Patent Application
20090183994
This invention relates to hydrogen generation by photo-electrolysis of water with solar light using band gap engineered nano-tubular titanium dioxide photo-anodes. The titanium dioxide nanotubes are formed by anodization of a titania substrate in an acidified fluoride electrolyte, which may be conducted in the presence of an ultrasonic field or mixed by conventional mixing. The electronic band-gap of the titanium dioxide nanotubes is engineered by annealing in a non-oxidizing atmosphere yielding oxygen vacancies and optionally doping various elements such as carbon, nitrogen, phosphorous, sulfur, fluorine, selenium, etc. Reducing the band gap results in absorption of a larger spectrum of solar light, including the visible region, and therefore generates increased photocurrent leading to higher rate of hydrogen generation.
Photoelectrolysis of water using visible light was first demonstrated by Fujishima and Honda with a single crystal rutile wafer. (See A. Fujishima and K. Honda, Nature 238 (1972) 37-38). Thermally or electrochemically oxidized Ti foils were used as anodes by the same authors in a subsequent paper and an energy conversion efficiency of more than 0.4% was observed. (See A. Fujishima, K. Kohayakawa and K. Honda, J. Electrochem. Soc., 122 (1975) 1487-1489). Recently Khan et al. demonstrated a maximum photoconversion efficiency of 8.35% using a chemically modified n-type TiO2 film on Ti substrate. (See S. U. M. Khan, M. Al-Shahry, W. B. Ingel Jr., Science, 297 (2002) 2243-2245). The higher photoconversion efficiency was attributed to the lower bang gap energy (2.32 eV) of carbon doped n-TiO2-xCx type film synthesized by combustion of Ti metal sheet, which absorbed light at wavelengths below 535 nm. Band gap narrowing was observed in nitrogen doped TiO2 nano-particles also. (See R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269-271). Dye sensitized nano porous TiO2 films are being extensively researched and higher efficiency is reported. (See U. Bach et al., Nature 395 (1998) 583-585).
Recent research focus is on nanocrystalline semiconductors to construct high efficiency photoelectrochemical cell. Nanocrystalline materials of tungsten trioxide, iron oxide and cadmium sulfide have been investigated as potential materials for solar water splitting. (See C. Santato, M. Ulmann and J. Augustynski, J. Phys. Chem., B105 (2001) 936-940, S. U. M. Khan, J. Akikusa, J. Phys. Chem. B103 (1999) 7184-7189, and G. Hodes, I. D. J. Howell, L. M. Peter, J. Electrochem. Soc., 139 (1992) 3136-3140). In these materials, charge separation is envisaged to occur at the semiconductor-electrolyte interface (by different rates of charge transfer to the solution) and not at the electrode as a space charge layer cannot be present at the electrode (each nano-crystal is an electrode) because of the size constraint. The type of semiconductivity of the nano-crystalline film is found to depend on the nature of the charge (hole or electron) scavenger present in the electrolyte. (See M. Gratzel, Nature 414 (2001) 338-344). By altering the dimensions of the nanomaterial, the quantum size effect is reported to be used to control the band gap and enhanced absorption coefficient has been observed due to quantum confinement. (See W. U. Huynh, J. J. Dittner, A. P. Alvisatos, Science 295 (2002) 2425-2427).
Al, Ti, Ta, Nb, V, Hf, W, Zr are all classified as “valve metals” because their surface is immediately covered with a native oxide film of a few nanometers when exposed to oxygen containing surroundings. These metals are widely used to synthesize their respective metal oxide nanotubes through anodization process (See G. P. Sklar, K. Paramguru, M. Misra and J. C. LaCombe, Nanotechnology, 16 (2005) 1265-1271., H. Tsuchiya, J. M. Macak, A. Ghicov, L. Taveira and P. Schmuki, Corrosion Science, 47 (2005) 3324-3335., I. Sieber, H. Hildebrand, A. Friedrich and P. Schmuki, Electrochem. Commun., 7 (2005) 97-100., and H. Tsuchiya, J. M. Macak, I. Sieber, L. Taveira, A. Ghicov, K. Sirotna and P. Schmuki, Electrochem. Commun., 7 (2005) 295-298.). Among all the different valve metals, there is great technological interest in titanium due to its versatility, which makes possible different applications. On the other hand, titanium oxide has many technologically relevant applications such as gas sensors, photovoltaics, photo and thermal catalysis, photoelectrochromic devices, and immobilization of biomolecules (See S. Liu and A. Chen, Langmuir, 21 (2005) 8409-8413., D. V. Bavykin, E. V. Milsom, F. Marken, D. H. Kim, D. H. Marsh, D. J. Riley, F. C. Walsh, K. H. El-Abiary and A. A. Lapkin, Electrochem. Commun., 7 (2005) 1050-1058., D. V. Bavykin, A. A. Lapkin, P. K. Plucinski, J. M. Friedrich and F. C. Walsh, J. Catal., 235 (2005) 10-17., K. S. Raja, M. Misra and K. Paramguru, Mater. Lett., 59 (2005) 2137-2141., S. Oh and S. Jin, Mater. Sci. Engg. C, 2006, in press., and K. S. Raja, V. K. Mahajan and M. Misra, J. Power Soursec, 2006, in press.).
Over the past several years preparation of nanoporous TiO2 tubes by anodization process has the main attention of the scientific community due to its easy of handling and simple preparation method than the TiO2 nanoparticles. Over the years, several electrolytic combinations are being used for the anodization of titanium (See J. Zhao, X. Wang, R. Chen and L. Li, Solid State Commun., 134 (2005) 705-710., C. Ruan, M. paulose, O. K. Varghese, G. K. Mor and C. A. Grimes, J. Phys. Chem. B, 109 (2005) 15754-15759., J. M. Macak, K. Sirotna and P. Schmuki, Electrochem. Acta, 50 (2005) 3679-3684., H. Tsuchiya, J. M. Macak, L. Taveira, E. Balaur, A. Ghicov, K. Sirotna and P. Schmuki, Electrochem. Commun., 7 (2005) 576-580., J. M. Macak, H. Tsuchiya and P. Schmuki, Angew. Chem. Int. Ed., 44 (2005) 2100-2102., and Q. Cai, M. Paulose, O. K. Varghese and C. A. Grimes, J. Mater. Res., 20 (2005) 230-236.).
Among the available photosensitive materials, TiO2 semiconductors (anatase and rutile) are highly stable and relatively inexpensive. Therefore, titanium dioxide is considered potential material for photo-anodes. In general, nanocrystalline TiO2 materials are typically synthesized through chemical route as powders and subsequently coated on a conductive substrate. The nanocrystalline anodes have been fabricated by coating TiO2 slurry on conducting glass, spray pyrolysis, and layer by layer colloidal coating on glass substrate followed by calcinations at an appropriate temperature. (See J. van de Lagemaat, N.-G. Park, A. J. Frank, J. Phys. Chem. B104, (2000) 2044-2052). The disadvantages of these processes are: lower mechanical bond strength between glass substrate and TiO2 coating, agglomeration of nanoparticles, poor control of coating parameters, poor electrical connectivity between particles etc. Further, it was suggested that instead of interconnected 3-D type nanoparticles, fabrication of vertical standing nanowires of TiO2 could improve the photoconversion efficiency. (See S. U. M. Khan, T. Sultana, Solar Energy Materials & Solar Cells 76 (2003) 211-221). Anodization of titanium metal substrate in acidified fluoride solution results in formation of ordered arrays of TiO2 nanotubes. These vertically oriented TiO2 nanostructures have better mechanical integrity and photoelectric properties than those of TiO2 nanocoating prepared by slurry casting route.
