A method for producing metal oxide nanoparticles

Abstract

NOT VISIBLE DUE TO STATUS OF PATENT

Description

[0001] A METHOD FOR PRODUCING METAL OXIDE NANOPARTICLES

[0002] INTRODUCTION

[0003] This invention relates to a method for producing metal oxide nanoparticles comprising contacting at least two reagents, wherein at least one of the reagents is in a frozen state, and wherein contact between the reagents are controlled through controlled phase transition of the frozen reagent and agitation of the reaction mixture.

[0004] BACKGROUND

[0005] The preparation and application of metal oxide nanoparticles as catalysts in organic syntheses reactions is an important area of research which has drawn significant attention in recent years. These materials possess interesting chemical and physical properties that are very different from the corresponding bulk phase, which enable them to serve as very effective catalytic agents for a variety of reactions.

[0006] High specific surface area of the particles, efficient dispersion of the catalyst within the reacting medium and the ease of recovery through magnetic separation (for magnetic particles) are some of the benefits of utilizing these materials.

[0007] There are a number of methods available for the synthesis of metal oxide nanocatalysts of this nature including sol-gel processing, solution precipitation and the water oil microemulsion method. Solvothermal methods offer good control over the particle size and shape, but the catalysts are produced in low yields and the methods require a number of synthesis steps.

[0008] Among the various methods reported for the synthesis of e.g. magnetite, the chemical co-precipitation of Fe2+and Fe3+salts by addition of sodium hydroxide is the simplest and cheapest and thus attractive for large-scale production. Ultrasonic agitation of the reaction medium during co-precipitation can also aid in the formation of uniform nanosized particles by preventing agglomeration. However, the conventional co-precipitation method still produces nanoparticles that are non-uniform in shape and much larger than the alternative methods. The size and shape of the nanocrystallites can be controlled by the rate of addition of NaOH to the iron solution, and this typically involves drop-wise addition of the hydroxide. However, even with this approach there exists localized areas of high concentration of hydroxide in the iron solution, which promotes the nucleation and rapid growth of iron oxide particles. Owing to its simplicity the solution method is preferred for large- scale production of nanoparticles, but a better method of contacting the reagents is required.

[0009] US Patent 3,681 ,011 describes a “cryo-coprecipitation” method of preparing mixed metal oxide particles. In this method, solid spheroids of metal salts are prepared and immersed in an aqueous hydroxide solution at low temperature (0-5eC). As is stated therein, “precipitation appears to occur initially at the surface of the spheroid and progress radially towards the centre”. The fundamental mechanism in this patent appears to be akin to the progressive conversion model of solid-fluid noncatalytic reactions. This method does not allow for control of the shape and size of the nanoparticles formed.

[0010] Accordingly, there is a need for an improved method for producing metal oxide nanoparticles that address at least some of the shortcomings of known methods including improved particle size, improved particle size distribution, and desirable particle shape.

[0011] SUMMARY OF THE INVENTION

[0012] According to a first aspect to the present invention there is provided a method for producing metal oxide nanoparticles, the method comprising: a) contacting at least a first reagent comprising metal ions and a second reagent comprising an alkali salt solution to form a reaction mixture, b) wherein at least one of the reagents are initially provided in a frozen state, and c) controlling the rate of contact of the at least one frozen reagent with other reagents in the reaction mixture by controlling the temperature of the reaction mixture and / or agitation applied to the reaction mixture to control the phase transition of the frozen reagent, thereby to control the size, size distribution and shape of the metal oxide nanoparticles.

[0013] In one embodiment, the alkali salt solution is a NaOH solution.

[0014] In a preferred embodiment, the second reagent is provided in a frozen state. Preferably, the metal ions are selected from the group consisting of Fe, Ni, Cu, Ag, Au, Pd, Pt, Sn, Zn, and combinations thereof.

[0015] Preferably, the metal ions are selected from Fe, Ni, Cu, and combinations thereof.

[0016] In one embodiment, the temperature of the reaction mixture is controlled at a temperature in the range of from more than about 0eC to about 25eC.

