Degenerately Doped Metal Oxide Nanocrystals as Plasmonic and

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Degenerately Doped Metal Oxide Nanocrystals as Plasmonic and Chemoresistive Gas Sensors Marco Sturaro, Enrico Della Gaspera, Niccolò Michieli, Carlo Cantalini, Seyed Mahmoud Emamjomeh, Massimo Guglielmi, and Alessandro Martucci ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09467 • Publication Date (Web): 18 Oct 2016 Downloaded from http://pubs.acs.org on October 26, 2016

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ACS Applied Materials & Interfaces

Degenerately Doped Metal Oxide Nanocrystals as Plasmonic and Chemoresistive Gas Sensors Marco Sturaro1, Enrico Della Gaspera2,*, Niccolò Michieli3, Carlo Cantalini4, Seyed Mahmoud Emamjomeh4, Massimo Guglielmi1, Alessandro Martucci1,* 1

Università di Padova, Dipartimento di Ingegneria Industriale, Padova, Italy

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RMIT University, School of Science, Melbourne, VIC, Australia

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Università di Padova, Dipartimento di Fisica e Astronomia, Padova, Italy

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Università di L’Aquila, Dipartimento di Ingegneria Industriale, L’Aquila, Italy

* Enrico Della Gaspera: [email protected] * Alessandro Martucci: [email protected]

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ABSTRACT Highly doped wide band gap metal oxides nanocrystals have recently been proposed as building blocks for applications as transparent electrodes, electrochromics, plasmonics, and optoelectronics in general. Here we demonstrate the application of gallium doped zinc oxide (GZO) nanocrystals as novel plasmonic and chemiresistive sensors for the detection of hazardous gases including hydrogen (H2) and nitrogen dioxide (NO2). GZO nanocrystals with a tunable surface plasmon resonance in the near infrared are obtained using a colloidal heat-up synthesis. Thanks to the strong sensitivity of the plasmon resonances to chemical and electrical changes occurring at the surface of the nanocrystals, such optical features can be used to detect the presence of toxic gases. By monitoring the changes in the dopant-induced plasmon resonance in the near infrared, we demonstrate that GZO thin films prepared depositing an assembly of highly doped GZO colloids are able to optically detect both oxidizing and reducing gases at mild (< 100 °C) operating temperatures. Combined optical and electrical measurements show that trivalent dopants within ZnO nanocrystals enhance the gas sensing response compared to undoped ZnO. Moreover, improved sub-ppm NO2 gas sensitivity is achieved by activating the sensors response through combined purple-blue (λ=430nm) light irradiation and mild heating at 75 °C. In addition, these thin films based on degenerately doped semiconductors are highly transparent in the visible range, enabling the fabrication of “invisible” gas sensors. The use of highly doped semiconductive nanocrystals for both IR plasmonic and chemiresistive sensors represent a marked advancement towards the development of highly sensitive and selective devices.

KEYWORDS transparent conductive oxides; doped zinc oxide; nanocrystal ink; optical gas sensors; surface plasmon resonance.


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INTRODUCTION Zinc oxide is one of the most studied materials for gas sensing applications because of its chemical sensitivity to many analytes, high thermal stability, suitability to doping, non-toxicity, abundance in nature and low cost. Previous works demonstrated the capability of ZnO to detect toxic gases and organic vapors, which represent severe hazards in both domestic and industrial environments1-5. Doping has been shown to influence the optical and electrical properties of ZnO, with beneficial effects for its catalytic and sensing performances6-9. In particular, trivalent cations as substitutional dopants for zinc within ZnO have been extensively studied, mostly for application in transparent conductors. In fact, doping ZnO with trivalent cations such as Ga3+ or Al3+ greatly enhances its conductivity without altering the transparency in the visible spectral range, turning highly resistive, undoped ZnO into a transparent conductive oxide (TCO)10. These aliovalent dopants are also responsible for the generation of a localized surface plasmon resonance (LSPR) in the near infrared, arising from the increased free electron concentration. While the main applications of TCOs are in optoelectronic devices including solar cells11, touch panel displays12-13, and energy efficient windows14, they have recently proved to be interesting materials also for gas sensing applications: in fact, indium tin oxide (ITO) has already been tested for the detection of hydrogen15 and volatile organic compound (VOC)16; gallium and aluminum doped zinc oxide (GZO and AZO, respectively) have also been used for electrical17-20 sensing of hydrogen, methanol, nitrogen dioxide, hydrogen sulfide and optical21 sensing of hydrogen. However, the optical features provided by the near infrared LSPR in these degenerately doped metal oxides have never been used as sensing platforms. It is well-known that the LSPR of metal nanostructures strongly depends on a variety of factors, including dielectric properties of the surrounding medium and chemical reactions occurring at the surface of the metals, which make the LSPR extremely interesting for sensing applications. Several examples can be found in the literature showing gas, organic molecules and refractive index sensing through the analysis of the optical changes in the LSPR peaks of metal (mainly Au and Ag) nanostructures22-25. However, for gas sensing applications the plasmonic metal nanostructures are ACS Paragon Plus Environment

