Ethanol Gas Sensing Mechanism in ZnO Nanowires: An ab Initio

Oct 8, 2014 - *E-mail: [email protected] (G.C.). ... play an important role in environmental monitoring, chemical process control, and person...
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Ethanol Gas Sensing Mechanism in ZnO Nanowires: An ab Initio Study K. K. Korir,†,‡ A. Catellani,‡,§ and G. Cicero*,†,‡ †

Dipartimento di Scienza Applicata e Tecnologia, Politecnico di Torino, I-10129 Torino, Italy CNR-IMEM, Parco Area delle Scienze, I-43010 Parma, Italy § CNR-Istituto Nanoscienze, Centro S3, I-41125 Modena, Italy ‡

S Supporting Information *

ABSTRACT: Solid-state nanostructured gas sensors based on oxide materials play an important role in environmental monitoring, chemical process control, and personal safety. Yet, the underlying operating mechanism is still not well comprehended, while a deeper understanding would possibly lead to the engineering of sensing elements with enhanced sensitivities. Here we present ab initio density functional theory calculations that provide a comprehensive description of the ethanol sensing mechanism for ZnO nanowires: our results reveal that the competitive adsorption at the nanostructure surfaces between the analyte and the oxygen molecules present in the atmosphere induces a switching in surface conductance between semiconducting and conducting behavior that is related to the ethanol concentration and can be detected electronically, thus disclosing the sensing mechanism.

1. INTRODUCTION ZnO is a direct wide band gap (∼3.4 eV at 300 K) semiconductor, which can be synthesized into a rich variety of nanostructured morphologies including nanowires (NWs), nanotubes, nanocombs, and nanorings.1 ZnO has a wide range of potential applications in the field of energy harvesting and sensing. In particular, ZnO NWs have been identified as promising candidates for gas sensing due to their excellent response and recovery characteristics,2−4 and they have potential applications in detecting polluted, toxic or flammable and other industrially relevant gases.5 Consequently, in recent years a great interest has been fueled in understanding the electrical response of ZnO nanosensors, which have been subject of intense research.6−8 Among the many different chemicals, the ability to detect ethanol gas is highly desirable as it is used in a variety of commercial products, as well as industrial processes. Ethanol sensors based on ZnO nanostructures have been demonstrated by a number of groups:9−12 sensing is based on changes in electrical conductivity upon gas detection. In particular, it has been shown that the conductivity of ZnO NWs increases when the wires are exposed to ethanol. Notwithstanding this large amount of practical demonstrations, and beyond a general consensus that metal oxide based gas sensors rely on changes in electrical conductivity upon interaction with the surrounding atmosphere, since surface reactions influence the device conductivity,4,9,12 the microscopic reasons associated with these changes in electrical conductivity are still an open © 2014 American Chemical Society

question. The issue is further complicated by the fact that asgrown ZnO exhibits n-type conduction properties, whose origin is yet the subject of debate.8,13−20 Several models have been proposed to describe in particular ethanol sensing mechanism of ZnO nanostructures.9,21−23 Ethanol may undergo a dehydrogenation reaction when interacting with basic oxide surfaces such as ZnO and release electrons into the oxide conduction band inducing an increase of ZnO conductance. On the other hand, other studies have claimed that ethanol detection is controlled by oxygen molecules that, under ambient conditions, adsorb at the ZnO surfaces. In particular, it is believed that when a n-type oxide is exposed to air, molecular oxygen adsorbs on the oxide surfaces as negative ions by capturing free electrons from the conduction band thus generating an electron depleted region at the surface, and finally a lower conductance.9−12,21 On the contrary if O2 is removed from the surface (thermally or by other means), the trapped electrons are injected back to conduction band (CB), resulting in increased conductivity of the sample. The actual picture and understanding are however hardly accessible experimentally due to the lack of techniques able to directly probe surface evolution during detection. An ab initio approach would thus be highly desirable to disentangle different effects and assist in validation and development of a Received: July 25, 2014 Revised: September 24, 2014 Published: October 8, 2014 24533

