Oleg Lupan
email: oleg.lupan@mib.utm.md

Nicolai Ababii

David Santos-Carballal
email: d.santos-carballal@leeds.ac.uk

Maik-Ivo Terasa

Nicolae Magariu

Dario Zappa

Elisabetta Comini

Thierry Pauporté

Leonard Siebert

Franz Faupel

Thierry Pauporte

Alexander Vahl

Sandra Hansen

Nora H De Leeuw

Rainer Adelung

Tuning the Reactivity of Ultralow Power Heterojunction Sensors toward H2 and VOCs through Noble Metal Nanoparticle Functionalization

Keywords: Nanolayered materials, heterojunctions, semiconductor oxides, gas sensing, low-energy, ultralow power

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Introduction

Tuning the properties of semiconductor-powered devices by means of functional oxide nanomaterials is a state-of-the-art technological approach. Heterojunctions have gained attention for applications in optoelectronics and solar cells, but they can also be employed as versatile and excellent ultra-low power gas sensors owning to the high degree of phase control at the nanoscopic level [START_REF] Rasch | Highly selective and ultra-low power consumption metal oxide based hydrogen gas sensor employing graphene oxide as molecular sieve[END_REF][START_REF] Xue | Enhanced photoresponsivity of the MoS2-GaN heterojunction diode via the piezophototronic effect[END_REF][START_REF] Zappa | Metal oxide -based heterostructures for gas sensors"-A review[END_REF][START_REF] Hoppe | CuO-Cu2O)/ZnO:Al heterojunctions for volatile organic compound detection[END_REF][START_REF] Majhi | Recent advances in energy-saving chemiresistive gas sensors: A review[END_REF][START_REF] Kim | Highly Selective Sensing of CO, C6H6, and C7H8 Gases by Catalytic Functionalization with Metal Nanoparticles[END_REF]. Previous studies on the decoration of semiconducting metal oxide nanostructures with nanoparticles (NPs), have demonstrated the versatile nature of such combined systems, e.g. in gas, UV and temperature sensors or for the discrimination of volatiles [START_REF] Ababii | Effect of noble metal functionalization and film thickness on sensing properties of sprayed TiO2 ultra-thin films[END_REF][START_REF] Vahl | Tuning the Reactivity of Ultralow Power Heterojunction Sensors toward H2 and VOCs through Noble Metal Nanoparticle Functionalization[END_REF][START_REF] Postica | Tuning ZnO Sensors Reactivity toward Volatile Organic Compounds via Ag Doping and Nanoparticle Functionalization[END_REF]. For example, a significant increase in sensitivity by a factor of ~ 60 and selectivity towards 200 ppm of ethanol vapors was achieved following surface functionalization of columnar ZnO:Fe films using AgO/Ag nanoparticles [START_REF] Hoppe | CuO-Cu2O)/ZnO:Al heterojunctions for volatile organic compound detection[END_REF], which was confirmed by quantum mechanical calculations showing that (AgO)5 clusters change the surface chemistry of the Fe-doped zinc oxide ZnO (10-10) surface towards ethanol detection [START_REF] Hoppe | CuO-Cu2O)/ZnO:Al heterojunctions for volatile organic compound detection[END_REF]. Wang et al. [START_REF] Niu | NiO nanoparticle-decorated SnO2 nanosheets for ethanol sensing with enhanced moisture resistance[END_REF] found that tetragonal rutile SnO2 nanosheets decorated with an optimum amount of 3 mol% of NiO nanoparticles increases by five times the response to 100 ppm of C2H5OH at 260 °C, compared to the sensor containing only pure SnO2 nanosheets. These findings were assigned to the formation of a p-n junction between the NiO nanoparticles and SnO2 nanosheets [START_REF] Niu | NiO nanoparticle-decorated SnO2 nanosheets for ethanol sensing with enhanced moisture resistance[END_REF]. Moreover, surface functionalisation using nanoparticles of noble metals and their alloys has been used previously to fabricate sensors for the detection of VOC molecules [START_REF] Postica | Tuning ZnO Sensors Reactivity toward Volatile Organic Compounds via Ag Doping and Nanoparticle Functionalization[END_REF][START_REF] Vahl | Surface functionalization of ZnO:Ag columnar thin films with AgAu and AgPt bimetallic alloy nanoparticles as an efficient pathway for highly sensitive gas discrimination and early hazard detection in batteries[END_REF]. Titanium dioxide (TiO2) nanometer-sized quantum dots (QDs) have been grown by Liu and coworkers, who found that Ag decoration led to a 6-fold increase in the sensing response toward 10-100 ppm NH3 gas at room temperature [START_REF] Liu | A room temperature operated ammonia gas sensor based on Ag-decorated TiO2 quantum dot clusters[END_REF]. Moreover, Wu et al. [START_REF] Zhao | Improving electrocatalytic activities of FeCo2O4@FeCo2S4@PPy electrodes by surface/interface regulation[END_REF] found that for in-situ electrochemical activation by the surface/interface modulation and to enhance the catalytic performances of nanomaterials, novel bi-functional sandwich-like electrocatalysts of FeCo2O4@FeCo2S4@PPy or MCo2O4@MCo2S4@PPy can be used, which reveals that the bi-product interface can motivate more active sites than a single nanomaterials interface [START_REF] Zhao | Improving electrocatalytic activities of FeCo2O4@FeCo2S4@PPy electrodes by surface/interface regulation[END_REF][START_REF] Zhao | Bi-interface induced multi-active MCo2O4@MCo2S4@PPy (M=Ni, Zn) sandwich structure for energy storage and electrocatalysis[END_REF][START_REF] Zhao | Tuning the Reactivity of Ultralow Power Heterojunction Sensors toward H2 and VOCs through Noble Metal Nanoparticle Functionalization[END_REF][START_REF] Liu | Boosting energy storage and electrocatalytic performances by synergizing CoMoO4@MoZn22 core-shell structures[END_REF].

Cite this paper as: O Lupan, N. Ababii Some advantages of noble metal modified nanoparticles compared with pure precious metal catalysts are: the deposition time of nanoparticles is quite small, the density of nanoparticles can be directed only with the deposition time and no precursor solutions need to be prepared.

An excellent example for the requirement of reliable gas sensors is n-butanol, which is a stimulating and narcotic volatile compound, used extensively as a solvent and extracting agent of organic synthesis intermediates [START_REF] Wang | Highly selective n-butanol gas sensor based on mesoporous SnO2 prepared with hydrothermal treatment[END_REF]. However, exposure to n-butanol vapours has adverse health effects in humans, causing symptoms like dizziness, headaches, somnolence or dermatitis [START_REF] Ababii | Effect of noble metal functionalization and film thickness on sensing properties of sprayed TiO2 ultra-thin films[END_REF][START_REF] Wang | Highly selective n-butanol gas sensor based on mesoporous SnO2 prepared with hydrothermal treatment[END_REF][START_REF]Right to Know Hazardous Substance Fact Sheet n-butyl alcohol[END_REF]. The traditional and most frequently used method for the detection of n-butanol is gas chromatography (GC) [START_REF] Wang | Highly selective n-butanol gas sensor based on mesoporous SnO2 prepared with hydrothermal treatment[END_REF][START_REF] Maddox | Production of n-butanol from whey filtrate using clostridium acetobutylicum N.C.I.B[END_REF][START_REF] Cheng | High yield bio-butanol production by solvent-producing bacterial microflora[END_REF], which is a complicated, timeconsuming and costly technique. The current trend of developing solid state gas sensors of low energy consumption, has led to the use of metal oxide semiconductors as good candidates to fabricate highly sensitive, selective and reliable n-butanol gas sensors [START_REF] Hoppe | CuO-Cu2O)/ZnO:Al heterojunctions for volatile organic compound detection[END_REF][START_REF] Wang | Highly selective n-butanol gas sensor based on mesoporous SnO2 prepared with hydrothermal treatment[END_REF][START_REF] Huang | Large-scale synthesis of flowerlike ZnO nanostructure by a simple chemical solution route and its gassensing property[END_REF].