[0017] In a preferred embodiment, the temperature of the reaction mixture is controlled at a temperature in the range of about 1eC to about 20eC, about 5eC to about 15eC, or about 10eC to about 15eC.

[0018] In one embodiment, the agitation provided is selected from the group consisting of stirring, ultrasonic agitation, and combinations thereof.

[0019] In one embodiment, the ultrasonic agitation is provided at about 50 - 100 W.

[0020] In one embodiment, the metal ions are Fe ions, and the average particle size of the FeO nanoparticles is at least 70% less than the average particle size obtained using a conventional co-precipitation method wherein the reagents are added drop-wise in a liquid-liquid reaction.

[0021] In another embodiment, the metal ions are Cu ions, and the average particle size of the CuO nanoparticles is at least 45% less than the average particle size obtained using a conventional co-precipitation method wherein the reagents are added drop-wise in a liquid-liquid reaction.

[0022] In another embodiment, the metal ions are Ni ions, and the average particle size of the NiO nanoparticles is at least 15% less than the average particle size obtained using a conventional co-precipitation method wherein the reagents are added drop-wise in a liquid-liquid reaction. According to a second aspect of the present invention, there is provided metal oxide nanoparticles prepared according to the method of the invention as described.

[0023] According to a third aspect of the present invention, there is provided a method of catalysing a reaction, the method comprising providing a metal oxide nanoparticle catalyst prepared according to the method of the invention as described.

[0024] BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The invention will now be described in more detail with reference to the following non-limiting embodiments and figures in which:

[0026] Figure 1 shows the average iron oxide metal nanoparticle particle size. CR = control experiment (drop-wise addition of reagent), NA15 = non-agitated 15°C, NA10 = non-agitated 10°C, A15 = agitated 15°C, and A10 = agitated 10°C;

[0027] Figure 2 shows the average copper oxide metal nanoparticle particle size. CR = control experiment (drop-wise addition of reagent), NA15 = non-agitated 15°C, NA10 = non-agitated 10°C, A15 = agitated 15°C, and A10 = agitated 10°C;

[0028] Figure 3 shows the average nickel oxide metal nanoparticle particle size. CR = control experiment (drop-wise addition of reagent), NA15 = non-agitated 15°C, NA10 = non-agitated 10°C, A15 = agitated 15°C, and A10 = agitated 10°C;

[0029] Figure 4 shows the average iron oxide, copper oxide and nickel oxide metal nanoparticle particle size. CR = control experiment (dropwise addition of reagent), NA15 = non-agitated 15°C, NA10 = non-agitated 10°C, A15 = agitated 15°C, and A10 = agitated 10°C; Figure 5 shows the % dye removed using iron oxide metal nanoparticles. CR = control experiment (drop-wise addition of reagent), NA15 = non-agitated 15°C, NA10 = non-agitated 10°C, A15 = agitated 15°C, and A10 = agitated 10°C;

[0030] Figure 6 shows the % dye removed using copper oxide metal nanoparticles. CR = control experiment (drop-wise addition of reagent), NA15 = non-agitated 15°C, NA10 = non-agitated 10°C, A15 = agitated 15°C, and A10 = agitated 10°C;

[0031] Figure 7 shows the % dye removed using nickel oxide metal nanoparticles. CR = control experiment (drop-wise addition of reagent), NA15 = non-agitated 15°C, NA10 = non-agitated 10°C, A15 = agitated 15°C, and A10 = agitated 10°C; and

[0032] Figure 8 shows the average iron oxide, copper oxide and nickel oxide particle size. CR = control experiment (drop-wise addition of reagent), NA15 = non-agitated 15°C, NA10 = non-agitated 10°C, A15 = agitated 15°C, and A10 = agitated 10°C.

[0033] DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0034] The invention as described hereinafter should not be construed to be limited to the specific embodiments disclosed, with slight modifications and other embodiments intended to be included within the scope of the invention.

