Tantalum vanadate nanorods and methods of their make and use

Abstract

Tantalum vanadate (TaVO.sub.5) forms into nanostructures, particularly nanorods, which may range in length between 100 and 600 nm with a length:width ratio between 20:1 to 50:1, and, as a bulk material, have a bandgap of 1.5 to 3.00 eV. Such nanostructures may be prepared by the hydrothermal method.

Claims

1. A method of preparing a TaVO.sub.5 nanorod, the method comprising: heating, at a temperature in a range of from 150 to 300 C., for a time period in a range of from 5 to 72 hours, a mixture comprising a metavanadate, an acid, and a soluble tantalum compound; forming TaVO5 nanorods having an average length-to-width ratio of at least 15:1, wherein the acid comprises at least one selected from the group consisting of formic acid, acetic acid, glycolic acid, propionic acid, butyric acid, citric acid, methanesulfonic acid, oxalic acid, malic acid, glutaric acid, maleic acid, fumaric acid, phenol, benzoic acid, salicylic acid, and tartaric acid, and/or wherein the acid and the metavanadate comprise H.sub.3VO.sub.4.

2. The method of claim 1, wherein the metavanadate comprises NH.sub.4VO.sub.3.

3. The method of claim 1, wherein the acid comprises formic acid.

4. The method of claim 1, wherein the soluble tantalum compound comprises a tantalum halide.

5. The method of claim 1, wherein the soluble tantalum compound comprises TaCl.sub.5.

6. The method of claim 1, wherein the time period is in a range of from 12 to 36 hours, and wherein the temperature is in a range of from 170 to 220 C.

7. A composition, comprising: a plurality of nanorods, wherein at least 75 wt. % of a total weight of the nanorods is TaVO.sub.5.

8. The composition of claim 7, wherein the plurality has an average length in a range of from 100 to 600 nm.

9. The composition of claim 7, having a bandgap in a range of from 1.5 to 3.00 eV.

10. The composition of claim 7, wherein the plurality has an average length:width ratio in a range of from 20:1 to 50:1.

11. The composition of claim 7, wherein the plurality has a BET surface area in a range of from 40 to 100 m.sup.2/g.

12. The composition of claim 7, wherein the plurality has an average pore volume in a range of from 0.170 to 0.210 cm.sup.3/g.

13. The composition of claim 7, wherein the plurality has an average pore size in a range of from 8 to 15 nm.

14. The composition of claim 7, comprising at least 90 wt. % of the plurality, relative to a total composition weight.

15. The composition of claim 7, consisting essentially of the plurality.

16. A nanorod, comprising: TaVO.sub.5 in at least 75 wt. % of total nanorod weight.

17. The nanorod of claim 16, comprising at least 90 wt. % of TaVO.sub.5, relative to the total nanorod weight.

18. The nanorod of claim 16, having an energy-dispersive x-ray spectroscopy pattern including a predominant peak in a range of from 4.7 to 5.1 KeV, having a first intensity, a secondary peak in a range of from 1.5 to 1.9 KeV, having a second intensity, and a tertiary peak in a range of from 0.3 to 0.6, having a third intensity, the intensities being integrals of areas under respective peaks, wherein the first intensity is in a range of from 3 to 6-fold that of the third intensity, and wherein the first intensity is in a range of from 2 to 3-fold that of the second intensity.

19. A photocatalyst, comprising the nanorod of claim 16.

20. A heterojunction, comprising the nanorod of claim 16.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

(2) FIG. 1A to D show scanning electron microscope (SEM) images of TaVO.sub.5 as synthesized within the scope of the invention using formic acid at various zooms;

(3) FIG. 2A shows an SEM image of TaVO.sub.5 as synthesized within the scope of the invention using formic acid;

(4) FIG. 2B shows an SEM image of TaVO.sub.5 as synthesized within the scope of the invention using lactic acid;

(5) FIG. 2C shows an SEM image of TaVO.sub.5 as synthesized within the scope of the invention using hydrochloric acid;

(6) FIG. 3 shows an x-ray diffraction (XRD) pattern of TaVO.sub.5 nanorods according to the invention;

(7) FIG. 4 shows an energy-dispersive x-ray spectroscopy (EDX or EDS) pattern of TaVO.sub.5 nanorods according to the invention;

