Bismuth oxyhalide compounds useful as photocatalysts

Abstract

Mixed chloride-bromide bismuth oxyhalide compounds, with the molar ratio chloride:bromide being equal to or greater than 1:1, in the form of microspheres exhibiting flower-like surface morphology, are disclosed. Processes for preparing the compounds, formulations of the compounds and a method for purifying water using said compounds are also disclosed.

Claims

1. Mixed chloride-bromide bismuth oxyhalide compounds having the formula BiOCl.sub.yBr.sub.1-y wherein y is between 0.60 and 0.95, in the form of microspheres exhibiting flower-like surface morphology, said microspheres being characterized by the presence of individual thin sheets arranged radially in a petal-like manner, wherein two or more adjacent individual thin sheets are interconnected to form cells or channels which open onto the external surface of said microspheres, as evidenced by images produced by scanning electron microscopy, wherein said compounds are photocatalysts and wherein the time required for said photocatalysts to completely decompose Rhodamine B in solution is less than 18 minutes when said photocatalysts are added in an amount of 150 mg to 200 ml water containing 15 ppm Rhodamine B and irradiated with a 300 W Xenon lamp at wavelength of 422-740 nm, said lamp being located at a distance of 10 cm from said solution, and wherein the decomposition of the Rhodamine is determined by a UV-Vis spectrophotometer.

2. The mixed chloride-bromide bismuth oxyhalide compounds according to claim 1, wherein y is between 0.70 and 0.95.

3. A photocatalyst comprising a film applied on a substrate, wherein said film contains the mixed chloride-bromide bismuth oxyhalide compounds as defined in claim 1.

4. A magnetic, photocatalytically active composite comprising a magnetic core having a coating thereon and an outer layer comprising the mixed chloride-bromide bismuth oxyhalide compounds as defined in claim 1 provided on said coating.

5. A method for the purification of water, comprising combining the mixed chloride-bromide bismuth oxyhalide as defined in claim 1 and water contaminated with one or more organic compounds, and irradiating said bismuth oxyhalide with UV-Vis light or visible light, thereby degrading said organic contaminant(s).

6. A method according to claim 5, wherein the mixed chloride-bromide bismuth oxyhalide is provided within a film containing mixed chloride-bromide bismuth oxyhalide compounds, with the molar ratio chloride: bromide being equal to or greater than 1:1, in the form of microspheres exhibiting flower-like surface morphology, said microspheres being characterized by the presence of individual thin sheets arranged radially in a petal-like manner, wherein two or more adjacent individual thin sheets are interconnected to form cells or channels which open onto the external surface of said microspheres.

7. Mixed chloride-bromide bismuth oxyhalide photocatalyst according to claim 1, which decomposes Rhodamine B in less than 10 minutes in the solution.

8. Mixed chloride-bromide bismuth oxyhalide photocatalyst according to claim 7, which completely decomposes Rhodamine B in 2 to 10 minutes in the solution.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1A to 1G are images produced with a scanning electron microscope showing the flower-like surface morphology of BiOCl.sub.0.75Br.sub.0.25 prepared according to a preferred embodiment of the invention.

(2) FIGS. 2A to 2G are images produced with a scanning electron microscope showing BiOCl.sub.0.75Br.sub.0.25 particles with plate-like morphology.

(3) FIG. 3 is a characteristic X-ray powder diffraction pattern of the BiOCl.sub.0.5Br.sub.0.5 compound of the invention.

(4) FIG. 4 is a characteristic X-ray powder diffraction pattern of the BiOCl.sub.0.67Br.sub.0.33 compound of the invention.

(5) FIG. 5 is a characteristic X-ray powder diffraction pattern of the BiOCl.sub.0.75Br.sub.0.25 compound of the invention.

(6) FIG. 6 is a characteristic X-ray powder diffraction pattern of the BiOCl.sub.0.8Br.sub.0.2 compound of the invention.

(7) FIG. 7 is a characteristic. X-ray powder diffraction pattern of the BiOCl.sub.0.88Br.sub.0.12 (BiOCl.sub.0.875Br.sub.0.125) compound of the invention.

