Enhanced CO2 Sensing by Oxygen Plasma-Treated Perovskite-Graphene Nanocomposites.

Carbon dioxide (CO2) is a major greenhouse gas responsible for global warming and climate change. The development of sensitive CO2 sensors is crucial for environmental and industrial applications. This paper presents a novel CO2 sensor based on perovskite nanocrystals immobilized on graphene and functionalized with oxygen plasma treatment. The impact of this post-treatment method was thoroughly investigated using various characterization techniques, including Raman spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The detection of CO2 at parts per million (ppm) levels demonstrated that the hybrids subjected to 5 min of oxygen plasma treatment exhibited a 3-fold improvement in sensing performance compared to untreated layers. Consequently, the CO2 sensing capability of the oxygen-treated samples showed a limit of detection and limit of quantification of 6.9 and 22.9 ppm, respectively. Furthermore, the influence of ambient moisture on the CO2 sensing performance was also evaluated, revealing a significant effect of oxygen plasma treatment.

Scanning Photocurrent Microscopy in Single Crystal Multidimensional Hybrid Lead Bromide Perovskites.

We investigated solution-grown single crystals of multidimensional 2D-3D hybrid lead bromide perovskites using spatially resolved photocurrent and photoluminescence. Scanning photocurrent microscopy (SPCM) measurements where the electrodes consisted of a dip probe contact and a back contact. The crystals revealed significant differences between 3D and multidimensional 2D-3D perovskites under biased detection, not only in terms of photocarrier decay length values but also in the spatial dynamics across the crystal. In general, the photocurrent maps indicate that the closer the border proximity, the shorter the effective decay length, thus suggesting a determinant role of the border recombination centers in monocrystalline samples. In this case, multidimensional 2D-3D perovskites exhibited a simple fitting model consisting of a single exponential, while 3D perovskites demonstrated two distinct charge carrier migration dynamics within the crystal: fast and slow. Although the first one matches that of the 2D-3D perovskite, the long decay of the 3D sample exhibits a value two orders of magnitude larger. This difference could be attributed to the presence of interlayer screening and a larger exciton binding energy of the multidimensional 2D-3D perovskites with respect to their 3D counterparts.

Octahedral Molybdenum Iodide Clusters Supported on Graphene for Resistive and Optical Gas Sensing.

This paper reports for the first time a gas-sensitive nanohybrid based on octahedral molybdenum iodide clusters supported on graphene flakes (Mo6@Graphene). The possibility of integrating this material into two different transducing schemes for gas sensing is proposed since the nanomaterial changes both its electrical resistivity and optical properties when exposed to gases and at room temperature. Particularly, when implemented in a chemoresistive device, the Mo6@Graphene hybrid showed an outstanding sensing performance toward NO2, revealing a limit of quantification of about 10 ppb and excellent response repeatability (0.9% of relative error). While the Mo6@Graphene chemoresistor was almost insensitive to NH3, the use of an optical transduction scheme (changes in photoluminescence) provided an outstanding detection of NH3 even for a low loading of Mo6. Nevertheless, the photoluminescence was not affected by the presence of NO2. In addition, the hybrid material revealed high stability of its gas sensing properties over time and under ambient moisture. Computational chemistry calculations were performed to better understand these results, and plausible sensing mechanisms were presented accordingly. These results pave the way to develop a new generation of multi-parameter sensors in which electronic and optical interrogation techniques can be implemented simultaneously, advancing toward the realization of highly selective and orthogonal gas sensing.

Unraveling the Gas-Sensing Mechanisms of Lead-Free Perovskites Supported on Graphene.