The photoelectrolysis properties of anodized titanium oxide nanotubes have previously been studied and reported. (See, for example, U.S. Patent Publication No. 2005/0224360 to Varghese et al.). These types of studies have reported the photoelectrolysis properties of anodized titanium oxide nanotubes having 22 nm diameter, 34 nm wall thickness and 224 nm long (See G. K. Mor, K. Shankar, M. Paulose, O. K. Varghese, C. A. Grimes, Nanoletters 5 (2005) 191-195). In addition, 6 micrometer long TiO2 nanotubes have been shown to have less than 0.4% efficiency of water photoelectrolysis using simulated solar spectrum of light (AM 1.5) (see M. Paulose, G. K. Mor, O. K. Varghese, K. Shankar, C. A. Grimes, J. Photochem. Photobio. A: Chem. 178 (2006) 8-15).
Although research has addressed hydrogen generation by photoelectrolysis of water using visible light there remains a need for a more efficient and robust system for these processes. This invention answers that need through the use of novel nano-tubular titania substrates where the titanium dioxide nanotubes have the required band-gap for photo-electrolysis of water.
The invention relates to a method of making a nanotubular titania substrate having a titanium dioxide surface comprised of a plurality of vertically oriented titanium dioxide nanotubes containing oxygen vacancies. The method preferably includes the steps of anodizing a titanium metal substrate in an acidified fluoride electrolyte under conditions sufficient to form a titanium oxide surface comprised of self-ordered titanium oxide nanotubes, and annealing the titanium oxide surface in a non-oxidating atmosphere. The non-oxidating atmosphere may be a reducing atmosphere, such as nitrogen, hydrogen, or cracked ammonia.
The method may further include the step of doping the titanium oxide surface with a Group 14 element, a Group 15 element, a Group 16 element, a Group 17 element, or mixtures thereof. The electrolyte preferably includes a fluoride compound selected from the group consisting of HF, LiF, Naf, KF, NH4F, and mixtures thereof, and the electrolyte may be an aqueous solution, or an organic solution, such as a polyhydric alcohol selected from the group consisting of glycerol, EG, DEG, and mixtures thereof. The electrolyte may also be mixed by traditional magnetic stirring or may be ultrasonically stirred.
The invention further relates to a nanotubular titania substrate having an annealed titanium dioxide surface comprised of self-ordered titanium dioxide nanotubes containing oxygen vacancies. The nanotubular titania substrate preferably has a band gap ranging from about 1.9 eV to about 3.0 eV. In addition, the titanium dioxide nanotubes may be doped with a Group 14 element, a Group 15 element, a Group 16 element, a Group 17 element, or mixtures thereof, and may also be nitrogen doped, carbon doped, or both. The titanium dioxide nanotubes may also be further modified with carbon under conditions suitable to form carbon modified titanium dioxide nanotubes.
The invention also relates to a photo-electrochemical cell that uses the nanotubular titania substrate of the invention as an electrode. The invention further relates to a photo-electrolysis method for generating H2 that includes the step of irradiating a photo-anode and a photo-cathode with light under conditions suitable to generate H2, wherein the photo-anode is a nanotubular titania substrate of the invention. The light may be solar light. In addition, an acidic solution may be used in the photo-cathode compartment, and a basic solution may be used in the photo-anode compartment. The photo-cathode may be at least one substance selected from the groups consisting of a cadmium telluride (CdTe) coated platinum foil, a cadmium zinc telluride (CdZnTe) coated platinum foil, and anodized TiO2 nanotubes coated with nanowires of CdTe or CdZnTe.
The invention further relates to an electrochemical method of synthesizing CdZn or CdZnTe nanowires comprising pulsing cathodic and anodic potentials to grow the nanowires, wherein a nanoporous TiO2 template was used in combination with non-aqueous electrolyte. The non-aqueous electrolyte may be propylene carbonate. The invention also relates to a nanotubular titania substrate having CdTe or CdZnTe nanowires extending therefrom.
This invention relates to hydrogen generation by photo-electrolysis of water with solar light using band gap engineered nano-tubular titania photo-anodes. The titania nanotubes are formed by anodization of a titanium metal substrate in an electrolyte. The electronic band-gap of the titania nanotubes is engineered by annealing in a non-oxidizing atmosphere yielding oxygen vacancies and optionally by doping with various elements such as carbon, nitrogen, phosphorous, sulfur, fluorine, selenium etc. Reducing the band gap results in absorption of a larger spectrum of solar light in the visible wavelength region and therefore generates increased photocurrent leading to higher rate of hydrogen generation.
Nano-Tubular Titania Substrates
The invention relates to a nano-tubular titania substrate having a surface comprised of self-ordered titania nanotubes. The term “self-ordered titania nanotubes” refers to a titania (a titanium dioxide) surface comprised of a plurality of vertically-oriented titania nanotubes, such as shown in
In addition, the self-ordered titania nanotubes of the invention contain oxygen vacancies. That is, the titania has non-stoichiometric amount of oxygen relative to titanium metal in its +4 oxidation state, Ti+4, although TiO2 (Ti+4) is the predominant portion of the titania nanotubes. Creation of oxygen vacancies at the two-fold coordinate bridging sites in the titania nanotubes results in the conversion of Ti4+ to Ti3+. In other words, due to the oxygen vacancies, or non-stoichiometric amount of oxygen, in the titania, the titanium is present in its +4 and +3 oxidation states. This can also be viewed as the nanotubes of the titania surface comprising a combination of TiO2 and Ti2O3 (i.e. TiO2-x).
Nano-tubular titania substrates of the invention are prepared by anodization of a titanium metal substrate in an acidified fluoride electrolyte to form a surface comprised of self-ordered titania nanotubes followed by non-oxidative annealing. Non-oxidative annealing includes annealing in vacuum and “reductive annealing”, annealing of the titanium dioxide nanotubes in a reducing atmosphere. This gives the nano-tubular titania substrate a band gap in the range of about 1.9 to about 3.0 eV. The nano-tubular titania substrates of the invention are useful in generating hydrogen by photo-electrolysis of water by solar light. The preferential band gap for effective photoelectrolysis of water is 1.6-2.1 eV.
Titanium Metal Substrates
Any type of titanium metal substrate may be used to form the nano-tubular titania substrates of the invention. The only limitation on the titanium metal substrate is the ability to anodize the titanium metal substrate or a portion thereof to form the titania nanotubes on the surface. The titanium metal substrate may be titanium foil, a titanium sponge or a titanium metal layer on an other substrate, such as, for example, a semiconductor substrate, plastic substrate, and the like, as known in the art. Titanium metal may be deposited on a substrate using conventional film deposition techniques known in the art, including but not limited to, sputtering, evaporation using thermal energy, E-beam evaporation, ion assisted deposition, ion plating, electrodeposition (also known as electroplating), screen printing, chemical vapor deposition, molecular beam epitaxy (MBE), laser ablation, and the like. The titanium metal substrate and/or its surface may be formed into any type of geometry or shape known in the art. For example, the titanium metal substrate may be planar, curved, tubular, non-linear, bent, circular, square, rectangular, triangular, smooth, rough, indented, etc. There is no limitation on the size of the titanium metal substrate. The substrate size depends only upon the size of the annodization tank. For example, sizes ranging from less than a square centimeter to up to square meters are contemplated. Similarly, there is no limit on thickness. For example, the titanium metal may be as thin as a few nanometers.
