TITANIUM DIOXIDE COATING METHOD

Abstract

A titanium dioxide coating method is disclosed. An electrolyte containing Ti3+, an oxidant, and at least one of NO3− and NO2− is provided for an electrodeposition device, wherein the oxidant is configured for essentially oxidizing Ti3+ into Ti4+. A substrate is immersed into the electrolyte and electrically connected to the electrodeposition device. A cathodic current is applied to the substrate via the electrodeposition device for reduction of NO2− or NO3−. A titanium dioxide film is thus formed on the surface of the substrate. The thickness, porosity, and morphology of the titanium dioxide film can be controlled by varying the electroplating parameters, and relatively uniform deposits on various substrates of complex shapes can be obtained by use of low cost instruments. The resultant structure of Ti4+ species oxidized from Ti3+ by the oxidant can be used to control the deposition rate of TiO2.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a titanium dioxide coating method and the electrolyte used therein, and more particularly to an electrodeposition method for coating titanium dioxide and the electrolyte used therein.

2. Description of the Prior Art

Titanium dioxide, also known as titania, is widely recognized as an important electrode material in semiconductor photo-electrochemistry. Among the three main crystalline phases: anatase, rutile, and brookite TiO₂, the anatase form (A-TiO₂) is the most popular photo-electrode because the lowest unoccupied molecular orbital of dyes, such as N719, is very close to the conduction band of A-TiO₂.

In addition, A-TiO₂ generally shows relatively high reactivity and chemical stability under ultraviolet light excitation for water and air purifications, photocatalysts, gas sensors, electrochromic devices, and so on, further emphasizing its practical importance.

Several techniques were proposed for fabricating TiO₂, such as sol-gel, chemical vapor deposition, hydrothermal, electrospinning, anodizing, and electrodeposition.

Among these methods, cathodic deposition of TiO₂ becomes attractive because electrochemical deposition provides the advantages of controlling the thickness and morphology by varying the electroplating parameters, relatively uniform deposits on complex shapes, and use of low cost instrumentation.

Sotiropoulos et al. (Electrochimica Acta 51 (2006) 2076-2087) prepared TiO2 films from acidic aqueous solutions of TiOSO₄ and H₂O₂ by room temperature potentiostatic cathodic electrosynthesis. However, Sotiropoulos taught that TiOSO₄ was oxidized to Ti⁶⁺ by using a strong oxidant H₂O₂, which needs to be reduced to prepare the TiO₂ film.

Kim et al. (Electrochimica Acta 50 (2005) 2713-2718) taught a novel approach using TiCl₃ or TiCl₄ as the precursors for the electrodeposition of TiO₂ films. Kim mainly focused on the advantage in using CTAB and the pH value of the solution is roughly 3 in all the cases (Kim, p. 2714 Experimental section 2, paragraph 2).

Both of Sotiropoulos and Kim did not achieve high yield of titanium dioxide and it is now a current goal to develop a cathodic deposition method for coating titanium dioxide with higher yield in comparison with the prior arts.

SUMMARY OF THE INVENTION

The present invention is directed to provide an electrolytic method for coating titanium dioxide to gain the advantages of controlling the thickness, porosity, and morphology by varying the electroplating parameters, relatively uniform deposits on various substrates of complex shapes, and use of low cost instrumentations.

The present invention is directed to a cathodic deposition method for coating a titanium dioxide film with higher yield in comparison with the prior arts.

According to one embodiment, the present invention provides a titanium dioxide coating method, which includes following steps. An electrolyte containing Ti³⁺, an oxidant and at least one of NO₃⁻ and NO2⁻ is provided for an electrodeposition device, wherein the oxidant is configured for essentially oxidizing Ti³⁺ into Ti⁴⁺. A substrate is immersed into the electrolyte and electrically connected to the electrodeposition device. A cathodic current from the electrodeposition device is applied to the substrate for reducing NO₂⁻ or NO₃⁻ to generate extensive OH⁻ and to form titanium dioxide film on the surface of the substrate.

