I09-Surface and Interface Structural Analysis
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Galo J.
Paez
,
Christopher N.
Singh
,
Matthew J.
Wahila
,
Keith E.
Tirpak
,
Nicholas F.
Quackenbush
,
Shawn
Sallis
,
Hanjong
Paik
,
Yufeng
Liang
,
Darrell G.
Schlom
,
Tien-lin
Lee
,
Christoph
Schlueter
,
Wei-cheng
Lee
,
Louis F. J.
Piper
Diamond Proposal Number(s):
[13812, 25355]
Abstract: Recent reports have identified new metaphases of
VO
2
with strain and/or doping, suggesting the structural phase transition and the metal-to-insulator transition might be decoupled. Using epitaxially strained
VO
2
/
Ti
O
2
(001) thin films, which display a bulklike abrupt metal-to-insulator transition and rutile to monoclinic transition structural phase transition, we employ x-ray standing waves combined with hard x-ray photoelectron spectroscopy to simultaneously measure the structural and electronic transitions. This x-ray standing waves study elegantly demonstrates the structural and electronic transitions occur concurrently within experimental limits (±1K).
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May 2020
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Abstract: The importance of metal migration during multi-electron redox activity has been characterized, revealing a competing demand to satisfy bonding requirements and local strains in structures upon alkali intercalation. The local structural evolution required to accommodate intercalation in Y2(MoO4)3 and Al2(MoO4)3 has been contrasted by operando characterization methods, including X-ray absorption spectroscopy and diffraction, along with nuclear magnetic resonance measurements. Computational modeling further rationalized behavioral differences. The local structure of Y2(MoO4)3 was maintained upon lithiation while the structure of Al2(MoO4)3 underwent substantial local atomic rearrangements as the stronger ionic character of the bonds in Al2(MoO4)3 allowed Al to mix off its starting octahedral position to accommodate strain during cycling. However, this mixing was prevented in the more covalent Y2(MoO4)3 which accommodated strain through rotational motion of polyhedral subunits. Knowing that an increased ionic character can facilitate the diffusion of redox-inactive metals when cycling multi-electron electrodes offers a powerful design principle when identifying next-generation intercalation hosts.
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Apr 2020
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I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[22148]
Open Access
Abstract: The disproportionation of H2O into solar fuels H2 and O2, or water splitting, is a promising strategy for clean energy harvesting and storage but requires the concerted action of absorption of photons, separation of excitons, charge diffusion to catalytic sites and catalysis of redox processes. It is increasingly evident that the rational design of photocatalysts for efficient water splitting must employ hybrid systems, where the different components perform light harvesting, charge separation and catalysis in tandem. In this Topical Review, we report on the recent development of a new class of hybrid photocatalysts that employs MxV2O5 (M= p-block cation) nanowires in order to engineer efficient charge transfer from the photoactive chalcogenide quantum dots (QDs) to the water-splitting and hydrogen evolving catalysts. Herein, we summarize the oxygen-mediated lone pair mechanism used to modulate the energy level and orbital character of mid-gap states in the MxV2O5 nanowires. The electronic structure of MxV2O5 is discussed in terms of density functional theory and hard x-ray photoelectron spectroscopy (HAXPES) measurements. The principles of HAXPES are explained within the context of its unique sensitivity to metal 5(6)s orbitals and ability to non-destructively study buried interface alignments of quantum dot decorated nanowires i.e., MxV2O5 /CdX (X= S, Se, Te). We illustrate with examples how the MxV2O5 /CdX band alignments can be rationally engineered for ultra-fast charge-transfer of photogenerated holes from the quantum dot to the nanowires; thereby suppressing anodic photo-corrosion in the CdX QDs and enabling efficacious hydrogen evolution.
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Mar 2020
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I09-Surface and Interface Structural Analysis
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Zachary W.
Lebens-higgins
,
Hyeseung
Chung
,
Mateusz J.
Zuba
,
Jatinkumar
Rana
,
Yixuan
Li
,
Nicholas V.
Faenza
,
Nathalie
Pereira
,
Bryan D.
Mccloskey
,
Fanny
Rodolakis
,
Wanli
Yang
,
M. Stanley
Whittingham
,
Glenn G.
Amatucci
,
Ying Shirley
Meng
,
Tien-lin
Lee
,
Louis F. J.
Piper
Diamond Proposal Number(s):
[22250, 22148]
Abstract: Sensitivity to the `bulk' oxygen core orbital makes hard X-ray photoelectron spectroscopy (HAXPES) an appealing technique for studying oxygen redox candidates. Various studies have reported an additional O 1s peak (530-531 eV) at high voltages, which has been considered a direct signature of the bulk oxygen redox process. Here, we find the emergence of a 530.4 eV O 1s HAXPES peak for three model cathodes, Li2MnO3, Li-rich NMC, and NMC 442, that shows no clear link to expected oxygen redox. Instead, the 530.4 eV peak for these three systems is attributed to transition metal reduction and electrolyte decomposition in the near-surface region. Claims of oxygen redox relying on photoelectron spectroscopy must explicitly account for the surface sensitivity of this technique and the extent of the cathode degradation layer.
