Abstract:
A titanium dioxide coating method is disclosed. An electrolyte containing Ti 3+  and at least one of NO 3   −  and NO 2   −  is provided for an electrodeposition device. 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 NO 2   −  or NO 3   − . 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 complex shapes can be obtained by use of low cost instruments.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    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. 
         [0003]    2. Description of the Prior Art 
         [0004]    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 TiO 2 , the anatase form (A-TiO 2 ) 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-TiO 2 . 
         [0005]    In addition, A-TiO 2  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. 
         [0006]    Several techniques were proposed for fabricating TiO 2 , such as sol-gel, chemical vapor deposition, hydrothermal, electrospinning, anodizing, and electrodeposition. 
         [0007]    Among these methods, cathodic deposition of TiO 2  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 instrumentations. 
         [0008]    To sum up, it is now a current goal to develop a cathodic deposition method for coating titanium dioxide. 
       SUMMARY OF THE INVENTION 
       [0009]    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 complex shapes, and use of low cost instrumentations. 
         [0010]    The present invention is directed to a cathodic deposition method for coating a titanium dioxide film. 
         [0011]    The present invention is also directed to an electrolyte for coating titanium dioxide including Ti 3+  and at least one of NO 3   −  and NO 2   − . 
         [0012]    According to one embodiment, the present invention provides a titanium dioxide coating method, which includes following steps. An electrolyte containing Ti 3+  and at least one of NO 3   −  and NO 2   −  is provided for an electrodeposition device. 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 NO 2   −  or NO 3   −  and to form titanium dioxide film on the surface of the substrate. 
         [0013]    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 
         [0014]    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: 
           [0015]      FIG. 1  illustrates a flowchart of a titanium dioxide coating method according to one embodiment of the present invention; 
           [0016]      FIG. 2  illustrates LSV (linear sweep voltammetry) curves according to one embodiment of the present invention; 
           [0017]      FIG. 3A  illustrates first and second scans of LSV curves according to one embodiment of the present invention; 
           [0018]      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; 
           [0019]      FIG. 3C  illustrates an enlarged view of  FIG. 3B ; 
           [0020]      FIGS. 4A and 4B  illustrate SEM (Scanning Electron Microscope) images according to one embodiment of the present invention; 
           [0021]      FIGS. 4C and 4D  illustrate TEM (Transmission Electron Microscope) images according to one embodiment of the present invention; and 
           [0022]      FIGS. 4E and 4F  illustrate depth profiles of XPS (X-ray photoelectron spectra) according to one embodiment of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0023]      FIG. 1  illustrates a flowchart of a titanium dioxide coating method including following steps. Beginning at step S 1 , an electrolyte containing Ti 3+  and at least one of NO 3   −  and NO 2   −  initiates the redox reaction between Ti 3+  and NO 3   − /NO 2   −  to form Ti(IV) and NO 2   − /N 2 . This electrolyte is provided for an electrodeposition device. Next, at step S 2 , a substrate is then immersed into the electrolyte and at step S 3 , the substrate is electrically connected to the electrodeposition device. At step S 4 , a cathodic current is applied on the substrate via the electrodeposition device for reducing NO 2   −  or NO 3   −  to generate extensive OH −  for depositing TiO 2  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. 
         [0024]    The continuous reduction of NO 2   −  to N 2  and NH 3  generates extensive OH − , and effectively enhances the deposition of TiO 2  films on the surface of substrates. 
         [0025]    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. 
         [0026]    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. 
         [0027]    TiO 2  particulates are cathodically deposited onto graphite substrates from an electrolyte bath containing 0.47 M HCl, 25 mM TiCl 3  and 75 mM NaNO 3  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. 
         [0028]    According to one embodiment of the present invention, the redox reaction between Ti 3+  and NO 3   −  during preparation of the deposition solution is herein disclosed. Nitrates, acting as the oxidizers, were reduced to NO 2  (reddish-brown bubbles) when the transparent NaNO 3  solution was added into the purple TiCl 3  solution. Since NO 2  molecules are soluble in aqueous media, they will automatically convert into NO 3   −  and NO 2   − . This statement is supported by the observation that reddish-brown bubbles gradually disappear within 30-40 seconds and the purple TiCl 3  solution in presence of Ti 3+  is a colorless transparent solution indicating the formation of TiO 2+  (see equations 1 and 2) 
         [0000]      Ti 3+ +NO 3 —→TiO 2+ +NO 2    (1) 
         [0000]      2NO 2 +H 2 O→HNO 3 +HNO 2    (2) 
         [0029]    Curves  1 - 5  in  FIG. 2  correspond to the i-E responses measured from various electrolytes. As can be seen from the 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 H 2  evolution. On curves  3  and  4 , reduction starts in the more positive potential region, revealing the facile reduction of NaNO 2 . 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 NO 2   −  is responsible for the reduction in the more positive potential region with minor gas evolution, presumably due to the reduction of NO 2   −  into N 2  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 N 2  to NH 4   +  in such a negative potential range (see equations 3 and 4). 
