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oxide coupled with gold nanoparticles for localized surface plasmon resonance based gas sensor
Michela Cittadini a, Marco Bersani a, Francesco Perrozzi b, Luca Ottaviano Wojtek Wlodarski d, Alessandro Martucci a,*
` di Padova, Padova, Italy Dipartimento di Ingegneria Industriale, Universita ` dell’Aquila Via Vetoio 10, 67100 L’Aquila, Italy Dipartimento di Scienze Fisiche e Chimiche, Universita c CNR-SPIN Uos L’Aquila, Via Vetoio 10, 67100 L’Aquila, Italy d School of Electrical and Computer Engineering, RMIT University, Melbourne, Australia
A R T I C L E I N F O
A B S T R A C T
Article history: Received 6 September 2013 Accepted 14 December 2013 Available online 19 December 2013
An optical gas sensor was prepared by depositing graphene oxide ?akes over a monolayer of gold nanoparticles, chemically attached to a functionalized fused silica substrate. The coupling between ?akes and nanoparticles lead to optical changes upon exposure to different gases: in particular, we observed a shift of the surface plasmon resonance band in presence of both reducing and oxidizing gases. This effect can be explained in terms of a strong gold–graphene interaction and speci?cally of electron transfer between the gold nanoparticles and the two-dimensional sheet of the sp2-hybridized carbons of graphene oxide. ? 2013 Elsevier Ltd. All rights reserved.
In recent years, graphene has been attracting considerable attention due to its unique morphological and electronic properties [1,2]. Despite its interesting features some of the attention has diverted towards its partially oxidized form, graphene oxide (GO), that has emerged as an alternative to graphene for selected applications, thanks to its low cost, production scalability, ease of processing and good compatibility both with aqueous and organic solvents [1,3]. GO consists in atomically-thin graphene sheets that are covalently decorated with oxygen-containing functional groups, either on the basal plane or at the edges, so that it contains a mixture of sp2- and sp3-hybridized carbon atoms. In particular, tailoring of the size, shape and relative fraction of the sp2-hybridized domains of GO by chemical or thermal reduction provides opportunities for tailoring its optoelectronic properties. For example, as-synthesized GO is
insulating but just by varying the oxidation level, with a controlled de-oxidation, a partially reduced GO can act as a semiconductor . It has been shown that GO can act as a photocatalyst for the production of H2 from a 20 vol.% solution of methanol in water ; other studies  have explained this behavior by ab-initio modeling, suggesting that GO is particularly effective thanks to its clean band gap and providing criteria for the determination of the ideal oxidation state for a given application. Furthermore, its highly 2-dimensional nature, which determines huge surface-to-volume ratio, and ef?cient UV absorption make it a very promising material for photocatalysis  and suggest potential applications wherever its peculiar optoelectronic properties can be exploited. In this work we used partially reduced GO coupled with Au nanoparticles (NPs) for optical gas sensing with the aim of combining the semiconducting and photocatalytic activity behavior of the partially reduced GO with the Localized
* Corresponding author. E-mail address: firstname.lastname@example.org (A. Martucci). 0008-6223/$ - see front matter ? 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.12.048
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Surface Plasmon Resonance (LSPR) of gold NPs. The synergistic interplay between these materials resulted in an enhancement of the photocatalytic properties of GO , extending them to the visible range where the LSPR of Au NPs can be used as an optical probe. The LSPR is known to be extremely sensitive to the changes in the dielectric properties of the surrounding medium, a characteristic that has been widely exploited for the preparation of sensing devices [9,10]. Here we expect an even larger enhancement of this effect induced by the electronic coupling of Au NPs and GO and the interactions of GO with reducing and oxidizing gases. While the use of GO for gas sensing has been covered in multiple reports [11–13], with the GO–Au NPs system already employed as a resistive gas sensor , only very recently GO has been used in an optical ?ber array for the detection of vapors  and to the best of our knowledge this is the ?rst time that GO is used as sensing material in an optical sensor for the detection of reducing and oxidizing gases. Moreover Au NPs exhibit a well-established, wavelength-dependent optical response, which can be exploited as a means to improve the selectivity of the sensor using wavelength modulation.
to be partially reduced (see Section 3), such samples have been labeled rGO.
