal dispersions in IPA were not stable and a white precipitate was formed after a week. The characterization of the dispersions by dynamic light scattering (Figure 4) shows that nanoparticles have a smaller Stokes diameter (i.e. hydrodynamic diameter) in ethanol than in DMSO and water with their peak centered at 13.7 nm, 16.1 nm, and 24 nm, respectively.

Figure 4: Hydrodynamic diameter distribution of TiO2 nanoparticles dispersed in water, DMSO, or ethanol measured by dynamic light scattering. A single dispersion peak was observed for each solution.

Some of the significant properties of the different solvents are listed in Table 1. The electrophoretic mobility of TiO2 was measured for the different colloidal dispersions. It appears that nanoparticles dispersed in DMSO have the lowest mobility which is an order of magnitude lower than the one in aqueous media. 42

-1

According to Zeng et al., the Debye length (κ ) for a colloidal dispersion is given by: κ

Equation 2

( !" #$ %& '())

where ε0 is the vacuum permittivity, εr is the dielectric constant of the media, kB is the Boltzmann constant, T the absolute temperature, F is the Faraday constant, e is the elementary charge, a, qs, and ρN are the nanoparticle radius, surface charge, and number density respectively, I is the ionic strength of added salt, and * is the

Avogadro number. The surface charge density can be estimated by applying the Stokes-Einstein relation which relates the particle mobility and diffusion coefficient: q,

-./01

Equation 3

("

where η is the dynamic viscosity of the solution, r is the hydrodynamic radius, and µe is the electrophoretic mobility of nanoparticles. The calculated values, shown in Table 1, are strongly dependent on the solvent. The surface charge density determined for nanoparticles dispersed in water is consistent with those obtained by 43

Holmberg et al.,

-2

who reported surface charges ranging from 0.1 to 0.15 C·m for titanium dioxide nanoparticles

of about 4 nm in diameter. However, the surface charge density of nanoparticles dispersed in ethanol or DMSO shows values which are about an order of magnitude lower. As no salt was added to the dispersions, the ionic strength was estimated from paH+ measurements and the Debye length was approximated to 14.6 nm, 16.3 nm, -1

and 25.4 nm in water, DMSO, and ethanol, respectively. As κ 44

nanocrystals was calculated using the Hückel approximation: ζ

≫ a, the ζ-potential of dispersed anatase

-./0

Equation 4

(

Calculated ζ-potentials are positive in every investigated solvent and the highest value was obtained for nanoparticles dispersed in ethanol. DMSO is the solvent with the highest dipole moment and viscosity and an


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intermediate dielectric function and proved to enable very stable dispersions despite the low ζ-potential of -1

anatase nanocrystals (+16 mV). Complementarily, the electrical conductivity of the DMSO dispersion (7.7 µS·cm ) -1

and of the ethanol one (24.5 µS·cm ) were considerably lower than the one for the aqueous solution (180 µS·cm

-

1

).

Table 1: General properties of TiO2 colloidal dispersion in water, DMSO, and ethanol. Solvent

Water

DMSO

Ethanol

Dielectric constant εr

80.1

47.24

24.5

Dynamic viscosity η (cP)

1.002

1.996

1.144

56 (D) Dipole moment d

1.85

4.06

1.69

Hydrodynamic diameter (nm)

24

16.1

13.7

3.36

3.26

3.91

184.00 ± 3.35

7.65 ± 0.86

24.53 ± 0.15

2.887 ± 0.163

0.225 ± 0.009

0.894 ± 0.039

130.2 ± 7.4

13.6 ± 0.5

26.3 ± 1.1

Debye length Κ (nm)

14.6

16.3

25.4

Zeta potential ζ (mV)

61.2 ± 3.5

16.1 ± 0.6

71.3 ± 3.1

Proton activity

paH+ -1

Ionic conductivity σ (µS·cm ) -8

2

-1 -1

Electrophoretic mobility µe (x10 m ·V ·s ) -2

Surface charge qs (mC·m ) -1

Anodization of aluminum The DLS results revealed a rather large hydrodynamic diameter of dispersed anatase nanoparticles and large pores are therefore preferred in order to efficiently fill porous AAO by electrophoretic deposition. For this reason, anodization of aluminum was performed in phosphoric acid which is known for generating wide pores ranging 45

from 30 nm to 235 nm depending on the electrolyte temperature, H3PO4 concentration, and applied potential.

