Article Precipitation of Pt Nanoparticles inside -Track-Etched Capillaries

Shunya Yamamoto *, Hiroshi Koshikawa, Tomitsugu Taguchi and Tetsuya Yamaki Department of Advanced Functional Materials Research, National Institutes for Quantum and Radiological Science and Technology (QST), Takasaki, Gunma 370-1292, Japan; [email protected] (H.K.); [email protected] (T.T.); [email protected] (T.Y.) * Correspondence: [email protected]; Tel.: +81-27-346-9129

 Received: 8 January 2020; Accepted: 4 February 2020; Published: 6 February 2020 

Abstract: Ion-track-etched capillaries containing nanoparticles of precious (e.g., Pt, Au, and Ag) can be applied to plasmonic absorber materials. The precipitation of homogeneous and highly dispersed precious nanoparticles inside capillaries represents a key process. Ion-track-etched capillaries (diameter: ~500 nm, length: ~25 µm) were created in film by 350 MeV Xe (3 107 /cm2) and chemical etching (using a sodium hypochlorite solution). The films × with capillaries were immersed in an aqueous solution containing 0.1–10 mmol/LH2PtCl6 and 0.5 vol% C H OH, and then irradiated with a 2 MeV beam up to a fluence of 1.4 1016 e/cm2. 2 5 × The Pt particles inside the capillaries were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The precipitation of Pt nanoparticles and isolated aggregates inside the capillaries was confirmed by TEM. The Pt nanoparticles tended to aggregate under increasing concentrations of H2PtCl6 in the aqueous solution; meanwhile, no changes in nanoparticle size were noted under increasing electron beam fluence. The results suggest that the proposed method can be used to form metal nanoparticles in nanosized capillaries with a high aspect ratio.

Keywords: nanoparticles; Pt; ion irradiation

1. Introduction Aligned capillaries containing precious metal nanoparticles can be applied to plasmonic absorber materials. Nanoparticles of precious metals (e.g., Pt, Au, and Ag) have been widely employed as light absorbers with localized surface plasmon resonance. Plasmonic absorbers, which have the ability to convert light energy into heat energy, have been developed for solar steam generation [1–3]. By also assigning catalytic qualities to plasmonic absorbers, it would be possible to apply them to gas-reforming materials and exploit the Sun’s light energy. For example, the use of Pt catalysts would allow the generation of hydrogen gas from organic chemical hydride. The ability to form highly dispersed precious metal nanoparticles into a narrow space is essential for the fabrication of gas-reforming materials (which can convert light energy into heat energy through their plasmonic absorbers). Aligned capillaries made of flexible film can allow unidirectional gas flows and act as filters. High-aspect-ratio capillaries (diameter: several hundreds of nm, length: several tens of µm) containing catalytic and plasmonic nanoparticles have the following property: they remain in contact with catalytic and plasmonic nanoparticles over a period of time sufficient for the development of gas-reforming and heating reactions, respectively. Thus, aligned and high-aspect-ratio capillaries containing highly dispersed precious metal nanoparticles and made of flexible and visible-light transparent materials can be applied to develop high-performance gas-reforming materials capable of using the Sun’s light energy. A key process in this context is the precipitation of homogeneous

Quantum Beam Sci. 2020, 4, 8; doi:10.3390/qubs4010008 www.mdpi.com/journal/qubs Quantum Beam Sci. 2020, 4, 8 2 of 6 and highly dispersed precious metal nanoparticles inside the capillaries. Ion-track-etched capillaries created in films by ion beam irradiation and chemical etching have various applications. For example, they can be used as templates for metal [4,5], for the creation of perfect blackbody sheets [6,7], and (associated to precious metal nanoparticles) to that of plasmonic absorbers. Precious metal nanoparticles can be obtained by precipitating them from a solution using ionizing (e.g., high-energy electron beams, γ-rays, and ion beams) [8–10]: the penetration of an aqueous solution containing metal ions into a narrow space allows the precipitation of homogeneous and highly dispersed metal nanoparticles, without the need for heating and adding a reducing agent. In this study, we demonstrated the creation of ion-track-etched capillaries containing Pt nanoparticles in polyimide film by using swift heavy ion beam irradiation and chemical etching. In addition, we investigated the influence of different H2PtCl6 concentrations in the aqueous solution and of the electron beam fluence on the Pt nanoparticles precipitation inside the ion-track-etched capillaries. The results indicated the formation of homogeneous Pt nanoparticles and isolated aggregates inside the capillaries of polyimide film. Thus, it was demonstrated that a combination of swift heavy ion beam and high energy electron beam can be used to produce capillaries containing metal nanoparticles and made of flexible polymer film.

