Electronic Structure Of Oxygen Vacancies Research Articles

S-scheme homojunction and activate site engineering over TiO2 for highly efficient photocatalytic nitrogen fixation

The presence of efficient catalytic active sites and an ample supply of electrons are the two indispensable factors for achieving photocatalytic nitrogen fixation. Herein, the Fe-doped TiO2 S-scheme anatase/rutile homojunction (F-T-A/R) was synthesized as a brilliant photocatalyst for photocatalytic nitrogen fixation from N2 to NH3 under ambient atmosphere. The optimized F-T-A/R photocatalyst (A/R = 98:2, Fe-2 wt%) exhibits an unprecedented NH3 generation rate of 358 μmol gcat-1h−1 in water solvent and under simulated solar light irradiation. Experimental and theoretical studies reveal that the constructed S-scheme homojunction in F-T-A/R is conducive to fluent charge separation and adequate supply of electrons on rutile with higher nitrogen fixing activity, while the tuned local electronic structure of oxygen vacancy with Fe doping is engineered to strengthen active sites, synergistically enhancing the photocatalytic nitrogen fixation.

First-principles study of the effects of Vo on the magnetic and photocatalytic properties of bilayer anatase TiO2(001): C/N/S

In this study, we investigated the effect of Vo on the magnetic and photocatalytic properties of bilayer anatase TiO2 (001): C/N/S using first principles within the density functional theory framework. Through research, it has been discovered that the stability and electronic structure of oxygen vacancies can vary depending on their positions, ultimately resulting in diverse photocatalytic performances. When the distance between N and Vo is relatively close, bilayer TiO2(001) system exhibited a relatively low formation energy under Ti-rich conditions, a high cohesive energy, and no virtual frequency formed in the phonon spectrum. Thus, it has good stability. The bilayer Ti24O46N (001) system displayed the most evident red shift in the absorption spectrum, the largest electric dipole moment, the smallest work function, relatively high carrier activity, a notable difference in mobility between electrons and holes, reduced recombination of electrons and holes, an extended carrier lifetime, and superior oxidation reaction capability. Consequently, the bilayer Ti24O46N (001) surface system emerged as the most suitable catalyst for the photo-degradation of water to produce oxygen.

Impact of lanthanum doping on the electronic structure of oxygen vacancies in hafnium oxide

The electronic structure of oxygen vacancies in lanthanum-doped hafnium oxide (HfO2:La) is studied using the density functional theory. The simulation is carried out for the optimal structure of HfO2:La in three crystalline phases for an oxygen vacancy involved in the impurity charge compensation that is structural element, as well as for a second purposely added oxygen vacancy. It is shown that the HfO2 doping with La increases the oxygen vacancy content in such a way that the twice positively charged vacancy concentration increases and the neutral one decreases. The doping with La facilitates new oxygen vacancies formation in HfO2. Oxygen vacancies in HfO2:La and HfO2 have a similar electronic structure and are traps for electrons. However, the deeper trap and the electrostatic repulsion caused by the impurity in HfO2:La suppress the vacancies participation in the charge transport. Based on the obtained results, the mechanisms of HfO2-based resistive and ferroelectric memory improvements, when HfO2 doped with lanthanum, are discussed.

Tuning the local electronic structure of oxygen vacancies over copper-doped zinc oxide for efficient CO2 electroreduction

Oxygen vacancies in metal oxides can serve as electron trap centers to capture CO2 and lower energy barriers for the electrochemical CO2 reduction reaction (CO2RR). Under aqueous electrolytes, however, such charge-enriched active sites can be occupied by adsorbed hydrogen (H∗) and lose their effectiveness for the CO2RR. Here, we develop an efficient catalyst consisting of Cu-doped, defect-rich ZnO (Cu–ZnO) for the CO2RR, which exhibits enhanced CO Faradaic efficiency and current density compared to pristine ZnO. The introduced Cu dopants simultaneously stabilize neighboring oxygen vacancies and modulate their local electronic structure, achieving inhibition of hydrogen evolution and acceleration of the CO2RR. In a flow cell test, a current density of more than 45 ​mA ​cm−2 and a CO Faradaic efficiency of > 80% is obtained for a Cu–ZnO electrocatalyst in the wide potential range of −0.76 ​V to −1.06 ​V vs. Reversible Hydrogen Electrode (RHE). This work opens up great opportunities for dopant-modulated metal oxide catalysts for the CO2RR.