During the first instant of anodization, a compact oxide barrier layer (BL) is produced on the aluminum substrate before the formation of pores. The barrier layer thickness is proportional to the anodization voltage by a factor -1

usually ranging from 1 to 1.4 nm·V . The presence of this dielectric layer between the aluminum surface and the electrolyte is detrimental for performing further electrodeposition and much higher potentials have to be applied in order to compensate for the voltage drop across the barrier layer. In order to assess the influence of the pore diameter and the barrier layer thickness on the electrophoretic deposition process, three types of AAO layers were produced; namely, as-anodized AAO without any further modification (Figure 5a-d), AAO with a chemical pore widening allowing to increase the pore diameter and decreasing the barrier layer thickness (Figure 5e-h), and AAO for which the barrier layer was thinned down by progressively decreasing the anodization potential (abbreviated 46

as EC BL thinning),

leading to the formation of smaller branched pores in the barrier layer (Figure 5i-l). Image

analysis was performed in order to determine the pore size distribution (Figure 5d, h, l), the porosity and the pore density whereas the pore length and barrier layer thickness were directly measured from micrographs. The structural characteristics are listed in Table 2. Details about uncertainty determinations can be found in the “electronic supporting information” section. The thickness of the barrier layer of as-anodized layers was measured -1

to be 210 nm, corresponding to about 1.17 nm·V , and was decreased to 55 nm after an isotropic chemical pore widening of 1 h. In order to obtain a barrier layer of 55 nm without enlarging the pores, the anodization potential was exponentially decreased from 180 V to 47 V. A long thinning step of 3 h was chosen in order to obtain homogeneous barrier layer thickness. The homogeneity of the barrier layer thickness was ensured by monitoring 47

the current transient curve recorded during the thinning procedure.

During this period, the growth of the oxide

occurs at a much slower rate and the obtained pores were only about 300 nm longer than the one measured after a simple anodization.


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Figure 5: SEM micrographs of (a, b, e, f, i, j) the top surface and (c, g, k) the cross-section, and (d, h, l) pore size distribution of porous anodic aluminum oxide layers (a-d) directly after anodization, (e-h) after chemical pore widening, and (i-l) after electrochemical (EC) thinning of the barrier layer (BL).

Table 2: Characteristics of the different AAO layers measured from SEM micrographs.

AAO layer

As-anodized

Chem. etching

EC BL thinning

204 ± 5

316 ± 4

160 ± 8

Pore density (x 10 cm )

6.63 ± 0.05

6.60 ± 0.03

6.63 ± 0.03

Porosity (%)

21.7 ± 1.1

51.7 ± 1.4

14.4 ± 1.3

Pore length (µm)

3.16 ± 0.02

3.28 ± 0.02

3.45 ± 0.03

Porous volume (x 10-5 cm3/cm2)

6.86 ± 0.35

17.00 ± 0.47

4.97 ± 0.45

BL thickness (nm)

210 ± 16

55 ± 5

55 (calculated)

Average pore diameter (nm) 8

-2

Electrophoretic deposition: influence of the colloidal dispersion The influence of the solvent on the electrophoretic deposition of anatase nanocrystals was first assessed using AAO after chemical pore widening (Figure 6). The deposition was performed at 100 V·cm

-1

for 1800 s and the

samples were cleaved in order to observe the cross-section by SEM. During the depositions from the aqueous or IPA dispersions (Figure 6a-d), the nanoparticles tended to agglomerate on the surface of the AAO layer but did 48

not yield any significant deposit within the pores. Kusdianto et al.

