Zinc Oxide Nanorod Surface-Enhanced Raman Scattering Substrates Without and with Gold Nanoparticles Fabricated Through Pulsed-Laser-Induced Photolysis

applied sciences

Article Zinc Oxide Nanorod Surface-Enhanced Raman Scattering Substrates without and with Gold Nanoparticles Fabricated through Pulsed-Laser-Induced Photolysis

Chia-Man Chou 1,2 , Le Tran Thanh Thi 3,4, Nguyen Thi Quynh Nhu 3,4, Su-Yu Liao 5, Yu-Zhi Fu 3, Le Vu Tuan Hung 4,* and Vincent K. S. Hsiao 3,* 1 Division of Pediatric Surgery, Department of Surgery, Taichung Veterans General Hospital, Taichung 40705, Taiwan; [email protected] 2 Department of Medicine, National Yang-Ming University, Taipei 112, Taiwan 3 Department of Applied Materials and Optoelectronic Engineering, National Chi Nan University, Nantou 54561, Taiwan; [email protected] (L.T.T.T.); [email protected] (N.T.Q.N.); [email protected] (Y.-Z.F.) 4 Department of Applied Physics, Faculty of Physics and Engineering Physics, University of Science, Ho Chiç Minh City, Vietnam 5 Department of Electrical Engineering, National Chi Nan University, Nantou 54561, Taiwan; [email protected] * Correspondence: [email protected] (L.V.T.H.); [email protected] (V.K.S.H.)

Received: 18 June 2020; Accepted: 17 July 2020; Published: 21 July 2020

Abstract: We fabricated surface-enhanced Raman scattering (SERS) substrates using gold nanoparticle (AuNP)-decorated zinc oxide (ZnO) nanorods (NRs). Prior to decoration with AuNPs, ZnO NRs on the glass substrate fabricated using the sol–gel method could enhance the SERS signal for detecting 5 10− M rhodamine 6G (R6G). Microscopic analysis revealed that the thermal-annealing process for fabricating the seed layers of ZnO facilitated the growth of ZnO NRs with the highly preferred c-axis (002) orientation. A decrease in the diameter of ZnO NRs occurred because of the use of annealed seek layers further increased the surface-to-volume ratio of ZnO NRs, resulting in an increase in the SERS 5 signal for R6G of 10− M. To combine the localized surface plasmon resonance (LSPR) mode with the charge transfer (CT) mode, ZnO NRs were decorated with AuNPs through pulsed-laser-induced photolysis (PLIP). However, the preferred vertical (002) orientation of ZnO NRs was prone to the aggregation of AuNPs, which hindered the SERS signal. The experimental results revealed that ZnO NRs with the crystalline structure of horizontal (100) and (101) orientations facilitated the growth of homogeneous, independent and isolated AuNPs which serves as “hot spots” for SERS 9 signal of detecting R6G at a low concentration of 10− M. Comparing to previous fabrication of SERS substrate, our method has advantage to fabricate AuNP-decorated ZnO NR in a short time. Moreover, the optimization of the SERS behaviors for diﬀerent fabrication conditions of AuNPs using the PLIP method was investigated in detail.

Keywords: zinc oxide nanorods; surface-enhanced Raman scattering (SERS); gold nanoparticles; pulsed-laser-induced photolysis (PLIP)

1. Introduction Raman spectroscopy, a spectroscopic analysis based on the inelastic scattering of light (laser as the excited light source) from excited molecules, reveals information on the vibration of functional chemical bonds in molecules and enables reliable identiﬁcation of unknown species [1]. However,

