Tin Oxide Nanowires Decorated With Ag Nanoparticles for Visible

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Tin Oxide Nanowires Decorated With Ag Nanoparticles for Visible Light-Enhanced Hydrogen Sensing at Room Temperature: Bridging Conductometric Gas Sensing and Plasmon-Driven Catalysis Nicola Cattabiani, Camilla Baratto, Dario Zappa, Elisabetta Comini, M. Donarelli, Matteo Ferroni, Andrea Ponzoni, and Guido Faglia J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09807 • Publication Date (Web): 05 Feb 2018 Downloaded from http://pubs.acs.org on February 9, 2018

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Tin Oxide Nanowires Decorated with Ag Nanoparticles for Visible Light-Enhanced Hydrogen Sensing at Room Temperature: Bridging Conductometric Gas Sensing and Plasmon-Driven Catalysis Nicola Cattabiani,†,‡ Camilla Baratto,∗,‡,† Dario Zappa,†,‡ Elisabetta Comini,†,‡ Maurizio Donarelli,†,‡ Matteo Ferroni,†,‡ Andrea Ponzoni,‡,† and Guido Faglia†,‡ †SENSOR Lab, Dept. of Information Eng., University of Brescia, Via Branze 38, Brescia, Italy ‡SENSOR Lab, CNR-INO, Via Branze 45, Brescia Italy E-mail: [email protected] Abstract We demonstrate that conductometric gas sensing at room temperature with SnO2 nanowires is enhanced by visible and supra bandgap UV irradiation when and only when the metal oxide nanowires are decorated with Ag nanoparticles (diameter < 20 nm); no enhancement is observed for the bare SnO2 case. We combine spectroscopic techniques with conductometric gas sensing to study the wavelength dependency of the sensors response, showing a strict correlation between the Ag loaded SnO2 optical absorption and its gas response as a function of irradiation wavelength. Our results lead to the hypothesis that the enhanced gas response under UV-vis light is the effect

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of plasmonic hot electrons populating the Ag nanoparticles surface. Finally we discuss the chemiresistive properties of Ag loaded SnO2 sensor in parallel with the theory of Plasmon-Driven Catalysis, to propose an interpretative framework that is coherent with the established paradigma of these two separated fields of study.

1

Introduction

Semiconductor Metal Oxides (SMOX) like SnO2 , ZnO and WO3 in porous polycrystalline form have been established as the standard sensing films for amperometric gas sensors in the market since the 70’s. Their success is due to their high surface to volume ratio and to their sensitive band structure dynamics when exposed to both oxidizing and reducing gases. 1,2 A second generation of SMOX-based sensor started to emerge in the early 2000 as the research community focused on low dimensional single crystalline structures like nanowires, nanorods, nanobelts etc. 3,4 Nanostructured SMOX promised to have enhanced signals due to their surface-like nature and to be more stable at high operating temperature compared to the previous generation counterparts. 5,6 Such materials are typically functionalized with small nanoclusters made of noble metals via wet chemical processes and physical evaporation techniques. 7 These sensitizing metallic nanoparticles (MNP) play different roles in the gas sensing mechanism depending on the kind of interaction they engage with the SMOX surface they are attached to and/or with the target gas. 8

1.1

SMOX Sensitization

SMOXs sensitized by dispersing small amounts of noble metals have been extensively studied. 9 Due to the complexity of the chemo-physical processes involved in a sensing event, the sensitizing effect has found a variety of explanations. Nevertheless, the specialized literature 10,11 converges on the following two main scenarios:

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• Spillover Effect: In this scenario, target gas molecules get loosely adsorb on the metallic surface as ionic species and they migrate onto the SMOX surface; on the semiconductor surface they react with the adsorbed Oxygen releasing charges to the conduction band. • Electronic Sensitization: This second mechanism regards metals that spontaneously accept adsorbed Oxygen on their surface, for instance Ag. In this case, the additive extends the oxide depletion layer leading to a lower conductivity. 12 Here the catalysis actually occurs on the MNP surface itself, while the SMOX is mainly left to transduce the signal via a charge transfer between its conduction band and the MNP. In this work, we will consider SnO2 NW’s loaded with Ag MNP for which the electronic sensitization has been established as the working mechanism. 13

