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Brookite is now recognized as an active phase of TiO2 which exhibits superior activities compared with anatase and rutile in some photocatalytic reactions. However, there is still little research and knowledge on its electron properties as well as the crucial role of defects in brookite. Using the ab initio many-body Green’s function theory, we examined the quasiparticle structures of defects, including oxygen vacancies, Ti interstitials, and hydroxyl groups, in the bulk and the (210) surface of brookite. We discovered that small polarons may generate a deep defect band and a shallow one which are approximately 0.7 and 0.3 eV below the conduction band minimum (CBM). In brookite bulk, oxygen vacancy can only create a deep defect band which is 1.1 eV below CBM and induced by the σ bonds formed between Ti 3d orbitals. These features are quite distinct from those in anatase and rutile. We also found that introducing hydroxyl groups into brookite bulk would make the band gap narrow by at least 0.4 eV, which may help to enhance its visible light absorption. The calculated band gap and defect levels of reduced brookite are in excellent agreement with the experiments.

Many kinds of modification techniques have been developed to improve the quantum efficiency of the graphitic carbon nitride (CN) for photocatalytic water splitting in recent years. In this work, we theoretically propose to incorporate polyyne (−C≡C−)n between the heptazine ring and the tertiary amino group of CN to form covalent organic frameworks (COFs) as a new strategy to further enhance the photocatalytic ability of CN. These COFs allow for a fine-tuning of their electronic and optical properties by altering the quantity of C≡C bonds. They could fulfill the three ultimate goals of an ideal photocatalyst for water splitting, i.e., broad optical absorption, overall water splitting, and low exciton binding energy. Most strikingly, in the visible region, these COFs possess a lot of strong dipole-allowed exciton transitions whose binding energies are just at the level of a few tens of meV and whose electron and hole are well separated in space by ∼10 nm. This extremely low exciton binding energy is of the ...

Fission of the lowest-energy singlet exciton (S1) to two lowest-energy triplet excitons (T1) in pentacene has been expected to be a promising means for increasing the quantum efficiency of solar cells. Experiments find that S1 and T1 dissociate at quite different time scales at the donor/acceptor interface. Using the pentacene/TiO2 heterojunction as the model, we investigate the dissociation of pentacene excitons by a combination of many-body Green’s function theory and the time-dependent Schrodinger equation. Singlet and higher-energy triplet (Tn) excitons of pentacene could dissociate at the same timescale of ∼100 fs, benefiting from their capability to scatter into charge-transfer (CT) states and weakly bound charge-separated (CS) states across the interface. Resonance of pentacene excitons with CS states could facilitate the creation of free charge carriers. However, dissociation of T1 is hampered due to its poor density of states projected onto the interfacial states, preventing its scattering into C...

As a typical photocatalyst, TiO2 has been intensively studied for water splitting. However, the mechanism, especially how photogenerated holes participate in, behind the reaction remains highly deb...

Passivated codoping has never been reported to enhance the solar-to-hydrogen activity of g-C3N4, although it is an effective approach for other materials. In this letter, we use many-body Green’s function theory to analyze the electronic structures, optical absorption spectra, and spatial distribution of electron–hole pair of the doped g-C3N4. Our results suggest that the passivated codoping, such as B+O, could not only extend absorption toward visible range but also promote the separation of photogenerated hole and electron to different parts of g-C3N4. Improvement of photocatalytic activity can be realized only when the n-type and p-type dopants reside in different tris-triazine units of g-C3N4. This manifests the important role of codoping microstructure on solar-to-hydrogen efficiency. These results could provide a guideline for the design of more efficient artificial photocatalysts.

Two-dimensional (2D) piezoelectric materials have gained considerable attention since they could play important roles in the nanoelectromechanical systems. Herein, we report a first-principles study on the piezoelectric properties of monolayer group-V binary compounds with theoretically stable honeycomb phases (α-phase and β-phase). Our calculations for the first time reveal that a majority of the monolayers possess extremely high piezoelectric coefficients d11, i.e., 118.29, 142.44, and 243.45 pm/V for α-SbN, α-SbP, and α-SbAs, respectively, comparable to those of recently reported group-IV monochalcogenides (d11 = 75–250 pm/V) with an identical mm2 symmetry. It is found that the giant piezoelectric responses of α-phase monolayers as compared to those of β-phase monolayers are induced by their flexible structures and special symmetry. Meanwhile, the piezoelectric coefficients of α-phase monolayers are found to be surprisingly anisotropic and obey a unique periodic trend which is not exactly identical to ...

Two kinds of novel carbon nanotubes, namely, (N, 0) and (0, N) 6,6,12-graphyne nanotubes (6,6,12-GNTs), are constructed by rolling up the rectangular 6,6,12-graphyne sheets along two different sides into cylinders. The mechanical and electronic properties of 6,6,12-GNTs with varied N from 3 to 20 are investigated by using density functional theory. Unlike the single-wall carbon nanotubes, the Young’s moduli of 6,6,12-GNTs do not remain constant in the case of (N, 0), but the (0, N) tubes possess almost the same one around 0.32 TPa. The band structures and density of states are also exhibited in this work. When the tube sizes N are bigger than four, Dirac points appear at Fermi level in the band maps of (N, 0) type 6,6,12-GNTs following an even–odd law, while the (0, N) tubes are narrow-gap semiconductors with tiny band gaps between 5.5 and 247.3 meV.

As a promising photocatalytic material in water splitting and organic degradation, the polymeric graphitic g-C3N4 has attracted intensive research interest during the past decade due to the visible light response, nontoxicity, abundance, easy preparation, as well as high thermal and chemical stability. However, the low efficiency owing to the fast charge recombination limits its practical applications. In the present work, we systematically investigated the electronic structure and photocatalytic properties of layered g-C6N6/g-C3N4 heterostructure on the basis of first-principles calculations. The results show that the type-II heterojunction can be established between g-C6N6 and g-C3N4 monolayers due to a perfect lattice match and aligned band edges, facilitating the separation of photogenerated carriers. In addition, it is worthwhile to note that hole effective masses of g-C6N6/g-C3N4 heterostructure can be significantly reduced compared to pristine g-C3N4 due to orbital hybridization between the two mon...

Using the first principles ground state method, we systematically studied the DX center in bulk GaAs and GaAs(110). The DX center is found to be a metastable state in bulk GaAs and completely unstable in the top few layers of GaAs(110). We find that the charge states of SiGa defects on the GaAs(110) surface are localized, and the extra charge is mainly concentrated on the Si atom in the charged system. The localized extra charge of the defect on the surface affects the stability of the GaAs(110) surface. Our results show that the negative charging of SiGa would preferably occur at the surface, whereas the positively charged SiGa defect is unstable at the surface. Finally, our calculated STM images of clean GaAs(110) and charged Si:GaAs(110) using Tersoff–Hamann approximation [Tersoff, J.; Hamann, D. R. Phys. Rev. B1985, 31, 805–813] are in good agreement with experimental results. Our results show that the charge state of defect at the surface has an important influence on the STM image. In addition, we s...