2. Materials and Methods Ion-track-etched capillaries were produced in polyimide films by ion irradiation and chemical etching. Polyimide film purchased from DU PONT-TORAY(Kapton, thickness: 25 µm, Tokyo, Japan) was cut into pieces of 10 10 cm2; then, the film pieces were irradiated with 350 MeV 129Xe23+ ions × using an azimuthally varying field (AVF) cyclotron accelerator (Sumitomo Heavy Industries, Tokyo, Japan) located at the National Institutes for Quantum and Radiological Science and Technology (QST). A vacuum chamber with a turntable-type film-carrier was pumped down to 1 10-3 Pa during the × ion irradiation. An ion beam of 5 nA, covering an area of 10 10 cm2, irradiated the films at a × fluence of 3 107 ions/cm2. Uniform ion beam irradiations perpendicular to the film surface were × conducted by using an xy-scanner system with a scanning frequency of 5 Hz (x-scan) and 0.5 Hz (y-scan). Stopping and range of ions in matter (SRIM) calculations indicated that a 350 MeV xenon ion beam could reach a substantial penetration depth of 39 µm and a linear energy transfer of 12.5 MeV/µm in the polyimide films (density: 1.42 g/cm3)[11]. Such data suggested that 350 MeV xenon ions could completely penetrate 25-µm thick polyimide films. The irradiated films, which were cut into several small pieces, were etched in a sodium hypochlorite (NaClO), (FUJIFILM Wako Pure Chemical, Osaka, Japan) aqueous solution at 60 ◦C to create the capillaries. The diameter of such capillaries can be controlled by adjusting the etching time. In this particular case, the irradiated films were immersed into the solution for 10–40 min. After the chemical etching process, the polyimide film samples were washed with purified water and air-dried. The electrical conductivity of the samples was enhanced for subsequent scanning electron microscope (SEM), (JEOL, Tokyo, Japan) and transmission electron microscope (TEM), (JEOL, Tokyo, Japan) observations by heating (i.e., carbonizing) them in the presence of N2 for 3 h at 530 ◦C. Additionally, platinum nanoparticles were prepared from a solution by precipitation, using a high-energy electron beam. The heat-treated polyimide films were immersed in an aqueous solution containing 0.1–10 mmol/L hexachloroplatinic acid hexahydrate (H2PtCl6), (Kojima Chemicals, Saitama, Japan) and 0.5 vol% ethanol (C2H5OH), (FUJIFILM Wako Pure Chemical, Osaka, Japan), and then irradiated with a 2 MeV electron beam using a 2-MV electron accelerator (Nissin Electric, Kyoto, Japan) at located at the QST. The electron beam irradiation of films in an aqueous solution requires the passage of a high-energy electron beam through the films. The semiempirical depth-dose code EDMULT, indicated that a 2 MeV electron beam was sufficiently energetic to penetrate 6.7 mm of polyimide and 8.7 mm of water. The film samples were located at the bottom of a vessel that was filled with an aqueous solution up to 5 mm from its bottom, and then irradiated with an electron beam of 4.6 1011 e/cm2 s. Homogeneous electron beam irradiations were conducted on the samples using a × · Quantum Beam Sci. 2020, 4, 8 3 of 6