1.2

Plasmon-Driven Catalysis

From the point of view of optics, noble MNP’s are mainly characterized by having sharp Localized Surface Plasmon Resonances (LSPR’s) in the visible and near UV spectrum. This surface phenomenon has firstly captured the attention of the surface chemistry community which showed how to enhance the catalytic activity of these MNP’s by exciting their LSPR’s. Indeed, extensive literature can be found regarding the surprising power LSPR’s have to gather energy from the electromagnetic field and convert it into a form that is essential for certain catalytic reactions to occur. 14,15 As a matter of fact, when the size of these MNP’s is considerably less than the free mean free path of the electrons in the conduction band, the quantum of energy hωp associated to the oscillation of the free carriers at the plasmon frequency ωp is absorbed by a single electron which then becomes a “hot” one, in the sense that its kinetic energy rise well above the Fermi level; this effect is the quantal counterpart of a classical phenomenon well known in plasma physics as Landau Damping. On a MNP, Landau Damping occurs at the femtoseconds time-scale, before electron-electron and electron-ion interactions lead to thermalization. 3



Therefore, when a LSPR is excited, the MNP surface can be thought as populated with highly energetic hot electrons which modify the chemistry of the adsorbed species leading to an enhanced catalytic activity.

1.3

Plasmon Enhancing of SMOX sensors

So far, the research on plasmonic photocatalysis has been subjected to extensive theoretical investigation but it found applications mostly in photochemistry and energy harvesting. In spite of sharing a partial common background, namely the surface physics and chemistry of MNP’s, SMOX based gas sensing and plasmonic photocatalysis studies have not yet converged into a consistent framework where the possible benefits of plasmonic hot electrons are examined. Indeed, when it comes to improve SMOX gas sensors performances, the focus has been put on two routes: • to supply the sensor with an external source of thermal energy 16 and • to irradiate the SMOX with supra band-gap UV-light. 16–18 Both routes lead to a shift towards the products side of the surface chemical equilibrium by virtue of multiple mechanisms, the common one being the dissociation adsorbed molecular oxygen. With the present study, we disclose a third way to activate such dissociation mechanism. To undertake this route has some clear advantages over the standard ones. Consider, for instance, the fact that plasmonic hot electrons are generated at low intensity radiation so that a luminous environment might substitute power-consuming suppliers of thermal energy or UV radiation. Furthermore, sensing at room temperature and in a non hazardous radiation field is certainly suitable for biological applications and it does not compromise the physical and chemical stability of the sensing material.

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Similar attempts has appeared in the recent literature 19–21 but, in our opinion, these examples lack of strong experimental evidence and coherence in their explanations compared to the present study. In order to experimentally demonstrate that the enhancement is due to a plasmonic effect, we performed a spectroscopic study of the sensor response. We show that the conductometric response enhancement of the Ag decorated sensor has the same wavelength dependence of its UV-Vis optical absorption. The UV-Vis optical absorption of the Ag decorated device is caused by plasmon resonance. Therefore, the conductometric response enhancement of the Ag decorated device is due to plasmonic absorption. This evidence is strengthened by the control sample made by bare SnO2 , for which we did not observed the same behavior.

