a)

Figure 1. Atomic model of the SnO2-cassiterite (110) surface. Darker atoms correspond to Oxygen and Light correspond to Tin. Relevant NOx

adsorption sites are identified.

Applications of atomistic calculations to chemical gas sensing

Joan Daniel Prades, Albert Cirera, Joan Ramon Morante EME/XaRMAE/IN2UB,

Department d’Electrònica, Universitat de Barcelona, C/ Martí i Franquès 1, E-08028, Barcelona, SPAIN

dprades@el.ub.es

Abstract— Today, atomistic simulations can be applied to models (containing thousands of atoms) which are very close to real nanotechnology experiments. By combining both theory and experiment it is possible to reach unprecedented insight in applied fields such as chemical sensing. We present two practical cases in which deep insight in the materials properties, sensing mechanisms and optimum operation conditions was achieved thanks to complementing experimental techniques with first-principles simulations. First, we will show how simulations can clarify the sensing mechanisms in the detection of NO2 and NO with SnO2. Second, we will present how changes in the surface of metal oxides due to varying real operation conditions can be determined by combining simulations with luminescence techniques.

I. INTRODUCTION Sensing of chemical species implies the transduction of a

number of individual interactions between target molecules and sensing materials. Atomistic simulations provide detailed information at molecular scale not accessible with macro, micro or nanoscopic techniques. On the other hand, the recent interest of nanoparticles in sensing applications and the availability of faster codes and computational facilities draw near the real situations (i.e. sensors based on individual nanowire) and the simulation models (i.e. quantum simulation of a full nanowire). Today, all this make possible the use of complementary approaches combining simulations and experimental techniques. In the last years, we have focused our research on the exploration of complementary theoretical and experimental strategies manly centered on chemical sensing [1]. Some examples are the study of the Raman spectra of polymorphs in nanomaterials [2], the interpretation of the adsorption and desorption of target species from metal oxides [3-5] or the connection between their visible luminescence their gas sensing activity [6,7].

In this work we present two applications of first-principles calculations to gas sensing with metal oxides.

First, we present the successful simulation and interpretation of the Temperature Programmed Desorption (TPD) spectra of NOx on SnO2(110) [3,4]. Here, theoretical calculations demonstrated the important role of the content of oxygen at the surface on the adsorption/transduction efficiency of these metal oxides [5]. Second, atomistic simulations allowed us to understand that the visible luminescence of SnO2 (and other metal oxides) was related to the surface oxidation/reduction state [6,7].

II. METHODOLOGICAL DETAILS The first-principles methodology we used [1] was based

on density functional theory (DFT) as implemented in the SIESTA code [7]. We used the generalized gradient approximation (GGA) for the exchange-correlation functional and norm-conserving pseudo-potentials. The electron’s wavefunctions were expanded as linear combinations of atomic pseudo-orbitals of finite range. For this application, converged simulations were obtained considering double ζ plus polarization orbital basis-sets for all atomic species, a real space mesh cut-off of 250Ry and a

This work has been partially supported by the Spanish Governmentthrough the projects N–MOSEN (MAT2007-66741-C02-01) and NanoAmper (CIT-030000-2007-36), the UE through the projectNAWACS (NAN2006-28568-E), and the project MAGASENS andCROMINA. JDP is indebted to the MEC for the FPU grant.

1-4244-2581-5/08/$20.00 ©2008 IEEE 1501 IEEE SENSORS 2008 Conference

b)

Figure 2. Top: Experimental TPD spectra of NO and NO2 desorbing form a dehydroxylated SnO2(110) surface From ref. [9]. Bottom: Calculated TPD spectra for NO and NO2 considering different adsorption sites. From ref. [3]

5x5x7 Monkhorst-Pack set. We also considered spin polarization in the total energy computations, and corrected the basis set superposition error (BSSE) in the calculated adsorption energies. In order to find the atomic arrangements corresponding to a minimum energy of the system, we introduced structural relaxations by means of conjugate gradient minimization of the energy. To deal with surface stability and adsorption energy calculations, we modeled all surface geometries as three dimensionally periodic slab systems, generated from the relaxed bulk unit cell, with a vacuum width of 12 Å between surfaces to avoid interaction between periodic images of the slabs [1]. For adsorption energy and structural calculations, we used 2x1 slabs composed of five O(Sn2O2)O layers.

