Enhancing electrocatalytic hydrogen evolution by nickel salicylaldimine complexes with alkali metal cations in aqueous media

New salicylaldimine nickel complexes, comprising only earth-abundant elements, have been developed for electrocatalytic hydrogen evolution in aqueous media. The second-sphere ether functionalities on the periphery of the complexes enhance the electrocatalytic activity in the presence of alkali metal cations. The electrocatalysts demonstrate improved performances especially in the economical and sustainable seawater reaction medium.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Published versio

2D black phosphorous nanosheets as a hole transporting material in perovskite solar cells

We demonstrate for the first-time liquid exfoliated few layers of 2D Black phosphorus (BP) nanosheets as a hole transporting material (HTM) for perovskite based solar cells. The photoelectron spectroscopy in air (PESA) measurements confirm the low laying valence band level of BP nanosheets (−5.2 eV) favourable for hole injection from CH3NH3PbI3 (MAPbI3). Our results show that ∼25% improvement in power conversion efficiency (PCE) of η = 16.4% for BP nanosheets + Spiro-OMeTAD as an HTM as compared to spiro-OMeTAD (η = 13.1%). When BP nanosheets are exclusively utilised as an HTM, a PCE of η = 7.88% is noted, an improvement over the 4% PCE values observed for HTM free devices. Photoluminescence (PL) quenching of MAPbI3 and impedance measurements further confirm the charge extraction ability of BP nanosheets. The structural and optical characterization of liquid exfoliated BP nanosheets is discussed in detail with the aid of transmission electron microscopy, Raman spectroscopy, absorption spectroscopy and photo-electron spectroscopy.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Accepted versio

Mesoporous cerium oxide nanospheres for the visible-light driven photocatalytic degradation of dyes

A facile, solvothermal synthesis of mesoporous cerium oxide nanospheres is reported for the purpose of the photocatalytic degradation of organic dyes and future applications in sustainable energy research. The earth-abundant, relatively affordable, mixed valence cerium oxide sample, which consists of predominantly Ce7O12, has been characterized by powder X-ray diffraction, X-ray photoelectron and UV–vis spectroscopy, and transmission electron microscopy. Together with N2 sorption experiments, the data confirms that the new cerium oxide material is mesoporous and absorbs visible light. The photocatalytic degradation of rhodamin B is investigated with a series of radical scavengers, suggesting that the mechanism of photocatalytic activity under visible-light irradiation involves predominantly hydroxyl radicals as the active species

Slow cooling and highly efficient extraction of hot carriers in colloidal perovskite nanocrystals

Hot-carrier solar cells can overcome the Schottky-Queisser limit by harvesting excess energy from hot carriers. Inorganic semiconductor nanocrystals are considered prime candidates. However, hot-carrier harvesting is compromised by competitive relaxation pathways (for example, intraband Auger process and defects) that overwhelm their phonon bottlenecks. Here we show colloidal halide perovskite nanocrystals transcend these limitations and exhibit around two orders slower hot-carrier cooling times and around four times larger hot-carrier temperatures than their bulk-film counterparts. Under low pump excitation, hot-carrier cooling mediated by a phonon bottleneck is surprisingly slower in smaller nanocrystals (contrasting with conventional nanocrystals). At high pump fluence, Auger heating dominates hot-carrier cooling, which is slower in larger nanocrystals (hitherto unobserved in conventional nanocrystals). Importantly, we demonstrate efficient room temperature hot-electrons extraction (up to similar to 83%) by an energy-selective electron acceptor layer within 1 ps from surface-treated perovskite NCs thin films. These insights enable fresh approaches for extremely thin absorber and concentrator-type hot-carrier solar cells

Highly stable, luminescent core–shell type methylammonium–octylammonium lead bromide layered perovskite nanoparticles

A new protocol for the synthesis of a highly stable (over 2 months under ambient conditions) solution-processed core–shell type structure of mixed methylammonium–octylammonium lead bromide perovskite nanoparticles (5–12 nm), having spherical shape, color tunability in the blue to green spectral region (438–521 nm) and a high photoluminescence quantum yield (PLQY) of up to 92% is described. The color tunability, high PLQY and stability are due to the quantum confinement imparted by the crystal engineering associated with core–shell nanoparticle formation during growth.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Accepted versio

Evolution of hydrogen by few-layered black phosphorus under visible illumination

Recently, a new class of two-dimensional black phosphorus (BP) with a visible direct band gap is predicted as a potential candidate for photo-catalysis applications. Here, we present the first experimental evidence of hydrogen (H2) evolution from aqueous solution by using BP (nanosheets and nanoparticles) under visible light illumination. Our experimental results describe that liquid phase exfoliated BP nanosheets and BP nanoparticles exhibit suitable energy level alignments for electron transfer and further proton reduction reactions in the solution under visible light illumination. Density functional theory (DFT) calculations predict that the H2 evolution activity of bilayer BP is independent of edge or center positions, which is unique in BP as compared to those of other 2D materials.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Accepted versio