2

Methods

2.1

SnO2 Nanowires and Ag NPs Synthesis

SnO2 NW’s were grown with the vapor-liquid-solid method. Al2 O3 substrates were initially functionalized with Pt by means of RF sputtering for 5 s at a pressure of 5 mbar and power of 70W, resulting in a nominal thickness of 2 nm. Then, in a tubular furnace connected to a mass flow controller, the substrates have been kept at 880◦ C next to a crucible containing SnO2 powder at the temperature of 1370◦ C. The temperature-induced formation of Pt nanodroplets promotes the growth of the SnO2 NW’s which occurs when inverting the direction of a pure Ar flow of 100 sccm at 100 mbar of pressure for 1 min. Finally, the NW’s has been functionalized with Ag NP’s by RF sputtering in inert atmosphere for 1 s at 50 W, followed by rapid thermal treatment at 350◦ C in atmosphere. After Ag deposition, interdigitated Pt contacts were deposited by RF sputtering to contact the nanowire bundle.

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2.2

Scanning Electron Microscopy

The samples morphology were investigated by Scanning Electron Microscopy (SEM, Zeiss LEO 1525) in conventional mode (recording the secondary electrons with an InLens detector) with an accelerating voltage of 10kV, and with SEM in scanning-transmission imaging mode (STEM), with an accelerating voltage of 20 kV. The Ag decorated SnO2 nanowires were scratched from the alumina substrate dispersed in ethanol and drop cast on a carbon grid (conventionally used for transmission electron microscopy measurements) to record STEM images of the nanowires. The transmitted electrons are detected by an in-house designed and fabricated detector, formed by five annular concentric sections. The activation of specific sectors of the detectors allows to obtain bright-field (BF) or dark-field (DF) images. 22

2.3

Raman, UV-Vis

SnO2 nanowires were characterized by Raman spectroscopy. The spectra were collected using a modular micro-Raman confocal system from Horiba, equipped with a single monochromator (ihr320mst3) and a Peltier-cooled CCD camera. A solid state laser at 442 nm wavelength was used as the excitation source, along with interference filters on the laser lines and edge filters on the signal. The spectra were collected by a 100x objective operating with 1800 l/mm grating, with 60 s integration time and 3 accumulations. SnO2 nanowires were also deposited on fused-silica substrates for UV-Vis measurements. Transmittance (%T) and Reflectance (%R) spectra were recorded in a Shimadzu UV-2600 spectrophotometer.

2.4

Gas Sensing

To assess the sensing performances, the substrates were welded on TO39 case to allow electrical measurements. Both sensors showed ohmic behaviour in the range ±10V (See Supporting Informations, Fig. S1). Since both sensors showed ohmic behaviour, constant bias 6


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(1 V direct-current) was applied to the sensing film and the electrical current was measured with a picoammeter Keithley model 486. All the sensing measurements were performed at room temperature (RT), constant 50% Relative Humidity (RH) and total flow of 300 cm3 /min; each gas inlet was 500 ppm of H2 . The gas sensing experiments were performed with two setups: A. High intensity and narrow band source. In this setup we utilized a green LED with a peak emission around to the plasmonic dipole mode of the Ag-doped sample. The green LED is placed outside the test chamber onto its quartz optical window just few inches from the sensors. B. Spectroscopic mode. This operating mode is the study of the sensors response as a function of incident wavelength; a schematic representation of this set up is depicted in Fig.1 An ad-hoc built test chamber was placed in front of the exit slit of a serial gas out

Sensor

OPTICAL MODULE

gas in

Gas Air Lamp Monochromator H 2O

pA Lamp Controller

GAS MIXING SYSTEM

PC

Figure 1: Experimental setup for studying the gas response as a function of the incident wavelength. controlled lamp and monochromator. A home written LabVIEW program controls the lamp power, the monochromator output wavelength and the gas sensing system, in order to measure the gas response at constant illumination intensity in the 350-650 nm 7



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spectral range with 50 nm interval. At 350 nm the lamp has the dimmest emission and the monochromator has a low efficiency; therefore, at each wavelength, the light intensity is the maximum that we achieved at 350 nm, that is approximately 2 µW.

3

Results and Discussion

The SEM images reported in Fig.2 were obtained from the bundle of nanowires grown on the alumina substrate. Fig.2a shows that the Ag NPs are dispersed over the nanowires; the diameter of the Ag nanoparticles is lying below 20 nm. At this scale, the probability of plasmonic hot electrons creation on the Ag surface is maximized. 23 Fig.2b shows that the nanowires are several microns long with very high aspect ratios and form a spaghetti-like structure.