         [0000]      2NO 2   − +4H 2 O+6e→N 2 +8OH −   (3) 
         [0000]      N 2 +8H 2 O+6e→2NH 4   + +8OH −   (4) 
         [0030]    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 NO 2   − . Accordingly, NO 2   −  reduction in the designed deposition bath for generating concentrated OH −  at the vicinity of electrode surface is concluded to be an effective step in promoting the deposition of TiO(OH) 2  (see equation 5). The TiO(OH) 2  is then dehyrated to form TiO 2 . 
         [0000]      TiO 2+ +2OH − +xH 2 O→TiO(OH) 2 .xH 2 O   (5) 
         [0031]    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 NO 3   −  reduction. Based on equations 3 and 4, OH −  is mainly provided by the NO 2   −  reduction and the consequent N 2  reduction, resulting in the generation of NH 4+ . As a result, a slight increase in pH found in this formulated solution after TiO 2  deposition is reasonable because the OH − /electron ratio for the reduction of NO 2   −  and N 2  is 4/3, 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 − . 
         [0032]      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 N 2  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, NO 2   −  starts to be reduced to N 2 , no significant increase in mass is observed. The slight weight gain in this potential region is probably due to the NO 2   −  adsorption at the cathode. Based on the EQCM result, once the potential is negative enough to generate/accumulate concentrated OH − , TiO 2+  will combine with OH −  to form TiO 2  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 TiO 2  already deposited onto the graphite surface during the first scan for NO 2   − /N 2  reduction. 
         [0033]    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 TiO 2  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. 
         [0034]    It is favorable to prepare porous A-TiO 2  films by combining cathodic deposition from this designed Ti 3+ +NO 3   −  solution and post-deposition annealing. As illustrated in FIGS.  4 A and  4 B, TiO 2  films before and after annealing are porous and the particle size is roughly estimated to be 60-100 nm. The porous nature of TiO 2  films prepared in this invention is probably due to the extensive tiny bubble evolution during the deposition. The particulates are considered as aggregates of TiO 2  primary particles. 
         [0035]    The average size for as-deposited TiO 2  primary particles is about 6 nm, which is enlarged by post-deposition annealing (ca. 10 nm for TiO 2  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 TiO 2  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. ½) within the whole oxide matrix. 
         [0036]    This result confirms the formation of TiO 2  in the as-prepared and annealed films. Accordingly, combining cathodic deposition from this designed Ti 3+  +NO 3   −  solution and post-deposition annealing is favorable for preparation of porous A-TiO 2  films. 
         [0037]    The aforementioned embodiment exemplified the reaction from the electrolyte solution containing Ti 3+ +NO 3   − ; however, the redox reaction between Ti 3+  and NO 2   −  in an electrolyte solution can be used for depositing titanium dioxide films (See Equation 6 and Equation 3-5). 
         [0000]      6Ti 3+ +2NO 2 -+2H 2 O→6TiO 2+ +N 2 +4H +   (6) 
         [0038]    To sum up, a titanium dioxide coating method according to the present invention includes a cathodic deposition using an electrolytic solution containing Ti 3+  and at least one of NO 3   −  and NO 2   − , and a post-deposition annealing process, which is favorable for preparing porous A-TiO 2  films. The redox reaction between Ti 3+  and NO 3   − /NO 2   −  to form Ti(IV) and NO 2   − /N 2  prior to cathodic deposition effectively promotes the TiO 2  deposition. The continuous reduction of NO 2   −  to N 2  and NH 3  generates extensive OH −  and effectively enhances the deposition of TiO 2  for forming a TiO 2  film at the substrate surface. 
         [0039]    The porous, anatase structure of annealed TiO 2 , examined by FE-SEM, TEM, and SAED analyses is expected to be good for the dye-sensitized solar cell (DSSC) application. In addition, A-TiO 2  may be applicable for water and air purifications, photocatalysts, gas sensors, electrochromic devices, and so on. 
         [0040]    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.