Solvent-Assisted Tuning of the Size and Shape of CsPbBr3 Nanocrystals via Redispersion Process at Ambient Condition

Halide Perovskite Photocatalysts for Clean Fuel Production and Organic Synthesis: Opportunities and Challenges

The need to constrain the use of fossil fuels causing global warming is motivating the development of a variety of photocatalysts for solar-to-fuel generation and chemical synthesis. In particular, semiconductor-based photocatalysts have been extensively exploited in solar-driven organic synthesis, carbon dioxide (CO2) conversion into value-added products, and hydrogen (H2) generation from water (H2O) splitting. Recently, metal halide perovskites (MHPs) have emerged as an important class of semiconductors for heterogeneous photocatalysis owing to their interesting properties. Despite key issues with long-term stability and degradation in polar solvents due to their ionic character, there has been significant progress in halide perovskite-based photocatalysts with improving their stability and performance in the gas and liquid phases. This review discusses the state-of-the-art for using halide perovskite-based photocatalysts and photoelectrocatalysis in hydrogen production from water and halogen acid solutions, CO2 reduction into value-added chemicals, and various organic chemical transformations. The different types of halide perovskites used, design strategies to overcome the instability issues in polar solvents, and the efficiencies achieved are discussed. Furthermore, the outstanding challenges associated with the use of polar electrolytes and how the stability and performance can be improved are discussed.

Concentration Dependent Modulation in Optoelectronic Traits of Self-Collated CsPbBr3 Perovskites

Self-collation of perovskite nanocrystals into superstructures of larger length scales has been growing in research interest due to their dramatically enhanced performance in various nano-devices, modulating their optical and electrical traits. Herein, the unique concentration-dependent self-assembly of phenethylamine (PEA)-capped CsPbBr3 (PCPB) perovskites spanning a size range of nano to micron level without structural phase alteration is infered. By optimizing various synthetic parameters like PEA amount, and solvents, the self-coalescence in PCPB crystal growth is controlled. Furthermore, the highest-concentrated PCPB (C5) has improved the charge transfer (CT) efficiency to 1,4-Napthoquinone (NPQ), corroborated with stronger binding between C5 and NPQ, compared to the lowest-concentrated PCPB (C1). Incorporating NPQ into such concentration-dependent PCPB enhances their local conductance unveiling the CT-induced current rise, while the detrimental insulating property of PEA molecules reduces the conductance in C5 compared to C1. These outcomes offer a foundation for tailoring the properties of self-assembled perovskites for optoelectronic devices and energy conversion technologies.

Band Gap Tuning in Mercaptoacetic Acid Capped Mixed Halides Perovskites and Effect of Solvents on Their Fluorescence Dynamics: A Potential Sensor for Polarity

From synthesis to application, there are always certain interactions between the polar solvents and perovskite nanocrystals (NCs). To explain the effect of solvent polarity especially on the photoluminescence (PL) properties of NCs is highly desirable, especially for sensing applications. Herein We have synthesized the methylammonium lead mixed halides (MAPbCl3 - nBrn, where n = 0, 0.5, 1, 1.5) perovskite nanocrystals (NCs) at room temperature by using ligand-assisted re-precipitation (LARP) method, by employing mercaptoacetic acid (MAA) as a capping ligand. Different techniques have been employed to get information regarding the structural and optical properties of the synthesized material. Powder X-ray diffraction (PXRD) confirms the orthorhombic crystal structure of the MAPbCl3 - nBrn perovskite NCs. FT-IR (Fourier-transform infrared) analysis confirms the successful interaction of capping ligands with NCs. By increasing MABr precursor concentration during the synthesis of perovskite NCs, a red shift in the UV-Vis absorption and PL spectra has been observed. The steady-state photoluminescence (SSPL) and time-resolved photoluminescence (TRPL) techniques suggested that these perovskite NCs exhibit tunable PL relative to the substitution of Cl with Br in NCs. The comparative PL studies in non-polar (benzene) and polar (tetrahydrofuran (THF)) revealed that the PL properties are highly sensitive and selective toward the solvent chosen. All synthesized NCs possess longer PL lifetime in benzene than in THF. Relatively, perovskite NCs synthesized with 0.166 mM MABr precursor concentration show a longer PL lifetime (6.51 ns in benzene) as compared to other MABr concentrations. These studies not only propose that by controlling precursors concentration, one can synthesize NCs having tunable PL with a longer radiative PL lifetime, but also provide a comparative understanding of PL dynamics of NCs in different solvents.