Atomic Force Microscopy (AFM) images have been acquired in air using a Veeco Digital D5000 system equipped with silicon tip. Scanning Electron Microscopy (SEM) images have been acquired with a Zeiss-Gemini LEO 1530 system. X-ray photoemission spectroscopy (XPS) spectra have been acquired with a PHI 1257 spectrometer equipped with a monochromatic Al Ka source (hm = 1486.6 eV) with a pass energy of 11.75 eV, corresponding to an overall experimental resolution of 0.25 eV. The acquired XPS spectra have been ?tted with Voigt line shapes and Shirley backgrounds.
Gas sensing measurements
Synthesis of Au NPs
Gold colloids, were prepared according to the Turkevich method  by reducing HAuCl4 with trisodium citrate in water, and were successively capped with poly (vinyl pyrrolidone) (PVP, MW = 10,000) according to the method described by Della Gaspera et al. , resulting in a ?nal concentration of 30 mM in ethanol.
Preparation of Au monolayers
Au monolayers have been prepared on fused silica slides or silicon. The substrate was ?rst functionalized with (3-aminopropyl) trimethoxysilane (APTMS) as described in . To deposit the monolayer, we used Au NPs PVP capped using the procedure reported in . The monolayers were formed by spin-coating at 3000 rpm for 30 s the liquid suspensions of gold NPs directly onto the APTMS. In this study, we prepared Au monolayers with 2 different extents of surface coverage, hereafter indicated as low (L), and high (H). L and H samples have been obtained by spinning 15 and 30 mM Au NPs solution, respectively. The as-deposited monolayer samples were thermally treated at 150 °C for 30 min in air. Following this stabilizing treatment, the samples were used as substrates for the GO deposition.
Optical gas sensing tests were performed by conducting optical absorption measurements in the 200–900 nm wavelength range on ?lms deposited on SiO2 substrates using a Harrick gas ?ow cell (with 5.5 cm path length) coupled with a Jasco V-650 spectrophotometer. The operating temperature (OT) was set at 150 °C and the sensor was tested for both reducing and oxidant gases, (H2, CO, NO2) all balanced with synthetic air, with a ?ow rate of 0.4 L/min. In particular, the concentration of H2 was set at 10,000 and 100 ppm; the concentration of CO was set at 10,000 ppm; the concentration of NO2 was set at 1 ppm in order to minimize the effects related to its optical absorption in the analytical range. The substrate size was approximately 1 · 2 cm and the incident spectrophotometer beam was normal to the ?lm surface and covered a 9 · 1.5 mm area of the ?lm. The time spent by the sensor on achieving 90% of the total absorbance change is de?ned as the response time in the case of gas adsorption or as the recovery time in the case of gas desorption. For the UV irradiation it has been used a Hamamatsu LC6 lamp with an intensity of 460 mW cm2 at 365 nm. The samples were illuminated at a distance of 2 cm for 60 s.
Results and discussion
Structural and optical properties
Commercial (Graphene Supermarket ) GO solution with a concentration of 0.5 mg/mL has been spin coated on the Au monolayers in two successive depositions, the ?rst at 3000 rpm and the second at 2000 rpm in order to increase the ?akes density, and the resulting samples were annealed at 150 °C for 1 h in air. As the annealed GO samples resulted
The UV–Vis spectra of the gold monolayer are reported in Fig. 1. In accordance with previous publications, an increase of the surface coverage results in both an increase of intensity and a red shift of the Au NPs LSPR peak [20,21]. The increase in absorbance with higher surface coverage is simply related to the larger cross-section of the NPs interacting with the incoming beam, while the red shift of the plasmon absorption can be ascribed to the decrease in the interparticle distance: the stronger coupling of the localized plasmon on neighboring Au NPs results in the red-shift of the plasmon resonance . SEM images of the metal NP monolayers are reported in Fig. 2, from which it is possible to note that the Au NPs are
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0.02 400 500 600 700 800
Fig. 1 – UV–Vis absorption spectra of the AuH and AuL NPs monolayers deposited on fused silica. (A color version of this ?gure can be viewed online.)
