Optimizing chemical gas sensors using IR spectroscopy

Chemical gas sensors based on semiconductor metal oxides are very popular for indoor air-quality monitoring because of their tiny size and low cost. They detect harmful fumes through variations in electrical conductivity when the metal oxide's surface is exposed to oxidizing or reducing gases. Even though using semiconductor nanosized particles in sensor fabrication enhances sensitivity, tremendous worldwide research efforts have only led to incremental improvements in commercial devices marketed since 1968. One of the major bottlenecks is our lack of a fundamental understanding of the reversible chemical mechanism at the root of gas detection.

Performance improvements are partly facilitated by the development of sophisticated electronic systems and software for data processing. Moreover, the use of novel materials—and particularly nanomaterials—for sensor fabrication has significantly boosted sensitivity.^1,2 While physical characterization of the semiconductor surface is considered important for controlling sensor properties, surface chemistry has not attracted much attention until recently. However, the fundamental principle at the basis of gas detection requires correlating the semiconductor's surface chemical properties with its electrical characteristics. Therefore, we have developed a technique based on Fourier-transform IR (FTIR) spectroscopy that allows simultaneous study of surface reactions and the resulting changes in electrical conductivity.

Figure 1. Fourier-transform IR transmission spectra of tin oxide nanoparticles recorded in situ at 300^°C (a) under vacuum, after adsorption of (b) 50mbar oxygen and (c) 10mbar carbon monoxide, and (d) after evacuation. CO₂: Carbon dioxide. a.u.: Arbitrary units.

FTIR spectroscopy is widely used to study interatomic bonds. However the surface contribution of nanoparticles to the total IR absorption spectrum is nonnegligible because of their high surface-to-bulk ratio. Under specific experimental conditions,³ FTIR spectroscopy can be used to identify chemical groups linked to the particles' surface, and monitor in situ surface reactions when molecules are adsorbed. Free carriers also contribute to the semiconductor absorption spectrum over the total IR range. Thus, a variation in free-carrier concentration translates into a change in overall IR absorption.⁴ For semiconductor nanoparticles, FTIR spectroscopy enables us to investigate surface reactions during gas adsorption while simultaneously observing the resulting variation in free-carrier concentration.⁵ We used tin oxide nanoparticles as an example of an n-type semiconductor.

Figure 1 shows the IR spectrum of the tin oxide nanoparticles recorded in situ at 300^°C in different gaseous environments. We first kept the sample under vacuum and subsequently adsorbed oxygen. The resulting decrease in overall absorbance between spectra (a) and (b) indicates a drop in free-electron concentration (decrease in electrical conductivity). Free electrons are indeed trapped in localized states by the formation of ionosorbed oxygen species. When carbon monoxide (CO) is subsequently adsorbed—spectrum (c)—the absorbance increase corresponds to a rise in free-electron density (increase in electrical conductivity). In parallel, we observed the formation of new absorption bands of carbon dioxide (CO₂) molecules and carbonate groups. CO₂ first forms when CO reacts with ionosorbed oxygen, releasing electrons into the conduction band and leading to higher conductivity. Next, CO₂ reacts with O²⁻basic sites at the tin oxide surface to form CO₃²⁻ carbonate groups, with no change in conductivity. After evacuation, CO₂ and carbonates are eliminated and the spectrum baseline returns to its original state: see spectra (d) and (a), respectively. Our work has also proved that fluctuations in IR energy transmitted by the semiconductor during gas adsorption/desorption cycles can be directly related to the sensor response obtained from standard electrical measurements (see Figure 2).⁶

Figure 2. Transmitted IR energy (E_IR) by the tin oxide nanoparticles at 300^°C versus gas adsorption/desorption cycles. O₂: Oxygen. CO: Carbon monoxide.

FTIR spectroscopy can help identify the specific reversible surface reactions at the root of semiconductor-sensor gas detection. This will require simultaneously probing the semiconductor material at two depths, a few Ângströms (surface reactions) and a few tens of nanometers (electron-depleted layer in the semiconductor). Once these particular reactions have been identified, we may be able to tailor the surface chemistry of the semiconductor sensing material, either by adjusting the synthesis parameters or by subsequent surface treatment—thus optimizing sensor sensitivity and selectivity. Finally, the technique may benefit industry by enabling quick evaluation of the potential of semiconductor nanoparticle batches as soon as they are synthesized, before embarking on actual device fabrication.

Part of this work has been funded by the Commission of the European Communities under several European project grants.