Nanoparticles, uses thereof, and method for producing metallic nanoparticles

Abstract

In the present invention, there is provided a nanoparticle comprising a plasmonic metal and a catalytic metal, characterized in that: the plasmonic metal and the catalytic metal are both present at the surface of the nanoparticle; wherein the nanoparticle comprises 0.1 wt% - 5 wt% of said catalytic metal; and wherein said catalytic metal is capable of catalysing a hydrogen evolution reaction. The present invention is also directed to the preparation method and uses of said nanoparticle.

Description

Nanoparticles, uses thereof, and method for producing metallic nanoparticles FIELD

[0001] The present disclosure relates to the field of catalytic nanoparticles, uses thereof and synthesis methods thereof. BACKGROUND

[0002] Solar or light-driven chemistry has emerged as a promising alternative for a sustainable transition to a green economy by reducing the carbon footprint and replacing fossil fuels. Plasmonic catalysis, which combines catalytic and optical properties of metals at the nanoscale, can play a crucial role in accelerating and controlling various molecular transformations using light as the only or primary energy input.

[0003] In plasmonic nanoparticles, incident photons resonantly interact with the collective motion of electrons. This phenomenon is referred to as localized surface plasmon resonance (LSPR) and takes place at a characteristic frequency that is dependent on size, shape, structure, composition of the nanoparticles, and the dielectric constant of the surroundings.

[0004] In plasmonic catalysis, the localized surface plasmon resonance excitation by light in plasmonic nanoparticles leads to the generation of hot carriers as well as localized heating that promote enhanced reaction rates.

[0005] For plasmonic metals such as silver (Ag), gold (Au), copper (Cu), and aluminium (Al), the Localized Surface Plasmon Resonance (LSPR) excitation can take place at the visible and near-infrared ranges, opening up possibilities for the utilization of solar radiation to excite their LSPR.

[0006] It has been established that the LSPR excitation in plasmonic nanoparticles can accelerate the rate of several chemical transformations. This so-called plasmonic catalysisor plasmonic photocatalysis has been attracting strong interest in the research community due to its beneficial properties.

[0007] LSPR can be used to target molecular transformations that are important in the areas of green energy, production of commodity chemicals, fuels, and environmental remediation. Although there has been significant progress in plasmonic catalysis, control of the catalytic selectivity and further improvement of the catalytic efficiency remains a major challenge.

[0008] Metallic nanoparticles that incorporate both plasmonic metals, and catalytic metals that do not support LSPR excitation in the visible or near–infrared ranges, are at the forefront of plasmonic catalysis. These systems, known as antenna–reactor nanoparticles, use the plasmonic component of the nanoparticle to harvest light energy through LSPR excitation, which generates LSPR-excited charge carriers that enhance catalytic performance at the catalytic sites. By utilizing this approach, it is possible to extend plasmonic catalysis to transformations beyond those for which Ag and Au, the most common plasmonic nanoparticles, are typically catalytically active.

[0009] Although antenna-reactor nanoparticles have been described for a variety of plasmonic-catalytic combinations, the antenna-reactor nanoparticles such as core-shell and alloy-based nanoparticles suffer from the disadvantage that the deposition of the catalytic metal at the surface of the plasmonic component usually leads to a decrease in the absorption and thus plasmonic properties of the core component.

[0010] Moreover, these systems may not be optimal for improving metal utilization, which is particularly important for platinum (Pt) and other Pt-group metals. Although these metals are the least abundant of the earth’s elements, they are crucial to several catalytic applications. Therefore, the improvement of metal utilization and catalytic performance represents an important challenge.

[0011] The present invention offers a solution to this problem by providing a nanoparticle with ultra-low deposition of Pt onto an Au nanoparticle. The inventors have surprisingly discovered that in addition to the increased efficiency of Pt usage, the catalytic efficiency of the metallic nanoparticle improves dramatically in the use of hydrogen evolution reaction.

[0012] The beneficial effect of the present disclosure is not limited to photocatalysis but some embodiments of the present disclosure provide an improvement to catalytic reactions performed without visible light, for example in electrocatalysis. SUMMARY

[0013] The invention is defined by the features of the independent claims. Some specific embodiments are defined in the dependent claims.

[0014] According to a first aspect, there is provided a nanoparticle comprising a plasmonic metal and a catalytic metal, wherein the plasmonic metal and the catalytic metal are both present at the surface, such as in a surface layer, of the nanoparticle, wherein the nanoparticle comprises 0.1 wt% - 5 wt% of said catalytic metal; and typically said nanoparticle is capable of catalysing a hydrogen evolution reaction.

[0015] According to a second aspect, there is provided a use of the nanoparticle as a catalyst in a chemical reaction.

[0016] According to a third aspect, there is provided a method for producing a metallic nanoparticle comprising the steps of: a) providing a suspension of nanoparticles of a plasmonic metal, such as Au or Ag nanoparticles; b) optionally, adding L-ascorbic acid to the suspension of the nanoparticles; and c) adding catalytic metal ions, such as Pt(IV) metal ions, to the suspension in conditions suitable for producing metallic nanoparticles of the plasmonic metal and the catalytic metal.