Lead halide perovskites have been attracting great attention due to their outstanding properties and have been utilized for a wide variety of applications. However, the high toxicity of lead promotes an urgent and necessary search for alternative nanomaterials. In this perspective, the emerging lead-free perovskites are an environmentally friendly and harmless option. The present work reports for the first time gas sensors based on lead-free perovskite nanocrystals supported on graphene, which acts as a transducing element owing to its high and efficient carrier transport properties. The use of nanocrystals enables achieving excellent sensitivity toward gas compounds and presents better properties than those of bulky perovskite thin films, owing to their quantum confinement effect and exciton binding energy. Specifically, an industrially scalable, facile, and inexpensive synthesis is proposed to support two different perovskites (Cs3CuBr5 and Cs2AgBiBr6) on graphene for effectively detecting a variety of harmful pollutants below the threshold limit values. H2 and H2S gases were detected for the first time by utilizing lead-free perovskites, and ultrasensitive detection of NO2 was also achieved at room temperature. In addition, the band-gap type, defect tolerance, and electronic surface traps at the nanocrystals were studied in detail for understanding the differences in the sensing performance observed. Finally, a comprehensive sensing mechanism is proposed.

Photoactive Zr and Ti Metal-Organic-Frameworks for Solid-State Solar Cells.

Solid-state photovoltaic cells based on robust metal-organic frameworks (MOFs), MIL-125(Ti), MIL-125(Ti)-NH2 , UiO-67, Ru(bpy)2 -UiO-67, (bpy 2,2'-bipyridine) as active components and spiro-MeOTAD (MeOTAD 2,2',7,7'-tetrakis[N,N-di(p-methoxyphenyl)amino]-9,9'-spirobifluorene) as hole transporting layer have been prepared., The photovoltaic response of this material increases in the presence of bathochromic -NH2 groups on the linker or Ru (II) polypyridyl complexes light harvester. These results show that the strategies typically employed in photocatalysis to enhance the photocatalytic activity of MOFs can also be applied in the field of photovoltaic devices.

Expanding the photoresponse of multidimensional hybrid lead bromide perovskites into the visible region by incorporation of subphthalocyanine.

This work explores a new methodology to adsorb a subphthalocyanine molecule (SubPc) on a hybrid lead bromide perovskite crystal structure with the aim of extending its photoresponse into the visible region. This process consists in the preparation of multidimensional 2D-3D perovskites. The use of large organic cations allows the possibility to insert guest molecules in the crystal structure of the perovskite. In this work, layered and 3D materials are obtained modifying the ratio of the organic cations (A/R) in the perovskite structure (RNH3)2An-1BnX3n+1. The present results show that incorporation of metal-free subphthalocyanine in the interlayer space provided by the 2D phase is a valid procedure to enhance the photoresponse of the perovskite solar cells.

Optical properties of organic/inorganic perovskite microcrystals through the characterization of Fabry-Pérot resonances.

A precise knowledge of the optical properties, specifically the refractive index, of organic/inorganic perovskites, is essential for pushing forward the performance of the current photovoltaic devices that are being developed from these materials. Here we show a robust method for determining the real and the imaginary part of the refractive index of MAPbBr3 thin films and micrometer size single crystals with planar geometry. The simultaneous fit of both the optical transmittance and the photoluminescence spectra to theoretical models defines unambiguously the refractive index and the crystal thickness. Because the method relies on the optical resonance phenomenon occurring in these microstructures, it can be used to further develop optical microcavities from perovskites or from other optical materials.

The role of anions and cations in the gas sensing mechanisms of graphene decorated with lead halide perovskite nanocrystals.

We report the effects of both anions and cations in lead halide perovskite-graphene hybrids applied to gas sensing. Ultra-fast sensors that can work at room temperature are developed and studied to elucidate the role in the gas sensing mechanisms of different ions in perovskite nanocrystals.

Enhanced Photocatalytic Activity and Stability in Hydrogen Evolution of Mo6 Iodide Clusters Supported on Graphene Oxide.

