Electrophoretic deposition of BaTiO3 thin films from stable colloidal aqueous solutions

Available online at www.sciencedirect.com

ScienceDirect Journal of the European Ceramic Society 34 (2014) 2239–2247

Electrophoretic deposition of BaTiO3 thin films from stable colloidal aqueous solutions Elsy Bacha a , Raphael Renoud a , Hélène Terrisse b , Caroline Borderon a , Mireille Richard-Plouet b , Hartmut Gundel a , Luc Brohan b,∗ a

LUNAM University IETR (UMR CNRS 6164), Université de Nantes, 2 rue de la Houssinière, 44322 Nantes, France b Institut des Matériaux Jean Rouxel, Université de Nantes, CNRS, 2 rue de la Houssinière, 44322 Nantes, France Received 24 June 2013; received in revised form 11 February 2014; accepted 11 February 2014 Available online 11 March 2014

Abstract Polycrystalline BaTiO3 (BTO) thin films were electrodeposited on titanium foils from colloidal aqueous solutions of BTO nanoparticles which were synthesized by a hydrothermal process, under autogenous conditions. Two kinds of reactants have been studied: one involves an aqueous solution of titanium tetrachloride and barium hydroxide octahydrate without any added solvent, whereas the second was obtained by dispersing amorphous titanium oxide in different volumes of barium hydroxide aqueous solutions. The nanoparticles were characterized by X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM). The particle size distribution (PSD) and the zeta potential measurements indicate that the colloids in the solution are polydisperse when using the first synthesis method, and they tend to be quasi monodisperse in the case of the second synthesis route. A porous microstructure of the electrodeposited films was detected by SEM. Finally, the dielectric properties of the BTO thin films were investigated. © 2014 Elsevier Ltd. All rights reserved. Keywords: BaTiO3 (BTO); Hydrothermal synthesis; Aqueous colloidal solution; Electrophoretic deposition; Nanoparticles

1. Introduction Owing to its wide-ranging applications, barium titanate (BaTiO3 or BTO) has received extensive attention with respect to its promising electrical and optical properties.1–5 BTO thin films are of great interest on laboratory or industrial scales due to widespread applications in multilayer ceramic capacitors, dynamic random access memories, thermistors, sensors, actuators, photonic crystals and waveguide modulators.6–13 Despite the increasing number of processes described in the literature for the preparation of BaTiO3 thin films, such as vacuum-based deposition techniques (sputtering,14,15 pulsed laser deposition,16 metal-organic chemical vapor deposition (MOCVD)17,18 ) and Chemical Solution Deposition (sol–gel methods19,20 ), all these techniques require high temperatures

∗

Corresponding author. Tel.: +33 02 51 12 55 3. E-mail address: [email protected] (L. Brohan).

http://dx.doi.org/10.1016/j.jeurceramsoc.2014.02.023 0955-2219/© 2014 Elsevier Ltd. All rights reserved.

annealing (>500 ◦ C) for crystallization and/or by-products decomposition. Compared with these processes, electrochemical methods such as electrolytic deposition (ELD) and electrophoretic deposition (EPD) can also be used to create uniform films over large areas, and less waste is generated. Conductive substrates are required for this technique, which makes it suitable for BTO film deposition on metal. In the first case, a delicate balance of electrochemical and chemical reactions is necessary to achieve the desired film composition and morphology, and high temperature annealing or treatment with additional precursors may be needed. For the second way, the low-temperature synthesis of BTO thin films by nanoparticles electrophoretic deposition (EPD) offers advantages of process simplicity, low cost, and easy deposition with controlled particles size and layer thickness.21–25 However, the most common strategy involves organic precursors in solvothermal process to prepare BTO nanoparticles and organic additives were used, as dispersing agent, to improve the stability of suspensions.26–29 Incomplete

