G10.Au Nanoparticle Monolayers Covered With Sol Gel Oxide Thin Films

8/18/2019 G10.Au Nanoparticle Monolayers Covered With Sol Gel Oxide Thin Films

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Published: October 04, 2011

r 2011 American Chemical Society 13739 dx.doi.org/10.1021/la2032829 | Langmuir 2011, 27, 13739–13747

ARTICLE

pubs.acs.org/Langmuir

Au Nanoparticle Monolayers Covered with SolGel Oxide Thin Films:Optical and Morphological Study

Enrico Della Gaspera,† Matthias Karg,‡ Julia Baldauf,‡ Jacek Jasieniak,§ Gianluigi Maggioni,|| and Alessandro Martucci* ,†

†Dipartimento di Ingegneria Meccanica Settore Materiali, Universita di Padova, Via Marzolo, 9, 35131 Padova, Italy ‡School of Chemistry & Bio21 Institute, University of Melbourne, Parkville, VIC, 3010, Australia§CSIRO Materials Science and Engineering, Ian Wark Laboratory, Bayview Avenue, Clayton 3168, Australia )Dipartimento di Fisica, Universita di Padova c/o INFNLegnaro National Laboratories, Viale dell'Universita, 2 35020 Legnaro (Pd) Italy

’ INTRODUCTION

Noble metal nanoparticles (NPs) have recently been dis-persed inside numerous active metal oxide matrices extensively studied as high-performance materials for sensing,13 catalysis,4,5

and within optoelectronic devices.6,7 Conventionally, such na-nocomposites are prepared as thinlmcongurationthrough theuse of techniques such as sputtering, physical vapor deposition(PVD), and chemical vapor deposition (CVD). However, de-spite each of these techniques requiring expensive depositionequipment, they produce lms with poor control of NP size, sizedistribution, and spatial distribution within the lm.

A simpler methodology relies on a two-step process whereby

NPs are

rst chemically synthesized and dispersed in a hostmatrix.810 The embedding of monodisperse metallic NPs insidesolgel matrixes is an example of such a methodology, whichpotentially obviates the pitfalls of the above-mentioned techni-ques, while presenting a cheap and straightforward way to createnanocomposite materials with tunable optical and electronicproperties. Nevertheless, the practical deposition of homoge-neous composite thin lms is not trivial, because there are many diff erent parameters involved in achieving a stable colloidal NPdispersion, such as pH, solvent type, ligand chemistry, andcomplexing agents.11,12

For these reasons, in this work we have decided to developcomposite thin lms through a diff erent approach. We have

employed a multilayer process involving an initial deposition of Au NP monolayers and subsequent solgel lm deposition. By rst anchoring noble metal NPs to a suitable substrate, a widecombination of metal oxides and noble metal NPs could beinvestigated while circumventing all the problems related withcolloidal stability of the NPs. Moreover, if the metal NPs areoptically active in the visible range due to the surface plasmonresonance (SPR), the mutual proximity in a close-packed NPlayer may induce coupling of the plasmon frequencies, result-ing in potentially novel and interesting optical and electronicproperties.13

Moreover, control of the resonance conditions, like tailoringNP organization, dielectric environments, and their stacking, isrequired in many technological applications like optical sensorsand biosensors,1417 surface-enhanced Raman spectroscopy,18

deep-colored coatings,19 and catalysis.20,21

In this paper, we present a detailed characterization of Au NPmonolayers that are deposited with diff erent surface coverages,and are subsequently overcoated with an active metal oxide(TiO2 and NiO). The eff ect of the Au NP layer surface coverage,annealing temperature, and type of metal oxide coating has been

Received: June 1, 2011Revised: September 28, 2011

ABSTRACT: In this work, we provide a detailed study of the inuence of thermal annealing onsubmonolayer Au nanoparticle deposited on functionalized surfaces as standalone lms andthose that are coated with solgel NiO and TiO2 thin lms. The systems are characterizedthrough the use of UV vis absorption, X-ray diff raction (XRD), atomic force microscopy

(AFM), scanning electron microscopy (SEM), and spectroscopic ellipsometry. The surfaceplasmon resonance peak of the Au nanoparticles was found to red-shift andincrease in intensity with increasing surface coverage, an observation that is directly correlated to the complex refractive index properties of Au nanoparticle layers. The standalone Au nanoparticles sinter at200 C, and a relationship between the optical properties and the annealing temperature ispresented. Whenovercoatedwith solgelmetaloxidelms(NiO,TiO2), the optical propertiesof the Au nanoparticles are strongly aff ected by the metal oxide, resulting in an intense red shiftand broadening of the plasmon band; moreover, the temperature-driven sintering is strongly limited by the metal oxide layer.Optical sensing tests for ethanol vapor are presented as one possible application, showing reversible sensing dynamics andconrming the eff ect of Au nanoparticles in increasing the sensitivity and in providing a wavelength dependent response, thusconrming the potential use of such materials as optical probes.


