sensors Article ZnO Structures with Surface Nanoscale Interfaces Formed by Au, Fe2O3, or Cu2O Modifier Nanoparticles: Characterization and Gas Sensing Properties Milena Tomić 1,2 , Martha Claros 3 , Isabel Gràcia 1 , Eduard Figueras 1 , Carles Cané 1 and Stella Vallejos 1,3, * 1 Institute of Microelectronics of Barcelona (IMB-CNM, CSIC), Campus UAB, Cerdanyola del Vallès, 08193 Barcelona, Spain; milena.tomic@imb-cnm.csic.es (M.T.); isabel.gracia@imb-cnm.csic.es (I.G.); eduard.figueras@imb-cnm.csic.es (E.F.); carles.cane@imb-cnm.csic.es (C.C.) 2 Department of Electronic Engineering, Campus UAB, Autonomous University of Barcelona (UAB), Cerdanyola del Vallès, 08193 Barcelona, Spain 3 CEITEC-Central European Institute of Technology, Brno University of Technology, 61200 Brno, Czech Republic; martha.claros@ceitec.vutbr.cz * Correspondence: stella.vallejos@imb-cnm.csic.es; Tel.: +34-935-947700 Abstract: Zinc oxide rod structures are synthetized and subsequently modified with Au, Fe2 O3 , or Cu2 O to form nanoscale interfaces at the rod surface. X-ray photoelectron spectroscopy corroborates the presence of Fe in the form of oxide—Fe2 O3 ; Cu in the form of two oxides—CuO and Cu2 O, with the major presence of Cu2 O; and Au in three oxidation states—Au3+ , Au+ , and Au0 , with the content of metallic Au being the highest among the other states. These structures are tested towards   nitrogen dioxide, ethanol, acetone, carbon monoxide, and toluene, finding a remarkable increase in the response and sensitivity of the Au-modified ZnO films, especially towards nitrogen dioxide and Citation: Tomić, M.; Claros, M.; Gràcia, I.; Figueras, E.; Cané, C.; ethanol. The results for the Au-modified ZnO films report about 47 times higher response to 10 ppm Vallejos, S. ZnO Structures with of nitrogen dioxide as compared to the non-modified structures with a sensitivity of 39.96% ppm−1 Surface Nanoscale Interfaces Formed and a limit of detection of 26 ppb to this gas. These results are attributed to the cumulative effects of by Au, Fe2 O3 , or Cu2 O Modifier several factors, such as the presence of oxygen vacancies, the gas-sensing mechanism influenced by Nanoparticles: Characterization and the nano-interfaces formed between ZnO and Au, and the catalytic nature of the Au nanoparticles. Gas Sensing Properties. Sensors 2021, 21, 4509. https://doi.org/10.3390/ Keywords: zinc oxide; gold; iron oxide; copper oxide; interfaces; Schottky junctions; heterojunctions; s21134509 gas sensing; nitrogen dioxide Academic Editor: Ilaria Fratoddi Received: 8 June 2021 1. Introduction Accepted: 29 June 2021 Published: 30 June 2021 Gas sensors are applied in diverse scenarios, including industry, environmental pro- tection, medicine, food quality, public safety, etc., and their usage is projected to increase Publisher’s Note: MDPI stays neutral further as the Internet of Things (IoT) settles [1]. Amongst a host of available gas sensor with regard to jurisdictional claims in technologies based on nanomaterials, metal oxide semiconductors (MOX) based gas sen- published maps and institutional affil- sors are very promising due to their good sensitivity, small dimensions, compatibility with iations. microelectronic fabrication processes, and potentially low cost. These devices have shown sensitivity to various gases (e.g., CO, NO2 , NH3 , H2 S) and vapors (alcohols, ketones, alde- hydes, aromatic compounds), although their performance in terms of limits of detection and selectivity is still a subject of improvement [2–4]. Copyright: © 2021 by the authors. Since the gas sensing response and mechanism of MOX are surface-dependent, pre- Licensee MDPI, Basel, Switzerland. vious research on gas-sensitive MOXs and their functional properties (e.g., sensitivity, This article is an open access article selectivity, stability, speed of response) found it significant to reduce the particle size of distributed under the terms and these materials to improve their properties [5]. Other studies, moreover, showed that conditions of the Creative Commons crystalline structures with specific surface morphologies (e.g., rods, wires, plates, etc.) Attribution (CC BY) license (https:// are determinants to achieve better gas sensing properties [6]. The modification of these creativecommons.org/licenses/by/ nanosized crystalline structures with second-phase constituents, either noble metals or 4.0/). other MOXs, demonstrated another via to enhance the functionality of the MOX support Sensors 2021, 21, 4509. https://doi.org/10.3390/s21134509 https://www.mdpi.com/journal/sensors Sensors 2021, 21, 4509 2 of 24 to different gases and vapors. These improvements are generally connected with the increment of adsorption centers at the surface and the band energy structure modification due to the formation of Schottky-barrier-like junctions (metal–MOX) or heterojunctions (MOX–MOX) [7]. Consequently, the interest in finding synthetic methods that allow for crystalline MOXs with tuned second-phase constituents (metals or MOXs) has kept con- stant in the gas sensing field, as well as the use of methods that provide scalability to these materials and compatibility with state-of-the-art microfabrication processes for possible multi-purpose microelectronic devices [3,7]. Zinc oxide (ZnO) is an n-type semiconductor with a wide band-gap (EG = 3.3 eV), high chemical and thermal stability, and tunable surface morphology due to its stable wurtzite structure [8,9]. This MOX, in its pristine form and modified with noble metals (e.g., Au, Ag, Pt, or Pd) [10–13] or another MOX (e.g., CuO) [14], is one of the most representative gas-sensitive MOXs with proved sensitivity to gases, such as CO, H2 , H2 S, and NO2 , and vapors such as ethanol, methanol, and acetaldehyde [3]. ZnO was synthesized previously by different methods, including wet- and vapor-chemical routes, delivering films and powders with varied forms, e.g., nanoplates, nanospheres, nanowires, nanorods, nanotubes, nanoflowers, nanofibers, nanoneedles, and nanoribbons [3,4]. Recently, we developed a method to deposit ZnO by aerosol-assisted chemical vapor deposition (AACVD), which offers significant advantages over other CVD techniques, including atmospheric pressure and relatively low-temperature process, as well as the possibility of tuning the material morphology and structure [15,16]. This method allowed the formation of crystalline rod-like ZnO structures and their direct integration into microsensors [17]. The surface modification of MOX structures with second-phase constituents can also be achieved by different methods [7]; amongst them, AACVD [18–20] and impregnation [21,22] methods proved effective to modify structures with both metals and MOXs. Recently, for instance, AACVD was employed to modify ZnO with iron and copper [3], although the gas sensing performance of these systems was not evaluated. Similarly, impregnation procedures based on the reduction of a chemical inorganic precursor were used to incorporate Ag and Au particles over ZnO [23,24]. However, these procedures usually do not include gold and silver preformed nanoparticles, as in the present work, and they were not employed for modifying gas-sensitive ZnO structures. Here, we use these synthetic methods to develop crystalline ZnO structures modi- fied with Au, Cu2 O, and Fe2 O3 particles and go further by exploring their applicability on gas sensing. The work includes a broad analysis of the properties of these materi- als with nanoscale interfaces towards several gases, such as nitrogen dioxide, ethanol, acetone, carbon monoxide, and toluene, and a discussion of their influence on the gas sensing mechanism. 