[0017] According to a fourth aspect, there is provided a method comprising the steps of: a) providing a suspension of nanostructures of a plasmonic metal, such as nanoparticles of a plasmonic metal; and b) adding ions of a catalytic metal to the suspension obtained in step a) in conditions suitable for producing nanostructures comprising the plasmonic metal and the catalytic metal. Typically said nanostructures are capable of catalysing a hydrogen evolution reaction.

[0018] According to a fifth aspect, there is provided a method for producing a nanostructure, the method comprising the steps of: a) providing a solution of a precursor for a nanostructure, which precursor comprises a plasmonic metal; b) adding ions of a catalytic metal to the solution obtained in step a); and c) subjecting the solution comprising the precursor and the ions of the catalytic metal to conditions suitable for producing ananostructure comprising the plasmonic metal and the catalytic metal. Typically said nanostructures are capable of catalysing a hydrogen evolution reaction.

[0019] Various embodiments of the first, the second, the third, the fourth or the fifth aspect may comprise one or more features from the following list: ^ The nanostructure is a nanoparticle or a nanowire or a nanosheet. ^ Said nanostructure, such as a nanoparticle exhibits catalytic activity for the hydrogen evolution reaction. ^ A surface layer of the nanostructure comprises a mixture of the plasmonic metal and the catalytic metal. ^ The surface layer has a thickness of up to 3 nm, such as up to 1.5 nm. ^ A core or an inner part of the nanostructure comprises an amount of the plasmonic metal and may be substantially free from the catalytic metal. ^ Said plasmonic metal is gold (Au), silver (Ag) or tungsten (W). ^ Said catalytic metal is platinum (Pt), iridium (Ir), nickel (Ni), iron (Fe), molybdenum (Mo), or cobalt (Co). ^ Said plasmonic metal is gold (Au) and said catalytic metal is platinum (Pt). ^ Said plasmonic metal is silver (Ag) and said catalytic metal is platinum (Pt). ^ Said plasmonic metal is in the form of a metal oxide or a metal sulphide or a metal hydroxide or a metal oxyhydroxide. ^ Said catalytic metal is in the form of a metal oxide or a metal sulphide or a metal hydroxide or a metal oxyhydroxide. ^ Said plasmonic metal and said catalytic metal form an alloy on the surface of the nanoparticle. ^ The nanostructure, such as a nanoparticle comprises 0.1 wt% - 5 wt% of said catalytic metal, preferably 0.1 wt% - 3.0 wt% of said catalytic metal. ^ The nanostructure, such as a nanoparticle comprises < 2.5 wt% of said catalytic metal, preferably 0.1 wt% - 1.0% wt% of said catalytic metal. ^ Said nanostructure, such as a nanoparticle is a bimetallic nanostructure, such as a bimetallic nanoparticle. ^ Said bimetallic nanostructure, such as a bimetallic nanoparticle contains gold (Au) as said plasmonic metal and platinum (Pt) as said catalytic metal.^ The diameter of the nanoparticle or the nanowire is in the range of 5 – 40 nm, such as 5 – 20 nm, preferably about 14 nm. ^ The shape of the nanoparticle is spherical. ^ The catalytic metal, such as Pt, is enriched within 3 nm, such as within 1.5 nm from the surface of the nanostructure, such as a nanoparticle. ^ The catalytic metal, such as Pt, is enriched within a surface layer of the nanostructure, such as a nanoparticle. ^ Pt is enriched within 1.5 nm from the surface of the nanoparticle. ^ Said nanoparticle is obtained by: a) adding an acid, such as L-ascorbic acid, to a suspension of Au nanoparticles; and b) adding Pt(IV) metal ions to the suspension obtained in step a) in conditions suitable for producing metallic Au-Pt nanoparticles. ^ Said Pt(IV) metal ions are provided by H2PtCl6 reagent. ^ The concentration of H2PtCl6in the suspension is in the range of 2.0 – 3.0 mM, preferably about 2.44 mM. ^ The Au nanoparticles in said suspension are citrate capped. ^ The temperature of the suspension is at least 40 ° C, preferably at least 60 ° C, more preferably at least 70° C. ^ Citrate-capped Au nanoparticles are employed as seeds. ^ H2PtCl6 is employed as a precursor. ^ An acid, such as L-ascorbic acid is employed as a reducing agent. ^ The ratio of Au:Pt in the nanoparticle is in the range of 95:5 – 99.4:0.6. ^ Said catalyst facilitates the reaction by photocatalysis including plasmonic catalysis. ^ Said catalyst facilitates the reaction by electrocatalysis. ^ Said catalyst facilitates a hydrogen evolution reaction. ^ The ratio of Au:Pt in the suspension is in the range of 94:6 – 99.4:0.6. ^ Said Pt(IV) metal ions are provided by H2PtCl6 reagent. ^ The concentration of H2PtCl6 in the suspension is in the range of 2.0 – 3.0 mM, preferably about 2.44 mM. ^ The Au nanoparticles in said suspension are citrate capped. ^ The temperature of the suspension is at least 40 ° C, preferably at least 60 ° C, more preferably at least 70° C.^ Said nanoparticle comprises said catalytic metal, such as Pt, 0.1 wt% - 5 wt%, preferably 0.1 wt%-4.0 wt%, more preferably 0.1 wt% - 3.0 wt% of the total weight of the nanoparticle. ^ Said nanoparticle comprises said catalytic metal, such as Pt, 0.1 wt% - 2.5 wt%, preferably 0.1 wt% - 1.0% wt% of the total weight of the nanoparticle. ^ Said nanoparticle comprises a plasmonic metal selected from the group consisting of: gold (Au), silver (Ag), and tungsten (W). ^ Said nanoparticle comprises a catalytic metal selected from the group consisting of: platinum (Pt), iridium (Ir), nickel (Ni), iron (Fe), molybdenum (Mo), and cobalt (Co). ^ Said catalytic metal, such as Pt, is enriched within 1.5 nm from the surface of the nanoparticle. ^ Said catalysis is electrocatalysis, photocatalysis or plasmonic catalysis. ^ Said catalysis, electrocatalysis, photocatalysis or plasmonic catalysis is used in the hydrogen evolution reaction. ^ When the catalytic metal in the nanoparticle is Pt(IV), it can be provided by H2PtCl6. ^ In the synthesis method of the nanoparticles, the concentration of H2PtCl6in the reaction is in the range of 2.0 – 3.0 mM, preferably about 2.44 mM. ^ In the synthesis method of the nanoparticles, the concentration of L-ascorbic acid in the reaction is in the range of 2.0 – 3.0 mM, preferably about 2.66 mM. ^ In the synthesis method of the nanoparticles, the Au nanoparticles in the reaction are citrate capped. ^ In the synthesis method of the nanoparticles, the temperature of the reaction solution is at least 40 °C, preferably at least 60 °C, more preferably at least 70 °C, most preferably in the range of 40-80 °C. ^ In the synthesis method of the nanoparticles, the distribution of Pt in the nanoparticle is controlled by selecting a suitable concentration of H2PtCl6.