Given that one of the important advantages to improve the highly selective sensitivity for certain specific gases and VOCs is the functionalization of the semiconducting metal oxide nanostructures surfaces with different types of noble metals [START_REF] Kim | Highly Selective Sensing of CO, C6H6, and C7H8 Gases by Catalytic Functionalization with Metal Nanoparticles[END_REF], this study reports the fabrication of gas sensors for VOCs and H2 using a titania/cupric oxide/cuprite (TiO2/CuO/Cu2O/glass) heterostructure decorated with noble metal nanoparticles. To gain insight into the sensing mechanism, we also present calculations based on the density functional theory (DFT) of the ternary heterojunction TiO2(111)/CuO(1 ̅ 11)/Cu2O(111) decorated with Pd7, Ag7 and Ag6Pt nanoparticles, where we calculate the adsorption energies, structures and work functions after impregnation with the noble metal nanoclusters. Despite the lattice mismatch introduced by the deposition of the monoclinic CuO(1 ̅ 11) and TiO2(111) thin film overlayers on the Cu2O(111) surface substrate with hexagonal symmetry, this is a thermodynamically stable heterostructure [START_REF] Lupan | Tuning the Reactivity of Ultralow Power Heterojunction Sensors toward H2 and VOCs through Noble Metal Nanoparticle Functionalization[END_REF][START_REF] Cretu | Synthesis, characterization and DFT studies of zinc-doped copper oxide nanocrystals for gas sensing applications[END_REF]. The change of selectivity of the sensors is investigated by examining the binding energies and charge transfers for the interactions of molecular hydrogen (H2), ethanol (C2H5OH) and n- This work demonstrates that the introduction of noble metal nanoparticles on the surface of TiO2/CuO/Cu2O presents a suitable route towards developing gas sensing devices with ultra-low energy consumption. The nanoscopic scale of the functionalisation also allows tuning, modulating and controlling the selectivity and response of these sensors, which can be used for practical applications requiring the discrimination between different VOCs and H2 gas.

Experimental

Synthesis of the CuO-Cu2O ultra-thin layers (20-60 nm of thickness) was achieved by sputtering an ultra-thin film of copper metal (Cu film with 20, 30, 40, 50 and 60 nm thicknesses) on top of a clean glass under vacuum conditions. The glass samples coated with the ultra-thin film of copper metal were thermally treated at 420C in an electrical furnace for 30 or 60 min under normal ambient conditions, because in the range of temperatures of 350C and 450C the mixed crystal phases of CuO/Cu2O are formed [START_REF] Lupan | Tuning the Reactivity of Ultralow Power Heterojunction Sensors toward H2 and VOCs through Noble Metal Nanoparticle Functionalization[END_REF][START_REF] Zhou | Facile synthesis of highly stable and porous Cu2O/CuO cubes with enhanced gas sensing properties[END_REF]. Next, titania (TiO2) films of 20 nm thick were spray-deposited on top of the layers of the mixed copper oxide phases, thus preparing six additional sample sets. Afterwards, the samples of the TiO2/CuO/Cu2O heterostructure were treated in air at 420C for another 30 min (for the Cu20, Cu30, Cu40 samples) or 60 min (for the Cu50, Cu60 samples), depending on their thickness, before allowing them to cool down to room temperature. The next step comprised the sputtering of Au top-contacts on the surface of the fabricated TiO2/CuO/Cu2O samples by means of a meander-shaped mask. The TiO2/CuO/Cu2O thin films were decorated with Ag and AgPt alloy nanoparticles (Ag NP and AgPt NP) using a high vacuum deposition system (custom-made) with attached Gas Aggregation Source (GAS, Haberland type [START_REF] Haberland | Thin films from energetic cluster impact: A feasibility study[END_REF]). Details of the deposition of noble metal alloy nanoparticles can be found in our earlier work [START_REF] Ababii | Effect of noble metal functionalization and film thickness on sensing properties of sprayed TiO2 ultra-thin films[END_REF][START_REF] Vahl | Single target sputter deposition of alloy nanoparticles with adjustable composition via a gas aggregation cluster source[END_REF]. For the preparation of thin film composites with Pd NPs, a similar custom-constructed chamber for deposition under high vacuum and with attached Haberland-type GAS was used [START_REF] Postica | Tuning ZnO Sensors Reactivity toward Volatile Organic Compounds via Ag Doping and Nanoparticle Functionalization[END_REF][START_REF] Vahl | Surface functionalization of ZnO:Ag columnar thin films with AgAu and AgPt bimetallic alloy nanoparticles as an efficient pathway for highly sensitive gas discrimination and early hazard detection in batteries[END_REF][START_REF] Veziroglu | PdO nanoparticles decorated TiO2 film with enhanced photocatalytic and self-cleaning properties[END_REF] and a technological approach developed by Lupan et al. [START_REF] Lupan | PdO/PdO2 functionalized ZnO:Pd films for lower operating temperature H2 gas sensing[END_REF].

A TiO2/CuO/Cu2O nanocomposite thin film, decorated with AgPt NPs, was investigated using X-ray photoelectron spectroscopy (XPS, Omicron Nano-Technology GmbH, Al, 240 W). The obtained XPS data were charge referenced with respect to aliphatic carbon C1s at 285.0 eV and analysed ("CasaXPS, version 2.3.16") [START_REF] Postica | Tuning ZnO Sensors Reactivity toward Volatile Organic Compounds via Ag Doping and Nanoparticle Functionalization[END_REF][START_REF] Vahl | Surface functionalization of ZnO:Ag columnar thin films with AgAu and AgPt bimetallic alloy nanoparticles as an efficient pathway for highly sensitive gas discrimination and early hazard detection in batteries[END_REF][START_REF] Veziroglu | PdO nanoparticles decorated TiO2 film with enhanced photocatalytic and self-cleaning properties[END_REF]. For the electrical studies, different bias voltages were applied to the functionalized samples and measurements were taken continuously and registered using a computer-controlled Keithley2400 sourcemeter using the LabView software (National Instruments). The gas response was determined as the ratio ( 𝑅 𝑔 -𝑅 𝑎 𝑅 𝑎 • 100%), where Rg and Ra stand for the resistance of the device structures exposed to gas or air, accordingly [START_REF] Lupan | Tuning the Reactivity of Ultralow Power Heterojunction Sensors toward H2 and VOCs through Noble Metal Nanoparticle Functionalization[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 general technological flow process for the manufacture of the TiO2/CuO/Cu2O heterostructure-based sensor devices, which is represented in Figure 1, consists of surface functionalization using Pd, Ag or AgPt nanoparticles. In the insert of Figure 1