Quantum Beam Sci. 2020, 4, 8 3 of 6 2 scanningQuantum electron Beam Sci. 2020 beam, 4, 8 of 120 5 cm , which was irradiated perpendicularly to the samples’3 surface. of 6 × Thethe morphology Pt particles of inside the ion-track-etched these capillaries capillaries were characterized and of the by Pt particlesSEM (JSM-6700F, inside these JEOL) capillaries and TEM were (JEM-2100F, JEOL); in particular, cross-sectional TEM samples of the Pt particles were prepared characterizedthe Pt particles by SEM inside (JSM-6700F, these capillaries JEOL) were and TEMcharacterized (JEM-2100F, by SEM JEOL); (JSM-6700F, in particular, JEOL) cross-sectional and TEM with an ion slicer (IB-090600CIS, JEOL). TEM(JEM-2100F, samples of JEOL); the Pt in particles particular, were cross-sectional prepared with TEM an ionsamples slicer (IB-090600CIS,of the Pt particles JEOL). were prepared with an ion slicer (IB-090600CIS, JEOL). 3. Results3. Results and and Dissociation Dissociation 3. ResultsThe diameter and Dissociation of the ion-track-etched capillaries was controlled by ion-irradiating the polyimide The diameter of the ion-track-etched capillaries was controlled by ion-irradiating the polyimide films over different chemical etching times (10 min, 20 min, and 40 min). Figure 1 shows the SEM films overThe di diameterfferent chemicalof the ion-track-etched etching times capillaries (10 min, was 20 min, controlled and 40 by min). ion-irradiating Figure1 showsthe polyimide the SEM images of some ion-track-etched capillaries in polyimide films obtained by applying 350 MeV xenon imagesfilms of over some different ion-track-etched chemical etching capillaries times in (10 polyimide min, 20 min, films and obtained 40 min). by Figure applying 1 shows 350 MeVthe SEM xenon ionimages irradiation of some (3 ion-track-etched × 107 ions/cm2) and capillaries different in chemicalpolyimide etching films obtainedtimes. The by films applying were 350coated MeV with xenon Au ion irradiation (3 107 ions/cm2) and different chemical etching times. The films were coated with byion sputteringirradiation to(3× avoid× 107 ions/cm charge-up2) and on different the surface. chemical Cylindrical etching holestimes. were The films formed were on coated the films with and Au Au by to avoid charge-up on the surface. Cylindrical holes were formed on the films and theirby sputtering diameter towas avoid enlarged charge-up by increasing on the surface. the etching Cylindrical time. The holes SEM were images formed demonstrated on the films that and a their diameter was enlarged by increasing the etching time. The SEM images demonstrated that a chemicaltheir diameter etching was time enlarged of 10–40 by min increasing enabled the the etching formation time. of The capillaries SEM images with diameters demonstrated of 300–800 that a chemicalnm.chemical Figure etching etching 2 shows time time a of cross-sectional of 10–40 10–40 min min enabled enabled SEM image thethe formationformationof ion-track-etched of of capillaries capillaries capillaries with with diameters created diameters in of one of300–800 of 300–800 the nm.heat-treatednm. Figure Figure2 shows 2 polyimide shows a cross-sectional a cross-sectional films after an SEM SEMetching image image time of of of ion-track-etchedio 20n-track-etched min: the capillaries capillaries capillaries presented created created a in diameter in one one of ofthe of the heat-treated∼heat-treated500 nm polyimideand polyimide were aligned films films after afterperpendicularly an an etching etching timetime to the ofof 20film min: surface, the the capillaries capillaries suggesting presented presented that the a diameter acapillaries diameter of of 500formed∼500 nm nm and along and were the were aligned trajectory aligned perpendicularly of perpendicularlya single Xe ion to inci the todent filmthe beam surface,film surface, perpendicular suggesting suggesting to that the thethat film. capillaries the Furthermore, capillaries formed ∼ alongtheformed theSEM trajectoryalong observations the trajectory of a singlerevealed of a Xe single ionthat incidentXe the ion capillariesinci beamdent beam perpendicular penetrated perpendicular the to films. the to the film. To film. Furthermore,ease Furthermore, the SEM the SEMobservationsthe observations SEM observations (considering revealed revealed thattheir theconventional that capillaries the capillaries magnification penetrated penetrated the and films. the thepreparation To films. ease theTo of SEM easecross-sectional observationsthe SEM (consideringsamples),observations we their let(considering conventionalthe Pt nanoparticles their magnification conventional precipitate andmagnification through the preparation ion-trac andk-etched the of preparation cross-sectional capillaries of withcross-sectional samples), diameters we let theofsamples), Pt ∼ nanoparticles500 nm. we let the precipitate Pt nanoparticles through precipitate ion-track-etched through ion-trac capillariesk-etched with diameterscapillaries with of 500 diameters nm. of ∼500 nm. ∼

nm nm Figure 1. Top-view SEM images of ion-track-etched capillaries in the polyimide films produced using different chemical etching times: (a) 10 min, (b) 20 min, and (c) 40 min. FigureFigure 1. Top-view1. Top-view SEM SEM images images of ion-track-etchedof ion-track-etched capillaries capillaries in thein the polyimide polyimide films films produced produced using diffusingerent different chemical chemical etching times:etching ( atimes:) 10 min, (a) 10 (b min,) 20 min,(b) 20 and min, (c and) 40 ( min.c) 40 min.