2. Materials and Methods ZnO rod structures were synthesized at 400 ◦ C on Si/SiO2 tiles (40 mm × 40 mm) and Si-based transducing platforms (40 mm × 40 mm) via the AACVD of ZnCl2 (50 mg, ≥98.0%, Sigma–Aldrich, St. Louis, MO, USA) dissolved in ethanol (5 mL, ≥99.9%, PanReac, Chicago, IL, USA), as described earlier [17]. The transducing platforms contained 100 pairs of Cr/Au (40/200 nm thick) interdigitated electrodes (5 × 2000 µm) with an electrode gap of 5 µm isolated from the Si substrate by a SiO2 layer. The ZnO structures were modified with Au, Ag, Fe2 O3 , and Cu2 O in a second step using AACVD or impregnation with preformed colloidal nanoparticles. The AACVD of FeCl3 ·6H2 O (3 mg, ≥99.0%, Sigma–Aldrich) at 430 ◦ C and Cu(NO3 )2 ·6H2 O (3 mg, ≥99.9%, Sigma–Aldrich) at 450 ◦ C dissolved in acetone (3 mL, ≥99.9%, PanReac) and ethanol (3 mL, ≥99.9%, PanReac), respectively, allowed the incorporation of iron and copper oxides at the ZnO rods, similarly to our previous studies [25]. However, the AACVD at 400 ◦ C of ethanolic solutions (3 mL) of HAuCl4 ·3H2 O or AgNO3 in concentrations up to 19 mM did not show appropriate for the incorporation of Au and Ag at the ZnO rods, despite previous evidence of the AACVD of these solutions on Si and glass tiles and other materials including WO3 structures [26,27]. Hence, the ZnO Sensors 2021, 21, 4509 3 of 24 structures were modified with Au and Ag by impregnation method using (preformed) colloidal nanoparticles synthesized by chemical reduction of HAuCl4 ·3H2 O (≥99.9%, Sigma–Aldrich) or AgNO3 (≥99.0%, Sigma–Aldrich) with Na3 C6 H5 O7 ·2H2 O (Ph Eur, BP, JP, USP, E 331, Sigma–Aldrich) [28] or NaBH4 (99.0%, Sigma–Aldrich) [29], respectively. The as-synthesized nanoparticles were washed with distilled water by centrifugation (12,000 rpm, 30 min) to eliminate the unreacted chemicals before the impregnation process, which consisted of immersing the ZnO samples (60 s) into the solution with suspended preformed Au or Ag colloids heated at 60 ◦ C. The samples were annealed in synthetic air at 450 ◦ C for 1 h. An examination of the samples’ morphology before and after annealing using SEM (Scanning Electron Microscopy—Zeiss Auriga series, 3 kV) did not show noticeable changes. The samples synthesized on Si tiles were used to perform most of the physical and chemical analysis to avoid possible interferences from the Cr/Au electrodes, whereas the Si-based transducing platforms were used primarily for gas sensing tests after performing SEM and confirm the integration of structures with similar characteristics to those registered on the Si tiles. An analysis of the samples synthesized on Si-tiles involved the examination of morphology by TEM (Transmission Electron Microscopy—FEI Tecnai, 200 kV), including STEM (Scanning Transmission Electron Microscopy) mode with elemental analysis by EDS (Energy Dis- persive Spectroscopy) and HRTEM (High-Resolution Transmission Electron Microscopy) mode. The crystalline structure was analyzed by XRD (X-ray Diffraction—Bruker, AXS D8 Advance, Cu Kα radiation operated at 40 kV and 40 mA), and the chemical composition by XPS (X-ray Photoelectron Spectroscopy—Kratos Axis Supra spectrometer, with Al/Ag monochromatic X-ray source). The structures were tested in a continuous flow system with dry air as the reference gas and ethanol, acetone, toluene, carbon monoxide, and nitrogen dioxide either in dry or humid ambient as analyte gases. After the exposure (10 min) to each gas analyte, the structures were recovered in the air for 30 min. Humidity was introduced to the system by bubbling through the water under controlled conditions at 25 ◦ C. The relative humidity (RH) just at the outlet of the test chamber was monitored throughout the whole test using a humidity/temperature sensor (SHT71, operating ranges from 0 to 100% RH, accuracy of ±3% RH and ±0.4 ◦ C) [25]. The sensor response (R) was defined as (Ra-Rg)/Ra for the reducing gases and (Rg-Ra)/Ra for the oxidizing gases. Ra and Rg represent the electrical resistance in air and after 10 min of gas exposure, respectively. The sensitivity was defined as the ratio of a sensor response change (∆R) to a given gas concentration change (∆C). The structures were tested, replicating the tests for each condition (analyte, concentration, and operating temperature) at least three times. Thus, the structures operated about 120 h for 4 weeks in total. 3. Results and Discussion The aerosol-assisted chemical vapor deposited ZnO films were characterized by a greyish color that uniformly covered the substrates. The morphological, structural, and chemical characteristics of these films were consistent with those observed in our previous work on ZnO [17]. Specifically, SEM images of the films displayed quasi-aligned rods (NRs) with lengths of approximately 1.5 µm and an average diameter of 200 nm. Similarly, TEM showed crystalline structures with a marked planar spacing of 0.26 nm and rod- like morphology. Further analysis of the films using XRD corroborated the presence of diffraction patterns corresponding to a hexagonal ZnO phase (P63mc space group, ICCD card No. 00-005-0664), and XPS confirmed the presence of typical Zn 2p and O 1s spectra characteristic for the ZnO films, as shown in Figure S1, (Supplementary Materials). The films modified with Au, Ag, Fe2 O3 , and Cu2 O in the second-step process showed morphological, crystalline, and chemical composition that corroborated the incorporation of these materials in the host ZnO structures, except for the Ag NPs modifications, which did not display clear evidence of Ag incorporation, neither by TEM nor by XPS, despite the apparent presence of Ag NPs at the ZnO surface after impregnation and annealing process modified with Au, Ag, Fe2O3, and Cu2O in the second-step process showed morphologi- cal, crystalline, and chemical composition that corroborated the incorporation of these materials in the host ZnO structures, except for the Ag NPs modifications, which did not Sensors 2021, 21, 4509 display clear evidence of Ag incorporation, neither by TEM nor by XPS, despite4 of the 24 ap- parent presence of Ag NPs at the ZnO surface after impregnation and annealing process (Figure S2, Supplementary Materials). Hence, hereafter, the results and discussion focus on the modification of ZnO with Au, Fe2O3, and Cu2O. (Figure S2, Supplementary Materials). Hence, hereafter, the results and discussion focus on the modification of ZnO with Au, Fe2 O3 , and Cu2 O. 3.1. Gold-Modified Zinc Oxide Films 3.1. Gold-Modified Figure 1a showsZinctheOxide Films morphology of the gold-modified zinc oxide films (Au@ZnO) Figure 1a shows the morphology recorded by SEM. The SEM images display of the the gold-modified zinc oxide ZnO structured films films (Au@ZnO) with uniform rod- likerecorded by SEM. morphology andThethe SEM images display presence the ZnO of equally structured distributed films NPs with due to uniform rod-like the incorporation of morphology and the presence of equally distributed NPs due to the Au NPs. XRD of the Au@ZnO films (Figure 1b) displays diffraction patterns connected incorporation of Au NPs. XRD of the Au@ZnO films (Figure 1b) displays diffraction patterns connected with the with the hexagonal ZnO phase, in agreement with our previous observations [17], and an hexagonal ZnO phase, in agreement with our previous observations [17], and an absence absence of Au diffraction peaks. This is probably connected with the high dispersion of of Au diffraction peaks. This is probably connected with the high dispersion of the Au NPs theover Au NPs overtheir the film, thesmall film, size, theirand small size, their lowand theirinlow amount amount in the film. the film. FigureFigure (a) SEM 1. (a)1.SEM images images (theinset (the inset shows shows aamagnified magnifiedimage of a single image Au decorated of a single ZnO rod).ZnO Au decorated (b) XRD rod).diffraction (b) XRDpattern diffraction of of pattern thethe Au@ZnO Au@ZnO structured film; all structured of the film; diffraction all of peaks inpeaks the diffraction the data in can the be indexed data can betoindexed a hexagonal to aZnO phase (P63mc hexagonal ZnO phase (P63mcspace group, space ICDD group, card card ICDD No. 00-005-0664). No. 00-005-0664). The STEM image of the Au@ZnO structures in Figure 2a confirms the rod-like mor- The STEM phology of ZnOimage of decoration and its the Au@ZnO withstructures in Figure randomly located 2a confirms spherical thediameters NPs with rod-like mor- phology betweenof 5ZnO and and 40 nm.itsHRTEM decoration with of these randomly particles located (Figure spherical 2b) reveals NPs with the presence diameters of highly between 5 and 40 nm. HRTEM of these particles (Figure 2b) reveals the presence ordered crystalline lattices with interplanar spacings of 0.26 nm and 0.23 nm, consistent of highly ordered crystalline with the hexagonallattices ZnO (002)with [17]interplanar and cubic Au spacings of 0.26 (111) planes nm and (Fm-3m 0.23 space nm, ICDD group, consistent card with theNo. 00-004-0784), hexagonal ZnOrespectively. (002) [17] and Similarly, the (111) cubic Au energy dispersive planes spectroscopy (Fm-3m (EDS)ICDD space group, line-scan card profiles in Figure No. 00-004-0784), 2c,d showSimilarly, respectively. the components recorded the energy on the particle dispersive along the(EDS) spectroscopy red line (Figure 2a) and confirm the relative locations of the elements and the line-scan profiles in Figure 2c,d show the components recorded on the particle along the incorporation of Au NPs at the ZnO rod surface. red line (Figure 2a) and confirm the relative locations of the elements and the incorpora- tion of Au NPs at the ZnO rod surface. SensorsSensors 2021, 2021, 21, 21, 4509 4509 5 of 24 5 of 24 Figure Figure 2. TEM2. TEM image image of an of an Au-modified ZnO Au-modified ZnO rod. rod.