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removal of the stabilizing agents can result in contamination that can negatively impact the electronic properties of the material due to carbonate or hydrogen carbonate adsorbed on particles. Aqueous solutions are highly desirable because they are costeffective, and water has no negative environmental impact. To overcome this problem, we report here: (i) the lowtemperature hydrothermal synthesis of phase-pure BaTiO3 perovskite using inorganic precursors; (ii) the preparation of stable colloidal suspensions in aqueous solution without organic molecules as dispersing agent; (iii) the nanoparticles electrophoretic deposition (EPD) of BTO thin films; (iv) the dielectric properties of the capacitor materials were finally investigated. 2. Experimental methods 2.1. Synthesis of the nanoparticles To study the influence of inorganic titanium precursor on the BTO particle size and dispersity, the BTO nanoparticles were synthesized by hydrothermal process starting from solid Ba(OH)2 ·8H2 O (Sigma–Aldrich) and two different titanium precursors with various Ba/Ti ratio. For first titanium reactant, we used a titanium oxychloridehydrochloric acid complex (TiOCl2 ·1.4HCl·7H2 O, [Ti4+ ] = 4.85 mol L−1 , Millennium Inorganic Chemicals, noted precursor A) which is a low cost salt, as opposed to other titanium alkoxide precursors, and easy to handle. Preliminary studies using diluted barium hydroxide in aqueous solution have shown that the low solubility of titanium oxide precursor hindered the crystallization of BTO. Therefore, Ba(OH)2 ·8H2 O and TiOCl2 ·1.4HCl·7H2 O, with molar ratio Ba/Ti = 2.8/1, closed to the stoichiometric proportion, were introduced in a Teflon-lined reactor (Model 4744, Paar Instrument Company) which was sealed then heated at 448 K for 72 h. The as-prepared product was washed with distilled water (100 mL) then filtered in order to eliminate the barium chloride species. Finally, the powder was dried in air at 70 ◦ C, overnight. The second titanium precursor is an amorphous powder which was obtained by precipitation of TiOCl2 ·1.4HCl·7H2 O by NH3 in an aqueous solution, as previously reported.30 The freshly prepared precipitate was recovered by filtration, washed with distilled water and then dried overnight in an oven at

70 ◦ C. To define the H2 O/TiO2 ratio, we performed thermogravimetric analysis and obtained TiO2 ·0.67H2 O, hereafter noted precursor B. To probe the reactivity of the barium hydroxide with titanium precursor the R = Ba/Ti molar ratio has been tuned. Three different solutions were prepared by addition of Ba(OH)2 ·8H2 O in large excess such that in each solution the cations molar ratio R was 1.35, 1.70, and 2.40, hereafter noted B1, B2, and B3, respectively. Three volumes 0.5 mL, 1 mL, and 2 mL of saturated barium octahydrate in aqueous solution (Ba(OH)2 ·8H2 O/water = 95 g/100 mL (80 ◦ C)) were added to the previously amorphous TiO2 ·0.67H2 O powder and solid Ba(OH)2 ·8H2 O in a stoichiometric ratio. The solutions were then poured into separate TeflonTM -lined stainless steel reactors, filled to 3–6% of the 50 mL capacities of the vessels. The reactors were sealed and placed in a forced-air convection oven at 473 K for 24 h in air. After cool down, the resultant powders were filtered and washed with hydrochloric acid solution (0.1 mol L−1 ) three times, to eliminate the Ba(OH)2 ·8H2 O in excess and the barium carbonate byproduct resulting from the reaction of Ba(OH)2 ·8H2 O with atmospheric CO2 . The washed powders were then dried at 70 ◦ C for 15 h in air. The experimental conditions for the hydrothermal synthesis of BTO are summarized in Table 1. 2.2. Colloidal aqueous solutions and electrodeposition The dried powders, (0.72 g of BTO) were dispersed into 30 mL aqueous solutions of hydrochloric acid (10−2 mol L−1 ), to stabilize the BTO nanoparticles in colloidal solutions. The electric charging of the particle was controlled by adjusting the suspension until a pH value about 2 was reached. This pH has been chosen to manipulate and control the electric charge of the particle. It corresponds to an electrostatic stabilization which occurs directly at the double or triple layer. The as-prepared colloidal solutions were stable on several weeks and used as electrolyte in the following EPD step. The nanoparticles electrophoretic deposition (EPD) of BTO thin films was performed with a DC voltage of 10 V, applied during 10 min between the titanium foil (deposition substrate) and a platinum electrode. The titanium foil has been chosen because it is consistent with the BTO films and it is low cost. The size of each electrode was 2.5 cm × 2.5 cm and prior to EPD experiments, the Ti substrates were only washed with alcohol then