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generally coupled to a semiconductive matrix (usually a metal oxide) which is responsible for the reaction with the target analyte, while the metal acts as optical probe, providing an optically detectable signal. In this work we show that the LSPR peak of highly doped ZnO nanocrystals (NCs) can be used to detect both oxidizing and reducing gases, combining the reactivity of ZnO towards gaseous analytes, and the optical signatures provided by the plasmonic resonance. Previous results on plasmonic semiconductors used for detecting metal ions in water - although through an aggregation-induced shift of the LSPR and not through a modulation due to a change in charge carrier concentration26 - and redox reactions in liquid27,28 show promise for the application of doped semiconductors as plasmonic sensors. We focus our study on hydrogen (H2) and nitrogen dioxide (NO2) gases, because their rapid and accurate detection is extremely important for several industrial and environmental applications. For example, hydrogen gas is odorless, colorless and forms a highly explosive mixture with oxygen gas: therefore, hydrogen sensors are vital in any application using hydrogen for both process monitoring and safety purposes. Nitrogen dioxide is an important agent in many industrial processes and also an environmental pollutant with highly toxic effects to human health. We demonstrate detection of hydrogen and nitrogen dioxide gases using Ga-doped ZnO nanocrystal assemblies by monitoring the gas-induced changes of the LSPR peak at mild operating temperatures (< 100 °C) as depicted in Scheme 1. In addition, the LSPR frequency and linewidth can be easily tuned varying dopant concentration29,30 and distribution31, enabling a wide range of operational parameters. Moreover, we validate the reaction mechanism and the effect of dopant concentration through conductometric gas sensing measurements, which show excellent sub-ppm sensitivity even at room temperature.


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Scheme 1. GZO NCs are synthesized with colloidal techniques (a) and used to fabricate nanocrystalline thin films (b). The LSPR peak in the near infrared is sensitive to changes in the charge carrier concentration which are promoted by oxidizing and reducing gases (c). Optical and electrical sensors are demonstrated (d).

This is the first demonstration of a plasmonic gas sensor based on degenerately doped metal oxide nanocrystals, and constitutes a stepping stone towards highly efficient sensing devices, which are also amenable to emerging applications including transparent and flexible sensors.

EXPERIMENTAL PROCEDURE Zinc oxide NCs were synthetized through a previously reported heat-up colloidal method29. In a typical synthesis of undoped ZnO NCs, 1.26 g Zn stearate (ZnSt2), 3.2 g 1-dodecanol (1-DDOL) and 1.76 g oleic acid (OA) were mixed together in a round bottom flask, using 21 mL 1-Octadecene (ODE) as non-coordinating solvent. To obtain doped nanoparticles, a certain amount of Gallium acetylacetonate (Ga(Acac)3) was added (36 mg and 73 mg to obtain, for example, nominal molar doping levels of 5% and 10%). The round bottom flask was then connected to a schlenk line, degassed at room temperature and maintained under inert (nitrogen) atmosphere for the duration of the whole reaction. The solution was first heated to 130 °C, it was maintained at this temperature for 45 min, and eventually it was heated to 240 °C under strong stirring with a heating rate of at 10 °C/min. After 3 hours at 240 °C to ensure efficient dopant incorporation, the solution was cooled