[0035] Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

[0036] As used herein, throughout this specification and in the claims which follow, the singular forms “a”, “an” and “the” include the plural form, unless the context clearly indicates otherwise. The terminology and phraseology used herein is for the purpose of description and should not be regarded as limiting. The use of the terms “comprising”, “containing”, “having”, “including”, and variations thereof used herein, are meant to encompass the items listed thereafter, and equivalents thereof as well as additional items.

[0037] As used in this specification, the term “frozen” or “frozen reagent” should be understood to refer to the phase transition process or condition wherein a reagent, that is typically in a liquid form at ambient conditions, turns into or is provided in a solid form when its temperature is lowered to below its freezing point.

[0038] The present invention provides for a method for producing metal oxide nanoparticles comprising contacting at least two reagents, wherein at least one of the reagents is initially provided in a frozen state, and wherein contact between the reagents are controlled through controlled melting or phase transition of the frozen reagent and agitation of the reaction mixture, thereby to control the size, size distribution and shape of the resultant metal oxide nanoparticles. The phase transition process, including the rate of the phase transition of the frozen reagent, is carefully controlled through the selection of appropriate parameters including the starting temperate of the liquid (nonfrozen reagents), the ratio and volumes of the frozen and liquid reagents, the temperature difference between the temperature of the reaction mixture and the temperature of the temperature control arrangement, and the particular agitation arrangement and agitation parameters.

[0039] One of the reagents may be an alkali salt solution, wherein the alkali salt solution is selected from the group consisting of sodium hydroxide, potassium hydroxide, calcium hydroxide, magnesium hydroxide, ammonium hydroxide, and combinations thereof. In one embodiment, the alkali salt solution is sodium hydroxide. These alkali salt solutions are reasonably expected to behave in a similar manner in respect of its frozen state and phase transition from the frozen to the liquid state. According to the present invention, in order to control the contact and the resultant reaction between the reagents in the reaction mixture, at least one of the reagents is provided in a frozen state. Depending on the temperature of the remaining reaction mixture, temperature control provided by the particular reaction setup, and the various agitation parameters, the rate of melting, in other words the phase transition, of the frozen reagent can be controlled as required.

[0040] In one embodiment, the alkali salt solution may be provided in the frozen state.

[0041] The metal ions are selected from the group consisting of Fe, Ni, Cu, Ag, Au, Pd, Pt, Sn, Zn, and combinations thereof. Iron, Nickel and Copper have been exemplified since these metal nanoparticular are of particular commercial interest. Iron oxides are used as conventional catalysts, photocatalysts and in medical diagnostics, for example. Copper oxides are preferred for environmental remediation applications, while nickel oxides are extensively used in hydrogen containing systems, for example. However, the person of ordinary skill will know that there would be reasonable expectation of success in employing the method of the invention, as described herein, with different metal ions including Ag, Au, Pd, Pt, Sn, Zn, Co, Ti, Cr, Ru, and Rh. In one embodiment, for example, the alkali salt is provided in the frozen state with the metal salt solution in the liquid phase. Accordingly, the freezing characteristics of the metal salt is not a factor. The skilled person would therefore have a reasonable expectation that the reaction will proceed in the same manner as a conventional liquid-liquid reaction would proceed, only with the technical advantage that contact of the reagents are managed through the controlled phase transition of the frozen reagent. Once the phase transition is effected by the selected temperature and agitation, for example, the precipitation reaction occurs through controlled contacting of the reagents in the normal manner.

[0042] The temperature of the reaction mixture is controlled at a temperature in the range of about 5eC to about 25eC. Preferably, the reaction mixture is controlled at a temperature in the range of about 5eC to about 20eC, about 5eC to about 15 -C, or about 10eC to about 15eC. The temperature of the reaction mixture, as used herein, refers to the average temperature of the entire reaction mixture including the effect of the reagent that is undergoing the phase transition from the frozen to the liquid state. The temperature of the reaction mixture can be controlled using any standard method known in the art of temperature control on industrial scale apparatus. The temperature range of about 5eC to about 25eC, is provided of one example of the implementation of the underlying invention, that is the controlled phase transition of one of the reagents in the reaction. The temperature and the rate of temperature increase will naturally dependent on the reagents used, the particular reactor apparatus design, and the volumes of the reagents used in the particular reaction, to name a few. However, it will be appreciated that this all of these changes will still fall within the scope of the underlying invention towards the preparation of metal oxide nanoparticles.

