Au-decorated black TiO 2 produced via laser ablation in liquid
S.O. Gurbatov, E. Modin, V. Puzikov, P. Tonkaev, D. Storozhenko, S. Sergeev, N. Mintcheva, S. Yamaguchi, N. Tarasenka, A. Chuvilin, S. Makarov, S.A. Kulinich, A. A. Kuchmizhak
AAu-decorated black TiO produced via laser ablation in liquid S.O. Gurbatov,
1, 2
E. Modin, V. Puzikov, P. Tonkaev, D. Storozhenko, S. Sergeev, N. Mintcheva, S. Yamaguchi, N. Tarasenka, A. Chuvilin, S. Makarov, S. A. Kulinich,
1, 8 and A. A. Kuchmizhak
1, 2, ∗ Far Eastern Federal University, Vladivostok, Russia Institute of Automation and Control Processes, Far Eastern Branch,Russian Academy of Science, Vladivostok 690041, Russia CIC nanoGUNE, Donostia, San Sebastian 20018, Spain ITMO University, St. Peterburg 197101, Russia Department of Chemistry, University of Mining and Geology, 1700 Sofia, Bulgaria Department of Physics, Tokai University, Hiratsuka, Kanagawa 259-1292, Japan B. I. Stepanov Institute of Physics, Minsk, Belarus Research Institute of Science and Technology, Tokai University, Hiratsuka, Kanagawa 259-1292, Japan
Rational combination of plasmonic and all-dielectric concepts within unique hybrid nanomateri-als provides promising route toward devices with ultimate performance and extended modalities.However, spectral matching of plasmonic and Mie-type resonances for such nanostructures can onlybe achieved for their dissimilar characteristic sizes, thus making the resulting hybrid nanostruc-ture geometry complex for practical realization and large-scale replication. Here, we producedunique amorphous TiO nanospheres simultaneously decorated and doped with Au nanoclustersvia single-step nanosecond-laser ablation of commercially available TiO nanopowders dispersed inaqueous HAuCl . The fabricated hybrids demonstrate remarkable light-absorbing properties (aver-aged value ≈ , as well as plasmon resonances of the decorating Au nanoclusters,which was confirmed by combining optical spectroscopy, advanced electron energy loss spectroscopy,transmission electron microscopy and electromagnetic modeling. Excellent light-absorbing and plas-monic properties of the produced hybrids were implemented to demonstrate catalytically passiveSERS biosensor for identification of analytes at trace concentrations and solar steam generator thatpermitted to increase water evaporation rate by 2.5 times compared with that of pure water underidentical one-sun irradiation conditions. I. INTRODUCTION
Collective resonant oscillations of conduction elec-trons (also known as localized surface plasmon resonance(LSPR)) in noble-metal nanoparticles (NPs) provide acommon and reliable way to control electromagneticfields at nanoscale. Such oscillations facilitate resonantabsorption of incident energy that can be converted tostrongly enhanced electromagnetic fields around the NPsurface or dissipate via nonradiative damping to inducelocalized heating. Both electromagnetic and photother-mal effects have found numerous practical applications inphotovoltaics, solar energy conversion, biomedicine, sens-ing, etc . Alternatively, NPs made of various materialswith high refractive index (high- n ) and low dissipativelosses in the visible and near-IR spectral range (for ex-ample, Si, Ge and TiO ) emerged recently as alternativeplatform for nanoscale light management . In particu-lar, such nanomaterials support Mie-type resonances thatpermit to concentrate electromagnetic energy inside theNP bulk giving rise to efficient photothermal conversionand highly enhanced nonlinear optical effects .Rational combination of plasmonic and all-dielectricconcepts to design hybrid nanostructures is expected toprovide a promising route toward advanced nanomateri-als with optimized optical response and extended opera-tion range . Meanwhile, spectral matching of LSPRsand Mie-type resonances of noble-metal and high- n NPsis crucial for optimal performance. Unfortunately, in the visible and near-IR spectral ranges such resonant match-ing can be achieved for dissimilar characteristic dimen-sions of both types of NPs (in particular, 100-500 nm fordielectric NPs and less than 50 nm - for plasmonic ones).This makes the NP geometry quite complex for practi-cal realization, even with time- and money-consuminglithography-based techniques.Laser ablation in liquids (LAL) has emerged as apromising high-performance and green approach fornanomaterial preparation . When compared withwet-chemistry methods, LAL represents a simple and en-vironmentally friendly technology that can be carried outunder normal environmental conditions without exter-nal stimuli. Intense pulsed laser radiation generates ex-tremely high local pressures, temperatures and quench-ing rates, thus providing experimental conditions for pro-duction of nanostructures with different phase composi-tion (including unique meta-stable phases) , complexchemical composition and morphology . However,so far only a few