Fig. 3 – SEM image of rGO deposited on silicon.
Fig. 2 – SEM image of the AuH monolayer deposited on silicon.
homogeneously dispersed on a micron scale and lie almost entirely on a single monolayer. A mean particle diameter D = 14 ± 1 nm has been estimated. SEM pictures of the rGO deposited by spin coating on a silicon substrate are reported in Fig. 3. The typical ?akes dimensions range from hundreds of nm to few lm. The substrate coverage is not complete, and some overlapping of the ?akes is noticeable as well. These observations are con?rmed by the AFM images of the metal NP monolayers coated with rGO (Fig. 4); showing that there are regions in which the NPs are not coated with rGO ?akes (marked with a circle in the ?gure). Yet most of the NPs are covered by rGO (square marked region in the picture) and in particular the area covered by rGO ?akes can be recognized by the enhanced smoothness both in the height image (on the left) and in the phase image (on the right). The measured surface roughness is 3.2 nm on the rGO ?akes and 4.5 nm on the bare Au NPs.
The optical absorption spectra of the different samples are reported in Fig. 5. It is possible to notice that the deposition of the rGO ?akes on top of the Au NPs induces a small red shift of the LSPR peak, which can be attributed to the variation of the dielectric constant of the NPs environment and/or to the coupling of the localized surface plasmon of the gold NPs with the surface plasmon polariton of the GO . XPS measurements are reported in Fig. 6. The carbon/oxygen ratio was found to be 3.1, while the nominal ratio was 1.9. This means, according to literature, that the annealing in air at 150 °C leads to the partial reduction of GO [24,25]. The C1s signal of our rGO consists of ?ve different chemically shifted components which can be deconvoluted into: C@C/CAC in aromatic rings (284.6 eV); CAOH (285.8 eV); CAOAC (286.8 eV); C@O (288.1 eV); C@O(OH) (289.2 eV); and p–p* satellite peak (shake-up) (290.8 eV) (Fig. 6). These assignments are in agreement with previous works [26,27]. The CAO bonds come from epoxy and hydroxyl groups in the basal plane. Our observations indicate that the majority of oxygen species consist of CAO (in the form of CAOH and CAOAC, respectively 16.3 and 8.5 at.%) while more oxidized species such as C@O and C@O(OH) are present in lesser amounts. In Fig. 6 it is also presented the O1s signal that appears at a binding energy in the range 529–535 eV, and can be deconvoluted, like in C1s signal, into C@O (530.6 eV), CAOAC (531.8 eV) and CAOH (533.2 eV). As we already said, the majority of oxygen species consist of CAO (40.7 and 46.6 at.%) while more oxidized species such as C@O are present in lesser amounts (7.8 at.%).
Gas sensing tests
Fig. 7 shows the absorption spectra of AuH-rGO when exposed to air and 100 ppm H2 or 1 ppm NO2 in air: to better visualize the gas effect, the Optical Absorbance Change (OAC) parameter, de?ned as the difference between absorbance during gas exposure and absorbance in air (OAC = AbsGas ? AbsAir) is also reported in the insets, together with the OAC for bare rGO. For all the prepared samples, CO gas did not induce any variation of the absorption. It is possible to notice that H2 (which is a reducing gas, Fig. 7a) induces a blue-shift of the Au LSPR peak, while the interaction with NO2 (which is an oxidizing gas, Fig. 7b)
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Fig. 4 – AFM height (a) and phase (b) images of rGO deposited on AuH monolayer on silica glass substrate. Square marked area represents regions where the Au NPs are covered by rGO ?akes. Circle marked area represents regions where the Au NPs are not covered by rGO ?akes. Height pro?les of a region where the Au NPs are covered (c) and not covered (d) by rGO ?akes. (A color version of this ?gure can be viewed online.)