Altering Hydrogenation Pathways in Photocatalytic Nitrogen Fixation by Tuning Local Electronic Structure of Oxygen Vacancy with Dopant.

To avoid the energy-consuming step of direct N≡N bond cleavage, photocatalytic N2 fixation undergoing the associative pathways has been developed for mild-condition operation. However, it is a fundamental yet challenging task to gain comprehensive understanding on how the associative pathways (i.e., alternating vs. distal) are influenced and altered by the fine structure of catalysts, which eventually holds the key to significantly promote the practical implementation. Herein, we introduce Fe dopants into TiO2 nanofibers to stabilize oxygen vacancies and simultaneously tune their local electronic structure. The combination of in situ characterizations with first-principles simulations reveals that the modulation of local electronic structure by Fe dopants turns the hydrogenation of N2 from associative alternating pathway to associative distal pathway. This work provides fresh hints for rationally controlling the reaction pathways toward efficient photocatalytic nitrogen fixation.

Altering Hydrogenation Pathways in Photocatalytic Nitrogen Fixation by Tuning Local Electronic Structure of Oxygen Vacancy with Dopant

AbstractTo avoid the energy‐consuming step of direct N≡N bond cleavage, photocatalytic N2 fixation undergoing the associative pathways has been developed for mild‐condition operation. However, it is a fundamental yet challenging task to gain comprehensive understanding on how the associative pathways (i.e., alternating vs. distal) are influenced and altered by the fine structure of catalysts, which eventually holds the key to significantly promote the practical implementation. Herein, we introduce Fe dopants into TiO2 nanofibers to stabilize oxygen vacancies and simultaneously tune their local electronic structure. The combination of in situ characterizations with first‐principles simulations reveals that the modulation of local electronic structure by Fe dopants turns the hydrogenation of N2 from associative alternating pathway to associative distal pathway. This work provides fresh hints for rationally controlling the reaction pathways toward efficient photocatalytic nitrogen fixation.

The color center singlet state of oxygen vacancies in TiO2.

Oxygen vacancies are ubiquitous in TiO2 and play key roles in catalysis and magnetism applications. Despite being extensively investigated, the electronic structure of oxygen vacancies in TiO2 remains controversial both experimentally and theoretically. Here, we report a study of a neutral oxygen vacancy in TiO2 using state-of-the-art quantum chemical electronic structure methods. We find that the ground state is a color center singlet state in both the rutile and the anatase phases of TiO2. Specifically, embedded coupled cluster with singles, doubles, and perturbative triples calculations find, for an oxygen vacancy in rutile, that the lowest triplet state energy is 0.6 eV above the singlet state, and in anatase, the triplet state energy is higher by 1.4 eV. Our study provides fresh insights into the electronic structure of the oxygen vacancy in TiO2, clarifying earlier controversies and potentially inspiring future studies of defects with correlated wave function theories.

Simulation of the Atomic and Electronic Structure of Oxygen Vacancies and Polyvacancies in ZrO2

Cubic, tetragonal, and monoclinic phases of zirconium oxide with oxygen vacancies and polyvacancies are studied by quantum chemical modeling of the atomic and electronic structure. It is demonstrated that an oxygen vacancy in ZrO2 may act as both an electron trap and a hole one. An electron added to the ZrO2 structure with an oxygen vacancy is distributed between two neighboring Zr atoms and is a bonding orbital by nature. It is advantageous for each subsequent O vacancy to form close to the already existing ones; notably, one Zr atom has no more than two removed O atoms related to it. Defect levels from oxygen polyvacancies are distributed in the bandgap with preferential localization in the vicinity of the oxygen monovacancy level.

