Functionalized zinc oxide nanoparticles for photocatalytic water splitting

Abstract

The present invention relates to a photocatalyst composition having visible light activity for hydrogen production through water splitting. More particularly, the present invention discloses a photocatalyst composition comprising a zinc oxide nanoparticles and a conjugated organic moiety selected from the group consisting of oligothiophenes, azo dyes, and perylenes.

Claims

1. A process for the preparation of a photocatalyst composition, wherein said process comprises: (i). refluxing KOH in methanol at 60° C. for 30 minutes to obtain a refluxed KOH solution, adding, drop wise, a dissolved solution of zinc acetate, prepared by adding Zn(OAc).sub.2.2H.sub.2O in methanol with stirring at a room temperature of 25° C., to the refluxed KOH solution maintained at 60° C. to obtain an alkaline zinc acetate solution; and (ii). Oil in-situ adding a conjugated organic moiety to the alkaline zinc acetate solution of (i) to obtain the photocatalyst composition, the photocatalyst composition consisting of: 95-97 wt % zinc oxide nanoparticles; 2-4 wt % conjugated organic moiety; and 1 wt % co-catalyst, wherein the conjugated organic moiety is a perylene.

2. The process for the preparation of the photocatalyst composition according to claim 1, wherein the composition is photoactive in visible light.

3. The process for the preparation of the photocatalyst composition according to claim 1, wherein the co-catalyst is selected from the group consisting of platinum, silver, and nickel (II) oxide.

4. The process for the preparation of the photocatalyst composition according to claim 1, wherein the perylene is selected from the group consisting of 9,10 perylenetetracarboxylic acid, N,N′-bis(p-carboxyl-phenylene) perylene-3,4:9,10-perylene bisimide, perylene-3,4,9,10-tetracarboxylic acid, 3,3′-(1,3,8,10-tetraoxo-1,3,8,10-tetrahydroanthra[2,1,9-def:6,5,10-d′e′f′]diisoquinoline-2,9-diyl) dibenzoic acid, and 5,5′-(1,3,8,10-tetraoxo-1,3,8,10-tetrahydroanthra[2,1,9-def:6,5,10-d′e′f′]diisoquinoline-2,9-diyl) diisophthallic acid.

Description

BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS

(1) FIG. 1 depicts TEM images of (a) ZnONPs, (b) ZnO grafted with 4ABBN, (c) ZnO grafted with 3ABBN.

(2) FIG. 2 depicts Absorption spectrum of ZnO-4ABBN composite (with 1:0.5 ratio based on surface ZnO).

(3) FIG. 3 depicts UV-vis spectrum and photoluminescence spectrum at excitation wavelength of 325 nm of pristine ZnO NPs of sizes 10+2 nm (left) and 35+15 nm (right).

(4) FIG. 4 depicts TEM images of ZnO-4ABBN composites of sizes 3.5+1.5 nm (top) and 35+15 nm (bottom).

(5) FIG. 5 depicts UV-vis spectrum (top) and photoluminescence spectrum at excitation wavelength of 260 nm (bottom) of 4ABBN and ZnO-4ABBN composites for sizes of 3.5+1.5 nm and 35+15 nm. The spikes observed are due to excitation overtone at 2λ.

(6) FIG. 6 depicts Hydrogen evolution for 4 h for (left) ZnO/Pt—10+2 nm (black) and 35+15 nm (grey), (middle) ZnO-4ABBN-3.5+1.5 nm (black) and 35+15 nm (grey), (right) Pt/ZnO-4ABBN-3.5+1.5 nm (black) and 35+15 nm (grey).

DETAILED DESCRIPTION OF THE INVENTION

(7) The present invention provides a photocatalyst composition comprising a zinc oxide nanoparticles and a conjugated organic moiety for water splitting in visible light to obtain hydrogen.

(8) In an aspect, the present invention provides a photocatalyst composition comprising a zinc oxide nanoparticles, a conjugated organic moiety and may optionally contain a co-catalyst for water splitting in visible light to obtain hydrogen.