Appl. Sci. 2020, 10, 5015; doi:10.3390/app10145015 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 5015 2 of 11 because of the extremely small scattering intensity and persistent problem of photoluminescence of Raman spectroscopy, which increases the background noise and results in a low signal-to-noise ratio. The surface-enhanced Raman scattering (SERS) technique, first observed by Fleischman in 1974 [2], can efficiently enhance the Raman signal through localized surface plasma resonance (LSPR) from metals. When electromagnetic waves travel along the surface of a metal plate with a wave frequency smaller than the plasma frequency of electrons in the metal, the interaction between the wave and the electron clouds amplifies the electric field locally, leading to large-intensity Raman signals [1]. Dielectrics—especially nanostructured semiconductors—exhibit enhanced Raman signals due to a series of optical effects, such as light absorption and trapping, photo-induced charge transfer (CT) and optical resonance, occurring in the dielectric or between the targeted molecules and dielectric [3]. The zinc oxide (ZnO) nanostructure, a unique material exhibiting semiconductor and piezoelectric dual properties [4], has proven to be a suitable SERS substrate [5,6]. The enhancement of Raman signals is due to the inherent nanostructure with a large surface-to-volume ratio and the CT between the absorbed analyte and substrate [7]. A combination of metallic and ZnO nanostructures further amplifies the Raman signal because of the resulting effective CT and LSPR effect. Metallic nanostructures, such as silver (Ag) or gold (Au), facilitate not only additional absorbance due to LSPR, but also effective electromagnetic enhancement and CT [7]. We fabricated a SERS substrate comprising ZnO nanorods (ZnO NRs) decorated with Au nanoparticles (AuNPs). Previous studies have demonstrated Ag–ZnO [8–19] or Au–ZnO [20–25] hybrid nanostructures with enhanced Raman signals. However, the deposition process of metallic nanostructures is complex and time consuming. We introduced a pulsed-laser-induced photolysis (PLIP) method for fabricating AuNPs on ZnO NRs in the short time of 30 min without further treatment. The ZnO NRs fabricated using the sol–gel method on glass substrate were immersed in an auric acid (HAuCl4) and hydrogen peroxide (H2O2) aqueous solution, and AuNPs were grown on the surface of ZnO NRs. Under pulsed-laser irradiation of only eight minutes, H2O2 reduced Au ions, forming AuNPs solution. By controlling the surface morphology of ZnO NRs and growth conditions 9 of AuNPs, a 10− M concentration of rhodamine 6G (R6G) was feasibly detected. The experimental results indicate that thermal annealing promotes the growth of ZnO NRs in the (002) direction and enhances the Raman signal. However, ZnO NRs in the (002) direction are not preferred for the growth of individual AuNPs to enhance the Raman signal. ZnO NRs with the vertical c-axis (002) orientation are prone to AuNP aggregation, and the Raman signal is weakened because of the nonhomogeneous distribution of AuNPs on the ZnO NR surface. Under the same AuNP growth conditions, by using the PLIP method, ZnO NRs with the horizontal (100) and (101) orientations promote the growth of individual and homogeneous dispersed AuNPs, which offers the “hot spot” for enhancing the Raman signal. Moreover, the optimization of the SERS behaviors for different fabrication conditions of AuNPs using the PLIP method was investigated and found out short pulsed laser irradiation of eight minutes and chemical bath immersion time of 15–30 min are suitable for AuNP-decorated ZnO SERS substrate.

2. Materials and Methods ZnO seed layers were ﬁrst fabricated using the sol-gel method [26]. In detail, the precursor for preparing the seed layers was a mixture of 15 mL sol solution containing 0.75 M zinc acetate dihydrate (Zn(CH COO) 2H O) mixed with the solvent containing equal quantities of monoethanolamine 3 2 · 2 (HOCH2CH2NH2) and 2-methoxyethanol (CH3OCH2CH2OH). The solution was stabilized at room temperature for 24 h. The dip coating of the seed layer was performed at a rate of 3 cm/min after immersing the glass slide in the sol solution for 10 min. The process was repeated three times. After the dip-coating process, the substrate was dried and annealed on a hot plate at 100–250 ◦C for 30 min, leading to the gelation of the sol solution on the substrate surface and the formation of the seed layer. The growth of ZnO NRs was prepared by immersing the sample in a chemical bath containing a solution of zinc nitrate hexahydrate (Zn(NO ) 6H O) and methenamine (C H N ) at the same 3 2 · 2 6 12 4 0.1-M concentration at 85 ◦C for 2 h. The growth of AuNPs on ZnO NRs was facilitated using the PLIP Appl. Sci. 2019, 9, x FOR PEER REVIEW 3 of 11

