Hydrogen on In<sub>2</sub>O<sub>3</sub>: Reducibility, Bonding, Defect Formation, and Reactivity

Bielz, Thomas; Lorenz, Harald; Jochum, Wilfrid; Kaindl, Reinhard; Klauser, Frederik; Klötzer, Bernhard; Penner, Simon

doi:10.1021/jp1017423

Cited by 114 publications

(162 citation statements)

References 21 publications

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“…Besides, a new band and a shoulder appear at 406 and 325 cm −1 , respectively, which are attributed to nearsurface species of reduced indium oxide, in agreement with the literature. 22 This assignment is consistent with their disappearance when the sample is exposed to synthetic air (see Figure 1). In addition, in the presence of ethanol new bands appear at 938 and 2939 cm −1 ; however, the hydroxyl band at 3643 cm −1 shows an intensity decrease, and the band at 3658 cm −1 completely disappears.…”

Section: ■ Results and Discussionsupporting

confidence: 78%

Ethanol Gas Sensing by Indium Oxide: An Operando Spectroscopic Raman-FTIR Study

Sänze

Heß

2014

J. Phys. Chem. C

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The ethanol gas-sensing mechanism of indium oxide has been investigated in detail by Raman spectroscopy in combination with resistance measurements of the indium oxide sensor material and Fourier transform infrared (FTIR) gas-phase analysis. The observed surface species depend on the gas environment and sensor temperature. Raman spectra taken at lower operating temperatures of the sensor (190°C) during ethanol gas sensing reveal the presence of surface acetate and reduced indium oxide. Addition of oxygen to the feed leads to the formation of surface formate-like species besides acetate. At elevated temperatures of 325°C an increase in the amount of the gas-phase products acetaldehyde, ethene, carbon oxide, and water is observed. Under these conditions the sensor surface is characterized by carbon species if oxygen is absent. The sensor signal is correlated with the nature of the adsorbates, the presence of surface hydroxyl groups, and the indium oxide oxidation state. The proposed gas-sensing mechanism is corroborated by detailed analysis of the spectroscopic and resistance response of indium oxide during exposure to acetaldehyde and ethene. Our results demonstrate the importance of detailed spectroscopic studies under working conditions to unravel the mode of operation of gas sensors. ■ INTRODUCTIONSemiconducting metal oxides have been used widely as gas sensor materials because of their high sensitivity to a large variety of target gases and their simple fabrication. 1−3 A metal oxide gas sensor reversibly changes its resistance in the presence of a target gas, which is caused by the adsorption of gas molecules on the semiconductor's surface. As a mechanistic explanation the ionosorption of the adsorbates is assumed, transferring electrons from or to the sensor's conduction band. Alternatively, the sensor behavior can be explained by reduction and reoxidation of the (sub)surface, producing and eliminating oxygen vacancies. The vacancies can be ionized, thereby releasing electrons to the conduction band. In both proposed mechanisms the oxygen from the air plays an important role, either as an ionosorbed species or as an oxidation source. Moreover, the sensor signal may be strongly influenced by the presence of the preadsorbed species (e.g., ionosorbed oxygen or hydroxyl groups). While there has been considerable progress in the field, a detailed understanding of the mode of operation of metal oxide gas sensors is still lacking. To this end, the development of experimental approaches will be essential, which (i) are applicable under realistic operating conditions of the gas sensor and (ii) allow for a correlation of the sensor response with adsorbed species, changes of the metal oxide material, and gas-phase composition (operando approach). 4,5 Despite the potential of Raman spectroscopy for studying gas sensors at work, only a few in situ Raman studies on metal oxide gas sensors have been published, 6−10 which in part were done in combination with resistance measurements. 7−9 For nanocrystalline SnO 2 7 and CuO/...

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Section: ■ Results and Discussionsupporting

confidence: 78%

Ethanol Gas Sensing by Indium Oxide: An Operando Spectroscopic Raman-FTIR Study

Sänze

Heß

2014

J. Phys. Chem. C

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“…After the isothermal period, a total amount of 3500 μmol m –2 consumed hydrogen on LSF and about 2000 μmol m –2 on STF has been determined, which is approximately 300 times the amount measured on highly reducible “simple” oxides, such as In 2 O 3 , under comparable experimental conditions. 84 …”