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13740 dx.doi.org/10.1021/la2032829 |Langmuir 2011, 27, 13739–13747

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assessed in detail, as well as the inuence of the top layer inlimiting the temperature-driven Au NP sintering and growth.Optical sensing tests for ethanol vapor detection were alsocarried out to exemplify one possible application of thesenanoscale architectures that can be employed in several otheroptochemical and optoelectronic devices. The versatility of the

described approachenables it to be easily extended to other typesof metal NPs (Ag, for example) or semiconducting NPs (likequantum dots as CdSe@CdS or PbSe), and to a variety of solution-based coatings, from oxides to polymers.

’EXPERIMENTAL SECTION

All chemicals used in the sample preparation have been purchasedfrom Sigma-Aldrich and used without any further purication.

Au NPs of about 14 nm mean diameter were prepared with theTurkevich method.22 Brie y, 12 mL of 1% trisodium citrate (>99%)aqueous solution was added to a 200 mL boiling solution of 0.5 mMHAuCl4 trihydrate (99.9%) in Milli-Q water. After the solution turned ared-wine color,it wasstirred at boiling point foran additional 15 minand

then was cooled down to room temperature. Separately, 11-mercap-toundecanoic acid (MUA, 95%) was dissolved in 10 mL of water and0.25 mL of ammonium hydroxide solution (33%) yielding a 2 mMconcentrated solution and then added as a complexing agent. Theresulting colloidal suspension was then puried and concentratedthrough a precipitation/redispersion process that has been previously described.11 The glass substrate was functionalized with (3-aminopro-pyl)trimethoxysilane (APTMS, 97%) using the method reported in ref 23. Brie y, the substrates were dipped in a 1% APTMS solution intoluene at 60 C for 5 min, and subsequently washed with fresh tolueneanddriedin a nitrogen stream. Then, Au NP monolayers were formedby spin-coating the liquid suspensions of gold NPs directly onto the

APTMS monolayers. In this study, we prepared Au NP monolayers with 3 diff erent extents of surface coverage, hereafter indicated as low

(L), medium (M), and high (H). The as-deposited Au NP monolayersamples were thermally treated at 100 C for 1 h in air. Following thisstabilizing treatment, the samples were used as substrates for solgelthin lm deposition.

NiO solgel solutions were prepared as follows: 300 mg of nickelacetate tetrahydrate (98%) was dissolved in 2 mL of methanol (99.8%),and then 0.18 mL of diethanolamine (99%) was added. The amine actsas a complexing agent, as conrmed from the change in color of thesolution (from bright green to dark green blue, due to the formation of thecomplex between Ni2+ ions andnitrogenatoms of theamine24). Thesolution was stirred for 30 min prior to deposition.

TiO2 solgel solutionswere prepared as follows: 0.55 mL of titanium butoxide (97%) was added to 0.47 mL of ethanol (99.8%) under stirring;0.27 mL acetylacetone (99%) was subsequently added and the solution

was stirred for 10 min, and then 0.12 mL of water was slowly addedunder vigorous stirring. After 20 min, 2.2 mL of ethanol was added, andthe solution was directly used for the deposition process.

All solgel samples were deposited by spin-coating at 2500 rpm for30 s on either SiO2 (HSQ300, Heraeus) or Si (Æ100æ oriented, p-type

boron-doped, Silicon Materials) substrates, with and without the Au NPmonolayers, and annealed in a muffle furnace at 500 C for 1 h in air,obtaining crystalline inorganic oxidelms of about 5060 nm thickness.

A complete list of the samples prepared in this study which utilized AuNPs underlayers is provided in Table 1.

The lms deposited on SiO2 substrates were characterized by XRDusing a Philips diff ractometer equipped with glancing-incidence X-ray optics. The analysis was performed at 0.5 incidence, using Cu K α Niltered radiation at 30 kV and 40 mA. The average crystallite size wascalculated from the Scherrer equation after tting the experimentalproles with Lorentzian curves: the diff raction peaks used for theanalyses are {111} at 38.2 and {200} at 44.4 for Au (JCPDS no.040714), {111} at 37.2 and {200} at 43.3 for NiO (JCPDS no.471049), {101} at 25.3 and {200} at 48.1 for TiO2 (JCPDS no.841285). The surface structure of the nanocompositelms depositedonSi substrates was investigated with an xT Nova NanoLab scanningelectron microscopy (SEM). AFM height proles of samples deposited

on Si substrates were recorded with a Veeco Multimode AFM operatingin tapping mode. Transmission electron microscopy (TEM) measure-ments of the metal NPs deposited on a carbon-coated copper grid weretaken with a Philips CM10 TEM; the size distribution of the NPs has

been evaluated with Fiji-Image JA 1.44b image analyzer softwaremeasuring a minimum of 150 particles.