Other advantages of the present invention will become apparent from the following descriptions taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the accompanying advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:

FIG. 1 illustrates a flowchart of a titanium dioxide coating method according to one embodiment of the present invention;

FIG. 2 illustrates LSV (linear sweep voltammetry) curves according to one embodiment of the present invention;

FIG. 3A illustrates first and second scans of LSV curves according to one embodiment of the present invention;

FIG. 3B illustrates the corresponding EQCM (electrochemical quartz crystal microbalance) responses of the first and second scans of LSV in FIG. 3A according to one embodiment of the present invention;

FIG. 3C illustrates an enlarged view of curve 1 in FIG. 3B;

FIG. 3D illustrates a SEM image of titanium dioxide depth for 3 cycles according to one embodiment of the present invention.

FIGS. 4A and 4B illustrate SEM (Scanning Electron Microscope) images according to one embodiment of the present invention;

FIGS. 4C and 4D illustrate TEM (Transmission Electron Microscope) images according to one embodiment of the present invention;

FIGS. 4E and 4F illustrate depth profiles of XPS (X-ray photoelectron spectra) according to one embodiment of the present invention;

FIG. 5A illustrates the LSV curves according to one embodiment of the present invention;

FIG. 5B illustrates the corresponding EQCM (electrochemical quartz crystal microbalance) responses of the LSV curves in FIG. 5A according to one embodiment of the present invention; and

FIG. 5C illustrates the dependence of TiO₂ mass on the cycle number of CV according to one embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates a flowchart of a titanium dioxide coating method including following steps. Beginning at step S1, an electrolyte with pH values ≦2 and containing Ti³⁺, an oxidant and at least one of NO₃⁻ and NO₂⁻. The Ti³⁺ is essentially oxidized into Ti⁴⁺ by the oxidant and NO₃⁻/NO₂⁻ is the OH⁻ provider. This electrolyte is provided for an electrodeposition device. Next, at step S2, a substrate is then immersed into the electrolyte and at step S3, the substrate is electrically connected to the electrodeposition device. At step S4, a cathodic current is applied on the substrate via the electrodeposition device for reducing NO₂⁻ or NO₃⁻ to generate extensive OH⁻ for depositing TiO₂ films on the surface of substrates. The cathodic current can be applied by galvanostatic (constant dc current), potentiostatic (constant voltage), potentiodynamic, or galvanodynamic methods, or in the pulse voltage or pulse current modes.

In one preferred embodiment, an electrolyte with pH values <1 is provided for titanium dioxide deposition. Ti³⁺ may be obtained from dissolution of titanium, for example by dissolving with H₂O₂ and ammonia.

The oxidants can be divided into two groups, strong and weak oxidants. When the weak oxidants are employed, Ti³⁺ can only be oxidized to Ti⁴⁺, even excess oxidants are added. When the strong oxidants are employed, a stoichiometric ratio between Ti³⁺ and oxidants is required to oxidize Ti³⁺ to Ti⁴⁺ which cannot be further oxidized to Ti⁶⁺. Referring to Table 1, weak oxidants that essentially oxidize Ti³⁺ into Ti⁴⁺ are provided and include without limitations to NO₃⁻, NO₂⁻, S₂O₈²⁻, ClO₄⁻, ClO⁻, BrO₄⁻, BrO⁻, IO₄⁻ or IO⁻. The strong stoichiometric oxidants include without limitations to H₂O₂ or O₃.

TABLE 1
Oxidants that essentially oxidize Ti3+ into Ti4+
	Oxidant	Ti3+→Ti4+	@Ti4+→Ti6+	Color
	NO3−	Yes	No	transparent
	NO2−	Yes	No	transparent
	*XO4−	Yes	No	transparent to
				pale yellow%
	*XO−	Yes	No	transparent to
				pale yellow%
	S2O8−	Yes	No	transparent to
				pale yellow%
	H2O2	Yes#	Yes	Tangerine
	O3	Yes#	Yes	Tangerine
	*X represents Cl, Br, I
	#Stoichiometric ratio
	@excess oxidant
	%turning pale yellow when excess oxidant is present

The continuous reduction of NO₂⁻ or NO₃⁻ to N₂ and NH₃ generates extensive OH⁻, and effectively enhances the deposition of TiO₂ films on the surface of substrates.