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significantly shift the ZnO Fermi level in conduction band. Among these, the most promising candidate is interstitial hydrogen8,16,17,19 which may be incorporated during ZnO growth intentionally or unintentionally and it is stable upon heating to at least 550 °C, as shown experimentally. Indeed, it was revealed that interstitial hydrogen forms shallow donor states which are ionized at room temperature, thus strongly contributing to ZnO electron conductivity. Moreover, hydrogen is subsurface region may achieve a concentration five times larger than in the bulk thus further contributing to surface free electron concentration.17 To make contact with the experimental evidence that ZnO NWs show intrinsic n-type behavior, we have first calculated the structural and electronic properties of H-doped ZnO slabs. Hydrogen dopant was introduced in the ZnO slab at an interstitial site in the fifth layer from the outermost surface plane with concentration of about 0.70% at.: upon relaxation, H forms a strong O−H bond 0.98 Å long. This is accomplished by a local distortion of the lattice network described by an out of plane shift of the Zn atom initially bonded to the oxygen atom that binds to H and which is pushed away by about 1 Å from its initial position (see Figure S1, Supporting Information); the shift is associated with minor distortions localized around the interstitial site, in agreement with similar calculations for bulk ZnO.27 The presence of hydrogen near the surface does not change the structural properties of the outer layer (the one exposed to air during gas sensing experiments): in particular, we observe that the ZnO (11̅00) surface is still characterized by a simple relaxation of the outermost atoms, with Zn and O species moving inward by about 0.34 and 0.003 Å, respectively. Moreover the ZnO bonds at surface are buckled of 10.6°, in agreement with existing results24−26 on the undoped surface. On the contrary, the presence of hydrogen modifies the surface electrical properties, as shown in Figure 1a: while the clean surface is semiconducting, as demonstrated by the location of the Fermi level (EF) in the band gap, introduction of hydrogen shifts EF in CB, in an otherwise barely modified DOS; hence, the ZnO surface becomes conductive, in agreement with previous experimental data and theoretical calculations on bulk-

better model. Up to now, most theoretical studies have been focused almost exclusively on providing fundamental understanding of the surface/ethanol interaction3,24,25 neglecting the role of intrinsic n-type character of ZnO in the gas sensing mechanism; yet its understanding would possibly lead to engineering better sensing devices. Here we address the combined problem of coexisting species (O2 and ethanol) and sensing mechanism via DFT calculations: we focus on the interaction between molecules in the gas phase and the ZnO (11̅00) surface, which is the most stable and abundant in ZnO NWs.26 In particular, we discuss the role of oxygen molecules present in the atmosphere during detection on the sensitivity of the ethanol gas sensor. To evaluate the effect of the intrinsic n-type character of ZnO NWs on the adsorption mechanism and chemical response, interstitial hydrogen is included in our calculations: indeed, as it was recently shown,16,17,27,28 H can be easily incorporated in the ZnO matrix, where it forms stable hydroxides near the surface, acting as source of n-type doping.