Oblique angle attenuated total reectance (ATR) FTIR was per-formed on Thermo Scientic Nicolet 6700 FT-IR spectrometer with aHarrick VariGATR attachment. Samples were prepared on polishedsilicon wafer, with bare silicon being used as the reference. Measure-ments were performedat an incidenceangle of 62 from normal. Opticalabsorption spectra of samples deposited on SiO2 substrates were measuredin the 3002000 nm range using a Jasco V-570 spectrophotometer.

Transmittance at normal incidence and ellipsometry quantitiesΨ andΔ

of samples deposited on SiO2 substrates were measured using a J.A. Woollam V-VASE spectroscopic ellipsometer in vertical conguration,at two diff erent angles of incidence (60 , 70) in the wavelength range3001700 nm. Optical constants n and k were evaluated fromΨ ,Δ , andtransmittance data using WVASE32 ellipsometry data analysis software,tting the experimental data with Gaussian and Cauchy oscillators forabsorbing and nonabsorbing spectral regimes, respectively.

Optical sensing tests for ethanol detection were performed inreection mode on samples deposited on SiO2 substrates using acustom-built stainless steel cell provided with a heater that enabledgas sensing tests up to 150C.Thereectionspectra were collectedat anincident angle of 90 with a reection probe composed of a tight bundleof seven optical bers (six illumination bers around one read ber),connected to a OceanOptics USB2000 spectrophotometer. For ethanolsensing, a nitrogen stream owed inside through saturated ethanol, andif needed, the nal stream was diluted with pure nitrogen, to lower theethanol concentration. The nal ow rate was set constant at 0.5 L/min.

’RESULTS AND DISCUSSION

Au Nanoparticle Monolayer. In Figure 1, we show SEM and AFM images of Au NPs on silicon that correspond to the low (a,d), medium (b,e), and high (c,f) surface coverage samplesprepared in this study, as well as a TEM micrograph of the as-prepared Au colloids (i). It can be clearly seen that at low surfacecoverages the Au NPs are homogeneously dispersed on amicrometer scale, while at higher surface coverage, the formationof NP islands are observed. High-resolution topographic (g) and

Table 1. Sample Formulations Based on Au NPs SurfaceCoverage and Top Layer Composition

sample name Au NPs amount top layer

AuL Low -

AuM Medium -

AuH High -

AuLN Low NiO AuMN Medium NiO

AuHN High NiO

AuLT Low TiO2

AuMT Medium TiO2

AuHT High TiO2


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cross-sectional (h) SEM images show that the Au NPs predo-minantly exist within a monolayer, and only in the high surfacecoverage samples, few multilayered nanoparticles are detected.

To estimate the surface coverage of submonolayer coatings,the ratio between the projected surface area of the Au NPs andthe total area of the analyzed surface is considered. The meanparticle diameter evaluated from TEM images is D = 14( 1 nm(see Figure 1i, and it was also conrmed by the SEM images

(Figure 1ah). AFM overestimates the actual particle size( D = 41 ( 4 nm) due to convolution of the measurement withthe nite angle of the tip.25 For this reason, the Au NP surfacecoverage could only be accurately evaluated from the SEMcharacterization. The values obtained from the surface coverageanalysis were 62%, 34%, and 6% for the high, medium, and low coverage samples, respectively. These values highlight that thesubstrate functionalization and the Au NPs deposition processesprovide a simple and reproducible way to deposit metal NPs withsubmonolayer covering.

During the deposition process, the MUA functionalized AuNPs must anchor to the APTMS derivatized surface. This may arise due to (i) a simple electrostatic interaction between the

surface of Au NPs and the amino functionalities on the substrateand/or (ii) the formation of an amide bond between the aminoand carboxylate groups which originate from the APTMS andMUA, respectively. It is known that such a reaction is strongly favored in the presence of activating agents, such as a mixture of pentauorophenol and 1-(3-dimethylaminopropyl)-3-ethylcar- bodiimide hydrochloride (EDC),26 but the formation of theamide bond in the absence of an activating agent is difficult at

room temperature.27To investigate the anchoring mechanism, we have employed

ATR FT-IR measurements to study APTMS-functionalizedsubstrates before and after Au NP monolayer deposition. Ascan be seen in Figure 2, the APTMS-functionalized substrate without Au NPs shows a broad absorption peak at 1650 cm1 ,two very weak peaks at 1465 cm1 and 1379 cm1 , and a furthertwo weak peaks at 2851 cm1 and 2922 cm1 , while Au NPslayer exhibit also a series of peaks in the 13001900 cm1 range.The band at 1650 cm1 can be ascribed to strong in-plane NH2scissoring absorptions,28 and it is consistent with the presence of APTMS. The vibrational peak at 1379 cm1 can be attributed tosymmetric rocking of HCH bonds, arising from the APTMS

Figure 1. AFM (a,b,c) and SEM (dh) images of Au NPs layers with diff erent surface coverage: Low = 0.06 (a,d), Medium = 0.34 (b,e), High = 0.62(c,f). Image (g) is a higher magnication micrograph image (f); image (h) is a cross-sectional micrograph showing that Au NPs are on one single layer.Image (i) is a TEM micrograph of the Au colloids used for the nanocomposites preparation.