0.20 substrate rGO AuH AuH-rGO
0.00 400 500 600 700 800
Fig. 5 – UV–Vis absorption spectra of rGO, AuH and AuH-rGO samples. The absorption spectrum of the bare glass substrate is also reported for comparison. (A color version of this ?gure can be viewed online.)
induces a red-shift of the Au LSPR peak. The OAC spectra reported in the insets of Fig. 7a and b show that the bare rGO samples are not sensitive to any gas, thus demonstrating that the presence of the Au NPs is necessary in order to have a detectable variation of the absorbance. On the other hand, bare Au monolayers showed very small variations (see the insets of Fig. 7), indicating that rGO enhances and ampli?es the interaction of the gas with the Au NPs. Dynamic tests have been conducted in the 500–600 nm wavelength range, corresponding to the LSPR peak region. The dynamic gas sensing tests are reported in Figs. 8 and 9, showing a variation of the absorbance ranging from 0.1%
Fig. 6 – C1s XPS spectra (top) and O1s XPS spectra (bottom) collected on single-layered GO thin ?lm deposited on silicon and annealed in air at 150 °C. The deconvolution of the experimental peaks showing the contribution of the different components is also shown.
(for 100 ppm) up to 0.5% (for 10,000 ppm) for H2 and a variation of 0.1% for 1 ppm NO2, while CO could not be detected.
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AuH-rGO AuH rGO
1.004 air H2
air CO air
-0.0004 400 450 500 550 600 650
Air H2 500 550 600 650 700 750
0.998 0 1000 2000 3000 4000 5000 6000 7000
Fig. 9 – Time-resolved tests for AuH-rGO sample performed at 528 nm after exposure to multiple air-gas (10,000 ppm H2, 10,000 ppm CO, 1 ppm NO2) cycles.
AuH-rGO AuH rGO
Air NO2 500 550 600 650 700 750
Fig. 7 – Optical absorption spectra of AuH-rGO when exposed to air and 100 ppm H2 in air (a) and when exposed to air and 1 ppm NO2 in air (b). The insets show the OAC of rGO, AuH and AuH-rGO. (A color version of this ?gure can be viewed online.)
H2 H2 air air 0.01% 0.01%
1.002 1.000 0 1000 2000 3000 4000 5000
Fig. 8 – Time-resolved tests for AuH-rGO sample performed at 526 nm after exposure to multiple air-H2 cycles.
As this is the ?rst time the GO is used as sensitive material for optical gas sensor based on the variation of optical absorbance, it is not possible to make a comparison with other
published data on GO. On the other hand, optical gas sensor based on the variation of the Au LSPR has been extensively studied [9,44–46]. The variation of the absorbance induced by the different gas reported in the present study is of the same order of magnitude of those reported for Au NPs dispersed in TiO2 ?lm  which thickness is three times higher than our Au-rGO samples. The prepared samples have been tested several time for about 1 year, obtaining very reproducible results in terms of Au LSPR peak shift and variation in absorbance. From the time resolved tests reported in Fig. 9 it is also possible to notice that both reducing and oxidizing gas induce a reversible variation of the absorbance indicating that the exposure to the different gas is not altering permanently the Au-rGO sample. On the other hand Raman measurements performed before and after gas exposure did not show any signi?cant variation (see Fig. S1 in the Supplementary information). The different wavelength dependence of the target gas showed in in the insets of Fig. 7, can be used for developing a selective gas sensor. As demonstrated in Fig. 9, at 528 nm the sensor gives a positive absorbance change for H2, a negative variation for NO2, and no variation for CO. The behaviors of the different samples in the gas sensing tests are reported in Table 1. The AuH-rGO samples showed the best performances; none of the tested samples was responsive to CO. The reported response and recovery times are in agreement with already published data for conductometric gas sensors . Bare Au NPs monolayer gave much slower dynamic repose to H2 and NO2 with respect to the Au monolayer coated with rGO, con?rming the importance of the presence of rGO. In order to understand the gas sensing interaction mechanism it is worth to analyze the interaction of reducing and oxidant gas with metal oxides. In the case of metal oxides based gas sensors, atmospheric oxygen adsorbs preferentially on the surface defects of the oxides, resulting in changes of the conductivity and the optical properties according to the n- or p-type nature of the oxide
