text:
Prof. Dr Oleg Lupan

Vasile Postica

Niklas Wolff

Jun Su

Frédéric Labat

Ilaria Ciofini

Heather Cavers

Prof. Dr Rainer Adelung

Oleksandr Polonskyi

Franz Faupel

Lorenz Kienle

Bruno Viana

Thierry Pauporté
email: thierry.pauporte@chimieparistech.psl.eu

Low-Temperature Solution Synthesis of Au-Modified ZnO Nanowires for

Keywords: Au-modified ZnO NW, gas sensor, nanosensor, hydrogen, electrochemical deposition

In this work, the low temperature one-step electrochemical deposition of arrayed ZnO nanowires (NWs) decorated by Au nanoparticles (NPs) with diameters ranging between 10-100 nm is successfully reported for the first time. The AuNPs and ZnO NWs were grown simultaneously in the same growth solution in consideration of the HAuCl4 concentration. Optical, chemical and structural characterization were performed in detail, showing high crystallinity of the NWs, as well as the distribution of Au NP on the surface of ZnO NWs demonstrated by transmission electron microscopy (TEM). Individual Au NPs-functionalized ZnO NWs (Au-NP/ZnO-NWs) were incorporated into sensor nanodevices using a FIB/SEM system. The gas sensing measurements demonstrated excellent selectivity to hydrogen gas at room temperature with a gas response, Igas/Iair, as high as 7.5 to 100 ppm for Au-NP/ZnO-NWs possessing a AuNP surface coverage of ~ 6.4%. The concentration of HAuCl4 in the growth solution was observed to have no impact on the gas sensing performances. This highlights the significant influence of the total Au/ZnO interfacial area establishing Schottky contacts for the achievement of high performances. The most significant performance of H2 response was observed for gas concentrations higher than 500 ppm of H2 in the environment, which was attributed to surface metallization of ZnO NW during exposure to hydrogen. For this case an ultra-high response of 32.9 and 47 to 1000 and 5000 ppm of H2 was obtained, respectively. Spin-polarized periodic density functional theory calculations were performed on Au/ZnO bulk and surface functionalized models, validating the experimental hypothesis. The combination of H2 gas detection at room temperature, ultra-low power consumption and small size, make these devices ideal candidates for hydrogen gas leakage detection, as well as hydrogen gas monitoring (down to 1 ppm).

INTRODUCTION

During the last decade the synthesis of new nanomaterials for a wide range of applications, such as high-performance gas sensors, nanosensors, biosensors, photocatalysis, electronics, optoelectronics, etc., has been the focus of an intense research from the scientific community. [START_REF] Franklin | Nanomaterials in Transistors: From High-Performance to Thin-Film Applications[END_REF] In this context, zinc oxide micro-and nanostructures are ideal building blocks for high-performance hybrid nanomaterials as they possess superior electrical, optical and sensing properties, and can be synthesized by numerous cost-effective synthesis methods. [START_REF] Mishra | Tetrapod Materials for Functional Applications[END_REF][START_REF] Li | Au-ZnO Hybrid Nanopyramids and Their Photocatalytic Properties[END_REF][START_REF] Özgür | A Comprehensive Review of ZnO Materials and Devices[END_REF][START_REF] Katoch | 2/ZnO Inner/Outer Double-Layer Hollow Fibers for Improved Detection of Reducing Gases[END_REF][START_REF] Katoch | Bifunctional Sensing Mechanism of SnO2-ZnO Composite Nanofibers for Drastically Enhancing the Sensing Behavior in H2 Gas[END_REF][START_REF] Choi | Dual Functional Sensing Mechanism in SnO2-ZnO Core-Shell Nanowires[END_REF] Another essential advantage of ZnO is the possibility to fabricate a wide range of one-dimensional, two-dimensional and threedimensional morphologies in a controlled manner. [START_REF] Mishra | Tetrapod Materials for Functional Applications[END_REF][START_REF] Zhong Lin | Zinc Oxide Nanostructures: Growth, Properties and Applications[END_REF][START_REF] Pearton | Recent Advances in Processing of ZnO[END_REF] A lot of attention has been paid to onedimensional nanostructures, such as nanowires, nanotubes, nanorods and nanobelts, as they are excellent candidates for the fabrication of devices based on individual structures with unique optical and sensing properties. [START_REF] Huang | Room-Temperature Ultraviolet Nanowire Nanolasers[END_REF][START_REF] Li | Electronic Transport Through Individual ZnO Nanowires[END_REF][START_REF] Li | Oxygen Sensing Characteristics of Individual ZnO Nanowire Transistors[END_REF] It was demonstrated that surface reactions (adsorption/desorption of gaseous species) have a large influence on the electrical transport properties in individual nanostructures, [START_REF] Li | Oxygen Sensing Characteristics of Individual ZnO Nanowire Transistors[END_REF] which is advantageous for fabrication of highly sensitive gas nanosensors that are able to operate at room temperature. [START_REF] Kolmakov | Enhanced Gas Sensing by Individual SnO2 Nanowires and Nanobelts Functionalized with Pd Catalyst Particles[END_REF][START_REF] Lupan | Functionalized Pd/ZnO Nanowires for Nanosensors[END_REF] Different methods were used in order to improve the sensitivity and selectivity of these gas nanosensors, such as doping, [START_REF] Lupan | Silver-Doped Zinc Oxide Single Nanowire Multifunctional Nanosensor with a Significant Enhancement in Response[END_REF] surface modification with noble metal nanoparticles [START_REF] Kolmakov | Enhanced Gas Sensing by Individual SnO2 Nanowires and Nanobelts Functionalized with Pd Catalyst Particles[END_REF][START_REF] Lupan | Functionalized Pd/ZnO Nanowires for Nanosensors[END_REF] or with transition metal oxides, [START_REF] Lupan | Enhanced UV and Ethanol Vapour Sensing of a Single 3-D ZnO Tetrapod Alloyed with Fe2O3 Nanoparticles[END_REF][START_REF] Kuang | Enhancing the Photon-and Gas-Sensing Properties of a Single SnO2 Nanowire Based Nanodevice by Nanoparticle Surface Functionalization[END_REF] as well as the design of Schottky contacts. [START_REF] Hu | Fast-Response Nanowire Sensors by Using Schottky Contacts[END_REF] In this context, the surface deposition of Au NPs on the ZnO nanostructures for the formation of Au-NP/ZnO structures is very attractive for high-performance gas sensors and UV photodetectors. [START_REF] Gogurla | Multifunctional Au-ZnO Plasmonic Nanostructures for Enhanced UV Photodetector and Room Temperature NO Sensing Devices[END_REF][START_REF] Guo | High-Performance Gas Sensor Based on ZnO Nanowires Functionalized by Au Nanoparticles[END_REF][START_REF] Liu | Giant Improvement of the Performance of ZnO Nanowire Photodetectors by Au Nanoparticles[END_REF][START_REF] Zheng | Light-Controlled C2H2 Gas Sensing Based on Au-ZnO Nanowires with Plasmon-Enhanced Sensitivity at Room Temperature[END_REF] For example, Liu et al. fabricated UV photodetector based on individual ZnO NW modified with Au NPs that showed increased improvement of the device performance. [START_REF] Liu | Giant Improvement of the Performance of ZnO Nanowire Photodetectors by Au Nanoparticles[END_REF] However, to the best of our knowledge, no data on gas sensing properties of individual ZnO NWs decorated with Au-NPs has been reported in the literature yet. Thus, it is very important to study the gas sensing properties of individual Au-NP/ZnO nanostructures/nanowires, which is the main aim of the research presented in this work.

Herein, we report on a one-step electrochemical functionalization of the surface of ZnO NW arrays with Au-NPs for the fabrication of nanodevices with highly selective gas-sensing applications, where Au-NP-nucleation, growth and deposition was performed in the same process as the ZnO NW growth. Individual ZnO NWs decorated with Au-NPs were integrated into the nanosensor devices using a FIB/SEM system. This approach demonstrated highly selective and sensitive hydrogen gas sensing properties at room temperature.

EXPERIMENTAL SECTION

Synthesis of AuNP/ZnO NWs: The synthesis of AuNP/ZnO-NWs arrays was performed by electrochemical deposition in a classical three-electrode electrochemical cell, [START_REF] Lupan | Silver-Doped Zinc Oxide Single Nanowire Multifunctional Nanosensor with a Significant Enhancement in Response[END_REF][START_REF] Pauporté | Low-Temperature Preparation of Ag-Doped ZnO Nanowire Arrays, DFT Study, and Application to Light-Emitting Diode[END_REF][START_REF] Lupan | Effects of Annealing on Properties of ZnO Thin Films Prepared by Electrochemical Deposition in Chloride Medium[END_REF][START_REF] Lupan | Hydrothermal Treatment for the Marked Structural and Optical Quality Improvement of ZnO Nanowire Arrays Deposited on Lightweight Flexible Substrates[END_REF] using a glass substrate coated with polycrystalline F-doped SnO2 (FTO, 10 Ω/ ) films as a working electrode.

The growth solution was composed of zinc chloride (ZnCl2) at 0.2 mM, a supporting electrolyte (KCl) at 0.1 M and O2 at saturation. [START_REF] Pauporté | Low-Temperature Preparation of Ag-Doped ZnO Nanowire Arrays, DFT Study, and Application to Light-Emitting Diode[END_REF] The concentrations of HAuCl4 (Sigma-Aldrich, >99.9%) in the growth solution was varied from 0.1 μM to 2.0 μM in order to study the influence on the growth of ZnO NWs arrays with Au-NPs. Upon deposition, the substrate was rotated at a constant speed of ω = 300 rotations/min (rpm). [START_REF] Lupan | Silver-Doped Zinc Oxide Single Nanowire Multifunctional Nanosensor with a Significant Enhancement in Response[END_REF][START_REF] Pauporté | Low-Temperature Preparation of Ag-Doped ZnO Nanowire Arrays, DFT Study, and Application to Light-Emitting Diode[END_REF][START_REF] Lupan | Effects of Annealing on Properties of ZnO Thin Films Prepared by Electrochemical Deposition in Chloride Medium[END_REF] The electrodeposition was performed at 85 or 90 ºC at constant applied potential, as indicated further for each sample investigated in this work, using an Autolab PGSTAT30 potentiostat/galvanostat monitored by the GPES AutoLab software. [START_REF] Pauporté | Low-Temperature Preparation of Ag-Doped ZnO Nanowire Arrays, DFT Study, and Application to Light-Emitting Diode[END_REF] More details are presented in previous works. [START_REF] Lupan | Silver-Doped Zinc Oxide Single Nanowire Multifunctional Nanosensor with a Significant Enhancement in Response[END_REF][START_REF] Pauporté | Low-Temperature Preparation of Ag-Doped ZnO Nanowire Arrays, DFT Study, and Application to Light-Emitting Diode[END_REF][START_REF] Lupan | Effects of Annealing on Properties of ZnO Thin Films Prepared by Electrochemical Deposition in Chloride Medium[END_REF][START_REF] Pauporté | Well-Aligned ZnO Nanowire Arrays Prepared by Seed-Layer-Free Electrodeposition and Their Cassie-Wenzel Transition after Hydrophobization[END_REF] Material characterization and computational details: The ZnO nanowires decorated with Au-NPs have been examined with methods of transmission electron microscopy (TEM). The specimens were prepared by scratching the deposit onto a lacey carbon/copper grid and placed into a double tilt TEM holder. Structural analysis was carried out by using a FEI Tecnai F30 G 2 STwin operated at 300 kV, equipped with a field emission gun. Selected area electron diffraction (SAED) patterns were recorded using apertures as small as 250 nm in diameter. The obtained SAED patterns were evaluated with the Digital Micrograph (Gatan, Inc.) analysis software. For the chemical analysis via energy-dispersive X-ray (EDX) spectroscopy, a Si/Li detector (EDAX System) was used. Scanning (S)TEM Z-contrast images were recorded by using a high angle annular dark field (HAADF) detector.

X-ray photoelectron spectroscopy (XPS, Omicron Nano-Technology GmbH) was utilized for surface characterization of deposited Au-NP/ZnO-NW samples in order to reveal the chemical composition of the sample surface. All the spectra were charge referenced after the measurements using the aliphatic carbon peak (285.0 eV) with the help of CasaXPS software.