(a) (a)Low-magnification Low-magnificationSTEM image, STEM (b) high-resolution image, TEM image, (b) high-resolution TEM andimage, EDS line-scan and EDS line-scan profiles profilescorresponding to the corresponding (c) ZnO to the rod and (c) ZnO rod(d) andAu(d) nanoparticles recorded Au nanoparticles along thealong recorded red line theinred (a). line in (a). XPS was used to analyze further the elemental and chemical composition of the Au- XPS was used to analyze further the elemental and chemical composition of the Au- modified ZnO films. The main core-level spectra and their corresponding components are modified ZnO films. The main core-level spectra and their corresponding components are shown in Figure 3. The Zn 2p core-level spectrum recorded on the Au@ZnO film (Figure 3a) shown shows inthe Figure 3. The typical Zn 2pZn 2p core-level doublet appearing spectrum recorded at characteristic on theenergies binding Au@ZnO filmeV (1022.3 (Figure 3a)for shows the typical Zn 2p doublet appearing at characteristic binding Zn 2p3/2 and 1045.4 eV for Zn 2p1/2 ), together with a shake-up peak at 1040.7 eV. energies (1022.3 eVThe for Zn 2p3/2 energy binding and 1045.4 eV for(∆BE separation Zn 2p=1/223.1 ), together with eV) of this a shake-up doublet peak at 1040.7 is in agreement with theeV. The binding energy literature and the ∆BE(ΔBE [30], separation = 23.1for registered eV) theofZn this 2p doublet core-levelispeaks in agreement with the litera- of the non-modified ZnO ture films [30], and (see theFigure ΔBE S1a, Supplementary registered Materials). for the Zn However, 2p core-level it is of peaks noticed that the Zn 2p ZnO the non-modified peaks of Au@ZnO are located on slightly higher binding energies films (see Figure S1a, Supplementary Materials). However, it is noticed (~0.7 eV) than those that theofZn 2p ZnO, probably due to the incorporation of Au NPs into the ZnO film. peaks of Au@ZnO are located on slightly higher binding energies (~0.7 eV) than those of ZnO, probably due to the incorporation of Au NPs into the ZnO film. The deconvolution of the O 1s XPS spectrum of the Au@ZnO films indicates the pres- ence of four peaks centered at 530.8 eV, 531.6 eV, 532.5 eV, and 533.4 eV (Figure 3b). The first component corresponds to the Zn–O bonds, the second to oxygen defects in the ma- trix of metal oxides related to oxygen vacancies, and the third and fourth to the physical or chemical adsorbed oxygen, hydroxides, and H2O on the surface of Au@ZnO film [31– 34]. The number of components is similar to those found on the non-modified ZnO films, although their positions are minimally shifted to higher energy values (Figure S1b, Sup- plementary Materials). Figure 3c displays the Au 4f spectrum of the Au@ZnO films. The deconvolution of cess at 310 °C in the same atmosphere used for the gas sensing tests (i.e., air and target gases, see Section 3.4) corroborated the stability of the gold nanoparticles incorporated at the ZnO surface and their chemical state. Quantification of the components related to gold indicate higher amounts of metallic gold (2.6%) compared to Au+ (1.8%) and Au3+ (0.7%) Sensors 2021, 21, 4509 6 of 24 this represents a 51%, 35%, and 14% of Au0, Au+, and Au3+, respectively, from the total gold content at the Au@ZnO sample. Figure 3.Figure 3. (a) (a) Zn 2p,Zn(b)2p, O(b) 1s,Oand 1s, and (c) Au (c) Au 4f 4f andandZn Zn3p3pcore-levels core-levels XPS XPSspectra recorded spectra on the recorded onAu@ZnO films. Grey the Au@ZnO films.dots Grey dots represent represent the rawthedata, raw data, the solid the solid yellow yellow line line corresponds to corresponds to the theenvelope/fitting envelope/fittingcurve, andand curve, the other colored the other curves curves colored to the to the deconvoluted deconvoluted components. components. The deconvolution of the O 1s XPS spectrum of the Au@ZnO films indicates the 3.2. Iron-Modified Zinc presence of four Oxide peaks Filmsat 530.8 eV, 531.6 eV, 532.5 eV, and 533.4 eV (Figure 3b). centered Thefirst The SEM images ofcorresponds component the iron-modified to the Zn–Ozincbonds, oxidethefilms (Fe2O second to3@ZnO) are presented oxygen defects in in the matrix of metal oxides related to oxygen vacancies, and the third Figure 4a. These images show the modification of the ZnO structures with particles of and fourth to the physical or chemical adsorbed oxygen, hydroxides, and H2 O on the surface of Au@ZnO flake-like morphology particularly concentrated on the top of the rods (inset in Figure 4a). film [31–34]. The number of components is similar to those found on the non-modified ZnO The films, nanoflakes although (NFs) their are varied positions arein size andshifted minimally do notto exceed 20 nmvalues higher energy in thick andS1b, (Figure 100 nm in diameter. The XRD Supplementary pattern of these films corroborates the presence of the hexagonal ZnO Materials). phase determined earlierthe Figure 3c displays [17] Auand the appearance 4f spectrum of newfilms. of the Au@ZnO low-intensity diffractions. The deconvolution of The first this spectrum at 33.0° indicate the presence 2θ corresponds of six plane to the (222) components of theassigned cubic Fe to2O different 3 (Ia−3 states spaceofgroup, gold ICSD (Figure 3c), Au0 (at 83.9 eV and 87.5 eV), Au+ (at 85.2 eV and 89.3 eV), and Au3+ (at 86.1 eV and 88.4 eV) [35,36]. Moreover, two additional components are located at 89.1 eV and 92.0 eV, which originate from the Zn 3p emission band [37]. The formation of Au oxidized species in gold nanoparticles was reported previously and is plausible, particularly in small well-dispersed clusters [38]. An evaluation of the Au@ZnO films after the aging process at 310 ◦ C in the same atmosphere used for the gas sensing tests (i.e., air and target gases, see Sensors 2021, 21, 4509 7 of 24 Section 3.4) corroborated the stability of the gold nanoparticles incorporated at the ZnO surface and their chemical state. Quantification of the components related to gold indicate higher amounts of metallic gold (2.6 at.%) compared to Au+ (1.8 at.%) and Au3+ (0.7 at.%); this represents a 51%, 35%, and 14% of Au0 , Au+ , and Au3+ , respectively, from the total gold content at the Au@ZnO sample. 3.2. Iron-Modified Zinc Oxide Films The SEM images of the iron-modified zinc oxide films (Fe2 O3 @ZnO) are presented in Figure 4a. These images show the modification of the ZnO structures with particles of flake- Sensors 2021, 21, 4509 like morphology particularly concentrated on the top of the rods (inset in Figure 4a). The 7 of 24 nanoflakes (NFs) are varied in size and do not exceed 20 nm in thick and 100 nm in diameter. The XRD pattern of these films corroborates the presence of the hexagonal ZnO phase determined earlier [17] and the appearance of new low-intensity diffractions. The first at Card 33.0◦No. 108905) andto 2θ corresponds is the consistent withofprevious (222) plane the cubic Fe Fe22OO33structures (Ia−3 space deposited by AACVD group, ICSD Card [39]. The second and third at 39.4° 2θ and 44.0° 2θ are connected with No. 108905) and is consistent with previous Fe2 O3 structures deposited by AACVD [39]. the rhombohedral ZnSiO 3 (R-3 space The second group, and third at 39.4ICSD ◦ 2θ Card No. and 44.0 340575) ◦ 2θ and cubic are connected Fe2the with O4Zn (Fd−3m space rhombohedral group, ZnSiO 3 ICSD Card group, (R-3 space No. 91940), mostNo. ICSD Card likely present 340575) and at theFeinterfaces cubic O 2 4 Zn (Fd of − Fe 3m O 3/ZnO 2space and group, ZnO/Si ICSD (from Card the No. substrate), 91940), mostrespectively. likely present This is consistent at the interfaces ofwith our Fe2 O previous 3 /ZnO observation and ZnO/Si (from for similarly structured the substrate), films [25]. respectively. This is consistent with our previous observation for similarly structured films [25]. Figure 4. (a) SEM images (the inset shows a magnified image of the top of the Fe decorated ZnO rod) and (b) XRD diffraction Figure 4. (a) pattern SEM of the Feimages (the inset shows a magnified image of the top of the Fe decorated ZnO rod) and (b) XRD diffrac- 2 O3 @ZnO structured film. tion pattern of the Fe2O3@ZnO structured film. STEM images of the Fe2 O3 @ZnO structures (Figure 5a) showed a rod-like morphology covered STEM byimages tiny particles. of the HRTEM Fe2O3@ZnOimages of these particles structures (Figure (Figure 5a) 5b) revealed showed crystalline a rod-like morphol- structures ogy coveredwith lattice by tiny spacingsHRTEM particles. of 0.26 nm and 0.28 images nm, particles of these assigned to the (002) (Figure 5b) plane of the revealed crys- hexagonal talline ZnO and structures withthe (222)spacings lattice plane ofofthe cubic 0.26 nm phase of the and 0.28 nm,Fe 2 O3 identified assigned to the by XRD, (002) plane ofrespectively. the hexagonalEDSZnOline-scan and profiles the (222)(Figure plane5c,d) recorded of the along the cubic phase red Fe of the line in Figure 5a 2O3 identified by confirm the presence of Zn and Fe in the rod area and only Fe in the XRD, respectively. EDS line-scan profiles (Figure 5c,d) recorded along the red outer area ofline the rod. in Fig- ure 5a confirm the presence of Zn and Fe in the rod area and only Fe in the outer area of the rod. ogy covered by tiny particles. HRTEM images of these particles (Figure 5b) revealed crys- talline structures with lattice spacings of 0.26 nm and 0.28 nm, assigned to the (002) plane of the hexagonal ZnO and the (222) plane of the cubic phase of the Fe2O3 identified by XRD, respectively. EDS line-scan profiles (Figure 5c,d) recorded along the red line in Fig- Sensors 2021, 21, 4509 ure 5a confirm the presence of Zn and Fe in the rod area and only Fe in the outer 8 of 24area of the rod. Sensors 2021, 21, 4509 8 of 24 Figure Figure 5. TEM 5. TEM image image of of a aZnO ZnOrodrodmodified modified with withFe 2O Fe 3 .3.