suggested hydrogen evolution could block the

pores and increase the local pH, inducing agglomeration on the surface. In the case of IPA, only short nanowires less than 1 µm long were deposited at the bottom of the pores whereas no deposition occurred inside the cavities using the aqueous dispersion. The electrophoretic depositions achieved from DMSO or ethanol solutions (Figure 6e-h) allow efficient pore filling. Nanoparticles agglomerated from the bottom of the pores and gave rise to nanowires as long as the pore length. Nanowires deposited from DMSO appear to be very compact along their


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whole length (Figure 6e-f). On the other hand, the 1D structures obtained in ethanol display a large number of voids on almost the whole nanopore length but for the last few hundreds of nanometers (Figure 6g-h). It is worth mentioning that the voids are located at the center of the pillars which means that nanoparticles settled preferentially at the electrolyte/alumina interface. Optical images of the top surface of TiO2 coated AAO samples demonstrate a whitening effect which depends on the pore filling and the presence of nanoparticles on the surface (see Figure S5 in “Supporting information”).

Figure 6: Cross-sectional views of (a, c, e, g) the top and (b, d, f, h) the bottom of chemically etched AAO layers after the electrophoretic deposition of TiO2 nanoparticles at 100 V·cm-1 for 1800 s in (a, b) water, (c, d) IPA, (e, f) DMSO, and (g, h) ethanol containing 0.15 g/L TiO2 nanoparticles.

Raman spectroscopy was performed on the overgrown TiO2 film (see Figure S6 in “Supporting Information”). The obtained spectrum only revealed the presence of anatase nanocrystals. The different peaks were fitted in order to accurately determine their position and width. The data associated to each peak are listed in Table S1 (in 49

“Supporting Information”). By comparing the obtained values with the results of Swamy et al.,

the position and

width of the peak associated to the Eg(1) vibration mode are consistent with 4 nm diameter nanoparticles, thereby confirming previous characterization of as synthesized nanoparticles and excluding subsequent crystal growth. Electrophoretic deposition: influence of the electric field Previous results (Figure 6) pointed out that DMSO and ethanol are the most suited solvent for filling porous AAO by electrophoretic deposition of TiO2 nanoparticles. A more detailed comparison has been made by performing electrophoretic deposition at different potentials in either DMSO (Figure 7) or ethanol (Figure 8). SEM observations of the overgrown coating obtained in DMSO (Figure 7a-c) show smooth films but with the presence -1

of cracks. Large and interconnected cracks are formed under a low electric field of 50 V·cm (Figure 7a). The film -1

deposited at 100 V·cm only revealed narrow short cracks that do not break the coating continuity (Figure 7b). Increasing the potential leads to the deterioration of the top coating again (Figure 7c) although the crack width -1

and length are smaller and cracks do not form a connected network as it is observed at 50 V·cm . For depositions carried out in ethanol (Figure 8a-c), all the top coatings present cracks and a rough and inhomogeneous surface with randomly distributed agglomerates that renders their appearance powdery. The top coating deposited at 100 -1

V·cm showed the lowest coverage due to extremely large cracks which can be more than 2 µm wide (Figure 8b). The top covering deposit was then polished away in order to visualize the top surface of the AAO and to estimate the fraction of pores that were filled. In the case of DMSO, SEM micrographs (Figure 7d-f) show that electrophoretic deposition occurred in every pores but the fraction of the pore that was filled increases with the -1

applied electric field. Indeed, at 50 V·cm , the diameter of the TiO2 nanowires is about half the one of the pore -1

-1

whereas all pores were completely filled at 200 V·cm . At 100 V·cm , an intermediate scenario is observed with some of the pores being completely filled whereas others were only partially filled and the deposition seemed to be localized at the AAO/electrolyte interface, giving rise to nanotubes. Observations of the corresponding crosssections (Figure 7g-i) are consistent with the previous finding and suggest that the compactness of the nanoparticle agglomerates increases with the applied potential. Indeed, the nanowires formed at 50 V·cm-1 do not -1

fill the whole pore section and are usually discontinuous. The ones formed at 100 V·cm exhibit a powdery aspect -1

whereas the one obtained at 200 V·cm appear more compact and smoother. A similar conclusion was drawn in the case of ethanol (Figure 8d-f), for which the fraction of the pore that was filled increases with the strength of