studies reported on LAL-generatedhybrid nanomaterials where plasmonic and dielectriccounterparts were combined within practically relevantdesign , yet without rigorous assessment of theirnanophotonic properties and practical applications.In this paper, unique amorphous TiO spherical-shaped NPs simultaneously decorated and doped withAu nanoclusters were produced via single-step ablationof commercially available TiO nanopowders in pres-ence of HAuCl with nanosecond (ns) laser pulses. a r X i v : . [ phy s i c s . a pp - ph ] S e p The fabricated hybrids demonstrated remarkable broad-band light-absorbing properties (averaged value ≈ , as well as LSPRs of the decorating Au nan-oclusters, which was confirmed by combining opticalspectroscopy, advanced EELS/TEM and electromagneticmodeling. When placed on a back reflecting mirror, theproduced Au@TiO NPs demonstrated excellent SERSperformance resulted from the coupled Mie resonance ofTiO spheres and the LSPRs of their decorating Aunanoclusters. Isolated Au@TiO NPs were found toprovide background-free chemically non-perturbing opti-cal identification of various analytes at initial concentra-tions down to 10 − M. Au@TiO NPs with their strongbroadband optical absorption make them promising forphotothermal conversion of the solar energy. As a proof-of-concept, by using a commercial cellulose membranefunctionalized with solar-energy absorbing Au@TiO ,we realized a lab-scale water steam generator which per-mitted to increase evaporation rate by 2.5 times com-pared with that of pure water. II. MATERIALS AND METHODSA. Fabrication of Au@TiO nanoparticles. Two types of commercially available TiO nanopow-ders (anatase powder from Wako Chemicals, 99.99 %pure, and P25 from Degussa, 99.5 % pure) with aver-age particle sizes 125 and 21 nm, respectively, were usedas supplied. First, NPs were dispersed in deionized wa-ter by means of ultrasonication to achieve 0.001% solu-tion. Then, the suspension (7.5 ml) was transferred to aquartz cuvette (3 x 3 x 6 cm ) and a certain amount ofaqueous tetrachloroauric acid (HAuCl , concentrationof 10 − M) was added. Finally, the suspension was mag-netically stirred and irradiated with focused nanosecondlaser pulses (Quantel Ultra 50, with 7 ns, 532 nm and20-Hz as pulse width, wavelength and repetition rate, re-spectively) for a certain period of time at laser fluence of20 J/cm . B. Characterization.
Scanning electron microscopy . SEM images ofAu@TiO NPs were obtained at accelerating voltage of5 kV and beam current of 50 pA (Helios NanoLab 450S,Thermo Fisher, USA). Backscattered electron detectorwas used to achieve atomic number-sensitive contrast forhighlighting Au particles.To obtain information about the internal structure andcomposition of the Au@TiO system, the cross-sectionsof NPs produced by focused ion beam (FIB) milling(beam current of 43 pA at an accelerating voltage of 30kV) were studied via SEM imaging. A platinum layer was locally deposited atop of Au@TiO nanospheres toprotect their surface structures during FIB milling (seeSupporting Information).More detailed characterization of composition and in-ner structure was carried out using transmission electronmicroscopy (TEM) combined with electron tomography.For TEM studies, particles were ultrasonicated in acetonefor 5 min and then 30 µ L droplet was placed on coppergrid with lacy carbon. TEM/scanning TEM (STEM)imaging, electron tomography and STEM-EDX (energydispersive x-ray spectroscopy, EDAX) chemical mappingexperiments were performed at an acceleration voltage of300 kV (Titan 60-300, Thermo Fisher, USA). The micro-scope was equipped with a monochromated X-FEG andspectrometer Quantum GIF (Gatan, USA) which allowedperforming electron energy loss spectroscopy (EELS)with energy resolution better than 50 meV. Probing ofthe surface plasmons around Au NPs was done by meansof high-resolution EELS technique combined with STEMimage acquisition. In this case, the microscope was tunedat a beam energy of 60 keV in STEM mode. The spec-trum images were recorded in the low loss region, includ-ing zero-loss peak. At each pixel of HAADF stem image,an EELS spectrum was stored with the length of 2048pixels and energy dispersion 0.01 eV.Three-dimensional particle morphology was character-ized by the electron tomography technique using high an-gular annular dark-field STEM (HAADF-STEM) imag-ing mode. The HAADF-STEM regime provides contrastthat is strongly dependent on the atomic number (?Z )in which Au NPs appear much brighter at HAADF-STEM images. Tomographic tilt series were acquired au-tomatically at angles between ?74 o and +74 o at 2 o tiltstep. Images were taken with a FEI Tomography 4.0 soft-ware in automatic mode; the dwell time for acquisitionwas set to 2 ?s for the images of 2048 x 2048 pixels withthe pixel size of 1.3 nm. The fiducial-less tilt-series align-ment and tomographic reconstructions with simultaneousiterative