Annodization of the Titanium Metal Substrates
Anodization of titanium metal substrates to form a surface of titantium dioxide (titania) nantotubes is known in the art. (See, for example, K. S. Raja, M. Misra, and K. Paramguru, Electrochem. Acta, 51, (2005) 154-165; O. K. Varghese, C. A. Grimes, J. Nanosci. Nanotech, 3 (2003) 277; D. gong, C. A. Grimes, O. K. Varghese, W. Hu, R. S. Singh, Z. Chem. J. Mater, Res. 16 (2001), 3331; R. Beranek, H. Hildebrand, P. Schmucki, Eletrochem. Solid-State Lett. 6 (2003) B12; Q. Cai, M. Paulose, O. K. Varghese, C. A. Grimes, J. Mater. Res. 20 (2005) 230; J. M. Macak, H. Tsuchiya, p. Schmucki, Angew. Chem., Int. ed. 44 (2005) 2; WO/2006/004686; and US 2005/0224360 A1. Each of these is incorporated here by reference.) Phosphoric acid and sodium fluoride or hydrofluoric acid may also be used to anodize titanium. (See K. S. Raja, M. Misra and K. Paramguru, Electrochem. Acta, 51 (2005) 154-165.). This procedure, generally speaking, takes about 45 minutes to get anodized titanium using 20V under magnetic stirring. The anodizing approach is able to build a porous titanium oxide film of controllable pore size, good uniformity, and conformability over large areas at low cost. The anodization time may be reduced by 50% or more using ultrasonic mixing. This ultrasonic mixing process of the invention (discussed below) also leads to better ordered and uniform TiO2 nanotubes compared to conventional stirring techniques. In addition, a barrier layer (i.e., the junction between the nanotubes and the titanium metal) forms during anodization. The barrier layer may be in the form of domes connected to each other (See, for example,
In general, titania nanotubes may be formed by exposing a surface of a titanium metal substrate to an acidified fluoride electrolyte solution at a voltage selected from a range from 100 mV to 40V, for a period of time ranging from about 1 minute to 24 hours, or more. Typically, the voltage used is about 20V and the anodization time is about 45 minutes to 8 hours. The acidified fluoride electrolyte is typically has a pH of less than about 6 and often a pH<4. Anodization under these conditions forms a titania surface comprised of a plurality of titanium dioxide nanotubes. Known anodization techniques may be used to anodize a titanium metal substrate to form a nano-tubular titania substrate having a surface comprised of self-ordered titanium dioxide nanotubes to be used in the practice of the invention. For example, a titanium metal substrate may be anodized using an aqueous or organic electrolyte, for example, 0.5 M H3PO4+0.14 M Na solution can be used for incorporating P atoms, 0.5-2.0 M Na(NO3)+0.14 M NaF solution or a 0.5-2.0 M NH4NO3+0.14 M NH4F with pH 3.8-6.0 for incorporating N atoms, or a combination of 0.5 M H3PO4+0.14 M NaF+0.05-1.0 M Na(NO3). The anodization preferably occurs at a temperature of 20-25° C. The titanium metal substrate is then anodized at 20 V for 20 minutes after observing a plateau current.
Optional Cleaning of the Titanium Metal Substrate
Prior to anodization to form the titania nanotubes, the titanium metal substrate may be cleaned and polished using standard metallographic cleaning and polishing techniques known in the art. Preferably, the titanium metal substrate is chemically and/or mechanically cleaned and polished as known in the art Mechanical cleaning is preferably done by sonication. Titanium foils are not polished after cleaning. As an example, a titanium metal surface may be incrementally polished by utilizing 120 grit emery paper down to 1200 grit emery paper followed by wet polishing in a 15 micron alumina slurry. After polishing, the valve metal substrate is thoroughly washed with distilled water and sonicated for about 10 minutes in isopropyl alcohol as known in the art. Performing such optional cleaning and polishing aids in consistency of the titanium metal substrates used in the invention, that is, it ensures the titanium metal substrates have uniform starting points (e.g., planar surfaces when desired). While it is preferred to use polished surfaces, any native oxides on the titanium metal substrates do not necessarily need to be removed in order for the titanium metal substrate to be used in the invention.
The Acidified Fluoride Electrolyte
The acidified fluoride electrolyte used in the anodization step may be an aqueous electrolyte, an organic electrolyte solution, or a mixture thereof. Fluoride compounds which may be used in the electrolytes are those known in the art and include, but are not limited to, hydrogen fluoride, HF; lithium fluoride, LiF; sodium fluoride, NaF; potassium fluoride, KF, ammonium fluoride, NH4F; and the like. It is preferred that the acidified fluoride electrolytes have a pH below 5, with a pH range of 4-5 being most preferred. Adjusting the pH may be done by adding acid as is known in the art. Inorganic acids such as sulfuric, phosphoric, or nitric acid, are generally preferred. Phosphoric acid and nitric acid are particularly preferred when phosphorous or nitrogen dopants are to be introduced as discussed below. Organic acids may be used to adjust pH and to introduce carbon as a dopant.
Any aqueous acidified fluoride electrolyte known in the art for the anodic formation of titanium dioxide nanotubes on titania substrates may be used in the practice of the invention. Suitable acidified fluoride electrolytes include, for example, a 0.5 M H3PO4+0.14 M NaF solution, a 0.5-2.0 M Na(O3)+0.14 M NaF solution, a 0.5-2.0 M NH4NO3+0.14 M NH4F, or a combination of 0.5 M H3PO4+0.14 M NaF+0.05-1.0 M NaNO3). Preferred aqueous acidified fluoride electrolytes are discussed below.
Any organic solvent, or mixture of organic solvents, which is capable of solvating fluoride ions and is stable under the anodization conditions may be used as an organic electrolyte. As mentioned above, the organic electrolyte may also be a miscible mixture of water and an organic solvent. It is preferred that at least 0.16 wt % water be present in an organic electrolyte because water participates in the initiation and/or formation of the nanotubes. Preferably, the organic solvent is a polyhydric alcohol such as glycerol, ethylene glycol, EG, or diethylene glycol, DEG. One advantage of using an organic electrolyte is that during the annealing step, the organic solvent is volatized and decomposes under the annealing conditions but also results in carbon doping of the titanium dioxide nanotubes.
Example 3 describes a method for anodizing titanium in ethylene glycol/glycerol organic solvents.
Mixing During Anodization
The formation of the titanium dioxide nanotubes is improved by mixing or stirring the electrolyte during anodization.
Conventional techniques for mixing or stirring the electrolyte may be used, e.g. mechanical stirring, magnetic stirring, etc. In a preferred embodiment, the mixing is achieved by ultra-sonicating the electrolyte solution during annodization. Sonication may be done using commercially available devices. Typical frequencies are about 40 kHz. As shown in
Preparation of Titanium Dioxide Nanotubes Using Ultrasonic Waves
Anodization completed using an ultrasonicator is more efficient that conventional techniques. For example, the use of an ultrasonicator gives rise to better ordered TiO2 nanotubes in a shorter time that mixing by conventional techniques. The synthesis time can typically be reduced up to 50% in this way. In addition, the pore openings and the length of the nanotubes can also be improved through ultrasonic mixing. For example, the length of the nanotubes can be increased to 700-750 nm.
Ultrasonic mediated anodization may be completed, for example, by washing Ti foil discs in acetone and securing the discs such that only small portions are exposed to an electrolyte. Nanotubular TiO2 arrays are formed by anodizing the Ti foils in an acidified fluoride electrolyte. During the anodization of the TiO2 arrays, an ultrasonicator was used to give mobility to the electrolytes, instead of a magnetic stirrer. After anodization, the anodized samples were washed in distilled water to remove the occluded ions from the anodized solutions and dried in oven and fabricated for photocatalysis of water. The various conditions used for anodization according to this method are listed in Examples 6 and 7 below. Various electrolytic combinations were used for this purpose both in aqueous and non-aqueous media.
