Black TiO2 nanomaterials have recently emerged as promising candidates for solar-driven photocatalytic hydrogen production. Despite the great efforts to synthesize highly reduced TiO2, it is apparent that intermediate degree of reduction (namely, gray titania) brings about the formation of peculiar defective catalytic sites enabling cocatalyst-free hydrogen generation. A precise understanding of the structural and electronic nature of these catalytically active sites is still elusive, as well as the fundamental structure-activity relationships that govern formation of crystal defects, increased light absorption, charge separation, and photocatalytic activity. In this Review, we discuss the basic concepts that underlie an effective design of reduced TiO2 photocatalysts for hydrogen production such as (i) defects formation in reduced TiO2, (ii) analysis of structure deformation and presence of unpaired electrons through electron paramagnetic resonance spectroscopy, (iii) insights from surface science on electronic singularities due to defects, and (iv) the key differences between black and gray titania, that is, photocatalysts that require Pt-modification and cocatalyst-free photocatalytic hydrogen generation. Finally, future directions to improve the performance of reduced TiO2 photocatalysts are outlined.

Photocatalysis with Reduced TiO2: From Black TiO2 to Cocatalyst-Free Hydrogen Production.

Based on electronic structure and atomic size considerations, nitrogen has been regarded as the most suitable impurity for p-type doping in ZnO. However, numerous experimental efforts by many different groups have not resulted in stable and reproducible p-type material, casting doubt on the efficacy of nitrogen as a shallow acceptor. Based on advanced first-principles calculations we find that nitrogen is actually a deep acceptor, with an exceedingly high ionization energy of 1.3 eV, and hence cannot lead to hole conductivity in ZnO. In light of this result, we reexamine prior experiments on nitrogen doping of ZnO.

http://absimage.aps.org/image/MAR10/MWS_MAR10-2009-001476.pdf

Why nitrogen cannot lead to p-type conductivity in ZnO

Open shell transition metal oxides are usually described as Mott or charge transfer insulators, which are often viewed as being disparate from semiconductors. Based on the premise that the presence of a correlated gap and semiconductivity are not mutually exclusive, this work reviews electronic structure calculations on the binary 3d oxides, so to distill trends and design principles for semiconducting transition metal oxides. This class of materials possesses the potential for discovery, design, and development of novel functional semiconducting compounds, e.g. for energy applications. In order to place the 3d orbitals and the sp bands into an integrated picture, band structure calculations should treat both contributions on the same footing and, at the same time, account fully for electron correlation in the 3d shell. Fundamentally, this is a rather daunting task for electronic structure calculations, but quasi-particle energy calculations in GW approximation offer a viable approach for band structure predictions in these materials. Compared to conventional semiconductors, the inherent multivalent nature of transition metal cations is more likely to cause undesirable localization of electron or hole carriers. Therefore, a quantitative prediction of the carrier self-trapping energy is essential for the assessing the semiconducting properties and to determine whether the transport mechanism is a band-like large-polaron conduction or a small-polaron hopping conduction. An overview is given for the binary 3d oxides on how the hybridization between the 3d crystal field symmetries with the O-p orbitals of the ligands affects the effective masses and the likelihood of electron and hole self-trapping, identifying those situations where small masses and band-like conduction are more likely to be expected. The review concludes with an illustration of the implications of the increased electronic complexity of transition metal cations on the defect physics and doping, using as an example the diversity of possible atomic and magnetic configurations of the O vacancy in TiO(2), and the high levels of hole doping in Co(2)ZnO(4) due to a self-doping mechanism that originates from the multivalence of Co.

https://iopscience.iop.org/article/10.1088/0953-8984/27/28/283203/pdf

Semiconducting transition metal oxides

The gallium vacancy, an intrinsic acceptor, is identified in β-Ga2O3 using electron paramagnetic resonance (EPR) Spectra from doubly ionized ( V G a 2 −) and singly ionized ( V G a −) gallium vacancies are observed at room temperature, without photoexcitation, after an irradiation with high-energy neutrons The V G a 2 − centers (with S = 1/2) have a slight angular variation due to a small anisotropy in the g matrix (principal values are 20034, 20097, and 20322) The V G a 2 − centers also exhibit a resolved hyperfine structure due to equal and nearly isotropic interactions with the 69,71Ga nuclei at two Ga sites (the hyperfine parameters are 128 and 163 mT for the 69Ga and 71Ga nuclei, respectively, when the field is along the a direction) Based on these g-matrix and hyperfine results, the model for the ground state of the doubly ionized vacancy ( V G a 2 −) has a hole localized on one threefold-coordinated oxygen ion The vacancy is located at one of the three neighboring gallium sites, and the r