(8) FIGS. 8A and 8B are images produced with a scanning electron microscope showing the morphology of BiOCl.sub.0.5Br.sub.0.5 particles prepared according to Keller et al. (supra).

(9) FIGS. 9A to 9C are images produced with a scanning electron microscope showing the flower-like surface morphology of BiOCl.sub.0.875Br.sub.0.125 prepared according to a preferred embodiment of the invention.

EXAMPLES

(10) Methods

(11) XRD measurements were performed on D8 Advance diffractometer (Bruker AXS, Karlsruhe, Germany) with a goniometer radius 217.5 mm, Gbel Mirror parallel-beam optics, 2 Sollers slits and 0.2 mm receiving slit. Low background quartz sample holder was carefully filled with the powder samples. XRD patterns from 5 to 85 2 were recorded at room temperature using CuK radiation (=0.15418 nm) with the following measurement conditions: tube voltage of 40 kV, tube current of 40 mA, step scan mode with a step size 0.02 2 and counting time of is per step for preliminary study and 12 s per step for structural refinement. The instrumental broadening was determined using LaB.sub.6 powder (NIST-660a).

(12) Morphological observations and identification of chemical composition were performed with the HRSEM-High Resolution Scanning Electron Microscope-Sirion (equipped with EDS LN2 detector, Oxford instruments, UK).

(13) Particle size was measured using Malvern Instruments-Mastersizer 2000 particle size analyzer.

(14) Surface area and Pore radius were determined by the N2 Brunauer-Emmett-Teller (BET) method NOVA-1200e).

Example 1

Preparation of BiOCl0.5Br0.5 in the Presence of an Acid

(15) Deionized water (80 ml), glacial acetic acid (35 ml) and bismuth nitrate (9.7 g) are added to a flask and mixed at room temperature for fifteen minutes until a clear, transparent solution is formed: CTAB (3.64 g dissolved in 10 ml of water) and CTAC (3.20 g in the form of 25 wt % aqueous solution) are added to the bismuth solution. The solution is then mixed at room temperature for additional 30 minutes. The precipitate formed is separated from the liquid phase by filtration, washed five times with ethanol and five times with water, in order to remove the non-reactive organic species. The solid product is then dried (in air). The weight of the dried solid collected is 7.2 grams. The product may be subjected to heating at 400 C. for approximately 1 hour.

(16) The X-ray powder diffraction pattern of the resultant bismuth oxyhalide is presented in FIG. 3. The entitled product exhibits X-ray powder diffraction pattern having characteristic peaks at 11.50, 23.05, 25:59, 32.48, 46.55 and 57.92 2 (0.05 2). The entitled product is characterized by average particle size of 11.30 m, surface area of 6.00 m.sup.2/g and pore radius of 16 .

Example 2

Preparation of BiOCl0.67Br0.33 in the Presence of an Acid

(17) Deionized water (60 ml), glacial acetic acid (35 ml) and bismuth nitrate (7.27 g) are added to a flask and mixed at room temperature for fifteen minutes until a clear, transparent solution is formed. CTAB (1.82 g dissolved in 10 ml of water) and CTAC (3.20 g in the form of 25 wt % aqueous solution) are added to the bismuth solution, for additional 30 minutes of mixing at room temperature. The precipitate formed is separated from the liquid phase by filtration, washed five times with ethanol and five times with water, in order to remove the non-reactive organic species. The solid product is then dried (in air). The weight of the solid collected is 6.6 grams. The product may be subjected to heating at 400 C. for approximately 1 hour.

(18) The X-ray powder diffraction pattern of the resultant bismuth oxyhalide is presented in FIG. 4. The entitled product exhibits X-ray powder diffraction pattern having characteristic peaks at 11.62, 23.46, 25.67, 32.47, 32.91, 46.57 and 58.19 2 (0.05 2). The entitled product is characterized by average particle size of 4.96 m, surface area of 16.84 m.sup.2/g and pore radius of 18 .

