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 INFN Legnaro National Laboratories, Viale dell'Universita, 2 35020 Legnaro (Pd) Italy
)
‡
ABSTRACT: In this work, we provide a detailed study of the influence of thermal annealing on submonolayer Au nanoparticle deposited on functionalized surfaces as standalone films and those that are coated with solgel NiO and TiO2 thin films. The systems are characterized through the use of UVvis absorption, X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), and spectroscopic ellipsometry. The surface plasmon resonance peak of the Au nanoparticles was found to red-shift and increase 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 at 200 C, and a relationship between the optical properties and the annealing temperature is presented. When overcoated with solgel metal oxide films (NiO, TiO2), the optical properties of the Au nanoparticles are strongly affected by the metal oxide, resulting in an intense red shift and 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 and confirming the effect of Au nanoparticles in increasing the sensitivity and in providing a wavelength dependent response, thus confirming the potential use of such materials as optical probes.
’ INTRODUCTION Noble metal nanoparticles (NPs) have recently been dispersed 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 nanocomposites are prepared as thin film configuration through the use of techniques such as sputtering, physical vapor deposition (PVD), and chemical vapor deposition (CVD). However, despite each of these techniques requiring expensive deposition equipment, they produce films with poor control of NP size, size distribution, and spatial distribution within the film. A simpler methodology relies on a two-step process whereby NPs are first chemically synthesized and dispersed in a host matrix.810 The embedding of monodisperse metallic NPs inside solgel matrixes is an example of such a methodology, which potentially obviates the pitfalls of the above-mentioned techniques, while presenting a cheap and straightforward way to create nanocomposite materials with tunable optical and electronic properties. Nevertheless, the practical deposition of homogeneous composite thin films is not trivial, because there are many different parameters involved in achieving a stable colloidal NP dispersion, such as pH, solvent type, ligand chemistry, and complexing agents.11,12 For these reasons, in this work we have decided to develop composite thin films through a different approach. We have r 2011 American Chemical Society
employed a multilayer process involving an initial deposition of Au NP monolayers and subsequent solgel film deposition. By first anchoring noble metal NPs to a suitable substrate, a wide combination of metal oxides and noble metal NPs could be investigated while circumventing all the problems related with colloidal stability of the NPs. Moreover, if the metal NPs are optically active in the visible range due to the surface plasmon resonance (SPR), the mutual proximity in a close-packed NP layer may induce coupling of the plasmon frequencies, resulting in potentially novel and interesting optical and electronic properties.13 Moreover, control of the resonance conditions, like tailoring NP organization, dielectric environments, and their stacking, is required in many technological applications like optical sensors and 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 NP monolayers that are deposited with different surface coverages, and are subsequently overcoated with an active metal oxide (TiO2 and NiO). The effect of the Au NP layer surface coverage, annealing temperature, and type of metal oxide coating has been Received: June 1, 2011 Revised: September 28, 2011 Published: October 04, 2011 13739
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Table 1. Sample Formulations Based on Au NPs Surface Coverage and Top Layer Composition sample name
Au NPs amount
top layer
AuL
Low
-
AuM AuH
Medium High
-
AuLN
Low
NiO
AuMN
Medium
NiO
AuHN
High
NiO
AuLT
Low
TiO2
AuMT
Medium
TiO2
AuHT
High
TiO2
assessed in detail, as well as the influence of the top layer in limiting the temperature-driven Au NP sintering and growth. Optical sensing tests for ethanol vapor detection were also carried out to exemplify one possible application of these nanoscale architectures that can be employed in several other optochemical and optoelectronic devices. The versatility of the described approach enables it to be easily extended to other types of metal NPs (Ag, for example) or semiconducting NPs (like quantum 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 purchased from Sigma-Aldrich and used without any further purification. Au NPs of about 14 nm mean diameter were prepared with the Turkevich method.22 Briefly, 12 mL of 1% trisodium citrate (>99%) aqueous solution was added to a 200 mL boiling solution of 0.5 mM HAuCl4 trihydrate (99.9%) in Milli-Q water. After the solution turned a red-wine color, it was stirred at boiling point for an additional 15 min and then was cooled down to room temperature. Separately, 11-mercaptoundecanoic acid (MUA, 95%) was dissolved in 10 mL of water and 0.25 mL of ammonium hydroxide solution (33%) yielding a 2 mM concentrated solution and then added as a complexing agent. The resulting colloidal suspension was then