[0020] The results characterizing the present disclosure show that plasmonic enhancement is dependent on both the composition of the nanoparticles and the surface distribution of the catalytic metal on the plasmonic metal core such as Pt on the Au core. An ultra-low catalytic Pt content surprisingly favours higher light absorption (as shown from the UV-VIS spectra in FIGURE 2A) which enable a higher plasmonic enhancement. Although the Pt content in the more Pt-rich nanoparticles is relatively low in comparison tocontents disclosed in prior art and the nanoparticles still display an extinction band in the visible spectrum, the plasmonic enhancement was low.

[0021] This illustrates the surprising effect the composition and surface distribution of Pt have for the optimization of plasmon-enhanced performance of an Au-Pt antenna- reactor nanoparticle, wherein an extremely dilute distribution of Pt is required for improved plasmonic enhancement.

[0022] The present disclosure shows that the deposition of ultra-low loadings of Pt at the surface of Au nanoparticles combines the catalytic and plasmonic properties of Pt and Au, respectively. The ultra-low loading of Pt at the Au surface avoids the decrease in the optical absorption of Au (plasmonic properties) during Pt deposition so that plasmonic effects of Au are retained. The effect of this is a dramatic improvement of catalytic efficiency of Au-Pt antenna-reactor nanoparticle systems.

[0023] Upon LSPR excitation, hot electrons and holes are generated. Hot electrons flow to the Pt sites, where they can participate in the activation of adsorbed H2O species, thus further accelerating the Hydrogen Evolution Reaction (HER) and leading to a decrease in overpotential. In this case, hot holes are harvested by the electrolyte. In this system, the presence of a lower Pt content and more dilute Pt distribution contributes to enhanced light absorption by the plasmonic component and better dissipation of the LSPR energy to the Pt sites, thus enabling higher HER activity.

[0024] In one embodiment, for example under visible light excitation, an eight-fold improvement in the photocatalytic performance of the hydrogen evolution reaction over pure Pt nanoparticles can be observed.

[0025] In another embodiment, for example without visible light excitation, a six-fold improvement in the electrocatalytic performance of the hydrogen evolution reaction over pure Pt nanoparticles can be observed.

[0026] In at least one embodiment, an ultra-low deposition of catalytic metal on a plasmonic core does not form a thick catalytic metal shell. A too thick catalytic shell may interfere the energy flow from the plasmonic metal to the catalytic metal, preventing full utilization of the plasmonic facilitation of the catalytic reaction.