III. NOX ITERACION WITH SNO2 Gas sensor performance depends on the surface chemical

activity of active materials. The theoretical study of surface-adsorbate interactions provides a valuable tool for understanding the chemistry of gas sensors. Chemical transduction involves many stages, including the adsorption of the target species on specific adsorption sites and charge transfer from the compound to the sensing material. These processes must be studied in order to understand the sensing process.

A. Selection of the Surface Orientation It is commonly agreed that the facets of a crystal are

those that minimize the surface free energy γsurf [1]. Consequently, for a given material, the most common (and relevant) facet orientation will have the lowest γsurf. Therefore, we calculated the surface energies of several low index facets of SnO2-cassiterite in order to select the surface on which the adsorption processes would be analyzed. We concluded that the (110) orientation has the lowest surface free energy (γsurf = 1.01±0.20 J/m2) and hence, we centered the rest of our study on this surface orientation.

Figure 1 shows one of the slab models used for adsorption onto the SnO2-cassiterite(110) surface. This figure clearly shows that is possible to distinguish between bridging oxygen (OBridg) and in-plane oxygen (OInPlane). When any one of these oxygen atoms is removed, a surface oxygen vacancy is generated. Henceforth, we will refer to these as OBridgVac and OInPlaneVac respectively.

B. Available Adsorption Sites The stability of the previous sites at different operation

conditions (i.e. temperature and oxygen content in air) was studied with ab initio thermodynamics calculations. In short, under ambient conditions, the stoichiometric surface configuration is the most stable (as we expected). When the temperature is raised above 270ºC a single OBridgVac may form and even at higher temperatures (above 480ºC) the formation of a single isolated OInPlaneVac is plausible. At temperatures above 640ºC multi-vacant configurations are the most probable. This predicted evolution is compatible

with known experimental behavior [2,3]. All this suggests that temperature can be used to technologically adjust the surface oxygen vacancy type and concentration.

C. Adsorption, Charge Transfer and Desorption The interaction between target gases and metal oxides

surfaces can be described in terms of the adsorption energy Eads and the charge captured Δq(NO2) by the molecule. To estimate both, we built models of 1) the clean surface slab, 2) the molecule, and 3) the surface plus the molecule system. For all three of these models, total energy and charge population analysis calculations were performed, allowing us to evaluate the energy Eads and charge Δq(NOx) balance of the adsorption process. For example, in the case of Eads we computed:

ETinitial = ET(clean surface) + ET(molecule)

ETfinal = ET(clean surface+molecule) (1)

Eads = ΔET = ETfinal – ETinitial

According to our criteria, the more negative the energy Eads the stronger the exothermic adsorption and the higher Δq(NO2) results in bigger charges captured by NO2.

To validate the different Eads values obtained at different adsorption sites, these results were compared with previously reported temperature programmed desorption (TPD) data [8]. obtaining an excellent agreement. Additionally, the combination of experiment and theory leads to remarkable conclusions:

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0 10 20 30 40 50 60 70 80 90 100-2.3

-2.2

-2.1

-2.0

-1.9

adso

rptio

n en

ergy

[eV

]

reduction percentage [%]

1.0

1.2

1.4

1.6

1.8

2.0

char

ge tr

ansf

erer

d to

NO

2 [e- ]

Figure 3. Effects of the surface reduction percentage on the energetics (adsorption) and the charge transfer (transduction) of the most favorable adsorption configuration of NO2 onto the SnO2(110) surface. This illustrates the dramatic influence of the oxygen content at the surface on the potential sensing performance. From ref. [5]

b)

Figure 4. a) Typical cathodoluminescence spectra (visible region) of thermal treated SnO2 nanowires. Four bands can be clearly distinguished. b) Based on DFT calculations, four radiative recombination paths were identified. All of them were related to the presence of surface oxygen vacancies. From ref. [6]

1) NO tends to adsorb onto non-vacant sites at the SnO2(110) surface.

