Possible Charge-Transfer-Induced Conductivity Enhancement in TiO2 Microtubes Decorated with Perovskite CsPbBr3 Nanocrystals.

Halide perovskite CsPbBr3 quantum dots (QDs) were synthesized via supersaturated recrystallization process and deposited on the surface of TiO2 microtubes forming local nano-heterostructures. Structural, morphological, and optical characterizations confirm the formation of heterostructures comprised of TiO2 microtube decorated with green-emitting CsPbBr3 nanocrystals. Optical characterizations reveal the presence of two band gap energies corresponding to CsPbBr3 (2.34 eV) and rutile-TiO2 (2.97 eV). Time-resolved photoluminescence decays indicate different charge dynamics when comparing both samples, revealing the interaction of CsPbBr3 QDs with the microtube surface and thus confirming the formation of local nano-heterostructures. The voltage-current measurements in the dark show an abrupt decrease in the electrical resistivity of the CsPbBr3/TiO2 heterostructure reaching almost 95% when compared with the pristine TiO2 microtube. This significant increase in the electrical conductivity is associated with charge transfer from perovskite nanocrystals into the semiconductor microtube, which can be used to fine tune its electronic properties. Besides controlling the electrical conductivity, decoration with semiconducting nanocrystals makes the hollow heterostructure photoluminescent, which can be classified as a multifunctionalization in a single device.

Ab Initio Simulations and Materials Chemistry in the Age of Big Data.

In this perspective, we discuss computational advances in the last decades, both in algorithms as well as in technologies, that enabled the development, widespread use, and maturity of simulation methods for molecular and materials systems. Such advances led to the generation of large amounts of data, which required the creation of several computational databases. Within this scenario, with the democratization of data access, the field now encounters several opportunities for data-driven approaches toward chemical and materials problems. Specifically, machine learning methods for predictions of novel materials or properties are being increasingly used with great success. However, black box usage fails in many instances; several technical details require expert knowledge in order for the predictions to be useful, such as with descriptors and algorithm selection. These approaches represent a direction for further developments, notably allowing advances for both developed and emerging countries with modest computational infrastructures.

Iron and oxygen vacancies at the hematite surface: pristine case and with a chlorine adatom.

Defect complexes play critical roles in the dynamics of water molecules in photoelectrochemical cell devices. For the specific case of hematite (α-Fe2O3), iron and oxygen vacancies are said to mediate the water splitting process through the localization of optically-derived charges. Using first-principles methods based on density-functional theory we show that both iron and oxygen vacancies can be observed at the surface. For an oxygen-rich environment, usually under wet conditions, the charged iron vacancies should be more frequent. As sea water would be an ideal electrolyte for this kind of device, we have also analyzed the effect of additional chlorine adsorption on this surface. While the chlorine adatom kills the charged oxygen vacancies, entering the void sites, it will not react with the iron vacancies, keeping them active during water splitting processes.

Surface Fe vacancy defects on haematite and their role in light-induced water splitting in artificial photosynthesis.

Haematite (α-Fe2O3) is a potential candidate for photo-electrochemical water splitting. It is abundant and its electronic properties fit those needed for this kind of device. However, very little is known about the intermediate steps of this photon-induced water splitting process. We propose here that surface iron vacancies can be the main defects responsible for the activity of haematite in the photoelectrochemical reaction. We perform DFT+U calculations and explicitly add holes to show that these defects are common in iron-terminated (0001) surfaces. As holes tend to be localized at these centers, they should be available for the dissociation of water under sunlight. Our calculations also reveal that the water adsorption energy close to the vacancy is 1 eV stronger than far from it, and when the formation of multi-holes is considered, a thermodynamically stable water dissociation mechanism can be developed. We determined that both Fe[double bond, length as m-dash]O and Fe-OOH intermediate steps are stable, although Fe-OOH quickly leads to the formation of O2, having therefore a very short lifetime. Phonon calculations on these structures reveal the appearance of peaks in the 800-900 cm-1 frequency range only for the intermediate steps, connected to Fe[double bond, length as m-dash]O vibrations, in agreement with recent measurements.

High temperature activation of hematite nanorods for sunlight driven water oxidation reaction.

Here we show that chlorine species originating from commonly used iron precursors annihilate the hematite nanorod photocurrent by providing recombination pathways. Although hematite nanorod films could be obtained by thermal decomposition of the iron oxyhydroxide phase (ß-FeOOH), indistinguishable photocurrent responses under dark and sunlight irradiation conditions were observed until the nanorods were annealed (activated) at 750 °C. The annealing led to the elimination of observable chlorine species and allowed photocurrent responses of 1.3 mA cm-2 at 1.23 V vs. RHE, which is comparable to the best results found in the literature, suggesting that residual chlorine species from the synthesis can act as electron traps and recombination sites for photogenerated holes.