(b)

(a)

Figure 2: (a) SEM images of Ag nanoparticles anchored on SnO2 nanowires and (b) Ag decorated SnO2 NW’s bundle at lower magnification.

Fig. 3 is a BF STEM image of a small amount of Ag@SnO2 NW sample deposited on the carbon-film support for the observation in the transmission mode. The top panel shows a larger amount of nanowires: the bigger particles that can be seen at the connection between two nanowires are often seen also in pure SnO2 samples. The STEM images on the lower panel highlight the Ag decoration over the nanowires; indeed, the round features marked by the white arrows are related to the Ag NP’s. The diameter of the NP’s measures below 20

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nm.

Figure 3: Top and down panel: bright field and dark field, respectively, STEM images of a SnO2 NW with Ag nanoparticles.

The crystallinity of these nanowires has been investigated by Trasmission Electron Microscopy in a previous publication. 24 SnO2 nanowire is in the cassiterite tetragonal phase; the growth of nanowire occurs along the [100] direction. We analysed the ensemble of nanowires by Raman spectroscopy, to gain information on the crystalline phase. SnO2 has the tetragonal rutile structure (space group D14 4h , P4/mnm) with two SnO2 molecules per unit cell. There are five Raman active vibration modes. 25 In Fig. 4 the most intense ones (namely A1g at 633 cm−1 and B2g at 774 cm−1 ) are visible, confirming the tetragonal rutile structure. Figure 5 shows the optical Absorption (%A = 100 − %T − %R) of a SnO2 NW’s sample before and after the Ag deposition. Ag NP’s do not affect the SnO2 band gap and UV Absorption. Instead, two plasmonic peaks appear around 500 and 350 nm; the former is the dipolar mode of Ag that is red-shifted by the high refractive index of the nanowires, while 9



A1 g

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Figure 4: Raman spectrum of SnO2 sample 3% Ag, showing the A1g and B2g peaks of SnO2 (peaks marked with asterisk are due to alumina substrate). Other bands are ascribable to sample fluorescence. the high energy mode at 350 nm is the quadrupolar mode due to the symmetry-breaking at the SnO2 interface. 26,27 Given the drastically different optical absorption features Ag NP’s add to the SnO2 NW’s, we took on strategy A (as described in 2.4) to establish whether visible light has different effects on the two sensors. From the raw data displayed in Fig.6a we can qualitatively appreciate the radically different behaviour of the two sensors in respect to light and gas exposure. To precisely examine these data we preemptively define our signals as: • I0,0 the electric current in air and dark • I0,λ the current in air when the light is on • Ig,0 the current in gas and dark • Ig,λ the current in gas when the light is on so to define what are the relevant quantities to compare, namely the Responses: 10


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Figure 5: UV-Vis spectra of bare and Ag decorated SnO2 nanowires

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Figure 6: (a) Raw electric current signals from bare and Ag decorated sensors. Grey columns represents H2 injection while green columns green LED light exposure. (b) Superimposed gas responses in dark and light of bare and Ag decorated SnO2 NW’s. The data are sampled from the second and third gas injection of Fig.6a.