Table 1 Experimental conditions of the hydrothermal synthesis route for the elaboration of the BTO nanoparticles. Starting materials Ba(OH)2 ·8H2 O

Titanium oxide precursors

A

4.6441 g

B1

1.3432 g

B2

1.3388 g

B3

1.3372 g

TiOCl2 ·1.4HCl·7H2 O (1.07 mL) TiO2 ·0.67H2 O (0.3937 g) TiO2 ·0.67H2 O (0.3931 g) TiO2 ·0.67H2 O (0.3938 g)

Saturated Ba(OH)2 ·8H2 O solution at T = 80 ◦ C (mL)

Ba/Ti molar ratio

Temperature time

No

2.8:1

0.5

1.35:1

1

1.70:1

2

2.40:1

448 K 72 h 473 K 24 h 473 K 24 h 473 K 24 h

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water without an electropolishing step. The distance between the cathode and the anode was fixed to 1 cm. After deposition, the films were dried at 373 K for 30 min in air. No annealing has been performed to have the properties of the real BaTiO3 obtained by the hydrothermal method rather than improved by annealing. 2.3. Characterization The X-ray diffraction (XRD) data were collected on a Siemens D5000 (powders) and a Bruker D8 diffractometer (films) in a Bragg–Brentano geometry with a CuK␣ and a CuK␣1 , respectively. A JEOL 6400F scanning electron microscope (SEM) with a cold cathode field emission gun operating at 7 kV was used for the morphological characterization of powders and films. The particle size distribution in films was obtained from the analysis of SEM images (70000×) from a 1.3 ␮m × 1.8 ␮m sample area, integrating one hundred particles. Taking into account this magnification, we estimated that the suitable width of the columns has been fixed to 25 nm. To optimize the stability of colloids, the particle size and zeta-potential were measured in a standard polycarbonate tank equipped with two gold electrodes at a controlled temperature (T = 25 ◦ C) using a Malvern Zetasizer Nano ZS equipped with a He–Ne laser (λ = 633 nm) (see supporting information providing details regarding the calculation of the zeta potential from the electrophoretic mobility). FTIR spectra were recorded using a Bruker Vertex 70, equipped with a specular reflection accessory. In order to perform the dielectric measurements, gold electrodes of 0.5 mm × 0.5 mm surface area were evaporated through a shadow mask on the films thus forming metal–ferroelectric–metal capacitors. The permittivity and the dielectric losses (tan δ) were measured using an Agilent 4294A impedance analyzer. 3. Results and discussion 3.1. Characterization of the nanoparticles Fig. 1a shows the powder X-ray diffraction (XRD) patterns of the as-synthesized sample A, before and after washing with distilled water. Beside the BTO phase, identified by comparison with JCPDS file No. 89–2475, several intense peaks can be attributed to barium chloride hydrate (BaCl2 ·(H2 O)2 (open squares (¤)) which was easily eliminated after washing. However, small amount of barium carbonates, (BCO, marked by *) are still detected together with some traces of TiO2 , as shown in the inset of Fig. 1a (expanded view of 2θ = 35–40◦ ). The crystallization of TiO2 results from a loss of barium hydroxide which is transformed into carbonate and is no more available for building the perovskite framework. The XRD data of the powders (Fig. 1b), prepared from the precursors B1–B3, indicate mainly a well-crystallized BTO phase with narrow diffraction peaks and baryum carbonate, as impurity. In both cases, reaction of barium hydroxide species with atmospheric CO2 was proposed as