(9) The photocatalyst composition has dual advantage of minimized bulk recombination sites using semiconductor nanoparticles and spatial separation and reduced surface sites using conducting organic linkers.

(10) The present invention provides a photocatalyst composition comprising a zinc oxide nanoparticle, a conjugated organic moiety and may optionally contain a co-catalyst for water splitting in visible light to obtain at least 2 milli moles hydrogen/hour/g of composition.

(11) The present invention provides a composition wherein the conjugated organic moiety is selected from the group consisting of oligothiophenes, azo dyes, porphyrins, hydroxyquinolones, and perylenes.

(12) The photocatalyst composition containing perylenes as conjugated organic moiety are selected from the group consisting of 9,10-perylenetetracarboxylic acid, N,N′-Bis (p-carboxyl-phenelyne) perylene-3,4:9,10-perylene bisimide, perylene-3,4,9,10-tetracarboxylic acid, 3,3′-(1,3,8,10-tetraoxo-1,3,8,10-tetrahydroanthra[2,1,9-def:6,5,10-d′e′f′]diisoquinoline-2,9-diyl) dibenzoic acid, and 5,5′-(1,3,8,10-tetraoxo-1,3,8,10-tetrahydroanthra[2,1,9-def:6,5,10-d′e′f′]diisoquinoline-2,9-diyl) diisophthallic acid.

(13) The photocatalyst composition containing oligothiophenes as conjugated organic moiety are selected from [2,2′:5″,2″-terthiophene]-5-carboxylic acid or [2,2′:5″,2″-terthiophene]-5-5″-dicarboxylic acid.

(14) The photocatalyst composition containing azo dyes as conjugated organic moiety wherein azo dye is (E)-4-((2-hydroxynaphthalen-1-yl) diazenyl) benzoic acid.

(15) The present invention provides a photocatalyst composition optionally contain a co-catalyst is selected from the group consisting of Pt, Ag, NiO, alcohol, Glycerol and inorganic moieties which acts as scavenger.

(16) The surface modification of ZnO nanoparticles is identified as a method of modulating surface sites advantageously. ZnO nanoparticles of two different sizes are surface modified with a conjugated organic moiety to enable electron conduction and transfer. Enhanced H.sub.2 evolution from water methanol mixtures was observed in the composite systems compared to pristine ZnO under visible light irradiation without any co-catalyst. The system is also marginally active in water splitting in pure water without any sacrificial agents. Photophysical characterisation indicates that even though reducing size into nanoregime affects the band gap detrimentally, modifications by simple conjugated organic molecules assist in enhanced visible light activity. The experimental observations are corroborated with computational studies, which also point to a localization of valence band maximum of the interface on the organic moiety and conduction band minimum on ZnO.

(17) The invention has double pronged advantage with this approach, (i) surface decoration of nanoparticles with organic ligands is known to reduce the defect sites and (ii) these organic moieties will be capable of absorbing and transferring electrons or holes preferentially away from the surface recombination sites depending on the band positions. Further, catalytic sites like metal nanoparticles or organometallic centres can also be linked to photogenerating semiconductor nanoparticles through these conducting organic moieties.

(18) The present invention provides a process for the preparation of ZnO nanoparticles via precipitation method comprising: a. refluxing KOH in methanol to obtain a refluxing KOH solution; b. preparing Zn (OAc).sub.2.2H.sub.2O in methanol by stirring at room temperature in the range of 20 to 30° C. and adding the dissolved solution of zinc acetate into the refluxing KOH solution of step (a) with vigorous stirring to obtain the ZnO nanoparticles.

(19) The present invention provides a process for the synthesis of conjugated organic moiety (azo dye) (E)-4-((2-hydroxynaphthalen-1-yl) diazenyl) benzoic acid (4ABBN) comprising: a. adding distilled water and concentrated sulfuric acid to a solution of 4-amino benzoic acid and cooling to obtain a cold reaction mixture; b. adding a solution of sodium nitrite in distilled water to the above cold mixture of step (a) with stirring to complete the diazotization and obtain the diazotized product; c. adding β-naphthol very slowly with vigorous stirring to the diazotized product of step (b) to obtain the desired reddish orange coloured product precipitate.