In one embodiment, a post annealing step is further performed after forming the titanium dioxide film on the surface of the substrate, wherein the post annealing step is carried out at about 100-800° C.

The following descriptions of specific embodiments of the present invention have been presented for purposes of illustrations and description, and they are not intended to be exclusive or to limit the present invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention can be defined by the Claims appended hereto and their equivalents.

TiO₂ particulates are cathodically deposited onto graphite substrates from an electrolyte bath containing 0.47 M HCl, 25 mM TiCl₃ and 75 mM NaNO₃ in an electrodeposition device according to an embodiment of the present invention. A pretreatment procedure of graphite substrates may be performed and the detailed description thereof is herein omitted.

According to one embodiment of the present invention, the redox reaction between Ti³⁺ and NO₃⁻ during preparation of the deposition solution is herein disclosed. Nitrates, acting as the oxidizers, were reduced to NO₂ (reddish-brown bubbles) when the transparent NaNO₃ solution was added into the purple TiCl₃ solution. Since NO₂ molecules are soluble in acidic aqueous media, they will automatically convert into NO₃⁻ and NO. This statement is supported by the observation that reddish-brown bubbles gradually disappear within 30-40 seconds and the purple TiCl₃ solution in presence of Ti³⁺ is a colorless transparent solution indicating the formation of TiO²⁺ (see equations 1 and 2)

Ti³⁺+NO₃₋→TiO²⁺+NO₂  (1)

3NO₂+H₂O→2HNO₃+NO  (2)

Curves 1-5 in FIG. 2 correspond to the i-E responses measured from various electrolytes. As can be seen from curves 1 and 2, reduction commences at potentials negative to −0.6 V and no gas evolution is found at potentials positive to −0.6 V. However, a rapid generation of many bubbles is clearly observed when potentials are negative to −0.6 V, indicating H₂ evolution. On curves 3 and 4, reduction starts in the more positive potential region, revealing the facile reduction of NaNO₂. In addition, minor gas evolution commences from 0.4 V to −0.4 V with a low current density, while gas evolution ceases in the potential range from −0.4 V to −1.2 V and occurs dramatically again at potentials behind −1.2 V. The above results indicate that NO₂⁻ is responsible for the reduction in the more positive potential region with minor gas evolution, presumably due to the reduction of NO₂⁻ into N₂ molecules. Since gas evolution temporarily disappears in the potential range from −0.4 V to −1.2 V. This result suggests a further reduction of N₂ to NH₄⁺ in such a negative potential range (see equations 3 and 4).

2NO₂⁻+4H₂O+6e→N₂+8OH⁻  (3)

N₂+8H₂O+6e→2NH₄⁺+8OH⁻  (4)

On curve 5, gas evolves gently at about −0.1 V, disappears at ca. −0.4 V and, dramatically evolves again at potentials negative to −1.2 V, which completely follows the gas evolution-disappearance phenomena measured from the solution containing NO₂⁻. Based on equations 1 and 2, reduction of NO₃⁻ in the designed deposition bath for generating concentrated OH⁻ at the vicinity of electrode surface is very similar to the reduction of NO₂⁻ (see equation 5). Accordingly, reduction of NO₂⁻ or NO₃⁻ is concluded to be an effective step in promoting the deposition of TiO(OH)₂ (see equation 6). The TiO(OH)₂ is then dehyrated to form TiO₂ (see equation 7).

