Rhodium Oxide Surface-Loaded Gas Sensors

Abstract

In order to increase their stability and tune-sensing characteristics, metal oxides are often surface-loaded with noble metals. Although a great deal of empirical work shows that surface-loading with noble metals drastically changes sensing characteristics, little information exists on the mechanism. Here, a systematic study of sensors based on rhodium-loaded WO3, SnO2, and In2O3—examined using X-ray diffraction, high-resolution scanning transmission electron microscopy, direct current (DC) resistance measurements, operando diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, and operando X-ray absorption spectroscopy—is presented. Under normal sensing conditions, the rhodium clusters were oxidized. Significant evidence is provided that, in this case, the sensing is dominated by a Fermi-level pinning mechanism, i.e., the reaction with the target gas takes place on the noble-metal cluster, changing its oxidation state. As a result, the heterojunction between the oxidized rhodium clusters and the base metal oxide was altered and a change in the resistance was detected. Through measurements done in low-oxygen background, it was possible to induce a mechanism switch by reducing the clusters to their metallic state. At this point, there was a significant drop in the overall resistance, and the reaction between the target gas and the base material was again visible. For decades, noble metal loading was used to change the characteristics of metal-oxide-based sensors. The study presented here is an attempt to clarify the mechanism responsible for the change. Generalities are shown between the sensing mechanisms of different supporting materials loaded with rhodium, and sample-specific aspects that must be considered are identified.

Facile Synthesis of the Composites of Polyaniline and TiO2 Nanoparticles Using Self-Assembly Method and Their Application in Gas Sensing

“…[7]. Generally speaking, they have the advantages of small size, light weight, easy integration and low cost, and are widely used in industrial hazardous gas leakage detection, toxic or harmful gas detection, flammable and explosive gas early warning and other fields [8,9]. However, for their applications in flammable or explosive gas detection, semiconductor gas sensors still have some shortcomings such as long response time, low sensitivity and especially high working temperature.…”

Ozone Sensing by In2O3 Films Modified with Rh: Dimension Effect

“…Moreover, with a certain degree of coverage with Rh, the sensor response becomes even less than the sensor response observed for unmodified In 2 O 3 (see Figure 1 ). This explains why, when using the same metal Rh to modify the surface of metal oxides, in some cases one can observe an increase [ 3 ], and in others, a decrease in the sensor response to oxidizing gases [ 12 , 45 , 46 ]. It can be assumed that, in all likelihood, Staerz et al [ 12 , 46 ], who observed reduction in sensor response to oxidizing gases after surface modification with Rh, have used too high a concentration of Rh.…”

“…Another reason may be related to the difference in surface modification methods. Typically, the chemical methods used in [ 12 , 46 ] tend to form agglomerates on the surface of a solid rather than individual atoms [ 14 ]. Indeed, scanning transmission electron microscopy of studied samples has shown that rhodium on the surface of metal oxides was in the form of Rh 2 O 3 particles and clusters with the size about 1–3 nm [ 12 ].…”

“…We also do not know how important the size effect is for rhodium-modified sensors. The results obtained in the study of the properties of gas sensors based on metal oxides modified with rhodium are too ambiguous [ 3 , 11 , 12 ].…”

Enhanced NO2-Sensing Properties of Au-Loaded Porous In2O3 Gas Sensors at Low Operating Temperatures

“…This results in an increase in the sensor resistance. Among some attractive NO 2 -sensing materials, In 2 O 3 has the advantage of the detection towards oxidizing gases at low temperatures [16,31,32] due to its high conductivity [17,18,23,32,33].…”