Promoting solution-phase superlattices of CsPbBr3 nanocrystals

We present a size-selective method for purifying and isolating perovskite CsPbBr₃ nanocrystals (NCs) that preserves their as-synthesized surface chemistry and extremely high photoluminescence quantum yields (PLQYs). The isolation procedure is based on the stepwise evaporation of nonpolar co-solvents with high vapor pressure to promote precipitation of a size-selected product. As the sample fractions become more uniform in size, we observe that the NCs self-assemble into colloidally stable, solution-phase superlattices (SLs). Small angle X-ray scattering (SAXS) and dynamic light scattering (DLS) studies show that the solution-phase SLs contain 1000s of NCs per supercrystal in a simple cubic, face-to-face packing arrangement. The SLs also display systematically faster radiative decay dynamics and improved PLQYs, as well as unique spectral absorption features likely resulting from inter-particle electronic coupling effects. This study is the first demonstration of solution-phase CsPbBr₃ SLs and highlights their potential for achieving collective optoelectronic phenomena previously observed from solid-state assemblies.

Cost-Effective Strategy for the Synthesis of Air-Stable CH3NH3PbX3 (X = Cl, Br, and I) Quantum Dots with Bright Emission

Lead halide perovskite quantum dots (QDs) are known as prospective optoelectronic device materials because of their excellent luminescence, extraordinary photoelectric performance, and specific octahedron framework. Herein, we report a cost-effective approach for synthesizing highly stable CH₃NH₃PbBr₃ QDs in low-polarity binary solvents without nitrogen protection. The CH₃NH₃PbBr₃ QDs are tunable from 1.2 to 4.2 nm by adjusting the proportion of oleic acid and oleylamine as capping ligands. The photoluminescence quantum yield of CH₃NH₃PbBr₃ QDs can reach 87.4%. The fluorescence can maintain over 80% of its earliest emission intensity under the atmosphere after 5 days, which is much better than that (∼10%) of QDs with ligand-assisted reprecipitation. The possible reaction mechanism of preparing CH₃NH₃PbBr₃ QDs was also addressed. Notably, such a strategy can be applied extensively in the preparation of other lead halide perovskite QDs. Furthermore, the as-prepared thick PMMA-coated CH₃NH₃PbBr₃ QDs were further conjoined with a red luminescence powder on a blue InGaN chip to obtain a powerful efficiency (45.4 lm W^-1) warm white light-emitting diode.

Lead-Free, Water-Stable A3 Bi2 I9 Perovskites: Crystal Growth and Blue-Emitting Quantum Dots [A=CH3 NH3 + , Cs+ , and (Rb0.05 Cs2.95 )+ ]

Despite the great success in the increase in the power conversion efficiency of lead halide perovskite solar cells, the toxicity of lead and the unstable nature of the materials are still major concerns for their wider implementation at the industrial level. Herein, large-size single crystals (SCs) are developed in HI solution by using a temperature lowering method and nanocrystals (NCs) of A₃ Bi₂ I₉ perovskites [where A=CH₃ NH₃ ⁺ (MA)⁺ , Cs⁺ , and (Rb_0.05 Cs_2.95 )⁺ ] are formed in ethanol (EtOH) and toluene (TOL). The stability of A₃ Bi₂ I₉ perovskite is investigated by immersing the SCs for 24 h and pellets for 12 h in water. Moreover, the A₃ Bi₂ I₉ perovskite NCs displays a promising photoluminescence quantum yield of 17.63 % and a long lifetime of 8.20 ns.