purified and concentrated through a precipitation/redispersion process that has been previously described.11 The glass substrate was functionalized with (3-aminopropyl)trimethoxysilane (APTMS, 97%) using the method reported in ref 23. Briefly, the substrates were dipped in a 1% APTMS solution in toluene at 60 C for 5 min, and subsequently washed with fresh toluene and dried in a nitrogen stream. Then, Au NP monolayers were formed by spin-coating the liquid suspensions of gold NPs directly onto the APTMS monolayers. In this study, we prepared Au NP monolayers with 3 different extents of surface coverage, hereafter indicated as low (L), medium (M), and high (H). The as-deposited Au NP monolayer samples were thermally treated at 100 C for 1 h in air. Following this stabilizing treatment, the samples were used as substrates for solgel thin film deposition. NiO solgel solutions were prepared as follows: 300 mg of nickel acetate tetrahydrate (98%) was dissolved in 2 mL of methanol (99.8%), and then 0.18 mL of diethanolamine (99%) was added. The amine acts as a complexing agent, as confirmed from the change in color of the solution (from bright green to dark greenblue, due to the formation of the complex between Ni2+ ions and nitrogen atoms of the amine24). The solution was stirred for 30 min prior to deposition. TiO2 solgel solutions were 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 added under vigorous stirring. After 20 min, 2.2 mL of ethanol was added, and the solution was directly used for the deposition process. All solgel samples were deposited by spin-coating at 2500 rpm for 30 s on either SiO2 (HSQ300, Heraeus) or Si (Æ100æ oriented, p-type boron-doped, Silicon Materials) substrates, with and without the Au NP monolayers, and annealed in a muffle furnace at 500 C for 1 h in air, obtaining crystalline inorganic oxide films of about 5060 nm thickness. A complete list of the samples prepared in this study which utilized Au NPs underlayers is provided in Table 1. The films deposited on SiO2 substrates were characterized by XRD using a Philips diffractometer equipped with glancing-incidence X-ray optics. The analysis was performed at 0.5 incidence, using Cu Kα Ni filtered radiation at 30 kV and 40 mA. The average crystallite size was calculated from the Scherrer equation after fitting the experimental profiles with Lorentzian curves: the diffraction peaks used for the analyses 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 nanocomposite films deposited on Si substrates was investigated with an xT Nova NanoLab scanning electron microscopy (SEM). AFM height profiles of samples deposited on Si substrates were recorded with a Veeco Multimode AFM operating in tapping mode. Transmission electron microscopy (TEM) measurements of the metal NPs deposited on a carbon-coated copper grid were taken with a Philips CM10 TEM; the size distribution of the NPs has been evaluated with Fiji-Image JA 1.44b image analyzer software measuring a minimum of 150 particles. Oblique angle attenuated total reflectance (ATR) FTIR was performed on Thermo Scientific Nicolet 6700 FT-IR spectrometer with a Harrick VariGATR attachment. Samples were prepared on polished silicon wafer, with bare silicon being used as the reference. Measurements were performed at an incidence angle of 62 from normal. Optical absorption spectra of samples deposited on SiO2 substrates were measured in the 3002000 nm range using a Jasco V-570 spectrophotometer. Transmittance at normal incidence and ellipsometry quantities Ψ and Δ of samples deposited on SiO2 substrates were measured using a J.A. Woollam V-VASE spectroscopic ellipsometer in vertical configuration, at two different angles of incidence (60, 70) in the wavelength range 3001700 nm. Optical constants n and k were evaluated from Ψ, Δ, and transmittance data using WVASE32 ellipsometry data analysis software, fitting the experimental data with Gaussian and Cauchy oscillators for absorbing and nonabsorbing spectral regimes, respectively. Optical sensing tests for ethanol detection were performed in reflection mode on samples deposited on SiO2 substrates using a custom-built stainless steel cell provided with a heater that enabled gas sensing tests up to 150 C. The reflection spectra were collected at an incident angle of 90 with a reflection probe composed of a tight bundle of seven optical fibers (six illumination fibers around one read fiber), connected to a OceanOptics USB2000 spectrophotometer. For ethanol sensing, a nitrogen stream flowed inside through saturated ethanol, and if needed, the final stream was diluted with pure nitrogen, to lower the ethanol concentration. The final flow 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 samples prepared in this study, as well as a TEM micrograph of the asprepared Au colloids (i). It can be clearly seen that at low surface coverages the Au NPs are homogeneously dispersed on a micrometer scale, while at higher surface coverage, the formation of NP islands are observed. High-resolution topographic (g) and 13740
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Figure 1. AFM (a,b,c) and SEM (dh) images of Au NPs layers with different surface coverage: Low = 0.06 (a,d), Medium = 0.34 (b,e), High = 0.62 (c,f). Image (g) is a higher magnification 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.