[0027] As shown, the present disclosure provides at least the following advantages to prior art: i) the utilization of Pt as a rare and expensive metal is more efficient; ii) the ultra- low deposition of Pt provides an increased catalytic effect to antenna-reactor nanoparticles; and iii) a way to tune reaction selectivity of some chemical reactions. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] FIGURE 1 illustrates the general approach for obtaining the nanoparticles according to at least some of the embodiments of present disclosure. Initially, citrate-capped Au nanoparticles (NPs) with a diameter of 14 nm were used as seeds for Pt deposition by employing H2PtCl6as the precursor and L–ascorbic acid as the reducing agent. The ratio of Au to H2PtCl6was adjusted to target two different bimetallic compositions, with the aim of depositing a monolayer or a sub-monolayer coverage of Pt on Au. This yielded NPs containing 6 and 0.6 wt% in terms of Pt. These NPs were named as Au94Pt6and Au99.4Pt0.6, respectively.

[0029] FIGURE 2 illustrates the enhanced HER performance of ultra-low Pt content nanoparticles. (A) LSV curves normalized by Pt mass for Au94.4Pt0.6 and Au99Pt6 NPs. The curve for pure Pt NPs is shown for comparison. Measurements were performed under dark and under 525 nm LED irradiation conditions (dark and LSPR excitation, respectively) and recorded at 5 mVs–1 in Ar–saturated 0.5 H2SO4. (B) Mass activity comparison for all samples under dark and light irradiation conditions at –0.1 V; and (C) Tafel plots calculated from LSV curves normalized by geometric area for Au99.4Pt0.6, Au94Pt6 and Pt / C. Pt / C; platinum on carbon.

[0030] As depicted in FIGURE 2B, the mass activity was enhanced by 6- and 5-folds relative to Au94Pt6 and Pt / C NPs, respectively (the corresponding mass activities values were 6.1, 1, and 1.1 A mg–1 Pt for Au99.4Pt0.6, Au94Pt6 and Pt / C NPs, respectively, under dark conditions). These results suggest that the control over the Pt composition and distribution of Pt at the Au surface improved the HER activities under dark conditions.

[0031] Under light irradiation (LSPR excitation), all catalysts presented higher current densities under 525 nm light irradiation relative to dark conditions. The LSV plots normalized by geometric area showed the magnitude of the plasmonic enhancement, which was also dependent on the surface distribution of Pt. Au99.4Pt0.6and Au94Pt6NPs displayedenhancements of 19% and 7%, respectively, in the current densities upon light illumination, while the Pt / C catalyst showed an 8% enhancement. In order words, while Au94Pt6 and even Pt / C NPs displayed only a slight enhancement in current densities under light excitation, the current densities for the Au99.4Pt0.6NPs under light illumination conditions became 2.5–folds higher relative to both Au94Pt6 and Pt / C NPs. In this case, the mass activity for Au99.4Pt0.6 NPs reached 7 mA µg–1 Pt becoming 7 and 6–folds higher relative to Au94Pt6 and Pt / C NPs, respectively.

[0032] FIGURE 3 illustrates (A and D) the general size and shape of the nanoparticles of the present disclosure, (B, C, E, and F) the distribution of Pt in the nanoparticles of the present disclosure and (G) the characterization of the Pt enriched region of the nanoparticles of the present disclosure. EMBODIMENTS

[0033] Embodiments of the present disclosure provide plasmonic nanostructures, such as plasmonic nanoparticles, methods of obtaining plasmonic nanostructures, such as plasmonic nanoparticles, and the like. Embodiments of the present disclosure can be used in catalysis, and in chemical and biological sensing.

[0034] Embodiments of the present disclosure provide plasmonic-catalytic nanoparticles, synthesis thereof and uses thereof.

[0035] Unless otherwise stated herein or clear from the context, any percentages referred to herein are expressed as percent by weight based on a total weight of the respective composition.

[0036] In the present context, the term “nanoparticle” is a nanometre-size particle of an inorganic material having a diameter between 1 nm and 100 nm. In the present context the preferable diameter of the nanoparticles is 2 nm – 50 nm, more preferably, the diameter of the nanoparticles is 5 nm – 20 nm.

[0037] In the present context, the term “nanowire” refers to a one-dimensional nanostructure in the form of a wire with a diameter in a scale ranging from 1 nm to 100 nm. More generally, nanowires can be defined as structures that have a thickness or diameter constrained to tens of nanometres or less and an unconstrained length.

[0038] In the present context, the term “nanosheet” refers to a two-dimensional nanostructure with a thickness in a scale ranging from 1 nm to 100 nm.

[0039] In the present context, the term “spherical nanoparticle” refers to an average shape of the nanoparticles of the present invention nanoparticles wherein the shape of any one nanoparticle in practise may have, for example, an approximately spherical shape, a slightly flattened shape or a three-dimensionally angulated shape. Exemplary illustrations of the shape and size of the nanoparticles of the present disclosure are presented in Figure 3A and 3D.

[0040] As used herein, citrate capped Au nanoparticles refer to Au nanoparticles having a layer of negatively charged citrate associated on the surface of the nanoparticle for stabilizing and reducing the Au nanoparticles and preventing aggregation of said Au nanoparticles.

