(Invited) Role of Surface p-n Heterojunctions in the Gas Sensing with Smox Based Devices - Operando Studies Insights
Authors: Barsan Nicolae 
Published: July 2021

In applications as well as in research, the majority of chemoresistive gas sensors developed for the detection of reducing gases is based on semiconducting metal oxides (SMOX), which are loaded or doped with an additional compound - typically noble or other transition metals. The introduction of these additives has proven to be the most suitable approach to overcome significant disadvantages of pristine SMOX gas sensing materials, namely the poor long-term stability, lack of selectivity, high operation temperature, i.e. energy consumption, and strong interference of humidity [1-4]. Because of their practical importance the understanding of the chemical and electrical role of additives is an essential topic of gas sensor research. The initial models, established in the 1980s [5-7], suffered because of the inability of the characterization methods to operate in conditions closed to the actual applications ones. Nevertheless, the proposed mechanisms based on solely chemical or electronic sensitization of SMOX by noble metal additives, namely the spillover or Fermi-level control sensitization mechanism, respectively, played an important role in the general advancement of the field. However, these models do not describe the complexity of the sensitization by metal or metal oxide additives, which arises from interplay of the materials reactivity and gas sensing performance and the mutual influence of surface chemistry and the electronic properties of the solid. Thus, in many cases one has to consider both, a chemical and an electronic contribution to the sensitization effect [8-10]. Besides the presence of a separate additive phase, doping, i.e. the presence of impurity atoms in the SMOX’s lattice, additionally influences the chemical and electrical properties of SMOX.
Today, the development of in-situ and operando methods provides a large tool-box to probe structural, chemical and electrical properties and processes during gas sensor operation. These developments play a key role in improving the mechanistic understanding of gas sensing with loaded or doped SMOX and this contribution aims to present revised models based on reviewing and discussing current works on sensitization mechanisms in SMOX materials and to describe the complexity of sensitization mechanisms with regard to the chemical and electronic contributions [11]. A summary of findings shows that the sensitization mechanism of an additive will depend on the nature of the additive itself and the properties of the SMOX, i.e. the selection of the materials, as well as the structure of the additive, which can be controlled material synthesis or introduction procedure. In some cases, sensitization mechanisms will be additive, e.g. the introduction of additional electronic states and reactive surface sites by doping; but in other cases, one effect will dominate, e.g. electronically coupled additives, which deactivate the SMOX surface and control its space charge layer.
In order to enable a knowledge-based design of SMOX gas sensing materials, it is important to obtain detailed synthesis-structure-function-relationships and concepts of selecting a combination of SMOX and additive, which allow tuning the gas sensing properties. Modern synthesis methods allow an unprecedented control of structural properties and state-of-the-art characterization techniques allow monitoring gas sensing materials under realistic operation conditions. Thus, it is possible to systematically extent synthesis-structure-function-relationships to a large set of gas sensing materials operation conditions and analytes. This will provide a profound basis to rationally design and optimize gas sensing materials based on the understanding of the fundamental mechanisms and how they are linked to the selected materials and structure.

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