(a) 2O (a)Low-magnification Low-magnification STEM STEMimage, (b) high-resolution image, TEM TEM (b) high-resolution image, and EDS line-scan profiles corresponding to (c) Zn and (d) Fe recorded along the red line image, and EDS line-scan profiles corresponding to (c) Zn and (d) Fe recorded along the red line in (a). in (a). The surface chemical state of the Fe2 O3 @ZnO films characterized by XPS is presented The surface in Figure chemical 6. Similar to our state of theobservations previous Fe2O3@ZnO on films characterized Au@ZnO by Zn films, the XPS2piscore- presented inlevel Figure 6. Similar spectrum to Fe of the our previous observations on Au@ZnO films, the Zn 2p core-level 2 O3 @ZnO films showed a doublet (∆BE = 23.2 eV) with the Zn spectrum 2p3/2 andofZnthe2pFe 1/22 O @ZnO peaks 3 films at centered showed a doublet 1021.8 eV and 1044.9(ΔBE eV =(Figure 23.2 eV) 6a),with the Zn 2p respectively, in 3/2 and Znagreement 2p1/2 peaks centered with at 1021.8 the previous eV and literature [25].1044.9 The ZneV 2p(Figure 6a), core-level respectively, peaks from Fe2 Oin agreement 3 @ZnO show with thenearly identical previous positions literature [25].toThe those Znof2pZnO films and core-level a slight peaks fromshift Fe2to O3lower @ZnObinding show nearly energiespositions identical in relation to to those those ofofZnOAu@ZnO films films and (see Figure a slight S1a, shift to Supplementary lower Materials, binding energies in relation and Figure 3a). to those of Au@ZnO films (see Figure S1a, Supplementary Materials, and Figure 3a). The O 1s spectrum of the Fe2O3@ZnO films (Figure 6b) reveals the presence of four components. The first of them, with binding energy at 530.1 eV, corresponds to oxygen lattice bonds (Fe–O and Zn–O). The second component at 531.1 eV is assigned to the oxy- gen defects in the matrix of metal oxides due to oxygen vacancies. The third component, at 532.1 eV, and the fourth component, at 532.8 eV, are associated with the presence of physical or chemical adsorbed oxygen, hydroxides, and H2O on the surface of the struc- tures, similarly to Au@ZnO film. These results indicate the Fe3+ ion incorporation into the ZnO lattice in agreement with the previous O 1s spectrum recorded on Fe2O3@ZnO films [25]. This is also consistent with the XRD results, which demonstrated the presence of the Fe2O4Zn compound at 44.0° 2θ (Figure 4b). Apart from the presence of one additional O 1s component in the Fe2O3@ZnO spectra compared to that of ZnO and Au@ZnO, one can also observe slight differences in the component positions, with the O 1s components of SensorsSensors 2021, 2021, 21, 21, 4509 4509 9 of 24 9 of 24 Figure 6. (a) 6. Figure Zn(a)2p,Zn (b)2p, O (b) 1s, and O 1s,(c) Fe (c) and 2p Fe core 2plevels XPS spectra core levels recorded XPS spectra on the recorded onFe 2OFe the 3@ZnO film.film. 2 O3 @ZnO The grey dots dots The grey represent the raw data, the represent the yellow raw data,solid line corresponds the yellow to the envelope/fitting solid line corresponds curve, and to the envelope/fitting theand curve, other thecolored curves other colored to thetodeconvo- curves the luted deconvoluted components.components. The O 1s spectrum 3.3. Copper-Modified of the Fe Zinc Oxide 2 O3 @ZnO films (Figure 6b) reveals the presence of four Films components. The first of them, with binding energy at 530.1 eV, corresponds to oxygen SEMbonds lattice images of the (Fe–O andcopper-modified Zn–O). The second zinc oxide films component also at 531.1 eV showed is assigned thetopresence the oxygen of NPs distributed defects in the matrix of metal oxides due to oxygen vacancies. The third component, at films at the top and along the rod walls (Figure 7a). The XRD pattern of these (Figure 7b)and 532.1 eV, showed the component, the fourth intense diffractions at 532.8 eV,that correspond are associated withtothe the hexagonal presence zinc oxide of physical or chemical adsorbed oxygen, hydroxides, identified above with additional diffractions at 32.9° and H O on the surface of the structures, 2 2θ, 39.4° 2θ, and 44.0° 2θ. The first is similarlyto connected tothe Au@ZnO film. These (110) plane of theresults indicate monoclinic the Fe3+ ion copper(II) incorporation oxide phase (C12/c1 into the ZnO space group, lattice in agreement with the previous O 1s spectrum recorded on Fe ICSD Card No. 98-062-8618). The second and third are assigned to the ZnSiO3 (R-3 space 2 O 3 @ZnO films [25]. This is also consistent with the XRD results, which demonstrated the presence of the group, ICSD Card No. 340575) and the cubic ZnCuO (0.85/0.15/1) compound (Fm-3m Fe2 O4 Zn compound at 44.0◦ 2θ (Figure 4b). Apart from the presence of one additional O space group, ICSD Card No. 181023), both present most likely at the interface of ZnO with 1s component in the Fe2 O3 @ZnO spectra compared to that of ZnO and Au@ZnO, one can thealso modifier observeNPs anddifferences slight the Si substrate, respectively. in the component The presence positions, with theof Odiffraction 1s components peaks of from theFe copper(I) oxide phase is not evident, probably due to the low amount of particles and 2 O3 @ZnO showing higher binding energies (~0.4 eV) than the O 1s components of ZnO their broad (Figure distribution. S1b, Supplementary However, their presence is not ruled out as the (101) diffraction Materials). of ZnO overlaps the highest intensityFigure The Fe 2p spectrum displayed in 6c shows diffraction twoofcharacteristic peak copper(I) oxide peaksphase at 711.1generally eV and 725.0ateV, appearing accompanied 36.4 2θ, e.g., forbythe twoICSD satellite Card peaks No.at00-005-0667. 719.2 eV and 733.0 eV obtained from the photoelectrons emitted from Fe 2p3/2 and Fe 2p1/2 , respectively. In total, the Fe 2p Sensors 2021, 21, 4509 10 of 24 spectrum contains eleven components: four multiplets of Fe 2p3/2 peak, two multiplets of Fe 2p1/2 peak, two satellite peaks, two surface peaks, and one characteristic pre-peak usually present in the Fe 2p3/2 spectrum. The results and the binding energies of the peaks are in good agreement with the literature and suggest a Fe3+ oxidation state for iron [25,40,41]. The total content of iron in the sample amounts is 7.5 at.%. 3.3. Copper-Modified Zinc Oxide Films SEM images of the copper-modified zinc oxide films also showed the presence of NPs distributed at the top and along the rod walls (Figure 7a). The XRD pattern of these films (Figure 7b) showed the intense diffractions that correspond to the hexagonal zinc oxide identified above with additional diffractions at 32.9◦ 2θ, 39.4◦ 2θ, and 44.0◦ 2θ. The first is connected to the (110) plane of the monoclinic copper(II) oxide phase (C12/c1 space group, ICSD Card No. 98-062-8618). The second and third are assigned to the ZnSiO3 (R-3 space group, ICSD Card No. 340575) and the cubic ZnCuO (0.85/0.15/1) compound (Fm-3m space group, ICSD Card No. 181023), both present most likely at the interface of ZnO with the modifier NPs and the Si substrate, respectively. The presence of diffraction peaks from Sensors 2021, 21, 4509 the copper(I) oxide phase is not evident, probably due to the low amount of particles and10 of 24 their broad distribution. However, their presence is not ruled out as the (101) diffraction of ZnO overlaps the highest intensity diffraction peak of copper(I) oxide phase generally appearing at 36.4 2θ, e.g., for the ICSD Card No. 00-005-0667. Figure 7. (a) SEM images (the inset shows a magnified image of the Cu decorated ZnO rods) and (b) XRD diffraction pattern of the copper-modified zinc oxide structured films. Figure 7. (a) SEM images (the inset