2NO₃⁻+6H₂O+10e→N₂+12OH⁻  (5)

TiO²⁺+2OH⁻+xH₂O→TiO(OH)₂.xH₂O  (6)

TiO(OH)₂.xH₂O→TiO₂+(x+1)H₂O  (7)

The mechanism proposed in this invention not only reasonably interprets the gas evolution/disappearance phenomena but also explains the slight increase in bath pH after the deposition, which is different from the slight decrease in pH found in previous case of NO₃⁻ reduction. Based on equations 3, 4, and 6, OH⁻ is mainly provided by the NO₂⁻ or NO₃⁻ reduction and the consequent N₂ reduction, resulting in the generation of NH₄⁺. As a result, a slight increase in pH found in this formulated solution after TiO₂ deposition is reasonable because the OH⁻/ electron ratios for the reduction of NO₂⁻, NO₃⁻, and N₂ are equal to 4/3, 6/5, and 4/3, respectively, which are larger than the proton/electron ratio (equal to 1) for oxygen evolution at the anode. Moreover, the deposition rate in this formulated solution is very fast, attributable to the massive generation of OH⁻, the catalytic reduction of NO₂⁻ and NO₃⁻ by TiO(OH)₂ and TiO₂, and the guarantee of TiO²⁺ formation via the redox reaction between Ti³⁺ and oxidants such as NO₃⁻/NO₂⁻.

FIG. 3A illustrates the first and second scans of LSV (linear sweep voltammetry) curves and FIG. 3B illustrates the corresponding EQCM (electrochemical quartz crystal microbalance) responses of the first and second scans of LSV measured from the designed solution in order to precisely obtain the onset potential of deposition. A comparison of the i-E and mass-E responses indicates that there is always an incubation period for N₂ evolution in the positive potential range, e.g., from 0.2 to −0.7 V and from 0.1 to −0.65 V for the first and second sweeps, respectively. Although in the incubation range, NO₂⁻ and NO₃⁻ start to be reduced to N₂, no significant increase in mass is observed. The slight weight gain in this potential region is probably due to the NO₂⁻/NO₃⁻ adsorption at the cathode. Based on the EQCM result, once the potential is negative enough to generate/accumulate concentrated OH⁻, TiO²⁺ will combine with OH⁻ to form TiO₂ and an obvious weight gain is visible behind this onset potential of deposition (−0.85 and −0.65 V for the first and second scans, respectively). Also note the positive shift in the onset potential of deposition during the second scan. This phenomenon is probably due to the electrocatalytic property of TiO(OH)₂ and TiO₂ already deposited onto the graphite surface during the first scan for NO₃⁻/NO₂⁻/N₂ reduction.

Referring to FIG. 3D, the present invention achieve ca. 20 μm (5.4, 7.4 and 7.6 μm for 3 cycles). The dashed lines in FIG. 3D indicate the boundary between deposit and substrate as well as the boundaries of TiO₂ deposits between each CV cycle, respectively. The catalytic effect of TiO(OH)₂ and TiO₂ for the NO₃⁻, NO₂⁻, and N₂ reduction is also one of the main reasons why the present invention achieved a much higher yield of titanium dioxide (in comparison to 4 μm for 20 cycles for Kim et al.). In addition, the usage of weak oxidants, such as NO₃⁻ and NO₂⁻ even in excess, guarantees the formation of TiO²⁺, which is also one of the main reasons why the present invention achieved a much higher yield of titanium dioxide.

The electrodes were cleaned in an ultrasonic DI water bath and dried under a cool air flow after cathodic deposition. After cleaning and drying, some electrodes were annealed at 400° C. in air for 1 hr. The morphologies were examined by a FE-SEM (Field-Emission Scanning Electron Microscope, FE-SEM). The EQCM study was performed by an electrochemical analyzer, CHI 4051A in a one-compartment cell. The microstructure and SAED (selected area electron diffraction, SAED) patterns of as-deposited and annealed TiO₂ deposits were observed through a TEM (FEI E.O Tecnai F20 G2). The depth profiles of Ti and O were measured by an X-ray photoelectron spectrometer (XPS, ULVAC-PHI Quantera SXM), employed Al monochromator (hv=1486.69 eV) irradiation as the photosource.