Computational

We have used the Vienna Ab initio Simulation Package (VASP) [START_REF] Kresse | Tuning the Reactivity of Ultralow Power Heterojunction Sensors toward H2 and VOCs through Noble Metal Nanoparticle Functionalization[END_REF][START_REF] Kresse | Ab initio molecular-dynamics simulation of the liquid-metalamorphous-semiconductor transition in germanium[END_REF][START_REF] Kresse | Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set[END_REF][START_REF] Kresse | Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set[END_REF] [34,35]. Long-range dispersion interactions were included using the semi-empirical method of Grimme, with the Becke-Johnson damping [D3-(BJ)] [START_REF] Grimme | A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu[END_REF][START_REF] Grimme | Effect of the damping function in dispersion corrected density functional theory[END_REF], which is required for the realistic simulation of the bulk and surface properties of several different materials [START_REF] Postica | Tuning ZnO Sensors Reactivity toward Volatile Organic Compounds via Ag Doping and Nanoparticle Functionalization[END_REF][38][START_REF] Ungerer | Tuning the Reactivity of Ultralow Power Heterojunction Sensors toward H2 and VOCs through Noble Metal Nanoparticle Functionalization[END_REF][START_REF] Botha | Mixing thermodynamics and electronic structure of the Pt₁₋ₓNiₓ (0 ≤ x ≤ 1) bimetallic alloy[END_REF][START_REF] Santos-Carballal | Ab initio investigation of the thermodynamics of cation distribution and of the electronic and magnetic structures in the LiMn₂O₄ spinel[END_REF][START_REF] Posada-Pérez | CO₂ Interaction with Violarite (FeNi₂S₄) Surfaces: A Dispersion-Corrected DFT Study[END_REF][START_REF] Postica | Tuning doping and surface functionalization of columnar oxide films for volatile organic compounds sensing: Experiments and theory[END_REF][START_REF] Reguera | Synthesis, Crystal Structures, and Properties of Zeolite-Like T₃(H₃O)₂M(CN)₆]₂•uH₂O (T=Co, Zn; M=Ru, Os)[END_REF][START_REF] Ramogayana | Ethylene carbonate adsorption on the major surfaces of lithium manganese oxide Li1-xMn2O4 spinel (0.000 < x < 0.375): a DFT+ U -D3 study[END_REF]. The inner electron included up to the 3s level for Cu; 2p level for Ti; 4f level for Pt; 4s level for Pd and Ag; and 1s level for O, C and N, and were described by the projector augmented wave (PAW) method [START_REF] Blöchl | Projector augmented-wave method[END_REF][START_REF] Kresse | From ultrasoft pseudopotentials to the projector augmentedwave method[END_REF], whereas the electron of the H atom was treated as valence. The valence states were expanded in a periodic plane-wave basis set with a cut off at 400 eV for the kinetic energy. An energy threshold defining selfconsistency of the electron density was set at 10 -5 eV. An efficient conjugate-gradients technique based on the Newton line optimizer [START_REF] Sheppard | Optimization methods for finding minimum energy paths[END_REF][START_REF] Hestenes | Methods of conjugate gradients for solving linear systems[END_REF] was used for the relaxation of the geometries, which reached the ground state when the Hellmann-Feynman forces on all atoms were smaller than 0.01 eV•Å -1 . We have used the Hubbard approximation [START_REF] Anisimov | Spin bags, polarons, and impurity potentials in La2-xSrxCuO4 from first principles[END_REF] in the form of the Dudarev et al. [START_REF] Dudarev | Electronenergy-loss spectra and the structural stability of nickel oxide: An LSDA+U study[END_REF] scheme to improve the description of localized d states in this type of system, where the standard GGA functionals fail [START_REF] Moreira | Effect of Fock exchange on the electronic structure and magnetic coupling in NiO[END_REF]. The values for the onsite Coulomb interaction term in this study were Ueff = 4.0 eV for Cu and 4.4 eV for Ti [START_REF] Lupan | Tuning the Reactivity of Ultralow Power Heterojunction Sensors toward H2 and VOCs through Noble Metal Nanoparticle Functionalization[END_REF][START_REF] Cretu | Synthesis, characterization and DFT studies of zinc-doped copper oxide nanocrystals for gas sensing applications[END_REF][START_REF] Calderon | The AFLOW standard for high-throughput materials science calculations[END_REF][START_REF] Setyawan | Tuning the Reactivity of Ultralow Power Heterojunction Sensors toward H2 and VOCs through Noble Metal Nanoparticle Functionalization[END_REF][START_REF] Setyawan | High-throughput combinatorial database of electronic band structures for inorganic scintillator materials[END_REF][START_REF] Wang | Assessing the Thermoelectric Properties of Sintered Compounds via High-Throughput Ab-Initio Calculations[END_REF][START_REF] Taylor | A RESTful API for exchanging materials data in the AFLOWLIB.org consortium[END_REF]. However, no Hubbard Hamiltonian was applied to the d states of the noble metal NPs, as they are already delocalised given the electron conducting properties of these materials [START_REF] Andersen | Electronic structure of the fcc transition metals Ir, Rh, Pt, and Pd[END_REF][START_REF] Speier | Bremsstrahlung isochromat spectra and density-of-states calculations for the 3d and 4d transition metals[END_REF]. These criteria allowed convergence of the total electronic energy to within 1 meVatom -1 .

The electronic integrations of the face-centred cubic (fcc) transition metal phases were calculated in the reciprocal space using their primitive unit cells and a Γ-centred Monkhorst-Pack (MP) sampling grid [START_REF] Monkhorst | Special points for Brillouin-zone integrations[END_REF] containing 17 × 17 × 17 k-points, which was sufficient to simulate correctly their conducting electronic properties [START_REF] Andersen | Electronic structure of the fcc transition metals Ir, Rh, Pt, and Pd[END_REF][START_REF] Speier | Bremsstrahlung isochromat spectra and density-of-states calculations for the 3d and 4d transition metals[END_REF]. The nanoparticle-functionalised TiO2(111)/CuO (1 ̅ 11) /Cu2O(111) heterostructure was simulated using a Γ-centred MP mesh [START_REF] Monkhorst | Special points for Brillouin-zone integrations[END_REF] [START_REF] Lupan | Tuning the Reactivity of Ultralow Power Heterojunction Sensors toward H2 and VOCs through Noble Metal Nanoparticle Functionalization[END_REF][START_REF] Cretu | Synthesis, characterization and DFT studies of zinc-doped copper oxide nanocrystals for gas sensing applications[END_REF]. The isolated molecules were modelled in a cell with broken symmetry and dimensions of 20×21×22 Å 3 , considering only the Γ point of the Brillouin zone. The electronic partial occupancies of the isolated molecules were determined using the Gaussian smearing method with a smearing width of 0.05 eV. The method of Methfessel-Paxton order 1 with a sigma value of σ = 0.2 eV was applied to the fcc metal phases [START_REF] Methfessel | High-precision sampling for Brillouin-zone integration in metals[END_REF], which ensured a negligible variational quantity, i.e. electronic entropy [START_REF] Mermin | Thermal Properties of the Inhomogeneous Electron Gas[END_REF]. Moreover, the tetrahedron method with Blöchl corrections [START_REF] Blöchl | Improved tetrahedron method for Brillouinzone integrations[END_REF] was used for the calculation of the electronic and magnetic properties and to obtain very accurate total energies for the nanoparticle functionalised heteroepitaxial junctions before and after molecular adsorption of the gases.

Cite this paper as

X-ray photoelectron spectroscopy, Micro-Raman and UV-Visible-NIR spectroscopy characterization of CuO/Cu2O and TiO2/CuO/Cu2O nanocomposite

The X-ray photoelectron spectra (XPS) of a TiO2/CuO/Cu2O nanocomposite decorated with AgPt NPs are shown in Figure 3. Cu, O, Ti, Na and C elements were detected in both the nanocomposite as well as in the TiO2/CuO/Cu2O reference thin film, see the overview spectra in Figure 3a. The presence of Cu, O and Ti originates from the TiO2/CuO/Cu2O base layer, although the signal from carbon is due to contamination of the top surface with C from the atmosphere, e.g. from carbohydrates [START_REF] Postica | Tuning ZnO Sensors Reactivity toward Volatile Organic Compounds via Ag Doping and Nanoparticle Functionalization[END_REF][START_REF] Vahl | Surface functionalization of ZnO:Ag columnar thin films with AgAu and AgPt bimetallic alloy nanoparticles as an efficient pathway for highly sensitive gas discrimination and early hazard detection in batteries[END_REF][START_REF] Veziroglu | PdO nanoparticles decorated TiO2 film with enhanced photocatalytic and self-cleaning properties[END_REF]. The observation of the signal corresponding to Na is due to diffusion from the glass substrates, which were used for the Cite this paper as: O Lupan, N. Ababii [START_REF] Ababii | Effect of noble metal functionalization and film thickness on sensing properties of sprayed TiO2 ultra-thin films[END_REF][START_REF] Li | Tuning the Reactivity of Ultralow Power Heterojunction Sensors toward H2 and VOCs through Noble Metal Nanoparticle Functionalization[END_REF][START_REF] Fonné | Interdiffusion between silica thin films and soda-lime glass substrate during annealing at high temperature[END_REF]. Such peaks were not detected from samples deposited on quartz substrates (not shown here).

Figure 3b depicts the high-resolution spectra of the Cu-2p, Ti-2p, Ag-3d and Pt-4d lines.

A closer look at the Cu-2p line reveals clear satellite peaks for both the Cu-2p3/2 and Cu-2p1/2 lines, which are relocated to higher binding energies. The Cu-2p3/2 line, as well as the corresponding satellites, were assigned to the nanocompositesee deconvolution of 2p3/2 with satellite peaks into 3 subpeaks. The observed satellite peaks are generally regarded as a signature for the occurrence of the CuO/Cu2O heterojunction with CuO exposed at the surface [START_REF] Vahl | Tuning the Reactivity of Ultralow Power Heterojunction Sensors toward H2 and VOCs through Noble Metal Nanoparticle Functionalization[END_REF][START_REF] Postica | Tuning ZnO Sensors Reactivity toward Volatile Organic Compounds via Ag Doping and Nanoparticle Functionalization[END_REF]. Accordingly, the signal from copper in the spectrum can be assigned to the existence of CuO/Cu2O in the base layer.