cross-sectional (h) SEM images show that the Au NPs predominantly exist within a monolayer, and only in the high surface coverage 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 and the total area of the analyzed surface is considered. The mean particle diameter evaluated from TEM images is D = 14 ( 1 nm (see Figure 1i, and it was also confirmed by the SEM images (Figure 1ah). AFM overestimates the actual particle size (D = 41 ( 4 nm) due to convolution of the measurement with the finite angle of the tip.25 For this reason, the Au NP surface coverage could only be accurately evaluated from the SEM characterization. The values obtained from the surface coverage analysis were 62%, 34%, and 6% for the high, medium, and low coverage samples, respectively. These values highlight that the substrate functionalization and the Au NPs deposition processes provide a simple and reproducible way to deposit metal NPs with submonolayer covering. During the deposition process, the MUA functionalized Au NPs 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 substrate and/or (ii) the formation of an amide bond between the amino and carboxylate groups which originate from the APTMS and MUA, respectively. It is known that such a reaction is strongly favored in the presence of activating agents, such as a mixture of pentafluorophenol and 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC),26 but the formation of the amide bond in the absence of an activating agent is difficult at room temperature.27 To investigate the anchoring mechanism, we have employed ATR FT-IR measurements to study APTMS-functionalized substrates before and after Au NP monolayer deposition. As can 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 further two weak peaks at 2851 cm1 and 2922 cm1, while Au NPs layer exhibit also a series of peaks in the 13001900 cm1 range. The band at 1650 cm1 can be ascribed to strong in-plane NH2 scissoring absorptions,28 and it is consistent with the presence of APTMS. The vibrational peak at 1379 cm1 can be attributed to symmetric rocking of HCH bonds, arising from the APTMS 13741
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Figure 2. FT-IR spectra of APTMS-functionalized silicon substrates uncovered (a) and covered (b) with Au NPs layer.
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 the surfaces of the Au NPs. However, this vibration cannot be resolved because it is convoluted with an intense peak at 1411 cm1. The peak at 1465 cm1 is distinctive of the asymmetric CH bending originating from the organic carbon chain of both APTMS and MUA molecules, and it is recognizable in both spectra as well. The two peaks at 2922 and 2851 cm1 are ascribable to CC and CH bond vibrations in CH2 groups as reported in several publications.2932 Some authors claim that a peak at about 2950 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 rather speculative. These peaks are again due to the presence of organic chains of both APTMS and MUA molecules, and it is consistent that they are more intense in the MUA-containing sample, since more material 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 a further confirmation 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 the amide bond26 or to NH scissoring modes of adsorbed amine on metals.36 The CdO stretching of the amide group has been found at 1660 cm1 by Whitesides and co-workers,26 but in our samples, this peak overlaps with primary amine vibrations. The last peak at 1411 cm1 is difficult to assign. In the past, it has been ascribed to CH vibrations, but the data available are controversial. Bertilsson and Liedberg37 have observed two peaks at 1418 cm1 and 1470 cm1 which they have related to CH vibrations of the organic chain of thiols self-assembled on a gold surface. These frequencies are relatively close to those in this study, thus providing a potential origin of the vibrational peak observed at 1411 cm1. The bonds that form during the deposition process enable submonolayer coatings of Au NPs to be deposited. As the temperature of such Au NP deposited substrates increases, these bonds may be insufficient to prevent particles from diffusing over the surface and ultimately coalescing. An understanding of such effects is vital for the application of Au NP based films. With this in mind, we have investigated the thermal stability of bare Au NP
Figure 3. SEM images of Au NP layers annealed at different temperatures: (a) 100 C; (b) 200 C; (c) 300 C; (d) 400 C.