reconstruction (SIRT) techniques were done us-ing in-house Digital Micrograph (Gatan, USA) scripts .Reconstructed volumes had a voxel size of 5.2 nm . Theintensity-based segmentation with a local threshold crite-rion and manual supervision was used. Depending on theintensity value of pixels, they were assumed as belongingto the Au nanoparticles (bright) or TiO sphere (grey).Segmentation of Au and TiO phases, subsequent 3-Drendering, and statistical calculations were done by usingFEI Avizo 8.1 software. Optical and Raman spectroscopy . Absorption coeffi-cient A=1-R was measured in 200-1700 nm spectral rangewith an integrating sphere spectrophotometer (Cary Var-ian 5000) at a spectral resolution of 1 nm. Halogen anddeuterium lamps were used as radiation sources for Vis-NIR and UV range, respectively. The bandgap value E g was determined by the position of the fundamental ab-sorption edge according to Tauc equation:( (cid:126) νF ( R )) /n = B ( (cid:126) ν − E g ) (1)where (cid:126) is a Planck’s constant, ν is the oscillation fre-quency of electromagnetic waves, B is a constant showingslope of the linear fit, while F(R) = (1 - R) /2R repre-sents Kubelka-Munks function. The value of the expo-nent denotes the nature of the interband electronic tran-sitions - for direct allowed transitions is equal to n=1/2.The basic procedure for Tauc analysis is to acquire opti-cal absorbance data that cover a range of energies belowand above the bandgap transition. E g value was deter-mined by extrapolating the linear part of the Tauc curve( (cid:126) ν ) /n on the (cid:126) ν axis.Raman spectra of pristine TiO NPs, as well asproduced and post-annealed AuTiO hybrids, wereacquired with commercial Raman microscope (AlphaWiTec) at 532 nm (2.33 eV) pump. Similar devicewas used to probe the SERS performance of AuTiO NPs. The Au@TiO NPs were functionalized with sev-eral types of practically relevant analytes, namely or-ganic dyes (Rhodamine 6G and Acridine orange), med-ical drags (Diphendramine hydrochloride) and histolog-ical marker molecules (4,6-diamidino-2-phenylindole di-hydrochloride, DAPI). The NPs were added into the10 − M analyte solutions and stored there for 2 h. Then,suspensions were drop-cast onto a bulk Ag mirror. SERSsignal was obtained only from isolated NPs on the sub-strate and averaged over 50 similar measurements foreach analyte to account for random distribution of Aunanoclusters.
FDTD modeling.
Finite-difference time domain calcu-lations (Lumerical Solutions Ltd.) were undertaken to as-sess plasmonic properties of the Au@TiO hybrids. Thestructures were excited with linearly polarized broadbandsource. Exact 3D models of the NPs were reconstructedfrom the corresponding STEM images produced duringthe EELS studies. The dispersion functions for Au andTiO were obtained from Palik . C. Modelling of radiation-induced heating ofAu@TiO particles. Theoretical description of nanoparticle heating (in-duced by either solar or laser radiation) was carried outnumerically with a commercial software package (ComsolMultiphysics). A numerical model was first constructedfor a plane wave irradiation of a bare and Au-decoratedellipsoidal TiO NP (the diameter along the long axis is21 nm) in water. Distribution of electromagnetic fields inthe simulated volume excited by a single laser pulse (532nm wavelength and 25 mJ pulse energy) was calculatedby solving wave equation in the frequency domain. Then,the following heat equation in a time domain during thesingle-pulse irradiation was calculated: ρC p δTδt + ∇ · ( − κ ∇ T ) = (cid:126)J · (cid:126)E, (2)where the right side of this equation describes the heatsource with the current density J and electric field am- plitude E , whereas the left side corresponds to tem-perature evolution in time and space with correspond-ing parameters: C p is the heat capacity at constantpressure, ρ is the material density, and κ is the ther-mal conductivity. We suppose the refractive indexes are n Au =0.56+3.44i, n water =1.335, n T iO = 2.54+0.0001i.Thermal conductivities for TiO , gold and water aretaken as κ T iO =7 W/ ( m · K ) , κ Au =320 W/ ( m · K )and κ water =0.6 W/ ( m · K ) . Similar procedure wasused to calculate heating efficiency of the Au@TiO hy-brids of variable diameter in water and on the glass sub-strate. For these calculations, an excitation source withcentral wavelength at 550 nm and intensity 0.1 W/cm was considered providing reliable approximation of solarirradiation. We define thermal boundary conditions asa heat flux through the computational domain surface q = h ( T ext − T ) , where h is the heat transfer coeffi-cient, T ext = 293 K is the external temperature, T isthe calculated temperature near boundary of the compu-tational domain. We assume that temperature gradientis weak because in real experiment there are a lot of par-ticle nearby. From these considerations we estimate valueof h equals 0.22 W/(m · K).
