We report here a series of nontoxic and stable bismuth-based perovskite nanocrystals (PeNCs) with applications for photocatalytic reduction of carbon dioxide to methane and carbon monoxide. Three ...bismuth-based PeNCs of general chemical formulas A3Bi2I9, in which cation A+ = Rb+ or Cs+ or CH3NH3 + (MA+), were synthesized with a novel ultrasonication top-down method. PeNC of Cs3Bi2I9 had the best photocatalytic activity for the reduction of CO2 at the gas–solid interface with formation yields 14.9 μmol g–1 of methane and 77.6 μmol g–1 of CO, representing a much more effective catalyst than TiO2 (P25) under the same experimental conditions. The products of the photocatalytic reactions were analyzed using a gas chromatograph coupled with a mass spectrometer. According to electron paramagnetic resonance and diffuse-reflectance infrared spectra, we propose a reaction mechanism for photoreduction of CO2 via Bi-based PeNC photocatalysts to form CO, CH4, and other possible side products.
We designed an S-heterojunction system with a perovskite nanocrystal, Cs1–x FA x PbBr3 (CF), coupled with a bismuth oxyiodide (BiOI) nanosheet to form a perovskite heterojunction (PHJ) photocatalyst. ...On the basis of femtosecond transient absorption measurements, the pristine CF sample has two charge recombination periods, 100 and 900 ps, corresponding to surface and bulk trap-state relaxations, respectively. When CF was in contact with BiOI to form an S-heterojunction, rapid interfacial charge recombination occurred to show two decay components with time coefficients 1 and 35 ps, responsible for the electron–hole recombination in the surface and bulk states, respectively. We observed a new photoinduced absorption band on the blue side of the photobleach band of PHJ that gives relaxation more rapid than that of pristine CF, presumably due to doping of bismuth cations creating defect states to enhance the charge recombination that leads to photocatalytic performance for the PHJ catalyst poorer than for the pristine CF sample.
The ability to control interparticle forces not only improves the existing nanoparticle (NP) functionalities but paves the way for newer properties as well. A proof of concept in this direction is ...presented here, wherein the regulation of interparticle forcesrevealing controlled aggregationhas been successfully translated into the trapping and scavenging of toxic ions. A perfect balance between the attractive and repulsive forces is achieved by tuning the + and − ligands on the surface of heterogeneously charged metal NPs. The NP–ion aggregates are stable for ∼2 days, with a visible color change (Δλmax = 12–15 nm), which makes them available for scavenging from the site of action. The incorporation of “potent” forces like repulsions, rather than a mere dilution of attractive forces, is necessary to ensure the formation of controlled aggregates. The net surface charge of NPs is conveniently modified to trap ions irrespective of their charge and binding strength. More importantly, the regulation of interparticle forces imparts a new function of selectivity toward trapping of toxic ions in a carboxylate functionalized NP system. Thus, the present work introduces a conceptually unprecedented approach to impart long-term stability (∼2 days) to NP–ion aggregates by controlling the interparticle forces.
Overcoming the issue of the stability of tin‐based perovskites is a major challenge for the commercial development of lead‐free perovskite solar cells. To attack this problem, a new organic cation, ...azetidinium (AZ), is incorporated into the crystal structure of formamidinium tin triiodide (FASnI3) to form the mixed‐cation perovskite AZxFA1‐xSnI3. As AZ has a similar size to FA but a larger dipole moment, hybrid AZxFA1‐xSnI3 films exhibit variation in optical and electronic properties on increasing the proportion of AZ. Trifluoromethylbenzene (CF3C6H5) serves as antisolvent to fabricate smooth and uniform perovskite films for the devices with an inverted planar heterojunction structure. The device performance is optimized to produce the greatest efficiency at x=0.15 (AZ15), for which a power conversion efficiency of 9.6 % is obtained when the unencapsulated AZ15 device is stored in air for 100 h. Moreover, the device retains 90 % of its initial efficiency for over 15 days. The significant performance and stability of this device reveal that the concept of mixed cations is a promising approach to stabilize tin‐based perovskite solar cells for future commercialization.