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• Rg,0 =

Ig,0 I0,0

the response to gas in dark

• Rg,λ =

Ig,λ I0,λ

the response to gas when the light is on

First of all, let us note that the Ag decorated sensor has a baseline conductance at least two order of magnitude lower than the bare one, and a higher Rg,0 . This is just the well known effect of the Ag electronic sensitization as mentioned in 1.1. The green light irradiation increased the conductance of the bare and of the Ag decorated SnO2 nanowires sensors. Since the energy of impinging green light is lower than that of SnO2 band gap, we claim this effect to be caused by light induced desorption of physisorbed Oxygen molecules. As seen in Figure 6a, the effect is higher in case of Ag@SnO2 sensor, due to the high photon absorption observed for the Ag@SnO2 sensor in the green region (Figure 5). As a consequence of plasmonic photoabsorption by Ag nanoparticles, the photoresponse in air I0,λ /I0,0 of the Ag@SnO2 sensor is remarkably higher than the bare one. The superimposed responses from both sensors are reported in Fig.6b, demonstrating that Rg,λ > Rg,0 only for the Ag decorated sensor, while in case of the bare one we have a slight quenching. This effect has been observed at different H2 concentrations (calibration curve in the Supporting Information, Fig. S2). The gas response of the Ag decorated sensor in light is around 33% higher than in dark. The response (recovery) times were calculated as the time needed to reach 70% of the steady state value in gas (air). Response times in green light are 430 s for the Ag decorated sensor and 480 s for the bare sensor, indicating that the Ag decorated sensor is slightly faster than the bare sensor. Recovery times are longer, being of the order of tens of minutes for both sensors. As expected response and recovery times are quite long due to room temperature operation of the sensor; countermeasures like exposing to UV light in order to reduce the recovery times will be investigated. 28 18 The results from the spectroscopic analysis of Rgλ (setup B, see 2.4) strengthen the hypothesis regarding the plasmonic nature of this enhancement. In Fig.7 we plot the en∗ hancement factor defined as the ratio between the maximum response Rg,λ attained at the

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∗ wavelength λ and the maximum response in dark Rg,0 .

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Figure 7: Enhancement factor as a function of wavelength. A value above 1 corresponds to an actual light-induced enhancement, while a value below 1 means a light-induced quenching. The Ag decorated sensor shows an enhancement factor that nicely matches its absorption spectrum (Fig.5), with the two plasmonic peaks at 500 and 350 nm. On the other hand, the enhancement factor of the bare SnO2 sensor always lies below 1, meaning that its response gets quenched at all λ’s. This degradation of the response can be attributed to light-induced oxygen desorption from defect states of the SnO2 which leads to an increase in I0,λ that, in case of the bare sensor, it is not counterbalanced by an increasing catalytic activity. The case of the Ag decorated sensor is different insofar the detrimental oxygen desorption must be counterbalanced by a light-induced process that raises the catalytic efficiency. We claim this process to be the creation of hot-electrons on the Ag nanoparticles at the LSPR wavelength. 29 As a matter of fact, plasmonic hot electrons are capable to convert 13



the adsorbed oxygen species from the rather inert molecular O− 2 form into the highly more reactive O− . 30 Assuming the Oxygen species adsorbed on the Ag NPs in dark are in form of O2− , the plasmon-assisted sensing of hydrogen is based on the following reactions: prior to hydrogen exposure, plasmonic hot electrons can dissociate chemisorbed Oxygen on Ag

− − O− 2 + ehot → 2O

these activated oxygen ionss then reacts with Hydrogen during gas exposure

O− + H2 → H2 O + e−

Also, the plasmonic hot electrons can further shift the previous reaction towards the products side by catalysing the required dissociation of H2 .

4

Conclusions

We have demonstrated how gas sensing with metal oxide loaded with metallic nanoparticles is intimately connected to Plasmon-Driven Catalysis. We thoroughly compared the responses of bare and Ag loaded SnO2 nanowires sensors produced entirely by physical bottom-up methods, to show how plasmonics drives conductometric gas sensing at room temperature. Our results and methods certainly have straightforward applications in both fields of gas sensing and photochemistry, and the fact that our interpretation is coherent with the conventional principles of these two fields ought to be stressed the most.

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Associated Content The Supporting Information is available free of charge at http://pubs.acs.org. Sensors IV curves and calibration curves.

Acknowledgement The research leading to these results were partially funded by the NATO Science for Peace and Security Programme under grant N◦ 9085043. We also acknowledge Prof. V. Golovanov for fruitful discussion.

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Tin Oxide Nanowires Decorated With Ag Nanoparticles for Visible

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