(20) The process for the synthesis of (E)-4-((2-hydroxynaphthalen-1-yl) diazenyl) benzoic acid (4ABBN) is shown in scheme 1.

(21) The invention describes the strategy which involves surface grafting of ZnO nanoparticles with a conjugated organic linker to assist electron conduction. The activities of two ZnO nanoparticles with sizes ˜3.5 and 35 nm surface modified with the conjugated molecule was compared. The enhanced activity of the smaller nanoparticles can be attributed to reduced bulk defects consequent to decrease in particles size which minimizes electron-hole recombination, even though effect of increased surface area cannot be ruled out. The conjugated organic linker, 4-((2-hydroxynaphthalen-1-yl) diazenyl) benzoic acid (4 ABBN) has also facilitated visible light absorption as shown by UV-vis spectroscopy. Such photocatalysts without the use of expensive dyes or co-catalysts may pave the way to a more efficient and cost effective method of H.sub.2 generation.

(22) The stability of the organic linker during reaction was tested by following the absorbance spectrum. Even though the absorbance intensity was found to decrease, this might not due to structural degradation of 4ABBN but some form of agglomeration and settling of the composite as observed by TEM. This was confirmed by continuous irradiation of the catalyst for longer durations; H.sub.2 evolution after 8 and 12 h were found to be 152.9 mmol/g and 154 mmol/g for ZnOSG-4ABBN composite.

(23) The detrimental effect of Pt on ZnO-4ABBN composites is also an interesting observation. The blocking of ZnO surface as well as to catalyzing undesired back reactions. This also shows that surface modification with 4ABBN has definitely enhanced the H.sub.2 evolution ˜20 fold. To test the role of methanol, measurements were done in water without adding methanol. H.sub.2 evolution from pure water splitting was observed albeit to a small extent (1.23 mmol/g) for 4 h irradiation. This shows that methanol plays the role of enhancing the H.sub.2 evolution. Methanol is known to assist in water splitting on semiconductor oxides by acting not only as a hole scavenger but also as electron enricher which enhances the hydrogen evolution. The radical formed after proton abstraction from CH.sub.3OH by the valence band hole injects one electron to the conduction band leading to the formation of formic acid and formaldehyde and eventually CO or CO.sub.2. There is no presence of CO or CO.sub.2 in detectable levels by gas chromatography. Presence of formic acid or formaldehyde, potential products in hole scavenging reactions of methanol, also was ruled out by liquid chromatography analysis of the reaction mixture.

(24) Further, from computational studies it is clear that the photoexcitation occurs in the azo compound hence bringing up the possibility of HOMO of 4ABBN interacting with CH.sub.3OH. This indicates that the utilization of methanol probably proceeds through a photochemical pathway involving HOMO of the azo compound rather than a simple current doubling mechanism involving valence band of ZnO and methanol contributes to H.sub.2 production.

EXAMPLES

(25) The following examples are given by way of illustration and therefore should not be construed to limit the scope of the invention.

Example 1: Preparation of ZnO Nanoparticles

(26) A. By Precipitation Method

(27) For smaller nanoparticles, precipitation method was followed. In a typical procedure, KOH (56 mM, 75 mL) in methanol was refluxed at 60° C. for 30 min. In another reaction mixture, Zn(OAc).sub.2.2H.sub.2O (56 mM, 25 mL) in methanol was prepared by stirring at room temperature (25° C.). The dissolved solution of zinc acetate was then added drop wise through an addition funnel into the refluxing KOH solution maintained at 60° C. with vigorous stirring. The formation of ZnONPs started immediately and the solution becomes clear.