Fig. 1. XRD diagrams of as-synthesized BaTiO3 powders from precursors. (a) A before and after washing with distilled water solution, (b) B1–B3 and (c) expanded view of the (0 0 2) and (2 0 0) XRD peaks of the samples A and B1 to B3 ((*) BaCO3 ; (¤) BaCl2 ·(H2 O)2 ; (+) TiO2 ).

the explanation for the presence of BaCO3 in fine-grain BaTiO3 powders. A more detailed view of the XRD patterns reveals a splitting of (2 0 0) peak around 2θ = 45◦ (Fig. 1c), suggesting a lowering of symmetry from cubic to tetragonal. To quantify the relative contribution of the tetragonal and cubic phases, a Rietveld refinement was performed from the XRD diagram of the BTO powder synthesized from precursor A. As previously reported31 the

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Fig. 2. SEM images of the BaTiO3 powders prepared from precursors (a) A (the elongated rods were identified to be BaCO3 ) and (b) B2 (see Table 1).

refined proportions were 65% tetragonal and 35% cubic phases. In addition, the particle sizes were also found to be 67 ± 3 nm and 73 ± 9 nm for the tetragonal and cubic phases respectively. The SEM images of the BTO synthesized by hydrothermal method (Fig. 2a) reveal crystallites with two grain size populations: one around 50 nm and another one close to 100 nm for sample A. Fig. 2a shows also some needle-shaped particles which are the typical morphology of the BaCO3 powder. For the other samples, the distribution in size seems to be bimodal, as shown for sample B2 in Fig. 2b, with one population below 100 nm and the other one larger than 200 nm.

(±7–10 mV) reveal the heterogeneity of the charge repartition at the particle surface. The evolution of ζ will be analyzed latter when examining the particle size in the electrodeposited thin films. The volume particle size distribution (PSD), recorded from Dynamic Light Scattering (DLS), is shown in Fig. 4a, indicating that the colloidal BTO suspension obtained from precursor A is polydisperse. Two main populations can be distinguished: one presenting an average particle size of about 70 nm, with sizes ranging between 20 and 100 nm, and the second one centered on 130 nm, with diameters ranging between 80 and 200 nm.

3.2. Colloidal suspensions and electrophoretic deposition Colloidal suspensions, containing BTO nanoparticles, were stabilized in aqueous solution of hydrochloric acid without addition of organic dispersing agent. At pH 2, the zeta potential measurement performed on the colloidal suspensions obtained from the samples A and B1–B3 reveal a large surface charge distribution for each sample, with a mean value of about 46 mV, 40 mV, 32 mV and 50 mV respectively (Fig. 3). These values indicate that the colloidal suspensions are globally stable (|ζ| > 30 mV)32 and that the nanoparticles are positively charged in acidic medium. However, the large zeta potential distributions

Fig. 3. Zeta potential of the colloidal suspensions prepared from the precursors A and B1–B3.

Fig. 4. Particle size distribution of the BTO nanoparticles in colloidal suspensions prepared from (a) precursor A and (b) precursors B1–B3.

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Fig. 5. XRD diagrams of the electrodeposited BTO thin films from samples A and B1–B3 ((−) Ti; (+) TiO).