[0043] Contact of the reagent undergoing the phase transition from the frozed to the liquid state with the remaining reagents in the reaction mixture including the alkali salt solution, in one example, is also partially controlled by the method of agitation and the selected agitation parameters. The method of agitation may be any one or more of those known in the art. In particular embodiments, agitation of the reaction mixture is provided by conventional stirring, ultrasonic agitation, and a combination of stirring and ultrasonic agitation thereof. Ultrasonic agitation will be selected based on the other parameters of the method including reagents, relative ratios and volumes, temperature control, etc. In one example, the ultrasonic agitation is provided at about 50 - 100 W. Without thereby wishing to be bound by any particular theory, it is believed that ultrasonic agitation assists in controlling the phase transition, i.e. the transformation of the frozen reagent, such that the reaction between the alkali salt and metal salt can occur at the interface between phases. Further, the ultrasonic agitation prevents post-reaction agglomeration of nanoparticles in the liquid medium, ensuring that the final product is of a uniform size distribution and of a specific shape (both dependent on temperature and agitation rate). The present invention will now be described more fully hereinafter with reference to some of the non-limiting embodiments of the invention.

[0044] General experimental procedures

[0045] The materials used in the experiments which follow included super-fine grade steel wool, copper shavings, 10.2M hydrochloric acid (HCI) and 12.3M nitric acid (HNO3), sodium hydroxide pellets, hydrogen peroxide (30 vol%), nickel powder (99.7 wt%) and methylene blue (99 wt%). All the agents were used as is.

[0046] A standard 1 M NaOH solution was prepared by dissolving 40g of sodium hydroxide pellets in 1 L of deionized water. Ice cube trays were used to freeze 10ml cubes of the 1 M NaOH solution in the laboratory freezer.

[0047] For the iron ion solution, 10.2M HCL was diluted to produce 1 L solutions having concentrations of 3.9M and 2.6M respectively. Two samples of 1 g of steel wool were weighed out and added to two separate 50 ml beakers. 12.5 ml of the 3.9M HCI solution was added to the first beaker to produce a Fe2+ion solution, and 25ml of the 2.6M HCI solution was added to the second beaker to produce a Fe3+ion solution. In each case, the steel wool was left to dissolve completely (approximately 15 to 20 minutes). The resulting solutions were filtered separately under suction to remove any remaining carbon solids. 1 ml of hydrogen peroxide was added dropwise to the 25 ml solution to convert the Fe3+ions to the Fe2+state. The change in state was identified by the solution changing from a light green to a golden orange colour. The solutions were combined and formed a bright yellow solution.

[0048] For the copper solution, approximately 2 grams of the pure copper shavings were weighed out and placed in a beaker. The 12.3M nitric acid provided was diluted to a 2M solution and 35ml was added dropwise until the copper had dissolved. The resultant copper ion solution was filtered under suction to remove any undissolved copper shavings. For the nickel solution, approximately 2 grams of the pure nickel powder (99.7%) was weighed out and placed in a beaker. The nitric acid provided was diluted to a 2M nitric acid solution and 25ml added dropwise to the nickel powder until the nickel powder was dissolved and formed a dark green nickel ion solution. The mixture was then filtered under suction to remove any undissolved nickel.

[0049] Comparative Example 1 : Co-precipitation preparation of Fe, Cu, and Ni metal nanoparticles

[0050] The comparative example experiments were conducted by precipitation at ambient conditions (1 atm, 25eC).

[0051] 20ml of deionized water and 20ml of 1 M NaOH was added dropwise to the previously prepared iron ion solution. The formation of a black precipitate was observed. The pH was tested to allow for the formation of the required iron hydroxide. Sodium hydroxide was added dropwise until a pH of 14 was obtained. The total amount of sodium hydroxide added to the solution was about 300ml. The resulting solution was then aerated using a pump for about 1 hour. Thereafter, the solution was filtered under suction for about 1 hour. The resulting precipitate was left to dry overnight to form the final product.