(8) FIG. 5A to C show respective x-ray photoelectron spectroscopy (XPS) patterns of Ta, V, and O, in TaVO.sub.5 nanorods according to the invention;

(9) FIG. 6 shows a diffuse reflectance UV-vis spectrum of TaVO.sub.5 nanorods according to the invention;

(10) FIG. 7 shows N.sub.2 adsorption-desorption isotherms of TaVO.sub.5 nanorods according to the invention;

(11) FIG. 8 shows the photocatalytic degradation of rose bengal using TaVO.sub.5 nanorods according to the invention; and

(12) FIG. 9 shows the evolution of absorption spectra of rose bengal over time in the presence of TaVO.sub.5 nanorods according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(13) Aspects of the invention may provide compositions comprising: a plurality of nanorods, wherein at least 75, 80, 85, 90, 91, 92, 92.5, 93, 94, 95, 96, 97, 97.5, 98, 99, 99.1, 99.5, or 99.9 wt. % of a total weight of the nanorods is tantalum vanadate (TaVO.sub.5). Inventive compositions may comprise at least 75, 80, 85, 90, 92.5, 95, 97.5, 98, 99, 99.1, 99.5, or 99.9 wt. % of the plurality of nanorods, relative to a total composition weight. Inventive compositions may consist essentially of the plurality of nanorods, meaning that for certain applications compositions will limit further contents to those which do not diminish the bandgap to less than 75, 85, or 90% of 1.5 eV, and/or that such further components do not cause to nanorods to lose more than 50, 75, or even 90% of the catalytic efficiency of pure TaVO.sub.5-comprising nanorods, and/or that such further components cause a loss of the nanorod morphology.

(14) Inventive compositions generally have a bandgap in a range of from 1.5 to 3.00, 1.6 to 2.8, 1.7 to 2.6, 1.8 to 2.5, 1.9 to 2.4, 2.0 to 2.3, or 2.1 to 2.2 eV. Upper and lower bandgap endpoints may be any of the prior values or may be at least 1.55, 1.65, 1.75, 1.85, 1.95, 2.05, or 2.15 eV, and/or no more than 3.05, 2.9, 2.75, 2.7, 2.55, 2.45, 2.33, or 2.25 eV.

(15) The plurality of nanorods may have an average length in a range of from 50 to 1000, 100 to 600, 150 to 550, 200, to 500, 275 to 375, 250 to 350, or 300 to 400 nm. The plurality of nanorods may have an average length:width (L:W) ratio, which may also be considered a length to cross-sectional diameter ratio or aspect ratio, in a range of from 20:1 to 50:1, 25:1 to 40:1, or 30:1 to 40:1. The average L:W ratio of inventive rods may be any of these ratios or at least 15:1, 17.5:1, 22.5:1, 27.5:1, or 32.5:1, and/or no more than 100:1, 75:1, 60:1, 45:1, 40:1, or 35:1. The nanostructures, e.g., nanorods, may have a cylindrical, square, hexagonal, and/or octagonal prismic structure, e.g., with a circular, ovular, square (non-square rectangular), rhombic, trapezoidal, triangular, pentagonal, hexagonal, and/or octagonal cross-section, orthogonal to the length of extension of the nanostructure. The nanostructure may also be spherical, plate-like, and/or needle-like, i.e., with an extended conical shape, particularly wherein one end has a larger cross-section than the other. In addition to these, or separately from them, multi-pronged morphologies are also considered, such as star-shaped, propeller-shaped, cross-shaped, x-shaped, and/or fractal-shaped morphologies.

(16) Nanorods within the scope of the invention may have a variety of lengths and diameters, though generally dimensionally under 1,000 nm. Typical average nanorod lengths may be at least 10, 25, 50, 100, 150, 200, 225, 250, 275, 300, 325, or 350 nm, and/or generally no more than 1,000, 750, 600, 550, 500, 475, 450, 425, 400, or 375 nm, in any combination. The standard deviation from the average length, which may depend upon the method of synthesis and/or the rate of crystallization, may be in a range of from 10 to 250, 25 to 200, 33 to 175, or 50 to 150 nm.

