Photocatalytic conversion of solar energy to fuels, such as hydrogen, is attracting enormous interest, driven by the promise of addressing both energy supply and storage. Colloidal semiconductor nanocrystals have been at the forefront of these efforts owing to their favourable and tunable optical and electronic properties as well as advances in their synthesis. The efficiency of the photocatalysts is often limited by the slow transfer and subsequent reactions of the photoexcited holes and the ensuing high charge recombination rates. Here we propose that employing a hydroxyl anion/radical redox couple to efficiently relay the hole from the semiconductor to the scavenger leads to a marked increase in the H2 generation rate without using expensive noble metal co-catalysts. The apparent quantum yield and the formation rate under 447 nm laser illumination exceeded 53% and 63 mmol g(-1) h(-1), respectively. The fast hole transfer confers long-term photostability on the system and opens new pathways to improve the oxidation side of full water splitting.

Scheme 1.Structures of "graphitic carbon nitrides." Shown are the 1D polymer melon (left), the fully condensed 2D counterpart (middle), and the 2D network PHI. KSCN HClScheme 2. Simplified reaction scheme of the compound synthesized in this work, showing melon and its conversion to NCN-CN x by a postsynthetic reaction using KSCN melt, and its acid-induced hydrolysis to urea-CN x .

Cadmium chalcogenide nanocrystals combined with co-catalyst nanoparticles hold promise for efficient solar to hydrogen conversion. Despite the progress, achieving high efficiency is hampered by high charge recombination rates and sample degradation. Here, we vary the decoration of platinum nanoparticles on CdS nanorods to demonstrate the important role of pathways for the photoelectrons to the co-catalyst. Contrary to expectations, the shortening of the path, by increasing the number of co-catalyst particles, increases the transfer rate but decreases the photocatalytic performance. This is because subsequent electron transfer to the acceptor is much slower; therefore, the recombination rate with the nearby holes increases. We show that with tip-decorated nanorods, the quantum yield of H2 production can reach and sustain nearly 90%, provided an efficient mechanism of mediated hole extraction is employed. The approach demonstrates that highly efficient photocatalysts may be prepared with only a minimal amount of co-catalyst and thereby suggests future pathways for solar to H2 conversion with semiconductor nanocrystals.

Copper(I)-based catalysts, such as Cu2S, are considered to be very promising materials for photocatalytic CO2 reduction. A common synthesis route for Cu2S via cation exchange from CdS nanocrystals requires Cu(I) precursors, organic solvents, and neutral atmosphere, but these conditions are not compatible with in situ applications in photocatalysis. Here we propose a novel cation exchange reaction that takes advantage of the reducing potential of photoexcited electrons in the conduction band of CdS and proceeds with Cu(II) precursors in an aqueous environment and under aerobic conditions. We show that the synthesized Cu2S photocatalyst can be efficiently used for the reduction of CO2 to carbon monoxide and methane, achieving formation rates of 3.02 and 0.13 μmol h(-1) g(-1), respectively, and suppressing competing water reduction. The process opens new pathways for the preparation of new efficient photocatalysts from readily available nanostructured templates.

Perovskite nanocrystals have emerged as an interesting material for light-emitting and other optoelectronic applications. Excitons are known to play an important role in determining the optical properties of these nanocrystals and their energetic levels as well as quantization properties have been extensively explored. Despite this, there are still many aspects of perovskites that are still not well-known, for example, the homogeneous and inhomogeneous line widths of the energetic transitions, quantities that cannot be directly extracted by linear absorption optical spectroscopy on nanocrystal ensembles. Here, we present temperature-dependent absorption and four-wave mixing (FWM) experiments on thick methylammonium lead iodide (MAPI) perovskite nanoplatelets exhibiting bulk-like absorption and emission spectra. Dephasing times T 2 of excitons are determined to lie in the range of several hundreds of femtoseconds at low temperatures. This value enables us to distinguish between the homogeneous and inhomogeneous contribution to the total broadening of the excitonic transitions. These turn out to be predominantly inhomogeneously broadened at low temperatures and homogeneously broadened at room temperature. Furthermore, we find excitonic quantum beats, which allow for the determination of the exciton binding energy and we extract E B = 25 ± 2 meV in the low temperature regime, in good agreement with other reports.

Strong coupling of plasmons and excitons can form hybrid states, the so-called "plexcitons". Although plasmons have a low quality factor, the exceptionally high coupling strength with molecular aggregates, in particular Jaggregates, allows the realization of strong interaction. Despite several studies in recent years showing the formation of plexcitonic states, their nature, especially at very short times, is still insufficiently investigated. In this article, we identify the nonlinear optical behavior of plexcitons formed on gold nanorods coated with J-aggregated cyanine molecules at short times by transient absorption spectroscopy and a simple Lorentz oscillator model. We control the spectral overlap of the two resonances and analyze the effect of detuning as well as the effect of off-and on resonance excitation on the hybrid states. We demonstrate that at ultrashort time scales plexcitons show tunable plasmonic and excitonic nonlinear performance according to the hybridization model. In a first approach, we discover a way to optically manipulate the quality factor and study the effects on the coupled hybrid states. As a second approach, we find that the coupling strength can also be influenced on an ultrashort time scale in the strong coupling regime when plexcitons are excited.

The crucial separation of photocarriers in solar cells can be efficiently driven by contacting semiconductor phases with differing doping levels. Here we show that intrinsic doping surges in methylammonium lead iodide (MAPbI) crystals as a response to environmental basicity. MAPbI crystals were passivated with polybases to induce the deprotonation of its methylammonium ions (MA). Stable crystals showed marked increases in photoluminescence and radiative decay, attributed to the presence of unbalanced charges acting as doped carriers. This emulates in a controlled manner the proton-withdrawing conditions of polycrystalline films, where excess basic precursors are found between grains. Raman spectroscopy showed the collective alignment of MA cations within the intrinsically doped lattices, thus revealing the buildup of electric fields. On this basis, we propose a mechanism for the formation of doping-gradients toward grain boundaries, potentially explaining the extended photocarrier lifetimes and diffusion lengths observed in perovskite solar cells.

CdSe/CdS core-shell nanocrystals with controlled CdS shell thickness and CdSe core size were synthesized for several different values of these two parameters. The particles in aqueous dispersion were in situ decorated with Ni nanoparticles and evaluated for photocatalytic hydrogen generation capacity. The highest H