shows a magnified image of the Cu decorated ZnO rods) and (b) XRD diffraction pattern of the copper-modified zincThe oxide structured STEM films. images in Figure 8a,b show the crystalline copper-modified and HRTEM zinc oxide rods covered by spherical NPs. These particles display lattice spacings of 0.26 nm The and 0.25STEM andcorresponding nm, first HRTEM images to thein Figure (002) plane8a,b show of the the crystalline hexagonal ZnO phasecopper-modified and second zinc tooxide either rods covered the (002) plane by spherical of the NPs.copper(II) monoclinic These particles oxide ordisplay lattice (111) cubic spacings copper(I) of 0.26 oxide nm and 0.25 nm, first corresponding to the (002) plane of the hexagonal ZnO phase and phase inferred by XRD. The EDS line-scan profiles of the components recorded along the red line second in Figure to either the8a(002) demonstrate plane ofthe thepresence of Zncopper(II) monoclinic (Figure 8c)oxide and Cuor(Figure 8d) incopper(I) (111) cubic the particles. These profiles also suggest that the CuO-Cu O NPs are between oxide phase inferred by XRD. The EDS line-scan profiles of the components recorded 2 5 and 25 nm in size. along the red line in Figure 8a demonstrate the presence of Zn (Figure 8c) and Cu (Figure 8d) in the particles. These profiles also suggest that the CuO-Cu2O NPs are between 5 and 25 nm in size. nm and 0.25 nm, first corresponding to the (002) plane of the hexagonal ZnO phase and second to either the (002) plane of the monoclinic copper(II) oxide or (111) cubic copper(I) oxide phase inferred by XRD. The EDS line-scan profiles of the components recorded along the red line in Figure 8a demonstrate the presence of Zn (Figure 8c) and Cu (Figure Sensors 2021, 21, 4509 11 of 24 8d) in the particles. These profiles also suggest that the CuO-Cu2O NPs are between 5 and 25 nm in size. Sensors 2021, 21, 4509 11 of 24 Figure Figure 8. TEM8. TEM images images of aofcopper-modified a copper-modifiedZnOZnOrod.rod. (a) (a) Low-magnification Low-magnification TEMTEM image, (b)(b) image, high-resolution TEMTEM high-resolution image, image, and EDS line-scan and EDS profiles line-scan of the profiles marked of the markedline linein in(a) (a) corresponding corresponding toto(c)(c) ZnZn and and (d)(d) Cu.Cu. AnAn XPS analysis of the copper-modified zinc oxide films reported characteristic Zn XPS analysis of the copper-modified zinc oxide films reported characteristic Zn 2p3/2 and Zn 2p1/2 core level peaks at 1021.9 eV and 1045 eV, respectively, separated by 2p3/2 and Zn 2p1/2 core level peaks at 1021.9 eV and 1045 eV, respectively, separated by 23.1 23.1 eV and with a shake-up peak at 1040.3 eV (Figure 9a), similarly to previous studies [25]. eVThese and with a shake-up results peak at are comparable to 1040.3 those foreVFe(Figure 9a), similarly to previous studies [25]. 2 O3 @ZnO as the Zn 2p doublet is located These almostresults at theare comparable same to those binding energies for for Fe2O3@ZnO as the the copper-modified zincZn 2p doublet oxide and Fe2 Ois located al- 3 @ZnO. most at the samethe Consequently, binding energiesoffor Zn 2p doublet thethe copper-modified copper-modified zinc oxide zinc oxide and films is Fe2O3at@ZnO. located Consequently, higher and lower the binding Zn 2p doublet of the energies than copper-modified those registered for thezinc ZnOoxide films is films, and Au@ZnO located at respectively higher (see binding and lower Figure S1a, Supplementary energies than those Materials, and Figure registered for the3a). ZnO and Au@ZnO films, respectively (see Figure S1a, Supplementary Materials, and Figure 3a). The O 1s spectrum (Figure 9b) of the copper-modified zinc oxide films contains four components. These are located at 530.4 eV (from Cu–O and Zn–O bonds), 531.2 eV (from oxygen defects in the matrix of metal oxides related to oxygen vacancies), and 532.1 eV and 533.0 eV (from chemisorbed oxygen, hydroxides, and H2O on the surface of the cop- per-modified zinc oxide film). These components are in agreement with the O1s spectra of the previously described materials. The copper-modified zinc oxide films also displayed two doublets with components in Cu 2p3/2 at 932.6 eV and 934.2 eV, and Cu 2p1/2 at 952.3 eV and 953.9 eV (Figure 9c). The doublet with lower intensities (at 932.6 eV and 952.3 eV) indicates the presence of a Cu+ Sensors 2021, 21, 4509 12 of 24 Sensors 2021, 21, 4509 12 of 24 Figure 9. 9. (a) (a) Zn Zn 2p, 2p,(b) (b)OO1s, 1s,and and(c) (c)Cu Cu2p2pcore core levels levels XPS XPS spectra spectra forfor CuCu 2O@ZnO 2 O@ZnO film. film. Grey Grey dots dots represent represent the the rawraw data, data, the the solid yellow line corresponds to the envelope/fitting curve, and the other colored curves to the deconvoluted solid yellow line corresponds to the envelope/fitting curve, and the other colored curves to the deconvoluted components. compo- nents. The O 1s spectrum (Figure 9b) of the copper-modified zinc oxide films contains four 3.4. Gas Sensing components. Testsare located at 530.4 eV (from Cu–O and Zn–O bonds), 531.2 eV (from These oxygenGasdefects sensingin the matrix tests of theof metal oxides films structured related(i.e., to oxygen vacancies), Fe ZnO, Au@ZnO, and 2O532.1 3@ZnO, eV and 533.0 Cu eV (from 2O@ZnO) werechemisorbed carried outoxygen, to various hydroxides, and Hdioxide, gases (nitrogen 2 O on the surfaceacetone, ethanol, of the copper- carbon modified zinc monoxide, andoxide film).byThese toluene) usingcomponents dc resistance aremeasurements. in agreement with the O1s spectra To determine of the the thermal previously described materials. dependence of the response, the structures were initially tested to 80 ppm of ethanol at Theoperating various copper-modified zinc oxide temperatures, from films 200 also to 310displayed °C. These tworesults doublets with components showed that the dy- in Cu 2p namic and at 932.6 eV and 934.2 eV, and 3/2magnitude of the response improved 1/2 Cu 2p at 952.3 eV and 953.9 as the operating temperature eV (Figure 9c). increased, The doublet with lower intensities (at 932.6 eV and 952.3 eV) indicates registering the highest and fastest responses at the highest temperature tested. Therefore, the presence of + a Cusubsequent oxidationtests statewere fromcarried Cu2 O,outwhile the°Cdoublet the at 310 using with higher intensities four samples (at 934.2type of each structure eV and 953.9 eV) corresponds to Cu 2+ oxidation state from CuO [42]. The additional broad and performing at least three replicates for each condition. peakFigure with low intensity in the 10 summarizes the area between obtained 940towards results eV and 950 eV could 10 ppm be assigned of nitrogen dioxide to and the satellite peak coming from the Cu 2+ component [43]. This is consistent with the XRD and 80 ppm of ethanol, acetone, carbon monoxide, and toluene. Overall, results reveal a sig-+ TEM analysis nificant above, inofwhich improvement the ZnO both species due response weretoalso inferred. Quantification its modification with Au NPs. of One the Cu can and Cu 2+ species at the surface of the samples suggest the presence of a higher amount of also notice that the ZnO rods modified with Fe2O3 nanoflakes show an improvement com- Cu2 O (0.93 at.%) than CuO (0.13 at.%) at the surface of the samples; i.e., 87.7% of Cu2 O and pared to the non-modified ZnO, although not as high as that reported for the ZnO modi- 12.3% of CuO from the total content of copper oxides at the copper-modified zinc oxide fied with Au. In contrast, the response observed for the ZnO structures modified with Sensors 2021, 21, 4509 13 of 24 sample. Therefore, hereafter the discussion centers on the major copper oxide component (i.e., Cu2 O), and the copper-modified zinc oxide films are abbreviated as Cu2 O@ZnO. 3.4. Gas Sensing Tests Gas sensing tests of the structured films (i.e., ZnO, Au@ZnO, Fe2 O3 @ZnO, and Cu2 O@ZnO) were carried out to various gases (nitrogen dioxide, ethanol, acetone, carbon monoxide, and toluene) by using dc resistance measurements. To determine the thermal dependence of the response, the structures were initially tested to 80 ppm of ethanol at various operating temperatures, from 200 to 310 ◦ C. These results showed that the dynamic and magnitude of the response improved as the operating temperature increased, regis- tering the highest and fastest responses at the highest temperature tested. Therefore, the subsequent tests were carried out at 310 ◦ C using four samples of each structure type and performing at least three replicates for each condition. Figure 10 summarizes the obtained results towards 10 ppm of nitrogen dioxide and Sensors 2021, 21, 4509 80 ppm of ethanol, acetone, carbon monoxide, and toluene. Overall, results reveal a significant improvement of the ZnO response due to its modification with Au NPs. One can also notice that the ZnO rods modified with Fe2 O3 nanoflakes show an improvement compared to the non-modified ZnO, although not as high as that reported for the ZnO modified Cu with Au. In contrast, the response observed for the ZnO structures modified 2O NPs showed a decrement of the response compared to the non-modified ZnO with Cu2 O NPs showed a decrement of the response compared to the non-modified tures. ZnO structures. 