Oxygen Polyvacancies as Conductive Filament in Zirconia: First Principle Simulation

The atomic and electronic structure of oxygen vacancy and polyvacancy in the cubic, tetragonal and monoclinic zirconium oxide were investigated using quantum-chemical density functional theory simulation. It is shown that the neutral oxygen vacancy in crystalline zirconia can act as electron and hole trap. An electron added to ZrO2 structure with the oxygen monovacancy has a bonding charge density character. The defect levels position as well as thermal and optical trap ionization energies are consisted with the previously defined experimentally values. Each subsequent vacancy is formed near the already existing one, and no more than 2 removed oxygen atoms are related to Zr atom. The levels from oxygen polyvacancies are distributed in the bandgap preferentially localized close to the monovacancy level. The ability of oxygen vacancy chain in crystalline ZrO2 to act as a conductive filament is discussed.

Electronic Structure of Oxygen Deficient Noncentrosymmetric Orthorhombic Hf0.5Zr0.5O2

High-κ hafnia/zirconia solid solutions Hf0.5Zr0.5O2 are promising for advanced microelectronics. It is known that the noncentrosymmetric orthorhombic (Pbc21) Hf0.5Zr0.5O2phase exhibits ferroelectric properties. This new ferroelectric material is of significant interest due to a potential possibility of replacement of traditional perovskite-based ferroelectrics in ferroelectric random access memory (FeRAM) [1]. In the present study the electronic structure of oxygen vacancies (VO) in o-Hf0.5Zr0.5O2 was investigated with quantum-chemical simulations using the Quantum-ESPRESSO code. The simulations are based on the density functional theory with B3LYP hybrid exchange-correlation functional. We use the periodic (96 atom) supercells model. The structure was obtained by the replacement of half of the Hf atom to Zr in the orthorhombic HfO2 cell [2]. Three types of VO with fourfold «HHZZ» and threefold coordinated «HHZ», «HZZ» were investigated. 20 nm thick orthorhombic Hf0.5Zr0.5O2 were synthesized by ALD technique with following rapid thermal annealing at 400 °C. X-ray photoelectron spectra was obtained using synchrotron radiation with a photon energy of 720 eV for the analysis of the chemical composition and 200 eV to record the spectrum of the valence band. Film stoichiometry was calculated as ratios of the integrated intensities of the main XPS lines of the metal atoms show the stoichiometry close to Hf0.5Zr0.5O2. The calculated band gap of o-Hf0.5Zr0.5O2 is 5.65 eV. The Kohn–Sham levels of five VO charged states (q = -2, -1, 0, 1, 2) localized in the band gap indicate that any type of oxygen vacancies can capture both electrons and holes. Note, that the oxygen vacancies with a trapped electron is a magnetic defect. The added electron localizes itself in an energy well due to strong neighboring atom relaxation (polaron effect). The values of thermal (E th) and optical (E opt) ionization energies were calculated for different charge states as described in [3]. It was found that the oxygen vacancy can act as an amphoteric localization center (trap) for charge carriers, i.e. it can capture both electrons and holes. The calculated E th and E opt values are in satisfactory agreement with the values of thermal (W t=1.25 eV) and optical (W opt=2.5 eV) trap energies, obtained from experiments on charge transport. Charge localization on the oxygen vacancy is confirmed by the spatial distribution of the electron density in VO and its first coordination sphere. The positive charge is distributed approximately evenly between the metal atoms nearest to VO, while the negative charge is distributed unevenly, and only a couple of atoms are bounded. The position of each subsequent atom for removing was found by examining of all the possible variants and determining the lowest defect formation energy. The oxygen atoms in o-Hf0.5Zr0.5O2, which are removed, have an HHZZ type and are placed at a significant distance from each other. The forming energy of triply coordinated VO (about 6.4 eV) is substantially higher than that for fourfold coordinated VO (about 6.1 eV). The formation of closely spaced VO (i.e. polyvacancy) in o-Hf0.5Zr0.5O2 is not profitable energetically. On the contrary, for the cubic, tetragonal and monoclinic HfO2 and ZrO2 formation energies of three- and fourfold coordinated VO is not so much different from each other, the it is energy profitable to form oxygen vacancies close to each other (there is some clustering of vacancies). Probably, this is the reason why the o-Hf0.5Zr0.5O2 film etching with Ar+ ions does not lead to metal enrichment, whereas the etching of amorphous HfO2 and ZrO2 leads to the generation high concentration of VO in the surface layer of the films. As a result of Hf0.5Zr0.5O2 etching with Ar+ ions, the XPS lines corresponding to Hf 4f and Zr 3d in the metallic state are not observed, and theoretically predicted XPS peak does not appear above the edge of the valence band. It is concluded that the original Hf0.5Zr0.5O2 films are not enriched with metal, and etching with Ar+ ions does not lead to depletion of oxygen. However, the experimental data do not exclude the oxygen vacancies presence in the Hf0.5Zr0.5O2 film, since XPS technique is insensitive to defects with density less than 0.1%. The work was supported by the Ministry of Education and Science of the Russian Federation, project #RFMEFI57514X0027 (Hf0.5Zr0.5O2 films synthesis), and by Russian Science Foundation, grant #14-19-00192 (XPS measurements, calculations). The modeling was performed on a cluster of the Institute of Semiconductor Physics SB RAS. [1] J.Müller, T.S.Böscke, U.Schröder, et al.Nano Lett. 12, 4318 (2012). [2] Q.Zeng, A.R.Oganov, A.O.Lyakhov, et al.Acta Cryst. C 70, 76 (2014). [3] D.Muñoz Ramo, J.L.Gavartin, A.L.Shluger et al. Phys. Rev. B 75, 205336 (2007).