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-1

the electric field. At 50 V·cm , the deposit appears very powdery and only a small fraction of the pore section is filled by TiO2 nanoparticles. Increasing the electric field leads to an increase of the filled fraction and seem to -1

improve the compactness and smoothness of the deposit. However, even at 200 V·cm , the pores are not fully filled and cracks and voids are still observed at the nanowires top surface. Interestingly, observations of the corresponding cross-sections show that the deposited nanowires are usually hollow over more than 2.5 µm from 23

the bottom of the pores. The formation of nanotubules within AAO was already reported by Wang et al.

for the

electrophoretic deposition of ZnO from aqueous dispersions. This result was attributed to either the formation of hydrogen gas or the shrinking of the deposit due to quick drying. The applied potential does not seem to have a significant influence on the nanowire or nanotube morphology and compactness in the studied range.

Figure 7: SEM micrographs (a-c) of the covering TiO2 layer, (d-f) of the top surface after removal of the overgrowth, and (g-i) of the crosssection of chemically etched AAO layers after the electrophoretic deposition of TiO2 nanoparticles at (a, d, g) 50 V·cm-1, (b, e, h) 100 V·cm1

-1

, and (c, f, i) 200 V·cm for 1800 s with a colloidal solution of 0.15 g/L TiO2 nanoparticles in DMSO.

Figure 8: SEM micrographs (a-c) of the covering TiO2 layer, (d-f) of the top surface after removal of the overgrowth, and (g-i) of the cross-1

section of chemically etched AAO layers after the electrophoretic deposition of TiO2 nanoparticles at (a, d, g) 50 V·cm , (b, e, h) 100 V·cm 1

, and (c, f, i) 200 V·cm-1 for 1800 s with a colloidal solution of 0.15 g/L TiO2 nanoparticles in ethanol.

The current transient curves recorded during the electrophoretic deposition of TiO2 in AAO are presented in Figure 9. As a general trend, the current density decreases with time as titania nanowires and a covering film are successively grown, thereby increasing the electrical resistance of the substrate. The current density in DMSO was about half the one obtained in ethanol at the same potential, which is probably due to the higher electrical conductivity of the ethanol dispersion. It also appears that depositions performed at 200 V·cm-1 displayed a much higher and unstable current density which is presumably due to the reduction of the solvent. The deposition at -1

100 V·cm in ethanol also showed some jumps in the current density that may result from the formation of cracks in the film, leading to a decrease in the film resistivity. The deposits were weighted directly after rinsing and drying in order to measure the total deposited mass. The overgrowth was then removed mechanically and the samples


-

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were weighted again to estimate the mass deposited within the pores. Knowing the mass of anatase present in the pore as well as the porous volume, the filling ratio of the nanopores can be assessed. In addition to that, the quantity of Coulomb involved in the deposition process can be integrated from the current transient curves and the deposited average mass of anatase nanocrystals per Coulomb can then be calculated. Assuming that the mass deposited per Coulomb is constant during the deposition process, one can plot the deposited mass as a function of the time (Figure 9 dashed lines) and determine the time required for filling the pores. This procedure is described in more details in the “electronic supporting information” (Figure S1). These results are presented in Table 3. The fact that TiO2 weight deposited at 50 V·cm-1 and 100 V·cm-1 is higher in ethanol than in DMSO demonstrates that higher ζ-potential and electrophoretic mobility allow for quicker deposition but also involve about twice as much Coulombs. Weight measurements also show that the total amount of deposited TiO2 increases with the applied electric field except in the case of ethanol for which a decrease in mass is observed at -1

200 V·cm . It may be explained by the fact that the top coating was powdery at this potential and a part of the deposit may have been washed away during the rinsing step. However, it does not exclude the possible impact of 50

parasitic reduction of ethanol which may lead to the formation of ethane or methane gas,

thereby obstructing

the pores. On the other hand, parasitic reduction of DMSO leads to the formation of liquid apolar dimethyl sulfide