The nanodevices based on single nanowires were fabricated by procedure developed previously by Lupan et al. [START_REF] Lupan | Silver-Doped Zinc Oxide Single Nanowire Multifunctional Nanosensor with a Significant Enhancement in Response[END_REF][START_REF] Lupan | Fabrication of ZnO Nanorod-Based Hydrogen Gas Nanosensor[END_REF][START_REF] Lupan | Versatile Growth of Freestanding Orthorhombic α-Molybdenum Trioxide Nano-and Microstructures by Rapid Thermal Processing for Gas Nanosensors[END_REF] The gas sensing measurements were performed at room temperature (RT, ~ 25 °C) and under normal ambient air (relative humidity of ~ 25 %), following the procedure reported in previous works. [START_REF] Lupan | Silver-Doped Zinc Oxide Single Nanowire Multifunctional Nanosensor with a Significant Enhancement in Response[END_REF][START_REF] Lupan | Tunable and Ultra-Light Aerographite Sensor for Climate Relevant Gas Monitoring[END_REF] The electrical measurements were continuously monitored and collected using a computer-controlled Keithley 2400 sourcemeter through LabView software (National Instruments) by applying 1 V bias voltage. [START_REF] Lupan | Tunable and Ultra-Light Aerographite Sensor for Climate Relevant Gas Monitoring[END_REF] The gas response (S) was defined as the ratio of the current under exposure to gas (Igas) and current under exposure to ambient air (Iair), i.e. S = Igas/ Iair. The theoretical detection limit was estimated using the method reported in detail by Dua et al. using a signal/(noise) 2 ratio. [START_REF] Dua | All-Organic Vapor Sensor Using Inkjet-Printed Reduced Graphene Oxide[END_REF] Computational details about DFT modeling and choice of the surface models based on experimental evidences are given in Supporting Information (Text S1 and S2, respectively).

RESULTS AND DISCUSSIONS

Growth of samples

Figure S1a shows the first voltammetry cycles recorded on the FTO substrate in the ZnO electrodeposition solutions with various HAuCl4 concentrations: (1) 0.6 µM; (2) 1.0 µM; and (3) 2.0 µM. The bath was composed of zinc chloride at 0.2 mM, a supporting electrolyte (KCl) at 0.1 M and O2 at saturation. [START_REF] Pauporté | Low-Temperature Preparation of Ag-Doped ZnO Nanowire Arrays, DFT Study, and Application to Light-Emitting Diode[END_REF] As can be observed, a small increase in cathodic current appears at above -0.4, -0.3 and -0.22 V for samples with HAuCl4 concentrations of 0.6 µM, 1.0 µM and 2.0 µM, respectively. This reduction peak was also observed and by Argiubi et al. in the case of electrochemical reduction of Au(III) on the surface of screen-printed carbon electrodes. Therefore, it can be attributed to the reduction of Au. [START_REF] Argoubi | Optimized Design of a Nanostructured SPCE-Based Multipurpose Biosensing Platform Formed by Ferrocene-Tethered Electrochemically-Deposited Cauliflower-Shaped Gold Nanoparticles[END_REF] The cathodic wave appears at above -0.8, -0.8 and -0.75 V, respectively, with the further increase of negative-going scan (see Figure S1a), and can be attributed to the reduction of Zn 2+ to the metal Zn. [START_REF] Pauporté | Well-Aligned ZnO Nanowire Arrays Prepared by Seed-Layer-Free Electrodeposition and Their Cassie-Wenzel Transition after Hydrophobization[END_REF] The slight increase in measured current density is observed with increasing in HAuCl4 concentrations, which might indicate that Au-ions act as electrocatalyst for the electrochemical reaction (

- -→ + + OH e O H O 4 4 2 2 2
). [START_REF] Pauporté | Low-Temperature Preparation of Ag-Doped ZnO Nanowire Arrays, DFT Study, and Application to Light-Emitting Diode[END_REF] Based on these curves, the values of constant potential for the deposition of samples were determined (see

Table S1).

The chronoamperometry curves, measured while the FTO substrate electrode was rotated at 300 rpm are shown in Figure S1b. It is clear from this figure that the deposition current density decreases sharply in the initial period, and gradually stabilizes. This can be attributed to the process of nucleation of ZnO NWs and Au NPs. Taking into account the values of current density for pure ZnO NWs, by adding HAuCl4 to the growth solution a dramatic increase of the deposition cathodic current density is obtained, which was also reported for ZnO:Ag NWs [START_REF] Pauporté | Low-Temperature Preparation of Ag-Doped ZnO Nanowire Arrays, DFT Study, and Application to Light-Emitting Diode[END_REF] and Pd/ZnO NWs. [START_REF] Lupan | Functionalized Pd/ZnO Nanowires for Nanosensors[END_REF] Thus, we believe that the Au-NPs decorating the deposited ZnO NWs serve as good electrocatalysts for the electrochemical reactions. The diameters of the NWs vary in the 100 -200 nm range (see Figure S2). With increasing HAuCl4 concentration, essentially no change in the size of the NWs was observed (see Figure 1 and Figure S2). While unmodified ZnO NWs arrays are vertically oriented, [START_REF] Pauporté | Low-Temperature Preparation of Ag-Doped ZnO Nanowire Arrays, DFT Study, and Application to Light-Emitting Diode[END_REF] the Au-NP decorated ZnO NWs are tilted from the vertical direction with increased surface texture (see Figure 1). This effect increases with the HAuCl4 concentration, which indicates more difficultly with the nucleation of the nanowires and that NWs grow from the same point forming flower-like structures, which were also observed for Ag-doped ZnO NW grown by the same method. [START_REF] Pauporté | Low-Temperature Preparation of Ag-Doped ZnO Nanowire Arrays, DFT Study, and Application to Light-Emitting Diode[END_REF] It is important to mention that at higher magnifications, Au-NPs deposited on the surface of the ZnO NWs could be observed (see red circles in Figure S3). These clusters are described in more details, using TEM measurements, in the following. The cathodic electrodeposition of Au NPs from the dissolved precursor can be described by the following equation: 32 (1)

Morphological and structural properties

In the case of samples grown using 2.0 μM of HAuCl4, the deformation of NWs morphology can be observed, as well as a lack of Au-NPs (see Figure 1f and Figure S4). In this case, the applied potential was -0.95 V vs saturated calomel electrode (SCE). The rugged surface structure can be a result of the formation of native defects, such as vacancies, during crystal growth in the solution, which can migrate to the surface leading to indentation of the surface. This effect was also observed for Pd-modified ZnO NWs, [START_REF] Lupan | Functionalized Pd/ZnO Nanowires for Nanosensors[END_REF] as well as for Cl-and Sb-doped ZnO NWs. [START_REF] Liang | Optical and Electrical Properties of Electrodeposited Sb-Doped ZnO Nanorod Arrays[END_REF][START_REF] Wang | Cl-Doped ZnO Nanowires with Metallic Conductivity and Their Application for High-Performance Photoelectrochemical Electrodes[END_REF] However, the samples grown at -0.65 V vs SCE showed no deformation on surface of the NWs while Au-NPs were observed on their surface (see Figure S5). This can be tentatively explained by the fact that the lower applied bias of -0.65 V vs SCE is closer to the kinetic regime of gold deposition (-0.5 V vs SCE in the case of HAuCl4), as was determined using currentpotential curves by Wijnhoven et al. 35 .

XRD /2 investigations were performed to study the influence of HAuCl4 concentration on the crystallinity of the Au-NP/ZnO-NW arrays. Figure 2a shows the XRD data in the 10-90° 2 range with a scanning step of 0.02° for Au-NP/ZnO-NW arrays, grown using different concentrations of HAuCl4 (0.0, 0.3, 1.5 and 2.0 μM). Reflections at 31.79º, 34.44º, 36.28º, 47.65º, 56.63º, 62.88º and 72.54º were assigned to the (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3) and (0 0 4) planes of wurtzite ZnO (PDF card # 36-1451). While reflections noted with red "*" were assigned to SnO2 (i.e. to the FTO substrate) according to PDF card # 01-077-0488. [START_REF] Lupan | Effects of Annealing on Properties of ZnO Thin Films Prepared by Electrochemical Deposition in Chloride Medium[END_REF] To study the effect of doping on the crystallinity of the ZnO NWs, the intensity of the (0 0 2) and (1 0 1) reflections was monitored (see Figure 2b). A slight decrease in intensity of the diffraction peaks (0 0 2) and (1 0 1) was observed corresponding to an increase in HAuCl4 concentration up to 2.0 μM. The ionic radii of Au and Zn are r(Au 3+ ) = 0.085 nm and r(Zn 2+ ) = 0.074 nm, respectively. [START_REF] Sahu | Enhanced UV Absorbance and Photoluminescence Properties of Ultrasound Assisted Synthesized Gold Doped ZnO Nanorods[END_REF] Therefore, the Au incorporation in ZnO lattice should lead to an expansion of lattice, shifting the XRD diffraction peaks. [START_REF] Sahu | Enhanced UV Absorbance and Photoluminescence Properties of Ultrasound Assisted Synthesized Gold Doped ZnO Nanorods[END_REF] However, in our case no shift in the position of the (0 0 2) and (1 0 1) reflections, with the HAuCl4 concentration, was observed (see

Figure 2b

). Moreover, additional low intensity peaks were observed for Au-NP/ZnO-NW arrays at 38.17º and 44.42º, which can be assigned to cubic Au with Fm-3m space group (a = 4.0786 Å, PDF card # 03-065-2870, see Figure 2c). [START_REF] Guo | High-Performance Gas Sensor Based on ZnO Nanowires Functionalized by Au Nanoparticles[END_REF] The weak reflections of Au indicate that only a small amount of gold on the nanometer scale is present. [START_REF] Guo | High-Performance Gas Sensor Based on ZnO Nanowires Functionalized by Au Nanoparticles[END_REF] These reflections increase in intensity with increasing HAuCl4 concentration (see Figure 2c). Therefore, it can be concluded that the Au-NPs are deposited on the surface of ZnO NWs during the process of electrochemical deposition and that likely no significant doping occurs. [START_REF] Lupan | Functionalized Pd/ZnO Nanowires for Nanosensors[END_REF] This could be due to the low solubility of Au in ZnO, [START_REF] Pearton | Recent Advances in Processing of ZnO[END_REF][START_REF] Lee | Possible p-Type Doping with Group-I Elements in ZnO[END_REF] only a few works have reported Au-doped ZnO with evident proof. [START_REF] Sahu | Enhanced UV Absorbance and Photoluminescence Properties of Ultrasound Assisted Synthesized Gold Doped ZnO Nanorods[END_REF][START_REF] Xu | Chemical States of Gold Doped in ZnO Films and Its Effect on Electrical and Optical Properties[END_REF] Figure 3 shows the results of the TEM analysis of the sample grown using 2.0 μM HAuCl4 at -0.65 V vs SCE. Figure 3a shows a STEM HAADF image of ZnO microcrystals decorated with Au nanodots which were observed ranging from 10-100 nm in size. An EDX elemental map (Figure 3b) of the area within the red frame correlates the high Z-contrast positions with X-ray signals denoted to the Au M-shell. The electron diffraction pattern depicted in Figure 3c shows the superposition of structural reflections of the ZnO crystal in [211 ̅̅̅ 0] orientation, and reflection spots originating from the (1 1 1) and (0 0 2) planes of fcc gold, with d = 0.231 nm and d = 0.201 nm, respectively. [START_REF] Morris | Standard X-ray Diffraction Powder Patterns: Section 13-Data for 58 Substances[END_REF] No systematic orientation relationship between the ZnO and the Au lattice could be identified. For the Au nanodots, multiple twinning [START_REF] Johnson | Effects of Elastic Anisotropy on Strain Distributions in Decahedral Gold Nanoparticles[END_REF][START_REF] Seo | Shape Adjustment between Multiply Twinned and Single-Crystalline Polyhedral Gold Nanocrystals: Decahedra, Icosahedra, and Truncated Tetrahedra[END_REF] as well as an amorphous shell coating the nanodot could be evidenced by high-resolution imaging.

