Preparation of titanium dioxide thin films by indirect-electrodeposition

Preparation of titanium dioxide thin films by indirect-electrodeposition

Accepted Manuscript Preparation of titanium dioxide thin films by indirectelectrodeposition Masaya Chigane, Tsutomu Shinagawa, Jun-ichi Tani PII: DOI...

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Accepted Manuscript Preparation of titanium dioxide thin films by indirectelectrodeposition

Masaya Chigane, Tsutomu Shinagawa, Jun-ichi Tani PII: DOI: Reference:

S0040-6090(17)30214-6 doi: 10.1016/j.tsf.2017.03.031 TSF 35879

To appear in:

Thin Solid Films

Received date: Revised date: Accepted date:

7 August 2016 10 March 2017 16 March 2017

Please cite this article as: Masaya Chigane, Tsutomu Shinagawa, Jun-ichi Tani , Preparation of titanium dioxide thin films by indirect-electrodeposition. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Tsf(2017), doi: 10.1016/j.tsf.2017.03.031

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ACCEPTED MANUSCRIPT Preparation of titanium dioxide thin films by indirectelectrodeposition Masaya Chigane,* Tsutomu Shinagawa and Jun-ichi Tani

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Electronic Materials Research Division Osaka Municipal Technical Research Institute

*Corresponding author

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Fax: +81-6-69638099

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E-mail: [email protected]

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1-6-50, Morinomiya, Joto-ku, Osaka 536-8553, Japan

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ACCEPTED MANUSCRIPT Abstract

Titanium oxide films were prepared on a quartz glass substrate by the cathodic galvanostatic electrolysis of a solution of titanium bis(ammonium lactato)dihydroxide and ammonium nitrate at 323 K using a stainless steel electrode as a “dummy electrode” located This novel film preparation method, named

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in the vicinity (within ~1 mm) of the substrate.

“indirect-electrodeposition” is effective for nonconductive substrates without catalysts or reducing agents.

During indirect electrodeposition, the films are deposited on the area of the

substrate above the electrolyte solution surface. by X-ray photoelectron spectroscopy.

The deposited film was identified as TiO2

The amorphous as-deposited film was converted to

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the anatase-type crystalline TiO2 phase by calcination at 723 K.

Optical bandgap of the film

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was evaluated with Kubelka-Munk function from diffuse reflection spectra.

Keywords Titanium oxide films

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Titanium bis(ammonium lactato)dihydroxide Electrodeposition

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Capillary action

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Direct transition

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ACCEPTED MANUSCRIPT 1. Introduction Titanium dioxide (TiO2) thin films are commonly used as photocatalyst and photoanodes for dye-sensitized solar cells (DSSCs) [1, 2]. DSSCs have gained increasing attention as alternatives or supplements to traditional silicon-based solar cells because of their low manufacturing costs and amenability toward constructing flexible cell architectures [3].

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Since the electrodeposition process from aqueous solution is environmentally benign, it is suitable for the production method of environment–conscious devices: solar cells, photocatalysts.

We have studied electrodeposition of titanium oxide (TiOx) films from

aqueous solutions for the anodes of DSSC [4]. Especially we reported the preparation of thick TiO2 films (>5 μm) via a repeated process employing electrolysis of an aqueous solution

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of a titanium bis(ammonium lactato)dihydroxide (TALH) and calcination.

These TiO2 films

achieved a DSSC conversion efficiency of 5.15 %, demonstrating the validity of the The film deposition process

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electrodeposition process for comparably thick TiO2 films [5]. can be described by the following sequence of reactions:

(i)

NO3− + H2O + 2e− → NO2− + 2OH−

(ii)

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(NH4) 2Ti(OH)2(C3H4O3)2 → Ti(OH)2(C3H4O3)22− + 2NH4+

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Ti(OH)2(C3H4O3)22− + 2OH− → Ti(OH)4 + 2C3H4O32−

Ti(OH)4 → TiO2 + 2H2O

(iii)

(iv).