[0041] In the present context, the term “plasmonic catalysis” refers to a type of catalysis that uses the collective oscillations of free electrons in metal nanoparticles to drive chemical reactions.

[0042] In the present context, the term “electrocatalysis” refers to a type of catalysis wherein a specific electrochemical interaction is facilitated by metallic nanoparticles.

[0043] As used herein, the term “antenna-reactor nanoparticle” refers to a nanoparticle wherein a plasmonic metal, for example Au or Ag, is combined with a catalytic metal, for example Pt, to enable improved catalytic efficiency of said catalytic metal to convert light efficiently to chemical energy.

[0044] As used herein, the term “enriched” refers to the distribution of the catalytic metal in the Au nanoparticle, wherein a catalytic metal enriched region is a layer or location of the nanoparticle where said catalytic metal is present in higher concentration than outside of said layer or location.

[0045] The term “ultra-low catalytic metal content” refers to the deposition of the catalytic metal in very low amounts onto a nanoparticle. At least some of the embodiments of the present disclosure are determined to have such ultra-low catalytic metal contents.

[0046] As used herein, the term “core” refers to a part of a nanoparticle on which the catalytic metal, preferably Pt, is deposited. Preferably the core is made of plasmonic metal such as Au or Ag.

[0047] As used herein, the term “shell” refers to the outer region or surface layer of the metallic nanoparticle containing most of the catalytic metal that is deposited onto the core. The shell thickness can be determined by a measurement, wherein the precision of the measurement of said thickness is limited by the uncertainty of the measurement device and methods thereof. The shell does not explicitly define a discrete border between a region where only the catalytic metal or only the core metal is present.

[0048] In some embodiments, the present nanostructures may be in the form of nanoparticles, nanowires or nanosheets, preferably in the form of nanoparticles.

[0049] In an embodiment, plasmonic nanoparticle catalysts for improved photocatalysis is provided.

[0050] In a preferred embodiment, the plasmonic metal of the nanoparticle of the present disclosure is selected from the group consisting of: gold (Au), silver (Ag), and tungsten (W). Gold (Au) is particularly preferred as said plasmonic metal.

[0051] In another preferred embodiment, the catalytic metal of the present disclosure is selected from the group consisting of: platinum (Pt), iridium (Ir), nickel (Ni), iron (Fe), molybdenum (Mo), or cobalt (Co). Platinum (Pt) is particularly preferred as said catalytic metal.

[0052] In another preferred embodiment, said metal (i.e. plasmonic or catalytic metal) is in the form of a metal oxide or a metal sulphide.

[0053] In another preferred embodiment, said plasmonic metal and said catalytic metal form an alloy on the surface of the nanoparticle.

[0054] In another preferred embodiment, the nanoparticle comprises 0.1 wt% - 5 wt% of said catalytic metal, preferably 0.1 wt% - 3.0 wt% of said catalytic metal.

[0055] In another preferred embodiment, said nanoparticle comprises < 2.5 wt% of said catalytic metal, preferably 0.1 wt% - 1.0% wt% of said catalytic metal.

[0056] In particular, the present disclosure is directed to a nanoparticle comprising Au and Pt, wherein Au and Pt are both present at the surface of the nanoparticle, preferably in a form of an alloy.

[0057] In a preferred embodiment, said nanoparticle comprises 0.1 wt% - 5 wt% of Pt, preferably 0.5 wt% - 3.0 wt% of Pt. More preferably, the nanoparticle comprises < 2.5 wt% of Pt, preferably 0.1 wt% - 1.0% wt% of Pt.

[0058] In another preferred embodiment, the diameter of the nanoparticle is in the range of 5-20 nm, more preferably about 14 nm.

[0059] In another preferred embodiment, the shape of the nanoparticle is spherical.

[0060] In another preferred embodiment, Pt is enriched within 1.5 nm from the surface of the nanoparticle.

[0061] In an embodiment, the nanoparticle of the present invention is obtained by: a) adding L-ascorbic acid to a suspension of Au nanoparticles; and b) adding Pt(IV) metal ions to the suspension obtained in step a) in conditions suitable for producing metallic Au-Pt nanoparticles.

[0062] In a preferred embodiment, said Pt(IV) metal ions in the suspension are provided by H2PtCl6reagent.

[0063] In another preferred embodiment, the concentration of H2PtCl6 in the suspension is in the range of 2.0 – 3.0 mM, preferably about 2.44 mM.

[0064] In another preferred embodiment, the Au nanoparticles in said suspension are citrate capped.

[0065] In another preferred embodiment, the temperature of the suspension is at least 40 ° C, preferably at least 60 ° C, more preferably at least 70° C.

[0066] In another preferred embodiment, the nanoparticles are obtained so that i) citrate-capped Au nanoparticles are employed as seeds, ii) H2PtCl6 is employed as a precursor, and iii) L-ascorbic acid is employed as a reducing agent.

[0067] In another preferred embodiment, the nanoparticle is a bimetallic nanoparticle and the ratio of Au:Pt in the nanoparticle is in the range of 95:5 – 99.4:0.6.