2. COMPUTATIONAL DETAILS All calculations were performed within the density functional theory approach by solving the Kohn−Sham equations29 as implemented in the Quantum Espresso suite.30 The generalized gradient approximation of the exchange and correlation potential was employed using the Perdew, Burke, and Ernzerhof (PBE) functional.31 The core−electrons were replaced by ultrasoft pseudo potentials as described in Vanderbilt’s formulation,32 and the electronic wave functions (charge density) were expanded in a plane wave basis with an energy cutoff of 28 Ry (280 Ry). The ZnO(11̅00) surface was modeled with 12 ZnO layers slabs in a periodic supercell; unless otherwise stated, the results presented are obtained with (2 × 2) and (3 × 2) surface supercells for undoped and H doped surfaces, respectively. A thick vacuum layer (∼15 Å) was included in the direction perpendicular to the surface to avoid fictitious interaction between periodic images. Integration over the Brillouin zone (BZ) was performed on a (4 × 4 × 1) kpoint Monkhorst and Pack33 mesh and structures were relaxed until forces on the atoms were smaller than 0.02 eV/Å. To avoid spurious electrostatic interactions between adjacent replicas, adsorbates were symmetrically introduced on both top and bottom slab surfaces, at an initial distance of about 3 Å from the outer substrate atoms. Since the application of DFT to ZnO is known to produce a severe underestimation of the electronic band gap due to unphysical interaction between the Zn d-band and the O p-band, we employed the DFT+U scheme to properly compute the electronic properties of the structures relaxed at the PBE level. We used Hubbard U values of 12.0 eV for the Zn 3d orbitals and 6.5 eV for the O 2p orbitals. These values have been shown to effectively heal DFT band gap underestimation for ZnO.34,35 Finally, spin polarization was included in calculations involving oxygen molecules. 3. RESULTS 3.1. The n-doped ZnO(11̅00) Surface. A long-standing controversial issue when comparing theoretical and experimental results on ZnO is the intrinsic n-character of the experimental samples. Although once attributed to oxygen vacancies (VO), recent experimental16,17,20 and theoretical results27,28 have shown that while VO alone is not responsible for n-type doping, other point defects or complexes can

Figure 1. Density of state (DOS) for the (a) undoped (red) and Hdoped (black) ZnO(11̅00) surface, (b) oxygen molecule, and (c) ethanol molecule adsorbed on the H-doped ZnO(11̅00) surfaces. The Fermi level is indicated by the dotted (dotted dashed line) for the Hdoped (undoped) system. 24534

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like systems.16,27 Indeed hydrogen acts as a donor impurity thus populating the CB, with an electron concentration directly determined by its dosage, as it occurs in bulk. We have also considered the possible presence of oxygen vacancies at the ZnO surface to verify if this kind of defects would also lead to donor states responsible for n-type behavior. Our calculations show that surface vacancies generate deep occupied levels (DOS not shown here) in the ZnO gap which are not ionized at room temperature and, as such, do not contribute to ZnO free electron concentration. Henceforth, in our discussion we will focus on hydrogen doped surfaces which show n-type conductivity, consistently with as-grown ZnO: in the following we discuss the results for gas adsorption on ZnO surfaces in the presence of Hinterstitials at 0.7 at. % concentration (the data for the adsorption on the undoped surface are very similar and are reported in Table 1 for completeness). In particular we first

Figure 2. Side view of the optimized O2/ZnO(11̅00) interface (a) and ethanol/ZnO(110̅ 0) interface (b) represented along the [12̅10] (left structures) and [0001̅] (right structures) directions. Only two bilayers of the ZnO slab are represented beside the adsorbate molecules. Gray, red, yellow, and white spheres represent zinc, oxygen, carbon, and hydrogen atoms, respectively.

Table 1. Adsorption Distance (dZn−O) in Å and Adsorption Energy per Molecule (Eads) in eV for the Interaction of O2 and Ethanol (EtOH) with the Clean and H-Doped ZnO(110̅ 0) Surfacea dZn−O (Å) ZnO(110̅ 0) ZnO:H(11̅00) a

Figure 3) shows that it derives from the lowest unoccupied state of the isolated O2 molecule (π*): upon oxygen/surface

Eads (eV/mol)

O2,bridge

O2,on‑top

EtOH

O2,bridge

O2,on‑top

EtOH

2.18 2.13

2.16 2.17

2.05 2.05

−0.32 −0.35

−0.16 −0.17

−0.92 −0.96

In the case of O2 both bridge and on-top configuration are reported.