(28) B. By Sol Gel Method

(29) Bigger ZnONPs were synthesized by sol gel method. Zinc acetate dihydrate (0.02389 mol) was treated with ethanol (144 mL) at 60° C. This was completely dissolved in about 30 min. Oxalic acid dihydrate (0.04878 mol) was dissolved in ethanol (98 mL) at 50° C. and was added slowly under stirring to the warm ethanolic solution of zinc acetate. A thick white gel formed, which was kept for drying at 80° C. for 20 h. The gel was calcined at 400° C. for 2 h to yield ZnONPs of size 20-40 nm.

Example 2: Preparation of Conjugated Organic Moiety

(30) A. Synthesis of (E)-4-((2-hydroxynaphthalen-1-yl)diazenyl) benzoic acid (4ABBN)

(31) To a solution of 4-amino benzoic acid (1.35 g, 0.009843 moles), distilled water (8.44 mL) and concentrated sulfuric acid (2.1 mL) was added drop wise. The mixture was cooled to 0° C. A solution of sodium nitrite (1.006 g, 0.01458 moles) in 3 mL of distilled water maintained at 0° C., was added to the above cold mixture drop wise with good stirring. After 20 min the diazotization was complete, which was verified by adding a solution of 4-(N,N-dimethylamine) benzaldehyde which generated colour if undiazotized aromatic amine is still present. To the diazotized product obtained, β-naphthol (2.012 g, 0.01395 moles) was added very slowly with vigorous stirring over 15 min. The reaction mixture was further stirred for 1 h and left to stand overnight (10 hr). A reddish orange coloured precipitate was observed which was purified by column chromatography. (Scheme 1).

(32) ##STR00001##

(33) B. Synthesis of Perylene-3,4,9,10-Tetracarboxylic Acid

(34) A solution containing perylene-3,4,9,10-tetracarboxylic dianhydride (1.0 g, 0.254 mmol) and potassium hydroxide (1.0 g, 17.82 mmol) in water (15 mL) was heated with vigorous stirring at reflux. After 12 hours, the reaction mixture was cooled to room temperature. Acidification of the reaction mixture with 10% sulfuric acid (10 mL) yielded precipitation of product. The precipitate was collected by filtration, intensively washed with water and ethanol, and then dried in a vacuum to give perylene-3,4,9,10-tetracarboxylic acid (0.4 g,) as dark orange powders.

(35) ##STR00002##

(36) C. Synthesis of N,N′Bis(p-Carboxy-Phenylene) Perylene-3,4:9,10-Perylene Bisimide (PBI)

(37) A mixture containing perylene-3,4,9,10-tetracarboxylic dianhydride (300 mg, 0.7 mmol) and imidazole (30 g) and 2 ml chloroform were stirred at 90° C. After sometime, about 255 mg (1.8 mmol) of p-aminobenzoic acid was added and the mixture was refluxed for 130° C. for 24 h followed by acidification and filtration. The precipitate was intensively washed with water and ethanol, and then dried in a vacuum to give PBI.

(38) ##STR00003##

(39) D. Synthesis of 2,2′:5′,2″-Terthiophene

(40) Mg (0.56 g, 23.3 mmol) was placed in 10 mL of dry ether then cooled to 0° C. 2-bromothiophene (2.9 mL, 30.3 mmol) was added dropwise for half h, and the reaction mixture was stirred also for another half hour. This solution was then transferred slowly via a cannula to a mixture of 2,5-dibromothiophene (1.3 mL, 11.6 mmol) and Ni(dppp)Cl.sub.2 (305 mg, 0.466 mmol) in dry ether (50 mL) while cooling on ice. The reaction mixture was stirred for 16 h at room temperature and subsequently poured out in ice/water (150 mL) containing concentrated HCl (10 mL). The product was extracted with ether and the combined organic layers were washed with water and brine, successively. The organic extracts were dried over anhydrous MgSO.sub.4, evaporated and purified with column chromatography to give slight yellowish solid 2.1 g (8.3 mmol, yield 71%).