The colloidal suspensions for samples B1–B3 also present two populations of particles, which, however, are more clearly separated (Fig. 4b): one distribution with an average particle size between 80 and 90 nm is observed for each sample while the second particle size distribution is considerably larger and centered on 300 nm (samples B1 and B3) or 500 nm (sample B2). The volume distribution shows that the second population is minor (about 30% and 10% for samples B1 and B3, respectively), which means that these particles are highly less numerous than the particles between 80 and 90 nm. (In volume, this population represents about 60% for sample B1 and 75% for sample B3). It is worth noting that in all samples, 10% of the global population is attributed to impurities, like dust, with a diameter close to 4 ␮m. As already suggested by SEM images, the analysis of the different volume distributions reveals that one major population (of diameter between 70 and 90 nm) is present in all samples. The second population of larger diameter is more or less pronounced for the different precursors. In the case of the precursor B2, the second population concerns about 50% of the global distribution. Thus, the particles are significantly larger than those of the solutions B1 and B3. This result was corroborated by the SEM image of Fig. 2b for the smallest particle size, whereas some agglomeration of the largest ones may occur leading to DLS size larger than the size observed by SEM, especially for B1 and B2. Finally, a better monodispersity with less agglomeration is found for the sample prepared with the highest amount of solvent (precursor B3). Thin films were produced by electrophoretic deposition of the BTO nanoparticles from these colloidal suspensions, on a Ti foil by applying a DC voltage of 10 V for 10 min. The XRD diagrams of the layers obtained with the different colloidal solutions are shown in Fig. 5. Beside the peaks relative to BTO and the substrate (Ti), the XRD diagram reveals also two peaks, at 2θ = 36.2◦ and 2θ = 42.06◦ , which can be attributed respectively to the (1 1 1) and (2 0 0) peak of TiO.33 The TiO phase results from the partial oxidation of the Ti foil and is probably located at the interface of the Ti foil and the BTO layer. The barium

Fig. 6. FT-IR spectra of the BTO films prepared by EPD process from samples B1 to B3 (a) before heating and (b) after annealing at 400 ◦ C ((+) for Ti O vibrations).

carbonate impurity, observed in the case of the BTO powders (see Fig. 1a) is no more detected on XRD diagrams of thin-films. BaCO3 is significantly soluble in acidic medium at room temperature, the solubility being a function of pH, with the release of carbonic acid (H2 CO3 ), hydrogen carbonate (HCO3 − ), and barium ions (Ba2+ ) into solution, at pH 2. Compared to XRD, FT-IR was found to be by far the most sensitive technique for the detection of residual carbonate species.34 FT-IR spectra (Fig. 6) reveal its presence through a very weak peak characterized by a broad band at around 1430 cm−1 . The peak height is very small after dispersing the powders in dilute hydrochloric acid (pH 2), but seems to persist after heating at 400 ◦ C for 1 h. It could be attributed to the unidentate carbonate ligand (asymmetric stretch, 1480 cm−1 , symmetric stretch, 1370 cm−1 ).35 The broad peak in the lower frequency range, with a center at ∼600 cm−1 , corresponds to stretching vibrations in the TiO6 octahedra.36 A second peak at lower frequency has been assigned to the Ti O bending vibrations.37 In addition, these films also contain some weakly bound water molecules that are identified by the deformation and stretching vibrations at 1641 cm−1 and 3390 cm−1 , respectively. The presence of hydroxyl groups can also contribute to this stretching vibration. Hydroxide species and water have been easily eliminated by heat treatment at 400 ◦ C. The effect of this annealing on the dielectric properties will be discussed in the paragraph devoted to this characterization.

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Fig. 7. Typical SEM images of electrodeposited BTO films (a)–(c) surface of films prepared with samples B1–B3, (d) cross-sectional view of the BTO layer on Titanium foil, from sample A.

Figs. 7 and 8 show the representative SEM images and the corresponding histograms for BTO films obtained by varying the Ba/Ti ratio. As shown on SEM top and cross-sectional images of the electrodeposited films (Fig. 7), a granular and porous microstructure can be seen, even for the films obtained from the least disperse like solution. The porosity of thin film may be due to the evolution of hydrogen gas from water electrolysis of aqueous colloid which interferes during the EPD process and deteriorates the final microstructure. From the cross-section image, a thickness of approximately 1.6 ␮m is measured. Fig. 8a–c represents nanoparticle size distribution of the BTO layers obtained from analysis of at least 100 nanoparticles on multiple SEM micrographs. In the case of precursors B1 and B3, the main contribution is this of the small particles (85 nm), whereas two populations (91 ± 17 and 166 ± 44 nm) are deposited for precursor B2. During electrophoresis, the smallest particles are mainly deposited, as far as suspensions obtained from B1 and B3 are concerned. In addition for the latter precursor, the distribution is the narrowest one with a half width at half maximum equal to 24 nm. For B2, both populations are deposited and the spread in size is quite broad for the largest particles (166 ± 44 nm). The particle size distributions obtained from the SEM images and those determined by DLS could appear to present some discrepancies because DLS reveals the presence of larger particles especially when 1 mL of H2 O is used in the precursor solution (B2). However, both techniques show the quasi monodisperse population centered on approximately 85 nm for the precursors B1 and B3. Aggregation of particles when dispersed in water

