BODIPY Chemisorbed on SnO2 and TiO2 Surfaces for Photoelectrochemical Applications.

Advancement toward dye-sensitized photoelectrochemical cells to produce solar fuels by solar-driven water splitting requires a photosensitizer that is firmly attached to the semiconducting photoelectrodes. Covalent binding enhances the efficiency of electron injection from the photoexcited dye into the metal oxide. Optimization of charge transfer, efficient electron injection, and minimal electron-hole recombination are mandatory for achieving high efficiencies. Here, a BODIPY-based dye exploiting a novel surface-anchoring mode via boron is compared to a similar dye bound by a traditional carboxylic acid anchoring group. Through terahertz and transient absorption spectroscopic studies, along with interfacial electron transfer simulations, we find that, when compared to the traditional carboxylic acid anchoring group, electron injection of boron-bound BODIPY is faster into both TiO2 and SnO2. Although the surface coverage is low compared with carboxylic acids, the binding stability is improved over a wide range of pH. Subsequent photoelectrochemical studies using a sacrificial electron donor showed that this combined dye and anchoring group maintained photocurrent with good stability over long-time irradiation. This recently discovered binding mode of BODIPY shows excellent electron injection and good stability over time, making it promising for future investigations.

Concentration-Dependent Photocatalytic Upcycling of Poly(ethylene terephthalate) Plastic Waste.

Photocatalytic plastic waste upcycling into value-added feedstock is a promising way to mitigate the environmental issues caused by the nondegradable nature of plastic waste. Here, we developed a MoS2/g-C3N4 photocatalyst that can efficiently upcycle poly(ethylene terephthalate) (PET) into valuable organic chemicals. Interestingly, the conversion mechanism is concentration-dependent. For instance, at a low ethylene glycol (EG) concentration (7.96 mM), acetate is the main product. Unexpectedly, the conversion of PET water bottle hydrolysate with only 7.96 mM ethylene glycol (EG) can produce a 4 times higher amount of acetate (704.59 nmol) than the conversion of 300 mM EG (174.50 nmol), while at a higher EG concentration (300 mM), formate is the dominant product. Herein, a 40 times higher EG concentration (300 mM compared to 7.96 mM) would produce only ∼3 times more formate (179 nmol compared to 51.86 nmol). In addition, under natural sunlight conditions, comparable amounts of liquid and gaseous products are produced when commercial PET plastics are employed. Overall, the photocatalytic PET conversion process is quite efficient under a low concentration of EG in PET hydrolysate, indicating the enormous potential of this photocatalysis strategy for real plastics upcycling.

BODIPY and dipyrrin as unexpected robust anchoring groups on TiO2 nanoparticles.

Covalent attachment of molecules to metal oxide surfaces typically demands the presence of an anchoring group that in turn requires synthetic steps to introduce. BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) chromophores have long been used in dye-sensitized solar cells, but carboxylic acid groups typically had to be installed to act as surface anchors. We now find that even without the introduction of such anchors, the unmodified BODIPY can bind to TiO2 surfaces via its BF2 group through boron-oxygen surface bonds. Dipyrrin, the parent molecule of BODIPY, is also capable of binding directly to TiO2 surfaces, likely through its chelating nitrogen atoms. These binding modes prove to be even more robust than that of an installed carboxylate and offer a new way to attach molecular complexes to surfaces for (photo)catalytic applications since, once bound, we show that surface bound BODIPY and dipyrrin derivatives exhibit ultrafast photoinjection of electrons into the conduction band of TiO2.

Terahertz Conductivity of Semiconducting 2H and Metallic 1T Phases of Molybdenum Disulfide.

Molybdenum disulfide (MoS2) has been extensively studied in its commonly occurring semiconducting 2H phase. Recent synthetic advances have enabled the bulk synthesis of the catalytically promising metallic 1T phase. However, the conductivity of bulk 1T-MoS2 has not been well characterized to ascertain the carrier transport properties. Terahertz (THz) spectroscopy is an ideal technique for obtaining this crucial information because it is a noncontact method of measuring the conductivity of emerging materials with ultrafast time resolution. This work applies THz spectroscopy to bulk 2H-MoS2 and 1T-MoS2, representing the first application of the technique on the 1T phase, with measurements confirming the semiconducting character of 2H-MoS2 and the metallic character of 1T-MoS2. This study provides new insight into the metallic nature of bulk 1T-MoS2 and a direct comparison to the semiconducting 2H phase that, together with physical characterization to obtain material parameters, are important for optimizing applications in catalysis devices and beyond.

Single Copper Atoms Enhance Photoconductivity in g-C3N4.

Graphitic carbon nitride (g-C3N4) and its doped analogues have been studied over the past decade in part due to their promising applications in heterogeneous photocatalysis; however, the effect of doping on the photoconductivity is poorly understood. Herein, we investigate Cu doped g-C3N4 (Cu-g-C3N4) and demonstrate via extended X-ray absorption fine structure that Cu+ incorporates as an individual ion. Time-resolved optical pump terahertz probe spectroscopy was utilized to measure the ultrafast photoconductivity in response to a 400 nm pump pulse and showed that the Cu+ dopant significantly enhances photoconductivity of the as-prepared powdered sample, which decays within 10 ps. Furthermore, a film preparation technique was applied that further enhanced the photoconductivity and induced a longer-lived photoconductive state with a lifetime on the order of 100 ps. This study provides valuable insight into the ultrafast photoconductivity dynamics of g-C3N4 materials, which is essential toward developing efficient g-C3N4 photocatalysts.