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Feb 2020
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I09-Surface and Interface Structural Analysis
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Jatinkumar
Rana
,
Joseph K.
Papp
,
Zachary
Lebens-higgins
,
Mateusz
Zuba
,
Lori A.
Kaufman
,
Anshika
Goel
,
Richard
Schmuch
,
Martin
Winter
,
M. Stanley
Whittingham
,
Wanli
Yang
,
Bryan D.
Mccloskey
,
Louis F. J.
Piper
Diamond Proposal Number(s):
[22250]
Abstract: Though Li2MnO3 was originally considered to be electrochemically inert, its observed activation has spawned a new class of Li-rich layered compounds that deliver capacities beyond the traditional transition-metal redox limit. Despite progress in our understanding of oxygen redox in Li-rich compounds, the underlying origin of the initial charge capacity of Li2MnO3 remains hotly contested. To resolve this issue, we review all possible charge compensation mechanisms including bulk oxygen redox, oxidation of Mn4+, and surface degradation for Li2MnO3 cathodes displaying capacities exceeding 350 mAh g–1. Using elemental and orbital selective X-ray spectroscopy techniques, we rule out oxidation of Mn4+ and bulk oxygen redox during activation of Li2MnO3. Quantitative gas-evolution and titration studies reveal that O2 and CO2 release accounted for a large fraction of the observed capacity during activation with minor contributions from reduced Mn species on the surface. These studies reveal that, although Li2MnO3 is considered critical for promoting bulk anionic redox in Li-rich layered oxides, Li2MnO3 by itself does not exhibit bulk oxygen redox or manganese oxidation beyond its initial Mn4+ valence.
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Jan 2020
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I09-Surface and Interface Structural Analysis
|
Diamond Proposal Number(s):
[22148]
Abstract: We synthesized a new class of heterostructures by depositing CdS, CdSe, or CdTe quantum dots (QDs) onto α-V2O5 nanowires (NWs) via either successive ionic layer adsorption and reaction (SILAR) or linker-assisted assembly (LAA). SILAR yielded the highest loadings of QDs per NW, whereas LAA enabled better control over the size and properties of QDs. Soft and hard x-ray photoelectron spectroscopy in conjunction with density functional theory calculations revealed that all α-V2O5/QD heterostructures exhibited Type-II band offset energetics, with a staggered gap where the conduction- and valence-band edges of α-V2O5 NWs lie at lower energies (relative to the vacuum level) than their QD counterparts. Transient absorption spectroscopy measurements revealed that the Type-II energetic offsets promoted the ultrafast (10−12–10−11 s) separation of photogenerated electrons and holes across the NW/QD interface to yield long-lived (10−6 s) charge-separated states. Charge-transfer dynamics and charge-recombination time scales varied subtly with the composition of heterostructures and the nature of the NW/QD interface, with both charge separation and recombination occurring more rapidly within SILAR-derived heterostructures. LAA-derived α-V2O5/CdSe heterostructures promoted the photocatalytic reduction of aqueous protons to H2 with a 20-fold or greater enhancement relative to isolated colloidal CdSe QDs or dispersed α-V2O5 NWs. The separation of photoexcited electrons and holes across the NW/QD interface could thus be exploited in redox photocatalysis. In light of their programmable compositions and properties and their Type-II energetics that drive ultrafast charge separation, the α-V2O5/QD heterostructures are a promising new class of photocatalyst architectures ripe for continued exploration.
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Dec 2019
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I09-Surface and Interface Structural Analysis
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Zachary W.
Lebens-higgins
,
David M.
Halat
,
Nicholas V.
Faenza
,
Matthew J.
Wahila
,
Manfred
Mascheck
,
Tomas
Wiell
,
Susanna K.
Eriksson
,
Paul
Palmgren
,
Jose
Rodriguez
,
Fadwa
Badway
,
Nathalie
Pereira
,
Glenn G.
Amatucci
,
Tien-lin
Lee
,
Clare P.
Grey
,
Louis F. J.
Piper
Diamond Proposal Number(s):
[22250, 22148]
Open Access
Abstract: Aluminum is a common dopant across oxide cathodes for improving the bulk and cathode-electrolyte interface (CEI) stability. Aluminum in the bulk is known to enhance structural and thermal stability, yet the exact influence of aluminum at the CEI remains unclear. To address this, we utilized a combination of X-ray photoelectron and absorption spectroscopy to identify aluminum surface environments and extent of transition metal reduction for Ni-rich LiNi0.8Co0.2−yAlyO2 (0%, 5%, or 20% Al) layered oxide cathodes tested at 4.75 V under thermal stress (60 °C). For these tests, we compared the conventional LiPF6 salt with the more thermally stable LiBF4 salt. The CEI layers are inherently different between these two electrolyte salts, particularly for the highest level of Al-doping (20%) where a thicker (thinner) CEI layer is found for LiPF6 (LiBF4). Focusing on the aluminum environment, we reveal the type of surface aluminum species are dependent on the electrolyte salt, as Al-O-F- and Al-F-like species form when using LiPF6 and LiBF4, respectively. In both cases, we find cathode-electrolyte reactions drive the formation of a protective Al-F-like barrier at the CEI in Al-doped oxide cathodes.