Can we mix it? Yes we can! Mixing 15 % of azetidinium (AZ) inside a FASnI3 perovskite crystal enhances the device performance to attain a power conversion efficiency of 9.6 % with excellent stability for the unencapsulated device, which retains 90 % of its initial performance for over 15 days.
Organic–inorganic lead halide perovskite nanocrystals have attracted much attention as promising materials for the development of solid-state light-emitting devices, but the existence of free or ...bound excitons or the formation of trap states remains under debate. We recorded the temperature-dependent electroabsorption (E-A) and electrophotoluminescence (E-PL) spectra, that is, electric-field-induced change in absorption and photoluminescence spectra, for methylammonium lead tribromide (MAPbBr3) colloidal perovskite nanocrystals, that is, quantum dots (QD), doped in a poly(methyl methacrylate) film in the temperature range of 40–290 K. Based on the results, the binding energy of the exciton (electron–hole pair) was estimated. The exciton binding energy of QD of MAPbBr3 estimated from the absorption and E-A spectra (∼17 meV) is nearly the same as that of a MAPbBr3 polycrystalline thin solid film, while the exciton binding energy estimated from the temperature-dependent PL spectra (∼70 meV) is much greater than that estimated from the absorption profile. The frequency dependence of the E-A intensity observed at 40 and 290 K for the modulated applied electric field indicates a slow ion migration in nanocrystals, which follows the modulation of the applied electric field at a frequency less than 500 Hz. The observed E-A spectra were analyzed with an integral method on assuming the Stark effect; the magnitudes of the changes in electric dipole moment and polarizability following photoexcitation were determined at each temperature from 40 to 290 K. E-PL spectra show that the PL of QD of MAPbBr3 is quenched on the application of an external electric field; the extent of quenching is much greater for trap emission than for exciton emission. Exciton–phonon scattering, which is responsible for the line broadening of the PL spectra, is also discussed based on the temperature-dependent PL spectra.
Femtosecond transient absorption spectral (TAS) investigations were performed to understand the carrier relaxation mechanism for perovskite nanocrystals Cs1–x FA x PbBr3 (CF, x = 0.45) and CsPbBr3 ...(CS), which served as efficient photocatalysts for splitting of CO2 into CO and O2 in the absence of water. Upon light irradiation for 12 h, formation of deep trap states was found for both CS and CF samples with spectral characteristics of the TAS photobleach (PB) band showing a long spectral tail extending to the long wavelength region. The charge recombination rates at the shallow surface states, bulk states, and deep-trapped surface state were found to be significantly retarded for the CF sample than for the CS sample, in agreement with the photocatalytic performances for CO product yields of the CF catalyst being greater by a factor of 3 compared to those of the CS catalyst.
We report time-resolved spectral properties of highly stable and efficient red-emitting hybrid perovskite nanocrystals with the composition FA0.5MA0.5PbBr0.5I2.5 (FAMA PeNC) synthesized by using the ...hot-addition method. The PL spectrum of the FAMA PeNC shows a broad asymmetric band covering 580 to 760 nm with a peak at 690 nm which can be deconvoluted into two bands corresponding to the MA and FA domains. The interactions between the MA and FA domains are shown to affect the relaxation dynamics of the PeNCs from the subpicosecond to tens of nanoseconds scale. Time-correlated single-photon counting (TCSPC), femtosecond PL optical gating (FOG), and femtosecond transient absorption spectral (TAS) techniques were employed to study the intercrystal energy transfer (photon recycling) and intracrystal charge transfer processes between the MA and the FA domains of the crystals. These two processes are shown to increase the radiative lifetimes for the PLQYs exceeding 80%, which may play a key role in enhancing the performance of PeNC-based solar cells.