describe the structural and electronic properties for the adsorption of O2 and ethanol molecules and then, based on the observed electronic features, draw conclusion on the occurring detection mechanism. 3.2. Oxygen Adsorption at the n-Doped ZnO(11̅00) Surface. In ambient conditions, oxygen is one of the dominant gases and it has a tendency to be adsorbed on the surface of metal oxides.36,37 As such, it is extremely plausible that surfaces of operating devices appear covered with oxygen. Thus, we first studied the adsorption of O2 molecules on the doped ZnO surface at 1/4 ML38 coverage and analyzed its effects on the electronic properties of the material. Two adsorption configurations were found for the interaction of O2 with the ZnO surface: the on-top adsorption geometry and the bridge configuration. In both systems, the OO bond appears to be weakened by the interaction with the surface, yet the molecule retains its molecular character since it is found in a triplet state. In the on-top configuration only one oxygen atom of the O2 molecule binds to a Zn atom of surface (distance of 2.17 Å) and the adsorption energy is −0.17 eV per molecule. In the most stable configuration, both O2 atoms bind to two neighboring Zn atoms on the surface (Zn−O distance of 2.13 Å), forming a bridging configuration, as shown in panel a of Figure 2. This structure corresponds to adsorption energy (Eads) of −0.35 eV per molecule and it is characterized by an increase of the oxygen molecule bond-length of about 5%. The most interesting effect induced by oxygen adsorption in H-doped ZnO corresponds to a shift of the Fermi level from the CB into the pristine energy gap of the clean oxide. Furthermore, by comparing panels a and b of Figure 1, it is evident that O2 adsorption generates occupied and empty states in the gap, which are responsible for a shift of EF position that is brought to coincide with one of these states. An analysis of the charge density pertaining to this orbital at the Fermi energy (see

Figure 3. Isosurface wave function plot of the electronic state located at the Fermi energy for the optimized O2/ZnO(11̅00) interface (same view of Figure 1a, left panel). Green (yellow) regions correspond to positive (negative) sign of the wave function which is the highest occupied molecular orbital of the O2/ZnO system. Red and gray spheres represent oxygen and zinc atoms, respectively.

interaction, that promotes electron transfer and trapping from the ZnO conduction band to the adsorbed O2, this state becomes filled, thus pinning the Fermi level of the system. This picture is further confirmed by the analysis of oxygen Löwdin charges, which increase of about 5% when the molecule attaches to the ZnO surface, hint of the charge transfer from the surface to the adsorbate. Our results indicate that when oxygen adsorbs at ZnO surfaces, the concentrations of free carriers in the conduction band of the oxide decreases and consequently the conductivity of the sample decreases, in agreement with experimental electrical measurements performed in oxygen rich atmosphere.9,39 3.3. Ethanol Adsorption at the n-Doped ZnO(11̅00) Surface. We then considered ethanol adsorption on the ndoped ZnO (11̅00) surface at 1/4 ML surface coverage.38 As shown in Figure 2b, adsorption of ethanol occurs in a geometry where the hydroxyl group (−OH) binds to the two surface atoms forming a Znsurf−O bond 2.05 Å long, and an Osurf−H bond 1.56 Å long, in a bridge configuration, in agreement with previous studies.24,25 This adsorption geometry is characterized by a binding energy of −0.96 eV, which reflects a much 24535

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by providing preferred adsorption sites.43 Our study on the other hand shows the contrary: in particular, we found that in the presence of VO, only the oxygen atom of ethanol hydroxyl groups can bind to the surface Zn, while hydrogen would remain dangling since the surface oxygen is missing. Our calculations on VO containing surfaces show a weaker interaction of ethanol compared to that of the undefected surface with an adsorption energy of −0.51 eV per molecule, which is much lower than that of the stoichiometric surface. Therefore, these results reveal that undefective substrates and stoichiometric surfaces should be preferred for the development of gas sensors with better performances.

