Jacek K. Stolarczyk

We present a synthesis method for colloidal nanocrystals exhibiting a one-dimensional Peierls-like distortion in the form of size-tunable colloidal and monodisperse Bi₁₃S₁₈I₂ nanorods. The Bi₁₃S₁₈I₂ nanorods exhibit an absorption onset around 1.6 eV and an excitonic transition around 1.1 eV. First-principles calculations demonstrate that this intrinsic excitonic transition originates from one-dimensional excitons localized in the Bi²⁺ columns formed within the Bi₁₃S₁₈I₂ lattice. In these columns, a Peierls-like distortion results in Bi²⁺ dimerization and the formation of a Peierls bandgap, which is intrinsic to Bi₁₃S₁₈I₂. This work demonstrates an exciting approach to induce excitonic properties in semiconductors without relying on traditional quantum confinement strategies, as well as opportunities to explore the spontaneous inherent symmetry breaking in nanocrystals. The Bi₁₃S₁₈I₂ nanorods also highlight the important role of colloidal chemistry in the discovery of complex materials and their optical properties and motivate further exploration of metal chalcohalide nanocrystals.

Photocatalytic water splitting is a promising approach to generating sustainable hydrogen. However, the transport of photoelectrons to the catalyst sites, usually within ps‐to‐ns timescales, is much faster than proton delivery (∼μs), which limits the activity. Therefore, the acceleration of abstraction of protons from water molecules towards the catalytic sites to keep up with the electron transfer rate can significantly promote hydrogen production. The photobasic effect that is the increase in proton affinity upon excitation offers means to achieve this objective. Herein, we design photobasic carbon dots and identify that internal pyridinic N sites are intrinsically photobasic. This is supported by steady‐state and ultrafast spectroscopic measurements that demonstrate proton abstraction within a few picoseconds of excitation. Furthermore, we show that in water, they form a unique four‐level lasing scheme with optical gain and stimulated emission. The latter competes with photocatalysis, revealing a rather unique mechanism for efficiency loss, such that the stimulated emission can act as a toggle for photocatalytic activity. This provides additional means of controlling the photocatalytic process and helps the rational design of photocatalytic materials.

Photocatalytic water splitting is a promising approach to generating sustainable hydrogen. However, the transport of photoelectrons to the catalyst sites, usually within ps‐to‐ns timescales, is much faster than proton delivery (∼μs), which limits the activity. Therefore, the acceleration of abstraction of protons from water molecules towards the catalytic sites to keep up with the electron transfer rate can significantly promote hydrogen production. The photobasic effect that is the increase in proton affinity upon excitation offers means to achieve this objective. Herein, we design photobasic carbon dots and identify that internal pyridinic N sites are intrinsically photobasic. This is supported by steady‐state and ultrafast spectroscopic measurements that demonstrate proton abstraction within a few picoseconds of excitation. Furthermore, we show that in water, they form a unique four‐level lasing scheme with optical gain and stimulated emission. The latter competes with photocatalysis, revealing a rather unique mechanism for efficiency loss, such that the stimulated emission can act as a toggle for photocatalytic activity. This provides additional means of controlling the photocatalytic process and helps the rational design of photocatalytic materials.

It usually requires high temperature and high pressure to reform methanol with water to hydrogen with high turnover frequency (TOF). Here we show that hydrogen can be produced from alkaline methanol on a light‐triggered multi‐layer system with a very high hydrogen evolution rate up to ca. 1 μmol s⁻¹ under the illumination of a standard Pt‐decorated carbon nitride. The system can achieve a remarkable TOF up to 1.8×10⁶ moles of hydrogen per mole of Pt per hour under mild conditions. The total turnover number (TTN) of 470 000 measured over 38 hours is among the highest reported. The system does not lead to any CO_x emissions, hence it could feed clean hydrogen to fuel cells. In contrast to a slurry system, the proposed multi‐layer system avoids particle aggregation and effectively uses light and Pt active sites. The performance is also attributed to the light‐triggered reforming of alkaline methanol. This notable performance is a promising step toward practical light‐driven hydrogen generation.