10.Responses Figure 10. Figure Responsestowards 10 ppm towards 10ofppm nitrogen dioxide and of nitrogen 80 ppmand dioxide of ethanol, 80 ppm acetone, carbon acetone of ethanol, monoxide, and toluene for the ZnO, Au@ZnO, Fe2 O3 @ZnO, and Cu2 O@ZnO sensors. monoxide, and toluene for the ZnO, Au@ZnO, Fe2O3@ZnO, and Cu2O@ZnO sensors. Figure 11 displays the results obtained by testing the sensors to various concentrations Figure of each 1110displays gas (1 to the results ppm for nitrogen dioxideobtained and 20 toby testing 80 ppm the sensors for ethanol, acetone,tocarbon various con monoxide, tions and gas of each toluene). (1 toOverall, 10 ppmthefor four structures nitrogen demonstrate dioxide and 20their tohighest 80 ppm responses for ethanol, a to nitrogen dioxide, despite the lower concentrations of nitrogen dioxide compared to carbon monoxide, and toluene). Overall, the four structures demonstrate their hig the other gases. The results also show a proportional increase in the response with the sponses to nitrogen gas concentration. dioxide, The tests despitethe corroborated the lower best concentrations response for the Au@ZnO of structures nitrogen dioxid pared among tothethe otherother gases. structures forThe results all tested gases also andshow applieda concentrations. proportional For increase in the re instance, with the gas concentration. The tests corroborated the best response for the A the Au@ZnO response to 10 ppm of nitrogen dioxide is about 47 times higher than that of ZnO (see Figure structures among11a,b). theThe difference other in the for structures response of these all tested two structures gases and applied to 80concentratio ppm of ethanol is similar, showing around 45 times higher response for Au@ZnO compared instance, the Au@ZnO response to 10 ppm of nitrogen dioxide is about 47 times than that of ZnO (see Figure 11a,b). The difference in the response of these two str to 80 ppm of ethanol is similar, showing around 45 times higher response for A compared to ZnO. For the low concentration, the difference in the responses of A carbon monoxide, and toluene). Overall, the four structures demonstrate their highest re- sponses to nitrogen dioxide, despite the lower concentrations of nitrogen dioxide com- pared to the other gases. The results also show a proportional increase in the response with the gas concentration. The tests corroborated the best response for the Au@ZnO Sensors 2021, 21, 4509 structures among the other structures for all tested gases and applied concentrations. For 14 of 24 instance, the Au@ZnO response to 10 ppm of nitrogen dioxide is about 47 times higher than that of ZnO (see Figure 11a,b). The difference in the response of these two structures to 80 ppm of ethanol is similar, showing around 45 times higher response for Au@ZnO to ZnO. For compared to the ZnO.low concentration, For the difference the low concentration, in the responses the difference of Au@ZnO in the responses and ZnO of Au@ZnO is still and ZnO significant. For example, is still significant. the responses For example, of Au@ZnO the responses sensors to of Au@ZnO 1 ppmtoof1nitrogen sensors ppm of dioxide and nitrogen 20 ppm dioxide and of 20 ethanol are approximately ppm of ethanol 37 and 25 are approximately 37times and 25higher, respectively, times higher, respec- in comparison to the response of ZnO. tively, in comparison to the response of ZnO. Sensors 2021, 21, 4509 14 of 24 Figure11. Figure 11.Response Responseof ofthe the(a) (a)ZnO ZnO(b) (b)Au@ZnO, Au@ZnO,(c) (c)Fe Fe22O O33@ZnO, @ZnO, and and (d) (d) Cu Cu22O@ZnO O@ZnO sensors towards various sensors towards various concentra- concentra- tions of each target analyte. tions of each target analyte. Figure 12 Figure 12 displays displays the the electrical electrical resistance resistance changes changes recorded recorded on on the theZnO, ZnO,Au@ZnO, Au@ZnO, Fe22O33@ZnO, Fe @ZnO, and and Cu22O@ZnO O@ZnO structures structures to to each each gas and concentration. Overall the struc- struc- tures indicate an n-type semiconducting behavior, reporting an increase in tures indicate an n-type semiconducting behavior, reporting an increase in the electrical the electrical resistance to resistance to nitrogen nitrogen dioxide dioxide (oxidizing (oxidizing gas) gas) and and aa decrease decrease inin the the resistance resistance inin front frontof of reducinggases reducing gasessuch suchas as ethanol, ethanol,acetone, acetone,carbon carbonmonoxide, monoxide,and andtoluene. toluene.TheTheresponses responsesare are reversible reversiblefor foreach eachanalyte analyteand andconcentration, concentration, showing showing a full recovery a full recoveryof the initial of the baseline initial base- resistance after after line resistance purging the target purging analytes. the target analytes. Sensors 2021, 21, 4509 15 of 24 Sensors 2021, 21, 4509 15 of 24 Figure12. Figure 12. Electrical Electrical resistance resistance changes changes to to various variousconcentrations concentrationsof of(a) (a)nitrogen nitrogendioxide, dioxide,(b) (b)ethanol, ethanol, (c) acetone, (d) carbon monoxide, and (e) toluene. (c) acetone, (d) carbon monoxide, and (e) toluene. Sensors 2021,21, Sensors2021, 21,4509 4509 16 16of of24 24 Figure 13 Figure 13sums sumsupupthetheresponse response(t(tRR)) and and recovery recovery(t(trr)) time time of of each each structure structuretowards towards nitrogen dioxide nitrogen dioxide and and ethanol. ethanol. As As itit can can bebe noticed, noticed, the theresponse responsetime timetotonitrogen nitrogendioxide dioxide does not does not show show significant changes among among the the four four structures, structures,showing showingperiods periodsbetween between4 4and and66min. min.However, However,thisthis is is different different for the recovery recovery time, time, which which demonstrates demonstrates faster faster recoveryfor recovery forthe theAu@ZnO Au@ZnO(5 (5min) min)and andFe O33@ZnO (8 min) structures Fe22O structures compared comparedto tothe theZnO ZnO (22 min) and Cu O@ZnO (15 min) structures. The results for ethanol, in (22 min) and Cu2 O@ZnO (15 min) structures. The results for ethanol, in contrast, reveal the 2 contrast, reveal the fastest fastest response response timetime for Au@ZnO for Au@ZnO (1.5 (1.5 min)min) and and nearlynearly similarsimilar recovery recovery timetime (4 min) (4 min) for for ZnO, ZnO, Au@ZnO, Au@ZnO, andand Fe2 OFe3 @ZnO. 2O3@ZnO. Figure13. Figure 13. (a) (a) Response Response and and (b) (b) recovery recoverytimes timesto to11ppm ppmof ofnitrogen nitrogendioxide dioxideand and20 20ppm ppmof ofethanol ethanol recordedwith recorded withthethenon-modified non-modifiedand andmodified modifiedZnO ZnOstructures. structures. A A summary summary of of the the sensitivity sensitivity ofof each each structure structure to to the the target target analytes analytes in in the the tested tested concentrations concentrations range is presented presented in inTable Table 1. 