EDX and XPS studies of the AuNP/ ZnO NWs

Analysis by energy dispersive X-ray spectroscopy (EDX) on a Zeiss SEM has been performed to give a rough estimate about the molar Au content in the nanowire arrays prepared in the presence of increasing HAuCl4 concentration. The results are presented in Table S1. The molar ratio between gold and zinc in the Au-NP/ZnO-NWs increased with the HAuCl4 concentration in the bath and was comparable with the ratio present in the deposition bath. Two samples of Au-NP/ZnO-NW arrays, grown using the addition of 0.6 and 0.9 µM of HAuCl4 to the ZnO NW growth solution, were analyzed by using XPS. Figure 4a shows a survey spectrum of a typical Au-NP/ZnO-NW array grown using a solution containing 0.6 µM of HAuCl4. The following elements were detected: C, O, Zn and Au and the relative atomic concentrations are shown below the spectrum. Generally, the deposition process does not contain carbon and so carbon is not expected to be present in the films. However, since the samples were exposed to the open air and no surface cleaning was performed prior XPS characterization, the carbon observed can be attributed to adventitious carbon resulting from exposure to the ambient atmosphere. [START_REF] Greczynski | C 1s Peak of Adventitious Carbon Aligns to the Vacuum Level: Dire Consequences for Material's Bonding Assignment by Photoelectron Spectroscopy[END_REF] The presence of the adventitious carbon (285.0 eV) allowed us to use it as a reference for energy calibration. Detailed deconvolution of C-1s spectrum (Figure 4b) shows that in addition to the adventitious carbon there are also two peaks present at 286.7 eV and 289.4 eV, corresponding to CO and CO2, respectively. [START_REF] Greczynski | C 1s Peak of Adventitious Carbon Aligns to the Vacuum Level: Dire Consequences for Material's Bonding Assignment by Photoelectron Spectroscopy[END_REF] Au-NP/ZnO-NW arrays grown using solutions with 0.6 and 0.9 µM of HAuCl4 contain around 0.4 at% and 0.6 at% of Au, respectively, as revealed by XPS analysis. This is slightly lower in comparison to the data measured using EDX (see Table S1). High resolution XPS spectra of Au-4d are shown in Figure 4c. The peaks observed at 352.26 and 334.72 eV can be attributed to metallic gold and are in a good agreement with the XPS data reported in the literature. [START_REF] Tian | Mechanisms and Applications of Plasmon-Induced Charge Separation at TiO2 Films Loaded with Gold Nanoparticles[END_REF][START_REF] Moulder | Handbook of X-ray Photoelectron Spectroscopy: A Reference Book of Standard Spectra for Identification and Interpretation of XPS Data[END_REF] As will be discussed later, samples containing different concentration of gold demonstrate different gas sensing behavior. In Figure 4d, the high resolution spectrum of O-1s core level is depicted. To get more insight into the chemical composition of the sample surface, the high resolution O-1s spectrum was de-convoluted using several components, following the proposed structure suggested in Ref. [START_REF] Lupan | Synthesis and Characterization of Ag-or Sb-Doped ZnO Nanorods by a Facile Hydrothermal Route[END_REF] . The resulted fitting is depicted in Figure 4d, and it contains the following three main components: ZnO (530.4 eV), defective ZnOx or Zn(OH)2 (531.5 eV) and OH groups or adsorbed oxygen (533.2 eV). [START_REF] Lupan | Functionalized Pd/ZnO Nanowires for Nanosensors[END_REF][START_REF] Lupan | Synthesis and Characterization of Ag-or Sb-Doped ZnO Nanorods by a Facile Hydrothermal Route[END_REF] Additionally, two smaller peaks located at 531.0 eV and 532.5 eV were attributed to adventitious CO and CO2, respectively. [START_REF] Lupan | Synthesis and Characterization of Ag-or Sb-Doped ZnO Nanorods by a Facile Hydrothermal Route[END_REF][START_REF] Armelao | Sol-Gel Synthesis and Characterisation of ZnO-Based Nanosystems[END_REF]

Optical properties of the AuNP/ZnO NWs

It was demonstrated that defects in ZnO can greatly affect the gas sensing properties. [START_REF] Ahn | Gas Sensing Properties of Defect-Controlled ZnO-Nanowire Gas Sensor[END_REF][START_REF] Li | Enhanced Sensing Properties of Defect-Controlled ZnO Nanotetrapods Arising from Aluminum Doping[END_REF] In this case, the measurements of optical properties can provide useful information related to defects in ZnO nanowires. Figure 5a shows UV-visible transmission spectra of the AuNPmodified ZnO NWs arrays grown using different concentrations of HAuCl4 in the growth solution.

While unmodified ZnO NW arrays are highly transparent in the near-IR region (close to 100%), by adding the HAuCl4 to the growth solution a decrease in transparency for the entire UV-visible spectrum can be observed. No shift of the band-edge, corresponding to an increase of HAuCl4 concentration, was observed. The optical band gap (Eg) was obtained from the intercept of (αhν) [START_REF] Mishra | Tetrapod Materials for Functional Applications[END_REF] vs. photon energy (hν) (see Figure 5b). It can be observed that no changes in value of Eg (~ 3.26 eV) are induced by an increase in the HAuCl4 concentration. Therefore, no evidence of doping was seen with these experiments.

The presence of Au-NPs in the Au-NP/ZnO-NW arrays was further demonstrated by the UV-vis absorption spectra. Figure 5c shows the absorbance spectra of Au-NP/ZnO-NW arrays grown using different concentrations of HAuCl4 in the growth solution. The gradual increase in the peak intensity at ~ 540 nm with higher concentration of HAuCl4 is observed (see Figure 5c), which is attributed to the surface plasmon resonance of Au NPs, [START_REF] Li | Au-ZnO Hybrid Nanopyramids and Their Photocatalytic Properties[END_REF][START_REF] Wang | Synthesis and Characterization of Water-Soluble and Bifunctional ZnO-Au Nanocomposites[END_REF] demonstrating the successful surface modification of ZnO NWs with Au NPs.

The photoluminescence (PL) spectra measured at room temperature for Au-NP/ZnO-NW arrays, grown using different concentrations of HAuCl4 in the growth solution, is presented in Figure 5d. No visible emission due to bulk and surface defects is observed, either for the unmodified ZnO NWs or Au-NP/ZnO-NWs, demonstrating that the synthesized NWs possess high structural quality, even with a deposition temperature of only 90 °C. [START_REF] Pauporté | Low-Temperature Preparation of Ag-Doped ZnO Nanowire Arrays, DFT Study, and Application to Light-Emitting Diode[END_REF] No shift of the near-bandedge (NBE) emission peak was observed, corresponding to an increase in HAuCl4 concentration (see Figure 5d). However, a gradual increase in intensity of NBE emission with increase in HAuCl4 concentration was observed. The emission is enhanced by Au as the surface is higher, despite stronger reabsorption at the wavelengths of the ZnO emission. As the absorbance increases with the HAuCl4 concentration as seen in Figure 5c, this unexpected enhancement of the UV emission may be attributed to the strong interfacial coupling between the metal Au NPs and the semiconductor ZnO NWs as observed in Ref. [START_REF] Wang | Synthesis and Characterization of Water-Soluble and Bifunctional ZnO-Au Nanocomposites[END_REF] , where Au surface electrons are transferred to the ZnO nanowires leading to an increase in electron density in the conduction band of ZnO resulting in an increased recombination rate. [START_REF] Wang | Synthesis and Characterization of Water-Soluble and Bifunctional ZnO-Au Nanocomposites[END_REF]

Theoretical insights on the structural and electronic modifications due to doping

From the above discussions, Au-doping in bulk of ZnO NW was not evidenced, however, surface modification of Au-NP/ZnO-NWs was highlighted. To shed light on possible Au/ZnO structures involved, we present data obtained on models of Au-doped bulk ZnO wurtzite and adsorption of Au clusters of different sizes on the clean (undoped) ZnO (10-10) surface, as an example of surface functionalization. As previously performed for the bulk case of Ag [START_REF] Pauporté | Low-Temperature Preparation of Ag-Doped ZnO Nanowire Arrays, DFT Study, and Application to Light-Emitting Diode[END_REF] and Pd, [START_REF] Lupan | Functionalized Pd/ZnO Nanowires for Nanosensors[END_REF] calculations have also been performed on: (i) pure ZnO, (ii) doped ZnO with inserted Au (Aui) at a 1.85 at% doping amount, to gain insight on the effect of the Au insertion into the wurtzite bulk structure on the electronic properties. The unit cell of the inserted Au:ZnO system is shown in S2.

Computed shortest metal to O distances (d1 to d6 in Table S2) indicate that when Au is inserted in the ZnO lattice, a highly distorted octahedron around Au can be evidenced, with distances ranging between 2.37 and 3.94 Å. The a and b wurtzite lattice vectors are slightly increased (+0.4%), while the c lattice vector is decreased of about -1.6%. From the data of Figure S7 and Table S2, it is clear that the ZnO band gap decreases upon insertion of Au, by at least 1.1 eV. Gold mainly contributes through 5d orbitals at the top of the valence band (VB) and at the bottom of the conduction band (CB), with bands possessing very low dispersion, indicating an almost atomic-like character. The lack of a shallow level at the bottom of the CB indicates that no p-type ZnO is obtained however, as already outlined for Pd [START_REF] Lupan | Functionalized Pd/ZnO Nanowires for Nanosensors[END_REF] or Ag [START_REF] Pauporté | Low-Temperature Preparation of Ag-Doped ZnO Nanowire Arrays, DFT Study, and Application to Light-Emitting Diode[END_REF] doping.

Adsorption of small Aun clusters (n ranging from 1 to 8) on a bulk ZnO (10-10) surface [START_REF] Kuang | Enhancing the Photon-and Gas-Sensing Properties of a Single SnO2 Nanowire Based Nanodevice by Nanoparticle Surface Functionalization[END_REF][START_REF] Dua | All-Organic Vapor Sensor Using Inkjet-Printed Reduced Graphene Oxide[END_REF] has also been considered (see Figure 6). Comments on the functionalized surface models can be found in Supporting Information Text S2.

From the data collected in Table S3, Supporting Information, the negative adsorption energies obtained indicate the stable adsorption of the Aun clusters in all the cases investigated, with Au6 showing the largest adsorption energy, probably due to the favorable bonding between adjacent clusters from one periodic cell to the other. In particular, while Au atoms of small clusters tend to occupy bridge sites between Zn and O surface atoms, in larger clusters, Au atoms tend to occupy hollow sites, with a clear hcp motif obtained for the Au7 and Au8 clusters. In addition, larger clusters tend to spread on the ZnO substrate, as evidenced by the dAu-Au distances reported in Table S3, which increase from the smallest to the largest clusters. Both bonding to surface Zn and O atoms can be evidenced, the shortest distances involving surface O atoms. A charge transfer from the Aun cluster to the ZnO substrate can also been evidenced, the Aun cluster being positively charged, with a charge value close to +1, as already highlighted in the case of the adsorption of Au9/TiO2 (001), [START_REF] Jiang | Density Functional Theory Study on the Metal-Support Interaction Between a Au9 Cluster and an Anatase TiO2(001) Surface[END_REF] for instance. Computed band gaps globally decrease upon increasing the size of the adsorbed Aun cluster, and all reported band gaps are lower than that of the clean ZnO (10-10) surface (3.88 eV). From Figure S8, Supporting Information, weakly-interacting Au5d levels contribute mainly to the band gap decrease commented above for the functionalized surface. Shallow levels can be observed in all cases, both at the top of the VB and at the bottom of the CB.

Together with the charge transfer mentioned above, this clearly outlines that the NP size strongly influences the ZnO electronic structure, with very different reactivity of the functionalized ZnO substrate possible, depending on the size of the chosen NP.