A Ti-2p3/2 peak in the range 459.6 eV -458.0 eV is usually assigned to Ti 4+ in TiO2. The evaluation of high resolution Ti-2p XPS binding energies reveals that the Ti-2p3/2 line is positioned around 458.3 eV. The peak positions of Ti-2p3/2 and Ti-2p1/2 and their separation of 5.6 eV indicate the presence of Ti in the form of TiO2 in the base layer [START_REF] Moulder | Handbook of X-ray Photoelectron Spectroscopy[END_REF][START_REF] Alexander | NIST X-ray Photoelectron Spectroscopy Database[END_REF].

Ag and Pt were detected alongside Cu, Ti, O and C in the TiO2/CuO/Cu2O nanocomposite decorated using the AgPt NPs. Spectra at higher resolution of the Ag-3d and Pt-4d lines were recorded in order to evaluate the composition of the AgPt NPs. We chose the Pt-4d lines for quantification due to the overlap of Pt-4f with intensive Cu-3p lines. In the case of the nanocomposite, the quantification of the AgPt NPs yields a platinum content of roughly 15 at% within the nanoparticles. A detailed investigation of AgPt NPs can be found in our earlier works [START_REF] Ababii | Effect of noble metal functionalization and film thickness on sensing properties of sprayed TiO2 ultra-thin films[END_REF][START_REF] Vahl | Surface functionalization of ZnO:Ag columnar thin films with AgAu and AgPt bimetallic alloy nanoparticles as an efficient pathway for highly sensitive gas discrimination and early hazard detection in batteries[END_REF].

In addition to the functionalisation of the TiO2/CuO/Cu2O nanocomposite with AgPt NPs, we also used Pd nanoparticles [START_REF] Lupan | PdO/PdO2 functionalized ZnO:Pd films for lower operating temperature H2 gas sensing[END_REF] to prepare the decorated sensors in the context of this 15 [START_REF] Sahai | Cu/Cu2O/CuO nanoparticles: Novel synthesis by exploding wire technique and extensive characterization[END_REF][START_REF] Willets | Localized Surface Plasmon Resonance Spectroscopy and Sensing[END_REF]. In the case of the metal nanoclusters, the local SPRs are pledged for the field improvement, which undergo surface-enhanced processes [START_REF] Sahai | Cu/Cu2O/CuO nanoparticles: Novel synthesis by exploding wire technique and extensive characterization[END_REF][START_REF] Willets | Localized Surface Plasmon Resonance Spectroscopy and Sensing[END_REF], that are only observed when using surface-enhanced Raman spectroscopy. Only phonon frequencies of the crystalline Cu2O, CuO and TiO2 layers are observed with our experimental set-up, as the group theory indicates that the Raman response is a function of the space group symmetry of the crystal [START_REF] Ferraro | Basic Theory[END_REF].

Figure S5 illustrate the Ultraviolet, Visible and Near InfraRed (UV-Vis-NIR) absorption spectra for the Pd/TiO2/CuO/Cu2O heterostructures, which confirm the energy structures and optical properties of this transparent semiconducting oxide. The Tauc plots for the Pd/TiO2/CuO/Cu2O heterostructures, presented in Figure S6, reveal a bandgap that widens with the reduction of the nanoparticle size from 30 to 10 nm. As expected, the absorption edge of 2.04 eV for the 20 nm sample is equal to the value published for bulk Cu2O [START_REF] Law | Resonance Response of the Quantum Vacuum to an Oscillating Boundary[END_REF][START_REF] Wooten | Optical properties of solids[END_REF][START_REF] Rakhshani ; O Lupan | Tuning the Reactivity of Ultralow Power Heterojunction Sensors toward H2 and VOCs through Noble Metal Nanoparticle Functionalization[END_REF][START_REF] Balamurugan | Size-dependent conductivity-type inversion in Cu2O nanoparticles[END_REF].

Gas sensing properties

Heterostructures formed by p-n semiconductors, especially those functionalized with noble metals have a high catalytic activity and can be used to enhance the surface reactions of chemical sensing materials [START_REF] Ababii | Effect of noble metal functionalization and film thickness on sensing properties of sprayed TiO2 ultra-thin films[END_REF][START_REF] Postica | Enhancement in UV Sensing Properties of ZnO:Ag Nanostructured Films by Surface Functionalization with Noble Metalic and Bimetallic Nanoparticles[END_REF]. It is known that the functionalization of oxides (such as ZnO, SnO2, TiO2, Cu2O and Fe2O3) with different noble metals or their alloys improves significantly their sensing performance. In this section the sensing properties of the TiO2/CuO/Cu2O heterostructure functionalized with the Pd, Ag or AgPt NPs will be discussed.

Gas sensing properties of Pd/TiO2/CuO/Cu2O heterostructures

The gas sensing characteristics of the TiO2/CuO/Cu2O samples with CuO/Cu2O thickness of 20 nm (Cu20) functionalized using Pd NPs (Pd-NPs/TiO2/CuO/Cu2O) were investigated towards different volatile organic compounds and hydrogen. Our main aim is to show Figure 4a shows the response versus operating temperature to hydrogen, n-butanol, 2propanol, ethanol, acetone, ammonia and CH4 by the TiO2/CuO/Cu2O samples after surface functionalization with the Pd-NPs (for a CuO/Cu2O specimen of 20 nm of thickness labelled as Cu20 set sample), where we observe a high and selective response to H2 at all working temperatures. The responses are 405%, 487%, 543% and 371% at the operating temperatures of 150°C, 200°C, 250°C and 300°C, respectively. The optimal operating temperature was found within the range 200-250C. Figure 4b at an operating temperature of 250°C can be seen in Figure S7b. Figure 4c shows that the current increases with the operating temperature for the Pd/TiO2/CuO/Cu2O material.

Figure 4d shows the dynamic response to different hydrogen concentrations (5, 10, 50, 100, 500 and 1000 ppm) of the functionalized Pd/TiO2/CuO/Cu2O (Cu20) samples, where a fairly high response of 88% is registered at 5 ppm. The dynamic response to H2 gas with concentrations of 5 and 50 ppm is presented in Figure S8.

Figure S9a presents the response to hydrogen, n-butanol, 2-propanol, ethanol, acetone and ammonia of the Pd/TiO2/CuO/Cu2O (for a substrate with CuO/Cu2O thickness of 30 nm, referred as Cu30) samples, which possess the highest response towards H2 gas (> 550%).

Gas sensing properties of Ag/TiO2/CuO/Cu2O

The surface functionalization with the Ag nanoparticles of the TiO2/CuO/Cu2O samples (CuO/Cu2O with thickness of 60 nm (Cu60)) were realized next to tune the response to VOCs, namely to change the selectivity from ethanol (in the case of the CuO/Cu2O samples) to n-butanol.