(17) The plurality of nanorods may have a BET surface area in a range of from 40 to 100, 45 to 95, 50 to 90, 55 to 85, 60 to 80, 62.5 to 77.5, 65 to 75, 67.5 to 72.5, or 69 to 71 m.sup.2/g. While not limited in theory, the BET surface area may be any of these or at least 35, 42.5, 47.5, 52.5, 57.5, 61, 62, 63, 64, 66, 67, 68, or 69 m.sup.2/g, and/or no more than 92.5, 82.5, 77, 76, 75, 73, m.sup.2/g. In addition or separately, the plurality of nanorods may have an average pore volume in a range of from 0.170 to 0.210, 0.175 to 0.205, 0.1775 to 0.2025, 0.180 to 0.200, 0.185 to 0.1975, 0.186 to 0.197, 0.187 to 0.196, 0.188 to 0.195, 0.189 to 0.1945, 0.190 to 0.194, 0.191 to 0.193 cm.sup.3/g. While not limited in theory, the average pore volume may be any of these or at least 0.165, 0.172, 0.174, 0.176, 0.177, 0.178, 0.179, 0.1825, 0.184, 0.1875, or 0.192 cm.sup.3/g, and/or no more than 0.215, 0.204, 0.203, 0.201, 0.199, 0.198, 0.1965, 0.1955, or 0.1935 cm.sup.3/g. In addition or separately, the plurality of nanorods may have an average pore size in a range of from 8 to 15, 9 to 14, 10 to 13, 11 to 12 nm. While not limited in theory, the average pore volume may be any of these or at least 7.5, 8.5, 9.5, 10.5, 10.75, or 11.5 nm, and/or no more than 20, 18, 17.5, 16, 13.5, or 12.5 nm.

(18) Inventive nanorod(s) may comprise TaV-oxides, particularly TaVO.sub.5, in at least 50, 75, 80, 85, 90, 92.5, 95, 97.5, 98, 99, 99.5, or 99.5 wt. % of total nanorod weight. Any of these inventive nanorods may be included in one or more photocatalyst(s) and/or heterojunction(s), either alone or in combination with other components.

(19) Methods of preparing a TaVO.sub.5 nanorod, may comprise: heating a mixture comprising metavanadate and tantalum ions, preferably in solution, particularly in a hydrothermal process, at a temperature in a range of from 150 to 300, 160 to 280, 165 to 260, 170 to 240, 172.5 to 220, 175 to 210, 177.5 to 200 C. The reaction may be conducted at any of these temperatures for a time period in a range of from 5 to 72, 10 to 60, 12 to 56, 15 to 50, 16 to 48, 18 to 36, or 20 to 28 hours. Reaction time periods may be in a range of from 12 to 36, 18 to 30, or 22 to 26 hours, optionally along with a temperature in a range of from 170 to 220, 172 to 212, 174 to 202, 176 to 192, or 178 to 182 C. or at about 180 C., and will generally be conducted above 120, 135, 150, 160, or 175 C.

(20) The reaction mixture may comprise a metavanadate, an acid, and a soluble tantalum compound. The fluidizing material, particularly a solvent, generally comprises at least 50, 75, 80, 90, 95, or 99% water, particularly distilled or deionized water, though cosolvents or alternates, such as alcohols, DMSO, DMF, acetonitrile, or ionic liquids may be used. The reaction is generally conducted in an autoclave, or some variety of a pressure reactor. The acid in the reaction may be inherent, e.g., acidified vanadate (H.sub.3VO.sub.4) in which case the vanadate and acid would be unified, or may comprise formic acid, acetic acid, glycolic acid, propionic acid, butyric acid, citric acid, methanesulfonic acid, oxalic acid, malic acid, glutaric acid, maleic acid, fumaric acid, phenol, benzoic acid, salicylic acid, and/or tartaric acid, particularly formic acid. The metavanadate used in inventive methods may comprise ammonium metavanadate (NH.sub.4VO.sub.3), and/or the acid may comprise formic acid (HCOOH), and/or the soluble tantalum compound may comprise a tantalum halide, i.e., chloride, bromide, or iodide, particularly tantalum pentachloride (TaCl.sub.5). An exemplary method of preparing TaVO.sub.5 nanorods within the invention may comprise heating a solution comprising ammonium metavanadate (NH.sub.4VO.sub.3), formic acid (HCOOH), and tantalum pentachloride (TaCl.sub.5) at a temperature between 150 to 300 C. or 120 to 280 C.