of the seed layer. The growth of ZnO NRs was prepared by immersing the sample in a chemical bath containing a solution of zinc nitrate hexahydrate (Zn(NO3)2 · 6H2O) and methenamine (C6H12N4) at Appl. Sci. 2020, 10, 5015 3 of 11 the same 0.1-M concentration at 85 °C for 2 h. The growth of AuNPs on ZnO NRs was facilitated using the PLIP method. The ZnO NR substrate was placed inside a glass vial containing 5 mL of 0.33- method.mM HAuCl The4 ZnO and NR2 mL substrate of thewas 10-mM placed H2 insideO2 aqueous a glass solution. vial containing The aqueous 5 mL of solution 0.33 mM was HAuCl then4 andirradiated 2 mL of using the10 a Nd:YAG mM H2O nanoseco2 aqueousnd solution. pulsed laser The aqueousoperated solutionin 532 nm was and then 47 irradiatedmJ laser energy using at a Nd:YAGdifferent nanosecondirradiation pulsedtime. Then, laser operatedthe samples in 532 were nm andkept 47 placing mJ laser inside energy glass at di ffvialserent at irradiation different time.immersion Then, thetimes. samples The process were kept of fabricating placing inside AuNP-decorated glass vials at diZnOfferent NRs immersion is illustrated times. in Figure The process 1. The ofX-ray fabricating diffraction AuNP-decorated (XRD) patterns ZnO of ZnO NRs NRs is illustrated were obtained in Figure in the1. 2 Theθ range X-ray of di 0°–90°ffraction on a (XRD) high- patternsresolution of ZnOX-ray NRs diffractometer. were obtained SERS in the measuremen 2θ range ofts 0 ◦were–90◦ onperformed a high-resolution using a micro-Raman X-ray diffractometer. system SERS(LabRAM measurements HR800; HORIBA were performed Jobin Yvon) using with a micro-Raman a helium–neon system laser (LabRAMas the excitation HR800; source HORIBA operating Jobin Yvon)at a 633 with nm a wavelength helium–neon and laser equipped as the excitation with a 40× source objective operating lens.at Scanning a 633 nm electron wavelength microscopy and equipped (SEM) withwas aconducted 40 objective using lens. a JEOL Scanning JSM-7800 electron F field-emission microscopy (SEM) scanning was electron conducted microscope using a JEOL (JEOL, JSM-7800 Tokyo, × FJapan). field-emission scanning electron microscope (JEOL, Tokyo, Japan).

Figure 1. Schematic of fabricating surface-enhanced Raman scattering (SERS) substrate using zincFigure oxide 1. Schematic (ZnO) nanorods of fabricating (NRs)decorated surface-enhanced with Au Raman nanoparticles scattering (AuNPs) (SERS) synthesized substrate using through zinc pulsed-laser-inducedoxide (ZnO) nanorods photolysis (NRs) decorated (PLIP). with Au nanoparticles (AuNPs) synthesized through pulsed- laser-induced photolysis (PLIP). 3. Results and Discussion 3. Results and Discussion 3.1. Surface Morphology, XRD and SERS Characteristic of ZnO NRs 3.1. SurfaceThe main morphology, advantage XRD of growingand SERS ZnO characteristic NRs through of ZnO dip NRs coating and decorating AuNP through PLIPThe is that main unlike advantage complex of sputtering growing ZnO deposition NRs through in high dip vacuum, coating the and two-step decorating process AuNP is a through simple techniquePLIP is that that unlike allows complex precise sputtering control over deposition the growth in high of ZnO vacuum, seed layers.the two-step However, process the is ZnO a simple seed layertechnique may havethat allows a random precise coplanar control orientation over the gr andowth a of c-axis ZnO (002) seed perpendicularlayers. However, to thethe substrate,ZnO seed resultinglayer may in have an incomplete a random c-axis coplanar orientation orientation after chemicaland a c-axis bath (002) immersion perpendicular [26]. Figure to 2thea illustrates substrate, theresulting SEM images in an incomplete of the surface c-axis morphology orientation of after ZnO chemical NRs, which bath reveals immersion the hexagonal [26]. Figure morphology 2a illustrates of µ ZnOthe SEM NRs images with a diameterof the surface and morphology length of approximately of ZnO NRs, 300 which nm reveals and 4 them, respectivelyhexagonal morphology and random of orientation.ZnO NRs with During a diameter the growth and of length ZnO NRs,of approximately nucleation ﬁrst 300 occurs nm and near 4 theμm, grain respectively boundaries and between random twoorientation. adjacent During particles the in growth the ZnO of seed ZnO layer NRs, [ 27nucl]. Noteation all first nanowire occurs arrays near the are c-axisgrain boundaries orientated becausebetween of two the adjacent polycrystalline particles nature in the of theZnO seed seed layers layer [27 [27].,28]. Not Figure all2 bnanowire depicts thearrays XRD are pattern c-axis oforientated ZnO NRs, because indicating of the a polycrystalline well-deﬁned crystalline nature of verticalthe seed c-axis layers (002), [27,28]. horizontal Figure 2b polar-terminated depicts the XRD (001)pattern and of horizontal ZnO NRs, nonpolar indicating low-symmetry a well-defined (101) crystalline faces. The vertical sharp XRD c-axis peaks (002), prove horizontal the growth polar- of theterminated ZnO wurtzite (001) and structure horizontal [29]. nonpolar Figure2c depictslow-symmetry the Raman (101) spectra faces. The of the sharp R6G XRD solution peaks at prove various the concentrations detected using ZnO NRs as the SERS substrate. Bare glass was also used as the substrate 3 for the control experiment, which exhibited no Raman signal for 10− M R6G. Strong Raman signals Appl. Sci. 2019, 9, x FOR PEER REVIEW 4 of 11