Charge Transport Mechanism of Stress Induced Leakage Current in Thermal Silicon Oxide

Write/erase operations of SONOS and TANOS flash memory with Si3N4-floating gates are driven by tunnel injections of electrons and holes through thin 1.8-8.0 nm (tunnel) SiO2 layer [Ed. by D. Brown, J. F. Brewer, IEEE Press, New York (1998); P.E. Blöchl, J.H., Stathis, Phys. Rev. Lett. 83, 372 (1999)]. The current through the tunnel oxide during erase/rewrite operations by applying strong (~10 MV/cm) electric field in the oxide increases the conductivity of the layer at low electric fields (~1 MV/cm). This is caused by the appearance of redundant current components through the tunnel oxide compared to the current flowing in a strong electric field on the mechanism of Fowler-Nordheim. This phenomenon is called Stress Induced Leakage Current, SILC. SILC limits reprogramming cycles number of the flash memory cells based on floating gate and the silicon nitride to 104-105. SILC also leads to accelerate the draining of charge in the flash memory elements in the data storage mode, that is, memory degradation characteristics. Despite the fact that the study of SILC is the subject of numerous articles [P.E. Blöchl, J.H., Stathis, Phys. Rev. Lett. 83, 372 (1999); T. Endoh et al., J. Appl. Phys. 86, 2095 (1999); K. Komiya, Y. Omura, J. Appl. Phys. 92, 2593 (2002); F. Jiménez-Molinos et al., J. Appl. Phys. 90, 3396 (2015)], the nature of this phenomenon is still a debatable issue. Recently a multiphonon model of charge transport in SILC was proposed [T. Endoh et al., J. Appl. Phys. 86, 2095 (1999); F. Jiménez-Molinos et al., J. Appl. Phys. 90, 3396 (2015)]. That model is not analytic and requires complicate numerical calculations to describe transport in SILC, and does not clarify the nature of the traps responsible for SILC. This study describes transport in SILC with phonon-assisted tunneling between traps, shows that oxygen vacancies are the charge traps in SiO2. SILC measurements were performed for test structures with poly-Si top electrodes and 7.5-nm-thick SiO2 on on n+ -Sip and p-Si substrates. Current-voltage characteristics were measured at different temperatures 25-70 °C before and after stress of 0.01 Q/cm2, 0.1 Q/cm2, 1 Q/cm2, 3 Q/cm2, 10 Q/cm2. The electronic structure of oxygen vacancy in SiO2 was calculated within the spin polarized density functional theory using the ab initio simulation code Quantum ESPRESSO [P. Giannozzi et al., J. Phys.: Condens. Matter 21, 395502 (2009)] with B3LYP hybrid exchange-correlation functional. The oxygen vacancy was generated by the removal of an O atom, followed by relaxation of remaining atoms in 100-atom supercell. The high-field (>7 MV/cm) current through the structure was limited by Fowler-Nordheim tunneling (Fig.1) [R.H. Fowler, L. Nordheim, Proc. R. Soc. A 119, 173 (1928)]. Low-field current (<6 MV/cm) was limited by phonon-assisted tunneling between traps (Fig.3) [K.A. Nasyrov, V.A. Gritsenko, J. Appl. Phys. 109, 093705 (2011)]. Experimental data was described quantitatively using thermal and optical trap energies of W t = 1.6 eV and W opt = 3.2 eV. Obtained value of W t is equal to a half of Stokes shift of photoluminescence near 4.4 eV exited by 7.6-eV-phonots on oxygen vacancy in SiO2 [V.A. Gritsenko, H. Wong, Crit. Rev. Solid State 36, 129 (2011)] and equal to calculated trap energy for trapped electrons and holes on oxygen vacancies; W opt is close to measured electron trap energy in SiO2 [K. Yamabe, Y. Miura, J. Appl. Phys. 51, 6258 (1980)]. Charge trap density before stress was less than 1020 cm-3. SILC stress caused arising of the trap density up to 7×1020/9.3×1020/1.05×1021/1.10×1021/1.15×1021 cm-3 depending on the stress (Fig.1). Annealing at 250 °C in O2 atmosphere during 120 hours leads to decreasing of the trap density to initial values (Fig.2). The work was partly supported by Russian Science Foundation, grant #16-19-00002. Figure 1

