Synthesis, Structure, and Thermal Instability of the Cu2Ta4O11 Phase

King, Nacole; Sommer, Roger D.; Watkins-Curry, Pilanda; Chan, Julia Y.; Maggard, Paul A.

doi:10.1021/cg500987c

Cited by 12 publications

(20 citation statements)

References 46 publications

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“…For example, when Cu 2 Ta 4 O 11 is heated in a vacuum, the following decomposition occurs: Cu 2 Ta 4 O 11 → Cu 2 O + 2 Ta 2 O 5 . This reaction, and other alternative decomposition pathways, have been observed in all Cu(I)-containing niobates, tantalates, and vanadates [4][5][6][15][16][17][18]. For example, Cu 2 Ta 4 O 11 is calculated to be metastable by ~0.04 eV atom −1 , and thus its decomposition reaction is thermodynamically favorable.…”

Section: Introductionmentioning

confidence: 84%

A Metastable p-Type Semiconductor as a Defect-Tolerant Photoelectrode

et al. 2021

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A p-type Cu3Ta7O19 semiconductor was synthesized using a CuCl flux-based approach and investigated for its crystalline structure and photoelectrochemical properties. The semiconductor was found to be metastable, i.e., thermodynamically unstable, and to slowly oxidize at its surfaces upon heating in air, yielding CuO as nano-sized islands. However, the bulk crystalline structure was maintained, with up to 50% Cu(I)-vacancies and a concomitant oxidation of the Cu(I) to Cu(II) cations within the structure. Thermogravimetric and magnetic susceptibility measurements showed the formation of increasing amounts of Cu(II) cations, according to the following reaction: Cu3Ta7O19 + x/2 O2 → Cu(3−x)Ta7O19 + x CuO (surface) (x = 0 to ~0.8). With minor amounts of surface oxidation, the cathodic photocurrents of the polycrystalline films increase significantly, from <0.1 mA cm−2 up to >0.5 mA cm−2, under visible-light irradiation (pH = 6.3; irradiant powder density of ~500 mW cm−2) at an applied bias of −0.6 V vs. SCE. Electronic structure calculations revealed that its defect tolerance arises from the antibonding nature of its valence band edge, with the formation of defect states in resonance with the valence band, rather than as mid-gap states that function as recombination centers. Thus, the metastable Cu(I)-containing semiconductor was demonstrated to possess a high defect tolerance, which facilitates its high cathodic photocurrents.

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Section: Introductionmentioning

confidence: 84%

A Metastable p-Type Semiconductor as a Defect-Tolerant Photoelectrode

et al. 2021

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“…Computational studies have found that structural rigidity and inhibited ion mobility are key to the strong kinetic stabilization of metastable phases . One example is given by Cu 2 Ta 4 O 11 → Cu 2 O + 2Ta 2 O 5 . After removal of the CuCl flux used in the synthesis of Cu 2 Ta 4 O 11 , the high mobility of Cu(I) cations (with ∼33% site vacancies) leads to the slow decomposition reaction at its surfaces at even room temperature.…”

Section: Mixed-metal Oxide (M′mo X ) Semiconductorsmentioning

confidence: 99%

Capturing Metastable Oxide Semiconductors for Applications in Solar Energy Conversion

Maggard

2021

Acc. Chem. Res.

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Metrics & MoreArticle Recommendations CONSPECTUS: Many small bandgap semiconductors have been discovered or predicted to exist beyond the edges of stability, that is, accessible only as metastable solids that are thermodynamically unstable. In many cases, these metastable semiconductors have been revealed to have technologically promising properties for solar energy conversion, such as in photocatalysis or in photovoltaics. This Account presents a review of research results selected from my group and others in recent years on these semiconductors. Notably, these include the chemical systems of mixed-metal oxides (i.e., M′MO x ; M = Ti(IV), Nb(V), or Ta(V) cation; M′ = Ag(I), Cu(I), Sn(II), Pb(II), or Bi(III) cation), which have diverse structure types and compositions. High photocatalytic activities have been found for the light-driven reduction or oxidation of water as p-or n-type photoelectrodes, respectively, or as suspended powders in aqueous solutions. These have exhibited new combinations of favorable semiconductor properties, such as deep visible-light absorption, near-optimal band edge energies, defect tolerance, and functional carrier mobilities and charge separation. As described herein, this set of properties is inextricably linked to their metastable nature, that is, the crystalline structures and compositions needed for these characteristics lead naturally to thermodynamic instabilities. This Account focuses on current research efforts that have begun unlocking the potential of these semiconductors via new recent advances in (1) synthetic approaches that enable their preparation and (2) the understanding of structure−property relationships discovered at the precipices of stability that lead to the improved semiconductor properties. For example, low-temperature reactions have been developed to facilitate greater kinetic control, such as with the use of molten salts, and have been a key factor in preparing many of these semiconductors. As a result, a plethora of promising new mixed-metal oxide systems have been uncovered that exhibit band gaps spanning the range of photon energies from ∼1.3 to >3.0 eV. Especially relevant for visible-light applications are the Cu(I)-and Sn(II)-containing semiconductors. For example, n-type Sn(II)-titanates and p-type Cu(I)-niobates can be synthesized by flux methods and exhibit some of the smallest known visible-light band gaps that also maintain suitable conduction and valence band edges for driving the water-splitting half reactions. Kinetic stabilization of these metastable semiconductors against thermally driven phase segregation is increased with the formation of solid solutions for both the M and M′ cation sites, leading to effective strategies to more finely tune their band gaps, band edge energies, and photoelectrochemical properties. Many unique and useful relationships are emerging between the synthesis and structures of metastable semiconductors and their physical properties, leading to more efficient solar energy conversion.