Figure 5a shows the response to several gases versus the OPTs of the surface functionalized Ag/TiO2/CuO/Cu2O (Cu60) samples (CuO/Cu2O with thickness of 60 nm). We observe that, at the low operating temperatures of 200C, the sample has n-type conductivity and shows sensing capabilities only towards 2-propanol and ethanol, because the electrical resistivity decreases after the molecular interactions. At the working temperatures OPTs of 250C, 300C and 350C, the sample already has p-type conductivity and is therefore selective to n-butanol, where the responses are 54%, 200%, and 163%, respectively. This phenomenon is dictated by the composition of the sample, which change the surface activity and reaction products [START_REF] Ababii | Effect of noble metal functionalization and film thickness on sensing properties of sprayed TiO2 ultra-thin films[END_REF][START_REF] Lupan | Tuning the Reactivity of Ultralow Power Heterojunction Sensors toward H2 and VOCs through Noble Metal Nanoparticle Functionalization[END_REF][START_REF] Cretu | Synthesis, characterization and DFT studies of zinc-doped copper oxide nanocrystals for gas sensing applications[END_REF] by band bending and formation of electron depletion or space charge layer in the conduction band, so the conductivity of the active materials either decreases or increases [START_REF] Mathew | A review on mechanisms and recent developments in p-n heterojunctions of 2D materials for gas sensing applications[END_REF]. The lower operating temperature may not influence the depletion layer of electrons in TiO2, and the CuO/Cu2O layer has a higher resistance, the current flows through the TiO2 layer which is an n-type semiconductor, but at a higher operating temperature the depletion layer of electrons in TiO2 and the accumulation layer of holes in CuO/Cu2O are also influenced, thus leading to the CuO/Cu2O layer predominating in the sensing mechanism leading to the increase of the heterojunction resistance to the application of the target gas [START_REF] Xuemei | Fabrication of Cubic p-n Heterojunction-Like NiO/In2O3 Composite Microparticles and Their Enhanced Gas Sensing Characteristics[END_REF]. Figure 5b shows the dynamic response towards hydrogen, n-butanol, 2-propanol, ethanol and acetone for the Ag/TiO2/CuO/Cu2O (Cu60) samples at the optimal OPT of 300C. Figure 5c shows the current-voltage performance of the Ag/TiO2/CuO/Cu2O (Cu60) samples, where the conductance increases with the operating temperature in the tested range. Figure 5d displays the dynamic response to 5, 10, 50, 100, 500 and 1000 ppm of nbutanol for the Ag/TiO2/CuO/Cu2O (Cu60) samples, demonstrating that a response of about 31% is registered even for the lowest concentration of this gas. The dynamic response to 5, 50 and 100 ppm of n-butanol of the Ag/TiO2/CuO/Cu2O samples with CuO/Cu2O thickness of 60 nm (Cu60) at the operating temperature of 300°C is presented in Figure S10. Figure 5e displays the dynamic response to 100 ppm of n-butanol and consecutively 2-propanol, acetone, ethanol in air, of Ag/TiO2/CuO/Cu2O samples. Figure 5f displays the dynamic response to 100 ppm of n-butanol and mixtures of alcohols in air, with and without n-butanol vapors, with 1000 ppm of Ag/TiO2/CuO/Cu2O samples.

Gas sensing properties of AgPt/TiO2/CuO/Cu2O

Next, the response to different compounds by the AgPt/TiO2/CuO/Cu2O (Cu60) samples were studied at various working temperatures to investigate whether we can tune the selectivity by means of changing the composition of the NPs. Figure 6a shows the response to hydrogen, n-butanol, 2-propanol, ethanol and acetone versus the OPT for the surface-functionalized AgPt/TiO2/CuO/Cu2O samples with CuO/Cu2O thickness of 60 nm (Cu60). At the OPT of 250C, 300C and 350C, the sample is selective to n-butanol and the responses are 78%, 118%, and 216%, respectively, suggesting that the optimal operating temperature is therefore 350C. Figure 6b illustrate the dynamic response to hydrogen, n-butanol, 2-propanol, ethanol and acetone of the AgPt/TiO2/CuO/Cu2O (Cu60) samples at the operating temperatures of 350C. Figure 6c illustrate the current-voltage characteristics of AgPt/TiO2/CuO/Cu2O (Cu60) samples, where we observe that the conductance increases with the operating temperature. Figure 6d shows the dynamic response to 5, 10, 50, 100, 500 and 1000 ppm of n-butanol for the Cu60 AgPt/TiO2/CuO/Cu2O samples with CuO/Cu2O thickness of 60 nm, where a response of 63% is registered for 5 ppm of n-butanol. Figure 6e displays the dynamic response to 100 ppm of n-butanol and consecutively 2-propanol, acetone, ethanol in air of AgPt/TiO2/CuO/Cu2O samples. Figure 6f displays the dynamic response to 100 ppm of nbutanol and mixtures of alcohols in air with and without n-butanol vapors with 100 ppm of AgPt/TiO2/CuO/Cu2O samples. The dynamic response of the AgPt/TiO2/CuO/Cu2O samples (Cu60) to 5, 50 and 100 ppm of n-butanol at the OPT of 350°C is presented in Figure S11. Thus, chemical sensitization facilitates the adsorption of certain gas molecules on the metal NP surface, thus inducing more interactions with pre-absorbed oxygen species, facilitating relevant chemical reactions and therefore an improved gas detection performance [START_REF] Majhi | Recent advances in energy-saving chemiresistive gas sensors: A review[END_REF][START_REF] Kim | Highly Selective Sensing of CO, C6H6, and C7H8 Gases by Catalytic Functionalization with Metal Nanoparticles[END_REF]. CuO/Cu2O thickness of 20 nm (Cu20). According to the figure, both the power consumption and the response to the H2 gas decreases when the applied voltage is decreased. However, we also observed that for an applied bias voltage of 10 mV the response increased. The power consumption obtained is 270, 140, 4.1 and 0.19 nW, and the responses are 588%, 404%, 310% and 390%, at the applied bias voltages of 250, 100, 50 and 10 mV, respectively. The response to different gases versus the applied bias voltage for the Pd/TiO2/CuO/Cu2O samples with CuO/Cu2O thickness of 20 nm (Cu20) is represented in Figure S12. The dependence on the power consumption and the response to 100 ppm of n-butanol vapors versus applied bias voltage for Ag/TiO2/CuO/Cu2O with CuO/Cu2O thickness of 60 nm are represented in Figure 7c, where it can be seen that the obtained power consumption is 170, 56, 23.5 and 17.6 nW, and the respective responses are 104%, 121%, 200% and 210%, at the applied bias voltages of 85, 50, 30 and 27.5 mV, respectively. For the AgPt/TiO2/CuO/Cu2O samples with CuO/Cu2O thickness of 60 nm (Cu60), the power consumption is 10.6, 8.4 and 7.2 nW, and the responses are 162%, 180% and 216% to 100 ppm of n-butanol vapors at the applied bias voltages of 12.5, 11.0 and 9.5 mV, respectively, shown in Figure 7d, where it can be seen that the obtained power consumption is 10.6 nW, 8.4 nW and 7.2 nW, and the respective responses are 162%, 180% and 216%, at the applied voltages of 12.5 mV, 11.0 mV and 9.5 mV, respectively. The dependence on the power consumption versus the applied bias voltage is represented in Figure S13 for the TiO2/CuO/Cu2O samples with CuO/Cu2O thickness of 20 nm (Cu20) decorated with Pd or Ag or AgPt NPs.

Comparison of the gas sensing properties and energy consumption of the heterostructures

The proposed gas sensing mechanism for our heterostructures after functionalization with noble metal nanoparticles is described in Text S1 (Supporting Information).

Cite this paper as

Deposition of nanoparticles: computational results

We have simulated the modification of the ternary TiO2(111)/CuO (1 ̅ 11) /Cu2O(111) heterostructure by adsorbing Pd7, Ag7 and Ag6Pt nanoparticles, see Figure 8, to subsequently assess their selectivity and sensitivity towards the detection of hydrogen (H2), ethanol (C2H5OH) and n-butanol (C4H9OH) vapors. The purpose of these simulations is to assist the interpretation of the trends of the gas response detected in experiments for our ternary heterojunctions functionalised using noble metal nanoparticles. The work function of the ternary heterojunction suffers a reduction of approximately 37% from its initial large value of Φ = 7.454 eV, after decoration by the noble metal nanoparticles, as listed in Table S1 and described in the Text S3 (Supporting Information).

The lower work function values calculated for the hetero-devices after surface functionalisation explain their enhanced chemical reactivity, compared to the material before deposition of the metal nanoclusters. Our simulated work functions show small differences of up to 0.14 eV, suggesting that the composition of the nanoparticles can be used to fine-tune the application of the nano-composite materials as chemical sensors.

Adsorption of molecules: computational results

Here, we have modelled the surface chemistry of the ternary heteroepitaxial junction materials decorated with the nanoparticles of noble metals or their alloys.

The binding energies for the interaction between the adsorbate molecules and the TiO2(111)/CuO (1 ̅ 11) /Cu2O(111) layered material functionalised with the Pd7 nanoparticle, show a preference for H2, releasing -1.494 eV upon interaction, see S2. S2. (n-C4H9OH) forms a vertical adsorption configuration on the heterostructure decorated using the Ag7 nanoparticles.