The electrodeposition of TiOx films from aqueous solutions of titanium species inherently possesses advantages such as low-cost process, the controllability of layer growth by passed charge [5] and possibility of fabrication on substrates of complex shape surface [6]. However, electrodeposition ordinarily can be used only for electrically conductive substrates and the highly resistive films such as TiOx [7] is limited in terms of the layer thickness that 3

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For non-conductive substrates, a chemical reduction using reaction (ii)

might be feasible, although such an electroless deposition would require a Pd catalyst [8]. Moreover, once the surface Pd layer has been covered with the TiOx thin film, the catalytic reduction would cease.

In the case of a representative method: chemical vapor deposition

(CVD), reaction products generated in the vicinity of the substrates attach the substrates, By analogy with CVD, we consider the

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move on the substrates and then form the films [9].

possibility of preparing oxide films by an “indirect-electrolysis” method which can be applied to various substrates without worrying about conductivity.

We intended to produce a film

by placing a conductive “dummy” electrode in the vicinity of the actual substrate and performing electrolysis on the electrode as shown in Fig. 1.

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In this study we prepared titanium dioxide thin films on non-conductive quartz glass substrates via “indirect-electrodeposition” and investigated the characteristics of the film

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deposition phenomena.

2. Experimental details

with a coulombmeter.

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Electrolyses were performed using a potentiostat/galvanostat (Hokuto Denko HZ7000) An austenitic Cr18-Ni8-type stainless steel (SUS304 according to the

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Japanese Industrial Standards; thickness 0.5 mm) cathode, which acted as the dummy A quartz glass plate and a platinum sheet were

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electrode, was galvanostatically polarized.

adopted as the substrate and counter electrode, respectively.

Figure 2 illustrates the

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configuration of the electrodes and substrate used during the electrolysis.

The exposure area

of the dummy electrode was limited to 15 × 30 mm2 with a plating masking tape (AS ONE Corporation, Japan, Code No. 2-2986-03; one-ply thickness 0.06 mm). electrode was fully covered with the tape.

The backside of the

At the two indicated positions on the masked area

(Fig. 2) multi-layered masking tape stacks (AS ONE Corporation, Japan, Code No. 2-298606; one-ply thickness 0.1 mm) were adhered and used as spacers.

The numbers of 0.1 mm-

thick masking tape spacers were 0, 2, 3, 5, 7, or 10, which changed the distance (D) between the dummy electrode and substrate to 0.06, 0.26, 0.36, 0.56, 0.76, or 1.06 mm, respectively. 4

ACCEPTED MANUSCRIPT The quartz glass substrate was placed on the spacer.

The composited masked dummy

electrode and substrate was wrapped with polytetrafluoroethylene sealing tape around.

In

this way, the dummy electrode and substrate, both with available areas of 15 × 30 mm2 were installed face to face. Deionized water (> 12 M cm; Millipore Corporation Elix Advantage 5) was used to Prior to electrolysis, the composite substrate was degreased by

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prepare the solutions.

immersion in 10 % NaOH solution for 10 min at 323 K.

An aqueous solution containing

0.05 mol dm–3 TALH (Aldrich; reagent grade) and 0.1, 0.2 or 0.5 mol dm−3 NH4NO3 at 323 K was used to film formation.

The composite substrate was immersed in the solution to a

depth of 5 mm exposing an area of 15 × 5 mm2 to the bulk solution.

Typically, TiOx films To

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were prepared by galvanostatic electrolysis at −7.5 mA for a prescribed charge.

determine the amount of deposited TiO2 on the substrates, the films were dissolved in 4 mL

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dm−3 H2SO4, 0.4 g dm−3 (NH4)2SO4 and 0.2 mL dm−3 H2O2 aqueous solution (30 %; Santoku Chemical Industries, Co., Ltd.) [10], and the absorbance at 408 nm was measured.

X-ray

photoelectron spectroscopy (XPS) analysis of the deposited films was conducted with a

line as the X-ray source.