Gas nanosensors based on individual Au-NP/ZnO-NWs

The nanosensor devices based on single Au-NP/ZnO-NWs were fabricated following the procedures reported by Lupan et al. [START_REF] Lupan | Silver-Doped Zinc Oxide Single Nanowire Multifunctional Nanosensor with a Significant Enhancement in Response[END_REF][START_REF] Lupan | Individual Hollow and Mesoporous Aero-Graphitic Microtube Based Devices for Gas Sensing Applications[END_REF] The Au-NP/ZnO-NWs were removed from the NW arrays grown on the original substrate via sonication in ethanol. In order to obtain a lower density of NWs on the chip for further integration into device the transfer to an intermediate SiO2 (300 nm)-coated Si substrate (SiO2/Si) was used. [START_REF] Lupan | Functionalized Pd/ZnO Nanowires for Nanosensors[END_REF] Next, the dispersion was diluted further to decrease the density of NWs on the substrate and for further transfer on special designed chip with pre-patterned Au/Cr electrical pads using a direct contact technique and a gentle rubbing few times in order to obtain a low density and good distribution of NWs on the chip. Finally, the NWs were connected to the Au/Cr electrical pads using Pt complex in FIB/SEM by maskless localized patterning feature, forming the Au/Pt/ZnO/Pt/Au structure. The schematic illustration of the device connection is presented in Figure S9. Figure 7a shows the typical image of the device fabricated from an individual Au-NP/ZnO-NW. The Au-NPs can be observed on the ZnO NW surface (see inset from on the formation of double-Schottky contacts which was observed for all samples. The formation of double-Schottky contacts is due to higher work function of Pt (ϕ = 6.1 eV) contacts compared to electron affinity of ZnO NW (Ea = 4.2 eV). [START_REF] Lupan | Functionalized Pd/ZnO Nanowires for Nanosensors[END_REF][START_REF] Lupan | Silver-Doped Zinc Oxide Single Nanowire Multifunctional Nanosensor with a Significant Enhancement in Response[END_REF] The asymmetry of I -V curves can be a result of the different contact barriers, different area of contacts and barrier height. [START_REF] Li | Electronic Transport Through Individual ZnO Nanowires[END_REF][START_REF] Li | Oxygen Sensing Characteristics of Individual ZnO Nanowire Transistors[END_REF][START_REF] Hu | Fast-Response Nanowire Sensors by Using Schottky Contacts[END_REF] Figure S10a shows the gas response to 100 ppm of H2 gas for individual ZnO NWs grown using different concentrations of HAuCl4. It is necessary to mention that no Au NPs were observed on the surface of these NWs. Also, for these ZnO NWs the diameter is 120 ± 10 nm in order to investigate only the potential influence of NW growth in different concentrations of HAuCl4. The results from Figure S10a demonstrate that the concentration of HAuCl4 does not significantly influence the gas response of individual ZnO NWs. This can be attributed to the fact that the ZnO NWs are not doped with Au, nor show large amounts of structural defects. Therefore we conclude that the main parameter that is important for the improvement of the gas response is the number of Au NPs on the surface of individual NWs, i.e. the surface ratio of nanoscale Schottky contacts to the pristine ZnO surface.

To achieve a considerable increase in gas response by surface-modification of the ZnO NWs with Au NPs, a gas sensing mechanism is proposed, which is attributed to both the electronic and chemical sensitization. [START_REF] Yamazoe | New Approaches for Improving Semiconductor Gas Sensors[END_REF] Figure 8 shows an illustration of the proposed gas sensing mechanism of devices based on individual Au NP-modified ZnO NWs. Under exposure in ambient air at room temperature, the oxygen species are adsorbed on the surface of the ZnO nanowires by capturing free electrons from the conduction band: [START_REF] Kolmakov | Enhanced Gas Sensing by Individual SnO2 Nanowires and Nanobelts Functionalized with Pd Catalyst Particles[END_REF][START_REF] Lupan | Silver-Doped Zinc Oxide Single Nanowire Multifunctional Nanosensor with a Significant Enhancement in Response[END_REF][START_REF] Yamazoe | New Approaches for Improving Semiconductor Gas Sensors[END_REF] O2 + e -→ O2 -(ad) (2) This reduction results in the formation of a region with higher electrical resistivity than the conductive core at the surface of the ZnO NW (see Figure 8a). [START_REF] Kolmakov | Enhanced Gas Sensing by Individual SnO2 Nanowires and Nanobelts Functionalized with Pd Catalyst Particles[END_REF][START_REF] Lupan | Silver-Doped Zinc Oxide Single Nanowire Multifunctional Nanosensor with a Significant Enhancement in Response[END_REF] The width of the electron depletion region was noted with Lair, while the diameter of conduction channel was noted as dair (see Figure 8a). Due to operation at room temperature the chemical sensitization effect, described above, is considerably reduced, i.e. adsorption of more oxygen species occurs via the "spillover effect", which was observed at high operating temperatures (thermally activated event, > 200 ºC). [START_REF] Chang | Outstanding H2 Sensing Performance of Pd Nanoparticle-Decorated ZnO Nanorod Arrays and the Temperature-Dependent Sensing Mechanisms[END_REF] In the case of room temperature conditions, the electronic sensitization is a dominant factor, which can greatly influence the electrical transport through the conduction channel due to formation of nanoscale Schottky barriers at the Au/ZnO interface (see Figure 8a). [START_REF] Kolmakov | Enhanced Gas Sensing by Individual SnO2 Nanowires and Nanobelts Functionalized with Pd Catalyst Particles[END_REF][START_REF] Gogurla | Multifunctional Au-ZnO Plasmonic Nanostructures for Enhanced UV Photodetector and Room Temperature NO Sensing Devices[END_REF][START_REF] Zheng | Light-Controlled C2H2 Gas Sensing Based on Au-ZnO Nanowires with Plasmon-Enhanced Sensitivity at Room Temperature[END_REF][START_REF] Yamazoe | New Approaches for Improving Semiconductor Gas Sensors[END_REF] As a result, the formation of Schottky barriers lead to an essential narrowing of the conduction channel (dgas) and a widening of the electron depletion region (Lgas, see Figure 8a).

Under exposure to H2 gas, the hydrogen molecules are oxidized by the adsorbed oxygen species as follow: 15,54

- - + → + e O H ad O H 2 1 2 2 2 1 2 ) ( (3)
The released electrons will contribute to the narrowing of the electron depletion region, or a widening of the conduction channel (see Figure 8b). Due to the exothermic nature of the reaction (1.8 kcal mol -1 ), the water molecules desorbs quickly from the surface. [START_REF] Lupan | Highly Sensitive and Selective Hydrogen Single-Nanowire Nanosensor[END_REF] In the case of Au-NP/ZnO-NWs, the modulation of the conduction channel diameter will be larger, which will lead to greater change in current flowing through the conduction channel, resulting in a higher gas response (see Figure 8b). [START_REF] Kolmakov | Enhanced Gas Sensing by Individual SnO2 Nanowires and Nanobelts Functionalized with Pd Catalyst Particles[END_REF][START_REF] Lupan | Silver-Doped Zinc Oxide Single Nanowire Multifunctional Nanosensor with a Significant Enhancement in Response[END_REF] Therefore, the formation of nanoscale Schottky barriers is an efficient way to highly improve the sensing properties of individual nanostructures such as nanowires. In this respect, works on AuNP/ZnO nanorod structures by Shingange et. al. [START_REF] Shingange | Highly Selective NH3 Gas Sensor Based on Au Loaded ZnO Nanostructures Prepared Using Microwave-Assisted Method[END_REF] and Hosseini et.al. [START_REF] Hosseini | Sensitive and Selective Room Temperature H2S Gas Sensor based on Au Sensitized Vertical ZnO Nanorods with Flower-Like Structures[END_REF] demonstrate that a narrow size distribution of Au-NPs with diameter between 5-15 nm and a uniform surface coverage yields the highest response at room temperature to H2 or H2S gas.

Figure 9a shows the behavior of the measured gas response to 100 ppm of H2 gas for individual Au-NP/ZnO-NWs, integrated in nanosensors, versus ratio of the Schottky contact area formed by Au NPs on the ZnO to the uncovered ZnO surface. This ratio is further called the "Au NPs surface coverage" and is determined from SEM images. The coverage was calculated by (SNP • 100%)/SNW, where SNP is the Schottky contact area formed by Au-NPs (calculated as πr 2 , with r the radius of the Au-NP approximated as half-spheres) on the surface of ZnO NW and SNW is the surface of ZnO NWs (calculated as 2πrh, where r and h are the radius and length of NW).

The gas response was measured at room temperature under 1 V applied bias voltage, in order to reduce the self-heating effect. [START_REF] Lupan | Silver-Doped Zinc Oxide Single Nanowire Multifunctional Nanosensor with a Significant Enhancement in Response[END_REF] All chosen NWs possess diameters of 120 ± 10 nm, in order to investigate only the influence of Schottky contact area. As expected, the gas response to hydrogen gas increased with the percentage of the Au covering surface. The number of Au-NPs follows the same tendency as the gas response demonstrating that the surface coverage of the Au-NPs is an essential factor in order to achieve a higher gas response. Further, the H2 gas response for individual Au-NP/ZnO-NWs with ~ 0.0, ~ 1.1, ~ 2.2, ~ 2.4, ~ 4.0 and ~ 6.4% Au-NPs coverage is 1.2, 2.4, 4.2, 6.8 and 7.5, respectively. The gas response at room temperature to different vapors of volatile organic compounds (VOCs, concentration of 100 ppm) is presented in Figure S10b and S10c. As shown, the gas response to VOC vapors such as ethanol, acetone, 2-propanol, n-butanol and methanol is lower than 1.1, demonstrating high selectivity of Au-NP/ZnO-NWs to hydrogen gas at room temperature. This high selectivity of H2 is also characteristic for unmodified ZnO NWs, [START_REF] Lupan | Silver-Doped Zinc Oxide Single Nanowire Multifunctional Nanosensor with a Significant Enhancement in Response[END_REF][START_REF] Lupan | Highly Sensitive and Selective Hydrogen Single-Nanowire Nanosensor[END_REF] therefore, no change in selectivity of individual NWs was observed with addition of Au-NPs, but a considerable increase in gas response was induced (by factor of 6.25). To conclude, it is believed that an even larger increase in the gas response to hydrogen gas could be achieved by larger surface coverage density of smaller Au-NPs and a narrower size distribution, regarding this and previous studies.

Figure 9b shows the gas response of one individual Au-NP/ZnO-NW (0.9 μM HAuCl4 and ~ 6.4% Au-NPs coverage) to different concentrations of H2 gas (from 20 to 500 ppm). The gas response to 20, 50, 100 and 200 ppm is 2.25, 3.7, 7.5 and 10.6, respectively. Under exposure to relatively higher concentrations of 500 ppm a rapid increase in the current value can be observed. A second short pulse of 500 ppm of H2 gas was applied and the same effect observed (see Figure 9b). Figure 9c shows the gas response to 1000 ppm and 5000 ppm of H2 gas, which is ~ 329 and ~ 470, respectively. As can be observed from Figure 9b the stabilization of current during exposure to hydrogen gas is slow, as well as recovery, resulting in a "saw-tooth"-like response which indicates on slow saturation rate and can be attributed to the charge accumulation and slow recovery rate of NW which operates at room temperature. [START_REF] Chan | Highly Sensitive Gas Sensor by the LaAlO3/SrTiO3 Heterostructure with Pd Nanoparticle Surface Modulation[END_REF] The increase and further decrease in current after introducing in the test chamber of the higher concentrations of hydrogen gas (> 500 ppm, see Figure 9c) can be attributed to the competition of two main mechanism involved in the gas sensing process, namely the surface metallization (which will be further discussed in detail) and the adsorption/desorption of the gas species on the surface of sensor. [START_REF] Chan | Highly Sensitive Gas Sensor by the LaAlO3/SrTiO3 Heterostructure with Pd Nanoparticle Surface Modulation[END_REF] Therefore, the individual Au-NPs/ZnO NW needs time to get a stabilized response after exposure to relatively low or high concentrations of hydrogen gas.