As indicated above, well ordered nanoporous TiO2 tubes can be obtained much more quickly with ultrasonic mixing than conventional mixing techniques (i.e. 20 minutes) under an applied external potential of 20 V using, for example, phosphoric acid and sodium fluoride electrolytes. The effect of different synthesis parameters viz., synthesis medium (inorganic, organic and neutral), fluoride source, applied voltage and synthesis time are discussed below. The pore diameters can be tuned from 30-120 nm by changing the annodization process parameters such as anodization potential and temperature. The pore diameter increases with anodization potential and fluoride concentration, and the diameter decreases with the electrolyte temperature. A 300-1000 nm thick self-organized porous titanium dioxide layer can be prepared by this procedure in a very quick time. Anodization by ultrasonic mixing is significantly more efficient than the conventional magnetic stirring. The anodizing approach discussed above is able to build a porous titanium oxide film of controllable pore size, good uniformity, and conformability over large areas at low cost. Generally, the anodization step occurs over period of 1-4 hours. However, by using ultrasonic mixing techniques, the anodization time can be reduced by more than 50%. It also leads to better ordered and uniform titanium dioxide nanotubes compared to the reported ones using conventional magnetic stirring. Examples 6 and 7 describe methods of ultrasonic mediated anodization of titanium. The results of Example 6 are illustrated in
Formation of the TiO2 Nanotubes
Generally speaking, the formation mechanism of the TiO2 nanotubes can be explained as follows. In aqueous acidic media, titanium oxidizes to form TiO2 (Equation 1).
Ti+2H2O→TiO2+4H+ (1)
The pit initiation on the oxide surface is a complex process. Though TiO2 is stable thermodynamically in a pH range between 2 and 12, a complexing species (F−) leads to substantial dissolution. The pH of the electrolyte is a deciding factor. The mechanism of pit formation due to F− ions is given by the equation 2;
TiO2+6F−+4H+→[TiF6]2−+2H2O (2)
This complex forming leads to breakage in passive oxide layer and the pit formation continues until repassivation occurs. (See J. M. Macak, H. Tsuchiya and P. Schmuki, Angew. Chem., Int. Ed., 44 (2005) 2100-2102., K. S. Raja, M. Misra and K. Paramguri, Electrochem. Acta, 51 (2005) 154-165., and G. K. Mor, O. K. Varghese, M. Paulose, N. Mukherjee and C. A. Grimes, J. Mater. Res., 18 (2003) 2588-2593.). The formation of the nanotubes goes through the diffusion of F− ions and simultaneous effusion of the [TiF6]2− ions. The faster rate of formation of TiO2 nanotubes using ultrasonic waves according to the invention can be explained by the mobility of the F− ions into the nanotubular reaction channel and effusion of the [TiF6]2− ions from the channel. The higher rate was further confirmed from current versus time plot (
Influence of Anodization Time
The growth of nanotubes can be improved as anodization time increases. For example, as shown in
Influence of Applied Potential
The applied potential may also affect nanotubes formation and pore size. As is described below in Example 10, the applied potential was varied from 5V to 20V by keeping the electrolytic solution and time constant, while mixing with ultrasonic waves.
Double Sided Anodization of Titanium
Another embodiment of the invention relates to a method of anodizing titanium on more than one side. This process, which is described in Example 11, consists of suspending titanium foil in an electrolytic solution under an applied voltage for a predetermined period of time. The resulting double-sided anodization exhibited a good photo activity of 0.4 mA from each side, whereas conventional single sided anodization has a photo activity of approximately 0.1 mA, without any treatment of the nanoporous titanium.
Non-Oxidative Annealing and Band-Gap Engineering
After the anodization step, the band gap of the nanotubular titanium dioxide layer may be reduced by annealing in a non-oxidating (a neutral or a reducing) atmosphere (e.g., nitrogen, hydrogen, cracked ammonia, etc.) and, depending upon the atmosphere, doping any combination of elements, such as, Group 14, 15, 16, and 17 elements, for example, carbon, nitrogen, hydrogen, phosphorous, sulfur, fluorine, selenium, and the like. The reduced band gap results in absorption of larger spectrum of light, particularly solar light in the visible wavelength region, and therefore generates increased photocurrent and efficiency, thereby leading to higher rate of hydrogen generation.
This “non-oxidative annealing,” that is annealing of the titanium dioxide nanotubes in a vacuum, a neutral atmosphere, or a reducing atmosphere. The annealing preferably occurs at a temperature of approximately 350° C. over a period of about 6 hours in any suitable annealing apparatus. Annealing in a non-oxidative, preferably a reducing atmosphere, allows the band gap to be engineered and retains and/or creates more oxygen vacancies in titania nanotubes. Neutral or reducing atmospheres include environments containing carbon, nitrogen, hydrogen, sulfur, etc. Annealing in a reducing atmosphere creates oxygen vacancies which lower the band gap of the titanium dioxide nanotubes. (See
As mentioned above, the non-oxidative annealing gives the a band gap in the range of about 1.9 to about 3.0 eV. The reduced band gap of the nano-tubular titania substrates of the invention makes them useful in generating hydrogen by photo-electrolysis of water by solar light. The preferential band gap for effective photoelectrolysis of water is 1.6-2.1 eV.
Doping the Titania Layer
As indicated above, the nanotubular titania substrate may be doped in any combination of elements, such as, Group 14, 15, 16, and 17 elements, for example, carbon, nitrogen, hydrogen, phosphorous, sulfur, fluorine, selenium, and the like. The doping may be conducted by conventional means known in the art, for example, by conventional diffusion techniques such as solid source diffusion, gas diffusion, and the like. In one embodiment, doping is preferably conducted via a thermal treatment, such as the annealing step, in carbon or nitrogen or sulfur containing environments. While either nitrogen-doping or carbon-doping may occur separately, it is preferred that both occur.
For example, in order to incorporate carbon, the anodized sample may be heated at 650-850° C. in a mixture of acetylene or methane/hydrogen/argon gases with a flow rate of 20 cc/minute, 40 cc/minute, and 200 cc/minute respectively using a Chemical Vapor Deposition Furnace. The total exposure time in carbon containing gas atmosphere varies from 5-30 minutes. This heat treatment of the anodized specimens in the carbon containing gas mixture resulted in incorporation of carbon in the nanotubes of TiO2 arrays, which will be hereinafter referred as carbon modified TiO2 nanotubes.
The size of the carbon modified TiO2 nanotubes were in the range of approximately 200-500 nm. Increasing the exposure time in the carbonaceous environment resulted in growth of carbon nanostructures within the TiO2 nanotubes. The amount of carbon incorporation increased with increase in treatment time and the color of the samples also changed from light gray to dark-gray. Treatments in acetylene for longer than 20 minutes resulted in a complete coverage of the TiO2 with the carbon nano-cone like features.
As another example, nitrogen doping may be conducted prior to the formation of the carbon modified TiO2 nanotubes. More specifically, doping of nitrogen is accomplished by heat-treating anodized (preferably in nitrate containing solutions) Ti samples at 350° C. for 3-8 hours in a nitrogen containing atmosphere. Commercial purity nitrogen/cracked ammonia may be passed over the anodized Ti surface at a flow rate of 150-1000 cc/minute inside a furnace maintained at 350° C. Similarly, doping of sulfur or selenium may be accomplished by heat-treating anodized samples embedded in sulfur or selenium powders at 300-650° C. for 1-6 hours. Optionally, the doping may be conducted on the nanotubular structure after the formation of the carbon modified TiO2 nanotubes.
In one embodiment, carbon modified TiO2 nanotubes may be formed after nitrogen doping. In this case, the doping of nitrogen can be accomplished by heat-treating the anodized (preferably in nitrate containing solutions) Ti samples at 350° C. for 3-8 hours in nitrogen atmosphere. Commercial purity nitrogen/cracked ammonia is passed at a flow rate of 150-1000 cc/minute inside a furnace maintained at 350° C. Similarly, doping of sulfur or selenium may be accomplished by heat-treating the anodized samples embedded in sulfur or selenium powders at 300-650° C. for 1-6 hours. In another embodiment, the nitrogen doping may be conducted on the nanotubular structure after the formation of the carbon modified TiO2 nanotubes.
Example 21 describes phosphorous doping and the benefits thereof. In particular, the nanotubular TiO2 arrays of the invention may be anodized in a various phosphate solutions, such as 0.5 M H3PO4+0.14 M NaF. Table 1 illustrates the various band-gaps that can be achieved in this manner. As is shown in
Table 1 below illustrates various band-gaps achieved by annealing and doping the TiO2 with different elements.