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Gallium Vacancies in β-Ga 2 O 3 Crystals

Graphene-induced formation of visible-light-responsive SnO2-Zn2SnO4 Z-scheme photocatalyst with surface vacancy for the enhanced photoreactivity towards NO and acetone oxidation

Optically stimulated luminescence (OSL) from Ag-doped Li2B4O7 crystals

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Triplet ground state of the neutral oxygen-vacancy donor in rutile TiO 2

An electron paramagnetic resonance(EPR) spectrum in neutron-irradiated ZnO crystals is assigned to the zinc-oxygen divacancy. These divacancies are observed in the bulk of both hydrothermally grown and seeded-chemical-vapor-transport-grown crystals after irradiations with fast neutrons. Neutral nonparamagnetic complexes consisting of adjacent zinc and oxygen vacancies are formed during the irradiation. Subsequent illumination below ∼150 K with 442 nm laser light converts these ( V Z n 2 − − V O 2 + )0 defects to their EPR-active state ( V Z n − − V O 2 + )+ as electrons are transferred to donors. The resulting photoinduced S = 1/2 spectrum of the divacancy is holelike and has a well-resolved angular dependence from which a complete g matrix is obtained. Principal values of the g matrix are 2.00796, 2.00480, and 2.00244. The unpaired spin resides primarily on one of the three remaining oxygen ions immediately adjacent to the zincvacancy, thus making the electronic structure of the ( V Z n − − V O 2 + )+ ground state similar to the isolated singly ionized axial zincvacancy. The neutral ( V Z n 2 − − V O 2 + )0 divacancies dissociate when the ZnO crystals are heated above 250 °C. After heating above this temperature, the divacancy EPR signal cannot be regenerated at low temperature with light.

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Identification of the zinc-oxygen divacancy in ZnO crystals

Electron paramagnetic resonance (EPR) is used to identify the singly ionized charge state of the Sn vacancy ( VSn−) in single crystals of Sn2P2S6 (often referred to as SPS). These vacancies, acting as a hole trap, are expected to be important participants in the photorefractive effect observed in undoped SPS crystals. In as-grown crystals, the Sn vacancies are doubly ionized ( VSn2−) with no unpaired spins. They are then converted to a stable EPR-active state when an electron is removed (i.e., a hole is trapped) during an illumination below 100 K with 633 nm laser light. The resulting EPR spectrum has g-matrix principal values of 2.0079, 2.0231, and 1.9717. There are resolved hyperfine interactions with two P neighbors and one Sn neighbor. The isotropic portions of these hyperfine matrices are 167 and 79 MHz for the two 31P neighbors and 8504 MHz for the one Sn neighbor (this latter value is the average for 117Sn and 119Sn). These VSn− vacancies are shallow acceptors with the hole occupying a diffuse wave...

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Sn vacancies in photorefractive Sn2P2S6 crystals: An electron paramagnetic resonance study of an optically active hole trap

Electron paramagnetic resonance (EPR) is used to identify four native defects in single crystals of CdSiP2. This nonlinear optical material is used in optical parametric oscillators to generate tunable output in the mid-infrared. The performance of these frequency-conversion devices is limited when infrared absorption bands associated with native defects overlap a pump wavelength. Cadmium, silicon, and phosphorus vacancies and also silicon-on-cadmium antisites are present in the as-grown undoped CdSiP2 crystals. Using near-band-edge 632.8 nm light from a He-Ne laser, a paramagnetic charge state, and thus an EPR spectrum, is formed at liquid-helium temperatures for three of the four defects. The EPR spectrum from the singly ionized silicon vacancy ( VSi−) is present without light and has five hyperfine lines due to equal interactions with the four neighboring 31P nuclei. In contrast, the photoinduced EPR spectrum from the singly ionized cadmium vacancy ( VCd−) has a three-line hyperfine pattern due to equa...

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Eric M. Golden

Papers

Optically stimulated luminescence (OSL) from Ag-doped Li2B4O7 crystals

Triplet ground state of the neutral oxygen-vacancy donor in rutile TiO 2

Identification of the zinc-oxygen divacancy in ZnO crystals

Sn vacancies in photorefractive Sn2P2S6 crystals: An electron paramagnetic resonance study of an optically active hole trap

Identification of native defects (vacancies and antisites) in CdSiP2 crystals