Solar electricity and fuel production with perylene monoimide dye-sensitised TiO2 in water

Ultrafast symmetry-breaking charge separation in Perylenemonoimide-embedded multichromophores: impact of regioisomerism

Symmetry-breaking charge separation (SB-CS) has recently evolved as an emerging concept offering its potential to the latest generation of organic photovoltaics. However there are several concerns that need to be addressed to reach the state-of-the-art in SB-CS chemistry, for instance, the desirable molecular geometry, interchromophoric distance and extent of electronic coupling. To shed light on those features, it is reported herein, that ortho-functionalized perylene monoimide (PMI) constituted regioisomeric dimer and trimer derivatives with varied molecular twisting and electronic conjugation have been synthesized. In steady-state photophysical studies, all the dimers and trimer derivatives exhibit a larger bathochromic shift in the emission spectra and a significant reduction of fluorescence quantum yield in polar DMF. Among the series of multichromophores, ortho- and self-coupled dimers display the strikingly different optical feature of SB-CS with a very fast charge separation rate (τCS = 80.2 ps) upon photoexcitation in DMF, which is unveiled by femtosecond transient absorption (fs-TA) studies. The SB-CS for two dimers is well-supported by the formation of PMI˙+ and PMI˙- bands in the fs-TA spectra. Further analysis of fs-TA data revealed that, among the other multichromophores the trimer also exhibits a clear charge separation, whereas SB-CS signatures are less prominent, but can not be completely disregarded, for the meta- and para-dimers. Additionally, the charge separation dynamics of those above-mentioned PMI derivatives are devoid of a kinetically favorable excimer or triplet formation. The evidence of a profound charge transfer phenomenon in the ortho-dimer is characterized by density functional theory (DFT) calculations on excited state electronic structures. The excitonic communications in the excited state electronic arrangements unravel the key role of dihedral twisting in SB-CS. The thermodynamic feasibility of CS (ΔGCS) and activation barrier (ΔG≠) of the derivatives in DMF are established from the Rehm-Weller equation and Marcus's theory, respectively. This work is an in-depth study of the effect of mutual orientation of PMIs and regioisomerism in determining sustainable guidelines for using SB-CS.

Green hydrothermal synthesis yields perylenebisimide-SiO2 hybrid materials with solution-like fluorescence and photoredox activity

In organic-inorganic hybrid materials' (HMs) synthesis, it is intrinsically challenging to, at the same time, achieve (i) the concomitant synthesis of the components, (ii) nanoscopic interpenetration of the components, and (iii) covalent linking of the components. We here report the one-pot hydrothermal synthesis (HTS) of inorganic-organic HMs consisting of perylene bisimide (PBI) dyes and silica, using nothing but water as the medium and directly from the corresponding bisanhydrides, n-alkyl amines, and alkoxysilane precursors. First, in the absence of a functionalized alkoxysilane for linking, a mixture of the products, PBI and SiO2, is obtained. This evinces that the two products can be synthesized in parallel in the same vessel. Except for minor micromorphological changes, the concomitant synthesis does not affect each component's physicochemical properties. The PBI/SiO2 mixtures do not show synergistic properties. Second, through adding the linker aminopropyltriethoxysilane (APTS), covalently-linked class II hybrids are obtained. These PBI@SiO2 class II hybrids show synergistic materials properties: increased thermal stability is obtained in combination with nanoscopic homogeneity. The PBI moieties are dissolved in the solid SiO2 matrix, while being covalently linked to the matrix. This leads to solution-like fluorescence with vibronic fine-structure of the dyes. Moreover, through tuning the SiO2 amount, the band gaps of the class II hybrid materials can be systematically shifted. We exploit these optoelectronic properties by using the PBI@SiO2 hybrids as heterogeneous and reusable photoredox catalysts for the reduction of aryl halides. Finally, we present a detailed small-angle X-ray scattering and powder X-ray diffraction study of PBI@SiO2 synthesized at various reaction times, revealing the existence of an ordered PBI-oligomeric silesquioxane-type intermediate, which subsequently further condenses to the final nanoscopically homogeneous PBI@SiO2 material. These ordered intermediates point at HTS' propensity to favor crystallinity (to date known for organic and inorganic compounds, respectively) to also apply to hybrid structures, and shed additional light on the long-standing question of structure formation in the early stages of sol-gel processes: they corroborate Brown's hypothesis (1965) that trifunctional hydroxysilanes form surprisingly well controlled oligomers in the early stages of polycondensation.