(41) ##STR00004##

(42) E. Synthesis of 2,2′:5′,2-Terthiophene-5-Carboxylic Acid

(43) To the synthesized 2,2′:5′,2″-Terthiophene, about 0.27 g of 5 formyl-2,2′:5′,2-terthiophene was dissolved in 50 ml of acetone and the temperature thereof was maintained at 15° C. CrO.sub.3/H.sub.2O/H.sub.2SO.sub.4 solution (0.9 g/12 ml/0.2 ml) was dropped in slowly and stirred for 4 hrs at 40° C. About 50 ml of water was added slowly, filtered and again washed with water and dried to obtain a yellowish solid.

(44) ##STR00005##

Example 3: Preparation of Photocatalyst Composition No—4ABBN Composite

(45) 4-((2-hydroxynaphthalen-1-yl) diazenyl) benzoic acid (23 μmol) was added in-situ during the synthesis of ZnO by precipitation method (0.142 mmoles), to the alkaline zinc acetate solution. The solution was stirred in methanol at room temperature (27° C.) for 1 h. The product (ZnOPP-4ABBN) was washed with methanol, centrifuged and collected. Similarly, 0.38 mmoles of 4ABBN was added to ZnONPs synthesized by sol-gel method (10 mmoles). The solution was stirred in ethyl acetate at 27° C. for 3 h and the product (ZnOSG-4ABBN) was collected after the removal of excess solvent under vacuum. As control samples, 1 wt % Pt cocatalyst loaded pristine ZnO as well as ZnOPP-4ABBN and ZnOSG-4ABBN also were synthesized by wet impregnation. Typically, 51 mg of H.sub.2PtCl.sub.6.6H.sub.2O was dissolved in minimum quantity of water and added to 198 mg of ZnO prepared by different methods, dried at 80° C. and finally calcined at 400° C. for 2 h. Appropriate amounts (4 wt %) of 4ABBN was added to the Pt loaded ZnO and stirred for 1 h, separated and dried.

Example 4: Experiment of Water Splitting

(46) The catalysts were studied for their photocatalytic activity by reactions that were carried out in a gas-closed system with 45 mL of the dead volume. The photocatalyst powder (50 mg) was dispersed in a reactant solution (25 mL) by a magnetic stirrer in an irradiation cell made of quartz. 20 mL pure water and 5 mL methanol were employed as reactant solutions for water splitting. Here methanol was taken as sacrificial reagent which gets oxidized by the resulting photogenerated holes. The light source was 400 W mercury lamp for UV and 400 W Tungsten lamp for visible irradiation. The amount of H.sub.2 evolved was determined using gas chromatography (Agilent GC with Carbosphere column and N.sub.2 as carrier gas). The reaction was carried for 1 to 8 hours. The evolving gas mixture was taken in a syringe at an interval of 1 hour.

Example 5: Hydrogen Evolution Results

(47) The ZnO Perylene composite/ZnO thiophene composite catalysts were studied for their photocatalytic activity by reactions carried out in a gas-closed system. These catalysts with their hydrogen evolution data are summarized in below table 1.

(48) TABLE-US-00001 TABLE 1 Hydrogen ZnO Perylene composite/ZnO thiophene composite evolution 8 mmol/g 9,10-perylene tetracarboxylic dianhydride 16 mmol/g N,N′-Bis(p-carboxyl-phenylene)perylene-3,4:9,10-perylene bisimide 12 mmol/g 3,3′-(1,3,8,10-tetraoxo-1,3,8,10-tetrahydroanthra[2,1,9-def:6,5,10- d′e′f′]diisoquinoline-2,9-diyl)dibenzoic acid 4.37 mmol/g 5,5′-(1,3,8,10-tetraoxo-1,3,8,10-tetrahydroanthra[2,1,9- def:6,5,10-d′e′f′]diisoquinoline-2,9-diyl)diisophthalic acid 0 6.82 mmol/g perylene-3,4,9,10-tetracarboxylic acid 5 mmol/g [2,2′,5′,2″-terthiophene]-5-carboxylic acid 3.68 mmol/g [2,2′,5′,2″-terthiophene]-5,5″-carboxylic acid Experimental setup and conditions: Catalyst amount-25 mg, maintain RT, visible light, head space analysis after freezing the solution by gas chromatography.