2) NO2 tends to adsorb onto SnO2(110) OInPlane and OBridgVac sites. The former site was found to be related to the low temperature TPD band (from 50 to 300ºC) while the energetics of the latter site were in accordance to the high temperature TPD signal (form 350ºC to 500ºC).

The very large responses to NO2 at room temperature or 100ºC would then indicate that the sensing–promoting sites are the OInPlane vacancies. On the other hand, we have also shown that the thermal treatment stimulates the generation of vacancies related to OBridg sites. Therefore, which is the influence of the reduction of OBridg sites on the overall sensing performance?

In order to obtain further insight into this issue, we studied the influence of the reduction percentage on the surface–adsorbate interaction from first principles. In we report the computed Eads and Δq(NO2) for a sensing-active adsorption at lower temperatures (NO2 onto an OInPlane site). The calculations predict that reduction slightly favors the adsorption (maximum of a 20% in energy) and almost doubles the charge transfer to the adsorbate. Our results demonstrate that the surface reduction influences both the energy exchange and charge transfers between surface and analyte. Concretely, the effect of removing OBridg atoms from the SnO2(110) surface by the heat-treatment on the NO2 adsorption onto the active sites in our measurements (OInPlane) is a slight modification of the desorption temperatures and a strong increase of the charge trapped by this adsorbate at the surface. A strong influence on the sensor performance by the OBridgVac generation (by thermally treating the SnO2 nanoparticles) then appears.

IV. SURFACE CONFIGURATION AND LUMINESCENCE For investigating the surface states in low-conductive

systems, cathodoluminescence (CL) is one of the best suited techniques [1]. It was shown in previous works [5] a that the visible CL spectrum of SnO2 is dominated by a broad signal composed of four different contributions centered around 1.90eV, 2.20eV, 2.37eV and 2.75eV respectively).

Basing on DFT calculations, we related these bands to radiative recombinations from the minimum of the conduction band and the SnO2-intrinsic bulk shallow levels towards intra-gap states near the top of the valence band corresponding to surface oxygen vacancies. For the most common low index surfaces of SnO2-cassiterite (namely (110), (100), (101), and (001)), the calculations revealed two families of states corresponding to two different angles between the oxygen site and the first neighboring tin atoms. Following the usual notation for the SnO2(110)-cassiterite surface (which is the most stable, abundant and deeply studied facet), the bands at 1.90eV and 2.20eV were related to bridging oxygen vacancies (OBridgVac) whereas the bands at 2.37eV and 2.75eV were attributed to in-plane oxygen vacancies (OInPlaneVac). These results are summarized in

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Figure 4. This model explained the CL spectra of a full set of SnO2 nanomaterials obtained by different synthesis methods and thermal processing.

Our results indicate that surface oxygen vacancies play a fundamental role in the interaction of metal oxide surfaces with gas molecules. In order to achieve further insight into the surface states associated to these sites, two challenging issues should be tackled in the next future. The first one is the role and magnitude of the surface relaxation associated to different charge states of these levels: significant changes in the atomic arrangements could motivate the revision of the crystallographic surface sites. Second, the experimental analysis of the spatial distribution of the luminescence could confirm its surface origin.

V. IDEAL EXPERIMENTAL SETUP FOR IDEAL THEORETICAL CONDITIONS

As previously show, full quantum atomistic calculation based on first principles provide a systematic and user independent methodology to obtain quantitative predictions based on the latest advances in theory. In spite of the huge approximations commonly assumed in many of their software implementations, current supercomputing facilities are needed to perform calculations with thousands of atoms which are close the smallest nanoparticles available. In the next years, advances in nanotechnology will be previously verified with real-size, fast, and cost effective in silico experiments.