Section: Results and Discussionmentioning

confidence: 99%

Water-Gas Shift and Methane Reactivity on Reducible Perovskite-Type Oxides

et al. 2015

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Comparative (electro)catalytic, structural, and spectroscopic studies in hydrogen electro-oxidation, the (inverse) water-gas shift reaction, and methane conversion on two representative mixed ionic–electronic conducting perovskite-type materials La0.6Sr0.4FeO3−δ (LSF) and SrTi0.7Fe0.3O3−δ (STF) were performed with the aim of eventually correlating (electro)catalytic activity and associated structural changes and to highlight intrinsic reactivity characteristics as a function of the reduction state. Starting from a strongly prereduced (vacancy-rich) initial state, only (inverse) water-gas shift activity has been observed on both materials beyond ca. 450 °C but no catalytic methane reforming or methane decomposition reactivity up to 600 °C. In contrast, when starting from the fully oxidized state, total methane oxidation to CO2 was observed on both materials. The catalytic performance of both perovskite-type oxides is thus strongly dependent on the degree/depth of reduction, on the associated reactivity of the remaining lattice oxygen, and on the reduction-induced oxygen vacancies. The latter are clearly more reactive toward water on LSF, and this higher reactivity is linked to the superior electrocatalytic performance of LSF in hydrogen oxidation. Combined electron microscopy, X-ray diffraction, and Raman measurements in turn also revealed altered surface and bulk structures and reactivities.

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“…Due to the small size of the In 2 O 3 crystals, it was difficult to completely avoid rhodium peaks; the rhodium counts could be minimized to about 1% of the In 3d 5/2 counts. For most measurements, a photon energy of 105 eV (4) 3.83920 (14) In2-O1(2) 2.1249 (2) was chosen because the Rh 4d levels have a cross-section minimum at this energy. 22 Sample preparation was identical to that used for STM analysis.…”

Section: Pesmentioning

confidence: 99%

Bulk and surface characterization of In2O3(001) single crystals

et al. 2012

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A comprehensive bulk and surface investigation of high-quality In 2 O 3 (001) single crystals is reported. The transparent-yellow, cube-shaped single crystals were grown using the flux method. Inductively coupled plasma mass spectrometry (ICP-MS) reveals small residues of Pb, Mg, and Pt in the crystals. Four-point-probe measurements show a resistivity of 2.0 ± 0.5 × 10 5 cm, which translates into a carrier concentration of ≈10 12 cm −3 . The results from x-ray diffraction (XRD) measurements revise the lattice constant to 10.1150(5)Å from the previously accepted value of 10.117Å. Scanning tunneling microscopy (STM) images of a reduced (sputtered/annealed) and oxidized (exposure to atomic oxygen at 300• C) surface show a step height of 5Å, which indicates a preference for one type of surface termination. The surfaces stay flat without any evidence for macroscopic faceting under any of these preparation conditions. A combination of low-energy ion scattering (LEIS) and atomically resolved STM indicates an indium-terminated surface with small islands of 2.5Å height under reducing conditions, with a surface structure corresponding to a strongly distorted indium lattice. Scanning tunneling spectroscopy (STS) reveals a pronounced surface state at the Fermi level (E F ). Photoelectron spectroscopy (PES) shows additional, deep-lying band gap states, which can be removed by exposure of the surface to atomic oxygen. Oxidation also results in a shoulder at the O 1s core level at a higher binding energy, possibly indicative of a surface peroxide species. A downward band bending of 0.4 eV is observed for the reduced surface, while the band bending of the oxidized surface is of the order of 0.1 eV or less.

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Hydrogen on In₂O₃: Reducibility, Bonding, Defect Formation, and Reactivity

Cited by 114 publications

References 21 publications

Ethanol Gas Sensing by Indium Oxide: An Operando Spectroscopic Raman-FTIR Study

Ethanol Gas Sensing by Indium Oxide: An Operando Spectroscopic Raman-FTIR Study

Water-Gas Shift and Methane Reactivity on Reducible Perovskite-Type Oxides

Bulk and surface characterization of In2O3(001) single crystals

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Hydrogen on In2O3: Reducibility, Bonding, Defect Formation, and Reactivity

Cited by 114 publications

References 21 publications

Ethanol Gas Sensing by Indium Oxide: An Operando Spectroscopic Raman-FTIR Study

Ethanol Gas Sensing by Indium Oxide: An Operando Spectroscopic Raman-FTIR Study

Water-Gas Shift and Methane Reactivity on Reducible Perovskite-Type Oxides

Bulk and surface characterization of In2O3(001) single crystals

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Product

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Hydrogen on In₂O₃: Reducibility, Bonding, Defect Formation, and Reactivity