FigureFigure 2. Cross-sectional2. Cross-sectional SEM SEM image image of ion-track-etched of ion-track-etched capillaries capillaries created created in a heat-treated in a heat-treated polyimide film after an etching time of 20 min. polyimideFigure 2. filmCross-sectional after an etching SEM time image of 20 of min. ion-track-etched capillaries created in a heat-treated polyimide film after an etching time of 20 min. ToTo investigate investigate the influencethe influence of H 2ofPtCl H26PtClon the6 on precipitation the precipitation of Pt nanoparticlesof Pt nanoparticles into the into capillaries, the we prepared film samples under different H PtCl concentrations2 6 (0.1, 0.5, 10 mmol/L) and observed capillaries,To investigate we prepared the influencefilm samples of Hunder2PtCl2 6different on6 the HprecipitationPtCl concentrations of Pt nanoparticles (0.1, 0.5, 10 mmol/L)into the themandcapillaries, by observed SEM. we Such them prepared observations by SEM. film Such samples confirmed observations under the precipitationdifferent confirmed H2PtCl ofthe Pt6 concentrationsprecipitation nanoparticles of (0.1, onPt the nanoparticles0.5, film 10 surfacemmol/L) on and insidetheand the filmobserved capillaries. surface them and Figureby inside SEM.3 shows theSuch capillaries. aobservations few cross-sectional Figu corenfirmed 3 shows SEM the a images precipitationfew cross-sectional of capillaries of Pt nanoparticles SEM in films images prepared onof undercapillariesthe difilmfferent surface in Hfilms2 PtCland prepared 6insideconcentrations: underthe capillaries. different (a) 0.1 HFigu2 mmolPtClre 6 3concentrations:/ L,shows (b) 0.5 a mmolfew (a)cross-sectional/L, 0.1 and mmol/L, (c) 10 mmolSEM(b) 0.5 images/ L.mmol/L, The of film 16 2 16 samplesandcapillaries (c) were 10 mmol/L.in irradiated films preparedThe with film an samplesunder electron different were beam irradiated H at2PtCl a fluence6 concentrations: with ofan 1.4 electron10 (a) beam e0.1/cm mmol/L, at. Ina fluence the (b) sample 0.5 of 1.4mmol/L, prepared × 10 × usingand 0.1 (c) mmol 10 mmol/L./LH2PtCl The6 ,film the Ptsamples nanoparticles were irradiated (size: < with5 nm) an with electron isolated beam aggregates at a fluence were of 1.4 distributed × 1016