[0068] In an embodiment, the present invention is directed to a use of the nanoparticle according to the present disclosure as a catalyst in a chemical reaction.

[0069] In a preferred embodiment, said catalyst facilitates the reaction by photocatalysis or plasmonic catalysis.

[0070] In another preferred embodiment, said catalyst facilitates hydrogen evolution reaction or nitrite reduction reaction.

[0071] In an embodiment, the present invention is also directed to a method for preparing a metallic nanoparticle comprising the steps of: a) adding L-ascorbic acid to a suspension of Au nanoparticles; and b) adding Pt(IV) metal ions to the suspension obtained in step a) in conditions suitable for producing metallic Au-Pt nanoparticles, wherein the ratio of Au:Pt in the suspension is in the range of 94:6 – 99.4:0.6.

[0072] EXAMPLES

[0073] In the methods of synthesis of the present materials, for each metal a suitable, corresponding precursor may be chosen. In the following, examples of synthesis methods employing particular precursors are provided.

[0074] Example 1. Synthesis of AuPt nanoparticles (NP)

[0075] The following is an exemplary embodiment for the preparation of at least one of the nanoparticles disclosed by the present invention

[0076] In a 250 mL round bottom flask, 100 mg of sodium citrate tribasic dihydrate was dissolved in 148 mL of water. The solution was stirred for 15 min at 105 °C in an oil bath. Then, 2 mL of HAuCl4(12.7 mM) was added, and the reaction was allowed to proceed for 30 min. A red suspension containing the Au NPs was obtained and allowed to cool down to room temperature under ambient conditions.

[0077] The deposition of Pt at the Au surface was performed by a seeded growth approach using ascorbic acid as a reducing agent.

[0078] In a 150 mL round bottom flask, 75 mL of the freshly prepared Au NPs suspension was heated to 70 °C under magnetic stirring for 15 min. Then, 35.2 mg of L– ascorbic acid was added to this suspension. After 30 min, a H2PtCl6 solution (2.44 mM) was added to generate the nanoparticles. The reaction mixture was kept under stirring for another30 min. After cooling down to room temperature, the nanoparticles were washed several times with water by successive rounds of centrifugation and removal of the supernatant.

[0079] Example 2.

[0080] The following is an exemplary embodiment for the measurement of the efficiency of the hydrogen evolution reaction catalysis.

[0081] Electrochemical experiments were performed in a three electrodes glass cell, with a glassy carbon rod (GCE) used as a working electrode (6 mm diameter, geometric area of 0.2827 cm2) and a high-area graphite rod used as the counter electrode. All the potentials were measured and displayed based on the reversible hydrogen electrode (RHE) prepared with the same solution of supporting electrolyte (0.1 M HClO4or 0.5 M H2SO4).

[0082] The electrolytes were prepared by diluting ultrapure HClO4 or H2SO4 in 18.2 MW cm water. The GCE electrode was cleaned by polishing it with alumina slurry, sonication in ultrapure water, and acetone (5 minutes each). Besides that, to avoid residual contamination from the synthesis, the nanoparticles were cleaned twice through water washing to remove synthesis reactants. After that, the clean GCE was modified by drop casting 30 μL of catalyst ink (AuPt nanoparticles and carbon black Vulcan XC–72R dispersed in H2O: IPA solution) resulting in a uniform film with an AuPt loading of 200 μg / cm2 and Vulcan carbon loading of 100 μg / cm2.

[0083] The electrochemical measurements were carried out at room temperature (25°C), using an Autolab PGSTAT 128 N equipped with Scan 250 modulus potentiostat. Before the experiments, the solution was purged with Argon 2.2, and during the data collection, this gas was kept in the cell headspace. For electrocatalytic studies, cyclic voltammograms were normalized by the platinum mass present in each catalyst accordingly with spectrometry absorption experiments.

[0084] For HER studies, linear sweep voltammetry measurements were performed at a scan rate of 5 mVs−1, and chronoamperometry measurements were recorded at –0.07 V (vs. RHE) under chopped illumination, both in Ar–saturated 0.5 M H2SO4solution. For the electrocatalytic studies under light irradiation conditions (LSPR excitation), the cell under irradiation of a LED PR 160L Kessil – Science 525 nm lamp.

[0085] Example 3. Synthesis of Ag nanoparticles

[0086] Ag nanospheres with ~25 nm were prepared by the polyol method.

[0042] In a typical procedure, 5 g of polyvinylpyrrolidone (PVP) was dissolved in 37.5 mL of ethylene glycol (EG). Then AgNO3(200 mg, 1.2 mmol) was added and mixed until complete dissolution. The resulting solution was heated to 125 °C for 2.5 h, leading to the appearance of a greenishyellow color, the mixture was allowed to cool down to room temperature, and then diluted to 125 mL with deionized water.

[0087] Example 4. Synthesis of AgPt nanoparticles

[0088] This can be achieved by employing Ag nanoparticles as seeds for Pt deposition in an analogous procedure as described for AuPt nanoparticles in Example 1. Also, the synthesis can be performed without any ascorbic acid.