growth of the ZnO wurtzite structure [29]. Figure 2c depicts the Raman spectra of the R6G solution Appl.at various Sci. 2020 concentrations, 10, 5015 detected using ZnO NRs as the SERS substrate. Bare glass was also used4 of 11as the substrate for the control experiment, which exhibited no Raman signal for 10−3-M R6G. Strong Raman signals were observed at 612, 772, 1184, 1314, 1366, 15111 and 1650 cm−1 on the ZnO NR SERS were observed at 612, 772, 1184, 1314, 1366, 1511 and 1650 cm− on the ZnO NR SERS substrate for substrate3 for 10−3-M R6G, which were attributed to xanthene ring bending and deformations, C–H 10− M R6G, which were attributed to xanthene ring bending and deformations, C–H bond bending, bond bending, N–H bond bending and CH2 waggling [22]. However, when the concentration of R6G N–H bond bending and CH2 waggling [22]. However, when the concentration of R6G was reduced to was5 reduced to 10−5 M, the Raman signal was considerably weak. 10− M, the Raman signal was considerably weak.

Figure 2. (a) SEM image of ZnO NRs fabricated using. the sol–gel method on a glass substrate; (b) XRD patterns of ZnO NRs fabricated by the unannealed seed layer, indicating both vertical (002) and Figure 2. (a) SEM image of ZnO NRs fabricated using the sol–gel method on a glass substrate; (b) horizontal (100) and (101) growth direction; (c) Raman signal of rhodamine 6G (R6G) using bare glass XRD patterns of ZnO NRs fabricated by the unannealed seed layer, indicating both vertical (002) and and ZnO NR-coated glass as surface-enhanced Raman scattering (SERS) substrates. horizontal (100) and (101) growth direction; (c) Raman signal of rhodamine 6G (R6G) using bare glass 3.2. XRD,and ZnO SERS NR-coated and Surface glass Morphology as surface-enhanc of Annealeded Raman ZnO scattering NRs (SERS) substrates. The quality of seed layers aﬀects the preferred orientation of ZnO NRs. Previous studies have 3.2. XRD, SERS and surface morphology of annealed ZnO NRs proved that surface morphology plays a critical role in the semiconducting nanostructure of the SERS substrateThe [quality4]. Therefore, of seed we layers annealed affects seed the layers preferred at various orientation temperatures of ZnO to NRs. achieve Previous superior studies ZnO have NRs SERSproved substrates. that surface Figure morphology3a illustrates plays the a critical XRD patterns role in the of semiconducting ZnO NRs fabricated nanostructure with annealed of the seedSERS layerssubstrate at various [4]. Therefore, annealing we temperatures. annealed seed The layers polar-terminated at various temperatures (100) and nonpolarto achieve low-symmetry superior ZnO (101)NRs facesSERS disappearedsubstrates. Figure in the ZnO3a illustrates NRs fabricated the XRD using patterns seed layersof ZnO annealed NRs fabricated at temperatures with annealed higher 3 thanseed 150layers◦C. Figureat various3b depicts annealing the corresponding temperatures Raman. The polar-terminated signal for 10 − M (100) R6G, whichand nonpolar indicated low- the enhancedsymmetry Raman (101) sensitivityfaces disappeared of ZnO NRs in grownthe ZnO using NRs annealed fabricated seed layers.using Figureseed layers S1 shows annealed the SERS at −34 behaviorstemperatures of ZnO higher NRs than grown 150 °C. using Figure annealed 3b depicts seed the layers corresponding at the R6G Raman concentration signal for of 1010− -MM R6G, and 5 10which− M. indicated Notably, thethe sample enhanced fabricated Raman using sensitivity ZnO seed of ZnO layers NRs annealed grown at using 150 ◦C annealed exhibited seed the highest layers. sensitivityFigure S1 forshows the SERSthe SERS signal. behaviors High annealing of ZnO temperature NRs grown was using favorable annealed for theseed growth layers ofat the the c-axis R6G (002)concentration orientation of and10−4-M contributed and 10−5 M. to theNotably, increased the sample SERS signal; fabricated however, using the ZnO SERS seed signal layers decreased annealed whenat 150 the °C annealingexhibited temperaturethe highest sensitivity was higher for than the 150 SERS◦C. Figuresignal.4 High depicts annealing the surface temperature morphology was offavorable ZnO NRs for fabricatedthe growth using of the di c-axisﬀerent (002) seed orient layersation annealed and contributed at various to temperatures. the increased SERS The sample signal; annealedhowever, at the 100 SERS◦C exhibited signal decreased a larger diameter when the (500 anne nm)aling of ZnOtemperature NRs than was that higher of the unannealedthan 150 °C. sample Figure (Figure2a). However, the diameter of ZnO NRs decreased to 100 nm at the annealing temperature of 150 ◦C when using as-prepared seed layers. The diameter of ZnO NRs slightly increased at the Appl. Sci. 2019, 9, x FOR PEER REVIEW 5 of 11

