Halogen effects on ordering and bonding of CH₃NH₃ ⁺ in CH₃NH₃PbX₃ (X = Cl, Br, I) hybrid perovskites:a vibrational spectroscopic study

This study reports Raman and infrared spectra of hybrid organic-inorganic MAPbX3 perovskites (MA = CH3NH3, X = Cl, Br, I) and their mixed-halide derivatives. Raman spectra were recorded at three laser wavelengths (514, 785, and 1064 nm) under on- and off-resonance conditions, as well as at room temperature and 100 K. The use of different excitation wavelengths allowed the unambiguous acquisition of "true" Raman spectra from the perovskites, without degradation or photoinduced structural changes. Low-frequency PbX vibrational modes were thoroughly identified by comparison of Raman and far-IR results. Red Raman frequency shifts for almost all MA vibrations from 200 to 3200 cm-1, and particularly intense for the torsional mode, were observed toward heavy halide derivatives, indicative of strengthening the interaction between halides and the organic cation inside the inorganic cage. Different MA-X bonding schemes are evidenced by torsional mode pairs emerging in the orthorhombic phase. MAPbBr3 was further characterized by variable temperature Raman measurements (100-295 K). Broadening of the MA rocking mode slightly above the tetragonal I to II phase transition is connected with disorder of the MA cation. Our results advance the understanding of perovksite materials properties (ferroelectric domain formation, anomalous hysteresis) and their use as efficient light absorbers in solar cells.</p

Halogen effects on ordering and bonding of CH₃NH₃ ⁺ in CH₃NH₃PbX₃ (X = Cl, Br, I) hybrid perovskites:a vibrational spectroscopic study

This study reports Raman and infrared spectra of hybrid organic–inorganic MAPbX₃ perovskites (MA = CH₃NH₃, X = Cl, Br, I) and their mixed-halide derivatives. Raman spectra were recorded at three laser wavelengths (514, 785, and 1064 nm) under on- and off-resonance conditions, as well as at room temperature and 100 K. The use of different excitation wavelengths allowed the unambiguous acquisition of “true” Raman spectra from the perovskites, without degradation or photoinduced structural changes. Low-frequency PbX vibrational modes were thoroughly identified by comparison of Raman and far-IR results. Red Raman frequency shifts for almost all MA vibrations from 200 to 3200 cm^–1, and particularly intense for the torsional mode, were observed toward heavy halide derivatives, indicative of strengthening the interaction between halides and the organic cation inside the inorganic cage. Different MA–X bonding schemes are evidenced by torsional mode pairs emerging in the orthorhombic phase. MAPbBr₃ was further characterized by variable temperature Raman measurements (100–295 K). Broadening of the MA rocking mode slightly above the tetragonal I to II phase transition is connected with disorder of the MA cation. Our results advance the understanding of perovksite materials properties (ferroelectric domain formation, anomalous hysteresis) and their use as efficient light absorbers in solar cells

A Review on the Visible Light Active Titanium Dioxide Photocatalysts for Environmental Applications

Development of visible light active (VLA) titania photocatalysts Fujishima and Honda (1972) demonstrated the potential of titanium dioxide (TiO 2) semiconductor mate-rials to split water into hydrogen and oxygen in a photo-electrochemical cell. Their work triggered the development of semiconductor photocatalysis for a wide range of environmental and energy applica-tions. One of the most significant scientific and commercial advances to date has been the development of visible light active (VLA) TiO2 photocatalytic materials. In this review, a background on TiO2 struc-ture, properties and electronic properties in photocatalysis is presented. The development of different strategies to modify TiO2 for the utilization of visible light, including non metal and/or metal doping, dye sensitization and coupling semiconductors are discussed. Emphasis is given to the origin of visible light absorption and the reactive oxygen species generated, deduced by physicochemical and photo-electrochemical methods. Various applications of VLA TiO2, in terms of environmental remediation and in particular water treatment, disinfection and air purification, are illustrated. Comprehensive studies on the photocatalytic degradation of contaminants of emerging concern, including endocrine disrupting compounds, pharmaceuticals, pesticides, cyanotoxins and volatile organic compounds, with VLA TiO2 are discussed and compared to conventional UV-activated TiO2 nanomaterials. Recent advances in bac-terial disinfection using VLA TiO2 are also reviewed. Issues concerning test protocols for real visible light activity and photocatalytic efficiencies with different light sources have been highlighted

Effect of Pt nanoparticle decoration on the H2 storage performance of plasma-derived nanoporous graphene

A nanoporous and large surface area (∼800 m2/g) graphene-based material was produced by plasma treatment of natural flake graphite and was subsequently surface decorated with platinum (Pt) nano-sized particles via thermal reduction of a Pt precursor (chloroplatinic acid). The carbon-metal nanocomposite showed a ∼2 wt% loading of well-dispersed Pt nanoparticles (<2 nm) across its porous graphene surface, while neither a significant surface chemistry alteration nor a pore structure degradation was observed due to the Pt decoration procedure. The presence of Pt seems to slightly promote the hydrogen sorption behavior at room temperature with respect to the pure graphene, thus implying the rise of “weak” chemisorption phenomena, including a potential hydrogen “spillover” effect. The findings of this experimental study provide insights for the development of novel graphene-based nanocomposites for hydrogen storage applications at ambient conditions