Photogeneration of Hydrogen
Photoelectrochemical cells known in the art may be used with a nano-tubular titanium anode of the invention to generate hydrogen. Generally, photoelectrochemical cells irradiates an anode and a cathode to generate H2 and O2. An schematic of an exemplary photoelectrochemical cell for generating hydrogen is illustrated in
While any suitable electrolyte solution known in the art may be used in the photoelectrochemical cell, preferred electrolyte solutions include aqueous basic, acidic or salt solutions with good ionic conductivity, for example, 1 M NaOH, 1 M KOH (pH˜14), 0.5 M H2SO4 (pH˜0.3) and 3.5 wt % NaCl pH˜7.2) aqueous solutions. The same electrolyte can be filled in both anode and cathode compartments. Alternately, anodic compartment can have higher pH solution such as KOH and cathodic compartment have acidic solution such as sulfuric acid. Specifically, with reference to
The Photo-Anode
While any suitable photo-anode may be used in typical photoelectrochemical cells known in the art, the photoelectrochemical cells of the invention preferably utilize nanotubular titania substrates of the invention, as discussed above, as the photo-anode.
The Photo-Cathode
Generally speaking, any photocathode known in the art may be used to generate hydrogen according to the invention. However, two preferred types of photocathodes include (1) cadmium telluride (CdTe) or cadmium zinc telluride (CdZnTe, or CZT) coated platinum foils, and (2) anodized TiO2 nanotubes coated with nanowires of CdTe or CdZnTe. The deposition is accomplished by depositing the elements at substantially the same time in an organic solvent and in an inert dry atmosphere (e.g., in an inert glove box). The solvent should have sufficient dielectric constant for the electrolysis. Exemplary solvents include, but are not limited to, propylene carbonate, acetonitrile, dimethyl sulfoxide (DMSO), tetrahydrofuran (ThF), and dimethyl formamide (DMF).
Typical electrolyte compositions include, for example, 10×10−3 M ZnCl2+5×10−3 M CdCl2+0.5 and 1.0×10−3 M TeCl4+25×10−3 M NaClO4 in propylene carbonate. 30×10−3 M NaClO4 may be used as a supporting electrolyte. It is preferred that the depositions be carried out in a controlled atmosphere inside a glove box, with ultra high purity argon being used as an inert atmosphere. The oxygen and moisture contents of the glove box were controlled at low levels. Nanowires of CdZnTe were deposited on the nanoporous TiO2 template by pulsing the potentials, and a typical pulsed-potentials cycle contained two cathodic, two anodic and one open circuit potential. All potentials were applied with respect to the cadmium reference electrode. Cathodic pulsed potential can be varied between −0.4V to −1.2 V, for example, and pulsed for 1 second. The anodic pulsed potentials were kept constant in all the test runs. The two anodic potentials used were 0.3V for 3 secs and 0.7V for 5 secs. The deposition time was typically around 30 minutes.
Both the photoanode and photocathode may be coated with the above-described electrodeposition technique. Optionally, a subsequent treatment may be used to stabilize the coating as known in the art. For example, a thermal treatment may be applied to the coating. Example 17 describes an exemplary method of coating CdTe or CdZnTe nanowires/thin films, and Example 18 describes a method of forming CdZnTe nanowires in a single step electrochemical synthesis using the nanoporous TiO2 template of the invention in a non-aqueous solution.
Photoelectrochemical Cells
By irradiating both an anode and cathode in an photoelectrochemical cell or by using acidic solution in the cathode compartment and a basic solution in anodic compartment, the external supply of electrical energy can be eliminated or minimized for higher rate of hydrogen generation. For example, Example 8 describes the use of photo-anodes in the invention.
where,
The efficiency of the system increased with increased external potential, because both the photocurrent and the potential between photo-anode and cathode also increased. The hydrogen evolution at the cathode and oxygen evolution at the anode could be visibly observed when anode was irradiated with light in addition to applied potential. When the light was cut-off maintaining the external potential, the evolution of gases stopped immediately and the measured current dropped to less than 20 microampere level from few milliamperes.
An exemplary nanotubular structure was formed as follows:
Step 1: A Ti metal surface was cleaned using soap and distilled water and further cleaned with isopropyl alcohol.
Step 2: The Ti material was immersed in an anodizing solution, as described below, at room temperature. Various combinations of solutions can be employed in order to incorporate doping elements such as nitrogen, phosphorous etc. For example 0.5 M H3PO4+0.14 M NaF solution can be used for incorporating P atoms, and 0.5-2.0 M Na(NO3)+0.14 M NaF solution or a 0.5-2.0 M NH4NO3+0.14 M NH4F with pH 3.8-6.0 can be used for incorporating N atoms. Combinations of 0.5 M H3PO4+0.14 M NaF+0.05-1.0 M Na(NO3) can also be used.
Step 3: A direct current (DC) power source, which can supply 40 V of potential and support 20 mA/cm2 current density, was connected to the Ti material and a platinum foil (Pt rod/mesh) having an equal or larger area of the Ti surface. The anodization set-up is schematically shown in
Step 4: The anodization voltage was applied in steps (0.5 V/minute) or was continuously ramped at a rate of 0.1 V/s from open circuit potential to higher values, typically 10-30 V. Generally, the voltage was ramped at a rate of 0.1 V/s and the typical final anodization potential was 20 V. This process resulted in a pre-conditioning of the surface to form nanoporous surface layer.
Step 5: After reaching the final desired anodization potential, the voltage was maintained, and the surface was anodized, at a constant value of 10-30 V, with 20V being preferred, to form the nano-pores/tubes (40-150 nm diameter). The current was continuously monitored and the anodization was stopped approximately 20 minutes after the current has reached a plateau value. The anodization process took about 45 minutes for solutions with pH<3 to get 400 nm long nanotubes. In pH 2.0 solutions, the steady state length of the TiO2 nanotubes was about 400 μm. Longer anodization times (>45 minutes) did not result in longer nanotubes (longer than the steady state length). Longer anodization times were allowed for higher pH solutions, which resulted in longer nanotubes. For example, in 0.5 M NaNO3+0.14 M NaF solution with pH 4.0, anodization for 4 hours resulted in 800 nm long nanotubes.
Step 6: The electrolyte was continuously stirred during the anodization process.
Step 7: The nano-pores obtained on the titanium surface after anodization are shown in
Titanium discs of diameter 16 mm and thickness 0.2 mm (0.2 mm thick, ESPI-metals, Ashland, Oreg., USA) were cleaned by sonication in acetone, isopropanol and methanol respectively and then rinsed in deionized water. The dried specimens were placed in a Teflon holder (from Applied Princeton Research, Oak Ridge, Tenn.) exposing only 0.7 cm2 of area to the electrolyte for anodization. The solution of 0.5 M H3PO4+0.14 M NaF was used for anodization, conducted at room temperature under a voltage of 20 V for 45 minutes with constant mechanical stirring. The morphologies of the resulting nano-porous titanium oxide were studied using a Hitachi S-4700 field emission scanning electron microscope (FESEM) and Shimadzu UV-VIS photospectrometer.
First, anodized titanium templates were prepared. Titanium discs having 16 mm diameters and a thickness of 0.2 mm (0.2 mm thick, ESPI-metals, Ashland, Oreg., USA) were cleaned by sonicating in acetone, isopropanol, and methanol respectively, and then rinsed in deionized water. The dried specimens were then placed in a Teflon holder (from Applied Princeton Research, Oak Ridge, Tenn.) exposing only 1 cm2 of area to the electrolyte for anodization.
Anodization was done in two types of organic solvents. The first was glycerol based and other was ethylene glycol based. The following combination of electrolytes were used:
(a) 0.5 wt. % NH4F & 8.75 wt. % Ethylene Glycol in Glycerol.
(b) 0.5 wt. % NH4F & 27.5 wt. % Ethylene Glycol in Glycerol.