1.These Theseresults resultsevidence evidencethe thehigh high sensitivity sensitivity of of Au@ZnO and Au@ZnO and Fe2O32@ZnOFe O 3 @ZnO structures to nitrogen dioxide with an estimated structures to nitrogen dioxide with an estimated limit of detec- limit of detection tion (LOD)(LOD) for each for each system system of 26ofand 26 and 114 114 ppb,ppb, respectively. respectively. TheThe results results also also suggest suggest an an improvement of the selectivity of ZnO to nitrogen dioxide by its modification with AuAu improvement of the selectivity of ZnO to nitrogen dioxide by its modification with or or FeFe NPs. NPs. The The partialselectivity partial selectivityofofAu@ZnO Au@ZnOtotoethanol ethanolamong among the the other other reducing reducing gases gasesisis also also worth worth itit of of mention. mention. −1 Table1.1. Sensitivity Table Sensitivity (∆R/∆C) (ΔR/ΔC) inin%%ppm ppm−1 to to ethanol, ethanol, acetone, acetone, carbon carbon monoxide, monoxide, andand toluene toluene for for con- concentrations between centrations between 20 and 20 and 80 ppm 80 ppm and nitrogen and nitrogen dioxidedioxide for concentrations for concentrations betweenbetween 1 and 101 ppm. and 10 ppm. Analyte ZnO Au@ZnO Fe2O3@ZnO Cu2O@ZnO Analyte Nitrogen dioxide ZnO 0.73 Au@ZnO 39.96 Fe2 O 3 @ZnO 7.48 Cu2 O@ZnO 0.22 NitrogenEthanol 0.73 0.05 39.96 1.79 7.48 0.27 0.04 0.22 dioxide Acetone 0.03 0.68 0.26 0.02 Ethanol 0.05 1.79 0.27 0.04 Carbon Monoxide 0.03 Acetone 0.02 0.68 0.55 0.26 0.15 0.02 0.02 CarbonToluene 0.02 0.31 0.08 0.002 0.02 0.55 0.15 0.02 Monoxide Toluene The 0.02 to reduce gases test of the sensors 0.31 0.08 revealed sensor in humid ambient 0.002 response losses by increasing the relative humidity (RH). In contrast, the test to nitrogen dioxide in humidTheambient showed test of the responses sensors withgases to reducing a tendency to increase, in humid ambientalthough revealed with poor sensor stabil- response ity and high signal noise. The greatest response changes in both cases were registered losses by increasing the relative humidity (RH). In contrast, the test to nitrogen dioxide by Sensors 2021, 21, 4509 17 of 24 in humid ambient showed responses with a tendency to increase, although with poor stability and high signal noise. The greatest response changes in both cases were registered by increasing the humidity from 0% RH to 20% RH. Above this value, the changes were negligible due to the saturation of the response to relative humidity, as noticed previously for similar AACVD ZnO structures [44]. The loss of the response for ethanol, acetone, and carbon monoxide is displayed in Figure S3, Supplementary Materials. These results show that the Au@ZnO sensors lose 30% of their response to ethanol and acetone and 18% to carbon monoxide, whereas the loss of response for the Fe2 O3 @ZnO sensors is about 20% towards ethanol and acetone and 2% to carbon monoxide. The changes in the response due to humidity for the Cu2 O@ZnO and ZnO sensors were minor, and the highest losses (8% and 2%) were recorded for Cu2 O@ZnO to ethanol and ZnO to carbon monoxide. 3.5. Gas Sensing Mechanism Chemoresistive gas sensors based on MOXs rely on the changes in the sensitive- material resistance upon exposure to target gases. These changes depend to a great extent on the gas analytes and their adsorption and desorption at the MOX surface. The gas sensing tests in this work demonstrate that the resistance changes of ZnO to nitrogen dioxide are significant compared to the other gases, and so that at first, we focus the discussion on the possible sensing mechanism of ZnO towards nitrogen dioxide. In the first phase, the sensing mechanism involves the interaction of the ZnO surface with air. Consequently, oxygen is adsorbed at the ZnO surface by trapping electrons from its conduction band and forming chemisorbed species in the form of O2 − , O− , or O2− . However, due to the high operating temperatures employed in the test, O− and O2− species tend to be dominant at the surface [45]. This process leads to the formation of an electron depletion layer at the surface and, accordingly, an increase in the ZnO electrical resistance (Figures 14a and 15a). In the second phase, when the sensing material is exposed to a target gas, such as nitrogen dioxide, the adsorbed oxygen species react with the gas. As nitrogen dioxide is a strong oxidizing agent with electrophilic properties, it acts as an electron acceptor, pulling out electrons from the conduction band (CB). Consequently, the electron depletion layer width increases (Figures 14a and 15a), leading to an increase in the sensor resistance. According to previous studies [46], the reaction between nitrogen dioxide and the formed oxygen species may be as follows: NO2 (gas) + O2− + 2e− → NO2− (ads) + 2O− (ads) (1) The sensing mechanism of the Au, Fe2 O3 , and Cu2 O modified ZnO structures, apart from the described above, involves additional synergistic effects due to the materials modifiers. These effects generally include spill-over of reactive species from the modifiers to ZnO and/or electronic effects due to the formation of extra nano-interfaces that facilitate the charge transfer during the gas–solid interactions at the surface. The modified materials can also have an impact on surface defects and oxygen vacancies (Ov) with respect to the pristine structure. This, in part, also influences the sensing performance of the material, to the extent that the oxygen vacancies act as donors modifying the baseline conductivity of the sensor material and consequentially the sensor response [47,48]. The XPS components associated with the oxygen vacancies in the results above indicate a higher amount of vacancies at the Au@ZnO films (19.1 at.%) compared to Cu2 O@ZnO (15.4 at.%), Fe2 O3 @ZnO (10.6 at.%), and pure ZnO (7.8 at.%) films, which partially explains the better gas sensing performances of the modified Au@ZnO and Fe2 O3 @ZnO. However, this is not consistent with the Cu2 O@ZnO results, which show a different behavior most likely influenced by the type of interface formed between these materials. The band energy of each system also has a role in the sensing response, and therefore below, we focus the discussion on the effect of each modifier material on the band bending and band alignment at the surface and the sensing properties of each ZnO-based system. mechanism of Au@ZnO films rely mainly on the metallic Au state. Hence, the perfor- mance improvements could be linked particularly to two phenomena—the formation of nano Schottky junctions between Au and ZnO and the catalytic activity of Au NPs. How- ever, the electronic influence of the Au oxidized species is not ruled out, which may en- Sensors 2021, 21, 4509 hance the interaction of the gold nanoparticles with ZnO and contribute to better gas18sens- of 24 ing performance, as observed previously [26]. Figure Figure 14. 14. Schematic Schematic representation representation of the possible of the possible sensing sensing mechanism mechanism of (a) ZnO, of (a) (b) Au@ZnO, ZnO, (c) Fe (b) Au@ZnO, 2 O3 @ZnO, and (d) (c) Fe2O Cu2 O@ZnO to3@ZnO, nitrogenand (d) Cu2O@ZnO to nitrogen dioxide. dioxide. Schottky junctions are formed when contacting a noble metal (e.g., Au) and a MOX (e.g., ZnO) with different work functions (WF), 4.2 eV (ZnO) [50] and 5.1 eV (Au) [51], and consequentially different Fermi level energies. Since the WF of Au is higher than that of ZnO, most of the energy required to withdraw an electron from bulk to surface is higher formation of Schottky junction and catalytic properties of Au. The formation of hetero- junctions also shows advantages in the sensing performance of the structures modified with Fe, although it shows an opposite effect for the structures modified with Cu, most likely due to the difference in the conductivity type of Fe2O3 (n-type) and Cu2O (p-type) Sensors 2021, 21, 4509 semiconductors. Generally, Fe2O3 and Cu2O could also act as catalyzers, but in light 19 ofofthe 24 dynamic of response observed, their catalytic activity is not the main parameter that in- fluences the response of these modified structures. Figure 15. Figure Energyband 15.Energy banddiagrams diagramsofof (a)(a) ZnO, ZnO, (b)(b) Au@ZnO, Au@ZnO, (c)2O (c) Fe Fe3@ZnO, 2 O3 @ZnO, and (d)andCu (d) Cu2 O@ZnO 2O@ZnO films films at at room room temperature temperature and before and before contactcontact between between ZnO with ZnO with Au, FeAu, 2O3Fe 2 OCu , or 3 , or 2O Cu 2 O column), (first (first column), after contact and inand after contact air in (second column), air (second and inand column), gasin(third column). gas (third column). 3.5.1. Gold-Modified Zinc Oxide Films The results above showed that the decoration of the pristine ZnO structures with Au improves the gas sensing performance of the first, similarly to the previous literature results for noble metal modified ZnO systems [24,49]. The chemical composition of the Au@ZnO films corroborated the incorporation of gold (Au0 , Au+ , Au3+ ) at the ZnO surface with a major presence of Au0 (2.6 at.%) than Au+ (1.8 at.%) or Au3+ (0.7 at.