PHOTOCATALYTICALLY ACTIVE PARTICULATE MATERIAL BASED ON ZNS, METHOD FOR THE PRODUCTION AND USE THEREOF

Abstract

A photocatalytically active particulate material includes a particle core of ZnS, particles of a nanoscale metal selected from Au, Ag, Pt, Pd, Cu or an alloy thereof loaded on the particle core, and a layer of Al2O3, SiO2, TiO2 or mixtures thereof on the loaded particle core.

Claims

1-12. (canceled)

13: A photocatalytically active particulate material comprising: a particle core of ZnS; particles of a nanoscale metal selected from Au, Ag, Pt, Pd, Cu or an alloy thereof loaded on the particle core; and a layer of Al.sub.2O.sub.3, SiO.sub.2, TiO.sub.2 or mixtures thereof on the loaded particle core.

14: The photocatalytically active particulate material as recited in claim 13, wherein a particle size d.sub.50 of the particle core is in the range of from 300 to 500 nm.

15: The photocatalytically active particulate material as recited in claim 13, wherein 0.5% to 1.5% by weight of the particles of the nanoscale metal selected from Au, Ag, Pt, Pd, Cu or the alloy thereof are loaded on the particle core with respect to a total weight of the photocatalytically active particulate material.

16: The photocatalytically active particulate material as recited in claim 13, wherein a particle size of the particles of the nanoscale metal selected from Au, Ag, Pt, Pd, Cu or the alloy thereof is 4 to 10 nm.

17: The photocatalytically active particulate material as recited in claim 13, wherein the layer of Al.sub.2O.sub.3, SiO.sub.2, TiO.sub.2 or mixtures thereof on the loaded particle core is at least 1.2% by weight calculated as the metal and with respect to a total weight of the photocatalytically active particulate material.

18: The photocatalytically active particulate material as recited in claim 13, wherein a thickness of the layer of Al.sub.2O.sub.3, SiO.sub.2, TiO.sub.2 or mixtures thereof on the loaded particle core is at least 2 nm.

19: A method for preparing the photocatalytically active particulate material as recited in claim 13, the method comprising: treating particle cores of ZnS with particles of a nanoscale metal selected from Au, Ag, Pt, Pd, Cu or an alloy thereof in an aqueous phase so as to obtain particles which are loaded on the particle cores; and coating the particles which are loaded on the particle cores with Al.sub.2O.sub.3, SiO.sub.2, TiO.sub.2 or mixtures thereof so as to obtain the photocatalytically active particulate material.

20: The method as recited in claim 19, wherein the particles of the nanoscale metal selected from Au, Ag, Pt, Pd, Cu or the alloy thereof are prepared via a pulsed laser ablation in a liquid or via a wet chemical method.

21: The method as recited in claim 19, wherein the coating of the particles which are loaded on the particle cores with Al.sub.2O.sub.3, SiO.sub.2, TiO.sub.2 or mixtures thereof is performed via an atomic layer deposition in a cyclic method.

22: The method as recited in claim 20, wherein the cyclic method includes performing at least five cycles.

23: The method as recited in claim 19, further comprising: calcining the photocatalytically active particulate material obtained at a temperature of from 400° C. to 600° C. for at least two hours.

24: A method of using the photocatalytically active particulate material as recited in claim 13 as a pigment in a plastic, the method comprising: providing the plastic; providing the photocatalytically active particulate material as recited in claim 13; and incorporating the photocatalytically active particulate material into the plastic.

25: A method of using the photocatalytically active particulate material as recited in claim 13 as a photocatalyst, the method comprising: providing the photocatalytically active particulate material as recited in claim 13; and using the photocatalytically active particulate material as the photocatalyst.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The present invention is described in greater detail below on the basis of embodiments and of the drawings in which:

[0018] FIG. 1 shows the normalized photoluminescence intensity (PL intensity) after 40 minutes of UV irradiation as a function of the deposited Al content and the calculated layer thickness (the normalization was carried out in each case with respect to the integral of the PL emission band at the time point to). In the three traces, the irradiated surface area is shown after the UV exposure: beyond 90%, no greying can be seen;

[0019] FIG. 2 shows a plot against time of normalized PL intensity during 40 minutes of UV irradiation as a function of deposited Al content. Every 90 seconds, an emission spectrum was recorded which was integrated and then normalized to the integral at time point t.sub.0;