Novel Ru(ii) sensitizers bearing an unsymmetrical pyridine-quinoline hybrid ligand with extended π-conjugation: Synthesis and application in dye-sensitized solar cells

Heteroleptic ruthenium(ii) sensitizers DV42 and DV51, encompassing a novel unsymmetrical pyridine-quinoline hybrid ligand with extended π-conjugation, were synthesized, characterized, and utilized in nanocrystalline dye-sensitized solar cells. Due to the extended conjugation of DV42 and DV51, the absorption of the corresponding sensitized TiO2 films extends into the red spectral range, shifted by 30-40 nm relative to the absorption of TiO 2 films sensitized with the standard Z907 ruthenium(ii) dye. Contact angle measurements of DV42- and DV51-sensitized TiO2 films suggest that these films are hydrophilic with contact angle values commonly observed upon sensitization with the standard N3 ruthenium(ii) dye. Electrochemical studies of the novel ruthenium(ii) dyes show that their first oxidation potentials lie well below the I-/I3- redox potential allowing easy regeneration. The excited-state oxidation potentials of both dyes lie above the TiO2 conduction band, permitting efficient electron injection from the excited dye molecules into the semiconductor conduction band. Liquid electrolyte dye-sensitized solar cells incorporating DV42- or DV51-sensitized TiO2 photoelectrodes afford overall power conversion efficiencies of 3.24 or 4.36% respectively. These efficiencies are up to 56% of the power conversion efficiencies attained by TiO2 photoelectrodes sensitized by the benchmark Z907 ruthenium(ii) dye under similar experimental conditions. © 2013 The Royal Society of Chemistry

Terpyridine- and 2,6-dipyrazinylpyridine-coordinated ruthenium(II) complexes: Synthesis, characterization and application in TiO2-based dye-sensitized solar cells

Two new ruthenium(II) complexes bearing terpyridine- or 2,6-dipyrazinylpyridine ligands have been prepared and characterized by one- and two-dimensional NMR techniques, ESI mass spectrometry, as well as by UV-vis, emission, FTIR, Raman, and cyclic voltammetry studies. The structure of the terpyridine-coordinated complex resembles that of black dye (tris(thiocyanato) (2,2′:6′,2″-terpyridyl-4,4′,4″-tricarboxylato) ruthenium(II) tris(tetra butylammonium) salt), while the spectral data of the 2,6-dipyrazinylpyridine-coordinated complex are consistent with an unanticipated, unsymmetrical binuclear structure. The ruthenium(II)/(III) oxidation potential of the terpyridine-coordinated dye was measured at +0.87 V (vs. Ag/AgCl), about 200 mV higher than the oxidation potential of black dye. According to a series of desorption experiments, both new dyes were found to adsorb on TiO2 to a greater extent than black dye. The photo-electrochemical properties of both dyes were investigated and compared to that of black dye; while the 2,6-dipyrazinylpyridine-coordinated complex was found to be a very poor sensitizer, the cells obtained from the terpyridine-coordinated dye show power-conversion efficiencies which are more than half of that attained by black dye. Finally, preliminary electron dynamics studies of the cells constructed with the terpyridine dye were carried out and the results are compared to that of black dye cells. © 2010 Elsevier B.V

Influence of Fluorine Plasma Treatment of TiO2 Films on the Behavior of Dye Solar Cells Employing the Co(II)/(III) Redox Couple

Fluorine plasma treatment was investigated as an appropriate means for the surface modification of TiO2 thin film electrodes and the optimization of their performance as photoanodes in dye solar cells (DSCs) employing the Co(II)/(III) redox shuttle and the organic D35 sensitizer. Detailed surface and structural characterization of the titania films by contact angle measurements, atomic force microscopy, profilometry, and Raman and UV-vis spectroscopy showed that high density SF6 plasma provoked severe film densification and thus an increase of the nanoparticles packing density, leaving intact the crystallinity, particle size, and optical bandgap. Surface fluorination of the TiO2 films was also identified by X-ray photoelectron spectroscopy. The combination of the above effects resulted in the enhancement of both photocurrent and power conversion efficiency of the corresponding DSCs at moderate plasma treatment durations, while the photovoltage decreased continuously as a function of the fluorine processing time. Electrochemical impedance spectroscopy analysis revealed a marked increase of the density and distribution of trap states due to fluorine induced surface states along with a systematic downward shift of the TiO2 conduction band, probably attributed to the electrostatic coupling of intercalated Li + cations with the polar Ti-F species at the TiO2 surface, in agreement with the Voc drop. In contrast, enhanced electron injection was inferred to underlie the observed Jsc and DSC performance improvements, as surface fluorination and the concomitant film densification slightly increased electron transport while hardly affecting dye loading capacity, light harvesting efficiency, and recombination kinetics, except for the case of prolonged plasma treatment. Effective control of the detrimental side effects of fluorine species can render this kind of plasma treatment a powerful method to tune the surface and electrical properties of TiO2 films and optimize the behavior and performance of the resulting DSC devices. © 2014 American Chemical Society