stronger interaction than that occurring for O2. Our results compare well with those of Spencer et al.25 As for the electronic properties of the ZnO/ethanol adsorbed system, our results show that the density of states around the band gap region is very similar to that of the clean n-doped surface. In particular, at variance with the case of oxygen adsorption, the position of EF is very close to that of the pristine n-type clean surface (as obtained by hydrogen doping, as shown in Figure 1c). For completeness, we have also calculated and analyzed the binding energy and the electronic properties of ethanol interacting with an undoped ZnO surface. Our calculations show that the geometry and the adsorption energy value are very similar to those of the H-doped surface (see Table 1); moreover, ethanol adsorption leaves unaltered the position of EF of the undoped ZnO surface in the ZnO band gap. Hence ethanol adsorption alone cannot be directly responsible of any change in surface conductivity both in the case of doped and undoped ZnO, at variance with what claimed in literature.25,40 Once characterized the surface electronic properties when oxygen or ethanol are separately adsorbed at the ZnO surface, we can then describe how ethanol gas sensing is realized using ZnO NWs. When n-type conducting ZnO NWs are exposed to ambient condition, which is normally rich in oxygen gas, their surfaces will be immediately covered by O2− species (Eads = −0.35 eV). The adsorbed oxygen captures electrons from CB, hence the substrate will be forced back to the semiconducting configuration (Figure 1b). When ethanol is introduced in the atmosphere, increased conductivity is observed: we attribute this effect to two subsequent phenomena occurring at the surface: (i) since ethanol has a higher adsorption energy (Eads = −0.96 eV) compared to oxygen, it is able to remove oxygen from the surface (ii) this is accompanied by freeing trapped electrons which are injected back to CB, finally leading to increased surface conductivity. For each ethanol that replaces an oxygen molecule on the ZnO surface, one electron is restored to CB. Therefore, surface carrier density will be closely linked to the concentration of ethanol in the atmosphere, and higher conductivities should be attained when O2 molecules are completely removed from the surface. In summary, the picture that we propose to describe ethanol sensing is mediated by the presence of oxygen molecules and it is based on competitive adsorption of these species occurring on the ZnO surface. Ethanol, being able to remove preadsorbed oxygen molecules, leads to enhanced surface conductivity, contrary to the models where it is treated as reducing agent.9,41,42 Indeed, further calculations that we performed for a surface exposed to both O2 and ethanol showed that when ethanol is introduced on a ZnO surface with oxygen preadsorbed, it is able to kick out oxygen molecules, thus reinforcing competitive adsorption as the essential key for describing ethanol gas sensing. It is worth noting that hydrogen doping has dramatic effects on the ZnO electrical properties, but has no significant effect on the surface/molecule interaction (adsorption energies change by less than 2% as shown in Table 1). Consistently with our predictions, experimental measurements have shown that for NWs with diameter ≤25 nm, adsorption of oxidizing or reducing gases on their surface is sufficient to alter the electrical properties of the entire nanowire,9,25 hence the possibility of achieving conductance switches only with NW surface chemistry. This effect can be traced back to the large surface to volume ratio. We finally mention that in the past few years, it has been claimed that oxygen vacancies (VO) improve ethanol detection

4. CONCLUSIONS Our ab initio results throw light on the ethanol sensing mechanism: Competitive adsorption between preadsorbed oxygen and ethanol on the surface of n-type ZnO is responsible for ethanol sensing, owing to ethanol’s ability to remove oxygen from the surface while increasing surface conductivity (release of electrons trapped by oxygen to CB). Furthermore, our approach shows the importance of explicitly considering in the calculations the n-type character of the sensor active elements (ZnO NWs) for a description of the sensor electrical response consistent with experiments. In our simulations, this was achieved by including in the ZnO structures interstitial hydrogen, which has been lately considered one of the most probable sources of the unintentional n-type doping. In conclusion, in small NWs, conductance switching can be achieved solely by surface chemistry, thus this may open the possibility of developing sensor with improved performances. More studies are needed to understand the sensing mechanism of other gases such as carbon or nitrogen oxides.

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ASSOCIATED CONTENT

S Supporting Information *

Structure of the hydrogen interstitial in ZnO. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (G.C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge funding from the European Union Seventh Framework Programme under Grant Agreement ITNNanowiring (Grant No. 265073). We acknowledge CINECA (ISCRA Project No. HP10BJWMPB, 2012) for the availability of high performance computing resources and support.

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