[0020] FIG. 3 shows PL spectra before and after UV irradiation of pure cobalt-free ZnS (A of FIG. 3) and of sachtolith L (B of FIG. 3) as well as images of the irradiated surfaces;

[0021] FIG. 4 shows the effect of calcining in N.sub.2 or ambient air (O.sub.2) on the photostability of pure cobalt-free ZnS and ZnS@Al.sub.2O.sub.3;

[0022] FIG. 5 shows the loss of ZnS by reaction with Ag.sup.+ ions for coated ZnS samples with different proportions of Al (% by weight);

[0023] FIG. 6 shows a photographic image of a ZnS—Au educt material and the coated samples;

[0024] FIG. 7 shows SEM images of selected samples of FIG. 6;

[0025] FIG. 8 shows a solid UV/vis spectra of different coated ZnS or ZnS—Au samples of FIG. 6;

[0026] FIG. 9: shows a normalized, relative PL intensity after 40 minutes of UV irradiation of ZnS, ZnS—Au and ZnS—Au@Al.sub.2O.sub.3 particles; and

[0027] FIG. 10 shows the photo-induced degradation of methyl orange under UV irradiation (100 W Hg—Xe lamp). The error bars shown the result from two irradiation experiments in each case.

DETAILED DESCRIPTION

[0028] Starting from this prior art, the present invention has pursued the approach of modifying spherical, cobalt-free zinc sulfide particles (number average d.sub.50 on the 400 nm scale) on the particle surface initially with laser-generated spherical gold nanoparticles (Au—NP approximately 5-8 nm) and therefore with approximately 0.5% to 1.5% by weight, in particular 0.8% to 1.2% by weight, more particularly with approximately 1% by weight of Au with respect to the total weight of the photocatalytically active particulate material, and finally, coating the ZnS—Au particles with Al.sub.2O.sub.3 (˜2-5 nm thick) by means of an ALD process. This constitutes a first embodiment of the present invention.

[0029] The particular feature of the present invention is that initially, a photostable ZnS material is obtained which at the same time still exhibits activity in respect of photocatalysis.

##STR00001##

[0030] In general, the present invention concerns a photocatalytically active particulate material: [0031] with a particle core of ZnS, [0032] with a loading of particles of nanoscale metal selected from Au, Ag, Pt, Pd, Cu or an alloy thereof on the particle core, and [0033] with a layer of Al.sub.2O.sub.3, SiO.sub.2, TiO.sub.2 or mixtures thereof around the loaded particle core.

[0034] The term “nanoparticle” or “nanoscale particle” in the context of the present invention should be understood to mean particles which have a diameter of less than 20 nm.

[0035] In this regard, the particulate material in accordance with the present invention has a particle size in the particle core (as a number average) d50 in the range from 300 to 500 nm, in particular 300 to 450 nm, more particularly 380 to 450 nm.

[0036] As a rule, the photocatalytically active particulate material has a loading of 0.5% to 1.5% by weight, in particular 0.8% to 1.2% by weight, with respect to the total weight of the photocatalytically active particulate material of nanoscale metal selected from Au, Ag, Pt, Pd, Cu or an alloy thereof.

[0037] In this regard, the number average particle size (d.sub.50) of the nanoscale metal selected from Au, Ag, Pt, Pd, Cu or an alloy thereof can, for example, be 4 to 10 nm, in particular 5 to 8 nm.

[0038] In accordance with the present invention, the layer of Al.sub.2O.sub.3, SiO.sub.2, TiO.sub.2 or mixtures thereof around the loaded particle core is present in a quantity of at least 1.2% by weight, in particular at least 1.4% by weight, calculated as the metal and with respect to the total weight of the photocatalytically active particulate material. Depending on the particle size, a layer thickness of at least 2 nm is thus produced, in particular at least 3 nm and, more particularly, at least 4 nm. In this regard, the layer thickness can, for example, be selected in a manner so that the particles of nanoscale metal selected from Au, Ag, Pt, Pd, Cu or an alloy thereof on the particle core “protrude” out of the layer of Al.sub.2O.sub.3, SiO.sub.2, TiO.sub.2 or mixtures thereof and conducts charge from the particle core to the surface of the coated particle. The thickness of the layer of Al.sub.2O.sub.3, SiO.sub.2, TiO.sub.2 or mixtures thereof on the loaded particle core and around the particles of nanoscale metal is as a rule selected so as to be smaller than the particle size and therefore is also smaller than the largest particle size. As an example, the thickness of the layer of Al.sub.2O.sub.3, SiO.sub.2, TiO.sub.2 or mixtures thereof is approximately 2 to 5 nm and the particle size (d.sub.50) of the nanoscale metal is in the range of 4 to 10 nm, in particular the 5 to 8 nm given above.