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organic chain.29 For the Au NP layer sample, this vibrational peak

should also be observed due to the HCH groups arising from both APTMS and the additional MUA molecules adsorbed to thesurfaces of the Au NPs. However, this vibration cannot be resolved because it is convoluted with an intense peak at 1411 cm1. Thepeakat1465cm1 is distinctive of theasymmetric CHbendingoriginating from the organic carbon chain of both APTMS andMUA molecules, and it is recognizable in both spectra as well.The two peaks at 2922 and 2851 cm1 are ascribable to CCand CH bond vibrations in CH2 groups as reported in severalpublications.2932 Some authors claim that a peak at about2950 cm1 can be due to amino groups,33,34 but since this peak is usually very weak and here it is overlapped with stronger CH vibrations, inferring its presence from these spectra will be ratherspeculative. These peaks are again due to the presence of organic

chains ofbothAPTMS andMUA molecules, and it is consistentthatthey are more intense in the MUA-containing sample, since morematerial is probed by the IR beam.

The peak at 1712 cm1 is assigned to the CdO stretching band arising from the carboxyl group of the MUA molecule,35 afurther conrmation of Au NPs being present.

The peak at 1560 cm1 appears only in the sample containing Au NPs, and it can be ascribed to NH bending modes of theamide bond26 or to NH scissoring modes of adsorbed amineon metals.36 The CdO stretching of the amide group has beenfound at 1660 cm1 by Whitesides and co-workers,26 but in oursamples, this peak overlaps with primary amine vibrations.

The last peak at 1411 cm1 is difficult to assign. In the past, ithas been ascribed to CH vibrations, but the data available are

controversial. Bertilsson and Liedberg37 haveobserved two peaksat 1418 cm1 and 1470 cm1 which they have related to CH vibrations of the organic chain of thiols self-assembled on a goldsurface. These frequencies are relatively close to those in thisstudy, thus providing a potential origin of the vibrational peak observed at 1411 cm1.

The bonds that form during the deposition process enablesubmonolayer coatings of Au NPs to be deposited. As thetemperature of such Au NP deposited substrates increases, these bonds may be insufficient to prevent particles from diff using overthe surface and ultimately coalescing. An understanding of sucheff ects is vital for the application of Au NP based lms. With thisin mind, we have investigated the thermal stability of bare Au NP

layers with medium surface coverage (about 34%) followingannealing at 100, 200, 300, and 400 C for one hour.

In Figure 3, we show SEM images that map out the evolutionof Au NPs layers with increasing temperature. A clear change inthe morphology of the layer can be seen at 200 C and above, while minor modications in the average particle size can be seenafter the 300 C and 400 C annealing temperatures. Fromanalysis of the average particle size, it can be noted that theoriginally monodisperse Au nanoparticles grow from 14 ( 1 nmto polydispersed nanoparticle ensembles of 32 ( 13 nm in sizefollowing annealing at 200 C; a progressive increase in the meandiameter can be seen after the 300 C (37( 15 nm) and 400 C

(43( 18 nm) annealing steps. This behavior canbe attributed tothe close packing of the as-synthesized Au NPs within themonolayer, which provides adequateinterparticle surface contact(Figure 3a) to enable temperature-driven sintering.38 Followingcoalescence, the interparticle distance increases (Figure 3b) andtherefore, further growth is hindered, or even prevented.

It is known that low molecular weight thiols desorb from goldsurfaces at temperatures below 200 C.39,40 This factor may suggest that the driving force for the observed Au NP sintering athigh temperatures is related to the desorption and/or decom-position of the organic molecules on their surface, i.e., MUA and APTMS. It has recently been reported41 that Au polymercoreshell structures when deposited on glass substrates andannealed at 700 C, produce Au NPs layers that adhere to the

substrate without signicant eff ect on the metal core size orspatial distribution. In this study, the surface coverage was sub-stantially higher, ensuring that the greatly reduced interparticlespacing may have additionally enabled more efficient sintering of the nanoparticles following theremoval of the capping agent. It isalso worth noting that both MUA and APTMS decompose below 500 C;42,43hence, the presenceof residual organic is unlikely in thesample annealed at 500 C.