layers with medium surface coverage (about 34%) following annealing at 100, 200, 300, and 400 C for one hour. In Figure 3, we show SEM images that map out the evolution of Au NPs layers with increasing temperature. A clear change in the morphology of the layer can be seen at 200 C and above, while minor modifications in the average particle size can be seen after the 300 C and 400 C annealing temperatures. From analysis of the average particle size, it can be noted that the originally monodisperse Au nanoparticles grow from 14 ( 1 nm to polydispersed nanoparticle ensembles of 32 ( 13 nm in size following annealing at 200 C; a progressive increase in the mean diameter can be seen after the 300 C (37 ( 15 nm) and 400 C (43 ( 18 nm) annealing steps. This behavior can be attributed to the close packing of the as-synthesized Au NPs within the monolayer, which provides adequate interparticle surface contact (Figure 3a) to enable temperature-driven sintering.38 Following coalescence, the interparticle distance increases (Figure 3b) and therefore, further growth is hindered, or even prevented. It is known that low molecular weight thiols desorb from gold surfaces at temperatures below 200 C.39,40 This factor may suggest that the driving force for the observed Au NP sintering at high temperatures is related to the desorption and/or decomposition of the organic molecules on their surface, i.e., MUA and APTMS. It has recently been reported41 that Au polymer coreshell structures when deposited on glass substrates and annealed at 700 C, produce Au NPs layers that adhere to the substrate without significant effect on the metal core size or spatial distribution. In this study, the surface coverage was substantially higher, ensuring that the greatly reduced interparticle spacing may have additionally enabled more efficient sintering of the nanoparticles following the removal of the capping agent. It is also worth noting that both MUA and APTMS decompose below 500 C;42,43 hence, the presence of residual organic is unlikely in the sample annealed at 500 C. Optical spectroscopy together with spectroscopic ellipsometry are useful for understanding the effects that the surface coverage, the thermal annealing, and the oxide coating presence have on the Au NPs. Figure 4a shows UVvis-NIR spectra for asdeposited Au NP layers with different surface coverage that have 13742
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Figure 5. Optical absorption spectra of (a) Au NP layers covered with NiO and annealed at 500 C. (b) Au NP layers covered with TiO2 and annealed at 500 C.
Figure 4. (a) Optical absorption spectra of Au NP layers annealed at 100 C. (b) Optical absorption spectra of AuM sample annealed between 100 and 400 C; the vertical dashed lines highlight the Au SPR peak recorded at 520 nm in water. The inset shows a picture of the samples annealed at 100 C (left) and at 400 C (right); the scale bar is in cm. (c) Refractive index and (d) extinction coefficient dispersion curves for Au NP layers annealed at 100 C.