Surface coating preparation of spherical magnetic materials

Magnetic materials are being increasingly used in anti-counterfeiting coatings, but the dark colors of magnetic materials greatly limit their applications. This necessitates the development of light-colored magnetic materials. In this study, the heterogeneous precipitation method was used to deposit a layer of titanium dioxide (TiO₂) on the surface of magnetic spherical metal particles, followed by the deposition of a layer of Ag by the reduction method, in order to achieve a light color. In the experiment, the particles were initially coated with a few tens of nanometers of TiO₂ with a strong shading effect, followed by a further coating of Ag of the same thickness with a similar shading performance. Not only did this achieve a lighter color, but there was no reduction in the magnetic properties of the material after the application of the coating. Scanning electron microscopy (SEM), scanning electron microscopy and energy-dispersive spectroscopy (SEM-EDS), X-ray diffractometry (XRD), and other methods were used to study the changes in morphology and composition before and after the magnetic material was coated. A magnetic tester was used to study the changes in magnetic strength before and after the magnetic material was coated.

Magnetic materials are being increasingly used in anti-counterfeiting coatings, but the dark colors of magnetic materials greatly limit their applications.

Extending photocatalysis to the visible and NIR: the molecular strategy

Photocatalysis exploits light to perform important processes as solar fuel production by water splitting, and CO2 reduction or water and air decontamination. Therefore, photocatalysis contributes to the satisfaction of the increasing needs for clean energy, environmental remediation and, most recently, sanification. Most of the efficient semiconductor nanoparticles (NP), developed as photocatalysts, work in the ultraviolet (UV) spectral region and they are not able to exploit either visible (Vis) or near infrared (NIR) radiation. This limitation makes them unable to fully exploit the broad band solar radiaton or to be applied in indoor conditions. Recently, different approaches have been developed to extend the spectral activity of semiconductor NP, like for example band-gap engineering, integration with upconversion NP and plasmonic enhancement involving also hot-electron injection. Nevertheless, the use of organic molecules and metal complexes, for enhancing the photoactivity in the Vis and NIR, was one of the first strategies proposed for sensitization and it is still one of the most efficient. In this minireview we highlight and critically discuss the most recent and relevant achievements in the field of photocatalysis obtained by exploiting dye sensitization either via dynamic or static quenching.

Charge Carrier Processes and Optical Properties in TiO2 and TiO2-Based Heterojunction Photocatalysts: A Review

Photocatalysis based technologies have a key role in addressing important challenges of the ecological transition, such as environment remediation and conversion of renewable energies. Photocatalysts can in fact be used in hydrogen (H₂) production (e.g., via water splitting or photo-reforming of organic substrates), CO₂ reduction, pollution mitigation and water or air remediation via oxidation (photodegradation) of pollutants. Titanium dioxide (TiO₂) is a “benchmark” photocatalyst, thanks to many favorable characteristics. We here review the basic knowledge on the charge carrier processes that define the optical and photophysical properties of intrinsic TiO₂. We describe the main characteristics and advantages of TiO₂ as photocatalyst, followed by a summary of historical facts about its application. Next, the dynamics of photogenerated electrons and holes is reviewed, including energy levels and trapping states, charge separation and charge recombination. A section on optical absorption and optical properties follows, including a discussion on TiO₂ photoluminescence and on the effect of molecular oxygen (O₂) on radiative recombination. We next summarize the elementary photocatalytic processes in aqueous solution, including the photogeneration of reactive oxygen species (ROS) and the hydrogen evolution reaction. We pinpoint the TiO₂ limitations and possible ways to overcome them by discussing some of the “hottest” research trends toward solar hydrogen production, which are classified in two categories: (1) approaches based on the use of engineered TiO₂ without any cocatalysts. Discussed topics are highly-reduced “black TiO₂”, grey and colored TiO₂, surface-engineered anatase nanocrystals; (2) strategies based on heterojunction photocatalysts, where TiO₂ is electronically coupled with a different material acting as cocatalyst or as sensitizer. Examples discussed include TiO₂ composites or heterostructures with metals (e.g., Pt-TiO₂, Au-TiO₂), with other metal oxides (e.g., Cu₂O, NiO, etc.), direct Z-scheme heterojunctions with g-C₃N₄ (graphitic carbon nitride) and dye-sensitized TiO₂.