Quantum Beam Sci. 2020, 4, 8 4 of 6 Quantum Beam Sci. 2020, 4, 8 4 of 6

e/cmQuantum2. In Beam the Sci. sample 2020, 4 ,prepared 8 using 0.1 mmol/L H2PtCl6, the Pt nanoparticles (size: < 5 nm) with4 of 6 on the wall of the ion-track-etched capillaries. Under increasing H2PtCl6 concentration, the number densityisolated of aggregates the Pt nanoparticles were distributed with on isolated the wall aggregates of the ion-track-etched increased and capillaries. the diameter Underof increasing aggregates 2 He/cm2PtCl. 6In concentration, the sample prepared the number using density0.1 mmol/L of theH2PtCl Pt 6,nanoparticles the Pt nanoparticles with isolated (size: < 5aggregates nm) with along the major axis direction reached several tens of nm (Figure3b). The sample prepared using 10 increasedisolated aggregates and the diameter were distributed of aggregates on the along wall theof the major ion-track-etched axis direction capillaries. reached several Under tens increasing of nm mmol/L (Figure3c) presented Pt aggregates with inhomogeneous shape and size on the wall of the (FigureH2PtCl 63 concentration,(b)). The sample the prepared number usingdensity 10 mmol/Lof the (FigurePt nanoparticles 3 (c)) presented with isolatedPt aggregates aggregates with capillaries. The size of these Pt aggregates reached 100 nm, suggesting that their size increased with inhomogeneousincreased and the shape diameter and size of aggregateson the wall along of the the capillaries.∼ major axis The direction size of these reached Pt aggregates several tens reached of nm the concentration of H PtCl in the aqueous solution. The influence of different electron beam fluences ∼(Figure100 nm, 3 (b)).suggesting The sample2 that6 theirprepared size usingincreased 10 mmol/L with the (Figure concentration 3 (c)) presented of H2PtCl Pt6 aggregatesin the aqueous with 15 2 15 2 16 2 (1.4 10 e/cm , 5.6 10 e/cm , and 1.4 10 e/cm ) on the precipitation15 2 of the15 Pt nanoparticles2 solution.inhomogeneous× The influence shape× ofand different size on electronthe wall× beamof the fluencescapillaries. (1.4 The × 10 size e/cm of these, 5.6 Pt× 10 aggregates e/cm , and reached 1.4 × into10∼ the10016 e/cm capillariesnm,2) onsuggesting the usingprecipitation that 0.5 mmoltheir of size/theLH Pt in2PtCl nanoparticcreased6 was with alsoles intothe investigated: concentrationthe capillaries the ofusing SEM H2PtCl observations0.5 6mmol/L in the aqueouH2 revealedPtCl6s thatwassolution. the also size Theinvestigated: of the influence Pt nanoparticles, theof different SEM observations theelectron number beam reveal density fluencesed andthat (1.4 sizethe × 10 ofsize15 thee/cm of Pt 2the, aggregates5.6 Pt × 10nanoparticles,15 e/cm did2, and not change1.the4 × undernumber1016 increasinge/cm density2) on the electron and precipitation size beam of the fluence.of thePt aggregatesPt nanopartic Formation didles of not into Pt nanoparticleschange the capillaries under seemsincreasingusing 0.5 to bemmol/L electron finished H beam2PtCl due6 to depletionfluence.was also ofFormation investigated: the precursor of Pt thenanoparticles below SEM a observations fluence seems of 1.4to revealbe fini1015edshede /thatcm due2 .the Overall,to sizedepletion of we the verifiedof Pt the nanoparticles, precursor that it is below possible the × to induceanumber fluence the densityof formation 1.4 ×and 1015 of sizee/cm homogenous of2. Overall,the Pt aggregates Ptwe nanoparticles verified did that not intoit changeis ion-track-etchedpossible under to increasinginduce capillaries the electronformation by beam electron of beamhomogenousfluence. irradiation Formation Pt reduction nanoparticles of Pt nanoparticles and controlling into ion-track-etch seems the to concentration be finiedshed capillaries due of Hto2 depletionPtClby 6electronin the of the aqueousbeam prec ursorirradiation solution. below 15 2 reductiona fluence andof 1.4 controlling × 10 e/cm the concentration. Overall, we ofverified H2PtCl 6that in the it isaqueous possible solution. to induce the formation of homogenous Pt nanoparticles into ion-track-etched capillaries by electron beam irradiation reduction and controlling the concentration of H2PtCl6 in the aqueous solution.