The possibility of accessing electrically to one single nanowire [10] makes this system a fine analogous to the conditions assumed in the models [1]. Specifically, the ideal atomic arrangements usually assumed in the simulation are a experimental challenge. As shown in this work, ideal and infinite surface with perfectly regular atomic arrangements are assumed to model the surface of the metal oxides. From the experimental point of view, defect free crystals or thin films with smooth surface at the atomic scale are complex to obtain. This way, nanowires represent a simpler way to obtain individual defect free crystals with well defined surfaces. Also, their high surface to volume ratio also highlights the contribution of the surface phenomena to their overall response.

VI. CONCLUSION We have shown that full atomistic first principles

calculations can be successfully applied to understand the mechanisms underlying chemical gas sensing with metal oxides. Specifically, the study of the gas-surface interaction between NOx and SnO2 provides deeper insight into the experimental desorption data. Based on this complimentary approach we determined that the most active adsorption sites are surface oxygen vacancies. We have also shown that luminescence is a powerful tool to determine the presence of these oxygen vacant sites at the surface of metal oxides. Finally, we recall that the availability of experimental setups to experimentally validate the data from atomistic

calculations is still a challenging issue but, advances in the nanomanipulation and nanocharacterization of nanowires makes them excellent candidates to validate theoretical results and models.

ACKNOWLEDGMENT The authors are grateful to Prof. F. Illas and Prof. K.M.

Neyman (Universitat de Barcelona, Spain) and Dr. J.M. Pruneda and Prof. P. Ordejón (ICMAB-CSIC, Spain) for many enlightening discussions.

REFERENCES [1] J.D. Prades, A. Cirera and J.R. Morante, “Applications of DFT

calculations to chemical gas sensors: design and understanding”, in the book “Quantum Chemical Calculations of Surfaces and Interfaces of Materials” (Editors: V. A. Basiuk and P. Ugliengo), American Scientific Publishers (2008).

[2] J.D. Prades, J. Arbiol, A. Cirera and J.R. Morante, “Concerning the 506 cm−1 band in the Raman spectrum of silicon nanowires”, Appl. Phys. Lett., vol. 91, pp. 123107, 2007.

[3] J.D. Prades, A. Cirera and J.R. Morante, “First-Principles Study of NOx and SO2 Adsorption onto SnO2(110)”, J. Electrochem. Soc., vol. 154, pp. H657-H680, 2007.

[4] J.D. Prades, A. Cirera, J.R. Morante, J.M. Pruneda, P.Ordejón, “Ab initio study of NOx compounds absorption on SnO2 surface”, Sensors and Actuators B, vol. 126, pp. 62-67, 2007.

[5] M. Epifani, J.D. Prades, E. Comini, E. Pellicer, M. Avella, P. Siciliano, G. Faglia, A. Cirera, R. Scotti, F. Morazzoni and J. R. Morante, “The role of surface oxygen vacancies in the adsorption and gas-sensing properties of metal oxide nanocrystals”, J. Phys. Chem. C, submitted.

[6] J.D. Prades, J. Arbiol, A. Cirera, J.R. Morante, M. Avella, L. Zanotti, E. Comini, G. Faglia and G. Sberveglieri, “Defect study of SnO2 nanostructures by cathodoluminescence analysis: Application to nanowires”, Sensors and Actuators B: Chemical, vol. 126, pp. 6-12, 2007.

[7] J.D. Prades, A. Cirera, J.R. Morante, A.Cornet “Ab initio insights into the visible luminescent properties of ZnO”, Thin Solid Films, vol. 515, pp. 8670-8673, 2007.

[8] J.M. Soler, E. Artacho, J.D. Gale, A. García, J. Junquera, P. Ordejón and D. Sánchez-Portal, “The Siesta method for ab initio order-N materials simulation”, J. Phys.: Condens. Matter, vol. 14, pp. 2745-2779, 2002.

[9] E. Leblanc, L. Perier-Camby, G. Thomas, R. Gibert, M. Primet, P. Gelin, “NOx adsorption onto dehydroxylated or hydroxylated tin dioxide surface. Application to SnO2-based sensors”, Sens. Actuators B, vol. 62, pp.67-72, 2000.

[10] F Hernandez-Ramirez, J.D. Prades et al., “Portable microsensors based on individual SnO2 nanowires”, Nanotechnol., vol. 18, pp. 495501, 2007.

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