[0052] 60ml of the 1 M NaOH solution and 60ml of distilled water was added consecutively to the copper ion solution. The 1 M NaOH solution was added dropwise to the solution until it turned cloudy blue and reached a pH of 13-14. A total of about 300ml of NaOH was used. The solution was aerated using a pump for about 1 hour and left overnight. The resultant solution was filtered, and the precipitate was left in the oven at 100eC until the blue particles turned completely black.

[0053] 1 M NaOH solution was added dropwise to the nickel nitrate ion solution until it turned light green and reached a pH of 13-14. The solution was aerated using the fish a pump for about 1 hour and left overnight. The nickel oxide solution was filtered, and the precipitate was left in the oven at 100eC for 1 hour. Example 2: Preparation of Fe, Cu, and Ni metal nanoparticles using at least one reagent in a frozen state (NaOH)

[0054] In the experiments conducted for example two, the 1 M sodium hydroxide solution was provided as 10ml cubes frozen cubes, and conducted at a temperature of 10eC and 15eC. However, it can reasonably be expected that similar results would be obtained if the metal ion solution would be provided in the frozen state. Further temperatures outside the range of 10eC and 15eC can reasonably be expected to provide comparable results, for example the temperature of the reaction mixture can be controlled between more than about 0eC to about 25eC. Depending on the reagents used, and the desired metal nanoparticle properties, the temperature of the reaction mixture may be controlled at a temperature in the range of between about 1eC to about 20eC, between about 5eC to about 15eC, or between about 10eC to about 15eC.

[0055] The previously prepared iron ion solution was placed in the temperature controlled water bath by clamping and allowed to reach thermal equilibrium. Once thermal equilibrium was reached, two of the frozen sodium hydroxide cubes were added and allowed to melt completely. Thereafter, 20ml of deionized water was added to the solution. The sodium hydroxide cubes were added two at a time and allowed to melt until the pH reached 14. The resulting solution was then aerated, filtered, and left to dry as with the comparative examples. The procedure was repeated at 10eC, and in an ultrasonic bath at 15eC and at 10eC.

[0056] The previously prepared copper ion solution was placed in a temperature controlled water bath at 10eC, and allowed to reach thermal equilibrium. Six of the previously prepared 1 M NaOH frozen cubes were added to the solution and were allowed to melt completely before 60ml of distilled water was added. Thereafter, more 1 M NaOH frozen cubes were added to the solution until it reached a pH of 13-14 and a blue precipitate had formed. In total, approximately 30 NaOH frozen cubes were added. The solution was aerated for one hour and left overnight. The copper oxide solution was filtered, and the particles were left in the oven at 100eC until the blue particles turned completely black. The procedure was repeated at 15eC, and with ultrasonic agitation at 10eC and at 15eC.

[0057] The previously prepared nickel ion solution was placed in a temperature controlled water bath at 10eC and allowed to reach thermal equilibrium. Thereafter, 1 M NaOH frozen cubes were added to the solution until it reached a pH of 13-14 and a green precipitate had formed. In total, approximately 30 NaOH frozen cubes were added. The solution was aerated for one hour and left overnight. The nickel oxide solution was filtered, and the particles were left in the oven at 100eC for 1 hour. The procedure was repeated at 15eC, and with ultrasonic agitation at 10eC and at 15eC.

[0058] Synthesis and Characterisation of Fe Nanoparticles

[0059] The iron nanoparticles produced using the method of the invention was compared to that produced by the conventional co-precipitation method. An EDX analysis was conducted and confirmed that the correct iron oxide nanoparticles were produced with no contaminants. The size, shape and size distribution of the particles were used as a means of comparison. The average magnetite particle sizes of these experiments are shown in Figure 1 .