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 in absorbance with higher surface coverage is simply related to the higher concentration of NPs interacting with the incoming beam, leading to an increase in absorption as described by the wellknown LambertBeer equation.44,45 Meanwhile, the red shift of the plasmon peak with increased surface coverage can be ascribed to the decrease in the mutual distance between Au NPs; this results in a stronger coupling of the plasmon resonances and a concordant red-shift of the plasmon resonance.46 In Figure 4b, we show absorption spectra of a Au NP layer with medium surface coverage (34%, the same coverage used for the SEM measurements shown in Figure 3) annealed between 100 and 400 C. The Au SPR peak clearly depends on the annealing temperature. Following the 200 C annealing step, the SPR peak reduces slightly in intensity, red shifts, and broadens. These observations are consistent with the initial coalescence step, which also causes necking between particles and the consequent formation of some elongated particles. However, with increasing annealing temperature, a progressive blue-shift and a more pronounced reduction in intensity is observed. This is due to the subsequent “spheroidization” of the as-coalesced particles
and their progressive increase in size. In turn, this leads to an increase of the average interparticle distance, causing a decrease in SPR coupling. Moreover, the number of particles decreases as a consequence of the sintering, and so does their optical cross section, causing a reduction in the absorbance of the sample. The chromatic effects of the thermal annealing can be seen in the picture shown in the inset of Figure 4b: the Au NP layer annealed at 100 C is blue-colored, as a consequence of the plasmon coupling (the wavelength corresponding to the SPR peak registered in aqueous solutions is at 520 nm, corresponding to a red-colored solution), while the Au NPs layer annealed at 400 C is pink, as a consequence of the plasmon decoupling after the particle growth and the consequent increase in the mutual distance. The absorption measurements, combined with the previously discussed SEM measurements, demonstrate that an annealing temperature of 200 C is sufficient to promote sintering and coarsening of close-packed Au NPs, leading to a respective change of their optical properties. For this reason, unless specified, all the characterizations of uncoated Au NP layers presented in the following refer to the samples annealed at 100 C. To understand further the observed absorption properties of the Au monolayers, we have combined spectroscopic ellipsometry and optical transmission measurements to determine their optical constants. Figure 4c,d shows the dispersion curves of the refractive index and the absorption coefficient of the Au NPs layers, respectively. The layers have been modeled as an effective medium thin film composed of Au NPs in air. As such, the resulting curves do not represent the dielectric function for Au itself, but they are the dielectric function for the layer itself. The refractive index dispersion curves (Figure 4c) show that increasing Au NP coverages results in a higher effective refractive index and in accordance with the KramersKronig relation exhibit a concordant red shifting of the second-order inflection point that is characteristic of the Au NPs SPR absorption. The spectral changes associated with the refractive indexes translate to a clear red shift and increased magnitude of the SPR peak extinction coefficients (Figure 4d) with increasing surface coverage, thus confirming the 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 13743
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Table 2. Mean Crystallite Diameters Calculated According to the Scherrer Equation for the Three Crystalline Phases Detected for the Samples Reported in Figure 6a diameter (nm) sample
Au
TiO2
NiO
Au Nanoparticle Monolayer Uncoated AuL
7.4 ( 0.4
/
/
AuM AuH
7.9 ( 2.6 7.5 ( 2.8
/ /
/ /
14.4 ( 2.0
/
/
AuM 400 C
Au Nanoparticle Monolayer Coated with 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
NiO
Figure 6. XRD patterns of (a) Au NPs layers stabilized at 100 C; for comparison purposes the sample annealed at 400 C is also reported; (b) Au NP layers covered with NiO and annealed at 500 C; (c) Au NP layers covered with TiO2 and annealed at 500 C. Theoretical diffraction lines for Au (black lines) NiO (dashed lines in figure (b)) and TiO2 (dashed lines in figure (c)) are reported at the bottom.