Polar Organic Solvent-Tolerant Perovskite Nanocrystals Permanently Ligated with Polymer Hairs via Star-like Molecular Bottlebrush Trilobe Nanoreactors

The key to exploiting perovskite nanocrystals (NCs) for long-term practical use in optoelectronic materials and devices lies in the ability to access stable NCs. Herein, we report the crafting of hairy perovskite NCs with a set of markedly improved stabilities by capitalizing on rationally designed star-like molecular bottlebrush trilobes as nanoreactors. An intriguing star-like molecular bottlebrush trilobe, poly(2-hydroxyethyl methacrylate)-graft-(poly(acrylic acid)-block-partially cross-linked polystyrene (denoted PHEMA-g-(PAA-b-cPS)) is synthesized. Subsequently, it is employed as a polymeric nanoreactor to direct the growth of green-emitting all-inorganic perovskite CsPbBr₃ NCs intimately and stably tethered by partially cross-linked PS "hairs" (i.e., cPS-capped CsPbBr₃ NCs). The resulting CsPbBr₃ NCs exhibit an array of impressive stabilities against UV irradiation, moisture, heat, and water, due to permanently ligated hydrophobic cPS "hairs" on the surface of CsPbBr₃ NCs as a result of the original covalent bonding between PAA and cPS blocks. More importantly, cPS-capped CsPbBr₃ NCs manifest outstanding stability in various polar organic solvents. Such greatly improved stability can be attributed to the reduced surface defects enabled by the favorable interaction (i.e., coordination interaction and hydrogen bonding) between CsPbBr₃ NCs and polar solvents, which dominates over their dissolution by polar solvents. Such exceptional stabilities impart the use of cPS-capped CsPbBr₃ NCs as a selective probe for tracing the presence of Cl^-/I^- in polar organic solvents. The amphiphilic nonlinear block copolymer nanoreactor strategy can afford easy access to stable perovskite NCs of interest with controlled compositions and surface chemistry. They may find applications in solar cells, LEDs, photodetectors, lasers, bioimaging, biosensors, etc.

Impact of cesium in methylammonium lead bromide perovskites: insights into the microstructures, stability and photophysical properties

The thermal and moisture instabilities of pure organic lead halide perovskites are the foremost concerns towards the commercialization of perovskite solar cells, which can be avoided by introducing an inorganic cation, such as cesium ion (Cs+) at the A-site of the perovskite crystals. In this report, the impacts of substituted Cs+ cations on the inherent properties such as microstructures, morphology, and photophysics of pure methylammonium lead bromide (MAPbBr3) perovskites have been investigated. Successful formation of mixed MA1-xCsxPbBr3 phases (with 0 ≤ x ≤ 1.0) was predicted from the theoretically calculated tolerance factor, which was further supported by the appearance of sharp diffraction peaks in X-ray diffraction (XRD) patterns without any additional peaks in the whole composition range. Substitution of Cs+ ions brings significant lattice contraction in the parent MAPbBr3 crystal due to the ion size disparity in the ionic radii between MA+ and Cs+ ions. We examine the vibrational signatures of the Raman bands related to the organic MA+ and infer the nature of interactions between the organic moiety and the surrounding inorganic cage as a function of Cs concentration. Raman spectroscopic analysis reveals structural distortion due to the altered H-bonding interaction of the N+-HBr- type between MA+ and the PbBr3- octahedral framework as a function of Cs content, which is responsible for the octahedral tilting in Cs substituted MAPbBr3. We also found hindered rotational motions of MA+ in the octahedral cage of mixed cationic systems, resulting in the orientational ordering of MA in the presence of Cs. These results certainly offer highly ordered mixed phase structures and promote superior thermal stability, as evident from the thermogravimetric analysis. The photoluminescence intensity becomes considerably enhanced at increased substitution levels, which highlights the capability of incorporated Cs+ cations in suppressing non-radiative recombination in a pure MA-based crystal, possibly related to the mitigation of trapping. The substitution of Cs+ with MAPbBr3 allows innovative strategies to improve the proficiency of tandem solar cells by modifying their structural and photophysical properties.

MA2CoBr4: lead-free cobalt-based perovskite for electrochemical conversion of water to oxygen

We have synthesized a lead-free stable organic–inorganic perovskite (MA₂CoBr₄) by using non-hazardous solvents such as methanol and ethanol, which are eco-friendly and safe to handle in comparison to DMF, toluene, etc.