D. Water steam generation.
Nanofluid was prepared by suspending 50 mg ofAu@TiO in 10 mL of distilled water. Also, similarsuspension was filtered through a commercial cellulosemembrane (pore diameter of 200 nm) and then dried invacuum. The produced nanofluid and “black” membraneloaded with Au@TiO NPs was placed on the surface ofdistilled water and irradiated by commercial solar simu-lator at 0.1 W/cm . A microbalance was used to detectweight loss of water after evaporation. The evaporationrate observed in both cases was compared with that ofdistilled water. Heating process was visualized with cal-ibrated IR camera. All experiments were performed at25 o C and relative humidity of 40 %.
III. RESULTS AND DISCUSSIONSA. Au-decorated black TiO : fabrication andstructural properties. Figure 1a schematically illustrates key processes un-derlying formation of Au@TiO NPs using the LALtechnique. In our experiments, aqueous suspension ofas-supplied TiO NPs (mainly anatase, average size of21 and 125 nm, respectively) mixed with aqueous solu-tion of HAuCl was irradiated by ns-laser pulses for afew hours resulting in formation of spherical TiO dec-orated with multiple nano-sized Au clusters (Fig. 1b).Typically, the average size of resulting Au@TiO hy-brids is always larger than the size of as-supplied NPs(for both types of starting nanopowders). Accordingly,
500 nm c b T, K
Bare TiO with Au NP n m R e d u c t i o n o f A u N P s o n t o T i O F o r m a t i on o f A u @ T i O v i a l a s e r- i ndu c ed m e l t i ng and f u s i on T, K n m TiO Au Average size, nm
Irradiation for 2 h
120 nm250 nm d
21 nm
125 nm
Pristine deionized water quartzcuvette ns-laser pulses Au@TiO NPs pristineNPs a
250 nm100 nm 250 nm100 nm
240 nm360 nm
Irradiation for 4 h
300 nm300 nm
FIG. 1.
Fabrication of Au@TiO NPs via laser ablation in liquid. (a) Schematic representation of fabrication processsuggesting irradiation of stirred water suspension of TiO NPs with ns-laser pulses. Insets explain relevant processes underlyingformation of Au@TiO NPs. (b) Representative false-color SEM image of NPs produced upon irradiation of suspension with21-nm-sized TiO NPs for 2 h at pulse energy 20 J/cm and 20 Hz pulse repetition rate. (c) Calculated temperature profilesin the vicinity of bare and Au-decorated TiO NP upon irradiation with single ns-laser pulse at 20 J/cm . Bare TiO NPis shown inside water medium. (d) Size distributions measured for pristine TiO NPs of both types (top) and Au@TiO product obtained from these NPs after irradiation for 2 and 4 h (middle and bottom). Each size distribution is illustrated byrepresentative SEM images . the observed increase in size and spherical shape of pro-duced NPs indicate melting and fusion as key processesinvolved in their formation.However, the as-supplied crystalline TiO NPs of bothtypes weakly absorb visible light, which results in theirsingle-pulse laser heating that is insufficient to reach themelting point of bulk TiO ( ≈ ). In particu-lar, 21-nm sized TiO nanoparticle suspended in watercan be homogeneously heated up to only 385 K upon ir-radiation with a single 7-ns laser pulse at fluence of 20J/cm , as confirmed by corresponding numerical model-ing (see Fig. 1c and Methods for simulation details). Ina sharp contrast, Au nanoclusters can efficiently absorblaser radiation at 532 nm that is close to the wavelengthof their localized plasmon resonance. The absorbed radi-ation is converted to Joule heating by resonantly excitedconduction electrons, while the generated heat can befurther transferred to surroundings. Similar modeling ofthe single-pulse laser heating of isolated 21-nm-big TiO NP decorated with an Au cluster with its diameter of 5nm shows that such a hybrid nanostructure can easilyreach temperatures exceeding 1900 K at the same irradi-ation laser fluence (Fig. 1d).To further support this idea, LAL experiments wereperformed with a similar suspensions of pristine TiO NPs at the same laser fluence and irradiation time but without addition of HAuCl . Careful SEM anal-ysis of obtained nanomaterials indicated no modifica-tion/melting of pristine NPs revealing the key role of Aunanoclusters decorating TiO NPs. The formation ofsuch Au NPs can occur even without laser-irradiationand is expected to be preferentially facilitated on TiO NPs via surface chemistry driven by active sites and laser-induced enhanced temperature near NP’s surface.As mentioned above, after irradiation for 2 h all as-supplied TiO NPs with irregular shapes were success-fully transferred to spherically-shaped Au@TiO hy-brids. For both types of used pristine NPs with theiraverage sizes of 21 and 125 nm, the obtained hybridshad the averaged diameters of 120 and 220 nm, respec-tively, indicating that fusion of several NPs is crucial(see Fig. 1d). Shorter irradiation time resulted in par-ticular presence of pristine TiO NPs in the obtainedproduct. However, irradiation of the TiO suspensionfor 4 h yielded in even larger size of obtained Au@TiO hybrids. The number of adsorbed Au nanoclusters andtheir size were also found to increase with laser irradia-tion time, as well as with the concentration of HAuCl in the irradiated dispersion. From practical applicationpoint of view, it is important to control both the averagesize of the spherical-shaped TiO NPs and their deco-ration degree. We showed that by varying the starting
TiO Au on the TiO surface Au inside TiO
100 nm
Ti-M a b c d -1 -1 [111][-211] e f O-MAu-L
TiO Au TiO Au FIG. 2.