[0039] The present invention is also directed towards a method for preparing the photocatalytically active particulate material, in which the particles of ZnS are treated with particles of nanoscale metal selected from Au, Ag, Pt, Pd, Cu or an alloy thereof in an aqueous phase and the particles obtained are coated with Al.sub.2O.sub.3, SiO.sub.2, TiO.sub.2 or mixtures thereof.

[0040] In the method in accordance with the present invention, particles of nanoscale metal selected from Au, Ag, Pt, Pd and Cu or an alloy thereof can, for example, be used which are respectively prepared via pulsed laser ablation in liquid as or by via a wet chemical method. In accordance with the present invention, pulsed laser ablation in liquid is carried out, for example, in accordance with the publication in Chem. Rev. 2017, 117, 3990-4103 or Photonik 43 (2011), No. 1, pp. 50-53 in a manner such that a high-energy pulsed laser beam is focused on a sheet of Au, Ag, Pt, Pd or Cu or an alloy thereof which is in an aqueous solution. The surface of the sheet metal is removed via the laser beam, whereupon the nanoparticles are formed which are obtained in the aqueous phase.

[0041] When using a wet chemical method in accordance with the present invention, the metal nanoparticles can, for example, be prepared via a reduction of the corresponding metal salt in an aqueous or organic phase with the aid of a reducing agent such as, for example, sodium citrate, hydrogen or sodium borohydride. As an example, this method has been described in the publications J. Am. Chem. Soc. 2006, 128, 3, 917-924 or Phys. Chem. Chem. Phys., 2011, 13, 2457-2487.

[0042] The layer of Al.sub.2O.sub.3, SiO.sub.2, TiO.sub.2 or mixtures thereof around the loaded particle core can, for example, be prepared by coating using atomic layer deposition in a cyclic process, wherein, for example, at least five cycles, in particular at least 12 cycles are carried out.

[0043] Finally, the photocatalytically active particulate material obtained may undergo calcining in a temperature range of 400° C. to 600° C. over a time period of at least two hours.

[0044] The photocatalytically active particulate material is particularly suitable for use as a pigment or as a photocatalyst.

[0045] In accordance with the present invention, the ZnS particles may be prepared using a standard method via precipitation from Na.sub.2SO.sub.4+ZnSO.sub.4 with a subsequent calcining.

[0046] In accordance with the present invention, the Au NPs may be obtained by a pulsed laser ablation in liquid (PLAL). In contrast to wet chemically synthesized NPs, particles prepared by PLAL have a “purer” surface because the use of precursors and ligands can be dispensed with. The Au NP particles may be generated in accordance with the publication in Chem. Rev. 2017, 117, 3990-4103 or Photonik 43 (2011), No. 1, pp 50-53.

[0047] In accordance with the present invention, the Atomic Layer Deposition (ALD) may be carried out with the precursors trimethylaluminum (TMA) and H.sub.2O at a temperature of 150° C. The number of cycles was varied between 5 and 50, whereupon between 0.6% and 5.8% by weight of Al was deposited. If desired, the Al.sub.2O.sub.3@ZnS—Au particles can subsequently be calcined further (T˜500° C.), further improving the photostability.

[0048] The present invention will now be explained in greater detail with the aid of the accompanying drawings.

[0049] As can be seen in FIGS. 1 to 4 for the coating of ZnS with Al.sub.2O.sub.3, this coating results in an increase in the photostability of the particles.

[0050] In the drawings, the results for coated zinc sulfides without Au NPs are shown (ZnS@Al.sub.2O.sub.3 particles). In order to assess the photostability, the photocorrosive Zn.sup.0 formation was investigated using photoluminescence spectroscopy (PL). To this end, the reduction in the photoluminescence intensity of the respective samples (in the form of ZnS pastes) during intensive UV irradiation was monitored (see FIG. 1). Untreated ZnS greyed extremely severely, and hence the PL intensity after the UV irradiation was only approximately 15%. After coating with Al.sub.2O.sub.3, an increase in the photostability could be obtained with increasing Al content. Proportions of Al above 1.4% by weight, which corresponded to a calculated layer thickness of 2 nm, obtain a near-complete maintenance of the PL intensity following the UV irradiation (intensities between 90-100%), when no further greying of the surface could be observed.