4 depicts the surface morphology of ZnO NRs fabricated using different seed layers annealed at various temperatures. The sample annealed at 100 °C exhibited a larger diameter (500 nm) of ZnO

Appl.NRs Sci.than2020 that, 10, 5015of the unannealed sample (Figure 2a). However, the diameter of ZnO NRs decreased5 of 11 to 100 nm at the annealing temperature of 150 °C when using as-prepared seed layers. The diameter of ZnO NRs slightly increased at the annealing temperature of 250 °C. Compared with the annealingcorresponding temperature SERS results of 250 ◦inC. ComparedFigure 3b, withapplyi theng corresponding the annealing SERS technique results when in Figure preparing3b, applying seed thelayers annealing decreased technique the diameter when preparing of ZnO NRs seed and layers further decreased increased the diameter the SERS of ZnOsignal NRs because and further of the increasedincrease in the the SERS surface-to-volume signal because of theratio increase of the in SERS the surface-to-volume substrate. Howeve ratior, ofwhen the SERSthe annealing substrate. However,temperature when was the higher annealing than temperature150 °C, the wasdiameter higher of than ZnO 150 NRs◦C, slightly the diameter increased, of ZnO which NRs was slightly not increased,favorable whichfor the wasRaman not signal favorable enhancem for theent Raman of the signal ZnO enhancement NR SERS substrate. of the ZnO NR SERS substrate.

FigureFigure 3.3. ((aa)) XRDXRD patternspatterns ofof annealedannealed ZnOZnO NRsNRs indicatingindicating thethe disappearancedisappearance ofof horizontalhorizontal (100)(100) andand (101)(101) growthgrowth directiondirection withwith thethe increaseincrease inin thethe annealingannealing temperaturetemperature appliedapplied onon thethe growthgrowth ofof seedseed 3 layers;layers; ((bb)) RamanRaman signalsignal ofof R6GR6G (10 (10−−3 M) using ZnO NR-coated glass asas thethe SERSSERS substrate,substrate, inin whichwhich ZnOZnO NRsNRs werewere fabricatedfabricated usingusing seedseed layerslayers annealedannealedat atdi differentﬀerent temperatures. temperatures. Appl.Appl. Sci. Sci. 20192020, 9,,10 x ,FOR 5015 PEER REVIEW 6 of6 of11 11