could be an explanation for the larger average size of the second population found by DLS (500 nm for precursor B2, 300 nm for precursors B1 and B3). This is confirmed by the zeta potential measurements which result in a potential that is 10–20 mV higher in the case of the suspensions prepared from the precursors B1 and B3 than that from precursor B2, indicating a lower stability of the latter dispersion. On the contrary, the small particles (85 nm) may remain well-dispersed in the aqueous solution due to their higher zeta potential. Complementary studies are necessary in order to identify the chemical nature of each kind of particle. 3.3. Dielectric properties Fig. 9 represents the dielectric properties of the BTO thin films electrodeposited from the colloidal solution prepared with the powder of precursor A. The permittivity is rather small and only little evolution (εr = 28–34) can be seen as the frequency varies from 10 MHz to 10 kHz while the dielectric losses are almost stable (5 × 10−2 ) in the same frequency range. At lower frequencies, and more particularly below 1 kHz, the losses increase rapidly, indicating the presence of a leakage current due to ionic-group diffusion in the material. The Argand diagram (ε (ω) as a function of ε (ω), not shown) allows us to estimate the contribution of this diffusion to the permittivity, using the relation (σ/iω)β where σ is the diffusion coefficient and β the dispersion parameter.38 In Fig. 10a, is plotted the real part of the permittivity with and without the diffusion effect. Diffusion plays a perceptible role only at frequencies

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Fig. 9. Dielectric permittivity εr and losses tan δ as a function of frequency for BTO thin film, electrodeposited from sample A.

of Lichtecker allows calculating the effective permittivity εeff of a composite material; we  have used the approach developed by Bouzit et al.41 ln εeff = i pi ln εi , where εi represents the relative permittivity of the material, i and pi its volumic fraction. The calculation also requires the information on the porosity of the film, which is not easily accessible experimentally. The preliminary ellipsometry experiments on BTO thin-films failed because of a too much thickness, 1.6 ␮m instead of 100–300 nm required. Therefore, this value was varied in the 25–40% range. The most

Fig. 8. Nanoparticle size distribution of the BTO layers obtained from analysis of at least 100 nanoparticles on multiple SEM micrographs. BTO electrodeposited from the colloidal solutions (a) B1, (b) B2, and (c) B3. Each distribution has been fitted by a Gaussian peak its position together with the half width at half maximum is indicated in the figures.

below 10 kHz. When considering only polarization, the maximum value of the dielectric losses is reached at about 25 kHz and tan δ ≈ 0.07 (Fig. 10b). The XRD powder diagram (Fig. 1c) shows that the BTO powder of precursor A contains grains which are crystallized either in the cubic (paraelectric) phase or in the tetragonal (ferroelectric) phase. Moreover, SEM images show a certain porosity of the layer (Fig. 7d). The relative permittivity of BTO in the cubic phase is almost constant in the frequency domain measured and is equal to εc = 5.24.39 On the other hand, the value of the permittivity in the tetragonal phase, εt , is more difficult to know because it strongly depends on the grain size. Zhao et al. work indicates a relative permittivity of about 750–1000 for a grain diameter of approximately 100 nm.40 The formula

Fig. 10. (a) Dielectric permittivity εr and (b) losses tan δ as a function of frequency, with and without the contribution of diffusion (BTO thin film, electrodeposited from sample A).