[0060] As can be seen from Figure 1 , the average particle size for the magnetite synthesized using the method of the invention was significantly and consistently smaller than that produced using the conventional coprecipitation method. The mean particle sizes were found to be 79.38nm, 19.04nm, 13.91 nm, 14.91 nm and 11.93nm for the control experiment, 15°C without agitation, 10°C without agitation, 15°C with agitation and 10°C with agitation, respectively. It is therefore clear that desirable particle size, and particle size distribution, was obtained even at 15°C without any agitation. Size distribution plots for each of the samples prepared employing the method of the invention showed that particle size distribution was significantly more uniform when compared to the co-precipitation method.

[0061] Without thereby wishing to be bound by any particular theory, it is believed that the difference in size of the magnetite (iron) nanoparticles can be attributed to how the reagents contact each other. In the co-precipitation method, NaOH was added dropwise and it was observed that localized areas of high concentration of NaOH existed in the iron solution. This promoted nucleation and the rapid growth of the particles, resulting in the production of nanoparticles that are more clustered, larger in size and of a spherical shape. In Figure 1 , the error bars reflect the population standard deviation with the population being all the particles produced at each of the experimental conditions across the three runs. It is clear that the particles produced in the co-precipitation method followed a wider distribution than that of the method of the invention utilising a frozen reagent and controlled contact between reagents.

[0062] In the method of the invention, the rate at which the frozen cubes melted was regulated by controlling the temperature of the solution, resulting in the reaction rate and diffusion being controlled, thereby producing particles of a smaller size and more uniform spherical shape than that of the conventional co-precipitation method. It is clear from Figure 1 that a lower synthesis temperature resulted in smaller particles. A decrease in the synthesis temperature from 15°C to 10°C led to a decrease in particle size by 26.79% for non-agitation and 19.99% with agitation. The use of agitation resulted in a further decrease in particle size for the same temperature, 21.69% for 15°C and 14.23% for 10°C.

[0063] Synthesis and Characterisation of Cu Nanoparticles

[0064] An EDX analysis was conducted and confirmed that the correct copper oxide nanoparticles were produced with no contaminants. A comparison of the average copper particle sizes for each method used are shown in Figure 2. As can be seen from Figure 2, the average particle size for the copper oxide synthesized using the method of the invention was significantly smaller than that produced using the conventional co-precipitation method. The mean particle sizes were found to be 147.86nm, 81.18nm, 65.60nm, 47.28nm and 33.26nm for the control experiment, 15°C without agitation, 10°C without agitation, 15°C with agitation and 10°C with agitation respectively. The lower temperature with agitation provided the smallest size particle of 33.26nm, although the result at 15eC without agitation is still notable. Size distribution plots for each of the samples prepared employing the method of the invention showed that particle size distribution was significantly more uniform when compared to the co-precipitation method. The size distributions obtained with the method of the invention were also found to be narrower at the lower temperatures with agitation. This was also confirmed with the size of the error bars as shown in Figure 2.

[0065] In the co-precipitation method, the average particle size was extremely large, clustered, had a wider distribution and a rod-like shape. Size distribution data showed that the majority of the particles lie in the region greater than 100nm, with a few particles below 100 nm. Utilising the method of the invention, the non-agitated experiments still produced rod-like shaped particles but relatively smaller than that of the co-precipitation method. The experiments at a lower temperature with agitation displayed smaller and more uniform spherical shaped particles, which are desirable due to its high specific surface area.

[0066] Synthesis and Characterisation of Ni Nanoparticles

[0067] As in the Fe and Cu experiments, an EDX analysis confirmed that the correct nickel oxide nanoparticles were produced with no contaminants. The average nickel nanoparticle particle sizes for each method are shown in Figure 3.

[0068] As can be seen from Figure 3, the average particle size for the nickel synthesized using the method of the invention was relatively smaller than that produced using the conventional co-precipitation method. The mean particle sizes were found to be 29.15nm, 24.68nm, 22.15nm, 17.24nm and 14.62 nm for the control experiment, 15°C without agitation, 10°C without agitation, 15°C with agitation and 10°C with agitation respectively. The lower temperature with agitation provided the smallest size particle of 14.62 nm.