monolayer on the optical properties of the nanocomposite is a broadening and red-shift of the SPR peak (see Figure 5a,b). This effect is found to be much more pronounced for TiO2 compared to NiO, due to the higher refractive index of the former compared to the latter (2.51 at 590 nm for TiO2,47 2.33 at 620 nm for NiO48), and also to the interaction between anatase crystals and the surface of Au NPs, causing spreading and scattering of conduction electrons as described previously.49,50 Moreover, looking particularly at the Low surface coverage samples coated with the two different oxide layers, distinct high and low wavelength SPR peaks emerge. The low wavelength peaks, 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 wavelength peaks appear in the Low surface coverage samples around 750 and 1000 nm for NiO and TiO2, respectively, and are found to progressively red shift and become more prevalent with increasing Au NPs surface coverage. These peaks are consistent with the surface plasmon coupling between Au NPs.38 We note that the apparent broad absorption band observed for pure TiO2 film is due to optical interference; hence, the Au SPR peaks of the Au containing samples overlap with this interference fringe. XRD measurements of the different samples studied are reported in Figure 6. Crystallization was evident in all samples, as testified by clearly identified diffraction peaks. The intensity of the Au peaks (JCPDS no. 040714) was in good agreement with the different surface coverage, and so with the amount of Au NPs present inside the films. For anatase TiO2 (JCPDS no. 841285), the peaks have similar intensities for each of the samples. This suggests that the Au monolayers used as substrates do not significantly alter the process of matrix formation for this metal oxide. In contrast, for NiO (JCPDS no. 471049) we observe broadened NiO diffraction peaks within increasing Au NP coverage. This factor suggests that Au NPs directly influence the crystallization of NiO, a point that will be further elucidated on shortly.
/
Au Nanoparticle Monolayer Coated with TiO2 20.1 ( 0.2
/
AuLT AuMT
7.5 ( 0.4 10.8 ( 0.2
19.2 ( 1.5 21.3 ( 5
/ /
AuHT
11.3 ( 0.4
22.6 ( 4.2
/
TiO2
a
/
The coated samples are annealed at 500 C.
Crystallite sizes evaluated through the Scherrer formula are listed in Table 2. As can be noticed, the crystallite size of Au NPs is slightly higher when the monolayer is covered with the solgel coating. This effect is related to the annealing process (500 C for 1 h) that must induce some coarsening or sintering of the particles. For comparison, the XRD spectrum of the uncovered, medium surface coverage, sample annealed at 400 C is reported as well in Figure 6a; a clear sharpening of the Au diffraction peaks is experienced as a consequence of the previously discussed NP sintering and growth processes. The crystallite size is about 8 nm for the uncovered Au layers annealed at 100 C, in the 811 nm range for the solgel-coated Au NP layer annealed at 500 C, and >14 nm for the uncovered Au NP layer with medium surface coverage annealed at 400 C (the same sample used for SEM and UVvis measurements reported in Figure 3d and Figure 4b, respectively). This comparison shows that the presence of the solgel films reduces or even prevents the growth of Au NPs. Notably, the difference in the mean crystallite size of Au NPs estimated from XRD peak broadening (78 nm) and the mean particle size evaluated from SEM (14 nm) indicates that the Au NPs are not monocrystalline. NiO and TiO2 crystallites sizes are in the 1417 nm and 1923 nm ranges, respectively. Interestingly, for NiO we find that the crystal size is slightly smaller for samples where the solgel solution 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 TiO2 crystals, where the data are randomly distributed and their difference is within the error bars. The trend observed in the NiO crystal size can be attributed to the small lattice mismatch between NiO and Au crystals.51 This factor can permit Au NPs to act as heterogeneous nucleating sites for NiO crystals. Therefore, as the number of nuclei is related to the number of Au NPs, i.e., to the surface coverage, higher Au NP concentrations result in smaller metal oxide crystal sizes for a given volume of deposited material. 13744
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Figure 8. ORC plots for the high surface coverage Au NP layers bare and covered with NiO and TiO2 films, and for pure NiO and TiO2 films when exposed to 180 ppm ethanol at 150 C OT. Zero value of response is highlighted with a dotted line. Figure 7. SEM images of Au NP monolayer with medium surface coverage: (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 of the film flipped over, exposing the Au NPs on the upper film surface.