Structure and composition of laser-fabricated Au@TiO NPs . (a,b) Representative TEM images of Au@TiO NPs. (c,d) HR-TEM images showing crystalline structure of an isolated Au nanocluster and TiO NP. Insets show thecorresponding FFT images. (e) EDX chemical composition mapping of Au@TiO NPs. (f) 3D model of an Au@TiO NPreconstructed using tomographic series of HAADF-STEM images. For clarity, Au NPs situated on the surface and inside theTiO nanosphere are highlighted by purple and green colors, respectively. size of pristine TiO NPs, irradiation time and HAuCl content, both the two parameters could be flexibly con-trolled. The related information is summarized in Fig.1d and in the Supporting Information.HR-TEM imaging was used to identify crystallinestructure of the obtained Au@TiO product. TEMimages of representative NPs are shown in Fig. 2(a-d), revealing completely disordered lattice in the TiO nanosphere and crystalline structure of decorating Aunanoclusters. FFT analysis of the selected HR-TEM images confirmed amorphous nature of the TiO nanospheres (insets in Fig. 3c,d). Additionally, Ramanspectroscopy was utilized to provide more informationregarding the crystallinity of TiO statistically aver-aged over multiple NPs before and after LAL process-ing. These studies permitted clearly to identify all mainanatase Raman bands for both types of as-supplied TiO NPs and low-intense bands at 440 and 610 cm − thatcan be attributed to a small amount of nanocrystallinerutile inclusions in the Au@TiO product (Supportinginformation). EDX mapping was used to confirm chemi-cal composition of the Au@TiO NPs (Fig. 2e).The formation mechanism suggests that Au NPs stim-ulating TiO melting can appear not only on the surfaceof Au@TiO product but also in its bulk upon fusion. Toclarify this, several FIB cross-sectional cuts of Au@TiO were first visualized using SEM revealing high-contrastnanoscaled inclusions that appeared much brighter com-pared with TiO surrounding and can be attributed toAu (see Supporting Information). More detailed studiesof the inner structures of Au@TiO NPs were carriedout via tomographic reconstruction of HAADF-STEM image series (see Methods for details). The reconstructed3D model of an isolated Au@TiO NP is shown in Fig.2f, where Au nanoclusters located on the surface and in-side the TiO NP are highlighted by different colors forclarity. Systematic studies of the produced Au@TiO hybrids indicated that the inner Au nanoclusters occupy ≈ nanosphere. Notewor-thy, such Au nanoclusters both embedded into TiO NPs and simultaneously decorating their surface are re-ported for the first time.
B. Optical and plasmonic properties of Au@TiO hybrids We started by probing the plasmonic response of Aunanoclusters capping amorphous spherical-shaped TiO using high-resolution EELS. The corresponding EELSspectrum measured from an isolated Au nanocluster sit-ting on the surface of a 225-nm-sized spherical TiO NP demonstrates a pronounced signal at 2.3 eV (Fig.3a). Several peaks with their maxima centered at 2.02,2.145 and 2.365 eV can be resolved indicating the multi-resonant nature of the electron plasma oscillations in theAu nanocluster. Mapping of the intensity of this signal(1.9-2.5 eV) confirmed that it originates from the Au nan-ocluster surface (Fig. 3b). Noteworthy, the EELS peaksat 5.7 and 10 eV can be attributed to low-loss edges ofTi.The size of the considered Au nanocluster is compara-ble with a doubled skin depth of gold ( ≈
30 nm) thussuggesting the dipolar approximation applicable for for- Energy, eV
62 12 161410 maxmin Au Wavelength, nm
500 1000 1500 p r i s t i ne T i O Au@TiO A b s o r p t i on , % EE L S S i gn a l i n t e n s i t y , r . u . Energy, eV EE L S s i gna l g E ≈3.26 eV g Photon energy, eV . . ( A h ν ) , ( e V ) TiO TiO Au E / E
101 @2.2 eV ( E / E ) , r . u .0 a b cd
100 nm
FIG. 3.