Figure 4. SEM images of ZnO NRs fabricated using seed layers annealed at (a) 100 ◦C, (b) 150 ◦C, Figure 4. SEM images of ZnO NRs fabricated using seed layers annealed at (a) 100 °C, (b) 150 °C, (c) (c) 200 ◦C and (d) 250 ◦C. 200 °C and (d) 250 °C. 3.3. SERS and Surface Morphology of AuNP-Decorated ZnO NRs 3.3 SERS and surface morphology of AuNP-decorated ZnO NRs Studies have shown the enhanced Raman signal using Au- or Ag-decorated ZnO NRs, in which theStudies 3D shape have of Aushown or Ag the o enhancedffers a “hot Raman spot” signal and enhances using Au- the or Raman Ag-decorated signalin ZnO the NRs, resonance in which and thenonresonance 3D shape of mode Au or [10 Ag,12 ,offers14,16]. a The “hot CT spot” occurs an ind theenhances interface the between Raman metal signal and in ZnO,the resonance with positively and nonresonancecharged metal mode and negatively[10,12,14,16]. charged The CT ZnO occurs [20,21 in]. the The interface Raman signal between of the metal analyte and absorbedZnO, with on positivelythe ZnO nanostructurecharged metal with and an negatively increased charged surface-to-volume ZnO [20,21]. ratio The was Raman further signal enhanced of the because analyte of absorbedadditional on absorption the ZnO nanostructure from the LSPR with mode an (resonance) increased andsurface-to-volume CT (nonresonance). ratio Therefore,was further to enhanced investigate becausethe ability of additional of the SERS absorption effect using from metallic the LSPR NPs, mode we decorated(resonance) ZnO and NRsCT (nonresonance). with AuNPs through Therefore, PLIP. toFigure investigate S2 depicts the ability the Raman of thesignal SERS ofeffect R6G using using metallic Au-decorated NPs, we ZnO decorated NRs at ZnO concentrations NRs with AuNPs ranging 6 9 throughfrom 10 −PLIP.to 10 Figure− M. ComparingS2 depicts tothe the Raman undecorated signal ZnOof R6G NR SERSusing substrateAu-decorated (Figure ZnO3b, annealedNRs at concentrationsT = 150 ◦C), AuNPs ranging did from not have10−6 to any 10− e9ff M.ect Comparing on the enhancement to the undecorated of Raman signal.ZnO NR Figure SERS5 illustratessubstrate (Figurethe SEM 3b, images annealed of Au-decoratedT=150 °C), AuNPs ZnO did NRs, not which have any indicated effect on the the nonhomogeneous enhancement of Raman distribution signal. of FigureAuNPs. 5 illustrates Moreover, the AuNPsSEM images aggregated of Au-decor on theated top ZnO and NRs, between which ZnO indicated NRs. the Studies nonhomogeneous have proved distributionthat the surface of AuNPs. morphology Moreover, of decorated AuNPs aggregat metalliced NPs on the plays top a and critical between role inZnO enhancing NRs. Studies the Raman have provedsignal [that10,12 the,14 surface,16]. Only morphology the 3D and of isolateddecorated metallic metallic NPs NPs enhanced plays a critical SERS eroleffectively in enhancing and further the Ramanincreased signal the [10,12,14,16]. Raman signal Only of the the analyte. 3D and The isol surfaceated metallic morphology NPs enhanced of ZnO NRs SERS directly effectively affects and the furthersize and increased the shape the of Raman grown AuNPs.signal of Therefore, the analyte. we grewThe surface AuNPs morphology on unannealed of ZnO NRs usingdirectly the affectssame PLIPthe size method. and the Figure shape6 aof depicts grown theAuNPs. Raman Therefore, signal of we R6G grew using AuNPs Au-decorated on unannealed ZnO NRs ZnO as NRs the usingSERS the substrate, same PLIP in which method. ZnO Figure NRs were 6a depicts fabricated the using Raman unannealed signal of seedR6G layers.using Au-decorated Notably, the Raman ZnO NRssignal as ofthe R6G SERS was substrate, enhanced in by which reducing ZnO the NRs full-width were fabricated of half maximum. using unannealed Figure6b illustratesseed layers. the Notably,SEM images the Raman of Au-decorated signal of R6G ZnO was NRs, enhanced which revealby reducing that AuNPs the full-w wereidth independently of half maximum. grown Figure on the 6b illustrates the SEM images of Au-decorated ZnO NRs, which reveal that AuNPs were Appl. Sci. 2020, 10, 5015 7 of 11 Appl. Sci. 2019, 9, x FOR PEER REVIEW 7 of 11 topindependently and sidewall grown of ZnO on NRs. the top The and surface sidewall morphology of ZnO NRs. of The Au-decorated surface morphology ZnO NRs of fabricated Au-decorated using annealedZnO NRs (Figure fabricated5) and using unannealed annealed seed (Figure layers 5) wasand comparedunannealed (Figure seed layers6b), which was compared revealed that(Figure the annealed6b), which seed revealed layers enhancedthat the annealed the growth seed of layers the ZnO enhanced NRs with the (002)growth c-axis of the orientation ZnO NRs and with improved (002) c- theaxis Raman orientation signal and because improved of higher the Raman surface-to-volume signal because ratio of andhigher density surface-to-volume of ZnO NRs. ratio However, and thedensity c-axis of orientation ZnO NRs. and However, high density the c-axis of ZnO orientatio NRs weren and favorable high density to the of growth ZnO NRs of closely were favorable packed and to aggregatedthe growth AuNPs of closely and packed inhibited and the aggregated growth of independentAuNPs and inhibited AuNPs, whichthe growth hinders of theindependent formation ofAuNPs, “hot spots” which for hinders SERS the [20 ].formation By contrast, of “hot ZnO spots” NRs for grown SERS using [20]. unannealedBy contrast, ZnO seed NRs layers grown resulted using in ZnOunannealed NRs with seed a large layers diameter resulted and in horizontal ZnO NRs (100)with anda large (101) diameter orientations, and horizontal which are ine(100)ffective and (101) SERS substratesorientations, because which of are the ineffective low density SERS and substrates surface-to-volume because of ratio. the low However, density AuNPsand surface-to-volume could be grown homogeneouslyratio. However, andAuNPs independently could be grown onZnO homogene NRs, suchously thatand AuNPsindependently offers e onffective ZnO NRs, “hot spots”such that for SERSAuNPs enhancement. offers effective Previous “hot spots” studies for haveSERS used enha complexncement. andPrevious time-consuming studies have methods used complex to achieve and 3Dtime-consuming metallic NPs on methods the ZnO to nanostructure achieve 3D metallic for enhancing NPs on the SERS ZnO signals nanostructure [10,12,14, 16for]. enhancing Our method SERS is a simplesignals method [10,12,14,16]. of fabricating Our method Au-decorated is a simple ZnO method NR SERS of substratesfabricating inAu-decorated a short time. ZnO NR SERS substrates in a short time.