Optical spectroscopy together withspectroscopicellipsometry are useful for understanding the eff ects that the surface coverage,the thermal annealing, and the oxide coating presence have onthe Au NPs. Figure 4a shows UV vis-NIR spectra for as-deposited Au NP layers with diff erent surface coverage that have

Figure 3. SEM images of Au NP layers annealed at diff erent tempera-tures: (a) 100 C; (b) 200 C; (c) 300 C; (d) 400 C.

Figure 2. FT-IR spectra of APTMS-functionalized silicon substratesuncovered (a) and covered (b) with Au NPs layer.


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been stabilized at 100 C. In accordance with previous reports,increasing surface coverage results in both an increased intensity and a red shift of the Au NPs SPR peak. 17,23 The increase inabsorbance with higher surface coverage is simply related to thehigher concentration of NPs interacting with the incoming beam,leading to an increase in absorption as described by the well-known LambertBeer equation.44,45 Meanwhile, the red shift of theplasmon peak with increasedsurface coverage canbe ascribedto the decrease in the mutual distance between Au NPs; thisresults in a stronger coupling of the plasmon resonances and a

concordant red-shift of the plasmon resonance.46In Figure 4b, weshow absorption spectra of a AuNP layer with

medium surface coverage (34%, the same coverage used for theSEM measurements shown in Figure 3) annealed between 100and 400 C. The Au SPR peak clearly depends on the annealingtemperature. Following the 200 C annealing step, the SPR peak reduces slightly in intensity, red shifts, and broadens. Theseobservations are consistent with the initial coalescence step, which also causes necking between particles and the consequentformation of some elongated particles. However, with increasingannealing temperature, a progressive blue-shift and a morepronounced reduction in intensity is observed. This is due tothe subsequent “spheroidization” of the as-coalesced particles

and their progressive increase in size. In turn, this leads to anincrease of the average interparticle distance, causing a decreasein SPR coupling. Moreover, the number of particles decreases asa consequence of the sintering, and so does their optical crosssection, causing a reduction in the absorbance of the sample. Thechromatic eff ects of the thermal annealing can be seen in thepicture shown in the inset of Figure 4b: the Au NPlayer annealedat 100 C is blue-colored, as a consequence of the plasmoncoupling (the wavelength corresponding to the SPR peak registered in aqueous solutions is at 520 nm, corresponding toa red-colored solution), while the Au NPs layer annealed at400 C is pink, as a consequence of the plasmon decoupling afterthe particle growth and the consequent increase in the mutualdistance.

The absorption measurements, combined with the previously discussed SEM measurements, demonstrate that an annealingtemperature of 200 C is sufficient to promote sintering andcoarseningof close-packed Au NPs, leadingto a respectivechangeof their optical properties. For this reason, unless specied, all thecharacterizations of uncoated Au NP layers presented in thefollowing refer to the samples annealed at 100 C.

To understand further the observed absorption properties of the Au monolayers, we have combined spectroscopic ellipsome-try and optical transmission measurements to determine theiroptical constants. Figure 4c,d shows the dispersion curves of therefractive index and the absorption coefficient of the Au NPslayers, respectively. The layers have been modeled as an eff ectivemedium thin lm composed of Au NPs in air. As such, the

resulting curves do not represent the dielectric function for Auitself, but they are the dielectric function for the layer itself. Therefractive index dispersion curves (Figure 4c) show that increas-ing Au NP coverages results in a higher eff ective refractive index and in accordance with the KramersKronig relation exhibit aconcordant red shifting of the second-order inection point thatis characteristic of the Au NPs SPR absorption. The spectralchanges associated with the refractive indexes translate to a clear redshift and increased magnitude of the SPR peak extinction coeffi-cients (Figure 4d) with increasing surface coverage, thusconrmingthe experimental results of the optical absorption measurements.

Au Nanoparticle Monolayer Coated with SolGel Film.

The effect of the solgel layer deposited on top of the Au NPs

Figure 4. (a) Optical absorption spectra of Au NP layers annealed at100 C. (b) Optical absorption spectra of AuM sample annealed

between 100 and 400 C; the vertical dashed lines highlight the AuSPR peak recorded at 520 nm in water. The inset shows a picture of thesamples annealed at 100 C (left) and at 400 C (right); the scale bar isin cm. (c) Refractive index and (d) extinction coefficient dispersioncurves for Au NP layers annealed at 100 C.

Figure 5. Optical absorption spectra of (a) Au NP layers covered withNiO and annealed at 500 C. (b) Au NP layers covered with TiO2 andannealed at 500 C.


