The disorder-engineered black TiO 2 nanocrystals have dramatically higher solar-driven photocatalytic activity for the decomposition of methylene blue, phenol, and acetaldehyde than compared to pristine TiO 2. After hydrogenation, an approximately 1 nm thick disordered outer layer formed around the surfaces of TiO 2 nanocrystals. The disorder-engineered TiO 2 nanocrystals show a dramatic color change to black ( Figure 12.8a and b) and a broad absorption across the entire visible region and into the near-infrared ( Figure 12.8c). These states can overlap with the conduction and valence band edges, forming a “band tail” and significantly extending the valence band and conduction band into the band gap region. The mid-gap states that form as a result of the induced lattice disorder have an energy distribution that is relatively wide and continuous. recently demonstrated a conceptually different approach to enhancing solar absorption by introducing disorder in the surface layers of nanophase TiO 2 through hydrogenation. Ĭompared to homogenous defect doping, Chen et al.
Both types of defects result in an increase in the visible-range absorption of the semiconductor. were able to introduce crystal defects in the form of Ti 3+ centers within the semiconductor lattice. On the other hand, Ti vacancies lead to acceptor levels above the valence band. Oxygen vacancies typically induce donor levels around 0.75–1.18 eV below the conduction band of TiO 2, compared to 1.23–1.56 eV for Ti 3+ interstitials.
Self-doping, or the introduction of defects through Ti or O vacancies as well as interstitials, can add acceptor and donor levels within the band gap of TiO 2. Additional investigations are ongoing to elucidate the origin of this band gap narrowing in further detail. The band gap narrowing due to N-doping can be attributed to the resulting oxygen vacancies, rather than orbital mixing between the N 2p and O 2p states. The separated holes and electrons can be used to decompose organic substances and reduce oxygen molecules, respectively. Upon visible light irradiation, electrons in the N 2p states can be excited to the conduction band, leaving behind holes in the N 2p states. N-doping forms an isolated state originating from N 2p, which is approximately 0.75 eV above the valence band edge. originally suggested that the band gap narrowing arises from the mixing of nitrogen 2 p states with the O 2p states forming a new valence band maximum. Nitrogen is a commonly studied dopant that has an atomic radius similar to oxygen and a lower electronegativity, which allows for sufficient replacement of oxygen and the formation of valence band states above the valence band of pristine TiO 2. Typically, doping TiO 2 involves the replacement of Ti atoms with transition metal cations (e.g., Cr, V, Fe, Ni) and O atoms with anions (e.g., N, S, C).
Wenxin Niu, in New and Future Developments in Catalysis, 2013 12.9.2 Doping of TiO 2 NanomaterialsĬrystal lattice impurities can substantially alter the optoelectronic properties of a semiconductor. Particularly intriguing for the future is to realize room-temperature quantum microwave amplifier with point defects in SiC. We demonstrate how these defects can be created and engineered in a controlled way, either by high energy or focused ion irradiation, but also how they can be optically initialized and readout with pulsed ODMR technique in a broad range of temperatures, including room temperature. Accordingly, several quantum sensing techniques can be developed and integrated in classical electronics and photonics, e.g., all-optical magnetometry and thermometry, which do not even require radio-frequency, as well as single-photon emitters. However, depending on the type of point defect in SiC, they can have different spin-multiplicity in the ground and excited state, namely, triplet ( S=1) or quadruplet ( S=3/2). Most recently, spin-carrying defects in this technologically mature wide band gap semi-conductor were considered as very interesting for quantum applications, being in many aspects similar to nitrogen vacancy centers in diamonds. Astakhov, Vladimir Dyakonov, in Defects in Advanced Electronic Materials and Novel Low Dimensional Structures, 2018 AbstractĬrystal lattice defects in silicon carbide (SiC) have attracted attention for decades, mainly due to their detrimental role in high-power and optoelectronic applications.
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