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down to room temperature under nitrogen. The NCs were then collected and purified through redispersion/centrifugation cycles using chloroform or toluene as solvents and acetone as nonsolvent. After purification, the NCs were redispersed in octane or toluene at a concentration of ~ 100 mg/mL. Spin coated films were deposited from these concentrated solutions at 1000 rpm for 30 s, and then stabilized on a hot plate at 150 °C for 15 min in air. This procedure was repeated to obtain desired film thickness. Thin films were then treated following a procedure previously described by our group32 in a 5% H2-95% Ar gas mixture. The samples have been labeled GZOX, where X is the nominal Ga doping express in atomic % with respect to Zn. X-ray diffraction (XRD) analyses on dry nanocrystalline powders were performed using a Bruker D8 Advance diffractometer using CuKα (0.15418 nm) radiation at 40 kV and 40 mA. Thin-film XRD measurements were performed at 3° incidence using a Philips PW1710 diffractometer equipped with grazing-incidence X-ray optics, using CuKα Ni-filtered radiation at 30 kV and 40 mA. The microstructure of the films was investigated with a Zeiss Sigma HD Field Emission Scanning Electron Microscope (FE-SEM) equipped with an energy dispersive X-ray spectrometer (EDX) for compositional analysis and a secondary electron InLens detector for imaging. Optical absorption spectra of samples were obtained in the 200–2500 nm range using a Jasco V-570 spectrophotometer; Fourier-Transform Infrared (FTIR) spectroscopy measurements were performed in the 400–4500 cm-1 range using a Jasco FT-IR 690 spectrometer on samples deposited on Si substrates. Optical gas sensing tests were performed by making optical absorption measurements in the 350–2500 nm wavelength range on films deposited on SiO2 glass substrates using a Harrick gas flow cell (with an optical path length of 5.5 cm) coupled with a Jasco V-570 spectrophotometer. The standard operating temperature (OT) was set between 80 and 200 °C and gases at concentrations of 1 vol% for H2 and of 1000 ppm for NO2 in dry air at a flow rate of 0.4 L/min were used. The incident spectrophotometer beam was set normal to the film surface and illuminated an area of ~13mm2.


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Electrical gas sensing tests were performed on films deposited onto a Si/Si3N4 supports, inserted in a chamber provided with Teflon tubing. Gases were mixed in a MKS147 multigas mass controller to obtain final gas target concentrations in the ranges 20 ppb - 1 ppm of NO2 and 50-1000 ppm of H2. NO2 gas concentrations in the downstream were measured by an Ansyco AS32M chemiluminescence analyzer from Environment S.A. An automated volt-amperometric system (Agilent 34970A Data Acquisition Unit) was utilized to evaluate changes in resistance under various operating temperatures and gas concentrations. A detailed description of electrical gas sensing equipment and procedure is presented in the Supporting Information.

RESULTS AND DISCUSSION

1. Microstructural and optical properties ZnO NCs with different doping levels were synthesized by varying the amount of the gallium precursor in the reaction mixture. As demonstrated previously29,30, the presence of the aliovalent dopant within ZnO triggers the formation of a LSPR feature in the near infrared, as evident from Figure 1a. The IR signatures of GZO nanocrystals are strongly dependent on the Ga doping, which also slightly affects the absorption properties in the visible spectrum, giving the doped NCs a distinctive green-blue color, with respect to the uncolored pure ZnO (see Figure S1 in the Supporting Information). To confirm the effective gallium doping, we evaluated the elemental composition of the GZO samples using SEM-EDX, and the results are also reported in Figure 1b. The Ga/Zn atomic ratios obtained by EDX are smaller than the nominal values calculated as the ratio between the amount of Ga and Zn precursors, which is in agreement with previous reports29,32. Therefore, the doping efficiency, which is defined as the real vs. nominal Ga doping, is always less than 100%, and is also found to decrease at higher doping levels. The GZO NCs synthesized here are all crystalline in hexagonal wurtzite structure (ICDD No. 361451) without any impurities or particular preferred orientation, as shown in Figure 1c. Due to the broad diffraction peaks of nanoscale particles, no detectable shift of the peaks between doped and ACS Paragon Plus Environment