It usually requires high temperature and high pressure to reform methanol with water to hydrogen with high turnover frequency (TOF). Here we show that hydrogen can be produced from alkaline methanol on a light‐triggered multi‐layer system with a very high hydrogen evolution rate up to ca. 1 μmol s⁻¹ under the illumination of a standard Pt‐decorated carbon nitride. The system can achieve a remarkable TOF up to 1.8×10⁶ moles of hydrogen per mole of Pt per hour under mild conditions. The total turnover number (TTN) of 470 000 measured over 38 hours is among the highest reported. The system does not lead to any CO_x emissions, hence it could feed clean hydrogen to fuel cells. In contrast to a slurry system, the proposed multi‐layer system avoids particle aggregation and effectively uses light and Pt active sites. The performance is also attributed to the light‐triggered reforming of alkaline methanol. This notable performance is a promising step toward practical light‐driven hydrogen generation.

Bismuth oxyiodide (BiOI) is a promising material for photocatalysis combining intriguing optical and structural properties. We show that excitation by a femtosecond laser pulse creates coherent phonons inducing a time-variant oscillating modulation of the optical density. We find that the two underlying frequencies originate from lattice vibrations along the [001] crystallographic axis, the stacking direction of oppositely charged layers in BiOI. This is consistent with a subpicosecond charge separation driven by a built-in dipolar field. This partially screens the field, launching coherent phonons. Further, we determine the two major dephasing mechanisms that lead to the loss of vibronic coherence: (i) the anharmonic decay of an optical phonon into two acoustic phonons and (ii) phonon-carrier scattering. Our results provide a direct demonstration of the presence of an electric field in BiOI along the [001] axis and show its role in efficient charge separation that is crucial for photocatalytic applications of BiOI.

Metal-halide perovskites have rapidly emerged as one of the most promising materials of the 21st century, with many exciting properties and great potential for a broad range of applications, from photovoltaics to optoelectronics and photocatalysis. The ease with which metal-halide perovskites can be synthesized in the form of brightly luminescent colloidal nanocrystals, as well as their tunable and intriguing optical and electronic properties, has attracted researchers from different disciplines of science and technology. In the last few years, there has been a significant progress in the shape-controlled synthesis of perovskite nanocrystals and understanding of their properties and applications. In this comprehensive review, researchers having expertise in different fields (chemistry, physics, and device engineering) of metal-halide perovskite nanocrystals have joined together to provide a state of the art overview and future prospects of metal-halide perovskite nanocrystal research.

Hydrogen production from methanol has attracted substantial interest because of the clean combustion of hydrogen and the convenience of methanol in storage and transportation. However, it requires high-temperature and high-pressure conditions to reform methanol with water to hydrogen with high turnover frequency (TOF, e.g. 104 moles of hydrogen per mole of Pt per hour). Here we show that hydrogen can be produced from anhydrous alkaline methanol with a remarkable TOF of 1.8×106 moles of hydrogen per mole of Pt per hour on a light-triggered multi-layer system under mild conditions. The performance is attributed to the use of anhydrous methanol as both the proton source and the hole scavenger in alkaline conditions. In contrast to a slurry system, we show that the proposed multi-layer system avoids particle aggregation, and it leads to the effective utilization of methanol, light and Pt active sites. This notable performance steps forward to the practical light-triggered hydrogen generation.

Carbon dots (CDs) are a promising nanomaterial for photocatalytic applications. However, the mechanism of the photocatalytic processes remains the subject of a debate due to the complex internal structure of the CDs, comprising crystalline and molecular units embedded in an amorphous matrix, rendering the analysis of the charge and energy transfer pathways between the constituent parts very challenging. Here we propose that the photobasic effect, that is the abstraction of a proton from water upon excitation by light, facilitates the photoexcited electron transfer to the proton. We show that the controlled inclusion in CDs of a model photobase, acridine, resembling the molecular moieties found in photocatalytically active CDs, strongly increases hydrogen generation. Ultrafast spectroscopy measurements reveal proton transfer within 30 ps of the excitation. This way, we use a model system to show that the photobasic effect may be contributing to the photocatalytic H₂ generation of carbon nanomaterials and suggest that it may be tuned to achieve further improvements. The study demonstrates the critical role of the understanding the dynamics of the CDs in the design of next generation photocatalysts.