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Highly mobile and reactive state of hydrogen in metal oxide semiconductors at room temperature.

Chen WP, He KF, Wang Y, Chan HL, Yan Z - Sci Rep (2013)

Bottom Line: Here we report a novel state of hydrogen with unusually high mobility and reactivity in metal oxides at room temperature.We show that freshly doped hydrogen in Nb₂O₅ and WO₃ polycrystals via electrochemical hydrogenation can reduce Cu²⁺ ions into Cu⁰ if the polycrystals are immersed in a CuSO₄ solution, while this would not happen if the hydrogenated polycrystals have been placed in air for several hours before the immersion.Time-dependent studies of electrochemically hydrogenated rutile single crystals reveal two distinct states of hydrogen: one as protons covalently bonded to oxygen ions, while the other one is highly unstable with a lifetime of just a few hours.

View Article: PubMed Central - PubMed

Affiliation: Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education and School of Physics and Technology, Wuhan University, Wuhan 430072, China.

ABSTRACT
Hydrogen in metal oxides usually strongly associates with a neighboring oxygen ion through an O-H bond and thus displays a high stability. Here we report a novel state of hydrogen with unusually high mobility and reactivity in metal oxides at room temperature. We show that freshly doped hydrogen in Nb₂O₅ and WO₃ polycrystals via electrochemical hydrogenation can reduce Cu²⁺ ions into Cu⁰ if the polycrystals are immersed in a CuSO₄ solution, while this would not happen if the hydrogenated polycrystals have been placed in air for several hours before the immersion. Time-dependent studies of electrochemically hydrogenated rutile single crystals reveal two distinct states of hydrogen: one as protons covalently bonded to oxygen ions, while the other one is highly unstable with a lifetime of just a few hours. Observation of this mobile and reactive state of hydrogen will provide new insight into numerous moderate and low temperature interactions between metal oxides and hydrogen.

No MeSH data available.


Related in: MedlinePlus

Crystal structure evolution of electrochemically hydrogenated TiO2.(a), XRD patterns of an electrochemically hydrogenated (001) rutile single crystal taken after a series of aging time. The hydrogenation time is 200 h, and the aging times are 0.3, 0.5, 0.8, 1.1, 1.7, 2.4, 3.4, 4.3, 5.4, 7, 12, 24, 33, 48, 96, and 150 h for the patterns from left to right. Inset: XRD patterns of an as-received (001) rutile single crystal and the electrochemically hydrogenated (001) rutile single crystal measured at an aging time of 0.3 h. (b), Lattice expansion of the (002) plane of the hydrogenated crystal compared to that of the undoped one as a function of aging time. The red curve is an exponential fit of the data: Δd = 0.0017exp(–t/39). Inset: the lattice expansion of the wide step indicated by a triangle in panel a. The red curve is also an exponential fit: Δd = 0.0058exp(–t/3).
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f2: Crystal structure evolution of electrochemically hydrogenated TiO2.(a), XRD patterns of an electrochemically hydrogenated (001) rutile single crystal taken after a series of aging time. The hydrogenation time is 200 h, and the aging times are 0.3, 0.5, 0.8, 1.1, 1.7, 2.4, 3.4, 4.3, 5.4, 7, 12, 24, 33, 48, 96, and 150 h for the patterns from left to right. Inset: XRD patterns of an as-received (001) rutile single crystal and the electrochemically hydrogenated (001) rutile single crystal measured at an aging time of 0.3 h. (b), Lattice expansion of the (002) plane of the hydrogenated crystal compared to that of the undoped one as a function of aging time. The red curve is an exponential fit of the data: Δd = 0.0017exp(–t/39). Inset: the lattice expansion of the wide step indicated by a triangle in panel a. The red curve is also an exponential fit: Δd = 0.0058exp(–t/3).

Mentions: We expect that it should be possible to reveal a more detailed evolution of this unstable hydrogen in metal oxide single crystals. As Nb2O5 and WO3 single crystals are not available, we turned to TiO2 rutile single crystals to conduct time-dependent study. These single crystals have nearly perfect crystal lattices that can eliminate the influence of crystal defects to doped hydrogen. For some metal oxides, phase transformation or crystal reformation occurs upon the hydrogen doping16. In this study, although we have tried to insert as much hydrogen as possible into the rutile single crystals, no new phases could be detected through XRD. Instead, the lattice of the rutile single crystals was found to be distorted to an extent beyond expectation when much hydrogen was inserted. Figure 2a shows XRD patterns of a hydrogenated rutile single crystal measured at different aging times, t, after 200 h of electrochemical hydrogen insertion. The (002) peak of as-received single crystals is sharp and highly symmetric (see the inset). In contrast, the (002) diffraction peak of the hydrogenated crystal becomes broad and is distorted to a very irregular shape, indicating non-uniform plane spacing or hydrogen distribution. An important observation is that there exists a wide step at the low angle side of the peak for short aging times, e.g., at 62.42° as indicated by the triangle for t = 0.3 h. We note that while the specific shape of the (002) peak could vary depending on the hydrogen insertion time, the broadening of the peak and the arising of a wide step always occur (see Supplementary Information, Fig. S1). Chester and Bradhurst have electrochemically inserted hydrogen into vacuum-reduced rutile single crystals and found that the hydrogen concentration gradient was much higher in the first few microns of the surface layer than in the bulk region (~100 μm deep)19. The wide step revealed in our study can be well explained in term of this layered structure: the concentration of hydrogen is particularly high at the surface, where the plane spacing is expanded by 0.54% at t = 0.3 h compared to that of the undoped (002) plane, while at the same time the expansion of the inner layer in the bulk region is only 0.16% as indicated by the distorted peak.