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undoped ZnO can be observed. The crystallite diameter of the NCs evaluated from the diffraction patterns using the Scherrer equation is ~10 nm, and becomes slightly larger with increased dopant concentration, as evident from Figure 1d. This is consistent with the previously observed role of dopant ions in defining shape and dimension in colloidal nanocrystals33,34, and specifically with trivalent dopants promoting a slight increase in the size of ZnO nanocrystals synthesized using various methods (including solvothermal and colloidal syntheses)29,35. Despite these small differences in size, all the synthesized NCs can be readily suspended in nonpolar solvents at high concentration obtaining stable colloidal “inks” that are suitable for thin films deposition. The films were prepared via spin coating as described in the Experimental section and subsequently annealed in reducing atmosphere at 450 °C. Prior to such annealing, the samples were exposed to UV light for 30 min, to remove organic compounds – mainly composed of surface ligands – and obtain purely inorganic coatings, as previously demonstrated32,36.

Figure 1. a) Optical absorption spectra for as-synthesized GZO colloidal solutions and annealed GZO films. b) Nominal vs real gallium doping evaluated using SEM-EDX for the different GZO films. c) XRD patterns for as-synthesized GZO NCs and annealed GZO films. The diffraction peaks are all indexed to wurtzite ZnO and the reference pattern (ICDD No. 36-1451) is reported at the bottom. d) Crystallite size for as-synthesized GZO NCs and annealed GZO films evaluated from XRD peaks using the Scherrer relationship.


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The combination of UV exposure and annealing under reducing gas atmosphere enhances the plasmonic absorption in the infrared without compromising the transparency in the visible range. In fact, while it is known that thermal treatment in air promotes a decrease of the IR absorption of TCO films caused by a decrease in oxygen vacancies concentration37, an annealing in reducing atmosphere promotes a blue shift of the LSPR peak resulting in an increased absorption in the near infrared (Figure S2 in the Supporting Information) and in an improvement of the TCOs performance38. However, if annealed under inert or reducing atmosphere, organic contaminants will decompose to carbonaceous residues that have a brownish color, which is undesirable for visibly transparent samples. Following a UV treatment, organics are fully removed and GZO films retain their transparency in the visible spectrum following the annealing at 450 °C (Figure S2 in the Supporting Information). Therefore, after such annealing procedure, GZO films that are transparent in the visible range and show a marked infrared absorption were obtained. Consistent with what was observed for the assynthesized NCs, the NIR absorption progressively increases with increased Ga doping, (Figure 1a). The presence of the LSPR peak has been confirmed using FTIR (Figure S3 in the Supporting Information), which shows a peak centered at ~5.5 µm for GZO5, at ~4.5 µm for GZO 10, at ~3.5 µm for GZO 20 and no optical features for undoped ZnO. The annealing treatment causes also an increase in crystallite size compared to the original NCs, due to thermally-induced crystal growth (Figure 1b and Table 1 in the Supporting Information). The high quality of the GZO thin films that can be inferred from their absorption spectra is further confirmed by microstructural analyses performed using SEM (Figure 2): all deposited films are uniform and homogeneous on a scale of several microns, and possess a similar morphology regardless of the amount of dopant. The images at high magnification show a transition from spherical particles to slightly elongated and faceted particles at increased Ga doping, which is fairly common for highly doped NCs. From the cross sectional images, we can measure the thickness of ACS Paragon Plus Environment

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the films, which is in the 500-600 nm range after three subsequent coating steps (see Figure S4 in the Supporting Information).

Figure 2. SEM images of a) ZnO, b) GZO5, c) GZO10 and d) GZO20 thin films after heat treatment in forming gas at 450 °C. Scale bars for main panels and insets are 1 µm and 100 nm, respectively.