For the molecular adsorptions on the sensor decorated by the Ag7 nanoparticles, the Bader charge transfers were calculated between -0.039 and -0.074 e -, see Table S2. These values indicate that the adsorbate loses electronic density to the heterojunction functionalised with the nanocluster, showing no clear correlation with the binding energies of the VOC and H2

molecules. The small values of electron density flow that we have calculated for this material support the low sensitivity that it displayed in the experiments. A discussion of the adsorption geometries and charge transfers for the least favourable interactions can be found in the Text S4 (Supporting Information).

Summary and Outlook

Changing the composition of the nanolayered CuO/Cu2O structure with a total thickness of 60 nm by first depositing 20 nm of TiO2 on top and subsequently Ag nanoparticles (5-15 nm of diameter), reduces of the energy consumption of the final gas sensing device to below 1 nW. This fabrication process also allows tuning the response spectrum of the sensor, thereby promoting the selectivity to 100 ppm of n-butanol vapours (response of over 200% at the operating temperature of 300°C). In the case of AgPt functionalization, the response of the sensor increases even higher to 220% for 100 ppm of n-butanol vapours at an operating temperature of 350C. In the case of Pd functionalization the selectivity of the sensor was changed from ethanol to hydrogen H2, and the sensing performance was enhanced compared to the other compounds (response in the range between 400-600% at the lower operating temperature of 150-250C).

A tentative surface reaction mechanism explaining the sensing behavior is proposed. At the interface of the TiO2/CuO/Cu2O heterostructure, the ethanol molecule first suffers a reduction and then is combined with the holes present in CuO, producing the CH3CHO intermediate, which finally reacts with the oxygen adsorbed on the n-type TiO2 overlayer. We have used thermodynamic arguments derived from DFT calculations to rationalize the stability of the heteroepitaxial material before and after surface functionalization by noble metal nanoparticles. The adsorption energies agree and support the gas sensing experiments. This manuscript presents the methodology for the nano-structuring and functionalizing of semiconducting oxides with tunable performances, as an alternative to clean room technology and micro-fabrication.

  is represented a cross-sectional structure of the multilayered devices comprising the Pd-, Agor AgPt-NPs/TiO2/CuO/Cu2O systems. Cite this paper as: O Lupan, N. Ababii, D. Santos-Carballal, M.-I. Terasa; N. Magariu; D. Zappa, E. Comini, Th. Pauporté, L. Siebert, F. Faupel, A. Vahl, S. Hansen, N. H de Leeuw, R. Adelung, Tuning the Reactivity of Ultralow Power Heterojunction Sensors toward H2 and VOCs through Noble Metal Nanoparticle Functionalization. NanoEnergy, 88 (2021) 106241 9

Fig. 1 .

 1 Fig. 1. (a) The technological flowchart for the manufacture of sensor device structures based on nanostructured layered films consisting of Pd, Ag or AgPt NPs on top of TiO2/CuO/Cu2O (Final device structure). Cross-section view of a device based on Pd, Ag or AgPt NPs on top of the TiO2/CuO/Cu2O -based sensor in insert from (a).

12 3. Results and Discussion 3 . 1

 1231 : O Lupan, N. Ababii, D. Santos-Carballal, M.-I. Terasa; N. Magariu; D. Zappa, E. Comini, Th. Pauporté, L. Siebert, F. Faupel, A. Vahl, S. Hansen, N. H de Leeuw, R. Adelung, Tuning the Reactivity of Ultralow Power Heterojunction Sensors toward H2 and VOCs through Noble Metal Nanoparticle Functionalization. NanoEnergy, 88 (2021) 106241 Morphological evolutions and composition analysis Scanning electron microscope (SEM) images are presented in Figure 2 for the mixed copper oxide CuO/Cu2O substrate coated with a TiO2 layer and functionalised with the Pd, Ag and AgPt noble metal nanoparticles. The surface of the TiO2/CuO/Cu2O material is covered with nanoclusters of 5-15 nm in diameter, which are forming uniformly interconnected nano-crystallites, which is favourable for sensor device applications. The SEM images of the CuO/Cu2O, TiO2/CuO/Cu2O and AgPt/TiO2/CuO/Cu2O samples are shown in Figures S1. To better understand the structures, the SEM images at high and low magnification are represented in Figures S2 and S3 in the Supporting Information.

Fig. 2 .

 2 Fig. 2. Higher magnification of SEM images of interconnected nano-crystallite TiO2/CuO/Cu2O covered with: (a) Pd nanoparticles, Cu20 set samples; (b) Ag nanoparticles, Cu60 set samples; (c) AgPt nanoparticles, Cu60 set samples, nanoclusters of 5-15 nm in diameter.

  work. The respective overview and high-resolution spectra corresponding to the TiO2/CuO structures with and without decoration by Pd NPs are shown in Figure S4 (Supporting Information). In general, the spectra of the AgPt nanoparticle-and Pd nanoparticledecorated sensors show strong similarities and mostly differ in the signals corresponding to the elemental Ag, Pt or Pd. The observation of the Pd-3d lines with peaks around 335.8 Cite this paper as: O Lupan, N. Ababii, D. Santos-Carballal, M.-I. Terasa; N. Magariu; D. Zappa, E. Comini, Th. Pauporté, L. Siebert, F. Faupel, A. Vahl, S. Hansen, N. H de Leeuw, R. Adelung, Tuning the Reactivity of Ultralow Power Heterojunction Sensors toward H2 and VOCs through Noble Metal Nanoparticle Functionalization. NanoEnergy, 88 (2021) 106241 14 eV (Pd-3d5/2) and 341.0 eV (Pd-3d3/2) indicates the successful decoration of the sensor structure by Pd nanoparticles [28].

Fig. 3 .

 3 Fig. 3. X-ray photoelectron spectra (XPS) of: (a) a AgPt/TiO2/CuO/Cu2O (red line) nanocomposite and an undecorated TiO2/CuO/Cu2O thin film (black line); (b) spectra at higher resolution for Cu-2p and Ti-2p lines as well as spectra for Ag-3d and Pt-4d lines. Micro-Raman spectra for: (c) the surface functionalized TiO2/CuO/Cu2O and Pd/TiO2/CuO/Cu2O samples with CuO/Cu2O thickness of 20 nm (Cu20) after being thermally treated at 420C for 30 min in ambient; (d) Pd/TiO2/CuO/Cu2O with CuO/Cu2O thicknesses of 20 nm (Cu20) and 30 nm (Cu30), respectively.

  Cite this paper as: O Lupan, N.Ababii, D. Santos-Carballal, M.-I. Terasa; N. Magariu; D. Zappa, E. Comini, Th. Pauporté, L. Siebert, F. Faupel, A. Vahl, S. Hansen, N. H de Leeuw, R. Adelung, Tuning the Reactivity of Ultralow Power Heterojunction Sensors toward H2 and VOCs through Noble Metal Nanoparticle Functionalization. NanoEnergy, 88 (2021) 106241 16experimentally that Pd-functionalization of such structures significantly increases the response and selectivity to hydrogen gas, even at low operating temperature. We also intend to demonstrate that it is possible to lower the energy consumption during operation and tune the reactivity toward H2 of the TiO2/CuO/Cu2O heterojunction sensors through functionalization by Pd nanoparticles.

Fig. 4 .

