The observation of low-energy edge photoluminescence and its beneficial effect on the solar cell efficiency of Ruddlesden−Popper perovskites has unleashed an intensive research effort to reveal its origin. This effort, however, has been met with more challenges as the underlying material structure has still not been identified; new modelings and observations also do not seem to converge. Using twodimensional (2D) (BA) 2 (MA) 2 Pb 3 Br 10 as an example, we show that threedimensional (3D) MAPbBr 3 is formed due to the loss of BA on the edge. This self-formed MAPbBr 3 can explain the reported edge emission under various conditions, while the reported intriguing optoelectronic properties such as fast exciton trapping from the interior 2D perovskite, rapid exciton dissociation, and long carrier lifetime can be understood via the self-formed 2D/3D lateral perovskite heterostructure. The 3D perovskite is identified by submicron infrared spectroscopy, the emergence of X-ray diffraction (XRD) signature from freezer-milled nanometer-sized 2D perovskite, and its photoluminescence response to external hydrostatic pressure. The revelation of this edge emission mystery and the identification of a self-formed 2D/3D heterostructure provide a new approach to engineering 2D perovskites for high-performance optoelectronic devices.
We present experimental evidence under low-dose conditions transmission electron microscopy for the unfolding of the evolving changes in carbon soot during mechanical milling. The milled soot shows evolving changes as a function of the milling severity or time. Those changes are responsible for the transformation from amorphous carbon to graphenes, graphitic carbon, and highly ordered structures such as morphed graphenes, namely Rh6 and Rh6-II. The morphed graphenes are corrugated layers of carbon with cross-linked covalently nature and sp2- or sp3-type allotropes. Electron microscopy and numerical simulations are excellent complementary tools to identify those phases. Furthermore, the TEAM 05 microscope is an outstanding tool to resolve the microstructure and prevent any damage to the sample. Other characterization techniques such as XRD, Raman, and XPS fade to convey a true identification of those phases because the samples are usually blends or mixes of the mentioned phases.
2 Graphical abstract Highlights: ZrO2-TiO2 heterojunction with high photocatalytic activity were obtained by solgel. Energy states at heterojunction interface altered the semiconducting properties. 5 mol% ZrO2 in ZrO2-TiO2 heterojunction propitiated a better electron-hole separation. Photocatalytic activity is improved by favoring the charge transfer process. Direct and indirect charge transfer process involved in phototocatalytic process. Abstract.In this paper, ZrO2-TiO2 composites were synthesized by sol-gel method with different ZrO2:TiO2 molar ratios (01:99, 05:95 and 10:90). The results identified two trends; at a low ZrO2 content, the incorporation of Zr 4+ into the TiO2 lattice was possible provoking generation of oxygen vacancies (1 mol % of ZrO2); while at higher ZrO2 contents, ZrO2-TiO2 heterojunctions were created (5 and 10 mol % of ZrO2). The photocatalytic activity was evaluated by measuring photodegradation of phenoxyacetic acid, 2,4-dichlorophenoxyacetic acid or 4-chlorophenol solutions. The ZT-5 composite shows the best performance attributed to surface states at the interface of ZrO2-TiO2 heterojunctions.These surface states act as traps for charge carriers favoring the spatial separation of electron-hole pairs until reaching a maximum in the composite with 5 mol% of ZrO2. The ZT-5 composite showed the most negative flat band potential and the highest donor density indicating that these surface states are in optimal concentration. At higher ZrO2 contents, charge carrier separation is less effective, which decreases the photocatalytic activity.Nevertheless, the molecule structure has an impact on the direct or indirect charge transfer process as was evidenced by EIS measurements.
CsPbBr is a stable, water-resistant, material derived from CsPbBr perovskite and featuring two-dimensional Pb-Br framework separated by Cs layers. Both compounds can coexist at nanolength scale, which often produces conflicting optical spectroscopy results. We present a complete set of polarized Raman spectra of nonluminescent CsPbBr single crystals that reveals the symmetry and frequency of nondegenerate Raman active phonons accessible from the basal (0 0 1) plane. The experimental results are in good agreement with density functional perturbation theory simulations, which suggests that the calculated frequencies of yet unobserved double degenerate Raman and infrared phonons are also reliable. Unlike CsPbBr, the lattice dynamics of CsPbBr is stable as evidenced by the calculated phonon dispersion. The sharp Raman lines and lack of a dynamic-disorder-induced central peak in the spectra at room temperature indicate that the coupling of Cs anharmonic motion to Br atoms, known to cause the dynamic disorder in CsPbBr, is absent in CsPbBr.
BACKGROUND Hydrogen, as a clean and renewable energy source has become very attractive due to the deterioration of the global environment. In this way photocatalytic water‐splitting for H2 production using light energy in the presence of semiconductors capable of absorbing such irradiation has become a promising approach for generation of H2. The intensity of incident solar energy on the earth's surface is 1000 W m−2 and only 4% of the total solar energy corresponds to UV light (40 W m−2), but this could be sufficient for H2 production from water splitting with very efficient semiconductors using the UV solar energy radiation. RESULTS The synthesis of Bi2S3–TiO2 composites using TiO2 sol–gel (3, 6 and 9 Bi2S3 wt%) was performed by a solvothermal method and these materials were evaluated under UV light irradiation (254 nm, 2 W) for photocatalytic hydrogen production from a water/methanol solution. The optimal loading was obtained for the Bi2S3–TiO2 composite at 6 wt% showing a production of 2460 µmol h−1 g−1 of hydrogen, increasing by a factor of 4 the production of bare TiO2 at 564 µmol h−1 g−1. The Bi2S3–TiO2 composite presented good stability after three complete cycles of reaction, proving resistance to corrosion effects. CONCLUSION Bi2S3–TiO2 presented a higher photocatalytic activity than bare TiO2 for an optimal content of 6 wt% Bi2S3–TiO2 .This improvement is attributed to enhanced absorption in the UV‐Vis region of the Bi2S3–TiO2 composite, and a higher transference of the charge carriers in the Bi2S3–TiO2 heterojunctions with a hindered recombination e−/h+. © 2016 Society of Chemical Industry
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