[0089] Example 5. Synthesis of WO3–xplasmonic nanostructures (nanosheets)

[0090] The WO3–x nanosheets were prepared by solvothermal method. Tungsten metal powder (2 mmol) was dissolved in H2O2(5 mL, 30%) under magnetic stirring, followed by the addition of 2–propanol (25 mL). The mixture was transferred to a Teflon–lined stainless– steel autoclave and maintained at 160 °C for 12 h. After reaching the room temperature naturally, the precipitate was centrifugated, washed with ethanol and ultrapure water (three times each) and finally dried at 40 °C.

[0091] Example 6. Synthesis of W18O49 plasmonic nanostructures (nanowires)

[0092] The W18O49 NWs sample was synthesized through a mild solvothermal method using WCl6as the precursor. Typically, 44 mg of WCl6was added into a beaker containing 30 mL of ethanol. The yellow mixture was stirred for 30 min, and then transferred into a 45– mL Teflon–lined autoclave, which was sealed and heated at 200 °C for 12 h. After the autoclave cooled down to room temperature, the product was collected by centrifugation and washed with ethanol for 3 times. Finally, it was dried at 60 °C in a vacuum oven for 12 h for further use.

[0093] The synthesis of Co, Cu or Pt–modified W18O49were performed by using the procedure describe above with the addition of the Co, Cu or Pt precursors to the mixture. After dissolving 44 mg of WCl6 in 30 mL of ethanol, cobalt(II) nitrate hexahydrate, copper(II) nitrate hemi(pentahydrate) or potassium hexachloroplatinate(IV) was added to the mixture in a metal / tungsten atomic ratio of 10%. Then the solution was transferred to aTeflon–lined stainless–steel autoclave and kept at 200 °C for 12 h. The precipitates were cleaned using the procedure describe above.

[0094] Example 7. Synthesis of Co–W18O49, Cu–W18O49 and Pt–W18O49 or Co– WO3–x, Cu– WO3–xand Pt– WO3–x

[0095] The synthesis of Co, Cu or Pt–modified W18O49 or WO3–x by using the procedure describe above with the addition of the Co, Cu or Pt precursors to the mixture during the synthesis. Cobalt(II) nitrate hexahydrate, copper(II) nitrate hemi(pentahydrate) or potassium hexachloroplatinate(IV) was added to the mixture in a metal / tungsten atomic ratio of 10%. Then the solution was transferred to a Teflon–lined stainless–steel autoclave and kept at 160 (for WO3-x) or 200 °C (for W18O49) for 12 h. The precipitates were cleaned using the procedure describe above for the synthesis of WO3–x and W18O49 nanostructures.

[0096] It is to be understood that the embodiments of the invention disclosed are not limited to the particular structures, process steps, or materials disclosed herein, but are extended to equivalents thereof as would be recognized by those ordinarily skilled in the relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting.

[0097] Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment.

[0098] As used herein, a plurality of items, structural elements, compositional elements, and / or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary. In addition, various embodiments and example of the present invention may be referred to herein along with alternatives for the various components thereof. It is understood that such embodiments, examples, and alternatives are not to be construed as de facto equivalents of one another, but are to be considered as separate and autonomous representations of the present invention.

[0099] Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided, such as examples of lengths, widths, shapes, etc., to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.

[0100] While the forgoing examples are illustrative of the principles of the present invention in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation can be made without the exercise of inventive faculty, and without departing from the principles and concepts of the invention. Accordingly, it is not intended that the invention be limited, except as by the claims set forth below.

[0101] The verbs “to comprise” and “to include” are used in this document as open limitations that neither exclude nor require the existence of also un-recited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of “a” or “an”, i.e. a singular form, throughout this document does not exclude a plurality. INDUSTRIAL APPLICABILITY

[0102] The nanoparticle and its various optional features disclosed by the present invention can be manufactured and are industrially applicable particularly in hydrogen evolution reaction catalysis as shown by the illustrations, examples and results reported herein. ACRONYMS LIST GCR Glassy Carbon Rodı8 HER Hydrogen Evolution Reaction LSPR Localized Surface Plasmon Resonance LSV Linear Sweep Voltammetry RHE Reversible Hydrogen Electrode

Claims

CLAIMS:

1. A nanoparticle comprising a plasmonic metal and a catalytic metal, characterized in that: the plasmonic metal and the catalytic metal are both present at the surface of the nanoparticle; wherein the nanoparticle comprises 0.1 wt% - 5 wt% of said catalytic metal; and wherein said nanoparticle is capable of catalysing a hydrogen evolution reaction.

2. The nanoparticle according to claim 1, wherein said plasmonic metal is gold (Au), silver (Ag) or tungsten (W).

3. The nanoparticle according to any of the preceding claims, wherein said catalytic metal is platinum (Pt), iridium (Ir), nickel (Ni), iron (Fe), molybdenum (Mo), or cobalt (Co).