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Kratos AXIS-Ultra DLD spectrometer, suing a monochromated Al K (1486.6 eV; 150 W) The binding energies of the Ti 2p and O 1s region photoelectron

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peaks were corrected for charging effects by referencing against the C 1s signal of

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adventitious hydrocarbon contaminants at 284.8 eV. X-ray diffraction (XRD) patterns of the films (as-prepared and calcined at 723 K for 30 min in air) were recorded on a RIGAKU

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RINT 2500 diffractometer (Cu K;  = 0.1541 nm; 40 kV; 100 mA), with an incident angle (θ) fixed at 2°.

The diffraction angle (2θ) was scanned in 0.1° increments with a counting

time of 10 s.

Morphological observations of the film cross-sections were performed by

field-emission scanning electron microscopy (FE-SEM; JEOL JSM-6700F) after coating the sample with a thin layer of W nanoparticles via vacuum deposition to improve conductivity. Transmission spectra and relative diffuse reflection (DR) spectra of samples in ultraviolet (UV)–visible range were measured by means of Shimadzu UV-3150 spectrometer.

An

integral sphere detector equipment (Shimadzu ISR-3100) was used for DR spectroscopy with 5

ACCEPTED MANUSCRIPT BaSO4 powder (Wako Pure Chemical Industries) as a reference reflector.

For the powder

sample, the film was prepared by indirect-electrodeposition at −7.5 mA for 600 C on quartz glass substrate and calcined at 723 K for 30 min in air, thereafter detached by scratching with spatula.

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3. Results and discussion Figure 3 shows the effects of the NH4NO3 concentration on the growth of the films during the indirect-electrodeposition process. reflect the averages of three samples.

Except for the data at 0 C (one), all plot data

The time course indicates successful film preparation

by the indirect-electrodeposition method. The faster deposition rate by the electrolysis of the

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solution containing the higher NH4NO3 concentration suggests a close relationship between the film deposition mechanism and the reduction of NO3− (Eq. 2), until ca. 75 C.

For the

passed charge.

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solutions containing 0.1 and 0.2 mol dm–3 NH4NO3, the films grow almost linearly against the The deposition rate from the 0.2 mol dm–3 NH4NO3 solution is

approximately twice that from 0.1 mol dm–3 NH4NO3. During deposition from the solution

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containing the highest NH4NO3 concentration (0.5 mol dm–3), several clump-like deposits were produced and tended to occlude the space between the dummy electrode and the

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substrate, resulting in flaking by the post-electrolysis rinse with water.

This is the main

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reason why the film growth from the solution appears to stop after ~100 C cm−2. we decided to adopt 0.2 mol dm−3 NH4NO3 for further studies.

Therefore,

Figure 3 (d) and (e) show

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SEM images of the film prepared from the solution containing NH4NO3 0.2 mol dm–3 at −7.5 mA for 150 C.

Surface photograph tends to emphasize roughness of surface of the film.

The film exhibited pumice-like porous structure composed of non-angled particles with the size of several micrometers. From the cross-sectional photograph the average thickness at four sampling spots was 2.3 μm. Figure 4 shows the appearance of the films prepared at various distances D between the dummy electrode and substrate.

In all cases, transparent or milky white films were

deposited in the area of the substrate above the solution surface, as indicated by the horizontal 6

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The films are formed in the liquid that rises in the space between the dummy electrode

and the substrate as a result of capillary action.

The height of the upper edge of the

deposited film tended to be lower according to the expansion of D.

This is related to the

capillary phenomenon, in which the height of the surface rise (h) of a liquid is in inverse

h

2 gr

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proportion to the inner diameter of the capillary tube (r)

(1)

where γ, ρ and g correspond to the surface tension, the density of the liquid, and gravitational acceleration, respectively [11].

The chronopotentiometry during indirect-electrodeposition

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showed that the potential of dummy electrode against Pt counter electrode changed between −2.5 V and −2.8 V, indicating high possibility of water decomposition.