(c) 0.4 wt. % NH4F in Ethylene Glycol.
The anodization was done by ramping the potential to 20V at a rate of 1V/s after which the potential was kept constant at 20V. The anodization was carried out for 45 minutes, 7 hrs., and 14 hrs. respectively in the case of the glycerol based electrolyte, and for 45 minutes and 7 hrs. in the case of the ethylene glycol based electrolyte. Each of the above samples were anodized at room temperature, and the morphologies of the resulting nano-porous titanium oxide were studied using a Hitachi S-4700 field emission scanning electron microscope (FESEM).
For the anodization in the glycerol based electrolyte, the FESEM image showed uniform coverage of titanium oxide nanopores on the surface. The tubes appeared to be arranged in the form of bundles (
For the anodization in the ethylene glycol based electrolyte, the surface looked more uniform and the tubes seemed to be spaced more uniformly over the surface. Also the bundles kind of arrangement mentioned in case of glycerol based electrolyte was not seen. As with glycerol based electrolytes, very long tubes ˜5 μm in length were obtained at a relatively short anodization time of 7 hrs. See
The titanium metal substrate was also anodized using an organic acid, ethylenediamine tetraecetic acid (EDTA), and ammonium fluoride. The electrolyte was prepared by mixing 0.5 wt % of ammonium fluoride in a saturated solution of EDTA and water. As is discussed above, a small amount of a common complexing agent, such as EDTA, may be added to allow for the formation of improved nanopores at a faster rate. The solubility of EDTA in water is 0.5 g/Lt at room temperature. The pH of the solution was monitored to be 4.1.
The titanium metal substrate may also be anodized in a neutral solution (water and ethylene glycol) instead of the inorganic acid (H3PO4) in 0.5 wt % sodium fluoride. Anodization in water as solvent gave rise to highly disordered nanotubular structure (
16 mm discs were punched out from a stock of Ti foil (0.2 mm thick, 99.9% purity, ESPI-metals, Ashland, Oreg., USA), washed in acetone, and secured in a polytetrafluoroethelene (PTFE) holder exposing only 0.7 cm2 area to the electrolyte. Nanotubular TiO2 arrays were formed by anodization of the Ti foils in 300 mL electrolyte solution of different concentrations of various electrolytes as described below.
A two-electrode configuration was used for anodization. A flag shaped Pt electrode (thickness: 1 mm; area: 3.75 cm2) served as cathode. The anodization was carried out at different voltages. The anodization current was monitored continuously. During anodization, an ultrasonicator was used to give mobility to the electrolytes, instead of a magnetic stirrer. The frequency applied during ultrasonication was approximately 40-45 kHz, with a frequency of about 42 kHz being preferred. The total anodization time was varied from 15 minutes to 75 minutes. The anodized samples were properly washed in distilled water to remove the occluded ions from the anodized solutions and dried in oven and fabricated for photocatalysis of water.
The various conditions used for anodization were as follows:
The chemical used in this example include Phosphoric acid (H3PO4, Sigma-Aldrich, 85% in water); Sodium fluoride (NaF, Fischer, 99.5%); Potassium fluoride (KF, Aldrich, 98%); Ammonium fluoride NH4F, Fischer, 100%), Ethylenediamine tetraacetic acid (EDTA, Fischer, 99.5%), and Ethylene glycol (EG, Fischer).
The nanoporous TiO2 templates were formed by punching out 16 mm discs from a stock of Ti foil (0.2 mm thick, 99.9% purity, ESPI-metals, USA), which was washed in acetone and secured in a polytetrafluoroethylene (PTFE) bolder exposing only 0.7 cm2 area to the electrolyte. Nanotubular TiO2 arrays were formed by anodizing the Ti foils in a 300 mL electrolyte solution (0.5 M H3PO4+0.14 M NH4F) using ultrasonic waves having a frequency of approximately 40-45 kHz, with about 42 kHz being preferred. A two-electrode configuration was used for anodization. A flag shaped Pt electrode (thickness: 1 mm; area: 3.75 cm2) served as a cathode. The anodization was carried out by the applied potential varying from 5V to 20V. During anodization, instead of a magnetic stirrer, ultrasonic waves were irradiated onto the solution to give the mobility to the ions inside the solution. The anodization current was monitored continuously. After an initial increase-decrease transient, the current reached a steady state value. The anodization was stopped after 20 minutes of reaching a steady state current value in lower pH electrolytes. The anodized samples were properly washed in distilled water to remove the occluded ions from the anodized solutions and dried in air oven and further characterized by scanning electron microscope (SEM; Hitachi, S-4700). Each of the above was mixed with ultrasonic waves.
To illustrate this invention, 1 cm2 anodes, for example, were irradiated with solar spectrum of light and the cathode was uncoated Pt with 7.5 cm2 surface area and was not exposed to extra-light irradiation, apart from room light. Generally, the surface area of the experimental photo-anodes ranged from 0.7 cm2-16 cm2 and the Pt-cathode was about 10 cm2. Using scaled up equipment larger area nano-tubular titanium dioxide-anodes can be prepared.
The light source was 300 W Xenon lamp manufactured by Newport Inc AM1.5 filter was used to simulate 1-sun intensity of ˜100 mW/cm2. The incident light intensity on the anode was ˜87 mW/cm2.
The photoanodes were investigated in the following conditions:
If 1 mA/cm2 current flows for one hour, the total volume of hydrogen evolved would be more than 0.4 ml. The maximum current observed in this invention was about 2.5 mA/cm2 at 0.7 V(Ag/AgCl) potential using 1-sun light intensity. The hydrogen generation rate will be more than 10 liters/m2 area of photo-anode per hour. This rate can be increased many folds by illuminating the photo-cathode also.
It can be seen from the figure that after 120 sec of anodization, small pits start to form on the surface of the titanium (
The uniformity and pore size of the nanotubes appears to improve as the applied potential increases. To confirm the effect of applied potential on the formation of nano-porous TiO2 structures, data was collected for various applied potentials from 5V to 20V by keeping the electrolytic solution (0.5 M H3PO4+0.14 M HF) and time (2700 sec) constant, and conducting the anodization under ultrasonic waves.
The following table shows the results obtained from the band gap and photocatalysis studies.
#at external potential of 0.5 V.
The results show that ultrasonic mediated anodization gives better result than the anodization by magnetic stirring. At lower applied potential ultrasonic samples gives almost similar photoactivity to the magnetic stirred samples at higher potential.
The electrode was prepared by taking a titanium foil of 1.5 cm2 area, which was connected to copper wire through a small copper foil and conductive epoxy. It was then suspended in the electrolytic solution of 0.5M H3PO4 and 0.14M NaF in distilled water for 45 minutes and applied potential of 20V. It showed very good photo activity of 0.4 mA from each side, whereas single sided anodization used to show around 0.1 mA, without any treatment of the nanoporous titanium.
16 mm discs were punched out from a stock of Ti foil (0.2 mm thick, 99.9% purity, ESPI-metals, Ashland, Oreg., USA), washed in acetone and secured in a polytetrafluoroethelene (PTFE) holder exposing only 0.7 cm2 area to the electrolyte. Nanotubular TiO2 arrays were formed by anodization of the Ti foils in 300 mL electrolyte solution of phosphoric acid and different fluoride salts. A two-electrode configuration was used for anodization. A flag shaped Pt electrode (thickness: 1 mm; area: 3.75 cm2) served as cathode. The anodization was carried out at different voltage. The anodization current was monitored continuously. During anodization, ultrasonication was used to give mobility to the electrolytes, instead of a magnetic stirrer. The total anodization time was varied from 15 minutes to 75 minutes. The anodized samples were properly washed in distilled water to remove the occluded ions from the anodized solutions and dried in oven and fabricated for photocatalysis of water. SEM images (
The various conditions used for anodization were as follows:
As is described above, various fluorides can be used to anodize titanium under ultrasonic treatment. NaF appears to be the most desirable for quick synthesis of the material, and NH4F appears to be a better source than NaF when considered for photoelectrochemical generation of hydrogen (
The combination of ethylene glycol and ultrasonic treatment yields very high quality ordered (hexagonal) nanotubes (
As is described above, good quality nanotubes can be prepared from ethylene glycol, diluted ethylene glycol and diethylene glycol under ultrasonic media. Various fluoride sources can be used but as the solubility of NH4F in glycol media is better than the others, NH4F is a better source in organic media. It is also observed that the photoactivity of ultrasonic treated materials is higher than the conventional magnetic stirring method.