The dependence of the gas response to the concentration of H2 gas is presented in Figure 9d. The error bars represent the standard deviation between the three measurements in the gas sensing tests of the same sensors. In the gas concentration region of 20 -200 ppm, the gas response is according to a power law relationship:

 2 H p S 
, where β = 0.72 is the slope of dependence curve [START_REF] Volanti | The Role of Hierarchical Morphologies in the Superior Gas Sensing Performance of CuO-Based Chemiresistors[END_REF][START_REF] Choi | H2 and C2H5OH Sensing Characteristics of Mesoporous p-Type CuO Films Prepared via a Novel Precursor-Based Ink Solution Route[END_REF] . In the case of gas concentrations higher than 500 ppm, a considerable increase in response can be observed. The theoretical detection limit of ~ 1 ppm was estimated using the signal/(noise) [START_REF] Mishra | Tetrapod Materials for Functional Applications[END_REF] ratio, as was reported by Dua et al. 30 Such a sharp increase in the response for gas concentrations higher than 500 ppm (pink region) can be explained using the semiconductor-to-metal transition of ZnO NW surface, which occurs in the presence of H2 gas molecules. [START_REF] Katoch | Highly Sensitive and Selective H2 Sensing by ZnO Nanofibers and the Underlying Sensing Mechanism[END_REF] The same effect was also observed for ultra-thin CuO NWs, [START_REF] Lupan | Influence of CuO Nanostructures Morphology on Hydrogen Gas Sensing Performances[END_REF] ZnO nanofibers, [START_REF] Kim | Design of Supersensitive and Selective ZnO-Nanofiber-Based Sensors for H2 Gas Sensing by Electron-Beam Irradiation[END_REF][START_REF] Abideen | An Ultra-Sensitive Hydrogen Gas Sensor Using Reduced Graphene Oxide-Loaded ZnO Nanofibers[END_REF] and ZnO-SnO2 composite nanofibers. [START_REF] Katoch | Grain-Size-Tuned Highly H2-Selective Chemiresistive Sensors Based on ZnO-SnO2 Composite Nanofibers[END_REF] In the presence of H2 the surface of ZnO will take on a metallic character. This is due to the fact that the 4s orbital of Zn in

ZnO is occupied by one electron (4s 1 ) and when hydrogen gas adsorbs on the surface this orbital becomes fully occupied (4s 2 ), causing a metallization of the ZnO on the surface. [START_REF] Katoch | Highly Sensitive and Selective H2 Sensing by ZnO Nanofibers and the Underlying Sensing Mechanism[END_REF][START_REF] Xu | Hydrogen and Oxygen Adsorption on ZnO Nanowires: A First-Principles Study[END_REF][START_REF] Wang | Hydrogen Induced Metallicity on the ZnO {10-10} Surface[END_REF] As shown in the results, in the presence of H2 gas the hydrogen molecules will adsorb on the surface of the ZnO NW (H2(ad) → 2H + + 2e -), and the surface depletion region of ZnO (resulting from the adsorption of oxygen molecules) will be narrowed and a thin metallic Zn (ϕZn = 4.3 eV) layer will be formed (Zn (4s [START_REF] Franklin | Nanomaterials in Transistors: From High-Performance to Thin-Film Applications[END_REF] ) + e -→ Zn (4s 2 ), see Figure 10a and10b). [START_REF] Katoch | Highly Sensitive and Selective H2 Sensing by ZnO Nanofibers and the Underlying Sensing Mechanism[END_REF] This process will drastically lower the resistance between the Pt contacts, leading to considerable increase in current of the device (which was observed in Figure 8b). When the H2 gas is evacuated from the test chamber, the metallic Zn layer will be converted back to ZnO (

) ( ) ( ) ( 2 2 1 s s g ZnO Zn O → +
), returning the current value to its original baseline (see Figure 10c). [START_REF] Katoch | Highly Sensitive and Selective H2 Sensing by ZnO Nanofibers and the Underlying Sensing Mechanism[END_REF] However, it is necessary to mention that surface metallization occur at relatively high temperatures, due to necessity of high energy for reduction of metal oxides. [START_REF] Kim | Design of Supersensitive and Selective ZnO-Nanofiber-Based Sensors for H2 Gas Sensing by Electron-Beam Irradiation[END_REF][START_REF] Abideen | An Ultra-Sensitive Hydrogen Gas Sensor Using Reduced Graphene Oxide-Loaded ZnO Nanofibers[END_REF][START_REF] Katoch | Grain-Size-Tuned Highly H2-Selective Chemiresistive Sensors Based on ZnO-SnO2 Composite Nanofibers[END_REF] In our case, the gas sensing is performed at room temperature by applying a bias voltage of 1 V, in order to reduce the self-heating effect. However, the Joule heating of individual Au-NPs/ZnO NW integrated into device is inevitable due to a high increase in current of device at exposure to high concentrations of hydrogen gas (see Figure 9b). Thus, we believe that the locally generated high temperature at exposure to high concentrations of hydrogen gas is responsible for surface metallization of NW. [START_REF] Prades | Ultralow Power Consumption Gas Sensors Based on Self-Heated Individual Nanowires[END_REF] Table S4 lists the results on H2 gas sensing of individual structures of metal oxides and other materials from literature in order to compare with our results. As can be observed, our results are superior to many of the reported data. Another important factor is fast response and recovery (~ 10 s) at exposure to high concentrations of H2 gas (5000 ppm), which is attributed to the semiconductor-to-metal surface transition process under H2 and leads to enhanced hydrogen response at concentrations higher than 500 ppm. This allows for its use in the fabrication of rapid detectors for hydrogen gas leakages. Due to its ease of production and its potential applications, the use of liquid and gaseous hydrogen has been developed in various fields such as hydrogenation processes, petroleum transformation, soldering, cryogenic cooling, or chemical substances production. [START_REF] Bévenot | Hydrogen Leak Detection Using an Optical Fibre Sensor for Aerospace Applications[END_REF] Under normal atmospheric conditions, gaseous hydrogen leakage in the air can lead to an explosive atmosphere easily ignited for hydrogen concentrations between 4% (lower explosive limit (LEL)) and 74.5% (upper explosive limit UEL)at room temperature and pressure. [START_REF] Bévenot | Hydrogen Leak Detection Using an Optical Fibre Sensor for Aerospace Applications[END_REF][START_REF] Okazaki | Sensing Characteristics of an Optical Fiber Sensor for Hydrogen Leak[END_REF] Thus, the fast detection of hydrogen leakage is very important for safety purposes, and is one of the major safety concerns in different fields due to the small ignition energy of only 0.02 mJ, and large velocity of flame propagation. [START_REF] Sumida | Distributed Hydrogen Determination with Fiber-Optic Sensor[END_REF]

CONCLUSIONS

In this work, the one-step electrochemical deposition of AuNP-modified ZnO NWs arrays is reported. The optical, chemical and structural investigations were performed in detail demonstrating high crystallinity of the NWs, and that doping of the NWs with Au does not occur.

The growth of Au NPs, with diameters varying from 10 to >100 nm on the surface of ZnO NWs was observed with TEM. Individual Au-NP/ZnO-NWs were integrated into sensor devices showing excellent hydrogen gas sensing properties, i.e. high gas response of ~ 7.5 to 100 ppm at room temperature and excellent selectivity against selected alcohols and ketones. It was observed that higher gas response can be achieved by using the individual Au-NP/ZnO-NWs with a higher coverage of Au NPs on the surface (in the case of NPs the gas response is ~ 7.5). At high concentrations of H2 gas, a considerable increase in gas response was observed, which could be due to surface metallization of the ZnO NW, resulting in ultra-high responses of 329 and 470 for

Figure 1

 1 Figure1shows SEM images of Au-NP/ZnO-NW arrays grown using different

Figure

  Figure S6 and corresponding band structures are shown in Figure S7, along with obtained data in

Figure 7a ).

 7a Figure 7a). The typical currentvoltage (I -V) characteristic of device based on an individual

Figure 1 .

 1 Figure 1. SEM images of Au-modified ZnO NWs grown using different concentrations of HAuCl4

Figure 2 .

 2 Figure 2. (a) XRD patterns of the Au-modified ZnO NWs arrays grown using different

Figure 3 .

 3 Figure 3. Structural and chemical analysis with TEM. (a) STEM dark field image of ZnO nano-

Figure 4 .

 4 Figure 4. XPS spectra of Au-NP/ZnO-NW arrays grown using different concentrations of

Figure 5 .

 5 Figure 5. (a) Transmission spectra; (b) plot of (αhν) 2 vs. photon energy (hν); (c) absorbance

Figure 6 .

 6 Figure 6. Side and top views of the unit cells of pure ZnO (10-10) surface with 8 layers, and of

Figure 7 .

 7 Figure 7. (a) Typical SEM image of device based on individual Au-NP/ZnO-NW. In inset is

Figure 8 .

 8 Figure 8. Tentatively proposed gas sensing mechanism for individual ZnO and Au-NP/ZnO-NW:

Figure 9 .

 9 Figure 9. (a) Measured gas response versus the Au NPs surface coverage of individual Au-

Figure 10 .

 10 Figure 10. Tentatively proposed gas sensing mechanism involving surface metallization for

ACKNOWLEDGMENTS

Dr. Lupan gratefully acknowledges PSL University, Chimie-ParisTech for invited professor position and CNRS Council for support as expert scientist at IRCP Chimie ParisTech, Paris. This research was sponsored partially by the German Research Foundation (DFG-Deutsche Forschungsgemeinschaft) under the schemes SFB1261, FOR2093 and by SFB859. Dr. Lupan acknowledges the Alexander von Humboldt Foundation for the research fellowship for experienced researchers 3-3MOL/1148833 STP at the Institute for Materials Science, Kiel University, Germany. This research was partly supported by the STCU within the Grant 6229. Dr. Ilaria Ciofini, Dr. Frédéric Labat and Jun Su would like to acknowledge the HPC resources of TGCC made available by GENCI (Grand Equipement National de Calcul Intensif) through project A0050810135. Jun Su gratefully acknowledges the China Scholarship Council (Grant number 201706150081) for PhD funding.

ZnO (10-10) surface and the different Aun/ZnO (10-10) systems considered. Summary of the hydrogen gas sensors based on individual structures of metal oxides and other materials.The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org

■ AUTHOR INFORMATION

Corresponding Authors

*E-mails: ollu@tf.uni-kiel.de oleg.lupan@mib.utm.md ra@tf.uni-kiel.de thierry.pauporte@chimieparistech.psl.eu

Notes

The authors declare no competing interest.


domain: chim phys

text:
Oleg Lupan

Vasile Postica

Dr Thierry Pauporté
email: thierry.pauporte@chimieparistech.psl.eu

Bruno Viana

Maik-Ivo Terasa

Rainer Adelung

Dr Eng Lupan

Keywords: Au-modified ZnO NW, gas sensor, nanosensor, hydrogen, individual nanowire Au-modified ZnO NW, gas sensor, nanosensor, hydrogen, individual nanowire

Room temperature gas nanosensors based on individual

Introduction

The demand for new room-temperature nanosensors has come from different fields and requirements, such as rapid detection of explosive and hazardous gases, compounds and various risky nanomaterials. Due to their high surface-to-volume ratio and crystallinity, the onedimensional (1-D) semiconducting oxide nanostructures, such as nanowires, nanobelts, nanofibers and nanotubes, have attracted great interest toward their integration in high performance sensing devices and nanodevices [START_REF] Cao | Engineering Light Absorption in Semiconductor Nanowire Devices[END_REF][START_REF] Hu | Fast-Response Nanowire Sensors by Using Schottky Contacts[END_REF][START_REF] Kolmakov | Enhanced Gas Sensing by Individual SnO2 Nanowires and Nanobelts Functionalized with Pd Catalyst Particles[END_REF][START_REF] Kolmakov | Detection of CO and O2 Using Tin Oxide Nanowire Sensors[END_REF][START_REF] Kind | Nanowire Ultraviolet Photodetectors and Optical Switches[END_REF][START_REF] Strelcov | Gas Sensor Based on Metal-Insulator Transition in VO2 Nanowire Thermistor[END_REF][START_REF] Hernandez-Ramirez | On the Role of Individual Metal Oxide Nanowires in the Scaling Down of Chemical Sensors[END_REF][START_REF] Peng | Low-Dimensional Nanostructure Ultraviolet Photodetectors[END_REF]. A lot of attention has been paid especially on devices based on individual 1-D nanostructures due to their unique opportunity to develop high sensitivity at room temperature, being one of the best candidates for realizing ultrasensitive gas sensors with ultralow power consumption [START_REF] Hu | Fast-Response Nanowire Sensors by Using Schottky Contacts[END_REF]. For example, Zhang et al. reported on the fabrication of a gas sensor based on an individual In2O3 NW able to detect NO2 down to the ppb level at room temperature [START_REF] Zhang | Detection of NO2 Down to ppb Levels Using Individual and Multiple In2O3 Nanowire Devices[END_REF]. Another huge advantage of individual 1-D nanostructures arises from their diameter comparable with the Debye length, which gives the possibility for an efficient modulation of the conduction channel under adsorption/desorption of gas molecules [START_REF] Kolmakov | Enhanced Gas Sensing by Individual SnO2 Nanowires and Nanobelts Functionalized with Pd Catalyst Particles[END_REF][START_REF] Hernandez-Ramirez | Insight into the Role of Oxygen Diffusion in the Sensing Mechanisms of SnO2 Nanowires[END_REF][START_REF] Postica | Schottky Diode Based on a Single Carbon-Nanotube-ZnO Hybrid Tetrapod for Selective Sensing Applications[END_REF]. Therefore, several studies demonstrated that in the case of individual 1-D nanostructures the size is the key factor which determines the performances of the devices [START_REF] Hernandez-Ramirez | On the Role of Individual Metal Oxide Nanowires in the Scaling Down of Chemical Sensors[END_REF][START_REF] Hernandez-Ramirez | Insight into the Role of Oxygen Diffusion in the Sensing Mechanisms of SnO2 Nanowires[END_REF][START_REF] Calarco | Size-Dependent Photoconductivity in MBE-Grown GaN-Nanowires[END_REF][START_REF] Gröttrup | Size-Dependent UV and Gas Sensing Response of Individual Fe2O3-ZnO:Fe Microand Nanowire Based Devices[END_REF][START_REF] Lupan | Highly Sensitive and Selective Hydrogen Single-Nanowire Nanosensor[END_REF][START_REF] Lupan | Ultra-Sensitive and Selective Hydrogen Nanosensor with Fast Response at Room Temperature Based on a Single Pd/ZnO Nanowire[END_REF][START_REF] Mathur | Size-Dependent Photoconductance in SnO2 Nanowires[END_REF][START_REF] Schütt | Single and Networked ZnO-CNT Hybrid Tetrapods for Selective Room-Temperature High-Performance Ammonia Sensors[END_REF]. Yang et al. demonstrated that smaller palladium NWs show accelerated response and recovery rates to H2 gas [START_REF] Yang | Smaller is Faster and More Sensitive: The Effect of Wire Size on the Detection of Hydrogen by Single Palladium Nanowires[END_REF]. Tonezzer and Hieu characterized SnO2 NWs with different diameter sizes as NO2 sensors and observed that thinner NWs show higher gas sensing performances, i.e. a higher gas response, a smaller detection limit and faster response and recovery times [START_REF] Tonezzer | Size-Dependent Response of Single-Nanowire Gas Sensors[END_REF].