[0069] Size distribution plots for each of the samples prepared employing the method of the invention showed that particle size distribution was significantly more uniform when compared to the co-precipitation method. The size distributions of the frozen reagent method were also found to be narrower at the lower temperatures with agitation. In the co-precipitation method, the average particle size was relatively small, but had a wider distribution and a spherical shape. The experiments utilising the method of the invention with agitation, displayed smaller and more uniform spherical-shaped particles.

[0070] Figure 4 provides a comparison of the dimensions of the different transition metal nanoparticles. As can be seen from Figure 4, the method of the invention utilising at least on frozen reagent to control the contact between reagents resulted in metal oxide nanoparticles that are smaller, more uniform and with a narrower particle size distribution compared to the traditional coprecipitation method. Embodiments of the invention have been shown for Fe, Cu, and Ni. However, the skilled person would have a reasonable expectation of success based on the present teaching, and without conducting any undue experimentation, to produce metal nanoparticles with desirable properties compared to standard methods for other metals including Ag, Au, Pd, Pt, Sn, Zn, and combinations thereof.

[0071] The choice of the precursor reagents, reaction pH, temperature and concentration of the precursors are possible process variables that will affect the relative rate of formation of the various hydroxides, and the resultant metal nano particle properties.

[0072] Example 3: Dye test performance of Fe, Cu, and Ni metal nanoparticles

[0073] Standard dye solutions of concentrations 20, 40, 60, 80 and 100 mg / l were prepared by dissolving the appropriate amount of methylene blue dye in 1 L of deionized water. The spectrophotometer was set to a wavelength of 662 nm, corresponding to that of methylene blue dye. The 100% adsorbance (A) / 0% transmission (T) function was selected. A cuvette was filled with deionized water and placed in the spectrophotometer to obtain a reference value. Thereafter, each standard solution was tested for the adsorbance reading. A straight-line plot of absorbance vs solution concentration was obtained, resulting in a successful calibration. 20 mg of the metal (iron, copper and nickel) nanoparticles was weighed out and added to 60 ml of the dye solution. The overhead stirrer speed was set to 500 rpm. The adsorbent-containing dye solution was stirred for about 1 hour at ambient conditions. Thereafter, the solution was extracted using a syringe before being added to the cuvette using a micro-filter for spectrophotometer adsorbance analysis. This procedure was repeated for each of the metal nanoparticles prepared in the control, 10°C and 15°C experiments. The dye removal tests were designed to test the performance of the synthesized metal nanoparticles. The selected parameters were the same for all runs to allow for the nanoparticles performances to be compared. The iron, copper and nickel oxide nanoparticles from each of the previously performed methods were selected for testing and repeated 3 times to ensure the results were consistent.

[0074] The dye removal performance of the various samples was tested and compared at an overhead stirrer speed of 500 rpm, and initial dye solution concentration of 60 mg / L, a contact time between the dye solution and the sample of 1 hour, the volume of dye solution used of 60 ml, and a sample dosage of 20 mg.

[0075] Iron oxide removal of dye from aqueous solution

[0076] The results for the % dye removed for each of the iron oxide nanoparticle sample experiments are shown in Figure 5. As can be seen from Figure 5, the % dye adsorbed for the iron (magnetite) nanoparticles synthesized using the method of the invention was significantly more than that produced using the conventional co-precipitation method. The % dye removed was found to be 29.87%, 49.90%, 69.93%, 66.06% and 83.57% for the samples produced by the control, 15°C without agitation, 10°C without agitation, 15°C with agitation and 10°C with agitation experiments respectively. Further, in Figure 5, the error bars reflect the sample standard deviation for the three runs conducted for each of the methods. Therefore, the error bars represent the difference in the % dye adsorbed for the three runs conducted for each method and gave a good representation of the reproducibility of the experiments. Copper oxide removal of dye from aqueous solution

[0077] The results for the % dye removed for each of the copper oxide nanoparticle sample experiments are shown in Figure 6. As can be seen from Figure 6, the % dye removed for the copper oxide nanoparticles synthesized using the method of the invention was more than that of the nanoparticles prepared using the conventional co-precipitation method. The % dye removed was found to be 16.73%, 27.51%, 36.94%, 44.85% and 56.13% for the samples produced in the control, 15°C without agitation, 10°C without agitation, 15°C with agitation and 10°C with agitation experiments respectively.