Example 3

Preparation of BiOCl0.75Br0.25 in the Presence of an Acid

(19) Deionized water (60 ml), glacial acetic acid (35 ml) and bismuth nitrate (7.35 g) are added to a flask and are mixed at room temperature for fifteen minutes until a clear, transparent solution is formed. CTAB (1.378 g dissolved in 10 ml of water) and CTAC (3.64 g in the form of 25 wt % aqueous solution) are added to the bismuth solution, for additional 30 minutes of mixing at room temperature. The precipitate formed is separated from the liquid phase by filtration, washed five times with ethanol and five times with water, in order to remove the non-reactive organic species. The solid product is then dried (in air). The weight of the solid collected is 6.7 grams. The product may be subjected to heating at 400 C. for approximately 1 hour.

(20) The X-ray powder diffraction pattern of the resultant bismuth oxyhalide is presented in FIG. 5. The entitled product exhibits X-ray powder diffraction pattern having characteristic peaks at 11.81, 25.69, 32.45, 46.47 and 54.01 2 (0.05 2). The entitled product is characterized by average particle size of 2.86 m, surface area of 25.62 m.sup.2/g and pore radius of 21 .

Example 4

Preparation of BiOCl0.8Br0.2 in the Presence of an Acid

(21) Deionized water (75 ml), glacial acetic acid (35 ml) and bismuth nitrate (9.18 g) are added to a flask and mixed at room temperature for fifteen minutes until a clear, transparent solution is formed. CTAB (1.378 g dissolved in 10 ml of water) and CTAC (4.85 g in the form of 25 wt % aqueous solution) are added to the solution, for additional 30 minutes of mixing at room temperature. The white precipitate formed is separated from the liquid phase by filtration, washed five times with ethanol and five times with water, in order to remove the non-reactive organic species. The solid is then dried (in air). The weight of the solid collected is 7 g. The product may be subjected to heating at 400 C. for approximately 1 hour.

(22) The X-ray powder diffraction pattern of the resultant bismuth oxyhalide is presented in FIG. 6. The entitled product exhibits X-ray powder diffraction pattern having characteristic peaks at 11.81, 25.68, 32.50, 46.58 and 58.22 (0.05 2). The entitled product is characterized by average particle size of 2.62 surface area of 25.75 m.sup.2/g and pore radius of 22 .

Example 5

Preparation of BiOCl0.875Br0.125 in the Presence of an Acid

(23) Deionized water (85 ml), glacial acetic acid (35 ml) and bismuth nitrate (14.69 g) are added to a flask and are mixed at room temperature for fifteen minutes until a clear, transparent solution is formed. CTAB (1.378 g dissolved in 10 ml of water) and CTAC (8.48 g in the form of 25 wt % aqueous solution) are added to the bismuth solution, for additional 30 minutes of mixing at room temperature. The precipitate thus formed is separated from the liquid phase by filtration, washed five times with ethanol and five times with water, in order to remove the non-reactive organic species. The solid is then dried (in air). The weight of the solid collected is 10.5 grams.

(24) The X-ray powder diffraction pattern of the resultant bismuth oxyhalide is presented in FIG. 7. The entitled product exhibits X-ray powder diffraction pattern having characteristic peaks at 12.04, 25.86, 32.56, 46.68 and 58.40 2 (0.05 2). The entitled product is characterized by average particle size of 1.98 m, surface area of 26.87 m.sup.2/g and pore radius of 24 .

Example 6

Preparation of BiOClyBr1-y Through a Hydrothermal Procedure in the Presence of a Base

(25) BiOCl.sub.yBr.sub.1-y powders (x=0.1, 0.2, 0.5, 0.7 and 0.8) were prepared according to the following general procedure:

(26) Bi(NO.sub.3).sub.3.5H.sub.2O (99.0%) (10 mmol) was mixed with distilled water (40 mL) and stirred at room temperature for 5 min. Subsequently, NH.sub.4OH (30.0%) (17 mL) was added dropwise. The resultant suspension was poured into a stainless steel Teflon-lined autoclave for the hydrothermal treatment. The autoclave was sealed, heated up to 130 C. and held for 14 hours, and was allowed to cool to room temperature. The product was filtered, washed thoroughly with distilled water, and then treated with adjusted amounts of HCl (37%) and concentrated HBr (48%) solutions. The final solid was separated by filtration and dried in air.