Optical properties of Au-decorated black TiO NPs . (a) High-resolution EELS spectrum of isolated Au nan-ocluster capping the TiO NP surface. Reference TEM im-age is shown as upper inset. Bottom inset provides close-uplook at EELS spectrum near the plasmon resonance featuresuperimposed with a calculated averaged near-field plasmonicspectrum of the Au nanocluster (green dashed curve).(b)EELS map showing signal intensity in the spectral range from1.9 to 2.4 eV. (c) Normalized EM-field amplitude E/E cal-culated at 2.2 eV pump. (d) Absorption spectra of pristineTiO and Au@TiO -based opaque coatings covering theglass slide. The insets show Kubelka-Munk representation ofthe absorption spectra with an indication of bandgap E g forAu@TiO and pristine TiO , as well as the optical imagesof both samples. mal analysis of its plasmon modes. Considering the di-electric permittivity of Au, the localized dipolar plasmonresonance of such a NP in vacuum should appear around2.35 eV redshifting to 2.1 eV when it attaches a high-refractive-index TiO surface. Also, the shape of the ob-tained Au nanoclusters (grown above or even penetratinginto their TiO support; see Supporting Information) istypically irregular suggesting a certain splitting of local-ized plasmon resonances. The resonance seen at 2.365eV can be attributed to the quadrupolar localized plas- monic modes appeared owing to the substrate-inducedredshift of the dipolar one. These inferences are consis-tent with the supporting FDTD calculations showing thenear-field plasmonic spectrum of the slightly elliptical Aunanocluster capping TiO NP as well as the normalizedamplitude of the electromagnetic near-fields under reso-nant excitation (at 2.1 eV; Fig. 3a,c).Statistical EELS studies averaged over various Au clus-ters found on TiO spheres indicate that the Au@TiO hybrids support localized plasmons in a rather broadspectral range spanning from 500 to 650 nm. Moreover,closely spaced Au clusters can act as plasmonic oligomerswith their resonance shifted to near-IR part of the spec-trum (see Supporting Information).Further, we assessed optical properties Au@TiO hy-brids in the visible and near-IR part of their spectra(Fig. 3d). For this purpose, the Au@TiO suspension(produced by laser irradiation of P25 pristine TiO for4h) was dried on a cover glass slide to form a uniformopaque coating. In a sharp contrast to the similar coat-ing made of both types of pristine TiO NPs, the colorof the Au@TiO nanomaterial appeared black indicat-ing high absorption in the visible spectral range. Corre-sponding measurements of absorption coefficient (A=1-R; R - diffuse reflectance) of both coatings revealed anorder of magnitude larger vis-to-IR absorption by the“black” Au-decorated amorphous TiO with respect toits “white” pristine precursors. Based on the obtaineddiffuse reflectance, we found a considerable decrease ofthe bandgap E g value from 3.26 (pristine TiO ) to 2.37eV (Au@TiO hybrids) for direct transitions accordingto Kubelka-Munk method (see inset in Fig. 3d). Note-worthy, the maximal absorption ( ≈ was observed at 2.2 eV that is consistentwith the numerical modeling and EELS studies. C. Use of Au@TiO hybrids for label-freebiosensing and photo-thermal conversion. The observed remarkable optical and plasmonic prop-erties of the prepared Au@TiO hybrids suggest sev-eral areas for their potential application. First of all,the plasmon-mediated EM field localized near Au parti-cles make such Au@TiO NPs attractive for label-freebiosensing based on the surface-enhanced Raman scatter-ing (SERS) effect. As the electromagnetic contributionto the SERS signal is predominant roughly scaling asforth power of the local electromagnetic field amplitudenormalized over the pump field amplitude E /E , max-imization of this value gives a general route for boostingSERS performance. Au nanoclusters produce enhancedplasmon-mediated EM fields under resonant visible-lightexcitation. However, the nanoscale size of such particlesresults in their low absorption cross-section. As a result,they efficiently interact only with a small fraction of in-cident pump laser beam with a diffraction-limited lat- kE E / E
13 16 110 a b c +Mie resonator +back reflector
TiO g l ass A u TiO MD Mie modeLSPR LSPR MD Mie mode reflectedincident
750 1150 1550
30 cps
750 1150 1550DAPIAcridine orangeDiphendramine Rhodamine 6G d egf
Au nanocluster
LSPR
50 cps 200 cps 150 cps -1 Raman shift, cm -1 Raman shift, cm HN NHN Н NH NH
NH OOO N NN NO N
FIG. 4.