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used in this case and by the grain size of the film. Indeed, more solvent implies a larger proportion of ionic groups which are normally localized at the grain boundaries, and smaller grains have more boundaries. At 10 kHz, the dielectric losses of the BTO films prepared with samples B1 and B2 (0.5 mL and 1 mL of water) present values of tan δ = 0.34 and 0.08 respectively. Finally, the sample B2 prepared with 1 mL of water seems to show an interesting compromise between the values of permittivity and losses. However, the dielectric permittivity is too low and the losses are too high for commercial exploitation and further improvements will be necessary. The processing parameters, especially the compactness of thin film, however, still have to be optimized in order to improve the dielectric properties. 4. Conclusions

Fig. 11. (a) Dielectric constant εr and (b) losses tan δ of the BTO thin films electrodeposited from samples B1 to B3.

realistic value for ε constant was obtained for 30%. Thus, the volumic fractions are: air 0.3, cubic BTO 0.245, tetragonal BTO 0.455 (Rietveld refinement). Using the measured value εeff = 30 at 1 MHz (Fig. 9), we obtain εt = 700, which is consistent with the value indicated by Zhao et al. The presence of ionic groups, identified by FT-IR spectrometry (Fig. 6), such as hydroxyls and a minor contribution of carbonates (H2 CO3 and HCO3 − adsorbed on the surface), due to the synthesis with a aqueous solvent, may be at the origin of the diffusion currents observed at low frequencies. In order to remove these ionic groups, the films were heat treated at 400 ◦ C. The OH− and H2 O peaks, respectively visible at 1641 cm−1 and 3390 cm−1 (Fig. 6a), are strongly attenuated by the heat treatment (Fig. 6b). Fig. 11 shows the dielectric permittivity and the dielectric losses after the heat treatment of the BTO thin films electrodeposited from the colloidal solutions prepared with the precursors B1–B3. The dielectric constant (Fig. 11a) of the BTO film from precursor B2 (1 mL of solvent) is higher (εr ≈ 115 at 10 kHz to 10 MHz) than this of the two other films (εr ≈ 85). A more important contribution of domain walls due to the larger grain size (determined by DLS measurements and SEM observation) could explain the difference, which has already been reported.42 The dielectric losses after heat treatment as a function of frequency are shown in Fig. 11b. At low frequencies, there is a strong increase of tan δ, especially in the case of precursor B3, indicating that there is still an important contribution of diffusion. This might be explained by the highest amount of solvent

In summary, a novel, reproducible, and simple aqueous solution-based process for the fabrication of the BTO layer involving hydrothermal synthesis of BTO nanoparticles with inorganic precursors at temperatures as low as 473 K, followed by electrodeposition of precursors in colloidal aqueous solution on a Ti substrate, has been presented; the capacitor device fabricated using this process exhibits a ferroelectric behavior with a dielectric loss value of tan δ = 0.08, at 10 kHz, and a dielectric permittivity above 100. This fabrication approach eliminates the need for processing steps such as vacuum-based deposition techniques and high-temperature post-deposition treatment for BTO crystallization and by-products decomposition. However, this process can be improved by control the viscosity of the colloidal aqueous solution or by using suitable surfactants to control the surface charge or the zeta potential. The initial results, presented here, demonstrate the promise of this alternative approach for the fabrication of thin-film BTO. This synthetic route is robust, low-cost, and environmentally friendly, and could be applied to many semiconductor systems for manufacturing of high quality electro-optic devices on various conductive substrates. Acknowledgements We thank N. Stephant, Service commun de microscopies, Faculté des Sciences, Nantes for help at SEM studies, P.-E. Petit for XRD studies. We also thank P. Derval for help in EPD equipment. References 1. Hennings D, Klee M, Waser R. Advanced dielectrics: bulk ceramics and thin films. Adv Mater 1991;3:334–40. 2. Yariv A. Optical electronics. 4th ed. Chicago: Saunders College Publishing; 1991. 3. Zhang Z, Sun X, Dresselhaus MS, Ying JY, Heremans J. Electronic transport properties of single crystal bismuth nanowire arrays. Phys Rev B 2000;61:4850–61. 4. Lines ME, Glass AM. Principles and applications of ferroelectrics and related materials. Oxford, UK: Oxford University Press; 2001. 5. Dawber M, Rabe KM, Scott JF. Physics of thin-film ferroelectric oxides. Rev Mod Phys 2005;77:1083–130.

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