2. Gas sensing Metal oxides in general have been extensively studied for gas sensing applications, due to the presence of native vacancies and defects in the oxides structure39. Such structural properties have been exploited for sensing purposes, especially because the electrical conductivity of metal oxides is found to be dependent on these vacancies and defects, which in turns are strongly affected by the presence of reactive gases40. However, the optical properties of the materials can also be affected by the analyte presence, and used for sensing purposes: previous studies have demonstrated this property for some oxides such as CuO, Co3O4, WO3 and NiO41-44. These sensing results were obtained at high temperature and with low sensitivity compared to electrical resistance measurements, and catalysts and/or optically active metallic nanoparticles have usually been introduced to increase the optical sensor performances45-47.


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ZnO is a non-stoichiometric oxide presenting a slight deficiency of lattice oxygen ions [ZnO1-x], which has been traditionally ascribed as the cause to its distinctive n-type behavior39. However, other studies have proposed additional mechanisms for the strong n-type characteristics of undoped ZnO, such as hydrogen multicenter bonds48. When synthesized as a nanocrystalline material using colloidal techniques, ZnO and Ga-doped ZnO have been demonstrated to show a series of lattice defects including oxygen vacancies, zinc vacancies, interstitial zinc and substitutional gallium, which are related to the dopant concentration and contribute to some defect-related emission49. In air, at equilibrium conditions, ambient oxygen adsorb on the oxide surface in the forms of O2−, O− or O2−, depending on the surface temperature. In particular, the stable oxygen ions are O2−, O− and O2− for temperatures T< 150°C, 150°C< T < 300°C and T > 300°C respectively50. Given that the sensing tests in this work have been performed at OT between 80°C and 150 °C, and considering that ambient oxygen is reported to adsorb primarily on lattice oxygen vacancies, the following adsorption reaction has to be considered:

+ + ⇌ ( − )

(1)

where represents electrons from the conduction band, .. , according to the Kröger-Vink

notation, represents a doubly charged positive oxygen vacancy and ( − ) a chemisorbed oxygen coupled with ..

Reaction (1) corresponds to the initial state of the GZO surfaces at

equilibrium in air in the temperature range 25 °C – 150 °C. Interactions with reducing (H2) and oxidizing (NO2) gases may be represented by the following51,52:

2 + ( − ) ⇌ 2 +

(2)

() + + ⇌ ( − )

(3)

According to these reactions, reducing and oxidizing gases will promote an injection or a removal of electron in the conduction band of ZnO, respectively. Besides the obvious effect on the change of the electrical resistance with the gas concentration, which is the main feature of a chemoresistive ZnO gas sensors, variations in carrier density can also affect the optical band gap – for n-type


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semiconductor like ZnO through a blue shift for reducing gas and a red shift for oxidizing gas – through the Burstein-Moss effect 53-55. Previous works have demonstrated that noble metal particles embedded in the metal oxide matrix can act as optical probes, using their LSPR shifts, caused by carrier density variation, instead of monitoring shifts in the band gap of the metal oxide25,28. Plasmonic metal oxides like GZO can open the possibility to obtain the same effect simply exploiting their LSPR features in the infrared, without the need to develop nanocomposite thin films composed of plasmonic NCs embedded within a metal oxide matrix. Moreover, the TCO plasmon resonance can be also easily tuned with dopant concentration, heat treatments or tailoring grains size and shape, and consequently can be exploited at different wavelengths for different sensing purposes29,31,55.

Figure 3. Optical absorption spectra for GZO10 sample when exposed to a) 1% vol H2 1% and b) 1000 ppm NO2 1000 ppm at 150 °C OT. A clear blue shift and red shift of the SPR absorption can be seen in a) and b), respectively. c) Optical absorbance change (OAC = AbsGas-AbsAir) for GZO10 at 150 °C when exposed to H2 and NO2.

