Staerz Anna, Boehme Inci, Degler David, Bahri Mounib, Doronkin Dmitry E, Zimina Anna, Brinkmann Helena, Herrmann Sina, Junker Benjamin, Ersen Ovidiu, Grunwaldt Jan-Dierk, Weimar Udo, Barsan Nicolae
Institute of Physical and Theoretical Chemistry (IPTC), University of Tuebingen, Auf der Morgenstelle 15, D-72076 Tuebingen, Germany.
European Synchrotron Radiation Facility (ESRF), 71 Avenue des Martyrs, 38043 Grenoble, France.
Nanomaterials (Basel). 2018 Nov 1;8(11):892. doi: 10.3390/nano8110892.
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.
为了提高其稳定性和调谐传感特性，金属氧化物通常会进行贵金属表面负载。尽管大量的实证研究表明，贵金属表面负载会极大地改变传感特性，但关于其机制的信息却很少。在此，我们展示了一项基于负载铑的WO₃、SnO₂和In₂O₃传感器的系统研究，该研究使用了X射线衍射、高分辨率扫描透射电子显微镜、直流（DC）电阻测量、原位漫反射红外傅里叶变换（DRIFT）光谱和原位X射线吸收光谱进行检测。在正常传感条件下，铑簇被氧化。有大量证据表明，在这种情况下，传感主要由费米能级钉扎机制主导，即与目标气体的反应发生在贵金属簇上，改变其氧化态。结果，氧化铑簇与贱金属氧化物之间的异质结发生改变，从而检测到电阻变化。通过在低氧背景下进行的测量，可以通过将簇还原为金属态来诱导机制转换。此时，总电阻显著下降，目标气体与基础材料之间的反应再次显现。几十年来，贵金属负载一直被用于改变基于金属氧化物的传感器的特性。本文所呈现的研究旨在阐明导致这种变化的机制。我们展示了负载铑的不同支撑材料的传感机制之间的共性，并确定了必须考虑的样品特定方面。