nm

nm FigureFigure 3. 3.Cross-sectional Cross-sectional SEM SEM images images of of capillaries capillaries created created under under di differentfferent H H2PtCl2PtCl66concentrations: concentrations: (a) 0.1(a mmol) 0.1 mmol/L,/L, (b) 0.5 (b mmol) 0.5 mmol/L,/L, and and (c) 10 (c) mmol 10 mmol/L./L. Figure 3. Cross-sectional SEM images of capillaries created under different H2PtCl6 concentrations: FigureFigure(a) 0.14a mmol/L 4. shows (a) shows, ( twob) 0.5 cross-sectionaltwo mmol/L, cross-sectional and (c) TEM10 mmol/L. TEM images images of precipitatedof precipitated Pt Pt nanoparticles nanoparticles located located on on the wallthe of wall an ion-track-etched of an ion-track-etched capillary, capillary, which which was formed was formed in a heat-treated in a heat-treated polyimide polyimide film afterfilm after an etching an timeetching of 20Figure min.time 4. Theof (a) 20 Ptshows min. nanoparticles twoThe cross-sectionalPt nanoparticles with isolated TEM with aggregates, images isolated of precipitatedaggregates prepared under, preparedPt nanoparticles a H2 PtClunder6 concentration locateda H2PtCl on6 of 0.5concentrationthe mmol wall of/L an and ion-track-etchedof an0.5 electron mmol/L beam and capillary, fluencean electron which of 1.4 beamwas10 formed16 fluencee/cm in2 were ofa heat-treated1.4 distributed × 1016 e/cm polyimide homogeneously2 were filmdistributed after inside an × thehomogeneouslyetching ion-track-etched time of inside20 capillary.min. the The ion-track-etched ThePt nanoparticles size of the capillary Pt with nanoparticles isolated. The size aggregates (i.e.,of the their ,Pt prepared diameternanoparticles under in the (i.e.,a majorH 2theirPtCl axis6 16 2 direction)diameterconcentration was in the measured of major 0.5 mmol/Laxis based direction) and on high-magnificationan was electron measur beamed based fluence TEM on high-magnification images.of 1.4 × The10 isolatede/cm TEM were aggregates images. distributed The were homogeneously inside the ion-track-etched capillary. The size of the Pt nanoparticles (i.e., their composedisolated ofaggregates Pt nanoparticles were composed with sizes of

Figure 4. (a) Low- and (b) high-magnification cross-sectional TEM images of Pt nanoparticles precipitated into an ion-track-etched capillary in a heat-treated polyimide film. FigureFigure 4. 4.( a()a) Low- Low- and and ( b(b)) high-magnificationhigh-magnification cross-sectiona cross-sectionall TEM TEM images images of of Pt Pt nanoparticles nanoparticles

precipitatedprecipitated into into an an ion-track-etched ion-track-etched capillary capillary inin aa heat-treatedheat-treated polyimide film. film. Quantum Beam Sci. 2020, 4, 8 5 of 6

QuantumThe practicalBeam Sci. 2020 application, 4, 8 of Pt nanoparticles in ion-track-etched capillaries requires5 of 6 the precipitation of Pt nanoparticles in without heat-treated polyimide films by this method. The The practical application of Pt nanoparticles in ion-track-etched capillaries requires the precipitation of such nanoparticles was confirmed by using H PtCl at a concentration of 0.5 mmol/L precipitation of Pt nanoparticles in without heat-treated polyimide2 6 films by this method. The and an electron beam fluence of 1.4 1016 e/cm2. The cross-sectional TEM image in Figure5 shows the precipitation of such nanoparticles× was confirmed by using H2PtCl6 at a concentration of 0.5 mmol/L presence of Pt nanoparticles on the wall of an ion-track-etched capillary in one of the polyimide films. and an electron beam fluence of 1.4 × 1016 e/cm2. The cross-sectional TEM image in Figure 5 shows Overall,the presence our results of Pt indicate nanoparticles that the on proposed the wall of electron an ion-track-etched beam irradiation capillary reduction in one method of the polyimide can be used to createfilms. Overall, nanosized our capillaries results indicate containing that the Pt proposed nanoparticles electron in beam polyimide irradiation films. reduction method can b

FigureFigure 5. Cross-sectional5. Cross-sectional TEM TEM image image of precipitated of precipitated Pt nanoparticles Pt nanoparticles into an into ion-track-etched an ion-track-etched capillary in onecapillary of the in polyimide one of the films.polyimide films.