Figure 5. SEM images of Au-decorated ZnO NRs of diﬀerent magniﬁcation. Figure 5. SEM images of Au-decorated ZnO NRs of different magnification. Appl. Sci. 2020, 10, 5015 8 of 11 Appl. Sci. 2019, 9, x FOR PEER REVIEW 8 of 11

Figure 6. ((aa)) Raman Raman signal signal of of R6G R6G at at di differentfferent concentrations concentrations using using Au-decorated Au-decorated ZnO ZnO NRs NRs as asthe the SERS substratesubstrate where where the the ZnO ZnO NRs NRs was fabricatedwas fabric usingated theusing unannealed the unannealed seed layer; seed (b) corresponding layer; (b) correspondingSEM images of SEM Au-decorated images of ZnOAu-decorated NRs of di ZnOfferent NRs magnification. of different magnification.

3.4. SERS SERS and and surface Surface morphology Morphology of ofAuNP-decorated AuNP-Decorated ZnO ZnO NRs NRs by bychanging Changing the PLIP the PLIP conditions Conditions Finally, we evaluated evaluated the the experimental experimental conditions conditions of of fabricating fabricating AuNPs AuNPs using using the the PLIP PLIP method, method, followed byby thethe immersion immersion process, process, to to grow grow AuNPs AuNPs on ZnOon ZnO NRs. NRs. Figure Figure7a depicts 7a depicts the Raman the Raman spectra spectraof R6G usingof R6G AuNP-decorated using AuNP-decorated ZnO NRs ZnO as the NRs SERS as substratethe SERS for substrate AuNP fabricatedfor AuNP for fabricated different for laser differentirradiation laser times irradiation at a fixed times immersion at a fixed time immersion of 30 min. time The of results 30 min. indicate The results that a indicate short laser that irradiation a short lasertime ofirradiation 8 min is suitabletime of 8 for min fabricating is suitable AuNP-decorated for fabricating AuNP-decorated ZnO NR SERS substrate ZnO NRto SERS support substrate the LSPR to supportmode and the enhance LSPR mode the Raman and enhance signal. the A longer Raman laser signal irradiation. A longer time laser of irradiation 20 min had time the best of 20 ability min had of the theSERS best eff ectability with of the the highest SERS effect Raman with intensity the highest for the Raman same FWHMintensity from for thethe Ramansame FWHM peak. Thefrom sample the Ramanfabricated peak. with The the sample laser irradiationfabricated with time the for laser 30 min irradiation was not time favorable for 30 formin the was SERS not favorable effect, and for the theRaman SERS signal effect, decreased. and the Raman Figure sign7bal illustrates decreased. the Figure Raman 7b signal illustra oftes R6G the usingRaman AuNP-decorated signal of R6G using ZnO AuNP-decoratedNRs as the SERS substrateZnO NRs foras the AuNPs SERS fabricated substrate atfo dir AuNPsfferent immersionfabricated at times different at fixed immersion laser irradiation times attime fixed of 8laser min. irradiation A suitable time immersion of 8 min. time A suitable facilitates immersion the growth time of AuNPs,facilitates and the an growth immersion of AuNPs, time of and30 min an immersion was the most time suitable. of 30 min The was SERS the samplemost suit withable. a longerThe SERS immersion sample with time a exhibited longer immersion a decrease timein the exhibited Raman signal.a decrease Figure in the7c Raman depicts signal. the SEM Figure images 7c depicts of AuNP-decorated the SEM images ZnO of AuNP-decorated NRs for AuNPs ZnOfabrication NRs for at AuNPs an immersion fabrication time at ofan 60immersion min. Nonhomogeneous time of 60 min. Nonhomogeneous and aggregated AuNPs and aggregated growth on AuNPsZnO NRs growth could on be ZnO the cause NRs could of the be weak the Ramancause of signal. the weak We triedRaman to correlatesignal. We the tried size, to density correlate and the the size,degree density of agglomeration and the degree of of AuNPs, agglomeration observed of in Au theNPs, SEM observed images in (Figures the SEM6b images and7c), (Figure with the 6b SERSand Figureenhancement 7c), with and the found SERS out enhancement the size of AuNPs and foun hasd toout be the in ~50-nm-diameter, size of AuNPs has the to density be in ~50-nm- of AuNPs diameter,has to be lessthe thandensity 10 AuNPsof AuNPs per has ZnO to NR,be less and than the AuNPs10 AuNPs has per to beZnO isolated NR, and and the independently AuNPs has to grown be isolatedon each ZnOand independently NR without aggregation. grown on each ZnO NR without aggregation.