(27) Specifically, BiOCl.sub.yBr.sub.1-y, wherein y is about 0.5, was prepared as follows. The bismuth-containing solid (2.37 g) obtained following the hydrothermal procedure was mixed with 150 mL distilled water, 2 mL concentrated HCl solution and 2 mL concentrated HBr solution in a 250 mL beaker at room temperature for 30 minutes. The solid product was separated by filtration and air-dried to afford the product (2.52 g).

Example 7

Decomposition Test of an Organic Impurity in Aqueous Medium in the Presence of the BiOClyBr1-y Compounds of the Invention

(28) Each of the compounds of Examples 1 to 5 was tested for its photocatalytic activity in aqueous media with respect to the destruction of Rhodamine B. The photocatalytic activity of the compounds of Examples 1 to 5 was induced by means of four different irradiation methods: [1] exposure to direct sunlight; [2] Table PL lamp (11 W) located at a distance of 10 cm from the sample; [3] Xenon lamp (300 W) at wavelength 385-740 nm located at a distance of 10 cm from the sample; and [4] Xenon lamp (300 W) at wavelength 422-740 located at a distance of 10 cm from the sample.

(29) Each of the tested solutions contained 15 ppm of Rhodamine B and 150 mg of the photocatalyst. The time needed for the complete decomposition of Rhodamine B under each of the irradiation methods set forth above was determined by following the disappearance of the pink colored RhB by measuring its concentration at one minute intervals. The concentration of the remnant RhB in the solution during and after the irradiation was analyzed with a UV-Vis spectrophotometer (Varian EL-03097225). The results (expressed as the time required for RhB disappearance) are presented in Table 1.

(30) TABLE-US-00001 TABLE 1 Xenon lamp Xenon lamp at wavelength at wavelength Sunlight Table Lamp 385-740 nm 422-740 BiOCl.sub.0.5Br.sub.0.5 15 minutes 30 minutes 18 minutes 18 minutes (of Example 1) BiOCl.sub.0.67Br.sub.0.33 10 minutes 18 minutes 20 minutes 10 minutes (of Example 2) BiOCl.sub.0.75Br.sub.0.25 3 minutes 10 minutes 18 minutes 3 minutes (of Example 3) BiOCl.sub.0.80Br.sub.0.20 3 minutes 10 minutes 18 minutes 3 minutes (of Example 4) BiOCl.sub.0.875Br.sub.0.125 2 minutes 9 minutes 18 minutes 2 minutes (of Example 5)

Example 8 (Comparative)

Decomposition Test of an Organic Impurity in Aqueous Medium in the Presence of Either the BiOClyBr1-y Compounds of the Invention or Mixed Bismuth Oxyhalides of the Prior Art

(31) The following mixed bismuth oxyhalides were prepared according to prior art procedures:

(32) BiOCl.sub.0.5Br.sub.0.5 was synthesized according to the procedure described in the experimental section of Keller et al. [Zeitschrift fuer Naturforschung, B: Chemical Sciences, 60(12), 1255-1263 (2005)]. Mixtures of BiOCl and BiOBr were formed by blending mortared weighed amounts of BiOCl and BiOBr. The mixtures were then heated to 550 C. for 3 days, and were subsequently allowed to solidify. The BiOCl and BiOBr used as starting materials for this procedure were synthesized as described in Preparation 1 and Preparation 2 below, respectively. The SEM images of the resultant BiOCl.sub.0.5Br.sub.0.5 are provided in FIGS. 8A and 8B, which show that the product of Keller et al. exhibits non-organized plate-like surface morphology.