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“…The Cu 2 Ta 4 O 11 structure occurs in two polymorphs, namely, the monoclinic α-Cu 2 Ta 4 O 11 phase and the rhombohedral β-Cu 2 Ta 4 O 11 [64,65], shown in Figures 5 and 6. At temperatures of −50 • C to room temperature, the monoclinic α-Cu 2 Ta 4 O 11 phase is relatively stable.…”

Section: Cu 2 Ta 4 O 11mentioning

confidence: 99%

“…The linearly coordinated Cu cation and 7-coordinate Na cation are located at the 18d and 12c Wyckoff crystallographic sites, respectively. By comparison, the use of nanoparticle reactants and lower reaction temperatures of ≈625 • C to ≈700 • C were found necessary to obtain the Cu-richest composition, i.e., Cu 2 Ta 4 O 11 , in high crystalline purity [65]. This composition was found to undergo a transformation from a monoclinic structure (Cc) to a rhombohedral (R3c) crystal structure upon heating, i.e., from α-Cu 2 Ta 4 O 11 to β-Cu 2 Ta 4 O 11 , as described above [64].…”

Section: Niobates/tantalates Solid Solutionmentioning

confidence: 99%

“…Improved charge separation and photocatalytic/photoelectrochemical redox reactions have been proposed to be enhanced in these structures due to the preferential anisotropic diffusion of charge carriers along the BO 7 pentagonal bipyramid layers. The anisotropic charge diffusion is a result of the delocalization of the Nb 4d and the Ta 5d lowest-energy unoccupied crystal orbitals across the BO 7 pentagonal bipyramid layers, as confirmed by electronic structure calculations which show the largest band dispersion in these crystallographic directions [35,47,64,65,71,[122][123][124][125][126][127][128].…”

Section: Introductionmentioning

confidence: 97%

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Polymorphism and Structural Distortions of Mixed-Metal Oxide Photocatalysts Constructed with α-U3O8 Types of Layers

et al. 2017

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Abstract:A series of mixed-metal oxide structures based on the stacking of α-U 3 O 8 type pentagonal bipyramid layers have been investigated for symmetry lowering distortions and photocatalytic activity. The family of structures contains the general composition A m+ ((n+1)/m) B (3n+1) O (8n+3) (e.g., A = Ag, Bi, Ca, Cu, Ce, Dy, Eu, Gd K, La, Nd, Pb, Pr, Sr, Y; B = Nb, Ta; m = 1-3; n = 1, 1.5, 2), and the edge-shared BO 7 pentagonal pyramid single, double, and/or triple layers are differentiated by the average thickness, (i.e., 1 ≤ n ≤ 2), of the BO 7 layers and the local coordination environment of the "A" site cations. Temperature dependent polymorphism has been investigated for structures containing single layered (n = 1) monovalent (m = 1) "A" site cations (e.g., Ag 2 Nb 4 O 11 , Na 2 Nb 4 O 11 , and Cu 2 Ta 4 O 11 ). Furthermore, symmetry lowering distortions were observed for the Pb ion-exchange synthesis of Ag 2 Ta 4 O 11 to yield PbTa 4 O 11 . Several members within the subset of the family have been constructed with optical and electronic properties that are suitable for the conversion of solar energy to chemical fuels via water splitting.

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Synthesis, Structure, and Thermal Instability of the Cu₂Ta₄O₁₁ Phase

Cited by 12 publications

References 46 publications

A Metastable p-Type Semiconductor as a Defect-Tolerant Photoelectrode

A Metastable p-Type Semiconductor as a Defect-Tolerant Photoelectrode

Capturing Metastable Oxide Semiconductors for Applications in Solar Energy Conversion

Polymorphism and Structural Distortions of Mixed-Metal Oxide Photocatalysts Constructed with α-U3O8 Types of Layers

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