Electronic Structure of Oxygen Deficient Noncentrosymmetric Orthorhombic Hf0.5Zr0.5O2

The electronic structure of oxygen vacancies in noncentrosymmetric orthorhombic Hf0.5Zr0.5O2 was investigated with quantum chemical density functional theory simulations. The oxygen vacancies with different coordination in five charged states were investigated. The single electron levels of charged states is located in the band gap, as well as the values of trap thermal and optical ionization energies, indicate that any type of oxygen vacancies can capture both electrons and holes, i.e. they can act as an amphoteric localization center (trap) for charge carriers. The vacancy levels position in the band gap for an added electron is in excellent agreement with the experimental trap thermal ionization energy 1.25 eV for the amorphous Hf0.5Zr0.5O2 film. X-ray photoelectron spectroscopy revealed that high oxygen vacancies concentration generation by the Ar+ etching method is not appropriate for o-Hf0.5Zr0.5O2.

The quasi-particle electronic structure of charged oxygen vacancies in TiO2

We studied the quasi-particle electronic structure of oxygen vacancies V O in rutile TiO2 using G0W0 approximation on top of GGA + U as a method of choice. In order to determine the degree of electronic localization, we examined the combined G0W0@GGA + U scheme in which both G0W0 and GGA + U methods are individually one step toward a better inclusion of the non-locality of the exchange-correlation potential. Our G0W0@GGA + U results realize the weak nature of electron correlation. Moreover, it was shown that the U-dependency of the energy gap in the perfect bulk is slightly stronger than that of a defected sample while the U-dependence of V O induced levels in different charged states show different behaviors. Eventually, we concluded that applying G0W0@GGA + U, though it does not stabilize oxygen vacancy levels in the gap, however, it does correct the formation energies and transition levels more closely to the experimental measurements.

Electronic structure of oxygen vacancies in hafnium oxide

The electronic structure of oxygen vacancies and polyvacancies in HfO2 was studied theoretically from the first-principles calculations and experimentally, by X-ray photoelectron spectroscopy. The electronic structure calculations of crystalline HfO2 were performed within the hybrid density functional theory. The experimental photoelectron spectra indicate that both nonstoichiometric chemistry and Ar-ion bombardment of hafnia films lead to the generation of the defect states at 3.0 eV above the valence band. According to the calculations, these defect states are attributed to the oxygen vacancies.