 4 Fig. 4. (a) Gas response towards different compounds (hydrogen, n-butanol, 2-propanol, ethanol, acetone, ammonia and CH4) of the Pd-functionalized TiO2/CuO/Cu2O samples with CuO/Cu2O thickness of 20 nm (Cu20) at different operating temperatures (OPT); (b) Dynamic response to hydrogen of the Pd/TiO2/CuO/Cu2O samples with CuO/Cu2O thickness of 20 nm (Cu20) measured at several OPTs; (c) The I-V current-voltage characteristics of the Pd/TiO2/CuO/Cu2O samples with CuO/Cu2O thickness of 20 nm (Cu20) at different OPT; (d) Dynamic response to different concentrations of hydrogen (5, 10, 50, 100, 500

  shows the dynamic hydrogen response of the Pd-NPs/TiO2/CuO/Cu2O (Cu20) samples at different operating temperatures. The calculated response times (τr = 18.7, 20.4, 12.8 and 12.4 s) and recovery times (td = (> 50), 36.3, 10.6 and 4.2 s) are relatively small at the operating temperatures of 150C, 200C, 250C and 300C, respectively, see Figure S7a

  . The dynamic response to different gases (hydrogen, n-butanol, 2-propanol, ethanol, acetone, ammonia and CH4) of Pd-NPs/TiO2/CuO/Cu2O with CuO/Cu2O thickness of 20 nm (Cu20)

Figure

  FigureS9cdisplays the dynamic response to hydrogen of the Pd/TiO2/CuO/Cu2O (Cu30) samples at various OPTs and FigureS9dillustrates the dynamic response to hydrogen gas with three consecutive pulses at the operating temperatures 300C. These samples clearly

  Cite this paper as: O Lupan, N. Ababii, D. Santos-Carballal, M.-I. Terasa; N. Magariu; D. Zappa, E. Comini, Th. Pauporté, L. Siebert, F. Faupel, A. Vahl, S. Hansen, N. H de Leeuw, R. Adelung, Tuning the Reactivity of Ultralow Power Heterojunction Sensors toward H2 and VOCs through Noble Metal Nanoparticle Functionalization. NanoEnergy, 88 (2021) 106241 19

Fig. 5 .

 5 Fig. 5. (a) Gas response towards hydrogen, n-butanol, 2-propanol, ethanol and acetone for the TiO2/CuO/Cu2O samples after surface functionalization with Ag-nanoparticles (CuO/Cu2O with thickness of 60 nm (Cu60)); (b) Dynamic response to different compounds (hydrogen, n-butanol, 2-propanol, ethanol and acetone) of the Ag/TiO2/CuO/Cu2O (Cu60) samples at the operating temperature of 300C. (c) The current -

  Cite this paper as: O Lupan, N. Ababii, D. Santos-Carballal, M.-I. Terasa; N. Magariu; D. Zappa, E. Comini, Th. Pauporté, L. Siebert, F. Faupel, A. Vahl, S. Hansen, N. H de Leeuw, R. Adelung, Tuning the Reactivity of Ultralow Power Heterojunction Sensors toward H2 and VOCs through Noble Metal Nanoparticle Functionalization. NanoEnergy, 88 (2021) 106241 21

Fig. 6 .

 6 Fig. 6. (a) Gas response to different gases and volatiles (hydrogen, n-butanol, 2-propanol, ethanol and acetone) of the AgPt-NPs surface functionalized TiO2/CuO/Cu2O samples with CuO/Cu2O thickness of 60 nm (Cu60); (b) Dynamic response to hydrogen, n-butanol, 2-propanol, ethanol and acetone for AgPt/TiO2/CuO/Cu2O (Cu60) samples at the operating temperature of 350C. (c) Current-voltage characteristic of the AgPt/TiO2/CuO/Cu2O (Cu60) samples at different operating temperatures; (d) Dynamic response to 5, 10, 50, 100, 500 and 1000 ppm of n-butanol of AgPt/TiO2/CuO/Cu2O (Cu60) samples (applied bias voltage 9.5 mV). (e) Dynamic response to 100 ppm of n-butanol and consecutively 2-propanol, acetone,

  Cite this paper as: O Lupan, N. Ababii, D. Santos-Carballal, M.-I. Terasa; N. Magariu; D. Zappa, E. Comini, Th. Pauporté, L. Siebert, F. Faupel, A. Vahl, S. Hansen, N. H de Leeuw, R. Adelung, Tuning the Reactivity of Ultralow Power Heterojunction Sensors toward H2 and VOCs through Noble Metal Nanoparticle Functionalization. NanoEnergy, 88 (2021) 106241 23In this section, we compare the gas responses for the TiO2/CuO/Cu2O heterostructures functionalized with the different types of NPs-, as we aim to show the possibility of controlling their selectivity by using the technological approach developed in this work.

Fig. 7 .

 7 Fig. 7. (a) Gas response to different compounds (hydrogen, n-butanol, 2-propanol, ethanol and acetone) of the Pd/TiO2/CuO/Cu2O, Ag/TiO2/CuO/Cu2O, and AgPt/TiO2/CuO/Cu2O samples. Dependence of the power consumption and of the gas versus the applied bias voltage for: (b) Pd/TiO2/CuO/Cu2O (with CuO/Cu2O thickness of Cu20), (c) Ag/TiO2/CuO/Cu2O (with CuO/Cu2O thickness of Cu60), and (d) AgPt/TiO2/CuO/Cu2O (with CuO/Cu2O thickness of Cu60).

Figure 24 Figure

 24 Figure7ashows the gas response to hydrogen, n-butanol, 2-propanol, ethanol and acetone for the Pd/TiO2/CuO/Cu2O, Ag/TiO2/CuO/Cu2O, and AgPt/TiO2/CuO/Cu2O samples. The samples functionalized with Pd nanoparticles are the most sensitive to hydrogen, whereas the samples functionalized using the Ag and AgPt nanoparticles experience a change of selectivity and are the most sensitive to butanol.

  : O Lupan, N. Ababii, D. Santos-Carballal, M.-I. Terasa; N. Magariu; D. Zappa, E. Comini, Th. Pauporté, L. Siebert, F. Faupel, A. Vahl, S. Hansen, N. H de Leeuw, R. Adelung, Tuning the Reactivity of Ultralow Power Heterojunction Sensors toward H2 and VOCs through Noble Metal Nanoparticle Functionalization. NanoEnergy, 88 (2021) 106241 25

Fig. 8 .

 8 Fig. 8. (Top panels) Side view and (bottom panels) simulated scanning tunnelling microscopy (STM) images of the ternary TiO2(111)/CuO(1 ̅ 11)/Cu2O(111) heteroepitaxial junction functionalised using (a and d) Pd7; (b and e) Ag7; and (c and f) Ag6Pt nanoparticles. The ternary heterojunctions functionalised using the Pd7, Ag7 or Ag6Pt nanoparticle are displayed using the ball-and-stick representation. The STM images were generated using a bias of: (d) V = -0.5 eV, (e) V = -1.0 eV and (f) V = -1.5 eV, an electron density of (d) ρ = 0.0009 e Å -3 , (e) ρ = 0.0029 e Å -3 and (f) ρ = 0.0070 e Å -3 as well as a tip distance of (d) d = 2.25 Å, (e) d = 1.06 Å and (f) d = 2.17 Å. Inset illustrate enlargement of the STM image for the surface unit cell. Crystallographic directions are indicated with respect to the Cu2O(111) substrate. O atoms are in red, Cu atoms are in dark blue, Ti atoms are in light blue and Pd, Ag or Pt atoms are in grey.

Figure

  Figure9a-c displays the thermodynamically most stable adsorption geometries for the interaction of the three molecules investigated computationally on the exposed facet of the heterostructure containing the noble metal clusters. We found that the diatomic H2 molecule dissociates upon adsorption onto the equatorial Pd atoms of the nanoparticle, leading to the strongest interaction found in this work, see Figure9a. For this binding mode, our calculations suggest that the H atoms place themselves approximately along the[2 ̅ 11] direction at either sides of the axial Pd atom. Thus, each H adatom forms a binuclear complex by coordinating both one equatorial and the axial Pd atom at the average distances of 1.71 Å and 1.78 Å. FigureS15a-cshows the binding configurations of H2, C2H5OH and n-C4H9OH on the most favourable adsorption sites of the Pd7/TiO2(111)/CuO(1 ̅ 11)/Cu2O(111) sensor. For the heterostructure containing the Pd7 clusters, H2 interacts molecularly and nearly vertically at 1.75 Å above the 5-fold axial Pd site (see Figure S15a), where it shows the least favourable adsorption observed on this material. C2H5OH moved outwards during the geometry optimisation from the initial equatorial site, forming a laterally coordinate bond of 2.27 Å to the axial Pd atom in its most stable adsorption configuration, as shown in Figure S15b. The O atom from the largest molecular weight alcohol stays above the initial adsorption site, i.e. at 2.23 and 2.13 Å from

Fig. 9 .