Composite electrode of the carbon modified nanotubular TiO2, which was anodized in H3PO4+NaF and then carbon doped at 650° C. for approximately 5 minutes, showed a photocurrent density of 2.75 mA/cm2 under sunlight illumination at higher anodic potentials. This photocurrent density corresponds to hydrogen evolution rate of 11 liters/hr on a photo-anode with 1 m2 area. The gases evolved in the cathode and anode compartments were analyzed separately using gas chromatography and the ratio of hydrogen to oxygen was 2:1, indicating that carbon in the carbon-modified TiO2 sample was stable. Further, the hydrogen generation was stable for more than 72 hours. The long-term test was interrupted because of the limited life of the lamp. The carbon-modified TiO2 nanotubular samples with 0.5-16.0-cm2 geometric surface areas were evaluated and the photo current density remained constant irrespective of the surface area of the anode.
The carbon modified samples, which were anodized in H3PO4+NaF and then carbon doped at 650° C. for approximately 5 minutes, showed a better photoelectrochemical behavior than the inert atmosphere annealed samples. This improved behavior could be attributed to possibly two reasons, viz, 1. band gap states introduced by carbon and 2. presence of trivalent Ti interstitials and oxygen vacancy states introduced by the reducing environments. In this study, enhanced absorption in the visible wavelength suggests that carbon modification resulted in local band gap states. High-resolution XPS studies carried out on the nitrogen/hydrogen annealed samples and carbon modified TiO2 nanotubular samples suggested presence of Ti3+ species. The presence of Ti3+ cations in the TiO2 should be associated with oxygen vacancies in order to maintain the electro-neutrality.
The TiO2 nanotubes of the invention are considered to be n-type semiconductors. Mon-Schottky results also show the n-type behavior, as shown in
The charge carrier densities, calculated based on the Mott-Schottky analyses, were in the range of 1-3×1019 cm−3 for both the carbon modified and the nitrogen-annealed nanotubular samples. The charge carrier densities of as-anodized and oxygen-annealed samples were 5×1017 and 1.2×1015 cm−3 respectively. There was no significant difference (not in the orders of magnitude) in the charge carrier densities between the dark and the illuminated conditions except for the N2-annealed specimens. The reason could be attributed to the smaller percentage of UV portion of the incident light. UV irradiation is thought to improve the hydrophilic nature of the TiO2 by creating Ti3+ states and oxygen vacancies. In this way, the charge carrier density could increase by UV light illumination. If oxygen vacancies were produced during annealing in nitrogen or hydrogen atmosphere, the charge carrier density would be expected to increase, and this expected increase in charge density after the annealing treatments could be attributed to the oxygen vacancies introduced after annealing in the inert or reducing environments. However, the methods of the invention instead showed a decrease in the charge carrier density upon light illumination, and the flat band potentials did not change significantly. In addition, it was shown that the measured photo current density was not directly related to the charge carrier densities of the nanotubes, because the photo current density generated by the O2-annealed specimens (˜1.4 mA/cm2) was significantly higher than that of the as-anodized specimens in spite of the considerably lower charge carrier density. The presence of different phases, such as amorphous, anatase, and Futile, appear to influence the photo activity more than the charge carrier density.
0.001 to 0.01 M CdCl2+0.0001 to 0.0005 M TeCl4 (for coating CdTe) or 0.001 to 0.01 M CdCl2+0.001 to 0.01 M ZnCl2+0.0001 to 0.0005 M TeCl4 (for coating CdZnTe) salts were dissolved in 1 liter of propylene carbonate. All salts are reagent grade and anhydrous. The electrolyte was heated to 80-140° C., with a temperature of about 130° C. being most preferred.
A three electrode configured electrochemical cell was used for deposition of Cd—Te and Cd—Zn—Te nanowires/thinfilms. Advantageously, the invention deposits Cd—Zn—Te nanowires/thinfilms in a single step. As a non-aqueous solvent is used for electrodeposition, moisture and oxygen are controlled less than 1 ppm in the electrochemical cell. This was ensured by carrying out all the activities such as preparation of the electrolyte and electrodeposition inside a dry-controlled atmosphere chamber. A glove box purged with dry, high-purity argon gas is used for this purpose. The dry and oxygen free atmosphere is ensured by measuring the burning life of a perforated 25 W filament light bulb. If the bulb burns for more than two hours exposing the filament to the atmosphere of the glove box, the oxygen and moisture contents of the chamber are assumed to be less than 1 ppm.
Electrodeposition of CdTe and CdZnTe are carried out by pulsing the potentials between pre-determined deposition potentials and 200 mV anodic to open circuit potentials. These potentials were determined from the cyclic voltammetry studies. The deposition potentials ranged from −0.3 to −1.2 V with reference to Cd wire reference. Anodic potentials ranged from 0.1 to 0.5 V with reference to Cd wire. The pulsing (deposition) time ranged from 0.1 to 1 second. The background (anodic potential) time ranged from 2-10 seconds. The total cycle time of deposition process varies from 45 minute to 2 hour depending on the final thickness of the nanowire coating. The electrodeposition was carried out at 80-140° C.
The reference electrode used is a pure Cadmium wire of 1 mm diameter and 200 mm long immersed in propylene carbonate solution containing 0.01 M CdCl2 salt. The reference electrode compartment has a 10 mm diameter and 150 mm long glass tube with type E fine pores ceramic fritted end. The counter electrode is a flag type Pt foil with 10 cm2 area.
Electrodeposition of CdTe/CdZnTe on anodized Ti samples resulted in formation of nanowires nucleating from bottom of the nanotubes of TiO2. On Pt foils, electrodeposition resulted in thin films of CdTe/CdZnTe.
Energy Dispersive Analysis of X-Ray results indicated the composition of the CdZnTe nanowires to be 44 atomic % Cd, 8 at % Zn and 48 at % Te. CdTe coatings contained stoichiometric amounts of Cd and Te.
After the electrodeposition the coating is thoroughly washed in anhydrous methyl alcohol and dried. Then, the coating is annealed at 400-500° C. in flowing high purity argon gas atmosphere for 1-3 hours. After annealing, the sample is ready as photo-cathode.
CdTe and CdZnTe compound semiconductors are used widely in infra-red (IR), X-ray and gamma ray radiation detection applications and in solar cell panels. CdZnTe is considered more advantageous than CdTe in radiation detection because of wider band gap and higher resistivity, which renders low noise level. Preparation of CdZnTe in the form of nanowire arrays facilitates the use of large area detectors with minimized trap centers.
Therefore, a single step electrochemical method of synthesis of cadmium zinc telluride (CdZnTe) nanowires using nanoporous TiO2 template was developed using propylene carbonate (PC) as a non-aqueous electrolyte. Pulsed cathodic and anodic potentials resulted in growth of nanowires of CdZnTe with p-type semiconductivity. More negative cathodic potentials increased the Zn content. Increase in Zn content increased the charge carrier density of the nanowires. Annealing of the material at 350° C. for 1 h decreased the charge carrier density to the order of 1015 cm−3. Cyclic Voltammogram studies were carried out to understand the growth mechanism of CdZnTe. EDAX and XRD measurements indicated formation of a compound semiconductor with a stoichiometry of Cd1-xZnxTe, where x varied between 0.04 and 0.2. Variation of the pulsed-cathodic potentials could modulate the composition of the CdZnTe. More cathodic potentials resulted in increased Zn content. The nanowires showed an electronic band gap of about 1.6 eV. Mott-Schottky analyses indicated p-type semiconductor properties of both as-deposited and annealed CdZnTe materials. Increase in Zn content increased the charge carrier density. Annealing of the deposits resulted in lower charge carrier densities, in the order of 1015 cm−3.