Appl. Sci. 20192020, 910, x, 5015FOR PEER REVIEW 99 of of 11

6 Figure 7. RamanRaman signal signal of of R6G R6G (10 (10−6− M)M) at atvarious various concentrations concentrations using using Au-decorated Au-decorated ZnO ZnO NRs NRs as the as theSERS SERS substrate substrate for AuNP for AuNP fabrication fabrication for (a for) different (a) different laser laser irradiation irradiation times times at a fixed at a fixed immersion immersion time timeof 15 ofmin 15 and min ( andb) different (b) different immersion immersion time time at a atfixed a fixed laser laser irradiation irradiation time time of 8 ofmin 8 min during during PLIP; PLIP; (c) (correspondingc) corresponding SEM SEM images images of ofAu-dec Au-decoratedorated ZnO ZnO NRs NRs fabricated fabricated using using an an immersion immersion time time of of 60 min. 4. Conclusions 4. Conclusions We developed AuNP-decorated ZnO NRs as SERS substrates, in which AuNPs were fabricated We developed AuNP-decorated ZnO NRs as SERS substrates, in which AuNPs were fabricated through a chemically reduced and solution-based PLIP method. This method is a simple process and through a chemically reduced and solution-based PLIP method. This method is a simple process and was completed in 30 min. ZnO NR fabrication using annealed seed layers increased the SERS signal. was completed in 30 min. ZnO NR fabrication using annealed seed layers increased the SERS signal. Analysis of surface morphology indicates that the decrease in the ZnO NR diameter and increase in the Analysis of surface morphology indicates that the decrease in the ZnO NR diameter5 and increase in surface-to-volume ratio increased the sensitivity of Raman signal for detecting 10− M R6G. The SERS the surface-to-volume ratio increased the sensitivity of Raman signal for detecting 10−5-M R6G. The signal could be further improved by using AuNP-decorated ZnO NRs grown using unannealed ZnO SERS signal could be further improved by using AuNP-decorated ZnO NRs grown using unannealed seed layers. The use of unannealed seed layers facilitated the growth of ZnO NRs with non-preferred ZnO seed layers. The use of unannealed seed layers facilitated the growth of ZnO NRs with non- c-axis orientations and promoted the growth of homogeneous and individual AuNPs, which function preferred c-axis orientations and promoted the growth of homogeneous and individual AuNPs, as an effective “hot spots” for the Raman signal. Further studies of the AuNP fabrication using the PLIP which function as an effective “hot spots” for the Raman signal. Further studies of the AuNP method indicate that a short laser irradiation time of eight minutes, followed by a suitable chemical fabrication using the PLIP method indicate that a short laser irradiation time of eight minutes, followed by a suitable chemical immersion time of 15–30 min are optimum conditions. Our results Appl. Sci. 2020, 10, 5015 10 of 11 immersion time of 15–30 min are optimum conditions. Our results present an alternative method of fabricating effective SERS substrates in a simple wet chemical process combined with the short-time PLIP method.

Supplementary Materials: The following are available online at http://www.mdpi.com/2076-3417/10/14/5015/s1. Author Contributions: Conceptualization, L.V.T.H. and V.K.S.H.; methodology, L.T.T.T., N.T.Q.N., S.-Y.L. and Y.-Z.F.; formal analysis, C.-M.C.; resources, L.V.T.H. and V.K.S.H.; data curation, L.T.T.T. and N.T.Q.N.; writing—original draft preparation, C.-M.C.; writing—review and editing, C.-M.C. and V.K.S.H.; supervision, L.V.T.H. and V.K.S.H.; project administration, S.-Y.L. and Y.-Z.F. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Taichung Veterans General Hospital/National Chi Nan University Joint Research Program, Grant Number TCVGH-NCNU-1097902 and the Ministry of Science and Technology, Taiwan and Grant Number MOST-107-2221-E-260-016-MY3. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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