It is favorable to prepare porous A-TiO₂ films by combining cathodic deposition from this designed solution with lower pH value and post-deposition annealing. As illustrated in FIGS. 4A and 4B, TiO₂ films before and after annealing are porous and the particle size is roughly estimated to be 60-100 nm. The porous nature of TiO₂ films prepared in this invention is probably due to the extensive tiny bubble evolution during the deposition. The particulates are considered as aggregates of TiO₂ primary particles.

The average size for as-deposited TiO₂ primary particles is about 6 nm, which is enlarged by post-deposition annealing (ca. 10 nm for TiO₂ annealed at 400° C.) from FIGS. 4C and 4D. The lattice clearly visible in FIG. 4D and the diffraction rings in its inset indicate the anatase structure which is transformed from the amorphous, as-deposited TiO₂ by post-deposition annealing. FIGS. 4E and 4F illustrate the depth profiles of Ti, O, and C for as-deposited and annealed samples. Clearly, the atomic ratio of Ti/O is approximately constant (ca. 1/2) within the whole oxide matrix.

These results confirm the formation of TiO₂ in the as-prepared and annealed films. Accordingly, combining cathodic deposition from this designed solution and post-deposition annealing is favorable for preparation of porous A-TiO₂ films.

The aforementioned embodiment exemplified the reaction from the electrolyte solution containing Ti³⁺⁺ NO₃⁻; however, the redox reaction between Ti³⁺ and NO₂⁻ in an electrolyte solution can be used for depositing titanium dioxide films, too (See Equations 3, 4, 6, and 8).

6Ti³⁺+2NO₂₃₁ +2H₂O→6TiO²⁺+N₂+4H⁺  (8)

FIGS. 5A and 5B show the typical LSV and Δm-E curves measured at 25 mV s⁻¹ from 0 to −1.6 V (vs. Ag/AgCl) in diluted baths A and B, respectively. Bath A is defined as a deposition solution containing 30 mM H₂O₂, 60 mM TiCl₃, and 75 mM NaNO₃. Bath B is defined as a deposition solution containing 60 mM TiCl₃ and 135 mM NaNO₃. In FIG. 5A, the onset potential of reduction on both i-E curves is the same, −0.47 V, which is reasonably due to the same reaction, NO₃⁻ reduction on the EQCM electrode. The reduction currents on curve 1 are always higher than that on curve 2 at any specified potentials negative to −0.47 V although the concentration of NO₃ in both baths should be the same under the assumption that most NO₂ gases generated in bath B are not dissolved in the deposition bath. Accordingly, the formation of certain Ti^4′ hydroxyl species (e.g.,

in bath A is favorable for the NO₃⁻ reduction.

Referring to FIG. 5B, the mass of TiO₂ increases sharply from 0 to 70 ng in the potential region between −0.71 and −0.8 V and then, a gradual increase to 145 ng at potentials negative to −0.8 V on curve 1. On curve 2, significant increase in mass commences at ca. −0.68 V and then, a shoulder is found between −0.68 and −0.9 V. After that, a sharp increase in mass occurs from −0.9 to −1.0 V and a gradual increase from 70 to 130 ng at potentials negative to −1.0 V. Clearly, the TiO₂ deposition rate in bath A is obviously higher than that in bath B, attributable to the formation of Ti⁴⁺ hydroxyl species containing bridged OH groups in the solution. Such Ti⁴⁺ hydroxyl species (with olation) need fewer OH⁻ to form the polymeric oxy-hydroxyl Ti precipitates which will be converted to TiO₂ through dehydration. Accordingly, the formation of Ti⁴⁺ hydroxyl dimmers containing bridged OH groups favors the cathodic deposition of TiO₂.