In the case of the

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shortest distance (0.06 mm), film deposition was apparently negligible, because the H2 bubbles produced by the reduction of water confined in such a small space hindered smooth To determine the optimal conditions for this process,

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contact of TiOx with the solution.

further investigations of the relationship between the film deposition efficiency and D, as well

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as the distribution of the deposits are now in progress. Figure 5 shows the Ti 2p and O 1s core levels from the XPS spectra of the as-prepared The peak position at 458.8 eV observed in the Ti 2p3/2 region of the film (Fig. 4(a)) is

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film.

identical to those of the TiO2 standard at 458.8 eV [12], suggesting that the chemical state of

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the film is similar to that of TiO2. The main O 1s peak at near 530 eV (Fig. 4(b)) is assigned to titanium oxide (Ti–O–Ti). A broad O 1s shoulder at 531~533 eV corresponds to the oxygen bonding modes of hydrogen-containing Ti–OH terminal groups (at ca. 531 eV) [12, 13] and water molecules (H–O–H) at 532~533 eV [13], which remain on the surface of the grains of the films due to incomplete dehydration (Eq. (iv)).

Figure 6 shows the XRD

patterns of the films, as well as a pattern from an anatase TiO2 powder diffraction standard [14].

The as-prepared sample (Fig. 6(a)) exhibited no remarkable diffraction peaks,

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In contrast, the peaks of the calcined film (Fig. 6(b))

clearly indicate formation of the anatase phase.

Such wide area preparation of anatase TiO2

film demonstrates the development of photocatalytic films and materials for solar cells. Figure 7 shows the optical transmittance spectra of the TiOx films. Almost identical spectra for both the as-prepared and calcined films were observed, which demonstrate the

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gradual decrease of the transmittance in the visible region (presumably due to light scattering) and photo-absorption in the ultra-violet (UV) region light (at photon energy higher than about 3 eV). The insufficient photo-absorption for both films in the ultra-violet region is due to the thickness of the thin films.

Figure 7(d) shows diffuse reflectance (DR) spectrum of

the powder sample from calcined films. The decrease of DR at higher than 3 eV is due to Slightly higher transmittance of as-prepared film (Fig.7

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interband photo-absorption of TiO2.

(a)) than calcined one (Fig.7 (b)) indicates therefore ambiguous absorption edge because of its DR change is closely associated with absorption coefficient (α) using

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amorphous structure.

2R





(2)

S

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2  1  R F ( R) 

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Kubelka-Munk function

as shown in Fig. 7(e), where R and S indicate diffuse reflectance and scattering coefficient, Although all terms depend on energy (wavelength) of incident light, in

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respectively [15, 16].

the wavelength range between 310 nm (ca. 4 eV) and 410 nm (ca. 3 eV) concerning

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photoabsorption (Fig. 7(b)) the change of S value might be little compared with α. Therefore in relation to absorption S can be assumed to be constant and then we investigated band edge using α/S values.

The relationship of α and photon energy (hν) of semiconductors

near the absorption edge region for direct or indirect transition is given by

(h ) 2  A(h  E g )

(3)

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(h )1/ 2  A(h  E g )

(4)

respectively, where A and Eg are proportion constant and band gap energy [17].

It has not The

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been clarified whether interband transition of anatase TiO2 is direct or indirect [18, 19].

linear relationship of Eq. (3) and (4) indicate that in the plots of (αhν)2 or (αhν)1/2 versus hν the optical band gap can be determined by intersection of extrapolated straight line with hν axis [20−22].

We derived the optical band gaps of the calcined film as 3.2 eV and 3.0 eV

from intersection points of linear fitting line with (αhν/S)2 and (αhν/S)1/2 versus hν for direct It could be inferred the direct process

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and indirect process, respectively, as shown in Fig. 8.

is more suitable because 3.2 eV of Eg for anatase TiO2 is commonly accepted [1, 23, 24].

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Such an Eg value well indicates high potential to electron transfer layer for solar cells [25].

4. Conclusions

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A method involving the electrolysis of a solution containing TALH and NH4NO3, with a stainless steel “dummy” electrode has been revealed to be effective for the deposition of TiOx

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films on a quartz glass substrate in close proximity of the dummy electrode. The strong

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dependence of the TiO2 deposition on the concentration of NH4NO3 indicates that the deposition originates from the cathodic reaction of NO3−.