We analyzed the variation in the LSPR peak for GZO films exposed to 1% vol H2 and 1000 ppm NO2 at different operating temperatures in the range 25-150 °C and dry air as carrier gas. Higher


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temperatures (> 200°C) were found to drastically decrease the plasmonic absorption due to decrease in oxygen vacancies, and therefore led to baseline instability. After exposure to hydrogen (a reducing gas), the absorbance in the NIR range increases, while upon exposure to NO2 (an oxidizing gas) a decrease in optical absorption in the NIR is observed (Figure 3a and b). To highlight this difference, Figure 3c shows the Optical Absorbance Change parameter, which is defined as the difference between absorbance during gas exposure and absorbance in air (OAC = AbsGas – AbsAir = ∆Abs). This behavior is consistent with IR plasmonic resonance shifts due to electron density variations and band gap shifts in n-type semiconductor like ZnO. In the case of hydrogen, electrons are injected in the metal oxide according to reaction (2) causing an increase in the concentration of free carriers which generates a consequent blue shift of the LSPR peak and an increase of the optical band gap. In fact, the frequency of the LSPR is proportional to the square root of the free electron concentration, according to the formula 57,58:

=

! ( " # )

(4)

Therefore, an increase in electrons within ZnO following the oxidation of hydrogen will cause an increase in the LSPR frequency. Conversely, the interaction with oxidizing gases like NO2 causes the removal of electrons according to reaction (3), with the related red-shift of the plasmon peak and the reduction of the optical band gap. These LSPR shifts, which are driven by different gases, represent a great opportunity to improve sensor selectivity. Tests could be performed exploiting the change in absorbance both in the infrared range and in the band gap region. The different wavelength dependence of the OAC for H2 and NO2 highlighted in figure 3c can be also exploited for developing selective optical gas sensors. In fact, by choosing a proper wavelength (for example 2000 nm) the presence of H2 will give a positive variation of the absorbance while NO2 a negative variation.


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The OAC spectra and a time-resolved measurement at fixed wavelength (2400 nm) for 1000 ppm NO2 at 150 °C are presented in Figure 4a and 4b for ZnO-based samples as a function of Ga doping. No signal was detected for ZnO sample in the near infrared, while a reversible signal caused by the red shift of the near infrared plasmon is detected for all GZO samples. A trend was identified with doping concentration: higher doping level caused a more intense LSPR shift, and for GZO10 and GZO20 a quicker response and recovery was detected.

Figure 4. (a) Optical absorbance change (OAC = AbsGas-AbsAir) curves for NO2 1000 ppm exposure for ZnO and GZO samples at 150 °C. (b) Time-resolved absorption change for ZnO and GZO samples for two cycles Air/NO2 1000ppm at 2400 nm and OT = 150°C. c) Time-resolved absorption change for GZO10 film for two cycles Air/H2 (1% vol) and two cycles Air/NO2 (1000 ppm) at 2400 nm at two different operative temperatures (80 °C and 150 °C).

After assessing the ability of GZO samples to optically detect H2 and NO2, we have investigated in figure 4c the effect of different operating temperatures on the H2 and NO2 sensitivity of the GZO10 film. At mild OT (80 °C), no signal was observed for hydrogen, while a small but detectable response was achieved at 150 °C. Conversely, NO2 was detected by GZO samples at both at 80 °C and at 150°C OT, with a strong enhancement the intensity of the response and a drastic reduction in the detection times at higher temperatures.


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The chemoresistive response of the GZO films to NO2 and H2 gases was also investigated and compared with optical gas sensing measurements. Changes of the electrical resistance of ZnO-based metal oxides to NO2 and H2 gases have been extensively reported in literature by activating the sensor response by thermal59, visible60 or UV light irradiation61, in order to decrease the operating temperature towards mild (T < 100 °C) or even ambient (25 °C) conditions. It was demonstrated that the sensitivity of ZnO to oxidizing gases (i.e. NO2) is always larger with respect to reducing (i.e. H2), with detection limits as low as 20 ppb to NO2 and 100 ppm to H262. New strategies aimed to enhance the NO2 electrical response have been recently presented for SnO2/ZnO composites63 and WO364 nanofibers, making use of the combined actions of temperature and visible light. In this study we explored the effect of both temperature (in the OT range 25 °C - 100 °C), and visible light irradiation using purple-blue (PB) LED source (λ=430nm wavelength and 770 µW/cm2 intensity), on the electrical gas sensing properties of highly doped GZO films for the detection of NO2 (400 ppb) and H2 (250 ppm) gases. The resistance change during hydrogen exposure was always lower with respect to NO2 exposure, for all the conditions we investigated, as shown in Figure S5 and S6 in the Supporting Information. This behavior is in line with the smaller gas sensing response to H2 as respect to NO2 observed during optical sensing tests (see Figure 4). For this reason, we focus the next discussion on NO2 sensing. The following figures of merit have been introduced to characterize and compare “gas response properties” of the GZO samples: base line resistance (BLR): the resistance in dry air at equilibrium before any gas exposure; relative response (RR): the ratio (RG/RA) to 400 ppb NO2, where RA and RG represent the