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monolayer on the optical properties of the nanocomposite is a broadening and red-shift of the SPR peak (see Figure 5a,b). Thiseffect is found to be much more pronounced for TiO2 comparedto NiO, due to thehigher refractive index of theformer comparedto the latter (2.51 at 590 nm for TiO2 ,

47 2.33 at 620 nm forNiO48), and also to the interaction between anatase crystals andthe surface of Au NPs, causing spreading and scattering of conduction electrons as described previously.49,50

Moreover, looking particularly at the Low surface coveragesamples coated with the two diff erent oxide layers, distinct high

and low wavelength SPR peaks emerge. The low wavelengthpeaks, at about 590 and 630 nm for NiO and TiO2 , respectively,can be considered as arising from noninteracting randomly dispersed Au NPs inside those matrices. The high wavelengthpeaks appear in the Low surface coverage samples around 750and 1000 nm for NiO and TiO2 , respectively, and are found toprogressively red shift and become more prevalent with in-creasing Au NPs surface coverage. These peaks are consistent with the surface plasmon coupling between Au NPs.38 We notethat the apparent broad absorption band observed for pureTiO2 lm is due to optical interference; hence, the Au SPR peaks of the Au containing samples overlap with this inter-ference fringe.

XRD measurements of the diff erent samples studied are

reported in Figure 6. Crystallization was evident in all samples,as testied by clearly identied diff raction peaks. The intensity of the Au peaks (JCPDS no. 040714) was in good agreement withthe diff erent surface coverage, and so with the amount of Au NPspresent inside the lms. For anatase TiO2 (JCPDS no. 841285),the peaks have similar intensities for each of the samples. Thissuggests that the Au monolayers used as substrates do notsignicantly alter the process of matrix formation for this metaloxide. In contrast, for NiO (JCPDS no. 471049) we observe broadened NiO diff raction peaks within increasing Au NPcoverage. This factor suggests that Au NPs directly inuencethe crystallization of NiO, a point that will be further elucidatedon shortly.

Crystallite sizes evaluated through the Scherrer formula arelisted in Table 2. As can be noticed, the crystallite size of Au NPsis slightly higher when the monolayer is covered with the solgelcoating. This eff ect is related to the annealing process(500 Cfor1h)that must induce some coarsening or sintering of the particles.For comparison, the XRD spectrum of the uncovered, mediumsurface coverage, sample annealed at 400C is reported as well inFigure 6a; a clear sharpening of the Au diff raction peaks is

experienced as a consequence of the previously discussed NPsintering and growth processes. The crystallite size is about 8 nmfor the uncovered Au layers annealed at 100 C, in the 811 nmrange for the solgel-coated Au NP layer annealed at 500 C,and >14 nm for the uncovered Au NP layer with medium surfacecoverage annealed at 400 C (the same sample used for SEM andUV vis measurements reported in Figure 3d and Figure 4b,respectively). This comparison shows that the presence of thesolgel lms reduces or even prevents the growth of Au NPs.Notably, the diff erence in the mean crystallite size of Au NPsestimated from XRD peak broadening (78 nm) and the meanparticle size evaluated from SEM (14 nm) indicates that the AuNPs are not monocrystalline.

NiO and TiO2 crystallites sizes are in the 1417 nm and

1923 nm ranges, respectively. Interestingly, for NiO we nd thatthe crystal size is slightly smaller for samples where the solgelsolution is deposited on the high surface coverage Au monolayers,and slightly higher when deposited on the lower surface coverages,or on bare substrates. This behavior was not observed for TiO2crystals, where thedata arerandomly distributedand their diff erenceis within the error bars. The trend observed in the NiO crystal sizecan be attributedto the small latticemismatch between NiO andAucrystals.51 This factor can permit Au NPs to act as heterogeneousnucleating sites for NiO crystals. Therefore, as the number of nucleiis related to the number of Au NPs, i.e., to the surface coverage,higher Au NP concentrations result in smaller metal oxide crystalsizes for a given volume of deposited material.

Figure 6. XRD patterns of (a) Au NPs layers stabilized at 100 C;for comparison purposes thesampleannealedat 400C is alsoreported;(b) Au NP layers covered with NiO and annealed at 500 C; (c) Au NP

layerscovered with TiO2 andannealedat 500C. Theoretical diff

ractionlines for Au (black lines) NiO (dashed lines in gure (b)) and TiO2(dashed lines in gure (c)) are reported at the bottom.