1,7,9,10-Tetrasubstituted PMIs Accessible through Decarboxylative Bromination: Synthesis, Characterization, Photophysical Studies, and Hydrogen Evolution Catalysis

In this work, we present a new synthetic strategy for fourfold-substituted perylene monoimides via tetrabrominated perylene monoanhydrides. X-ray diffraction analysis unveiled the intramolecular stacking orientation between the substituents and semicircular packing behavior. We observed the remarkable influence of the substituent on the longevity and nature of the excited state upon visible light excitation. In the presence of poly(dehydroalanine)-graft-poly(ethylene glycol) graft copolymers as solubilizing template, the chromophores are capable of sensitizing [Mo3 S13 ]2- clusters in aqueous solution for stable visible light driven hydrogen evolution over three days.

Optofluidic Photonic Crystal Fiber Microreactors for In Situ Studies of Carbon Nanodot-Driven Photoreduction

Performing quantitative in situ spectroscopic analysis on minuscule sample volumes is a common difficulty in photochemistry. To address this challenge, we use a hollow-core photonic crystal fiber (HC-PCF) that guides light at the center of a microscale liquid channel and acts as an optofluidic microreactor with a reaction volume of less than 35 nL. The system was used to demonstrate in situ optical detection of photoreduction processes that are key components of many photocatalytic reaction schemes. The photoreduction of viologens (XV²⁺) to the radical XV^•+ in a homogeneous mixture with carbon nanodot (CND) light absorbers is studied for a range of different carbon dots and viologens. Time-resolved absorption spectra, measured over several UV irradiation cycles, are interpreted with a quantitative kinetic model to determine photoreduction and photobleaching rate constants. The powerful combination of time-resolved, low-volume absorption spectroscopy and kinetic modeling highlights the potential of optofluidic microreactors as a highly sensitive, quantitative, and rapid screening platform for novel photocatalysts and flow chemistry in general.

Photocatalytic H2 Production from Water by Metal-free Dye-sensitized TiO2 Semiconductors: The Role and Development Process of Organic Sensitizers

The utilization of solar energy to produce hydrogen from water is showing increased importance and desirability in the field of artificial photosynthesis to produce clean and sustainable fuels. In a typical three-component dye-sensitized semiconductor system for photocatalysis, the dye sensitizer plays an essential role of energy antenna for harvesting visible light and promoting the reduction reaction to generate hydrogen. In recent decades, a lot of attention has focused on metal-free organic sensitizers, which have the advantages of low cost, high molar extinction coefficient, good modifiability and, most importantly, ability to avoid the use of noble metal ions. This Review enumerates the design strategies, specific properties and photocatalytic performances of metal-free sensitizers in the past 30 years and concludes their evolution process. The advantages of different types of metal-free sensitizers are highlighted and the instructively enlightening experiences are systematic summarized.

Synthetic Organic Design for Solar Fuel Systems

From the understanding of biological processes and metalloenzymes to the development of inorganic catalysts, electro- and photocatalytic systems for fuel generation have evolved considerably during the last decades. Recently, organic and hybrid organic systems have emerged to challenge the classical inorganic structures through their enormous chemical diversity and modularity that led earlier to their success in organic (opto)electronics. This Minireview describes recent advances in the design of synthetic organic architectures and promising strategies toward (solar) fuel synthesis, highlighting progress on materials from organic ligands and chromophores to conjugated polymers and covalent organic frameworks.