[0051] A comparison of the photostability over PL intensities with cobalt-stabilized ZnS (sachtolith L) as the reference material is rather problematic because cobalt acts as a buffer for photogenerated charge carriers and therefore functions as what is known as a “killer” with respect to photoluminescence (see FIG. 3; for comparison purposes, on the left A, a pure cobalt-free ZnS is shown). With the aid of the images before and after coating, however, it can be seen that even the reference material sachtolith L greys very slightly under the action of the intensive UV. Via the Al.sub.2O.sub.3 coating alone, a higher photostability compared with cobalt doping could thus be obtained because in this, above 1.4% by weight Al, which corresponds to a calculated layer thickness of 2 nm, no further greying of the irradiated surface could be observed (compare FIGS. 1 and 2).

[0052] A further positive effect is exhibited by calcining the coated samples at 500° C. in ambient air or a nitrogen atmosphere. As can be seen in FIG. 4, the photostability increases slightly with calcining at 500° C. Calcining at 900° C., on the other hand, leads to a reduction in the photostability.

[0053] In order to evaluate whether the coated samples still have a sulfided surface or a dense Al.sub.2O.sub.3 layer is present, the reaction on the surface with Ag.sup.+ ions was investigated. The sulfided surface of ZnS reacts with Ag.sup.+ ions to form Ag.sub.2S, whereupon the surface turns brownish and the corresponding consumption of silver ions can be used to quantify the loss of ZnS (see FIG. 5). FIG. 5 shows that above 1.4% by weight Al, there is no longer a sulfide surface (no consumption of Ag.sup.+ ions) and thus the ZnS main body in the context of the coating has been completely covered with aluminum oxide.

[0054] FIGS. 6 to 10 show the results of coating ZnS—Au particles with Al.sub.2O.sub.3(ZnS—Au@ Al.sub.2O.sub.3 particles). In this regard, the particles are ZnS particles to the surfaces of which Au NPs have been applied (1% by weight) before coating by means of ALD (FIG. 6; 5, 12 & 50 cycles; this corresponds to calculated Al contents of 0.6%, 1.4% and 5.8% by weight). FIG. 7 shows selected SEM images; a homogeneous distribution of the AU NPs can be seen therein. FIG. 8 furthermore shows that surface plasmon resonance of the AU NPs has been obtained (at approximately 530 nm), which can be seen in the corresponding main body UV/vis spectra.

[0055] FIG. 9 shows the photostability of the ZnS and ZnS—Au educts, as well as ZnS—Au particles which were coated by atomic layer deposition for 5, 12 and 50 cycles. It can here be seen that uncoated ZnS—Au is somewhat more sensitive to light than pure ZnS. Beyond 12 ALD cycles (approximately 1.4% by weight Al), however, a sufficient photostability could be obtained.

[0056] The evaluation of the photoactivity of photostable samples was carried out by means of the photo-induced bleaching of methyl orange, wherein photogenerated charge carriers cause the degradation of the dye. In order to investigate the effect of the Au NPs, coated ZnS samples with the same Al content (approximately 1.4% by weight) with and without Au were investigated. A comparison of FIGS. 1 and 9 also shows that both samples have an identical photostability (PL intensity after UV irradiation approximately 90%).

[0057] FIG. 10 shows the degradation of methyl orange (conversion [%]) against the irradiation time (0-140 min). In the time period −60 to 0 min, no irradiation was carried out, in order to exclude any possible adsorption effects. It can be seen that ZnS with a layer of Al.sub.2O.sub.3 but without Au nanoparticles has a low activity as regards photo-induced dye degradation, wherein 16% of the dye had been bleached after 140 min. On the other hand, when Au NPs have been deposited on the ZnS surface prior to Al.sub.2O.sub.3 coating, almost three times as much dye degradation is obtained (45% after 140 min). The blank measurement (irradiation without catalyst) exhibited bleaching of only 1.7% after 140 min and can therefore be ignored.

[0058] The data thus shows that a photostable ZnS main body can be prepared via an Al.sub.2O.sub.3 coating, but it only exhibits a low photo-induced activity in respect of dye degradation.

[0059] Compared thereto, the combination of Au NPs and Al.sub.2O.sub.3 coating in accordance with the present invention results in a significant increase in activity, which is indicative of the interaction in respect of the charge carriers between ZnS main bodies and Au NPs.