Previous studies demonstrated that individual ZnO and Ag-or Cd-doped ZnO NWs, integrated into nanosensors using the same procedure, possess a good selectivity to hydrogen gas at room temperature compared to other reducing gases [START_REF] Lupan | Highly Sensitive and Selective Hydrogen Single-Nanowire Nanosensor[END_REF][START_REF] Lupan | Fabrication of ZnO Nanorod-Based Hydrogen Gas Nanosensor[END_REF][START_REF] Lupan | Silver-Doped Zinc Oxide Single Nanowire Multifunctional Nanosensor with a Significant Enhancement in Response[END_REF][START_REF] Lupan | Selective Hydrogen Gas Nanosensor Using Individual ZnO Nanowire with Fast Response at Room Temperature[END_REF]. The highly efficient approach to further increase the gas sensing properties and to improve the specificity of individual 1-D nanostructures is surface functionalization with noble metal nanoparticles [START_REF] Kolmakov | Enhanced Gas Sensing by Individual SnO2 Nanowires and Nanobelts Functionalized with Pd Catalyst Particles[END_REF][START_REF] Lupan | Ultra-Sensitive and Selective Hydrogen Nanosensor with Fast Response at Room Temperature Based on a Single Pd/ZnO Nanowire[END_REF][START_REF] Lupan | Functionalized Pd/ZnO Nanowires for Nanosensors[END_REF]. In this case, nanoscale Schottky barriers are formed at the interface of metal oxide nanostructures and noble metal NPs, which greatly improve the modulation of the conduction channel under adsorption/desorption of oxygen species, i.e. lead to a higher gas response [START_REF] Kolmakov | Enhanced Gas Sensing by Individual SnO2 Nanowires and Nanobelts Functionalized with Pd Catalyst Particles[END_REF][START_REF] Lupan | Ultra-Sensitive and Selective Hydrogen Nanosensor with Fast Response at Room Temperature Based on a Single Pd/ZnO Nanowire[END_REF][START_REF] Lupan | Functionalized Pd/ZnO Nanowires for Nanosensors[END_REF]. This gas sensing mechanism is widely attributed to electronic sensitization [START_REF] Kolmakov | Enhanced Gas Sensing by Individual SnO2 Nanowires and Nanobelts Functionalized with Pd Catalyst Particles[END_REF][START_REF] Yamazoe | New Approaches for Improving Semiconductor Gas Sensors[END_REF]. On the other hand, the chemical sensitization is another mechanism which greatly improves the quantity of the adsorbed oxygen species via the spillover effect, because noble metal NPs are far better oxygen dissociation catalysts than metal oxides [START_REF] Kolmakov | Enhanced Gas Sensing by Individual SnO2 Nanowires and Nanobelts Functionalized with Pd Catalyst Particles[END_REF]. For example, Kolmakov et al. demonstrated that Pd-functionalized SnO2 nanobelts exhibit a dramatic improvement in sensitivity toward oxygen and hydrogen due to the enhanced catalytic dissociation of the molecular adsorbate on the Pd nanoparticle surfaces and the subsequent diffusion of the resultant atomic species to the oxide surface [START_REF] Kolmakov | Enhanced Gas Sensing by Individual SnO2 Nanowires and Nanobelts Functionalized with Pd Catalyst Particles[END_REF]. Liao et al. fabricated a gas sensor based on individual Pt-functionalized CeO2 NWs, demonstrating that Pt incorporation can significantly increase the sensor response [START_REF] Liao | Single CeO2 Nanowire Gas Sensor Supported with Pt Nanocrystals: Gas Sensitivity, Surface Bond States, and Chemical Mechanism[END_REF]. However, the main disadvantage in all cases is the necessity of an additional technological step to functionalize the 1-D nanostructures with noble metal NPs. Recently, our group demonstrated the possibility to grow Au NPs-functionalized ZnO NW arrays via a single-step electrodeposition approach [START_REF] Lupan | Synthesis of Au-Modified ZnO NWs for Ultra-Sensitive Gas Nanosensor[END_REF]. In this case, the surface functionalization with Au NPs is performed in the same step of NWs growth by electrodeposition, which simply excludes the necessity of additional steps for surface functionalization. Moreover, while the gas sensing properties of sensors based on networks of Aufunctionalized nano-and microstructures were widely reported [START_REF] Feng | In-Situ Generation of Highly Dispersed Au Nanoparticles on Porous ZnO Nanoplates via Ion Exchange from Hydrozincite for VOCs Gas Sensing[END_REF][START_REF] Gogurla | Multifunctional Au-ZnO Plasmonic Nanostructures for Enhanced UV Photodetector and Room Temperature NO Sensing Devices[END_REF][START_REF] Guo | High-Performance Gas Sensor Based on ZnO Nanowires Functionalized by Au Nanoparticles[END_REF], the gas sensing properties of an individual Au-functionalized nanostructure or nanowire have not been reported. Although, the technological concept of individual functionalized nanowire integration was reported by Lupan et al., as well as the combination of Au NPs with ZnO nanostructures, we consider that detailed gas sensing properties of individual Au/ZnO NWs reported in this work are completely new and will be of high interest for the scientific community due to the lack of information in literature on a single functionalized semiconducting oxide nanowire covered with nanoscale-Schottky contacts at the Au NPs/ZnO NW interface. In this field it is important to identify an updated concept and models of nanosensors, which are essential for understanding the role of noble metal nanoclusters on semiconducting oxide nanowires and contribute to a design of fiable room-temperature gas sensors.

In this work, the individual and multiple networked Au NPs-functionalized ZnO NWs with nanoscale-Schottky contacts were integrated into gas sensing devices using a dual beam FIB/SEM system. The influence of the diameter and the effect of relative humidity on the room temperature gas sensing properties were investigated in detail. Moreover, the comparison of gas sensing characteristics for individual and multiple networked NWs was carried out in order to find out which configuration is more suitable for fabrication of high performance gas micro-and nanosensors.

Experimental part

The synthesis of ZnO NW arrays, modified by Au, was performed by electrochemical deposition in a classical three-electrode electrochemical cell, as was already reported in our previous works [START_REF] Lupan | Synthesis of Au-Modified ZnO NWs for Ultra-Sensitive Gas Nanosensor[END_REF][START_REF] Belghiti | Mechanistic Study of ZnO Nanorod Array Electrodeposition[END_REF][START_REF] Lupan | Hydrothermal Treatment for the Marked Structural and Optical Quality Improvement of ZnO Nanowire Arrays Deposited on Lightweight Flexible Substrates[END_REF]. In this study, only the samples grown using 0.9 µM of HAuCl4 (Sigma-Aldrich, >99.9%) in an electrochemical bath were used for nanosensor fabrication. The detailed morphological, structural, optical and chemical characterization of Au-modified ZnO NW arrays are presented in our previous work [START_REF] Lupan | Synthesis of Au-Modified ZnO NWs for Ultra-Sensitive Gas Nanosensor[END_REF]. In the present paper, only the gas sensing properties of the synthesized Au/ZnO NWs will be investigated.

The individual Au-modified ZnO NWs were integrated into nanosensor devices using a FIB/SEM system by a method developed by Lupan et al. [START_REF] Lupan | Fabrication of ZnO Nanorod-Based Hydrogen Gas Nanosensor[END_REF][START_REF] Lupan | Versatile Growth of Freestanding Orthorhombic α-Molybdenum Trioxide Nano-and Microstructures by Rapid Thermal Processing for Gas Nanosensors[END_REF]. The nanosensors device represents a SiO2 (350 nm)-coated Si substrate (SiO2/Si) with pre-patterned Au/Cr electrodes. The size of the chip is 10 mm × 10 mm. The configuration of the electrodes and optical images of the chip are presented in our previous work [START_REF] Lupan | Versatile Growth of Freestanding Orthorhombic α-Molybdenum Trioxide Nano-and Microstructures by Rapid Thermal Processing for Gas Nanosensors[END_REF], as well as schematically illustrated in Figure S1a-c.

On the same chip, maximum eight devices based on a single nanowire or multiple structures can be integrated (see Figure S1b) and measured using the two probes method. The integration of individual or multiple nanowires can be described as follows. The Au/ZnO NWs are released from the initial substrate by sonication in ethanol, followed by a transfer by touching an intermediate SiO2/Si substrate in order to obtain the lower density of NWs [START_REF] Lupan | Fabrication of ZnO Nanorod-Based Hydrogen Gas Nanosensor[END_REF][START_REF] Lupan | Versatile Growth of Freestanding Orthorhombic α-Molybdenum Trioxide Nano-and Microstructures by Rapid Thermal Processing for Gas Nanosensors[END_REF]. This allows the connection of multiple networked NWs to the Au pads by deposition of a Pt complex. In order to integrate the individual nanostructures, further dispersion to a lower density of NWs is necessary, which was used also for the integration of individual CuO, MoO3 and Fe2O3 NWs with a diameter down to 20-30 nm [START_REF] Lupan | Fabrication of ZnO Nanorod-Based Hydrogen Gas Nanosensor[END_REF][START_REF] Lupan | Versatile Growth of Freestanding Orthorhombic α-Molybdenum Trioxide Nano-and Microstructures by Rapid Thermal Processing for Gas Nanosensors[END_REF][START_REF] Lupan | Localized Synthesis of Iron Oxide Nanowires and Fabrication of High Performance Nanosensors Based on a Single Fe2O3 Nanowire[END_REF]. The lower density and uniform distribution of Au/ZnO NWs can be obtained by transfer to the SiO2/Si chip using a direct contact technique and a micro-manipulator, described in detail in our previous work [START_REF] Lupan | Versatile Growth of Freestanding Orthorhombic α-Molybdenum Trioxide Nano-and Microstructures by Rapid Thermal Processing for Gas Nanosensors[END_REF]. The respective SEM images of networked and individual Au/ZnO NWs is further presented in this work.

The gas sensing measurements were performed at room temperature (RT, ~ 25 °C) and in normal ambient air (relative humidity, RH of ~ 25 %) as was already reported in our previous works [START_REF] Lupan | Silver-Doped Zinc Oxide Single Nanowire Multifunctional Nanosensor with a Significant Enhancement in Response[END_REF][START_REF] Cretu | Synthesis, Characterization and DFT Studies of Zinc-Doped Copper Oxide Nanocrystals for Gas Sensing Applications[END_REF][START_REF] Lupan | Single and Networked CuO Nanowires for Highly Sensitive p-Type Semiconductor Gas Sensor Applications[END_REF]. The RH was continuously monitored using a standard hygrometer, while the bubbling system was used in order to generate different values of RH [START_REF] Cretu | Synthesis, Characterization and DFT Studies of Zinc-Doped Copper Oxide Nanocrystals for Gas Sensing Applications[END_REF]. The gas response (S) is defined as the ratio between the current under exposure to gas (Igas) and the current under exposure to ambient air (Iair), i.e. S = Igas/ Iair. The theoretical detection limit was estimated as reported in details by Dua et al. and Lu et al., i.e. using the signal-to-noise ratio [START_REF] Dua | Manohar Sanjeev All-Organic Vapor Sensor Using Inkjet-Printed Reduced Graphene Oxide[END_REF][START_REF] Li | Carbon Nanotube Sensors for Gas and Organic Vapor Detection[END_REF].