51

+

which involves the consumption of H . Additionally, the ion concentration in the pore may increase with

the applied voltage. The increase in the ionic strength and pH may lead to a decrease in the zeta-potential and thereby destabilize the dispersion near the electrode surface, inducing nanoparticle flocculation. The mass deposited per passed charge follows the same trend and is therefore not a constant. This observation suggests that the deposition mechanism does not only rely on the neutralization of the particle charge but may also involve the particle flocculation by accumulation or their electrochemical coagulation in the electrical double layer. It also appears that the mass deposited per passed charge is always higher in DMSO than in ethanol for equivalent electric field strength which can be explained by the lower surface charge density of particles in DMSO than in ethanol. Regarding the mass deposited within the pores, it appears that higher filling ratio was systematically obtained in DMSO. The evaluated filling ratio always increases with the applied potential except for the deposition performed in ethanol at 200 V·cm-1. The best filling ratio was therefore obtained in DMSO at 200 V·cm-1 and was evaluated as high as 62 %. Results show that the filling time reasonably decreases with increasing the potential except in the case of the electrophoretic deposition performed in ethanol at 200 V·cm-1. For depositions -1

-1

performed under high electric field (i.e. 200 V·cm in DMSO and above 100 V·cm in ethanol), it is interesting to note that the filling time matches with the increase in the cathodic current. This observation supports the claim that a parasitic reduction, probably limited by kinetics, takes place under such conditions and its contribution to the total current thereby increases with the active surface area of the substrate.

Figure 9: Current transients recorded during the electrophoretic deposition of TiO2 nanoparticles in chemically etched AAO layers with a colloidal dispersion of 0.15 g/L TiO 2 (a) in DMSO, and (b) in ethanol. The dashed lines correspond to the deposited mass as a function of time.

Table 3: Data for the electrophoretic deposition of TiO2 nanoparticles in chemically etched AAO with a colloidal solution of 0.15 g/L TiO2 in DMSO.


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Langmuir

Solvent

DMSO

Electric field (V·cm

50

100

200

50

100

200

0.30(6) ± 0.02(1)

0.76(4) ± 0.03(2)

4.88(9) ± 0.16(7)

0.40(7) ± 0.02(3)

1.01(9) ± 0.03(9)

0.61(1) ± 0.02(7)

0.239 ± 0.020

0.374 ± 0.022

1.520 ± 0.057

0.437 ± 0.024

0.874 ± 0.036

3.416 ± 0.122

1.27(9) ± 0.13(7)

2.04(2) ± 0.14(8)

3.21(7) ± 0.16(3)

0.93(2) ± 0.07(2)

1.16(5) ± 0.06(5)

0.17(9) ± 0.01(0)

0.12(7) ± 0.02(9)

0.25(5) ± 0.03(0)

0.40(7) ± 0.03(2)

0.10(2) ± 0.02(9)

0.15(3) ± 0.02(9)

0.17(8) ± 0.02(9)

19(.4) ± 4(.4)

38(.7) ± 4(.6)

61(.9) ± 5(.1)

15(.5) ± 4(.4)

23(.2) ± 4(.5)

27(.1) ± 4(.5)

1

)

Total mass (mg·cm-2) Total charge (C·cm-2) Mass per Coulomb (mg·C-1) Mass in pores (mg·cm-2) Filling ratio (%) Filling time (s) tmin - tmax

Ethanol

-

534

374

74

428

219

343

297 - 878

273 - 502

64 - 85

269 - 605

167 - 277

243 - 450

Electrophoretic deposition: influence of the AAO layer To evaluate the influence of the pore diameter and the barrier layer thickness independently, electrophoretic depositions were performed in as-anodized templates (i.e. small pore diameter and 210 nm thick barrier layer) and in templates after electrochemical thinning (i.e. small pores and a 55 nm thick barrier layer). Depositions were performed at 100 and 200 V·cm-1 which gave the best results in DMSO. SEM micrographs of as-anodized templates after deposition of TiO2 nanoparticles are presented in Figure 10 and the corresponding current -1

transients are plotted in Figure 11. Images from the top surface show that the pores are better filled at 200 V·cm , which is consistent with previous results. However, cross-sectional views display less compact and inhomogeneous -1