(33) BiOI.sub.0.8Cl.sub.0.2 was synthesized according to the procedure described in the experimental section of Wang et al. [Scripta Materials 56, p. 669-672 (2007)]. It is indicated in that paper that under visible light irradiation, the BiOI.sub.0.8Cl.sub.0.2 possesses the highest photocatalytic activity amongst the mixed iodide-chloride bismuth oxyhalides described in said paper.

(34) BiOBr.sub.0.75I.sub.0.25 was synthesized according to the procedure described in the experimental section of Wang et al. [catalysis Communications 9, p. 8-12 (2008)]. It is noted that the BiOBr.sub.0.75I.sub.0.25 is reported to be the strongest photocatalyst amongst the mixed bismuth bromide-iodide oxyhalides described in said paper.

(35) The photocatalytic activities of the mixed bismuth oxyhalides identified above, and of the BiOCl.sub.0.875Br.sub.0.125 according to Example 5 of the invention, were tested with respect to the decomposition of methyl orange (MO) in aqueous medium. To this end, aqueous solutions containing 10 ppm methyl orange and 100 mg of the tested photocatalyst in 200 ml of water were prepared and irradiated for a period of one hour, with either a UV-Vis light source (=385-740 nm) or a visible light source (422 nm). The degree of decomposition of the organic contaminant (MO) was determined after the one hour, irradiation period by means of measuring its concentration using a UV-Vis spectrophotometer (Varian EL-03097225)].

(36) The results are summarized in Table 2.

(37) TABLE-US-00002 TABLE 2 UV-Vis light Visible light irradiation irradiation (% decomposition) (% decomposition) BiOCl.sub.0.5Br.sub.0.5 1% 0% (Keller et al., 2005) BiOI.sub.0.8Cl.sub.0.2 61% 54% (Wang et al., 2007) BiOBr.sub.0.75I.sub.0.25 59% 50% (Wang et al., 2008) BiOCl.sub.0.875Br.sub.0.125 87% 91% (of Example 5)

(38) Notably, the mixed chloride-bromide bismuth oxyhalide of Keller et al., which is formed upon solidification of a molten mixture consisting of BiOCl and BiOBr, and is characterized by surface morphology as shown in FIGS. 8A-8B, has been found to be devoid of photocatalytic activity.

(39) The mixed iodide-chloride bismuth oxyhalide having plate morphology according to Wang et al (2007, supra), or the mixed bromide-iodide bismuth oxyhalide according to Wang et at (2008, supra) demonstrate photocatalytic activity. However, it is apparent from the results reported in Table 2 that the mixed chloride-bromide bismuth oxyhalide of the invention, with its flower-like surface morphology, demonstrates significantly better photocatalytic activity both in response to UV-Vis and visible light irradiation.

Example 9

Decomposition Test of an Organic Impurity in Aqueous Medium in the Presence of a BiOCly,Br1-y. Compound of the Invention or (Non-Mixed) BiOCl and BiOBr Prepared by the Method of the Invention

(40) Aqueous solutions of Rhodamine B were prepared (15 ppm of RhB dissolved in 200 ml water). The BiOCl.sub.0.875Br.sub.0.125 of Example 5 and pure (non-mixed) bismuth oxychloride and bismuth oxybromide, synthesized according to the procedures set forth in Preparation 1 and Preparation 2 below, respectively, were tested for their ability to completely degrade the dye, in response to either UV-Vis light irradiation or visible light irradiation. The amount of the photocatalyst introduced into each solution was 150 mg. The period of time needed to achieve full decomposition of the dye is set out in Table 3, with respect to the three tested compounds. This period of time was determined by following the disappearance of the pink colored RhB by means of measuring its concentration at one minute intervals. The concentration of the remnant RhB in the solution during and after the irradiation was analyzed with UV-Vis spectrophotometer (Varian EL-03097225).