Results and discussions

Influence of the Au/ZnO NW diameter and relative humidity on gas sensing properties

Figure 1a and Figure S1 show SEM images of Au/ZnO NWs with different radius (140, 115, 90 and 60 nm), used in this study in order to investigate the influence of the NW diameter on the gas sensing properties. All NWs were grown using 0.9 µM of HAuCl4 in electrochemical bath, which was demonstrated to be the optimal concentration in order to achieve the highest gas sensing performances [START_REF] Lupan | Synthesis of Au-Modified ZnO NWs for Ultra-Sensitive Gas Nanosensor[END_REF]. On the surface of all NWs, the Au NPs could be observed (see Figure S2). The gas response at room temperature versus the diameter of the Au/ZnO NW to 100 ppm of hydrogen gas, ethanol, acetone and methanol is presented in Figure 1b. As was demonstrated in our previous work [START_REF] Lupan | Synthesis of Au-Modified ZnO NWs for Ultra-Sensitive Gas Nanosensor[END_REF], the Au/ZnO NWs are highly selective to hydrogen gas, which can be also observed for NWs in a wide range of radius (from 60 to 140 nm, see Figure 1b), i.e. the selectivity does not depend on the diameter of the NWs. Also, it must be mentioned that for all individual NWs, the number of Au NPs was practically the same (5 ± 3 NPs), in order to exclude the influence of the NP density on the gas response (see Figure 1a andS2). The gas response to 100 ppm of ethanol, acetone and methanol vapors for Au/ZnO NW with 150 nm diameter is presented in Figure S3a, showing no deviation in current values. The gas response of Au/ZnO NWs with radius of 60, 75, 90, 115 and 140 nm to 100 ppm of H2 gas is 7.5, 4.7, 2.4, 1.4 and 1.2, respectively (see Figure 1b).

Figure 1c shows the dynamic gas response at room temperature to 100 ppm of H2 gas of individual Au/ZnO NWs with different radius (60, 90 and 140 nm). The calculated response and recovery times versus diameter of individual Au/ZnO NW is presented in Figure 1d. A decrease in recovery time with the decrease of the NW diameter can be observed, which is in agreement with other experimental results based on individual semiconducting oxide NWs nanosensors [START_REF] Lupan | Ultra-Sensitive and Selective Hydrogen Nanosensor with Fast Response at Room Temperature Based on a Single Pd/ZnO Nanowire[END_REF][START_REF] Tonezzer | Size-Dependent Response of Single-Nanowire Gas Sensors[END_REF]. In the case on Au/ZnO NW with a radius of 60 nm the response and recovery times are 140 s and 300 s, respectively (see Figure 1d).

The rise in H2 gas response by decreasing in NW diameter can be explained based on increased impact of surface phenomena (adsorption/desorption of gaseous species) on electron transport through the conduction channel of NW [START_REF] Hernandez-Ramirez | Insight into the Role of Oxygen Diffusion in the Sensing Mechanisms of SnO2 Nanowires[END_REF][START_REF] Gröttrup | Size-Dependent UV and Gas Sensing Response of Individual Fe2O3-ZnO:Fe Microand Nanowire Based Devices[END_REF][START_REF] Lupan | Ultra-Sensitive and Selective Hydrogen Nanosensor with Fast Response at Room Temperature Based on a Single Pd/ZnO Nanowire[END_REF][START_REF] Liao | Size Dependence of Gas Sensitivity of ZnO Nanorods[END_REF][START_REF] Paulowicz | Zinc Oxide Nanotetrapods with Four Different Arm Morphologies for Versatile Nanosensors[END_REF]. The thinner the NWs, the higher is modulation of surface electron depletion region under adsorption/desorption of oxygen species and then it results in a higher gas response (S) [START_REF] Lupan | Ultra-Sensitive and Selective Hydrogen Nanosensor with Fast Response at Room Temperature Based on a Single Pd/ZnO Nanowire[END_REF]:

2 2 / 1 ) ( 2 2 2                       - =         - = kT qV D D L D D S air S D air  (1)
Assuming that Lgas << D, where D is the diameter of NW, Lair and Lgas are the width of electron depletion region under air and gas, respectively. λD is the Debye length, T is the absolute temperature, k is the Boltzmann's constant, and VS(air) is the band bending induced by adsorption under ambient air. According to Eq. ( 1), the gas response was calculated assuming that Lair is 15 nm [START_REF] Fan | ZnO Nanowire Field-Effect Transistor and Oxygen Sensing Property[END_REF]. The results in comparison with experimental data are presented in Figure S3b, showing that for relatively thick NWs the experimental data are comparable with theoretical calculations.

However, for NWs with radius smaller than 100 nm the experimental data considerably deviate from theoretical results, which may be of high interest for further scientific investigations and new nano-physics. This can be explained based on gas sensing mechanism elaborated in previous work [START_REF] Lupan | Synthesis of Au-Modified ZnO NWs for Ultra-Sensitive Gas Nanosensor[END_REF], which is based on formation of nanoscale-Schottky contacts at Au NPs/ZnO NW interface due to higher work function of Au (φAu = 5.1 eV) compared to that of ZnO (φAu = 4.1 eV) [START_REF] Liu | Giant Improvement of the Performance of ZnO Nanowire Photodetectors by Au Nanoparticles[END_REF][START_REF] Joondong | ZnO Nanowire-Embedded Schottky Diode for Effective UV Detection by the Barrier Reduction Effect[END_REF].

This will lead a to narrower conduction channel (dair) under exposure to ambient air, and therefore to higher modulation of the conduction channel width (dgas) after exposure to H2 gas (see Figure 2).

Previous studies demonstrated that individual ZnO and doped ZnO NWs (Ag-and Cddoped) possess a good selectivity to hydrogen gas at room temperature [START_REF] Lupan | Highly Sensitive and Selective Hydrogen Single-Nanowire Nanosensor[END_REF][START_REF] Lupan | Fabrication of ZnO Nanorod-Based Hydrogen Gas Nanosensor[END_REF][START_REF] Lupan | Silver-Doped Zinc Oxide Single Nanowire Multifunctional Nanosensor with a Significant Enhancement in Response[END_REF][START_REF] Lupan | Selective Hydrogen Gas Nanosensor Using Individual ZnO Nanowire with Fast Response at Room Temperature[END_REF]. In our case, the surface functionalization with Au NPs induced no changes in the selectivity from hydrogen gas to other tested gases (ethanol, acetone and methanol, see Figure 1b). Thus, we can conclude that the main mechanism responsible for enhancement of gas sensing properties is an additional modulation of the conduction channel due to formation of nanoscale Schottky contacts at the interface of Au NPs and ZnO NWs (or so called electronic sensitization), while catalytic properties of Au NPs do not increase the rate of oxidation of ethanol, acetone or methanol molecules.

According to other results, the electronic sensitization does not induce the changes in selectivity and is only responsible for increasing the gas response [START_REF] Choi | Remarkable Improvement of Gas-Sensing Abilities in ptype Oxide Nanowires by Local Modification of the Hole-Accumulation Layer[END_REF][START_REF] Miller | Nanoscale Metal Oxide-Based Heterojunctions for Gas Sensing: A Review[END_REF].

Influence of relative humidity on gas sensing properties of individual Au/ZnO nanowires

In normal ambient conditions, the value of relative humidity can vary over a wide range, it is thus an important parameter which can reduce the gas sensing properties of metal oxide based gas sensors [START_REF] Kim | The Role of NiO Doping in Reducing the Impact of Humidity on the Performance of SnO2-Based Gas Sensors: Synthesis Strategies, and Phenomenological and Spectroscopic Studies[END_REF]. Figure 3a shows the dynamic gas response at 30% and 85% RH to 250 ppm of H2 gas at room temperature for an individual Au/ZnO NW with 75 nm radius. The higher values of RH were generated using a bubbling system, while the RH value was continuously monitored using a standard hygrometer [START_REF] Cretu | Synthesis, Characterization and DFT Studies of Zinc-Doped Copper Oxide Nanocrystals for Gas Sensing Applications[END_REF]. It is found that at higher value of RH, the value of the electrical baseline in the absence of H2 gas, was not clearly modified and that the gas response was slightly reduced from 35.7 for 30% RH to 32 for 85% RH. In the case of unmodified ZnO NWs (radius of 90 nm) the gas response to 100 ppm of H2 was essentially reduced from 2.3 for 30% RH to 1. 3b), which can explain the lower influence of water vapors on the gas response for the Au-modified ZnO NW. In general, the decrease in resistance of semiconducting oxide under a rise in the RH value can be explained based on hydroxyl poisoning, which was already discussed in other works [START_REF] Cretu | Synthesis, Characterization and DFT Studies of Zinc-Doped Copper Oxide Nanocrystals for Gas Sensing Applications[END_REF][START_REF] Kim | The Role of NiO Doping in Reducing the Impact of Humidity on the Performance of SnO2-Based Gas Sensors: Synthesis Strategies, and Phenomenological and Spectroscopic Studies[END_REF][START_REF] Lupan | Non-Planar Nanoscale p-p Heterojunctions Formation in ZnxCu1-xOy Nanocrystals by Mixed Phases for Enhanced Sensors[END_REF].

The influence of water vapors on the gas sensing mechanism of the ZnO NW and Au/ZnO NW (presented in cross-section) is tentatively proposed in Figure 4. Under exposure to dry air the diameter of the conduction channel will be narrowed due to adsorption of oxygen molecules (O2(g) + e -→ O2(ad) -, see Figure 4a,d) [START_REF] Lupan | Highly Sensitive and Selective Hydrogen Single-Nanowire Nanosensor[END_REF][START_REF] Lupan | Silver-Doped Zinc Oxide Single Nanowire Multifunctional Nanosensor with a Significant Enhancement in Response[END_REF]. By exposure of NWs in atmosphere with high concentration of water vapors, the H2O molecules are adsorbed on the surface of metal oxides in the molecular or hydroxyl form with donor effect and can replace the previously adsorbed oxygen species by generation of free electrons (see Figure 4b). This leads to a decrease in resistance and to a widening of the conduction channel (dRH) in the case of a ZnO NW (see Figure 4b) [START_REF] Lupan | Ultra-Sensitive and Selective Hydrogen Nanosensor with Fast Response at Room Temperature Based on a Single Pd/ZnO Nanowire[END_REF][START_REF] Kim | The Role of NiO Doping in Reducing the Impact of Humidity on the Performance of SnO2-Based Gas Sensors: Synthesis Strategies, and Phenomenological and Spectroscopic Studies[END_REF].

However, in the case of a Au/ZnO NW we can tentatively propose that Au NPs play the role of hydroxyl adsorbers on the surface of NPs, thereby providing an oxygen adsorption site for surface reactions in humid atmosphere (see Figure 4d,e), which do not lead to the change in the conduction channel (dair ≈ dRH) [START_REF] Suematsu | Surface-Modification of SnO2 Nanoparticles by Incorporation of Al for the Detection of Combustible Gases in a Humid Atmosphere[END_REF]. The same effect was observed for Fe2O3-functionalized ZnO tetrapods [START_REF] Lupan | Enhanced UV and Ethanol Vapour Sensing of a Single 3-D ZnO Tetrapod Alloyed with Fe2O3 Nanoparticles[END_REF], Al-doped SnO2 NPs [START_REF] Suematsu | Surface-Modification of SnO2 Nanoparticles by Incorporation of Al for the Detection of Combustible Gases in a Humid Atmosphere[END_REF], Sb-doped SnO2 [START_REF] Suematsu | Antimony-Doped Tin Dioxide Gas Sensors Exhibiting High Stability in the Sensitivity to Humidity Changes[END_REF], CuO-loaded SnO2 hollow spheres [START_REF] Choi | Ultraselective and Ultrasensitive Detection of H2S in Highly Humid Atmosphere Using CuO-Loaded SnO2 Hollow Spheres for Real-Time Diagnosis of Halitosis[END_REF], and NiO-doped SnO2 [START_REF] Kim | The Role of NiO Doping in Reducing the Impact of Humidity on the Performance of SnO2-Based Gas Sensors: Synthesis Strategies, and Phenomenological and Spectroscopic Studies[END_REF]. However, the interactions of water vapors with gold have been studied by many methods [START_REF] Wells | Adsorption of water on clean gold by measurement of work function changes[END_REF][START_REF] Hakamada | Water-adsorption effect on electrical resistivity of nanoporous gold[END_REF][START_REF] Sharma | Adsorption of water vapor on thin-gold electroplate on copper[END_REF]. For example, Wells and Fort studied the adsorption of water on clean gold by means of measurement of work function changes, demonstrating that water chemisorbs on the gold with an activation energy of 3 to 9 kcal/mole [START_REF] Wells | Adsorption of water on clean gold by measurement of work function changes[END_REF]. Sharma and Thomas measured the adsorption/desorption isotherms of water on the porous plate gold using the quartz crystal sorption (is that correct ??) monitor, showing no strong dependence on the temperature in the range from 22 ºC to 60 ºC [START_REF] Sharma | Adsorption of water vapor on thin-gold electroplate on copper[END_REF].