Electronic structure of oxygen vacancies inSrTiO3andLaAlO3

The electronic structure of oxygen vacancies in bulk perovskite oxides SrTiO${}_{3}$ and LaAlO${}_{3}$ is studied using the Heyd, Scuseria, Ernzerhof (HSE) hybrid density functional. In SrTiO${}_{3}$ the oxygen vacancy defect introduces a localized state comprised of $3{d}_{{z}^{2}}$ and $4{p}_{z}$ orbitals of the adjacent Ti atoms. This results in a bound state 0.7 eV below the conduction band edge. For LaAlO${}_{3}$, the oxygen vacancy creates a deep state 2.19 eV below the conduction band edge. The defect state is a hybrid of the adjacent La 5$d$ and the Al 3$p$ states. We compute the formation energies of the neutral oxygen vacancy defect V${}^{0}$ in bulk SrTiO${}_{3}$ and LaAlO${}_{3}$ to be 6.0 and 8.3 eV, respectively.

Electronic structure of δ-Ta2O5 with oxygen vacancy: ab initio calculations and comparison with experiment

Electronic structure of oxygen vacancies in Ta2O5 have been studied theoretically by first-principles calculations and experimentally by x-ray photoelectron spectroscopy. Calculations of δ-Ta2O5 were performed using density functional theory within gradient-corrected approximation with the +U approach. Results indicate that the oxygen vacancy causes a defect level in the energy gap at 1.2 eV above the top of the valence band. To produce oxygen vacancies, amorphous films of Ta2O5 were bombarded with Ar+ ions. XPS results indicate that the Ar-ion bombardment leads to the generation of the oxygen vacancies in Ta2O5 that characterize the peak at 2 eV above the valence band. The calculated spectrum of crystalline δ-Ta2O5 demonstrates qualitative correspondence with the XPS spectrum of the amorphous Ta2O5 film after Ar-ion bombardment.

Electronic structure of oxygen vacancy in crystalline InGaO3(ZnO)m

We perform first-principles theoretical calculations to investigate the defect properties of oxygen vacancy (V O ) in crystalline InGaO 3 (ZnO) m ( m =3). In a flat boundary structure, in which Ga atoms are located on a single plane, various configurations of V O exist. We find that neutral V O at a site near the In–O layer or in the ZnO area is energetically more favorable than those formed near and on the Ga–O layer. Although the defect levels vary with the type of metal ions in the neighborhood, V O defects act as deep donors, similar to that of bulk ZnO. Moreover, the O-vacancies exhibit the negative- U behavior, with the charge transition levels well below the conduction band minimum.

Atomic and electronic structure of oxygen vacancies and Nb-impurity in SrTiO3

We present the results of a first-principle full-potential linearized augmented plane wave (FLAPW) method to study the effect of defects on the electronic structure of SrTiO3. In addition, the relaxation of nearest neighbor atoms around those defects were calculated self-consistently. The calculations were performed using the local (spin) density approximations (L(S)DA), for the exchange-correlation potential. SrTiO3 was found to experience an insulator-to-metal transition upon the formation of oxygen vacancies or the substitution of Nb at the Ti site. The formation of oxygen divacancy disclosed additional states below the conduction band edge. The crystalline lattice relaxation showed displacements of atoms in rather large defective region. The magnitudes of atomic movements, however, were not large, normally not exceeding 0.15 A. Our results were compared to the available experimental observations.

First-principles calculations of defects near a grain boundary in MgO

The electronic structure of oxygen vacancy and proton defects close to grain boundaries in MgO are calculated using first principles methods. These defects, in various charge states, favorably segregate to grain boundaries and can trap electrons. The interplay between electron and defect segregation provides a mechanism for charge to build up at boundaries, for example, under irradiation or applied electrical voltage. The theoretical calculations presented provide insight into the complex electronic properties of metal-oxide grain boundaries that can be difficult to obtain by experiment alone but which are important for many applications.

Electronic structure of oxygen vacancies in La 2O 3, Lu 2O 3 and LaLuO 3

This paper presents first principles calculations of the energy levels of the oxygen vacancy in La 2O 3, Lu 2O 3 and LaLuO 3. The levels lie above the Si gap when aligned using the known band offsets. In hexagonal La 2O 3, Lu 2O 3 and LaLuO 3, oxygen vacancies with 4 neighbours have similar properties to those in HfO 2, where they are the main cause of trapping instability. Oxygen vacancies with 6 neighbours have no negative charged state so that they are less important for charge trapping.