Catalytic properties of the cluster compound (TBA)2[Mo6Ii8(O2CCH3)a6] (TBA = tetrabutylammonium) and a new hybrid material (TBA)2Mo6Ii8@GO (GO = graphene oxide) in water photoreduction into molecular hydrogen were investigated. New hybrid material (TBA)2Mo6Ii8@GO was prepared by coordinative immobilization of the (TBA)2[Mo6Ii8(O2CCH3)a6] onto GO sheets and characterized by spectroscopic, analytical, and morphological techniques. Liquid and, for the first time, gas phase conditions were chosen for catalytic experiments under UV-Vis irradiation. In liquid water, optimal H2 production yields were obtained after using (TBA)2[Mo6Ii8(O2CCH3)a6] and (TBA)2Mo6Ii8@GO) catalysts after 5 h of irradiation of liquid water. Despite these remarkable catalytic performances, "liquid-phase" catalytic systems have serious drawbacks: the cluster anion evolves to less active cluster species with partial hydrolytic decomposition, and the nanocomposite completely decays in the process. Vapor water photoreduction showed lower catalytic performance but offers more advantages in terms of cluster stability, even after longer radiation exposure times and recyclability of both catalysts. The turnover frequency (TOF) of (TBA)2Mo6Ii8@GO is three times higher than that of the microcrystalline (TBA)2[Mo6Ii8(O2CCH3)a6], in agreement with the better accessibility of catalytic cluster sites for water molecules in the gas phase. This bodes well for the possibility of creating {Mo6I8}4+-based materials as catalysts in hydrogen production technology from water vapor.

Vapor-Phase Photocatalytic Overall Water Splitting Using Hybrid Methylammonium Copper and Lead Perovskites.

Films or powders of hybrid methylammonium copper halide perovskite exhibit photocatalytic activity for overall water splitting in the vapor phase in the absence of any sacrificial agent, resulting in the generation of H2 and O2, reaching a maximum production rate of 6 µmol H2 × g cat-1h-1 efficiency. The photocatalytic activity depends on the composition, degreasing all inorganic Cs2CuCl2Br2 perovskite and other Cl/Br proportions in the methylammonium hybrids. XRD indicates that MA2CuCl2Br2 is stable under irradiation conditions in agreement with the linear H2 production with the irradiation time. Similar to copper analogue, hybrid methylammonium lead halide perovskites also promote the overall photocatalytic water splitting, but with four times less efficiency than the Cu analogues. The present results show that, although moisture is strongly detrimental to the photovoltaic applications of hybrid perovskites, it is still possible to use these materials as photocatalysts for processes requiring moisture due to the lack of relevance in the photocatalytic processes of interparticle charge migration.

Gas Sensing Properties of Perovskite Decorated Graphene at Room Temperature.

This paper explores the gas sensing properties of graphene nanolayers decorated with lead halide perovskite (CH3NH3PbBr3) nanocrystals to detect toxic gases such as ammonia (NH3) and nitrogen dioxide (NO2). A chemical-sensitive semiconductor film based on graphene has been achieved, being decorated with CH3NH3PbBr3 perovskite (MAPbBr3) nanocrystals (NCs) synthesized, and characterized by several techniques, such as field emission scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. Reversible responses were obtained towards NO2 and NH3 at room temperature, demonstrating an enhanced sensitivity when the graphene is decorated by MAPbBr3 NCs. Furthermore, the effect of ambient moisture was extensively studied, showing that the use of perovskite NCs in gas sensors can become a promising alternative to other gas sensitive materials, due to the protective character of graphene, resulting from its high hydrophobicity. Besides, a gas sensing mechanism is proposed to understand the effects of MAPbBr3 sensing properties.

Single Crystal Growth of Hybrid Lead Bromide Perovskites Using a Spin-Coating Method.

Synthesis and studies of single crystals of hybrid perovskite are important for achieving a better understanding of the optoelectronic phenomena occurring in this material and for improving ongoing applications. Here, we report on the growth of micrometer-size single crystals of methylammonium lead bromide (MAPbBr3) using the spin coating deposition method on a quartz substrate. We studied the influence of the rotation speed and the use of three different additives N-cyclohexyl-2-pyrrolidone, dimethyl sulfoxide, and 4-tert-butylpyridine on the crystal size and shape. The introduction of an additive in the precursor solution is revealed to be very useful for obtaining crystals with well-defined geometries and for decreasing the amount of defects. In this way, high-quality single crystals that sustain optical resonating modes were obtained and characterized by transmittance and photoluminescence measurements.