[0060] In addition to the use of laser-generated Au nanoparticles of the example, the system in accordance with the present invention can also be described with other conductive nanoparticles such as Ag, Pt, Pd, Cu and their alloys which can be prepared in an analogous manner by pulsed laser ablation (refer to Chem. Rev. 2017, 117, 3990-4103).

[0061] The use of an inert inorganic shell in accordance with the present invention in order to protect the particle surface, shown by way of example for Al.sub.2O.sub.3, can also be applied to the materials SiO.sub.2 and TiO.sub.2 which also constitute conventional materials in the context of atomic layer deposition (Crit Rev Solid State, 38:203-233, 2013).

Methods and Apparatus

Methods

Photocorrosion Determination

[0062] In order to investigate the photostability via photoluminescence spectroscopy, firstly, 300-400 mg of sample was ground and mixed with 150-200 mg of demineralized water. The paste obtained was placed on a plastic support, covered with a quartz glass slide, and inserted into the Fluorolog®-3 fluorescence spectrometer from HORIBA. Next, the sample was irradiated for 40.5 min at an excitation wavelength of 330±2 nm, wherein every 90 seconds, an emission spectrum was recorded from 350 to 650 nm (slit width to detector 1 nm). By integrating the respective emission bands and subsequent normalization to the integral at time point to, the relative reduction in the photoluminescence intensity with time could be determined; this constitutes a measure of the susceptibility to photocorrosive greying.

Determination of Al Content

[0063] In order to quantify the deposited quantity of Al, 300 mg of the respective sample was placed in a 250 mL dual-necked flask and 100 mL of 2N HCl was added. Next, the dispersion was heated to 90° C. and stirred for 3 h. During this time, the reaction solution was continuously flushed with N.sub.2 (2 L/h) in order to drive off the H.sub.2S which formed. The clear solution was then investigated by ICP-mass spectrometry in respect of the Al.sup.3+ concentration.

Investigation of Layer Density Using AgNO.SUB.3

[0064] 50 mg of the powdered sample was placed in 44 mL of demineralized water and dispersed for 2 minutes in an ultrasound bath. Next, 6 mL of 0.1M silver nitrate solution was added to the dispersion, with stirring. After one hour, the dispersion was centrifuged for 20 min at 5000 rpm and the clear supernatant was removed. Next, the Ag.sup.+ concentration of the supernatant was determined by a Volhard titration in order to quantify the quantity of silver ions which had not reacted with the ZnS surface. To this end, 10 mL of the supernatant was made up to 100 mL with distilled water. Next, as an indicator, a few drops of ammonium iron(III) sulphate solution (0.1M) were added which had been supplemented with concentrated nitric acid until the brown color of the solution disappeared. A 0.01M ammonium thiocyanate solution was used as the standard solution.

[0065] The relative loss of ZnS could then be calculated via the Ag.sup.+ concentration in the supernatant:

[00001]$\mathrm{loss}\mathrm{of}\mathrm{ZnS}\left[\mathrm{mol}-\%\right]=\frac{n_0\left({\mathrm{Ag}}^{+}\right)-n_t\left({\mathrm{Ag}}^{+}\right)}{2.Math.n\left(\mathrm{ZnS}\right)}.Math.100\%$ [0066] n.sub.0 (Ag.sup.+)=initial molarity of Ag.sup.+ ions [mol] [0067] n.sub.t (Ag.sup.+)=consumed molarity of Ag.sup.+ ions during the titration [mol] [0068] n (ZnS)=molarity of ZnS [mol]

SEM Investigation

[0069] The SEM measurements were carried out with the aid of the SU-70 scanning electron microscope from Hitachi. In the context of preparation, the powdered samples were initially placed in ethanol and dispersed in an ultrasound bath for 1 min. A few drops of the suspension were placed on a graphite wafer, which was then dried at 50° C. in a vacuum drying oven.

UV/Vis Spectroscopy

[0070] The solid body samples were measured using the Cary 400 spectrometer from Varian. The wavelength range was 400-800 nm with a resolution of 1 nm; spectralon was used as the white standard.

Particle Size Determination

[0071] The particle size determination was carried out with the aid of an analytical disk centrifuge from CPS Instruments (model DC 24000). The calibration was carried out using PCV particles (d=0.237 μm; standard), wherein the detection wavelength was 450 nm. The number average particle diameter (d.sub.50) was determined via the cumulative size distribution by mass.