Referring to FIG. 5C, Lines 1 and 2 show the dependence of TiO₂ mass on the cycle number of CV between 0 and −1.6 V from baths A and B, respectively. Clearly, the dependence of TiO₂ mass on the cycle number of CV from both deposition baths is linear. However, the slope of curve 1 is obviously higher than that of curve 2, revealing that the deposition solution containing H₂O₂ is more favorable for the cathodic deposition of TiO₂ in comparison with that containing NO₃⁻ only. Hence, the resultant structure of Ti⁴⁺ species oxidized from Ti³⁺ by the oxidant determines the deposition rate of TiO₂.

To sum up, a titanium dioxide coating method according to the present invention includes a cathodic deposition using an electrolytic solution containing Ti³⁺, an oxidant, and at least one of NO₃⁻ and NO₂⁻, and a post-deposition annealing process, which is favorable for preparing porous A-TiO₂ films. The redox reaction between Ti³⁺ and oxidant to form Ti⁴⁺ prior to cathodic deposition effectively promotes the TiO₂ deposition. The resultant structure of Ti⁴⁺ species oxidized from Ti³⁺ by the oxidant determines the deposition rate of TiO₂. The continuous reduction of NO₂⁻ or NO₃⁻ to N₂ and NH₃ generates extensive OH⁻ and effectively enhances the deposition of TiO₂ for forming a TiO₂ film at the substrate surface.

The porous, anatase structure of annealed TiO₂, examined by FE-SEM, TEM, and SAED analyses is expected to be good for the dye-sensitized solar cell (DSSC) application. In addition, A-TiO₂ may be applicable for water and air purifications, photocatalysts, gas sensors, electrochromic devices, and so on.

While the invention is susceptible to various modifications and alternative forms, a specific example thereof has been shown in the drawings and is herein described in detail. It should be understood, however, that the invention is not to be limited to the particular form disclosed, but to the contrary, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the appended claims.

Claims

1. A titanium dioxide coating method comprising: providing an electrolyte with a pH value ≦2 and containing Ti3+, an oxidant, and at least one of NO3− and NO2− for an electrodeposition device, wherein the oxidant is configured for essentially oxidizing Ti3+ into Ti4+;immersing a substrate into the electrolyte;electrically connecting the substrate to the electrodeposition device; andapplying a cathodic current to the substrate via the electrodeposition device whereby NO3− or NO2− is reduced to generate extensive OH− for forming a titanium dioxide film on the surface of the substrate.

2. The method as claimed in claim 1, wherein the oxidant is a weak oxidant which is unable to further oxidize Ti4+ to Ti6+.

3. The method as claimed in claim 2, wherein the weak oxidant comprises NO3− or NO2−.

4. The method as claimed in claim 2, wherein the weak oxidant comprises S2O82−, ClO4−, ClO−, BrO4−, BrO−, IO4− or IO−.

5. The method as claimed in claim 2, wherein a ratio of Ti3+ to the weak oxidant is equal to/above the stoichiometric ratio.

6. The method as claimed in claim 1, wherein the oxidant is a strong oxidant in a stoichiometric ratio to Ti3+.

7. The method as claimed in claim 6, wherein the strong oxidant comprises H2O2 or O3.

8. The method as claimed in claim 1, wherein the pH value of the electrolyte is less than 1.

9. The method as claimed in claim 1 further comprising a post annealing step after forming the titanium dioxide film.

10. The method as claimed in claim 9, wherein the post annealing step is carried out at about 100-800° C.

11. The method as claimed in claim 1, wherein the cathodic current is applied by galvanostatic (constant dc current), potentiostatic (constant voltage), potentiodynamic, or galvanodynamic methods, or in the pulse voltage or pulse current modes.

12. The method as claimed in claim 1, wherein OH− is generated by reduction of NO3− or NO2− at the cathode.

13. The method as claimed in claim 12, wherein TiO(OH)2 is generated from a reaction between Ti4+ and OH− and then dehydrated to form TiO2.

14. The method as claimed in claim 13, wherein the generation of OH− by NO3− or NO2− reduction at the cathode is catalyzed by TiO(OH)2 and TiO2.

15. The method as claimed in claim 1, wherein the Ti3+ is obtained from dissolution of titanium.