Table 2. Mean Crystallite Diameters Calculated According tothe Scherrer Equation for the Three Crystalline PhasesDetected for the Samples Reported in Figure 6a

diameter (nm)

sample Au TiO2 NiO

Au Nanoparticle Monolayer Uncoated

AuL 7.4 ( 0.4 / /

AuM 7.9 ( 2.6 / /

AuH 7.5 ( 2.8 / /

AuM 400 C 14.4 ( 2.0 / /

Au Nanoparticle Monolayer Coated with NiO

NiO / / 16.7 ( 0.4

AuLN 9.3 ( 2.1 / 15.2 ( 1.3

AuMN 11.1 ( 3.8 / 14.2 ( 0.2

AuHN 11.6 ( 3.2 / 13.7 ( 0.4

Au Nanoparticle Monolayer Coated with TiO2

TiO2 / 20.1 ( 0.2 /

AuLT 7.5 ( 0.4 19.2 ( 1.5 /

AuMT 10.8 ( 0.2 21.3 ( 5 /

AuHT 11.3 ( 0.4 22.6 ( 4.2 /a The coated samples are annealed at 500 C.


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To evaluate the morphology of the metal oxide coated Aumonolayer following annealing at 500 C, SEM analyses have been carried out. As can be seen from Figure 7, NiO coatedsamples are rougher and with more irregularities than thosecoated with TiO2 lms. To investigate the Au NP morphology below the TiO2 solgel lms, we gentlyscratch the surface with ascalpel. This enabled us to observe the underlying Au NPs(Figure 7c), as well as overturn the 5060 nm lm to expose

the underlayer containing the Au NP monolayer (Figure 7d). In both cases, the Au NPs could be easily detected due to theirhigher contrast (highlighted by arrows in Figure 7c).

From the SEM, the size of the Au NPs was estimated as being16.1( 1.8 nm;a value that is only slightly highercompared to theas-synthesized colloids. Thus, it can be concluded that the metaloxide lm is not damaging the Au NPs layer during the solgelsolution spinning process, andis also strongly limiting thegrowthof the metal particles by providing a physical diff usion barrier between neighboring particles.

Having characterized the Au NP layers covered with NiO andTiO2 solgel lms, we will now evaluate the practical use of suchsystems as a gas sensor to detect ethanol vapor. To ensure thestability of the Au monolayer without metal oxide layers, the

sensor was operated at a temperature (OT) of 150 C and itsoptical detection mode was reection (see Experimental sectionfor details). All tests were performed on the Au NP layers withhigh surface coverage. The uncovered sample was annealed at150 C for 6 h prior to gas sensing measurements. Followingthis annealing time, no changes to the optical absorptionspectrum could be detected, conrming its thermal stability atthis temperature.

The eff ect of ethanol vapors is shown in Figure 8. Thediff erence in reection intensity between the spectrum collectedduring ethanol exposure and during nitrogen exposure (opticalreection change, ORC = ReEtOH ReN2) is plotted as afunction of wavelength, and it is reported for uncovered Au NPs

layer, Au NPs layers covered with NiO and TiO2 , and pure NiOand TiO2 lms. As can be seen, outside the Au SPR peak wavelength range, the ORC is similar to the response of themetal-oxide matrix with no Au NPs monolayer. Interestingly, inthese ranges the ORC parameter is negative for NiO lm,positive for TiO2 lms, and null for the uncovered layer. TheSPR wavelength range shows a wavelength-dependent response,this being true for the uncovered Au NPs layer as well. This arisesas a consequenceof the role playedby the noble metal particles asoptical probes for the target analyte detection. Moreover, com-pared to the pure metal oxide lms, the covered Au NPs layersexhibit a higher response in a limited wavelength range. Thisprovides conrmation of the SPR enhancement to the sensing

behavior obtained by combining the close-packed Au NPs layerand the oxide active lms. In fact, especially for TiO2 , the ORCmaximum of the covered Au NPs layers is higher than the sum of the Au NPs layer and the oxide lm alone, so a synergetic eff ect between the two components is likely to occur. Moreover, theeff ect of the diff erent metal oxide can be also seen: the ethanoleff ect on NiO is to reduce the reection intensity, while itsinteraction with TiO2 causes an increase in reection. This factcan be explained by considering the diff erent electric nature of the two semiconducting oxides, i.e., NiObeing a p-type andTiO2an n-type semiconductor.

As reported in the literature, volatile organic compounds(VOCs) can be oxidized on the surface of semiconductingmaterials; for example, in the case of ethanol (C2H5OH), themain reaction mechanisms can be describedas the following:52,53

2C2H5OH þ O2 f 2CH3CHO þ 2H2O

C2H5OH f C2H4 þ H2O

In the rst reaction, ethanol is oxidized to acetaldehyde(CH3CHO) by dehydrogenation of the ethanol molecule anda subsequent reaction with oxygen leads to water formation. Thisreaction can proceed further, with successive oxidation of acetal-dehyde to aceticacid. The secondreaction is a direct dehydrationof ethanol to ethylene (C2H4) with water formation. There areother possible ethanol oxidation reactions leading, for example,

Figure 7. SEM images of Au NP monolayer with medium surfacecoverage: (a) covered with NiO annealed at 500 C; (bd) covered

with TiO2 annealed at 500 C. Au NPs as brighter spots are indicated by the arrows in(c); in(d), a fragment ofthe lm ipped over, exposing the

Au NPs on the upper lm surface.

