Developing Nature-Inspired Catalysts for a Sustainable Future
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RESEARCH
STUDY OF ADSORBED WATER FILM ON METAL OXIDES SURFACES
Water is one of the most prevalent chemicals on Earth, and thus it is no surprise that it is an adsorbate important in diverse fields such as catalysis, electrochemistry, tribology, geochemistry, atmospheric chemistry, and many others. Water is known adsorbs naturally on most transition metal oxide (TMO) at ambient conditions, and is known to impact diverse properties. For example, adsorbed water is known to affect electronic properties such as work function and electron affinity of oxides, which has been the subject of two major reviews. Water is also known to enhance catalytic rate of several gaseous reactions, such as CO oxidation, although the nature of this interaction is not known. Despite their similar lattice structure, the difference in the nature of water adsorption on α-Fe O and α-Al O leads to marked differences in the reactivity of these oxides towards geochemical processes, such as chemical speciation and heavy metal adsorption. These examples clearly illustrate the importance of adsorbed water in enabling many functional properties of oxides, but the fundamental nature of the dynamics of electronic and chemical interaction between adsorbed water and the semiconductor surface is still not clear.
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and CO , and thus enables electron exchange with the solid through electrochemical reactions between the dissolved species that otherwise would have not been possible with direct gas phase interaction.
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Focus of our current work is to understand how redox reactions in adsorbed water film can affect functional properties of metal oxide. Recently, we showed, using advanced spectroscopic techniques such as photoluminescence and infrared vibrational spectroscopy, conclusive proof for the existence of electrochemical surface transfer doping effects in hydrated metal oxides and showed the various manifestations of this phenomenon, such as its effect on metal-to-insulator electronic phase transitions, optical emission, and defect equilibrium processes. The proposed structure-property-function correlation provides a new electrochemical framework for understanding the adsorbed-water/oxide interaction and its effect on the catalytic and electronic properties.
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Related Publications:
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Q. Wang, A. Puntambekar, and V. Chakrapani,* “Gaseous Reactions in Adsorbed Water Present on Transition Metal Oxides,” J. Phys. Chem. C, 121, 13151 (2017)
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Q. Wang, A. Puntambekar, and V. Chakrapani,* “Co-adsorption of water and oxygen on GaN: Effects of charge transfer and formation of electron depletion layer,” J. Chem. Phys. 147, 104703 (2017)
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V. Chakrapani et al. “Charge Transfer Equilibria Between Diamond and an Aqueous Oxygen Electrochemical Redox Couple,” Science, 318, 1424 (2007).
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Funding: National Science Foundation, CBET: Award ID:1511733, “Adsorbed water on Transition metal oxide: Doping, Defects and Electrochemistry,”
A type of adsorbed water-substrate interaction and its role in affecting electronic properties was previously been demonstrated by us in hydrogen-terminated diamond that is known as “electrochemical surface transfer doping”. It has been shown that exposure of undoped, hydrogen-terminated diamond to humid air can induce the formation of a highly unusual p-type surface conductive layer on an otherwise insulating surface. This hole accumulation layer is a result of electron exchange between the valence band of diamond and an gaseous reaction occurring in the adsorbed water film that acts as a source or sink for electrons. Here, the water film facilitates the dissolution and solvation of ambient gases such as O
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PHASE TRANSITIONS IN STRONGLY CORRELATED OXIDES THROUGH ELECTROCHEMICAL GATING
Strongly correlated oxides have many unusual properties, such as high-temperature superconductivity, colossal magnetoresistance, spin polarization, and metal–insulator phase transitions (MIT), which arise as a result of complex intrinsic interactions between electrons, spins, orbitals, and phonons. The transitions between the insulating and metallic phases can be induced by different types of external stimuli, such as temperature, strain, pressure, electron doping, chemical doping, magnetic field, disorder, and light. In addition, electrochemical charging (or gating) has been used to suppress and induce phase transitions and for modulating number of other unusual properties of strongly-correlated oxides, such as high-temperature superconductivity, colossal magnetoresistance that have attracted significant attention due to their potential applications in diverse optoelectronic devices. However, the fundamental mechanism of electrochemical gating process is not well understood, especially the role of vacancy defects in inducing phase transitions. In our recent study, we reported a new type of phase transition in p-type, nonstoichiometric nickel oxide involving a semiconductor-to-insulator-to-metal transition (SIMT) along with the dramatic reversal of conductivity from p- to n-type at room temperature induced by electrochemical gating.
Our current research aims to elucidate the origin of these novel types of phase transitions in correlated oxides by probing the nature of interactions between vacancy defects and redox species during electrochemical gating.
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Related publications:
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Q. Wang, A. Puntambekar, and V. Chakrapani,* “Vacancy-induced Semiconductor-Insulator-Metal Transitions in Non-Stoichiometric Nickel and Tungsten Oxides,” Nano Lett. 16, 7067 (2016)
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A. Puntambekar, Q. Wang and V. Chakrapani,* “Electrochemical Charging of CdSe Quantum Dots: Effects of Intercalation versus Adsorption,” ACS Nano 10, 10988 (2016)
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Funding: National Science Foundation, DMR: Award ID: 1709649 “Phase Transitions in Strongly Correlated Oxides Modulated Through Electrochemical Gating,”