The titanium dioxides used were prepared by anodizing high purity titanium foils (0.1 mm thick, 99.999 wt % purity, ESPI-metals, Ashland, Oreg., USA). The surface area exposed for anodization was around 0.7 cm2. The anodization was carried in a solution of 0.5 M phosphoric acid, 0.14 M sodium fluoride and pH of 2.0. Anodization was carried out at 20 V and 25° C. for about 45 minutes. The resultant product obtained was nanoporous titanium dioxide with a pore diameter of 100 nm and pore length of 400-500 nm.
The non-aqueous medium used for deposition was propylene carbonate (PC). Propylene carbonate was chosen as a solvent because of its higher dielectric strength (65), higher dipole moment and charge acceptor number. Cyclic voltammetry (CV) studies were carried out to understand the growth mechanism's of CdZnTe. Both platinum and anodized nanoporous TiO2 were used as electrodes during cyclic voltammetry. The following electrolytes were used for cyclic voltammetry (CV) studies:
Both the CV and electrochemical deposition of CdZnTe nanowires were carried out in a three-electrode cell at 95±2° C. CV tests were carried out using both Pt and nanoporous TiO2 substrates at a potential sweep rate of 10 mV/s. 5 cm2 platinum foil in the shape of a flag was used as a counter electrode. A pure cadmium wire immersed in PC solution saturated with CdCl2 and contained in fritted end glass tube acted as a reference electrode. Here after this reference electrode will be referred as a cadmium wire reference electrode. Anodized titanium dioxide sample was used as the template for nanowire growth. 25×10−3 M NaClO4 was used as the supporting electrolyte. All depositions were carried out in a controlled atmosphere inside a glove box (Labconco, Model 50600-00). Ultra high purity argon was used as the inert atmosphere. The oxygen and moisture contents of the glove box were controlled at low levels so that a pierced 25 W incandescent bulb could burn at least for an hour inside the glove box environment, Nanowires of CdZnTe were deposited on the nanoporous TiO2 template by pulsing the potentials. A typical pulsed-potentials cycle contained two cathodic, two anodic and one open circuit potential, as depicted in
Scanning electron microscopy (SEM) and glancing angle X-ray powder diffraction (XRD) measurements were used to characterize the nanowires of CdZnTe. The chemical compositions of the nanowires were characterized by X-Ray energy dispersive analysis (EDAX). Further resistance measurements of the deposited film were also measured.
A Mott-Schottky analysis was carried out on the sample to study the electronic properties of the deposited films in annealed and as deposited conditions. The analysis was carried out in a 0.5 M sodium sulfate solution by adjusting pH to 2.0. Potential of the sample was scanned from +1 to −1 V with a scan step rate of −50 mV/s. The frequency used was 3000 Hz. The interfacial capacitance C was calculated by the system software (Z-Plot, Solartron) using the relation C=(−(1+D2)*Z″2πf)−1, in case of parallel capacitance circuit assuming presence of surface states at oxide-semiconductor interface or C=−1/(2πfZ″) in case of series capacitance; where, D=Z′/Z″, Z′=real part of impedance, Z″ is the imaginary part of the impedance and f is the frequency. Capacitance C is related, in turn, to the charge carrier density, NA, by the following equation:
Where e=elementary electron charge (positive for n-type and negative for p-type), ε0=permittivity in vacuum, ε=dielectric constant (11 for CdZnTe and 86 for TiO2), NA=charge carrier density, E=applied potential, EFB=flat band potential, K=Boltzmann constant, T=temperature.
According to Equation 1, the slope of 1/C2 vs. potential plot gives the charge carrier density, NA, from the relation:
Where m is the slope of the Mott-Schottky plot in the region of interest. A positive slope indicates n-type semiconductor and a negative slope p-type. The intercept 1/C2=0 on the potential axis gives the flat band potential EFB. All potentials were measured with respect to the Ag/AgCl reference electrode in saturated KCl.
This is about 0.7V more positive to that Cd2+/Cd reaction. Therefore, the cathodic currents can be attributed to the reduction of Te from the non-aqueous solution. Similarly anodic peaks occurred more positive to 0.7V which corresponded to the reverse reactions: Te2−→Te+2e−, and Te→Te4++4e−.
In CdTe solutions a cathodic current was observed at potentials more negative to +0.9V and at +0.4 V a change in the slope of cathodic current occurred as shown in
Te+2e−→Te2−
Cd2++Te2−→CdTe
These reactions suggest that already reduced Te species act as sites for CdTe deposition. At potentials more negative to 0 V, the Cd2+ ions also compete for electrons for electro reduction reaction of Cd2++2e−→Cd. Therefore there was a current plateau with increase in potential between 0 and −0.2V. The plateau region indicates slower kinetics of deposition at these potentials, which could be attributed to the competition for adsorption sites for deposition of either CdTe or Cd. At more negative potentials, the cathodic increased with steeper slope which could be attributed to additional deposition of Cd along with CdTe.
Reversing the CV sweep in anodic direction resulted in two faint peaks at +0.26V and +0.5V. The first peak was similar to the anodic stripping of Cd observed in pure Cd solution and the second peak could be labeled as the dissolution of Cd from the CdTe compound lattice. Third anodic peak at more positive potential than 0.66V is attributed to the stripping of Te.
Cyclic voltammetry in CdZnTe solutions was more or less similar to the results of CdTe as shown in
The peak current potentials were shifted positively as compared to that of CdTe stripping. Significant similarities were observed between CV of CdTe and ZnTe as shown in
When the CV was carried out on anodized TiO2 surface, not much difference was observed with the behavior of cathodic current waves. However, the anodic behavior was quite different with TiO2 nanoporous surface. In TiO2 surface only one anodic peak was observed, which occurred at 0.34V vs. Cd. This peak can be similar to the first anodic peak observed on Pt surface at 0.39V.
In order to understand the origin of the anodic strip, CV was carried out in CdZnTe solutions on TiO2 surface by switching the scan directions at various potentials. When the forward (Cathodic potential) was switched (reversed) after reaching +0.3V and −0.4V, no specific anodic peak current was observed as shown in
When the potential was switched at 0.3V, only Te deposition was observed as shown in
The observations are further supported by considering the anodic scans after different holding times at different constant cathodic potentials in CdZnTe solution.
The nanowires deposited were analyzed by XRD in as deposited as well as annealed condition.
Thus, It was observed that samples anodized in phosphate solutions showed relatively better optical absorption as compared to the samples anodized in nitrate solutions. It is envisaged that anodization in 0.5 M H3PO4+0.14 M NaF solution results in adsorption of phosphate ions at the outer walls of the TiO2 nanotubes and subsequent annealing in low oxygen pressure could cause diffusion of phosphorous species in the TiO2 lattice creating sub-band gap or surface states.
This application claims priority to U.S. Provisional Patent Application No. 60/715,163, filed Sep. 9, 2005, U.S. Provisional Patent Application No. 60/749,639, filed Dec. 13, 2005, U.S. Provisional Patent Application No. 60/750,335, filed Dec. 15, 2005, and U.S. Provisional Patent Application No. 60/794,853, filed Apr. 26, 2006, the disclosures of which are hereby incorporated herein by reference in their entirety.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US06/35252 | 9/11/2006 | WO | 00 | 12/10/2008 |
| Number | Date | Country | |
|---|---|---|---|
| 60715163 | Sep 2005 | US | |
| 60749639 | Dec 2005 | US | |
| 60750335 | Dec 2005 | US | |
| 60794853 | Apr 2006 | US |