Figure 8. ORC plots for the high surface coverage Au NP layers bareand covered with NiO and TiO2 lms, and for pure NiO and TiO2 lms

when exposed to 180 ppm ethanol at 150 C OT. Zero value of responseis highlighted with a dotted line.


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to the formation of diethyl ether or diethylacetal, butthey are lesscommon and require specic reaction conditions.53 The preferredpath is generally related to the type of the metal oxide (basicoxides usually promote the rst reaction, acid oxides the secondone), to the presence of adsorbed oxygen species on the surfaceof the oxide, or to the presence of oxygen in the gas phase (as can be seen, oxygen is necessary for the rstreaction). In anycase,theoxidation of the ethanol molecules will lead to electron injectioninto the metal oxide. In the case of TiO2 , the ethanol oxidation will lead to an increase in conductivity, because electrons areinjected into the conduction band of anatase. For NiO, theopposite is true, as oxidation will lead to a decrease in con-ductivity, due to electronhole recombination. As a conse-quence, it is reasonable to suppose that a diff erence in the

electronic properties of the metal oxides produces a diff erencein the reection intensity, and that this diff erence is of oppositesign according to the type of semiconducting material.

Within our experimental setup, we employ a low-resolutionspectrophotometer and collect light under reection mode.These factors contribute to low signal-to-noise ratios. Despitethis, the observed ORC trends clearly exemplify the synergeticeff ect of coupling Au NP layers and metal oxide lms to enhancethe sensing properties.

While the above experiments were obtained under staticconditions, we also performed dynamic tests at a xed wave-length of 594 nm (corresponding to the maximum of theresponse, as obtained from Figure 8) on the AuHT sample, themost sensitive among the tested ones. The results, which are

depicted in Figure 9, show a reversible signal during repeatedcycles of nitrogenethanol exposure to the sensor. Although thesensing dynamics are not ideal, because both response andrecovery times are occurring in a time scale of few minutes, theresults are promising, considering the low thickness of thesamples and the low resolution of the setup used. In fact, aneasily detectable variation in the reectivity during ethanolexposure has been observed, with good reproducibility afterrepeated nitrogen/ethanol cycles (see Figure 9). These resultsshow promise for applications of such materials in transmissionmode or in devices where the reection is enhanced, like SPR congurations or on the surface of unclad optical bers. One of the main issues with gas sensors is the cross sensitivity between

interfering vapors or gases: these nanocomposites are currently under investigation in order to analyze their gas sensing response when exposed to diff erent gases and vapors, according to theircomposition and the operative temperature, and the results areintended to be published in a separate paper. Nevertheless, thesepreliminary results are encouraging, considering also that thesynthetic approach described in this work can be easily extended

to a great variety of active layers, simply changing the type of NPsandthe materialfor thetop coating,tailoring theproperties of thenanocomposite by an appropriate choice of the active materialsand a proper optimization of their organization and spatialdistribution.

’CONCLUSIONS

We have demonstrated that Au NP layers covered withsolgel oxide lms constitute an eff ective design for materialsto be used in optoelectronic applications. Nearly monodisperse Au NPs were deposited on properly functionalized substrates with good control of the surface coverage. Detailed optical andmorphological studies have been presented, showing a relation-

ship between the Au NP surface coverage, annealing temperatureand optical properties of the uncovered monolayers; moreover,the bond formation between Au NPs and the APTMS function-alized substrate has been deduced from infrared spectroscopy measurements. The presence of a solgel oxide lm depositedon top of the Au NP layers aff ects the optical properties of thenanocomposite and also provides a physical barrier betweenneighboring Au NPs, strongly limiting the extent of theirtemperature-driven sintering. Preliminary gas sensing measure-ments on these systems show that ethanol vapor induces areversible and reproducible response, conrming the role of Au NPs in increasing the sensitivity of the oxide lm itself andproviding a wavelength-dependent response.

’AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

’ACKNOWLEDGMENT

This work has been supported through Progetto StrategicoPLATFORMS of Padova University. E.D.G. thanks FondazioneCARIPARO for nancial support. A.M. thanks the Universitiesof Melbourne and Padova for their support through the Uni- versity academic exchange program. J.J. acknowledges the Aus-tralian Research Council for support through the APD grantDP110105341. M.K. acknowledges the Alexander von Hum-

boldt foundation for a Feodor Lynen research fellowship.

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Figure 9. Dynamic tests for AuHT sample at 594 nm and 150 C OTunder repeated cycles of nitrogen180 ppm ethanol.


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