A diketopyrrolopyrrole dye-based dyad on a porous TiO2 photoanode for solar-driven water oxidation

Dye-sensitised photoanodes modified with a water oxidation catalyst allow for solar-driven O2 evolution in photoelectrochemical cells. However, organic chromophores are generally considered unsuitable to drive the thermodynamically demanding water oxidation reaction, mainly due to their lack of stability upon photoexcitation. Here, the synthesis of a dyad photocatalyst (DPP-Ru) consisting of a diketopyrrolopyrrole chromophore (DPPdye) and ruthenium-based water oxidation catalyst (RuWOC) is described. The DPP-Ru dyad features a cyanoacrylic acid anchoring group for immobilisation on metal oxides, strong absorption in the visible region of the electromagnetic spectrum, and photoinduced hole transfer from the dye to the catalyst unit. Immobilisation of the dyad on a mesoporous TiO2 scaffold was optimised, including the use of a TiCl4 pretreatment method as well as employing chenodeoxycholic acid as a co-adsorbent, and the assembled dyad-sensitised photoanode achieved O2 evolution using visible light (100 mW cm-2, AM 1.5G, λ > 420 nm). An initial photocurrent of 140 μA cm-2 was generated in aqueous electrolyte solution (pH 5.6) under an applied potential of +0.2 V vs. NHE. The production of O2 has been confirmed by controlled potential electrolysis with a faradaic efficiency of 44%. This study demonstrates that metal-free dyes are suitable light absorbers in dyadic systems for the assembly of water oxidising photoanodes.

Consecutive Charging of a Perylene Bisimide Dye by Multistep Low-Energy Solar-Light-Induced Electron Transfer Towards H2 Evolution

A photocatalytic system containing a perylene bisimide (PBI) dye as a photosensitizer anchored to titanium dioxide (TiO2 ) nanoparticles through carboxyl groups was constructed. Under solar-light irradiation in the presence of sacrificial triethanolamine (TEOA) in neutral and basic conditions (pH 8.5), a reaction cascade is initiated in which the PBI molecule first absorbs green light, giving the formation of a stable radical anion (PBI.- ), which in a second step absorbs near-infrared light, forming a stable PBI dianion (PBI2- ). Finally, the dianion absorbs red light and injects an electron into the TiO2 nanoparticle that is coated with platinum co-catalyst for hydrogen evolution. The hydrogen evolution rates (HERs) are as high as 1216 and 1022 μmol h-1  g-1 with simulated sunlight irradiation in neutral and basic conditions, respectively.

Efficient Fenton Degradation of Perylene Diimide Dye Promoted by a Catalytic Amount of Cucurbit[8]uril

Herein, we propose a new method for promoting the degradation of a perylene diimide (PDI) dye, through a Fenton reaction with cucurbit[8]uril (CB[8]) as a supramolecular catalyst. The CB[8] can encapsulate the hydrophobic moiety of the PDI dye and inhibit its aggregation in aqueous solutions, thus increasing the collision frequency between the PDI and oxidants to accelerate the reaction. As a result, the encapsulated PDI molecule is preferentially degraded, followed by freeing the cavity of CB[8] and enabling it to encapsulate another PDI molecule to realize a catalytic cycle. Hence, a catalytic amount of CB[8] is sufficient to accelerate the the Fenton degradation. It is anticipated that this work will extend the realm of supramolecular catalysis systems and enrich the field of degradation of polycyclic aromatic dyes.

Inverse Opal CuCrO2 Photocathodes for H2 Production Using Organic Dyes and a Molecular Ni Catalyst

Dye-sensitized photoelectrochemical (DSPEC) cells are an emerging approach to producing solar fuels. The recent development of delafossite CuCrO2 as a p-type semiconductor has enabled H2 generation through the coassembly of catalyst and dye components. Here, we present a CuCrO2 electrode based on a high-surface-area inverse opal (IO) architecture with benchmark performance in DSPEC H2 generation. Coimmobilization of a phosphonated diketopyrrolopyrrole (DPP-P) or perylene monoimide (PMI-P) dye with a phosphonated molecular Ni catalyst (NiP) demonstrates the ability of IO-CuCrO2 to photogenerate H2. A positive photocurrent onset potential of approximately +0.8 V vs RHE was achieved with these photocathodes. The DPP-P-based photoelectrodes delivered photocurrents of -18 μA cm-2 and generated 160 ± 24 nmol of H2 cm-2, whereas the PMI-P-based photocathodes displayed higher photocurrents of -25 μA cm-2 and produced 215 ± 10 nmol of H2 cm-2 at 0.0 V vs RHE over the course of 2 h under visible light illumination (100 mW cm-2, AM 1.5G, λ > 420 nm, 25 °C). The high performance of the PMI-constructed system is attributed to the well-suited molecular structure and photophysical properties for p-type sensitization. These precious-metal-free photocathodes highlight the benefits of using bespoke IO-CuCrO2 electrodes as well as the important role of the molecular dye structure in DSPEC fuel synthesis.