-1

agglomerates at 200 V·cm whereas deposition did not take place from the bottom of the pores at 100 V·cm but seemed to rather occur on the upper half of the pore length. Such partial filling was already reported for too weak electric field

22

although the precise mechanism responsible for this is not well understood. It may be explained by

a local destabilization in the colloidal dispersion which could result either from a local increase in the nanoparticle concentration or from a local increase in the paH+ (arising from cathodic reaction or in the electrical double layer at the pore wall). The current density recorded at 100 V·cm

-1

(Figure 11, red curve) is rather constant during

deposition, suggesting that the electrical resistance of the substrate does not significantly change during deposition which is likely due to a high porosity of the titania coating. On the other hand, the current density recorded at 200 V·cm

-1

(Figure 11, black curve) quickly decreases during the first 800 s of deposition and finally

converges to about the same value recorded at 100 V·cm

-1

after 20 min. Performing the same analysis as

previously allows estimating the filling ratio and filling time (Table 4). As observed previously, increasing the electric field leads to an increase in the filling ratio and a decrease in the filling time. However, it appears that the filling ratios are much lower using as-anodized templates and the filling time much shorter, which may be attributed to the thicker barrier layer and to the smaller pore diameter respectively. This effect will be discussed in more detail in the next section.


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Figure 10: SEM micrographs (a, d) of the top surface after removal of the overgrowth, and (b, c, e, f) of the cross-section of as-anodized AAO layer after the electrophoretic deposition of TiO2 nanoparticles at (a, b, c) 100 V·cm-1, and (d, e, f) 200 V·cm-1 for 1800 s with a colloidal solution of 0.15 g/L TiO 2 nanoparticles in DMSO.

Figure 11: Current transients recorded during the electrophoretic deposition of TiO 2 nanoparticles in as-anodized AAO layers with a colloidal dispersion of 0.15 g/L TiO2 in DMSO. The dashed lines correspond to the deposited mass as a function of time.

Table 4: Data for the electrophoretic deposition of TiO2 nanoparticles in as-anodized AAO with a colloidal solution of 0.15 g/L TiO 2 in DMSO. -1

Electric field (V·cm )

100

200

Total mass (mg·cm-2)

1.42(6) ± 0.05(2)

5.29(7) ± 0.18(1)

0.648 ± 0.029

1.712 ± 0.063

2.20(1) ± 0.12(7)

3.09(4) ± 0.15(6)

0.10(2) ± 0.02(9)

0.12(7) ± 0.02(9)

38(.3) ± 11(.0)

47(.9) ± 11(.1)

-2

Total charge (C·cm ) Mass per coulomb -1

(mg·C ) Mass in pores (mg·cm-2) Filling ratio (%) Filling time (s) tmin - tmax

80

9

49 - 117

6 - 13


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Langmuir

In an attempt to dissociate the effect of the barrier layer and the pore diameter, the barrier layer was thinned down after the second anodization step. The thinning potential was set at 47 V which leads to a barrier layer of the same thickness as it was measured for AAO after chemical pore widening (i.e. about 55 nm). The pore diameter was however half the one after chemical pore widening (i.e. 160 nm instead of 316 nm), meaning that the porosity was about fourfold lower. The SEM micrographs showing the top surface after removal of the overgrowth (Figure 12a, d) do not show significant differences in the pore filling and almost every pore appears to be filled at the AAO surface except for a few small pores of less than 100 nm diameter. Cross sectional views (Figure 12b, c, e, f) prove that nanoparticle deposition was initiated from the bottom of the pores and the branched part of nanopores appears to be well filled for both potentials. Although cross-sectional views do not expose conspicuous differences, SEM micrographs suggest that nanoparticles assemblies deposited at 100 V·cm-1 are less compact, especially along the upper half of the pore length. On the contrary, nanowires obtained at 200 -1