A graphene oxide (GO) film is functionalized with metal (Au) and metal‐oxide (MoOx) nanoparticles (NPs) as a hole‐extraction layer for high‐performance inverted planar‐heterojunction perovskite solar ...cells (PSCs). These NPs can increase the work function of GO, which is confirmed with X‐ray photoelectron spectra, Kelvin probe force microscopy, and ultraviolet photoelectron spectra measurements. The down‐shifts of work functions lead to a decreased level of potential energy and hence increased Voc of the PSC devices. Although the GO‐AuNP film shows rapid hole extraction and increased Voc, a Jsc improvement is not observed because of localization of the extracted holes inside the AuNP that leads to rapid charge recombination, which is confirmed with transient photoelectric measurements. The power conversion efficiency (PCE) of the GO‐AuNP device attains 14.6%, which is comparable with that of the GO‐based device (14.4%). In contrast, the rapid hole extraction from perovskite to the GO‐MoOx layer does not cause trapping of holes and delocalization of holes in the GO film accelerates rapid charge transfer to the indium tin oxide substrate; charge recombination in the perovskite/GO‐MoOx interface is hence significantly retarded. The GO‐MoOx device consequently shows significantly enhanced Voc and Jsc, for which its device performance attains PCE of 16.7% with great reproducibility and enduring stability.
Two functional graphene oxide (GO) films are fabricated by coating gold nanoparticles and molybdenum oxide nanoparticles (GO‐MoOx) on the GO sheets to serve as p‐contact electrode for inverted planar perovskite solar cells. The GO‐MoOx device forms delocalized holes to retard charge recombination and atains power conversion efficiency of 16.7%.
We report here a series of nontoxic and stable bismuth-based perovskite nanocrystals (PeNCs) with applications for photocatalytic reduction of carbon dioxide to methane and carbon monoxide. Three ...bismuth-based PeNCs of general chemical formulas A
Bi
I
, in which cation A
= Rb
or Cs
or CH
NH
(MA
), were synthesized with a novel ultrasonication top-down method. PeNC of Cs
Bi
I
had the best photocatalytic activity for the reduction of CO
at the gas-solid interface with formation yields 14.9 μmol g
of methane and 77.6 μmol g
of CO, representing a much more effective catalyst than TiO
(P25) under the same experimental conditions. The products of the photocatalytic reactions were analyzed using a gas chromatograph coupled with a mass spectrometer. According to electron paramagnetic resonance and diffuse-reflectance infrared spectra, we propose a reaction mechanism for photoreduction of CO
via Bi-based PeNC photocatalysts to form CO, CH
, and other possible side products.
Perovskite nanocrystals (PeNCs) are known for their use in numerous optoelectronic applications. Surface ligands are critical for passivating surface defects to enhance the charge transport and ...photoluminescence quantum yields of the PeNCs. Herein, we investigated the dual functional abilities of bulky cyclic organic ammonium cations as surface-passivating agents and charge scavengers to overcome the lability and insulating nature of conventional long-chain type oleyl amine and oleic acid ligands. Here, red-emitting hybrid PeNCs of the composition Cs
FA
PbBr
I
are chosen as the standard (Std) sample, where cyclohexylammonium (CHA), phenylethylammonium (PEA) and (trifuluoromethyl)benzylamonium (TFB) cations were chosen as the bifunctional surface-passivating ligands. Photoluminescence decay dynamics showed that the chosen cyclic ligands could successfully eliminate the shallow defect-mediated decay process. Further, femtosecond transient absorption spectral (TAS) studies uncovered the rapidly decaying non-radiative pathways; i.e., charge extraction (trapping) by the surface ligands. The charge extraction rates of the bulky cyclic organic ammonium cations were shown to depend on their acid dissociation constant (pKa) values and actinic excitation energies. Excitation wavelength-dependent TAS studies indicate that the exciton trapping rate is slower than the carrier trapping rate of these surface ligands.