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Dec 2019
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I09-Surface and Interface Structural Analysis
|
Jack E. N.
Swallow
,
Benjamin A. D.
Williamson
,
Sanjayan
Sathasivam
,
Max
Birkett
,
Thomas J.
Featherstone
,
Philip A. E.
Murgatroyd
,
Holly J.
Edwards
,
Zachary W.
Lebens-higgins
,
David A.
Duncan
,
Mark
Farnworth
,
Paul
Warren
,
Nianhua
Peng
,
Tien-lin
Lee
,
Louis F. J.
Piper
,
Anna
Regoutz
,
Claire J.
Carmalt
,
Ivan P.
Parkin
,
Vin R.
Dhanak
,
David O.
Scanlon
,
Tim D.
Veal
Diamond Proposal Number(s):
[18428]
Open Access
Abstract: Transparent conductors are a vital component of smartphones, touch-enabled displays, low emissivity windows and thin film photovoltaics. Tin-doped In2O3 (ITO) dominates the transparent conductive films market, accounting for the majority of the current multi-billion dollar annual global sales. Due to the high cost of indium, however, alternatives to ITO have been sought but have inferior properties. Here we demonstrate that molybdenum-doped In2O3 (IMO) has higher mobility and therefore higher conductivity than ITO with the same carrier density. This also results in IMO having increased infrared transparency compared to ITO of the same conductivity. These properties enable current performance to be achieved using thinner films, reducing the amount of indium required and raw material costs by half. The enhanced doping behavior arises from Mo 4d donor states being resonant high in the conduction band and negligibly perturbing the host conduction band minimum, in contrast to the adverse perturbation caused by Sn 5s dopant states. This new understanding will enable better and cheaper TCOs based on both In2O3 and other metal oxides.
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Sep 2019
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I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[16005]
Abstract: Zn
M
I
I
I
2
O
4
(
M
I
I
I
=
Co
, Rh, Ir) spinels have been recently identified as promising
p
-type semiconductors for transparent electronics. However, discrepancies exist in the literature regarding their fundamental optoelectronic properties. In this paper, the electronic structures of these spinels are directly investigated using soft/hard x-ray photoelectron and x-ray absorption spectroscopies in conjunction with density functional theory calculations. In contrast to previous results,
ZnCo
2
O
4
is found to have a small electronic band gap with forbidden optical transitions between the true band edges, allowing for both bipolar doping and high optical transparency. Furthermore, increased
d
−
d
splitting combined with a concomitant lowering of Zn
s
/
p
conduction states is found to result in a
ZnCo
2
O
4
(
ZCO
)
<
ZnRh
2
O
4
(
ZRO
)
≈
ZnIr
2
O
4
(
ZIO
)
band gap trend, finally resolving long-standing discrepancies in the literature.
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Aug 2019
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I09-Surface and Interface Structural Analysis
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Sebastian A.
Howard
,
Christopher N.
Singh
,
Galo J.
Paez
,
Matthew
Wahila
,
Linda W.
Wangoh
,
Shawn
Sallis
,
Keith
Tirpak
,
Yufeng
Liang
,
David
Prendergast
,
Mateusz
Zuba
,
Jatinkumar
Rana
,
Alex
Weidenbach
,
Timothy M.
Mccrone
,
Wanli
Yang
,
Tien-lin
Lee
,
Fanny
Rodolakis
,
William
Doolittle
,
Wei-cheng
Lee
,
Louis F. J.
Piper
Diamond Proposal Number(s):
[20647]
Open Access
Abstract: The discovery of analog LixNbO2 memristors revealed a promising new memristive mechanism wherein the diffusion of Li+ rather than O2− ions enables precise control of the resistive states. However, directly correlating lithium concentration with changes to the electronic structure in active layers remains a challenge and is required to truly understand the underlying physics. Chemically delithiated single crystals of LiNbO2 present a model system for correlating lithium variation with spectroscopic signatures from operando soft x-ray spectroscopy studies of device active layers. Using electronic structure modeling of the x-ray spectroscopy of LixNbO2 single crystals, we demonstrate that the intrinsic memristive behavior in LixNbO2 active layers results from field-induced degenerate p-type doping. We show that electrical operation of LixNbO2-based memristors is viable even at marginal Li deficiency and that the analog memristive switching occurs well before the system is fully metallic. This study serves as a benchmark for material synthesis and characterization of future LixNbO2-based memristor devices and suggests that valence change switching is a scalable alternative that circumvents the electroforming typically required for filamentary-based memristors.
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Jul 2019
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