V·cm look more compact and homogeneous all along the pore length. It can also be noted that most of these nanowires are still complete after mechanical cleavage of the sample, which emphasizes high mechanical stability. The corresponding current transients are presented in Figure 13 and coulometric and mass data are listed in Table 5. The total deposited mass at a given electric field is similar to that on as-anodized AAO but higher than that obtained with chemically etched AAO. However, the passed charge is also higher and the mass deposited per Coulomb is systematically lower. Although the mass present in these pores is significantly lower than the one deposited in large pores, the filling ratio is higher because of the much smaller porosity (i.e. ≈160 nm diameter pores). As previously observed, the filling ratio increases with the applied potential and was calculated to be 53 % -1

-1

at 100 V·cm and 79 % at 200 V·cm . It has to be noted, however, that a filling ratio of 79 % is theoretically not possible since the closest packing of equal spheres would only occupy about 74 % of the volume. This high value could be explained by the important calculated uncertainties, inhomogeneity in the nanoparticle size and shape, and/or the presence of impurities in the deposit. The filling time is shorter than that for filling wider pores but longer than the one determined for as-anodized AAO. It also appears that the filling time matches with the increase in the cathodic current density which suggests a parasitic faradaic reaction. Indeed, the current resulting from faradaic reactions (e.g. solvent reduction) would increase with the active surface area, which would increase when the pores are filled and local overgrowths form. The decrease in the current density that follows gradually after could be explained both by the formation of a flat overgrowth (i.e. coalescence of local overgrowths) and by the by the formation of a resistive TiO2 covering layer. Once the pores are filled, the current becomes very -1

unsteady at 200 V·cm .

Figure 12: SEM micrographs (a, d) of the top surface after removal of the overgrowth, and (b, c, e, f) of the cross-section of AAO layer -1

whose barrier layer was electrochemically thinned before the electrophoretic deposition of TiO2 nanoparticles at (a, b, c) 100 V·cm , and (d, e, f) 200 V·cm-1 for 1800 s with a colloidal solution of 0.15 g/L TiO2 nanoparticles in DMSO.


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Figure 13: Current transients recorded during the electrophoretic deposition of TiO2 nanoparticles with a colloidal dispersion of 0.15 g/L TiO2 in DMSO in AAO layers whose barrier layer was electrochemically thinned. The dashed lines correspond to the deposited mass as a function of time.

Table 5: Data for the electrophoretic deposition of TiO2 nanoparticles in as-anodized AAO with a colloidal solution of 0.15 g/L TiO2 in DMSO.

Electric field (V·cm-1)

100

200

Total mass (mg·cm )

1.07(0) ± 0.04(1)

5.29(7) ± 0.18(1)

Total charge (C·cm-2)

0.556 ± 0.027

1.812 ± 0.087

Mass per coulomb (mg·C )

1.92(4) ± 0.11(8)

2.92(3) ± 0.17(2)

Mass in pores (mg·cm-2)

0.10(2) ± 0.02(6)

0.15(3) ± 0.02(6)

52(.8) ± 15(.6)

79(.3) ± 16(.6)

-2

-1

Filling ratio (%) Filling time (s) tmin - tmax

106

37

70 - 160

31 - 42

For a same barrier layer thickness and electric field, the cathodic current density and the filling ratio are higher for smaller pores, i.e. smaller porosity. This effect can be explained by the influence of the porosity on the electric field strength localized within the pores. A schematic view of the electrochemical setup and its equivalent electrical circuit are represented on Figure 14. The voltage drop through the barrier layer is directly dependent on its thickness and current density. For simplification, the barrier layer and electrical double layers will be neglected, the surface area is constant between the working electrode and the counter electrode, i.e. A1 = AE = A2. The Debye length and electrical double layer thickness at the pore wall are considered to be very small compared to the pore size so that the electrolyte conductivity remains the same in the confined nanopores as in the bulk. The resistance of the pores REP can therefore be written as: R 9: ρ

9 ρ

Nonaqueous sol-gel synthesis of anatase nanoparticles and their

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