(21) Acids useful in the synthesis of nanorods according to the invention may be organic acids with molecular weights under 150, 100, 75, or 50 g/mol. Acids useful in the synthesis of nanorods according to the invention may include formic acid, acetic acid, glycolic acid, propionic acid, butyric acid, citric acid, tartaric acid, methanesulfonic acid, oxalic acid, malic acid, glutaric acid, maleic acid, fumaric acid, phenol, benzoic acid, salicylic acid, and the like. Formic acid is presently believed to be particularly useful in forming TaVO.sub.5 nanorods. In certain embodiments lactic and hydrochloric acids may be used, though these have initially been seen to inhibit nanorod formation under similar synthetic circumstances. Inorganic acids, such as hydrobromic acid, hydroiodic acid, sulfuric acid, sulfurous acid, phosphoric acid, perchloric acid, chloric acid, and the like, may likewise be useful.

(22) While they need not, the nanostructures, particularly nanorods and/or nanotubes, produced according to the invention may comprise at least 50, 60, 75, 85, 90, 92.5, 95, 97.5, 98, 99, 99.5, 99.9, 99.95, or 99.99 wt. % of a nanostructure total weight as TaVO.sub.5. An aspect of the invention may provide a nanotube, comprising an oxide of TaV in any amount discussed herein relative to the total nanostructure mass, made by any hydrothermal method described herein, particularly using formic acid, particularly using ammonium vanadate, particularly using tantalum chloride.

(23) The nanostructures, e.g., nanorods and/or nanotubes, may comprise no more than 15, 10, 7.5, 5, 4, 3, 2, 1, or 0.5 wt. %, relative to the total nanostructure weight, of Cu, Pd, Pt, Gd, Y, Zr, W, Hf, Ti, Zn, Mn, Mo, Nb, and/or Bi. Concretely, the nanostructures may comprise no more than 40, 33, 25, 20, 15, 10, 7.5, 5, 4, 3, 2, 1, or 0.5 wt. %, relative to the total nanostructure weight, of Cu, Pd, and Pt, and/or Gd and Y, and/or Zr, W, and Hf, and/or Ti, particularly TiO.sub.2, and/or Zn, particularly ZnO, and/or Mn, and/or Mo, and/or Nb or Bi.

(24) Put another way, relative to their total weight, the nanostructures, e.g., nanorods and/or nanotubes, may comprise no more than 15, 10, 7.5, 5, 4, 3, 2, 1, or 0.5 wt. % of Cu. Additionally or alternately, the inventive nanostructures may comprise no more than 15, 10, 7.5, 5, 4, 3, 2, 1, or 0.5 wt. % of Pd. Additionally or alternately, the inventive nanostructures may comprise no more than 15, 10, 7.5, 5, 4, 3, 2, 1, or 0.5 wt. % of Pt. Additionally or alternately, the inventive nanostructures may comprise no more than 15, 10, 7.5, 5, 4, 3, 2, 1, or 0.5 wt. % of Gd. Additionally or alternately, the inventive nanostructures may comprise no more than 15, 10, 7.5, 5, 4, 3, 2, 1, or 0.5 wt. % of Y. Additionally or alternately, the inventive nanostructures may comprise no more than 15, 10, 7.5, 5, 4, 3, 2, 1, or 0.5 wt. % of Zr. Additionally or alternately, the inventive nanostructures may comprise no more than 15, 10, 7.5, 5, 4, 3, 2, 1, or 0.5 wt. % of W. Additionally or alternately, the inventive nanostructures may comprise no more than 15, 10, 7.5, 5, 4, 3, 2, 1, or 0.5 wt. % of Hf. Additionally or alternately, the inventive nanostructures may comprise no more than 15, 10, 7.5, 5, 4, 3, 2, 1, or 0.5 wt. % of Ti, esp. TiO.sub.2. Additionally or alternately, the inventive nanostructures may comprise no more than 15, 10, 7.5, 5, 4, 3, 2, 1, or 0.5 wt. % of Zn, esp. ZnO. Additionally or alternately, the inventive nanostructures may comprise no more than 15, 10, 7.5, 5, 4, 3, 2, 1, or 0.5 wt. % of Mn. Additionally or alternately, the inventive nanostructures may comprise no more than 15, 10, 7.5, 5, 4, 3, 2, 1, or 0.5 wt. % of Mo. Additionally or alternately, the inventive nanostructures may comprise no more than 15, 10, 7.5, 5, 4, 3, 2, 1, or 0.5 wt. % of Nb. Additionally or alternately, the inventive nanostructures may comprise no more than 15, 10, 7.5, 5, 4, 3, 2, 1, or 0.5 wt. % of Bi.