(41) TABLE-US-00003 TABLE 3 UV-Vis light irradiation Visible light irradiation Time (min) Time (min) BiOCl 29 25 BiOBr 36 31 BiOCl.sub.0.875Br.sub.0.125 18 2 (of Example 5)

(42) It is noted that the pure (non-mixed) bismuth oxychloride and oxybromide, prepared by the process of the invention, exhibit fairly good photocatalytic activity. Nevertheless, the mixed chloride-bromide bismuth oxyhalide demonstrates superior activity, and is capable of accelerating the decomposition of the dye to a significantly greater extent than the pure (non-mixed) bismuth oxyhalide.

Example 10

Thin Film Formulation

(43) Tetraethyl orthosilicate (TEOS; 5.2 gram), de-ionized water (2.7 gram) and ethanol (6 gram) were mixed together in the presence of nitric acid (pH=2) at 60 C. for 20 minutes. Pluronic P123 (0.15 gram) and poly vinyl alcohol (0.18 gram), both dissolved in 4 gram of ethanol were then added and the stirring continued for an additional hour at 60 C. to form the glue solution siloxane.

(44) 0.4-0.7 grams of the photocatalyst (of Examples 1-5) was dispersed in 2 gram of ethanol, following which the dispersion was treated with ultrasonic bath for 2 minutes and was then added to the siloxane solution. The resultant mixture is left for 15 minutes under magnetic stirring.

(45) Microscope glass slides were carefully cleaned using acid piranha (a 3:1 mixture of sulfuric acid and hydrogen peroxide). Then, 2-3 drops of the photocatalyst-containing mixture were spread on the clean glass slide using a suitable coating rod or alternatively via the spin coating technique. The coated slide was calcined at oven for 3 hours up to 400 C. (5 C./min), then cooled gradually at room temperature.

(46) The resultant thin coating comprising BiOCl.sub.1-yBr.sub.y particles on a glass is highly photoactive, capable of decomposing RhB solution under sun light. 150 mg of the BiOCl.sub.1-yBr.sub.y particles in the coating decompose 15 ppm of an aqueous RhB under sun light irradiation within 2 minutes.

Example 11

Magnetic Formulation

(47) Magnetite and sodium silicate (dissolved in 200 ml aqueous acidic solution at molar ratios magnetite: silicate of about 3:1-5:1) are mixed over night at 80 C. The resultant solid particles are silica-coated magnetite.

(48) 0.9 gram of the resulting particles (SM) and 0.3 gram of dispersed BiOCl.sub.1-yBr.sub.y particles (of Examples 1-5) are transferred to autoclave for a hydrothermal treatment at 95 C. for 6 hours. The product thus formed is a composite of the formula BiOCl.sub.1-yBr.sub.y/(SiOSi).sub.x/Fe.sub.3O.sub.4, i.e., a magnetite core coated with silica layer(s), with the photocatalyst particles being provided on the silica coating.

Preparation 1

Preparation of BiOCl

(49) Deionized water (80 ml), glacial acetic acid (35 ml) and bismuth nitrate (9.7 g) are added to a flask and mixed at room temperature for fifteen minutes until a clear, transparent solution is formed. Afterwards, CTAC (6.40 g in the form of 25 wt % aqueous solution) is added to the bismuth solution. The resultant suspension is then mixed at room temperature for additional 30 minutes. The precipitate formed is separated from the liquid phase by filtration, washed five times with ethanol and five times with water, in order to remove the non-reactive organic species. The solid product is then dried (in air). The weight of the dried solid collected is 7.3 grams. The product may be subjected to heating at 400 C. for approximately 1 hour.

Preparation 2

Preparation of BiOBr

(50) Deionized water (80 ml), glacial acetic acid (35 ml) and bismuth nitrate (9.7 g) are added to a flask and mixed at room temperature for fifteen minutes until a clear, transparent solution is formed. Afterwards, CTAB (7.28 g dissolved in 10 ml watery is added to the bismuth solution. The resultant suspension is then mixed at room temperature for additional 30 minutes. The precipitate formed is separated from the liquid phase by filtration, washed five times with ethanol and five times with water, in order to remove the non-reactive organic species. The solid product is then dried (in air). The weight of the dried solid collected is 7.3 grams. The product may be subjected to heating at 400 C. for approximately 1 hour.