SERS performance of the Au@TiO hybridson a back-reflecting mirror . Average enhancement of theelectromagnetic field amplitude E/E calculated for isolatedAu NP in a free space (a), as well as Au NP attached tothe 240-nm diameter TiO sphere on a glass (b) and Ausubstrate (c). Series of averaged SERS spectra of Rhodamine6G (d), Acridine orange (e), Diphendramine (f) and DAPI(g). eral size typical for SERS experiments. In the producedAu@TiO hybrids, the excitation of decorating Au par-ticles can be improved via additional coupling of the in-cident field to the Mie resonances supported by TiO spheres with diameters comparable with the diffraction-limited ( ≈ λ /2) laser spot . To illustrate this, weperformed FDTD calculations showing a 2-fold increaseof the local E/E amplitude near an isolated Au parti-cle upon its attachment to a 240-nm-sized TiO spheresupporting magnetic-type dipolar resonance at plasmonexcitation wavelength (2.2 eV). Such improvement per-mits to expect contribution to SERS electromagnetic en-hancement which is at least an order of magnitude largercompared to those from a similar isolated Au particle ina free-space.Additionally, the geometry of the prepared Au@TiO hybrids permits to apply them with back-reflector mirror.In our case, this concept can be simply realized uponSERS studies by placing Au@TiO NPs on a smooth re-fectory mirror. In this respect, the TiO sphere acts asa support for Au nanoclusters providing required gap be-tween plasmonic NPs and reflecting mirror. Correspond-ing FDTD calculations of the local EM field enhancementgave more than a 3-fold enhancement of E/E (com-pared with the case of a Au particle in a free-space) nearthe isolated Au nanocluster yielding in further increaseof the SERS performance (Fig. 4c). Amorphous struc- ture of TiO spheres contributed to the background freeSERS studies.To assess the applicability of Au@TiO hybrids forreliable label-free optical identification at trace concen-trations, we probed the SERS signal from isolated NPswith an average diameter 240 ±
20 nm at 532 nm (2.33eV) as pumping wavelength. The Au@TiO NPs werefunctionalized with several types of practically relevantanalytes, namely organic dyes (Rhodamine 6G and Acri-dine orange), medical drags (diphendramine hydrochlo-ride) and histological marker molecules (4’,6-diamidino-2-phenylindole dihydrochloride, DAPI). Considering therather random size distribution and arrangement of dec-orating Au nanoclusters, for each type of the analyte westatistically averaged the SERS signal over at least 50NPs performing measurements only from isolated struc-tures. By laser pumping isolated Au@TiO NPs storedin the analyte solution for 2h and then drop-cast ontoa silver mirror, we found distinct SERS signals fromall tested analytes (Fig. 4d). The spectral position ofthe identified Raman bands were found to be in goodagreement with previously reported studies substantiat-ing isolated Au@TiO hybrids as simple all-in-one SERSplatforms with optimized EM response for reliable finger-printing at initial concentrations down to 10 − M .Noteworthy, NPs for SERS studies provide additionalflexibility for device designs. For example, quantitativeSERS measurements using NPs can be performed in liq-uids to detect and trace catalytic processes . Moreover,the analyte solution loaded with functional NPs can bedried on a substrate with specially designed wetting prop-erties. After solvent evaporation on such surface, thedeposition area for both the molecules and functionalNPs will be substantially reduced increasing local con-centration of the analyte already mixed with SERS-activeprobes . Realization of such NP-based sensor ac-tive device will become a subject of our forthcoming stud-ies.Moreover, several studies highlighted the excellent cat-alytic performance of laser-modified titania again visible-light driven degradation of dye molecules . In thiswork we used methylene blue and methylene orange or-ganic dye molecules to benchmark photocatalytic ac-tivity of the produced Au@TiO hybrids (see Sup-porting information). Remarkably, in comparison withthe as-supplied pristine TiO (P25 from Degussa) theAu@TiO hybrids with amorphous core demonstrateweak activity in photocatalytic degradation of both dyemolecules. Noteworthy, weak photocatalytic activity ofnanostructres are beneficial in designing SERS substratesfor studies where non-invasive and non-perturbing iden-tification of analytes is mandatory . It should be notedthat post-annealing of the Au@TiO product can beused to convert its amorphous core to rutile crystallinephase. Raman spectroscopy indicated appearance ofcharacteristic rutile bands (at 445 and 610 cm − ) inAu@TiO hybrids after their annealing at 500 o for 2h(see Supporting Information). However, more detailed
100 200 300 400 500050100150200
TiO NP diameter, nm Δ T , K i n w a t e r on g l a ss A u @ T i O p r i s t i ne T i O c m p r i s t i ne T i O A u @ T i O Time, s a bc A u @ T i O w a t e r + w a t e r d w a t e r A u @ T i O m e m b r ane
25 63
Localized heating Volumetric heating e o T, C A u @ T i O m e m b r ane w a t e r nano fl u i d
25 39 o T, C o T , C f FIG. 5.