resistance in dry Air and in Gas, respectively; adsorption/desorption time (τads/des): the time required to reach 90% of the full response at

equilibrium, during both gas NO2 adsorption and desorption; recovery percentage (RP): a measure of the sensor ability to recover its base line, calculated

as the percentage ratio (∆D/∆A) x 100, where ∆D and ∆A are the variations of the electrical ACS Paragon Plus Environment

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resistance during gas desorption and absorption, respectively, calculated within the time scale of the sensing cycle (see Figure 5b for its graphical representation).

Figure 5. Normalized resistance changes to 400 ppb NO2 exposure (yellow shaded box) for ZnO, GZO10 and GZO20 at different operating temperatures under dark (a-d) and Purple-Blue light irradiation (e-h) conditions.

Regarding these points, we first clarify that given the time scale of a sensing cycle, gas adsorption/desorption times can be eventually numerically quantified only if the equilibrium conditions are achieved. It may happen that equilibrium conditions are achieved (i.e. τads/des can be quantified), but the sensor resistance fails to regain its original base line value. This occurs in the case of irreversible adsorption, when gas molecules are bound so strongly to the surface that they do not desorb, regardless of the time scale of a sensing cycle. In some cases, no equilibrium conditions are achieved during desorption so that neither base line recovery is achieved nor τads/des times can be quantified. Figure 5 shows the normalized change of the electrical resistance of the ZnO, GZO10 and GZO20 samples to 400 ppb NO2 by increasing the OT from 25 °C to 100 °C under dark or under PB light illumination.


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By analyzing the electrical response at different OT and under dark conditions (Figure 5a-d), we observed that gallium doping enhances relative responses (RRs), improves the recovery percentages of the base line (RPs) and slightly increases the response times. Numerical figures of these parameters are reported in Table 1. RRs at room temperature are the highest with a maximum of RR = 47.7 for the GZO20, but with associated poor recovery of the base line during desorption, as represented by the blue plot of figure 5a. No gas response was recorded for undoped ZnO at room temperature. We found that at 75 °C OT and 20% gallium doping a reasonable compromise in terms of RR, RP and response times can be achieved, as shown in Figure 5 and Table 1.

Table 1 Comparison of RR, RP, τads and τdes to 400ppb NO2 in dark conditions and PB light illumination at different OT (25 °C – 100 °C). The notation (/) indicates that no adsorption or desorption equilibrium conditions have been achieved within the time scale of the experiment. OT (°C)

25

50

75

100

RR=RG/RA

RP=[∆D/∆A ]*100

[-]

[%]

Sample

τ ads

τ des

(min) Light

(min) Dark Light

Dark

Light

Dark

Light

Dark

ZnO

1

2.94

0

90

/

4

/

/

GZO10

3.52

2.44

0

80.66

32

30

/

/

GZO20

47.73

10.44

19.28

93.80

105

77

/

/

ZnO

1

2.65

0

88.15

/

4

/

/

GZO10 GZO20

1 5.80

5.31 27.50

15.77 69.62

88.74 98.01

/ 82

16 67

/ /

/ 119

ZnO

1.18

2.03

0

99,00

/

4

/

55

GZO10

4.29

8.81

87.47

99.33

19

15

/

115

GZO20

15.78

20.82

95.28

99.06

73

49

/

105

ZnO

1.46

1.58

54.59

89.34

/

4

/

35

GZO10

6.17

8.74

96.24

98.25

68

80

69

105

GZO20

10.81

24.41

97.52

99.23

99

100

86

105

Irradiation with PB light, as shown in figure 5e-h, further enhances RRs and remarkably increase the RPs, as respect to dark conditions. Given the positive effect of PB light irradiation, reasonable gas sensing features are maintained even at 50 °C OT, yielding for the 20% GZO, RR and RP values of 27.5 and 98.0% respectively. Moreover, undoped ZnO films activated with PB light show