After introduction of H2 gas in the test chamber, the oxidation of H2 molecules will occur [START_REF] Lupan | Ultra-Sensitive and Selective Hydrogen Nanosensor with Fast Response at Room Temperature Based on a Single Pd/ZnO Nanowire[END_REF][START_REF] Kim | The Role of NiO Doping in Reducing the Impact of Humidity on the Performance of SnO2-Based Gas Sensors: Synthesis Strategies, and Phenomenological and Spectroscopic Studies[END_REF]:

- - + → + e O H O H ad 2 ) ( 2 2 2 2 (2)
Therefore, the electrons will be released back to the NW and will widen the conduction channel (dgas, see Figure 4c,f). However, in the case of a Au/ZnO NW the modulation of conduction channel (dgas -dRH) will be higher due to the non influence of water vapors on the conduction channel in humid atmosphere. Thus, addition of Au NPs can significantly decrease the influence of water vapors on the gas sensing properties of ZnO NWs, which is very important for practical applications in ambient atmosphere.

Individual and multiple Au/ZnO NWs based micro-and nanodevices

Figure 5a shows the SEM images of devices based on multiple networked Au/ZnO NWs, while Figure 5b shows the currentvoltage (I -V) characteristics of devices based on individual (radius of 60 nm) and multiple networked Au/ZnO NWs, demonstrating the formation of double asymmetric Schottky contacts. Figure 5a shows that the multiple networked Au/ZnO NWs are connected between the two Au/Cr electrodes using Pt complex contacts deposited in a FIB/SEM system. In the case of multiple NWs the higher current value can be explained based on increased channels for current flow, which was also observed in other experimental results [START_REF] Zhang | Detection of NO2 Down to ppb Levels Using Individual and Multiple In2O3 Nanowire Devices[END_REF][START_REF] Khan | Nanojunction Effects in Multiple ZnO Nanowire Gas Sensor[END_REF].

Figure 5c shows the gas response to 100 ppm of H2 gas at room temperature for single (with a radius of 60 nm) and multiple Au/ZnO NWs (device from Figure 5a). In the case of multiple NWs higher response value of ~ 40 was obtained, compared to 7.5 for an individual NW.

Figure 5d shows the dynamic gas response at room temperature to different concentrations of H2

gas. The gas response to 30, 50, 100, 150, 175 and 300 ppm is 4.9, 9.6, 40, 50, 55 and 120, respectively (see Figure 5e). The gas response follows a power law relationship on gas concentration

 2 H p S 
, where β ~ 1.1 is the curve slope of the dependence [START_REF] Cretu | Synthesis, Characterization and DFT Studies of Zinc-Doped Copper Oxide Nanocrystals for Gas Sensing Applications[END_REF]. The theoretical detection limit of ~ 1 ppm was estimated using signal/noise ratio 2 , as was reported by Dua et al. [START_REF] Dua | Manohar Sanjeev All-Organic Vapor Sensor Using Inkjet-Printed Reduced Graphene Oxide[END_REF], which is smaller value compared to an individual Au/ZnO NW, reported previously [START_REF] Lupan | Synthesis of Au-Modified ZnO NWs for Ultra-Sensitive Gas Nanosensor[END_REF]. In order to check the reproducibility of a device based on multiple Au/ZnO NWs, three pulses of 300 ppm H2 gas were applied (see Figure 5f), which showed a deviation in response smaller than 5%.

The better gas sensing properties (higher gas response and lower detection limit) of multiple NWs compared to an individual NW were also observed by Zhang et al. for In2O3 NWs [START_REF] Zhang | Detection of NO2 Down to ppb Levels Using Individual and Multiple In2O3 Nanowire Devices[END_REF], as well as by Khan et al. for ZnO NWs [START_REF] Khan | Nanojunction Effects in Multiple ZnO Nanowire Gas Sensor[END_REF]. We tentatively attribute these improved gas sensing properties to the formation of potential barriers at NW/NW junctions, which is missing in the case of an individual NW [START_REF] Zhang | Detection of NO2 Down to ppb Levels Using Individual and Multiple In2O3 Nanowire Devices[END_REF][START_REF] Schütt | Single and Networked ZnO-CNT Hybrid Tetrapods for Selective Room-Temperature High-Performance Ammonia Sensors[END_REF][START_REF] Lupan | Single and Networked CuO Nanowires for Highly Sensitive p-Type Semiconductor Gas Sensor Applications[END_REF][START_REF] Lupan | Enhanced UV and Ethanol Vapour Sensing of a Single 3-D ZnO Tetrapod Alloyed with Fe2O3 Nanoparticles[END_REF][START_REF] Lupan | Individual Hollow and Mesoporous Aero-Graphitic Microtube Based Devices for Gas Sensing Applications[END_REF]. A proposed gas sensing mechanism is presented in Figure 6. In this case, the modulation of the potential barriers height (qVS1) between Au/ZnO NWs has the main role in enhancement of the gas sensing properties (see Figure 6) [START_REF] Zhang | Detection of NO2 Down to ppb Levels Using Individual and Multiple In2O3 Nanowire Devices[END_REF][START_REF] Lupan | Enhanced UV and Ethanol Vapour Sensing of a Single 3-D ZnO Tetrapod Alloyed with Fe2O3 Nanoparticles[END_REF][START_REF] Postica | Multifunctional Materials: A Case Study of the Effects of Metal Doping on ZnO Tetrapods with Bismuth and Tin Oxides[END_REF][START_REF] Postica | Morphology Dependent UV Photoresponse of Sn-Doped ZnO Microstructures[END_REF].

Under exposure in ambient air, due to adsorbed oxygen species (as was already described in the gas sensing mechanism for individual Au/ZnO NWs in Figure 2) on the surface of Au/ZnO NWs the electron depletion region will be formed, leading to the formation of potential barriers (qVS1, see Figure 6a). Under exposure to H2, the width of the electron depletion region is narrowed, leading to a decrease in the potential barriers height (qVS2, see Figure 6b). In the case of exposure to H2 gas molecules of the device based on networked NWs the decrease in the barrier height occurs due to a release of electron after oxidation of the hydrogen molecules [START_REF] Lupan | Ultra-Sensitive and Selective Hydrogen Nanosensor with Fast Response at Room Temperature Based on a Single Pd/ZnO Nanowire[END_REF][START_REF] Lupan | Silver-Doped Zinc Oxide Single Nanowire Multifunctional Nanosensor with a Significant Enhancement in Response[END_REF][START_REF] Lupan | Enhanced UV and Ethanol Vapour Sensing of a Single 3-D ZnO Tetrapod Alloyed with Fe2O3 Nanoparticles[END_REF]:

- - + → + e O H ad O H 2 2 2 ) ( 2 1 (3)
Consequently, the formed additional potential barrier leads to a more efficient modulation in the device resistance [START_REF] Zhang | Detection of NO2 Down to ppb Levels Using Individual and Multiple In2O3 Nanowire Devices[END_REF][START_REF] Paulowicz | Zinc Oxide Nanotetrapods with Four Different Arm Morphologies for Versatile Nanosensors[END_REF][START_REF] Lupan | Enhanced UV and Ethanol Vapour Sensing of a Single 3-D ZnO Tetrapod Alloyed with Fe2O3 Nanoparticles[END_REF][START_REF] Khan | Nanojunction Effects in Multiple ZnO Nanowire Gas Sensor[END_REF]. In this case the gas response exponentially depends on the variation of the potential barriers height (qΔVS = qVS1 -qVS2) [START_REF] Bârsan | Conduction Mechanisms in SnO2 Based Polycrystalline Thick Film Gas Sensors Exposed to CO and H2 in Different Oxygen Backgrounds[END_REF][START_REF] Postica | Multifunctional Device Based on ZnO:Fe Nanostructured Films with Enhanced UV and Ultra-Fast Ethanol Vapour Sensing[END_REF]:

         - =  air gas S I I kT V q ln 2 (4)
Thus, the device based on multiple networked Au/ZnO NWs demonstrates a more sensitive H2 gas response compared to the individual NW device.

Conclusions

In summary, the individual and multiple networked Au nanoparticles (NPs)-functionalized ZnO nanowires were integrated into nanosensors using a dual beam FIB/SEM system. These devices were studied systematically as gas sensors at room temperature at different RH values.

The nanowire diameter strongly influences the H2 gas sensing properties and we demonstrated that thinner NWs show a higher gas response and faster response and recovery times at room temperature, which is very important for practical applications. Also, the low impact of water vapors on the sensing properties of an individual Au/ZnO NW was observed, which is very important for operation under ambient conditions. While an individual Au/ZnO NW demonstrated the response of ~ 7.5 to 100 ppm of H2 gas, the multiple networked NW device showed a higher gas response of ~ 40. The multiple NWs also demonstrated a smaller theoretical detection limit of ~ 1 ppm at room temperature, compared to ~ 5 ppm for the individual nanowire device. The higher gas response was explained based on a gas sensing mechanism which includes the modulation of a potential barrier height at the interface of different Au/ZnO nanowires bridging external contacts.

These results provide useful information for the fabrication of high performance and highly selective H2 gas sensors.

  2 for 85% RH (see Figure S4). We also observed longer response (~ 190 s) and recovery times (~ 151 s) in the case of exposure in 85% RH, compared to exposure in 30% RH (response time of ~ 76 s and recovery time of ~ 128 s). The much lower influence of RH on the gas response of a Au-modified ZnO NW can be explained based on a smaller change in resistance under increase in the RH compared to an unmodified ZnO NW. Figure S5 shows the change in resistance of individual Au-modified and unmodified ZnO NWs under the change of the RH value from 30% to 85%.

  Figure 3b shows the response to RH for devices based on an individual Au-modified and unmodified ZnO NW, based on measurements from Figure S5. The response was defined as (RRH -R30%)/R30%) • 100%, where RRH and R30% are the resistance under different values of RH and at 30% RH, respectively. It can be observed that in the case of the Au-modified ZnO NW the change in resistance by a rise in the RH value is much smaller compared to an unmodified ZnO NW (see Figure

Figure 1 .

 1 Figure 1. (a) SEM image of an individual Au/ZnO nanowire with a radius of 140 nm. (b) Gas

Figure 2 .

 2 Figure 2. Illustration of the gas sensing mechanism for an individual Au/ZnO nanowire under

Figure 3 .

 3 Figure 3. (a) Dynamic gas response at 30% and 85% RH to 250 ppm of H2 for a Au/ZnO nanowire

Figure 4 .

 4 Figure 4. Schematic illustration of the water vapors influence on the gas sensing mechanism of a

Figure 5 .

 5 Figure 5. (a) SEM image of a device based on multiple Au/ZnO nanowires. (b) Currentvoltage

Figure 6 .

 6 Figure 6. Illustration of the gas sensing mechanism for multiple networked Au/ZnO nanowires

Figure S1 .

 S1 Figure S1. Schematic illustration of the chip used for the fabrication of the nanosensors: (a) overall view, (b) top view (with red dotted circles the regions where the structures can be integrated are indicated) and (c) front view.

Figure S2 .

 S2 Figure S2. (a) Comparative SEM images of a small area of individual Au/ZnO NWs with different

Figure S3 .

 S3 Figure S3. Dynamic response at room temperature of a Au/ZnO NW with 75 nm radius to different

Figure S5 .

 S5 Figure S5. The dependence of resistance for individual unmodified and Au-modified ZnO NWs

Acknowledgments

Dr. Lupan gratefully acknowledges PSL University, Chimie-ParisTech for invited professor position in 2018 and 2019 and the CNRS Council for support as expert scientist at IRCP Chimie ParisTech, Paris. This research was sponsored partially by the German Research Foundation (DFG) under the schemes FOR 2093 (AD 183/12-2). Katrin Brandenburg is acknowledged for her help in the proof-reading of the manuscript. This research was partly supported by the Technical University of Moldova.