Example 6: Characterization Studies of Catalyst

(49) The size quantization of ZnO NPs can change the spectral positions of the defect and exciton emissions also. Photophysical characterizations of pristine ZnO NPs display slight variations as expected of the size differences (FIG. 1). The absorption onset shifted from 356 nm to 367 nm as the particle size increased from 10 to 35 nm. Also, as expected, the exciton emission was highly suppressed in both particles with an enhanced green defect emission. No substantial change in the defect emission spectral position was observed.

(50) These NPs were separately surface modified with 4ABBN. The amount of 4ABBN to be used was roughly estimated based on the number of ZnO on the surface of the particles. For smaller particles using precipitation method, the ligand 4ABBN was added in-situ before separating the solid product, whereas, in case of bigger particles, the ligand was grafted to dry powder samples. Powder X-ray diffraction patterns show that the parent wurtzite phase is not altered during the surface modification. Corresponding TEM images (FIG. 2) show spherical ZnO nanoparticles, with fairly uniform size with mean diameter of 3.5±1.5 nm (by precipitation method) and 35±15 nm (by sol-gel method). The surface modification of ZnO (size ˜10 nm in pristine form) has assisted in reducing the agglomeration effect seen in naked ZnO. Also, surface modification appears to have decreased the ZnO particle size (3.5±1.5 nm) because of arrest of solution ripening possible for naked ZnO as a consequence of in-situ addition of the organic ligand.

(51) UV-vis absorption studies were carried out on 4ABBN as well as ZnO-4ABBN composite and compared with pristine ZnO (FIG. 3). 4ABBN is found to have sharp absorptions at 310 and 480 nm. Shoulder peaks around, 410 and 505 nm are also observed. The absorbance spectrum of the ZnO-azo composite system showed combined features of ZnO and dye with well-defined absorption peaks at 315 and 495 nm with some shoulder absorption peaks around 225, 270 and 370 nm. Considering the absorption edge at 325 nm as the band gap absorption, a slight blue shift is observed which may be attributed to a reduction in size observed after grafting. However, the composite showed visible light absorption contributed by the organic linker.

(52) PL studies also prove surface modification in which the green emission is suppressed. This evidences a modification of the surface sites thereby minimizing the trap sites for recombination. Fluorescence emission of 4ABBN in the UV region has also helped in enhancing the band gap emission of the composite. These characterizations indicate that even though trap sites, which may act as electron sinks, are minimized in the composites, some new energy levels in appropriate positions are created. These allow visible light absorption and an electron transfer probably from the organic linker to ZnO.

(53) The FIG. 6 shows H.sub.2 evolution under same conditions measured for pristine ZnO NPs (both 10±2 nm and 35±15 nm) modified with Pt as well as ZnO-4ABBN composites (both ZnOPP-4ABBN and ZnOSG-4ABBN) modified with Pt. Interestingly, both displayed lesser activities than ZnO-4ABBN composites without Pt; H.sub.2 evolution of Pt/ZnO was 1.7 mmol/g for 10±2 nm and 4.4 mmol/g for 35±15 nm and that of Pt/ZnO-4ABBN was 7.2 mmol/g for 3.5±1.5 nm and 9.2 mmol/g for 35±15 nm. There is enhanced (5 fold) H.sub.2 evolution from ultra small nanoparticles (˜3.5 nm) of ZnO surface modified with the organic molecule when compared to bigger surface modified ZnO NPs is observed.

Advantages of Invention

(54) a. Provides surface functionalized catalyst. b. Minimizes bulk recombination sites using semiconductor nanoparticles and spatial separation. c. Reduces surface sites using conducting organic linkers. d. It can have applications related to dye sensitized solar cells.