Investigating the feasibility of symmetric guanidinium based plumbate perovskites in prototype solar cell devices

In this work we have studied the feasibility of highly symmetric guanidinium (GA; CH6N3+) cation in perovskite based solar cells. It is an alternative to the methyl ammonium (MA; CH3NH3+) or formamidinium (FA; CH(NH2)2+) ions commonly utilized in the perovskite solar cells, may bring additional advantages due to its triad rotational symmetry and zero dipole moment (μ). We noticed that due to steric factors, GA preferably forms two-dimensional (2D) layer, where GA2PbI4 is favored monoclinic structure (E g = 2.5 eV) and the obtained device efficiency is η = 0.45%. This study provides a guideline for designing guanidinium based perovskite absorbers for solar cell devices. In addition, through further compositional engineering with mixed organic cations using GA may enhance the device performance.NRF (Natl Research Foundation, S’pore)ASTAR (Agency for Sci., Tech. and Research, S’pore)MOE (Min. of Education, S’pore)Accepted versio

Modulating excitonic recombination effects through one-step synthesis of perovskite nanoparticles for light-emitting diodes

The primary advantages of halide perovskites for light-emitting diodes (LEDs) are solution processability, direct band gap, good charge-carrier diffusion lengths, low trap density, and reasonable carrier mobility. The luminescence in 3 D halide perovskite thin films originates from free electron-hole bimolecular recombination. However, the slow bimolecular recombination rate is a fundamental performance limitation. Perovskite nanoparticles could result in improved performance but processability and cumbersome synthetic procedures remain challenges. Herein, these constraints are overcome by tailoring the 3 D perovskite as a near monodisperse nanoparticle film prepared through a one-step in situ deposition method. Replacing methyl ammonium bromide (CH3 NH3 Br, MABr) partially by octyl ammonium bromide [CH3 (CH2 )7 NH3 Br, OABr] in defined mole ratios in the perovskite precursor proved crucial for the nanoparticle formation. Films consisting of the in situ formed nanoparticles displayed signatures associated with excitonic recombination, rather than that of bimolecular recombination associated with 3 D perovskites. This transition was accompanied by enhanced photoluminescence quantum yield (PLQY≈20.5 % vs. 3.40 %). Perovskite LEDs fabricated from the nanoparticle films exhibit a one order of magnitude improvement in current efficiency and doubling in luminance efficiency. The material processing systematics derived from this study provides the means to control perovskite morphologies through the selection and mixing of appropriate additives.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Accepted versio

Benzyl alcohol-treated CH3NH3PbBr3 nanocrystals exhibiting high luminescence, stability, and ultralow amplified spontaneous emission thresholds

We report the high yield synthesis of about 11 nm sized CH3NH3PbBr3 nanocrystals with near-unity photoluminescence quantum yield. The nanocrystals are formed in the presence of surface-binding ligands through their direct precipitation in a benzyl alcohol/toluene phase. The benzyl alcohol plays a pivotal role in steering the surface ligands binding motifs on the NC surface, resulting in enhanced surface-trap passivation and near-unity PLQY values. We further demonstrate that thin films from purified CH3NH3PbBr3 nanocrystals are stable >4 months in air, exhibit high optical gain (about 520 cm-1), and display stable, ultralow amplified spontaneous emission thresholds of 13.9 ± 1.3 and 569.7 ± 6 μJ cm-2 at one-photon (400 nm) and two-photon (800 nm) absorption, respectively. To the best of our knowledge, the latter signifies a 5-fold reduction of the lowest reported threshold value for halide perovskite nanocrystals to date, which makes them ideal candidates for light-emitting and low-threshold lasing applications.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Accepted versio

Understanding the origins of nucleophilic hydride reactivity of a sodium hydride-iodide composite

Sodium hydride (NaH) has been commonly used as a Brønsted base in chemical syntheses, while it has rarely been employed to add hydride (H¢) to unsaturated electrophiles. We previously developed a procedure to activate NaH through the addition of a soluble iodide source and found that the new NaH–NaI composite can effect even stereoselective nucleophilic hydride reductions of nitriles, imines, and carbonyl compounds. In this work, we report that mixing NaH with NaI or LiI in tetrahydrofuran (THF) as a solvent provides a new inorganic composite, which consists of NaI interspersed with activated NaH, as revealed by powder X-ray diffraction, and both solid-state NMR and X-ray photoelectron spectroscopies. DFT calculations imply that this remarkably simple inorganic composite, which is comprised of NaH and NaI, gains nucleophilic hydridic character similar to covalent hydrides, resulting in unprecedented and unique hydride donor chemical reactivity.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Accepted versio