 9 Fig. 9. Adsorption of: (a) H2, and (b and c) n-C4H9OH on the TiO2(111)/CuO (1 ̅ 11) /Cu2O(111) heterostructure decorated with the Pd7, Ag7 and Ag6Pt clusters, respectively. Interatomic distances are shown and crystallographic directions are indicated with respect to the Cu2O(111) substrate. The ternary heterojunctions functionalised using the Pd7, Ag7 or Ag6Pt nanoparticles are displayed using the space-filling representation, whereas the adsorbates are shown using the stick representation. O atoms are in red, H atoms are in white, C atoms are in brown, Cu atoms are in dark blue, Ti atoms are in light blue and Pd, Ag and Pt atoms are in grey.

Figure

  FigureS15g-i depicts the structures of the molecular interactions with the surface sites of the heteroepitaxial material decorated with Ag6Pt that released the largest adsorption energies. We found that H2 remained perpendicularly atop the axial Pt atom at 1.91 Å, while it coordinated laterally the equatorial Ag site at the more elongated distance of 2.07 Å, which moved outwards upon adsorption, see FigureS15g. Our DFT modelling suggests that C2H5OH prefers to stay parallel to the surface, binding both adsorption sites at the same distance of 2.29 Å, regardless of the different chemical nature of the metal atom, as shown in FigureS15h. When n-C4H9OH interacts with the axial or equatorial adsorption sites, the large alkyl radical positions itself perpendicularly to the surface. Figure9cillustrates that n-C4H9OH has a bidentate coordination mode with Ag6Pt/TiO2(111)/CuO(1 ̅ 11)/Cu2O(111), by binding laterally to the equatorial Ag atom and by forming a hydrogen bond between the hydroxyl H and one of the surface O species.Moving this alcohol to the axial site forces a reduction of the interfacial O-Pt distance to 2.23 Å, which is nevertheless insufficient to counteract the loss of the hydrogen bond formed on the equatorial Ag site.Our calculated charge transfers indicate that most of the adsorbates donate electron density to the sensor containing the Ag6Pt nanoclusters, see TableS2. The electronic charges transferred by C2H5OH to the axial and equatorial sites of the nanoparticle are -0.085 and

  , D. Santos-Carballal, M.-I. Terasa; N. Magariu; D. Zappa, E. Comini, Th. Pauporté, L. Siebert, F. Faupel, A. Vahl, S. Hansen, N. H de Leeuw, R. Adelung, Tuning the Reactivity of Ultralow Power Heterojunction Sensors toward H2 and VOCs through Noble Metal Nanoparticle Functionalization. NanoEnergy, 88 (2021) 106241

	5

  Cite this paper as: O Lupan, N.Ababii, D. Santos-Carballal, M.-I. Terasa; N. Magariu; D. Zappa, E. Comini, Th. Pauporté, L. Siebert, F. Faupel, A. Vahl, S. Hansen, N. H de Leeuw, R. Adelung, Tuning the Reactivity of Ultralow Power Heterojunction Sensors toward H2 and VOCs through Noble Metal Nanoparticle Functionalization. NanoEnergy, 88 (2021) 106241

	butanol (n-C4H9OH) with the heteroepitaxial junctions functionalized by the noble metal
	nanoparticles.
	6

  Cite this paper as: O Lupan, N.Ababii, D. Santos-Carballal, M.-I. Terasa; N. Magariu; D. Zappa, E. Comini, Th. Pauporté, L. Siebert, F. Faupel, A. Vahl, S. Hansen, N. H de Leeuw, R. Adelung, Tuning the Reactivity of Ultralow Power Heterojunction Sensors toward H2 and VOCs through Noble Metal Nanoparticle Functionalization. NanoEnergy, 88 (2021) 106241

	2. Methods
	7

  Cite this paper as: O Lupan, N.Ababii, D. Santos-Carballal, M.-I. Terasa; N. Magariu; D. Zappa, E. Comini, Th. Pauporté, L. Siebert, F. Faupel, A. Vahl, S. Hansen, N. H de Leeuw, R. Adelung, Tuning the Reactivity of Ultralow Power Heterojunction Sensors toward H2 and VOCs through Noble Metal Nanoparticle Functionalization. NanoEnergy, 88 (2021) 106241

	8

  of 5 × 5 × 1 k-points, in line with previous

	Cite this paper as: O Lupan, N. Ababii, D. Santos-Carballal, M.-I. Terasa; N. Magariu; D.
	Zappa, E. Comini, Th. Pauporté, L. Siebert, F. Faupel, A. Vahl, S. Hansen, N. H de Leeuw,
	R. Adelung, Tuning the Reactivity of Ultralow Power Heterojunction Sensors toward H2
	and VOCs through Noble Metal Nanoparticle Functionalization. NanoEnergy, 88 (2021)
	106241
	works
	11

  , D. Santos-Carballal, M.-I. Terasa; N. Magariu; D. Zappa, E. Comini, Th. Pauporté, L. Siebert, F. Faupel, A. Vahl, S. Hansen, N. H de Leeuw, R. Adelung, Tuning the Reactivity of Ultralow Power Heterojunction Sensors toward H2 and VOCs through Noble Metal Nanoparticle Functionalization. NanoEnergy, 88 (2021) 106241

	deposition of the sensor structures
	13

Table

  Cite this paper as: O Lupan, N.Ababii, D. Santos-Carballal, M.-I. Terasa; N. Magariu; D. Zappa, E. Comini, Th. Pauporté, L. Siebert, F. Faupel, A. Vahl, S. Hansen, N. H de Leeuw, R. Adelung, Tuning the Reactivity of Ultralow Power Heterojunction Sensors toward H2 and VOCs through Noble Metal Nanoparticle Functionalization. NanoEnergy, 88 (2021) 106241

	S2.	Table	S2	also	lists	the	adsorption	energies	(Eads)	for
	Ag7/TiO2(111)/CuO (1 ̅ 11) /Cu2O(111) and Ag6Pt/TiO2(111)/CuO (1 ̅ 11) /Cu2O(111),
	which display a strong shift of preference towards n-C4H9OH, with calculated Eads = -1.129
	eV and -1.053 eV, respectively, on the equatorial site of the NPs. However, the adsorption
	of the largest molecular weight alcohol on the least coordinated axial site of the NPs leads
	to a reduced binding energy by around 50% compared to its equatorial Ag counterpart
	position. Our calculations reveal that the decreasing order of strength of molecular
	adsorption is Eads(n-C4H9OH) > Eads(C2H5OH) > Eads(H2) after replacing the axial Ag atom
	by Pt in Ag7, as shown in Table
		27

  Cite this paper as: O Lupan, N.Ababii, D. Santos-Carballal, M.-I. Terasa; N. Magariu; D. Zappa, E. Comini, Th. Pauporté, L. Siebert, F. Faupel, A. Vahl, S. Hansen, N. H de Leeuw, R. Adelung, Tuning the Reactivity of Ultralow Power Heterojunction Sensors toward H2 and VOCs through Noble Metal Nanoparticle Functionalization. NanoEnergy, 88 (2021) 106241

	31

Acknowledgements

This research was sponsored in part by the NATO Science for Peace and Security Programme (SPS) under grant G5634 "Advanced Electro-Optical Chemical Sensors" AMOXES. Dr. Lupan gratefully acknowledges PSL University, Chimie-ParisTech for invited professor positions in 2018 and 2019, 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 PAK 902 (SFB1261, FOR2093 & AD 183/16-1). This work was financially supported by the German Research Foundation (DFG) via the research unit FOR 2093 "Memristive devices for neuronal systems" through project A2. The authors would like to thank Dr. Oleksandr Polonskyi for the technical assistance and for fruitful discussion. This work has used the ARCHER UK National Supercomputing Service (http://www.archer.ac.uk) provided via our membership of the UK's HEC Materials Chemistry Consortium, funded by the Engineering and Physical Sciences Research Council (EPSRC grant EP/R029431). Work was also undertaken on ARC4, part of the High-Performance Computing facilities at the University of Leeds, United Kingdom. All data created during this research is provided in full in the results section of this paper. Cite this paper as: O Lupan, N. Ababii, D. Santos-Carballal, M.-I. Terasa; N. Magariu; D. Zappa, E. Comini, Th. Pauporté, L. Siebert, F. Faupel, A. Vahl, S. Hansen, N. H de Leeuw, R. Adelung, Tuning the Reactivity of Ultralow Power Heterojunction Sensors toward H2 and VOCs through Noble Metal Nanoparticle Functionalization. NanoEnergy, 88 (2021) 106241