%), suggesting the sensing mechanism of Au@ZnO films rely mainly on the metallic Au state. Hence, the performance improvements could be linked particularly to two phenomena—the formation of nano Schottky junctions between Au and ZnO and the catalytic activity of Au NPs. However, the electronic influence of the Au oxidized species is not ruled out, which may enhance the interaction of the gold nanoparticles with ZnO and contribute to better gas sensing performance, as observed previously [26]. Sensors 2021, 21, 4509 20 of 24 Schottky junctions are formed when contacting a noble metal (e.g., Au) and a MOX (e.g., ZnO) with different work functions (WF), 4.2 eV (ZnO) [50] and 5.1 eV (Au) [51], and consequentially different Fermi level energies. Since the WF of Au is higher than that of ZnO, most of the energy required to withdraw an electron from bulk to surface is higher compared to ZnO. Because of that, free electrons from ZnO are more prone to move from ZnO to Au until the Fermi energies become equal (Figure 15b). Therefore, in the first phase of the gas sensing mechanism, when Au@ZnO is exposed to air, the electron transferring from ZnO to Au leads to the adsorption of more oxygen species and, in turn, the formation of a thicker depletion layer compared to the pristine ZnO films for the same conditions. Consequently, the resistance of the Au@ZnO sensors is higher than that of the ZnO sensors. This is consistent with the electrical resistance registered for these sensors (Figure 12). In the second phase of the mechanism, when Au@ZnO is exposed to nitrogen dioxide, adsorbed oxygen species will react with the gas, and more electrons will be consumed from the conduction band of the modified ZnO films compared to the non-modified one. This leads to a proportional increase in the electron depletion layer width (Figures 14b and 15b) and, eventually, a higher increase in the Au@ZnO resistance compared to the ZnO sensors. Additionally, the catalytic activity of Au NPs, as reported previously in the litera- ture [52,53], may play a role in accelerating the kinetics of the surface reactions during the sensing mechanism. This is consistent with the better dynamics of the Au@ZnO sensor’s response (Figure 12), which shows faster response and recovery times, particularly for the reducing gases (Figure 13) in contrast to the ZnO sensors. 3.5.2. Iron- and Copper-Modified Zinc Oxide Films Although Fe2 O3 @ZnO and Cu2 O@ZnO rely on similar gas sensing mechanism princi- ples because both systems present the formation of nano-heterojunctions at the interface of the ZnO rods and the decorating MOX (Fe2 O3 or Cu2 O) particles, the difference between these materials lies in their type of conductivity. Cu2 O reports p-type semiconductor behav- ior [54], whereas Fe2 O3 may present either n- or p-type behavior [7]. Our previous studies, however, demonstrated that the Fe2 O3 films obtained via the AACVD of FeCl3 ·6H2 O have an n-type behavior with a typical increase in conductivity when exposed to reducing gases [39]. Therefore, hereafter we discuss the sensing mechanism of Fe2 O3 @ZnO and Cu2 O@ZnO, considering their n- and p-type behavior, respectively. Due to the different work function (5.88 eV [55]) and band-gap (2.2 eV [56]) of Fe2 O3 compared to ZnO (4.2 eV and 3.3 eV [57], respectively), and higher position of the CB of ZnO, free electrons from the CB of ZnO migrate to the CB of Fe2 O3 . This is the opposite for the holes, which migrate from the valence band (VB) of Fe2 O3 to that of ZnO until the Fermi levels of both materials reach equilibrium (Figure 15c). These processes increase the number of electrons near the surface of the system so that when Fe2 O3 @ZnO sensors are exposed to air, more oxygen species can be adsorbed at the surface in comparison to pristine ZnO as displayed in Figure 14c. Consequentially, in the gas stage (e.g., nitrogen dioxide), more gas molecules react with the adsorbed oxygen, and a higher amount of electrons is trapped, enhancing the response of the Fe2 O3 @ZnO sensors compared to Zn. Similarly, Cu2 O, with a work function of 5.0 eV and a band-gap of 2.1 eV [54], has a CB below that of ZnO, meaning that the transfer of electrons occurs from the ZnO side towards the Cu2 O side as shown in Figure 15d. Analogously to the Fe2 O3 @ZnO sensor, more electrons will provide more adsorbed oxygen on the surface and more active sites for gas molecules to react (Figure 14d). However, in the case of p-type semiconductors, such as Cu2 O, which have as main carriers holes, the ionosorption of oxygen favors the conduction by holes (Figure 15d), contrary to n-type semiconductors. Therefore, when the oxidizing gas NO2 comes in contact with a p-type semiconductor, NO2 will trap the electrons, which will increase further the numbers of holes at the interface [58] and decrease the resistance of the whole Cu2 O@ZnO system partially; a similar effect was reported previously for instance for ZnO decorated with p-type Cr2 O3 [59]. This is most likely the rationale behind Sensors 2021, 21, 4509 21 of 24 the loss of sensitivity to nitrogen dioxide for the Cu2 O@ZnO sensors compared to the pristine ZnO and the modified ZnO with gold and iron (see Table 1). The presented mechanism described the sensing behavior of the different ZnO systems towards oxidizing gases (e.g., nitrogen dioxide). Reducing gases, such as ethanol, behave in an opposite way, providing electrons to ZnO and thus decreasing its resistance, as can be noticed in the electrical resistance changes in Figure 12b–e. As we can conclude from the previous, all three systems are influenced by the incor- poration of nano-interfaces, which provide slightly different gas sensing mechanisms that favor especially the sensing performance of the structures modified with Au due to the formation of Schottky junction and catalytic properties of Au. The formation of hetero- junctions also shows advantages in the sensing performance of the structures modified with Fe, although it shows an opposite effect for the structures modified with Cu, most likely due to the difference in the conductivity type of Fe2 O3 (n-type) and Cu2 O (p-type) semiconductors. Generally, Fe2 O3 and Cu2 O could also act as catalyzers, but in light of the dynamic of response observed, their catalytic activity is not the main parameter that influences the response of these modified structures. 4. Conclusions Zinc oxide structures were deposited and modified with Au, Fe2 O3 , and Cu2 O nanoparticles. The results showed the formation of ZnO rods (approximately 1.5 µm long with an average diameter of 200 nm) as well as the incorporation of crystalline Au nanoparticles (diameters between 5 and 40 nm), Fe2 O3 nanoflakes (thickness smaller than 20 nm and diameter of 100 nm), and Cu2 O nanoparticles (diameters between 5 and 25 nm). Chemical composition determination by XPS revealed the presence of Fe in the form of oxide—Fe2 O3 ; Cu in the form of two oxides—CuO and Cu2 O, with the major presence of Cu2 O; and Au in three oxidation states—Au3+ , Au+ , and Au0 , with the content of metallic Au (2.6 at.%) being the highest among the other states. Gas sensing results showed the best response for the Au@ZnO sensors among the others, especially for nitrogen dioxide, with about 47 times higher response to 10 ppm of nitrogen dioxide compared to the ZnO sensors. The sensitivity (39.96% ppm−1 ) and limit of detection (26 ppb) reported for this modified material to nitrogen dioxide were also higher compared to the other sensor structures. This was attributed to a combination of factors, including the oxygen vacancies, the gas sensing mechanism influenced by the nano-interfaces formed between ZnO and Au, and possible catalytic effects of the Au nanoparticles. Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/s21134509/s1. Figure S1: (a) Zn 2p, and (b) O 1s core levels XPS spectra for non-modified ZnO film. Grey dots represent the raw data, the solid yellow line corresponds to the envelope/fitting curve, and the other colored curves to the deconvoluted components. Figure S2: SEM images (the inset shows a magnified image of a single Ag decorated ZnO rod) of the Ag@ZnO structured film. Figure S3: Responses towards 80 ppm of ethanol, acetone, carbon monoxide for the ZnO, Au@ZnO, Fe2 O3 @ZnO, and Cu2 O@ZnO sensors at 0% and 20% RH. Author Contributions: Conceptualization, M.T. and S.V.; methodology, M.T., M.C., I.G., E.F., C.C.; validation, M.T., I.G.; formal analysis, M.T., M.C., S.V.; writing—original draft preparation, M.T.; writing—review and editing, S.V., M.C.; funding acquisition, S.V., C.C. All authors have read and agreed to the published version of the manuscript. Funding: This work has been supported by the Spanish Ministry of Science and Innovation via Grant PID2019-107697RB-C42 (AEI/FEDER, EU) and the Ramón y Cajal programme, and the Czech Science Foundation (GAČR) via Grant No 20-20123S. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Sensors 2021, 21, 4509 22 of 24 Acknowledgments: This research has made use of the infrastructures of the Spanish ICTS Net- work MICRONANOFABS, partially supported by MINECO, and Czech CEITEC Nano Research Infrastructure, supported by MEYS CR (LM2018110). Conflicts of Interest: The authors declare no conflict of interest. References 1. Potyrailo, R.A. Multivariable sensors for ubiquitous monitoring of gases in the era of Internet of Things and industrial internet. Chem. Rev. 2016, 116, 11877–11923. [CrossRef] 2. Sun, Y.F.; Liu, S.B.; Meng, F.L.; Liu, J.Y.; Jin, Z.; Kong, L.T.; Liu, J.H. 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