Layer Thickness Determination

[0072] The thickness (d) of the applied layers can be calculated with the aid of the following formula based on the BET surface area of the ZnS main body, the density of the Al, Ti or Si species, and the deposited molarity of Al, Si or Ti:

[00002]$V_S=O_{\mathrm{BET}}.Math.d$$V_S=\frac{n_{A1}}{{\rho }_{\mathrm{molar}}}$$d=\frac{n_{A1}}{{\rho }_{\mathrm{molar}}.Math.O_{\mathrm{BET}}}$ [0073] V.sub.S: volume of deposited layer [m.sup.3] [0074] O.sub.BET: BET-surface area of ZnS main body (4.8 m.sup.2/g) [0075] d: thickness of layer [m] [0076] n.sub.Al: molarity of Al (Si, Ti) [mol] [0077] ρ.sub.molar: molar density of deposited layer (ρ.sub.Al: 5.134×10.sup.4 mol/m.sup.3; ρ.sub.Ti 5.297×10.sup.4 mol/m.sup.3; ρ.sub.Si: 4.411×10.sup.4 mol/m.sup.3)

Dye Degradation

[0078] In order to investigate the photo-induced dye degradation, initially, 21 mg of the powdered sample was dispersed in 84 mL of methyl orange solution (18 mg/L) for 1 min in an ultrasound bath and placed in a quartz glass reactor. Subsequently, the dispersion was stored for 60 min in the dark so that an adsorption-desorption equilibrium could be established. Next, irradiation was carried out via a 200 W He(Hg) arc lamp with an upstream neutral density filter (50%). In this regard, the reaction volume was stirred continuously and flushed with synthetic air (5 mL/min). At time points 0, 5, 15, 30, 45, 60, 80, 100 and 140 min, a volume of approximately 1.5 mL was removed and centrifuged for 10 min at 15000 rpm in order to obtain sedimentation of the catalyst material. Next, the supernatant was investigated in respect of the concentration of methyl orange using the Evolution 201 UV/vis spectrometer from Thermo Scientific and the degradation of the methyl orange was determined using the following formula:

[00003]$X=\frac{c_0-c_t}{c_0}=\frac{E_0-E_t}{E_0}$ [0079] E.sub.0 or E.sub.t: extinction at time point 0 or t [WLOG] [0080] X: conversion [WLOG] [0081] c.sub.0 or c.sub.t: concentration at time point 0 or t [mol/L]

PREPARATION EXAMPLES

Chemicals Used

[0082] Trimethylaluminum solution (97%; Sigma Aldrich)

[0083] Sheet gold (99.99%; 1 mm thick; Allgemeine Gold- and Silberscheideanstalt AG)

[0084] Sodium hydroxide pellets (>98%, Sigma Aldrich)

[0085] Silver nitrate powder (>99%, Sigma Aldrich)

[0086] Ammonium iron(III)sulphate solution (0.1N; Bernd Kraft)

[0087] 0.01M ammonium thiocyanate solution (0.1N Reag. Ph. Eur.; Bernd Kraft

[0088] Methyl orange powder (ACS Reagenz, dye content 85%; Sigma Aldrich

[0089] 2N hydrochloric acid (Reag. Ph. Eu; Fluka Analytical)

[0090] Nitrogen (99.999%, Alphagaz Air Liquide)

[0091] Synthetic air (99.999%, Alphagaz Air Liquide)

Syntheses

Synthesis of ZnS Particles

[0092] The zinc sulfide was prepared by means of continuous precipitation with the aid of ZnSO.sub.4 and Na.sub.2S solutions which were commercially available. For the precipitation, the two solutions were initially heated to 65° C. before mixing of both educts was then carried out in the reactor vessel. Sufficient mixing during the reaction was obtained via an appropriate stirrer (400 rpm). After precipitation, more Na.sub.2S solution was added, with stirring, to the reaction mixture obtained until the pH was 7-7.5. After this, the ZnS was separated from the solution with the aid of a Büchner funnel and the filter cake was dried for 8 h in a drying oven at 130° C. The ZnS obtained in this manner was then calcined in an electric tube furnace in ambient air. After calcining, the calcined sample was immediately quenched in approximately 1000 mL of water and dispersed (approximately 6400 rpm and 10 min), washed, and the solid was separated by a Büchner funnel. The filter cake obtained was then dried for approximately 1 h in the drying oven at 130° C. and then ground for 1 min using an IKA laboratory mill.