The films were deposited on the

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substrate area situated above the bulk solution surface where the solution spread due to capillary action; this suggests the feasibility of application to the film preparation onto large area substrates.

As the distance between the substrate and dummy electrode increased, the

deposition range on the substrate was reduced, according to the principles of capillary action. The results suggest the development of a novel film preparation process in which films can be prepared on non-conductive substrates without expensive catalysts or reducing agents.

This

work seems to introduce a third electrolytic plating method beyond electrodeposition and electroless deposition.

The conversion of the as-prepared films to anatase crystalline TiO2 9

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Acknowledgments

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This work was supported by JSPS KAKENHI Grant Number 25420782.

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York, 1997.

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hollow titanium dioxide shell thin films by electrophoresis and electrolysis for dye-

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sensitized solar cells, Electrochem. Solid-State Lett. 12 (2009) E5-E8.

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[13] M. Chigane, M. Ishikawa, XRD and XPS characterization of electrochromic nickel oxide thin films prepared by electrolysis–chemical deposition, J. Chem. Soc. Faraday

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[14] Joint Committee on Powder Diffraction Standards, Powder Diffraction File, International Center for Diffraction Data, (Swarthmore PA), set 21 no. 1272.

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[16] A. B. Murphy, Band-gap determination from diffuse reflectance measurements of 12

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titanium dioxide: Rutile, anatase, and brookite, Phys. Rev. B 51 (1995) 13023-13032.

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ACCEPTED MANUSCRIPT Figure Captions Schematic representation of the concept of indirect-electrodeposition.

Fig. 2

Configuration of the electrodes and substrate during indirect-electrodeposition.

Fig. 3

Time course of indirect electrodeposition from aqueous solutions containing 0.05

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Fig. 1

mol dm–3 TALH and (a) 0.1, (b) 0.2 and (c) 0.5 mol dm−3 NH4NO3 at 323 K.

(d)

Cross-sectional and (e) surface scanning electron micrographs of the film prepared at −7.5 mA for 150 C from 0.2 mol dm−3 NH4NO3 solution.

Fig. 4

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correspond to 1 μm.

The black bars

Photographs of the films on the quartz glass substrates formed by indirect

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electrodeposition for various dummy electrode–substrate distances, commonly at −7.5 mA for 75 C from the solution containing 0.05 mol dm–3 TALH and 0.2 mol dm−3 NH4NO3. The white horizontal line corresponds to the surface level of bulk

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solution.

(a) Ti 2p and (b) O 1s XP spectra of as-indirect-elecrodeposited TiOx film.

Fig. 6

XRD patterns of (a) as-prepared (at −7.5 mA for 150 C from 0.2 mol dm−3 NH4NO3

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Fig. 5

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solution) and (b) calcined TiOx (at 723 K for 30 min) films on quartz glass substrates, with plane indices of an authentic pattern of anatase-type TiO2 [14].

Fig. 7

Transmittance spectra of (a, solid line) as-prepared, (b, dashed line) calcined TiOx films and (c, dotted line) a quartz glass substrate.

The film was prepared by

indirect-electrodeposition at −7.5 mA for 45 C from an aqueous solution containing 0.05 mol dm–3 TALH and 0.1 mol dm−3 NH4NO3. (d) DR spectra and (e) KubelkaMunk function plots for the powdered sample transformed from DR spectrum 15

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Plots of (a) (ahν/S)2 and (b) (ahν/S)1/2 against UV-visible photon energy of powdered films transferred from a/S spectrum corresponding to Fig. 7(e).

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drawn to determine band gap energies.

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Fig. 8

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Linear lines are

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Highlight of the paper: “Preparation of titanium dioxide thin films by indirect-electrodeposition”.

The “indirect-electrodeposition” has been conceptualized and realized.



By the indirect-electrodeposition TiO2 films are formed on insulating substrates.



The capillary action leaded to enlargement of deposition area of the films.



As prepared films were crystallized to anatase type by calcination at 723 K.

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