To evaluate the morphology of the metal oxide coated Au monolayer following annealing at 500 C, SEM analyses have been carried out. As can be seen from Figure 7, NiO coated samples are rougher and with more irregularities than those coated with TiO2 films. To investigate the Au NP morphology below the TiO2 solgel films, we gently scratch the surface with a scalpel. This enabled us to observe the underlying Au NPs (Figure 7c), as well as overturn the 5060 nm film to expose the underlayer containing the Au NP monolayer (Figure 7d). In both cases, the Au NPs could be easily detected due to their higher contrast (highlighted by arrows in Figure 7c). From the SEM, the size of the Au NPs was estimated as being 16.1 ( 1.8 nm; a value that is only slightly higher compared to the as-synthesized colloids. Thus, it can be concluded that the metal oxide film is not damaging the Au NPs layer during the solgel solution spinning process, and is also strongly limiting the growth of the metal particles by providing a physical diffusion barrier between neighboring particles. Having characterized the Au NP layers covered with NiO and TiO2 solgel films, we will now evaluate the practical use of such systems as a gas sensor to detect ethanol vapor. To ensure the stability of the Au monolayer without metal oxide layers, the sensor was operated at a temperature (OT) of 150 C and its optical detection mode was reflection (see Experimental section for details). All tests were performed on the Au NP layers with high surface coverage. The uncovered sample was annealed at 150 C for 6 h prior to gas sensing measurements. Following this annealing time, no changes to the optical absorption spectrum could be detected, confirming its thermal stability at this temperature. The effect of ethanol vapors is shown in Figure 8. The difference in reflection intensity between the spectrum collected during ethanol exposure and during nitrogen exposure (optical reflection change, ORC = ReflEtOH ReflN2) is plotted as a function of wavelength, and it is reported for uncovered Au NPs
layer, Au NPs layers covered with NiO and TiO2, and pure NiO and TiO2 films. As can be seen, outside the Au SPR peak wavelength range, the ORC is similar to the response of the metal-oxide matrix with no Au NPs monolayer. Interestingly, in these ranges the ORC parameter is negative for NiO film, positive for TiO2 films, and null for the uncovered layer. The SPR wavelength range shows a wavelength-dependent response, this being true for the uncovered Au NPs layer as well. This arises as a consequence of the role played by the noble metal particles as optical probes for the target analyte detection. Moreover, compared to the pure metal oxide films, the covered Au NPs layers exhibit a higher response in a limited wavelength range. This provides confirmation of the SPR enhancement to the sensing behavior obtained by combining the close-packed Au NPs layer and the oxide active films. In fact, especially for TiO2, the ORC maximum of the covered Au NPs layers is higher than the sum of the Au NPs layer and the oxide film alone, so a synergetic effect between the two components is likely to occur. Moreover, the effect of the different metal oxide can be also seen: the ethanol effect on NiO is to reduce the reflection intensity, while its interaction with TiO2 causes an increase in reflection. This fact can be explained by considering the different electric nature of the two semiconducting oxides, i.e., NiO being a p-type and TiO2 an n-type semiconductor. As reported in the literature, volatile organic compounds (VOCs) can be oxidized on the surface of semiconducting materials; for example, in the case of ethanol (C2H5OH), the main reaction mechanisms can be described as the following:52,53 2C2 H5 OH þ O2 f 2CH3 CHO þ 2H2 O C2 H5 OH f C2 H4 þ H2 O In the first reaction, ethanol is oxidized to acetaldehyde (CH3CHO) by dehydrogenation of the ethanol molecule and a subsequent reaction with oxygen leads to water formation. This reaction can proceed further, with successive oxidation of acetaldehyde to acetic acid. The second reaction is a direct dehydration of ethanol to ethylene (C2H4) with water formation. There are other possible ethanol oxidation reactions leading, for example, 13745
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interfering vapors or gases: these nanocomposites are currently under investigation in order to analyze their gas sensing response when exposed to different gases and vapors, according to their composition and the operative temperature, and the results are intended to be published in a separate paper. Nevertheless, these preliminary results are encouraging, considering also that the synthetic approach described in this work can be easily extended to a great variety of active layers, simply changing the type of NPs and the material for the top coating, tailoring the properties of the nanocomposite by an appropriate choice of the active materials and a proper optimization of their organization and spatial distribution.