Particulate Photocatalysts for Light-Driven Water Splitting: Mechanisms, Challenges, and Design Strategies

Solar-driven water splitting provides a leading approach to store the abundant yet intermittent solar energy and produce hydrogen as a clean and sustainable energy carrier. A straightforward route to light-driven water splitting is to apply self-supported particulate photocatalysts, which is expected to allow solar hydrogen to be competitive with fossil-fuel-derived hydrogen on a levelized cost basis. More importantly, the powder-based systems can lend themselves to making functional panels on a large scale while retaining the intrinsic activity of the photocatalyst. However, all attempts to generate hydrogen via powder-based solar water-splitting systems to date have unfortunately fallen short of the efficiency values required for practical applications. Photocatalysis on photocatalyst particles involves three sequential steps: (i) absorption of photons with higher energies than the bandgap of the photocatalysts, leading to the excitation of electron-hole pairs in the particles, (ii) charge separation and migration of these photoexcited carriers, and (iii) surface chemical reactions based on these carriers. In this review, we focus on the challenges of each step and summarize material design strategies to overcome the obstacles and limitations. This review illustrates that it is possible to employ the fundamental principles underlying photosynthesis and the tools of chemical and materials science to design and prepare photocatalysts for overall water splitting.

Spectral Properties of PMMA Films Doped by Perylene Dyestuffs for Photoselective Greenhouse Cladding Applications

Luminescent polymethylmethacrylate (PMMA) films were prepared by the solvent-casting technique from polymer solution doped with different concentrations of red perylene dyestuffs (KREMER 94720 and KREMER 94739). The effect of the dye concentration on the structure and spectroscopic properties was studied using X-ray diffraction (XRD), transmission electron microscope (TEM) optical absorption, and fluorescence spectroscopy. The optimum dye concentration of photoselective PMMA films was determined by the fluorescence spectroscopy measurements and showed the best emission properties for the doping concentration 10⁻³ wt % of the investigated dyes. The accelerated photostability tests showed promising stability of the prepared films towards terrestrial solar ultraviolet radiation (UVA). The results endorsed a promising application of the investigated films in photoselective greenhouse cladding applications as the optimized film fluoresces at the action spectra of special chlorophyll a.

Mesoporous hollow black TiO2 with controlled lattice disorder degrees for highly efficient visible-light-driven photocatalysis

Black TiO₂ has received tremendous attention because of its lattice disorder-induced reduction in the TiO₂ bandgap, which yields excellent light absorption and photocatalytic ability. In this report, a highly efficient visible-light-driven black TiO₂ photocatalyst was synthesized with a mesoporous hollow shell structure. It provided a higher specific surface area, more reaction sites and enhanced visible light absorption capability, which significantly promoted the photocatalytic reaction. Subsequently, the mesoporous hollow black TiO₂ with different lattice disorder-engineering degrees were designed. The structure disorder in the black TiO₂ obviously increased with reduction temperature, leading to improved visible light absorption. However, their visible-light-driven photocatalytic efficiency increased first and then decreased. The highest value can be observed for the sample reduced at 350 °C, which was 2-, 1.4- and 5-fold that of the samples reduced at 320 °C, 380 °C and 400 °C, respectively. This contradiction can be ascribed to the varied functions of the surface defects with different concentrations in the black TiO₂ during the catalytic process. In particular, the defects at low concentrations boost photocatalysis but reverse photocatalysis at high concentrations when they act as charge recombination centers. This study provides significant insight for the fabrication of high-efficiency visible-light-driven catalytic black TiO₂ and the understanding of its catalysis mechanism.

Our work provides significant insights into the design of hollow black TiO₂ spheres and the mechanism accounting for their high-efficient visible-light-driven catalysis.

A heavy metal-free CuInS2 quantum dot sensitized NiO photocathode with a Re molecular catalyst for photoelectrochemical CO2 reduction

Heavy metal-free CuInS₂ QDs as a photosensitizer and a Re molecular catalyst have been successfully employed to co-sensitize a NiO photocathode for CO₂ reduction into CO.