Photo-thermal conversion with AuTiO hy-brids . (a) Calculated temperature increase ∆ T in the vicin-ity of Au@TiO NPs of variable diameter D in water andon a glass substrate under conditions close to one-sun irra-diation. (b) Measured temperature increase of the coatingsmade of black Au@TiO product and pristine white TiO material. (c) Optical and IR thermal images of correspondingcoatings arranged to form “FEFU” letters. (d) Schematic il-lustration of volumetric and localized heating with Au@TiO hybrids. (e,f) Comparative thermal images of bare distilledwater, Au@TiO -based nanofluid and distilled water cappedwith Au@TiO membrane. studies of post-annealing of Au@TiO NPs and its ef-fect on crystalline structure and photocatalytic activityof the product are out of the scope of this paper.Finally, the excellent broadband optical absorption ofthe prepared Au@TiO NPs make them promising forphotothermal conversion of the solar energy. In par-ticular, the maximal visible light absorption ( ≈ nanomaterial mediated by the localizedplasmon resonances in Au nanoclusters was observed at2.2 eV which perfectly matches the maximum of the so-lar energy spectrum. Our numerical calculations showedthat Au@TiO NPs with diameters ranging from 300to 500 nm in the form of water suspensions or continu-ous coatings on a substrate can be efficiently heated un-der optical excitation that is close to one-sun irradiationconditions (Fig. 5a). Thus, local heating provided byAu@TiO NPs allows for fast liquid-vapor phase tran-sition that can be applied for steam generation or waterdesalination .We performed comparative experiments run withcustom-built solar simulator heating the black Au@TiO product and pristine white TiO material arranged to form “FEFU” letters under one-sun irradiation ( ≈ ). These experiments showed that the blackAu@TiO coating reaches a maximal temperature ofabout 60 o C within 5 min in a sharp contrast with whiteTiO reaching only 35 o C as indicated by temperaturemeasurements with a calibrated IR-camera (see Fig. 5b).Further, using the Au@TiO product, we realized a lab-scale water steam generator based on both localized andvolumetric heating approaches (Fig. 5d). To do this, theweight loss of water during evaporation under one-sun ir-radiation (at 0.1 W/cm ) was monitored and evaluatedfor a Au@TiO -based nanofluid and cellulose-membranefunctionalized Au@TiO NPs (Fig. 5d). For both ap-proaches, we found that the evaporation rate increases 2-2.5 times compared with that for pristine distilled water,implying the potential applicability of light-absorbingAuTiO for solar steam generation and water desalina-tion. IV. CONCLUSIONS
Here, we produced unique amorphous TiO nanospheres simultaneously decorated and dopedwith Au nanoclusters via single-step ns-laser ablation ofcommercially available TiO nanopowders dispersed inpresence of aqueous HAuCl . The fabricated hybridsdemonstrate remarkable light-absorbing properties(averaged value ≈
96% in the visible and near-IRspectral range mediated by bandgap reduction of thelaser-processed amorphous TiO as well as LSPRs ofthe decorating Au nanoclusters that was confirmed bycombining optical spectroscopy, advanced EELS/TEMand electromagnetic modeling. Excellent light-absorbingand plasmonic properties of the produced hybridswere implemented to demonstrate catalytically passiveSERS biosensing for identification of analytes at traceconcentration, as well as solar steam generator thatpermits to increase water evaporation rate by 2.5 timescompared with that of pure water under identicalone-sun irradiation conditions. We also envision that theproduced Au@TiO product will be useful as transportlayers in third-generation solar cells, for which our NPspossess excellent optical properties, low cost, and abilityto be deposited by scalable wet-chemistry approaches.Similarly, the developed nanomaterials, being mixedwith a binder, can be further used for production ofhighly adhesive ultrablack coatings for optical deviceswhere even weak undesired reflections represent crucialissue. CONFLICTS OF INTEREST
There are no conflicts to declare.
ACKNOWLEDGEMENTS
Laser-related experiments were supported by RussianScience Foundation (grant no. 19-79-00214). A.K. and S.M. express their gratitude to the Ministry of Scienceand Higher Education of the Russian Federation (grantsnos. M-3258.2019.8 and MK-3514.2019.2) regarding per-formed calculations of temperature profiles and light-to-heat conversion efficiency. ∗ [email protected] V. Giannini, A. I. Fern´andez-Dom´ınguez, S. C. Heck, andS. A. Maier, Chemical Reviews , 3888 (2011). H. A. Atwater and A. Polman, in
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