(25) Inventive nanotubes or nanorods may have less than 25, 15, 10, 5, 2.5, 1, or 0.1 wt. % Li ion intercalation, though some uses may call for particularly these amounts, or more, of battery ion intercalation, such as Li, NiCd, Ni-M-H, etc.

(26) While they need not, nanostructures within the scope of the invention may comprise no more than 33, 20, 15, 10, 7.5, 5, 4, 3, 2, 1, or 0.5 wt. % of any metal besides Ta and V.

(27) TaVO.sub.5 nanostructures/nanorods within the scope of the invention, including the spherical or plate-like morphologies (and even amorphous), may have a narrow band gap useful in photocatalysis, e.g., for producing H.sub.2 by splitting H.sub.2O, degrading pollutants in the air and/or water, or formation for heterojunctions between other material(s) and could act as useful material to promote (photo)catalysis. Relevant pollutants which may be degraded by nanostructures according to the invention may include dyes, soaps, partially combusted hydrocarbons, oils, crude hydrocarbons, proteins, amino acids, amines, nitrogen-containing air pollutants, sulfur-containing air pollutants. As such, the TaVO5 nanostructures reported herein may be applied as coatings onto surfaces, such as canal sides or bases, roofs, building sides, stairs, ship and other watercraft surfaces and/or decks, aircraft hulls and/or wings and/or rotors. The metals of the inventive nanostructures may be provided as isotopes, such as Ta.sup.180, to use the nanostructures in irradiation therapies.

Example 1

(28) Preparation of TaVO.sub.5 (Tantalum Vanadate) Nanorods: 0.175 g NH.sub.4VO.sub.3 (Ammonium metavanadate) was weighed and added into 25 mL of deionized water in a PTFE lined autoclave; 0.5 mL of formic acid (HCOOH) was added dropwise to this solution. After addition of 0.179 g TaCl.sub.5 (Tantalum pentachloride), the mixture was stirred for 15 minutes and transferred into stainless steel autoclave, then heated at 180 C. for 24 hours. After 24 hours of reaction, the product was naturally cooled at room temperature. The cooled product was then centrifuged, washed with water and ethanol several times, and dried at 60 C. overnight. Spectroscopic and spectrometric analysis was then conducted on the product.

(29) A field emission scanning electron microscope (Hitachi, S-4800) was used to observe the morphologies of TaVO.sub.5 and to determine the elemental composition by X-ray energy-dispersive spectrometer. X-ray powder diffraction (XRD, Rigaku, Japan) analysis of TaVO.sub.5 was performed with a X-ray diffractometer, using copper radiation (=1.5418 ). An ultrahigh vacuum VG MultiLab 2000 X-ray photoelectron spectrometer was used to record the X-ray photoelectron spectra. A UV-Vis diffuse reflectance spectrum of the TaVO.sub.5 nanorods made according to Example 1 was recorded on a diffuse reflectance UV-Vis spectrophotometer (JASCO V-750). A Micromeritics ASAP 2020 PLUS nitrogen adsorption apparatus (USA) was employed for BET surface area determination. Before analysis, samples were degassed at 180 C., and the surface area was determined by using N.sub.2 adsorption data in the relative pressure (P/P0) range of 0.05 to 0.3.