4. Conclusions4. Conclusions ThisThis study study demonstrated demonstrated a a facile facile approachapproach toto the preparation of of ion-track-etched ion-track-etched capillaries capillaries containingcontaining Pt Pt nanoparticles nanoparticles in in polyimide polyimide films. films. First,First, swift heavy heavy ion ion beam beam irradiation irradiation and and chemical chemical etchingetching were were used used to to prepare prepare nanosized nanosized capillaries capillaries withwith a high high aspect aspect ratio ratio in in polyimide polyimide films. films. Then, Then, homogeneoushomogeneous and and highly highly dispersed dispersed Pt Pt nanoparticlesnanoparticles were precipitated precipitated inside inside th thee ion-track-etched ion-track-etched capillaries by electron beam irradiation reduction. In conclusion, we demonstrated that a capillaries by electron beam irradiation reduction. In conclusion, we demonstrated that a combination combination of ion and electron beam irradiation techniques can be applied to the production of of ion and electron beam irradiation techniques can be applied to the production of capillaries containing capillaries containing metal nanoparticles in flexible polymer films. metal nanoparticles in flexible polymer films. Author Contributions: Conceptualization, S.Y.; methodology, S.Y. and H.K.; investigation, S.Y. and T.T.; Authorwriting—original Contributions: draft Conceptualization,preparation, S.Y.; writing—review S.Y.; methodology, and editing, S.Y. S.Y. and and H.K.; H.K.; investigation, supervision, Y.T.; S.Y. project and T.T.; writing—originaladministration, draftY.T. All preparation, authors have S.Y.; read writing—review and agreed to the and published editing, version S.Y. and of H.K.;the manuscript. supervision, T.Y.; project administration, T.Y. All authors have read and agreed to the published version of the manuscript. Funding: This work was financially supported by JSPS KAKENHI Grant Number 18K04739. Funding: This work was financially supported by JSPS KAKENHI Grant Number 18K04739. Acknowledgments: The authors thank Chihiro Suzuki for her help with the TEM observations. Acknowledgments: The authors thank Chihiro Suzuki for her help with the TEM observations. Conflicts of Interest: The authors declare no conflicts of interest. Conflicts of Interest: The authors declare no conflict of interest. References References 1. Zhou, L.; Tan, Y.; Ji, D.; Zhu, B.; Zhang, P.; Xu, J.; Gan, Q.; Yu, Z.; Zhu, J. Self-assembly of highly efficient, 1. Zhou,broadband L.; Tan, Y.;plasmonic Ji, D.; Zhu, absorbers B.; Zhang, for P.; Xu,solar J.; Gan,steam Q.; Yu,generation. Z.; Zhu, J.Sci. Self-assembly Adv. 2016 of, highly2, e150127 efficient,, broadbanddoi:10.1126/sciadv.1501227. plasmonic absorbers for solar steam generation. Sci. Adv. 2016, 2, e150127. [CrossRef][PubMed] 2. 2. Liu,Liu, F.; F.; Lai, Lai, Y.; Y.; Zhao, Zhao, B.; B.; Bradley, Bradley, R.; R.; Wu,Wu, W.W. PhotothermalPhotothermal materials for for efficient efficient solar solar powered powered stea steamm generation.generation.Front. Front. Chem. Chem. Sci. Sci. Eng.Eng. 2019, 13,, 636, 636. doi:10.1007/s11705-019-1824-1. [CrossRef] 3. 3. Kosuga,Kosuga, A.; A.; Yamamoto, Yamamoto, Y.; Y.; Miyai, Miyai, M.;M.; Nishimura,Nishimura, Y.; Hidaka, S.; S.; Yamamoto, Yamamoto, K.; K.; Tanaka, Tanaka, S.; S.;Yamamoto, Yamamoto, Y.; Y.; Tokonami,S.;Tokonami, Iida,S.; Iida, T. A highT. A performance high performance photothermal photothermal film with sphericalfilm with shell-type spherical metallic shell-type nanocomposites metallic fornanocomposites solar thermoelectric for solar conversion. thermoelectricNanoscale conversion.2015, 7 ,Nanoscale 7580–7584. 2015 [CrossRef, 7, 7580–7584,][PubMed doi:10.1039/C5NR00943J.] 4. 4. Koshikawa,Koshikawa, H.; H.; Usui, Usui, H.; Maekawa,H.; Maekawa, Y. Thermally Y. Thermally stable andstable anisotropically and anisotropically conducting conducting membranes membra consistingnes of sub-micronconsisting of copper sub-micron wires copper in polyimide wires in ion polyimide track membranes. ion track membranes.J. Membr. Sci.J. Membr.2009, Sci.327 2009, 182–187., 327, 182–187, [CrossRef] 5. Apel,doi:10.1016/j.memsci.2008.11.023. P. Track etching technique in membrane technology. Radiat. Meas. 2001, 34, 559–566. [CrossRef] 5. Apel, P. Track etching technique in membrane technology. Radiat. Meas. 2001, 34, 559–566, doi:10.1016/s1350-4487(01)00228-1. Quantum Beam Sci. 2020, 4, 8 6 of 6

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