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gas response even at room temperature (Figure 5e) as compared to the undoped ZnO in dark (Figure 5a). Finally, the dynamic gas responses at 75 °C OT and at NO2 concentrations ranging from 100 to 400 ppb, are compared in Figure 6 in dark and under PB light. For all the investigated samples, by exposing all the films to PB light, base line resistances (horizontal dashed lines in fig. 6) always decrease with respect to dark. This behavior could be explained65,66, considering the photodesorption of surface oxygen (O2-ads) induced by the impinging photons, according to reaction (5):

( − ∙∙ ) + ℎ& ⇌ () + ∙∙ +

(5)

Considering that the binding energy of chemically-adsorbed oxygen atoms on defective SnO2 is around 1.67eV67, PB light with associated photon energy of 2.88eV, is suitable to activate the desorption mechanism of reaction (5), therefore explaining the decrease of the base line resistance. Moreover, GZO films with higher doping levels always show lower base line resistances. This is a consequence of both the aliovalent n-type Ga3+ doping and oxygen desorption as described in reaction (5). Regarding the larger RRs of the PB irradiated GZO20 as respect to the others, a possible explanation is that the gallium content increases the concentration of surface oxygen vacancies (∙∙ ), which according reaction (5), become available to react with NO2 (reaction (3)). The increase of free adsorption sites (∙∙ ) induced by light irradiation, could also explain why the GZO10 seems to saturate its response under dark, whereas under PB light illumination, steadily increases its resistance with increasing the NO2 concentration as reported in figure 6b. Analyzing also the RR vs. NO2 concentration plot presented in Figure 6d, we can conclude that the GZO20 yields higher RRs, higher sensitivity and better recovery of the base line, identifying it as the best sample for sub-ppm NO2 monitoring at mild temperatures (75°C).


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Figure 6. Dynamic gas responses to NO2 at 75 °C under dark and PB light irradiation of (a) ZnO; (b) GZO10; (c) GZO20. d) Relative response (RR = RG/RA) as a function of NO2 for the different samples measured under dark or under PB light irradiation.

CONCLUSIONS In conclusion, we have demonstrated plasmonic gas sensing using the infrared LSPR peak of degenerately doped semiconductors, specifically Ga-doped ZnO. Zinc oxide nanocrystals doped with varying amounts of gallium were synthetized using colloidal methods and used to deposit thinfilm coatings that have been used for plasmonic and chemoresistive sensing. Aliovalent dopants in ZnO induce the formation of a plasmonic resonance in the near infrared, which analogously to LSPR in metal nanoparticles, is affected by the presence of both reducing and oxidizing gases. We elucidated the role of Ga dopant, oxygen vacancies and overall sensing mechanism combining the optical gas sensing data with electrical measurements. Moreover, by exposing the GZO film to blue light during the sensing tests, we achieved room temperature sensitivity to sub-ppm NO2 concentrations. These very thin GZO films are optically transparent in the visible range and can be used as electrical and optical sensors for the detection of hazardous gases at low operating


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temperatures, paving the way to the fabrication of highly efficient, multifunctional “invisible” sensors. ASSOCIATED CONTENT

Supporting Information. Picture of the colloidal solution is reported in Figure 1S. Absorption spectra of films are reported in Figure 2S and 3S. SEM pictures of film cross-section are reported in Figure 4S. Conductometric gss sensing experiments are reported in figure 5S and 6S. The mean crystallite diameter evaluated from XRD and the EDX compositional analysis are reported in Table S1. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION

Corresponding Authors *Email: [email protected]. *Email: [email protected].

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interests. ACKNOWLEDGMENT EDG thanks RMIT University for the funding provided through the Vice-Chancellor’s Research Fellowships scheme. REFERENCES


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TOC GRAPHICS


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Degenerately Doped Metal Oxide Nanocrystals as Plasmonic and

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