[0078] Nickel oxide removal of dye from aqueous solution

[0079] The results for the % dye removed for each of the nickel oxide nanoparticle sample experiments are shown in Figure 7. As can be seen from Figure 7, the % dye removed for the nickel oxide nanoparticles synthesized using the method of the invention was more than that produced using the conventional co-precipitation method. The % dye removed was found to be 55.62%, 61.85%, 65.72%, 66.57% and 76.33% for the samples prepared using the control, 15°C without agitation, 10°C without agitation, 15°C with agitation and 10°C with agitation experiments respectively.

[0080] The aim of the dye removal experiments was to determine whether the metal nanoparticles produced by the method of the invention utilising at least one frozen reagent to control reagent contact was more effective in the removal of organic dye than the conventional co-precipitation method. The dye test was used as a representative test for metal nanoparticle performance, and it is reasonably anticipated that the underlying particle properties will lead to similar improved performance in other catalytic applications. Figure 8 shows a comparison of the results for the dye removal tests of the three metal oxide nanoparticles. It is clear from the results presented in Figure 8 that the metal nanoparticles prepared using the method of the invention led to improved result across all samples.

[0081] This above description of some of the illustrative embodiments of the invention is to indicate how the invention can be made and carried out. Those of ordinary skill in the art will know that various details may be modified thereby arriving at further embodiments, but that many of these embodiments will remain within the scope of the invention. For example, it will be clear to those of ordinary skill in the art, and having the benefit of the present teaching, that the invention is not limited to the exact material used herein but that various other metal ions and alkali salt solutions can be used.

Claims

CLAIMS1. A method for producing metal oxide nanoparticles, the method comprising: a) contacting at least a first reagent comprising metal ions and a second reagent comprising an alkali salt solution to form a reaction mixture, b) wherein at least one of the reagents are initially provided in a frozen state, and c) controlling the rate of contact of the at least one frozen reagent with other reagents in the reaction mixture by controlling the temperature of the reaction mixture and / or agitation applied to the reaction mixture to control the phase transition of the frozen reagent, thereby to control the size, size distribution, and shape of the metal oxide nanoparticles.

2. The method according to claim 2, wherein the alkali salt solution is a NaOH solution.

3. The method according to any one of the preceding claims, wherein the second reagent is provided in a frozen state.

4. The method according to any one of the preceding claims, wherein the metal ions are selected from the group consisting of Fe, Ni, Cu, Ag, Au, Pd, Pt, Sn, Zn, and combinations thereof.

5. The method according to claim 5, wherein the metal ions are selected from Fe, Ni, Cu, and combinations thereof.

6. The method according to any one of the preceding claims, wherein the temperature of the reaction mixture is controlled at a temperature in the range of about 5eC to about 25eC.

7. The method according to claim 6, wherein the temperature of the reaction mixture is controlled at a temperature in the range of about 5eC to about 20 -C, about 5eC to about 15 -C, or about 10eC to about 15eC.

8. The method according to any one of the preceding claims, wherein the agitation provided is selected from the group consisting of stirring, ultrasonic agitation, and combinations thereof.

9. The method according to claim 9, wherein the ultrasonic agitation is provided at about 50 - 100 W.

10. The method according to claim 1 , wherein the metal ions are Fe ions, and the average particle size of the FeO nanoparticles is at least 70% less than the average particle size obtained using a conventional co-precipitation method wherein the reagents are added dropwise in a liquid-liquid reaction.11 . The method according to claim 1 , wherein the metal ions are Cu ions, and the average particle size of the CuO nanoparticles is at least 45% less than the average particle size obtained using a conventional co-precipitation method wherein the reagents are added dropwise in a liquid-liquid reaction.

12. Metal oxide nanoparticles prepared according to the method of any one of claims 1 to 11 .

13. A method of catalysing a reaction, the method comprising providing a metal oxide nanoparticle catalyst prepared according to the method of any one of claims 1 to 11 .