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Table 1 – Sensing performances for the different samples. Sample AuH-rGO Gas H2 10,000 ppm H2 100 ppm CO 10,000 ppm NO2 1 ppm H2 10,000 ppm CO 10,000 ppm NO2 1 ppm DAbs 0.004 0.0007 – 0.0004 0.0015 – 0.0003 Response time (min) 1 2 – 12 1 – 15 Recovery time (min) 2 4 – 15 3 – 17
vacancies [29,30]. The interaction of the target gases with the absorbed oxygen induces a variation of the charge densities which can be monitored by measuring the variation in conductivity  or the variation of the optical absorption . Regarding graphene, it has been reported that oxygen can be adsorbed on it [33–35], changing its electronic  and optical properties . It has also been demonstrated that these effects are enhanced in the case of coupling with noble metal NPs . For metal oxides it has been demonstrated that UV light exposure can remove the adsorbed oxygen species leading to a variation of the charge densities [38,39]. This effect is reversible as soon as the UV source is removed and oxygen is restored in the environment. To analyze this effect we exposed rGO and Au-rGO samples to UV light keeping the samples under a nitrogen blanket. UV–visible spectroscopy was performed: after turning off the UV lamp while maintaining the sample under nitrogen, we observed an absorption increase (Fig. 10) due to removal of adsorbed oxygen. When air is re-introduced inside the cell, the effect is lost in a time scale of a few tens of minutes. The reducing gases (H2 and CO) react with the oxygen absorbed on the surface of the rGO and donate electrons, while oxidant gases (NO2) subtract electrons. In both cases there is a
change in the charge density of the rGO that can be monitored by measuring the variation in conductivity [28,34] or in the optical absorption, as reported for the ?rst time in the present paper. The presence of the Au NPs greatly enhances such effect. Changes in the Au NPs electron density can alter the plasmon frequency causing a shift in the LSPR band position . Moreover the presence of Au NPs can improve the visible-light photocatalytic activity through resonant energy transfer (RET) . The Au NPs convert the energy of incident photons into LSPR oscillations, transferring the energy to the rGO. In this way it is possible to excite electron–hole pairs with visible light, through the Au NPs-rGO coupling [42,43]. The sensing reactions for reducing (H2, CO) and oxidant (NO2) gases can be explained as follow. The reducing gases react with the oxygen absorbed on the surface of the GO and donate electrons. Oxidant gases instead, react with the oxygen ions absorbed on the surface of the GO matrix and subtract electrons. These reactions are catalyzed by the presence of Au NPS which enhance the visible-light photocatalytic activity of GO through RET. This injection or subtraction of electrons in and from the GO implies a shift in the LSPR due to the strong electromagnetic coupling between the metal NP and the GO sheets. The plasmon band blue-shifts with reducing gases and red-shifts with oxidant gas, as already observed for Au NPs dispersed in metal oxide matrices [44–46].
0.11 After UV Before UV Bare Glass Substrate 400 500 600 700 800
Fig. 10 – Optical absorption spectra of rGO under N2 atmosphere before and after exposure to UV light. The absorption spectrum of the bare glass substrate is also reported for comparison. (A color version of this ?gure can be viewed online.)
In this work we have investigated the optical gas sensing performances of rGO coupled with Au NPs thin ?lms towards H2 (10,000 ppm and 100 ppm), CO (10,000 ppm) and NO2 (1 ppm). The sensors showed good and reversible responses with fast kinetics towards H2 and NO2, while no detectable response was observed towards CO. In particular the AuH-rGO sample showed the best sensing performances thanks to the higher NPs concentration. It was also demonstrated that the coupling of Au NPs with rGO provided a wavelengthdependent sensing response for different gas, allowing the realization of a selective sensor. The mechanisms involved in the sensing of reducing (H2 and CO) and oxidant (NO2) gases using GO is just a combination between the photocatalytic behavior of GO in the visible, if coupled with Au NPs, and an exchange of electrons with the target gases through red-ox reactions mediated by the adsorbed oxygen.
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This work has been supported through Progetto Strategico PLATFORMS of Padova University. A.M. thanks Veneto Nanotech for ?nancial support through Progetto Idrogeno.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.carbon.2013. 12.048.
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