4. The nanoparticle according to any of the preceding claims, wherein said metal is in the form of a metal oxide or a metal sulphide or a metal hydroxide or a metal oxyhydroxide.

5. The nanoparticle according to any of the preceding claims, wherein said plasmonic metal and said catalytic metal form an alloy on the surface of the nanoparticle.

6. The nanoparticle according to any of the preceding claims, wherein the nanoparticle comprises 0.1 wt% - 5 wt% of said catalytic metal, preferably 0.1 wt% - 3.0 wt% of said catalytic metal.

7. The nanoparticle according to any of the preceding claims, wherein the nanoparticle comprises < 2.5 wt% of said catalytic metal, preferably 0.1 wt% - 1.0% wt% of said catalytic metal.

8. The nanoparticle according to any of the preceding claims, wherein said nanoparticle is a bimetallic nanoparticle.

9. The nanoparticle according to claim 8, wherein said bimetallic nanoparticle contains gold (Au) as said plasmonic metal and platinum (Pt) as said catalytic metal.

10. The nanoparticle according to any of the preceding claims, wherein the diameter of the nanoparticle is in the range of 5 – 40 nm, such as 5 – 20 nm.

11. The nanoparticle according to any of the preceding claims, wherein the shape of the nanoparticle is spherical.

12. The nanoparticle according to any of the preceding claims, wherein said catalytic metal, such as Pt, is enriched within 1.5 nm from the surface of the nanoparticle.

13. The nanoparticle according to any of the preceding claims, wherein said nanoparticle is obtained by: a) adding L-ascorbic acid to a suspension of Au nanoparticles; and b) adding Pt(IV) metal ions to the suspension obtained in step a) in conditions suitable for producing metallic Au-Pt nanoparticles.

14. The nanoparticle according to claim 13, wherein said Pt(IV) metal ions are provided by H2PtCl6reagent.

15. The nanoparticle according to claim 14, wherein the concentration of H2PtCl6 in the suspension is in the range of 2.0 – 3.0 mM, preferably about 2.44 mM.

16. The nanoparticle according to any one of claims 13-15, wherein the Au nanoparticles in said suspension are citrate capped.

17. The nanoparticle according to any one of claims 13-16, wherein the temperature of the suspension is at least 40 ° C, preferably at least 60 ° C, more preferably at least 70° C.

18. The nanoparticle according to any one of claims 13-17, wherein: ^ citrate-capped Au nanoparticles are employed as seeds, ^ H2PtCl6is employed as a precursor, and ^ L-ascorbic acid is employed as a reducing agent.

19. The nanoparticle according to any of the preceding claims, wherein the ratio of Au:Pt in the nanoparticle is in the range of 95:5 – 99.4:0.

6.

20. Use of a nanoparticle according to any of the preceding claims as a catalyst in a chemical reaction.

21. The use according to claim 20, wherein said catalyst facilitates the reaction by photocatalysis including plasmonic catalysis.

22. The use according to claim 20, wherein said catalyst facilitates the reaction by electrocatalysis.

23. The use according to any one of claims 20-22, wherein said catalyst facilitates a hydrogen evolution reaction.

24. A method for producing a metallic nanoparticle comprising the steps of: a) providing a suspension of nanoparticles of a plasmonic metal, such as Au or Ag nanoparticles; b) optionally, adding L-ascorbic acid to the suspension of the nanoparticles; and c) adding catalytic metal ions, such as Pt(IV) metal ions, to the suspension in conditions suitable for producing metallic nanoparticles of the plasmonic metal and the catalytic metal.

25. The method according to claim 24, wherein the ratio of Au:Pt in the suspension is in the range of 94:6 – 99.4:0.

6.

26. The method according to claim 24 or 25, wherein said Pt(IV) metal ions are provided by H2PtCl6reagent, and wherein the concentration of H2PtCl6in the suspension is in the range of 2.0 – 3.0 mM, preferably about 2.44 mM.

27. The method according to any one of claims 24-26, wherein the Au nanoparticles in said suspension are citrate capped.

28. The method according to any one of claims 24-27, wherein the temperature of the suspension is at least 40 ° C, preferably at least 60 ° C, more preferably at least 70° C.

29. The method according to any one of claims 24 to 28, wherein: ^ citrate-capped Au nanoparticles are employed as seeds, ^ H2PtCl6is employed as a precursor, and ^ L-ascorbic acid is employed as a reducing agent in the preparation method.

30. A method for producing a nanostructure, the method comprising the steps of: a) providing a solution of a precursor for a nanostructure, which precursor comprises a plasmonic metal; b) adding ions of a catalytic metal to the solution obtained in step a); and c) subjecting the solution comprising the precursor and the ions of the catalytic metal to conditions suitable for producing a nanostructure comprising the plasmonic metal and the catalytic metal, wherein a surface layer of the nanostructure comprises a mixture of the plasmonic metal and the catalytic metal, wherein said nanostructure exhibits catalytic activity for the hydrogen evolution reaction.

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