Figure 9. Dynamic tests for AuHT sample at 594 nm and 150 C OT under repeated cycles of nitrogen180 ppm ethanol.
to the formation of diethyl ether or diethylacetal, but they are less common and require specific reaction conditions.53 The preferred path is generally related to the type of the metal oxide (basic oxides usually promote the first reaction, acid oxides the second one), to the presence of adsorbed oxygen species on the surface of the oxide, or to the presence of oxygen in the gas phase (as can be seen, oxygen is necessary for the first reaction). In any case, the oxidation of the ethanol molecules will lead to electron injection into the metal oxide. In the case of TiO2, the ethanol oxidation will lead to an increase in conductivity, because electrons are injected into the conduction band of anatase. For NiO, the opposite is true, as oxidation will lead to a decrease in conductivity, due to electronhole recombination. As a consequence, it is reasonable to suppose that a difference in the electronic properties of the metal oxides produces a difference in the reflection intensity, and that this difference is of opposite sign according to the type of semiconducting material. Within our experimental setup, we employ a low-resolution spectrophotometer and collect light under reflection mode. These factors contribute to low signal-to-noise ratios. Despite this, the observed ORC trends clearly exemplify the synergetic effect of coupling Au NP layers and metal oxide films to enhance the sensing properties. While the above experiments were obtained under static conditions, we also performed dynamic tests at a fixed wavelength of 594 nm (corresponding to the maximum of the response, as obtained from Figure 8) on the AuHT sample, the most sensitive among the tested ones. The results, which are depicted in Figure 9, show a reversible signal during repeated cycles of nitrogenethanol exposure to the sensor. Although the sensing dynamics are not ideal, because both response and recovery times are occurring in a time scale of few minutes, the results are promising, considering the low thickness of the samples and the low resolution of the setup used. In fact, an easily detectable variation in the reflectivity during ethanol exposure has been observed, with good reproducibility after repeated nitrogen/ethanol cycles (see Figure 9). These results show promise for applications of such materials in transmission mode or in devices where the reflection is enhanced, like SPR configurations or on the surface of unclad optical fibers. One of the main issues with gas sensors is the cross sensitivity between
’ CONCLUSIONS We have demonstrated that Au NP layers covered with solgel oxide films constitute an effective design for materials to be used in optoelectronic applications. Nearly monodisperse Au NPs were deposited on properly functionalized substrates with good control of the surface coverage. Detailed optical and morphological studies have been presented, showing a relationship between the Au NP surface coverage, annealing temperature and optical properties of the uncovered monolayers; moreover, the bond formation between Au NPs and the APTMS functionalized substrate has been deduced from infrared spectroscopy measurements. The presence of a solgel oxide film deposited on top of the Au NP layers affects the optical properties of the nanocomposite and also provides a physical barrier between neighboring Au NPs, strongly limiting the extent of their temperature-driven sintering. Preliminary gas sensing measurements on these systems show that ethanol vapor induces a reversible and reproducible response, confirming the role of Au NPs in increasing the sensitivity of the oxide film itself and providing a wavelength-dependent response. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work has been supported through Progetto Strategico PLATFORMS of Padova University. E.D.G. thanks Fondazione CARIPARO for financial support. A.M. thanks the Universities of Melbourne and Padova for their support through the University academic exchange program. J.J. acknowledges the Australian Research Council for support through the APD grant DP110105341. M.K. acknowledges the Alexander von Humboldt foundation for a Feodor Lynen research fellowship. ’ REFERENCES (1) Della Gaspera, E.; Antonello, A.; Guglielmi, M.; Post, M. L.; Bello, V.; Mattei, G.; Romanato, F.; Martucci, A. J. Mater. Chem. 2011, 21, 4293. (2) Joy, N. A.; Settens, C. M.; Matyi, R. J.; Carpenter, M. A. J. Phys. Chem. C 2011, 115, 6283. (3) Kolmakov, A.; Klenov, D. O.; Lilach, Y.; Stemmer, S.; Moskovits, M. Nano Lett. 2005, 5, 667. (4) Larsson, E. M.; Langhammer, C.; Zoric, I.; Kasemo, B. Science 2009, 326, 1091. (5) Formo, E.; Lee, E.; Campbell, D. Nano Lett. 2008, 8, 668. 13746
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