Synthesis of Au Nanoparticles and Deposition thereof on ZnS

[0093] Colloids which had been prepared by pulsed laser ablation in liquid (PLAL) were used to support the laser-generated Au nanoparticles. The synthesis of the Au nanoparticles was carried out with the aid of a nanosecond Nd:YAG-Laser IS400-1 from Edgewave. To this end, an Au target (sheet Au with a thickness of 1 mm) was fixed in a flow chamber, wherein 0.5 mM of NaOH solution was pumped at a flow rate of 100 mL/min through the ablation chamber. The Au target was irradiated with the laser light (wavelength 1064 nm) via a quartz glass window in the flow chamber in a moving rectangular pattern; this was carried out via a scanner system (Sunny S-8210D, scan speed 2 ms.sup.−1) with a Linos F Theta lens (focal length 100 mm). For the pulsed laser beam, a repetition rate of 5 kHz and a pump current of 54 A were used. The Au colloid prepared in this manner was trapped in a downstream collecting container. In order to apply the Au nanoparticles to the zinc sulfide, 16 g of ZnS was added to 1 L of distilled water and dispersed for 1 min in the ultrasound bath, with stirring. Next, with stirring, 1.5 L of the previously prepared Au colloid (Au concentration 107.7 mg/L) was dripped into the ZnS suspension at a flow rate of 25 mL/min, corresponding to a mass loading of 1.0% by weight. The dispersions were then stirred for 60 min. Next, the particles were filtered off, washed twice with 500 mL of distilled water each time, and dried in the drying oven at 100° C. for 30 min.

[0094] In addition to using Au nanoparticles, laser ablation can also be used for other materials such as Ag, Pt, Pd, Cu and their alloys (Chem. Rev. 2017, 117, 3990-4103). For this, only the target of the desired material is used in the context of laser ablation. In this manner, the “Hedgehog particles” described here are not restricted to Au nanoparticles alone, but may also be prepared with nanoscale Ag, Pt, Pd, Cu and their alloys.

Coating with Al.sub.2O.sub.3

[0095] Atomic layer deposition (ALD) of Al.sub.2O.sub.3 was carried out using the commercially available Savannah® system from Veeco. Firstly, 2 g of ZnS or ZnS—Au powder was added to the rotating drum reactor, the system was evacuated and the reactor chamber was heated to 150° C. Next, the rotational speed of the rotating drum reactor was adjusted to 4 rotations per minute. In order to remove physisorbed water, a 45-minute drying step was carried out at an Ar flow rate of 20 sccm (carrier gas). Next, the two precursors, trimethylaluminum (TMA) and demineralized water, were added in alternation; they could be introduced into the ALD system in the gaseous form via cartridges (heated to a temperature of 25° C.). The table below describes the sequence for a single deposition cycle in detail:

TABLE-US-00001 [00002]

[0096] In the context of the experimental work, the number of deposition cycles was varied between 5 and 50 in order to vary the quantity of the aluminum species to be deposited. After coating was complete, the pressure in the reactor chamber was slowly increased with the aid of the Ar flow to ambient pressure and the sample material was removed.

[0097] In addition to coating with Al.sub.2O.sub.3, this method can also be used for preparing layers of SiO.sub.2 or TiO.sub.2. To this end, precursors such as, for example, titanium tetraethanolate, titanium tetramethanolate, 3-aminopropyltriethoxysilane or tetrachlorosilane may be used; The “Hedgehog particles” here described are thus not restricted to Al.sub.2O.sub.3 shells alone, but can also be coated with SiO.sub.2 or TiO.sub.2.

Calcining of Coated Samples

[0098] Selected samples were calcined following ALD coating in synthetic air or under a nitrogen atmosphere. To this end, 800 mg of each sample was transferred into a quartz glass crucible and inserted into the work tube (quartz glass) of the compact tube furnace from Carbolite. This could be flushed with an appropriate gas via two gas connections (volume flow rate: 8 L/h). Before calcining was begun, a 12 hour flushing period was carried out with the respective gas. Next, the temperature was increased at a heating rate of 5° C./min to 500° C. or 900° C. and held for 2 h. After cooling the furnace to room temperature, the sample material was removed.

[0099] The present invention is not limited to embodiments described herein; reference should be had to the appended claims.