Highly mobile and reactive state of hydrogen in metal oxide semiconductors at room temperature.

Chen WP, He KF, Wang Y, Chan HL, Yan Z - Sci Rep (2013)

Crystal structure evolution of electrochemically hydrogenated TiO2.(a), XRD patterns of an electrochemically hydrogenated (001) rutile single crystal taken after a series of aging time. The hydrogenation time is 200 h, and the aging times are 0.3, 0.5, 0.8, 1.1, 1.7, 2.4, 3.4, 4.3, 5.4, 7, 12, 24, 33, 48, 96, and 150 h for the patterns from left to right. Inset: XRD patterns of an as-received (001) rutile single crystal and the electrochemically hydrogenated (001) rutile single crystal measured at an aging time of 0.3 h. (b), Lattice expansion of the (002) plane of the hydrogenated crystal compared to that of the undoped one as a function of aging time. The red curve is an exponential fit of the data: Δd = 0.0017exp(–t/39). Inset: the lattice expansion of the wide step indicated by a triangle in panel a. The red curve is also an exponential fit: Δd = 0.0058exp(–t/3).
© Copyright Policy - open-access
Related In: Results  -  Collection

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Show All Figures
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f2: Crystal structure evolution of electrochemically hydrogenated TiO2.(a), XRD patterns of an electrochemically hydrogenated (001) rutile single crystal taken after a series of aging time. The hydrogenation time is 200 h, and the aging times are 0.3, 0.5, 0.8, 1.1, 1.7, 2.4, 3.4, 4.3, 5.4, 7, 12, 24, 33, 48, 96, and 150 h for the patterns from left to right. Inset: XRD patterns of an as-received (001) rutile single crystal and the electrochemically hydrogenated (001) rutile single crystal measured at an aging time of 0.3 h. (b), Lattice expansion of the (002) plane of the hydrogenated crystal compared to that of the undoped one as a function of aging time. The red curve is an exponential fit of the data: Δd = 0.0017exp(–t/39). Inset: the lattice expansion of the wide step indicated by a triangle in panel a. The red curve is also an exponential fit: Δd = 0.0058exp(–t/3).
Mentions: We expect that it should be possible to reveal a more detailed evolution of this unstable hydrogen in metal oxide single crystals. As Nb2O5 and WO3 single crystals are not available, we turned to TiO2 rutile single crystals to conduct time-dependent study. These single crystals have nearly perfect crystal lattices that can eliminate the influence of crystal defects to doped hydrogen. For some metal oxides, phase transformation or crystal reformation occurs upon the hydrogen doping16. In this study, although we have tried to insert as much hydrogen as possible into the rutile single crystals, no new phases could be detected through XRD. Instead, the lattice of the rutile single crystals was found to be distorted to an extent beyond expectation when much hydrogen was inserted. Figure 2a shows XRD patterns of a hydrogenated rutile single crystal measured at different aging times, t, after 200 h of electrochemical hydrogen insertion. The (002) peak of as-received single crystals is sharp and highly symmetric (see the inset). In contrast, the (002) diffraction peak of the hydrogenated crystal becomes broad and is distorted to a very irregular shape, indicating non-uniform plane spacing or hydrogen distribution. An important observation is that there exists a wide step at the low angle side of the peak for short aging times, e.g., at 62.42° as indicated by the triangle for t = 0.3 h. We note that while the specific shape of the (002) peak could vary depending on the hydrogen insertion time, the broadening of the peak and the arising of a wide step always occur (see Supplementary Information, Fig. S1). Chester and Bradhurst have electrochemically inserted hydrogen into vacuum-reduced rutile single crystals and found that the hydrogen concentration gradient was much higher in the first few microns of the surface layer than in the bulk region (~100 μm deep)19. The wide step revealed in our study can be well explained in term of this layered structure: the concentration of hydrogen is particularly high at the surface, where the plane spacing is expanded by 0.54% at t = 0.3 h compared to that of the undoped (002) plane, while at the same time the expansion of the inner layer in the bulk region is only 0.16% as indicated by the distorted peak.

Bottom Line: Here we report a novel state of hydrogen with unusually high mobility and reactivity in metal oxides at room temperature.We show that freshly doped hydrogen in Nb₂O₅ and WO₃ polycrystals via electrochemical hydrogenation can reduce Cu²⁺ ions into Cu⁰ if the polycrystals are immersed in a CuSO₄ solution, while this would not happen if the hydrogenated polycrystals have been placed in air for several hours before the immersion.Time-dependent studies of electrochemically hydrogenated rutile single crystals reveal two distinct states of hydrogen: one as protons covalently bonded to oxygen ions, while the other one is highly unstable with a lifetime of just a few hours.

View Article: PubMed Central - PubMed

Affiliation: Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education and School of Physics and Technology, Wuhan University, Wuhan 430072, China.

ABSTRACT
Hydrogen in metal oxides usually strongly associates with a neighboring oxygen ion through an O-H bond and thus displays a high stability. Here we report a novel state of hydrogen with unusually high mobility and reactivity in metal oxides at room temperature. We show that freshly doped hydrogen in Nb₂O₅ and WO₃ polycrystals via electrochemical hydrogenation can reduce Cu²⁺ ions into Cu⁰ if the polycrystals are immersed in a CuSO₄ solution, while this would not happen if the hydrogenated polycrystals have been placed in air for several hours before the immersion. Time-dependent studies of electrochemically hydrogenated rutile single crystals reveal two distinct states of hydrogen: one as protons covalently bonded to oxygen ions, while the other one is highly unstable with a lifetime of just a few hours. Observation of this mobile and reactive state of hydrogen will provide new insight into numerous moderate and low temperature interactions between metal oxides and hydrogen.

No MeSH data available.


Related in: MedlinePlus