(30) The hydrothermal synthetic approach in Example 1 above, for preparing TaVO.sub.5 surprisingly yielded TaVO.sub.5 nanorods. The TaVO.sub.5 nanorods of Example 1 had lengths of 300 to 400 nm, though reaction conditions may be modulated to obtain and vary characteristics of the nanorods of TaVO.sub.5, or alternate morphologies. XRD, EDX and XPS analysis was employed to confirm the formation of TaVO.sub.5. The BET surface area of TaVO.sub.5 nanorods from Example 1 was observed to be 69.78 m.sup.2/g, with a pore size of 11.53 nm, and a pore volume 0.192 cm.sup.3/g. The band gap of the TaVO.sub.5 nanorod material was found to be in a range of from 2.1 to 2.2 eV, which is an ideal band gap for visible light driven photocatalysis. Due to narrow band gap, these TaVO.sub.5 nanorod can also be used as heterojunctions between other nanomaterial for photocatalysis or catalytic applications. Another potential application of TaVO.sub.5 according to the invention was evaluated for photocatalytic degradation of rose bengal (4,5,6,7-tetrachloro-2,4,5,7-tetraiodofluorescein). Results indicate that TaVO.sub.5 is efficient and fast photocatalyst for the degradation for rose bengal and photocatalytic degradation was achieved within 45 minutes.

(31) Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views.

(32) In reference to FIG. 1A to 1D, show scanning electron microscope (SEM) images of TaVO.sub.5 as synthesized within the scope of the invention using formic acid at various scales, with FIG. 1A showing a 3 m scale, FIG. 1B showing a 1 m scale, FIG. 1C showing a 400 nm scale, and FIG. 1D showing a 300 nm scale. The nanorods illustrated in FIG. 1A to 1D illustrate morphologies arising from the synthetic method described in Example 1, using formic acid. FIG. 2A again shows the SEM image of TaVO.sub.5 as synthesized within the scope of the invention using formic acid. FIG. 2B shows an SEM image of TaVO.sub.5 as synthesized within the scope of the invention using lactic acid, illustrating an alternate morphology, and FIG. 2C shows the results using hydrochloric acid. The images in FIGS. 2B and 2C can be compared to the image in FIG. 2A.

(33) Table 1 describes the relative abundance and/or relative ratio of elements such as tantalum, vanadium and oxygen in the tantalum nanorods as obtained from the EDX spectrum.

(34) TABLE-US-00001 TABLE 1 Elements Weight % Atomic % O 39.35 76.19 V 30.75 18.70 Ta 29.89 5.12

(35) FIG. 3 shows an x-ray diffraction (XRD) pattern of TaVO.sub.5 nanorods preparable by the method in Example 1, while FIG. 4 shows an energy-dispersive x-ray spectroscopy (EDX or EDS) pattern of the same. FIG. 5A to C show respective x-ray photoelectron spectroscopy (XPS) patterns of Ta, V, and O, in TaVO.sub.5 nanorods obtainable according to the method in Example 1, while FIG. 6 shows a diffuse reflectance UV-vis (DR UV-vis) spectrum of these TaVO.sub.5 nanorods. FIG. 7 shows N.sub.2 adsorption-desorption isotherms of these TaVO.sub.5 nanorods.

(36) FIG. 8 shows the photocatalytic degradation of rose bengal (4,5,6,7-tetrachloro-2,4,5,7-tetraiodofluorescein) using TaVO.sub.5 nanorods according to the invention indicating a reduction of detected concentration within 15 minutes. The photocatalytic activity of the TaVO.sub.5 nanorods was evaluated through photocatalytic degradation of rose bengal under UV-visible light irradiation using Xenon lamp (300 W) as light source for 45 min. 50 mg of TaVO.sub.5 was dispersed in 50 mL of aqueous solution of rose bengal (10 mg/L). In order to ensure the adsorption-desorption equilibrium between catalyst and dye, solution was stirred in the dark for 30 min and then illuminated under Xenon lamp (300 W). After a 15 minute time interval, 4 mL of the suspension was collected and centrifuged to remove TaVO.sub.5 nanorod catalyst. The concentration of rose bengal was assessed using UV-Visible spectrophotometer (JASCO V-750) by measuring the absorbance at 544 nm. The degradation efficiency was calculated according to Equation 1:
Degradation rate (%)=(C.sub.0C)/C.sub.0100Eq. 1,
wherein C.sub.0 is the initial concentration of the rose bengal, and C is the time-dependent concentration of rose bengal upon irradiation. FIG. 9 shows the evolution of absorption spectra of rose bengal over time in the presence of TaVO.sub.5 nanorods according to the invention, wherein it can be observed that the 4,5,6,7-tetrachloro-2,4,5,7-tetraiodofluorescein UV-Vis absorbance is diminished with time.

(37) Numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.