Photo-to-Thermal Conversion Harnessing Low-Energy Photons Renders Efficient Solar CO2 Reduction.

Efficient photocatalytic solar CO2 reduction presents a challenge because visible-to-near-infrared (NIR) low-energy photons account for over 50% of solar energy. Consequently, they are unable to instigate the high-energy reaction necessary for dissociating C═O bonds in CO2. In this study, we present a novel methodology leveraging the often-underutilized photo-to-thermal (PTT) conversion. Our unique two-dimensional (2D) carbon layer-embedded Mo2C (Mo2C-Cx) MXene catalyst in black color showcases superior near-infrared (NIR) light absorption. This enables the efficient utilization of low-energy photons via the PTT conversion mechanism, thereby dramatically enhancing the rate of CO2 photoreduction. Under concentrated sunlight, the optimal Mo2C-C0.5 catalyst achieves CO2 reduction reaction rates of 12000-15000 µmol·g-1·h-1 to CO and 1000-3200 µmol·g-1·h-1 to CH4. Notably, the catalyst delivers solar-to-carbon fuel (STF) conversion efficiencies between 0.0108% to 0.0143% and the STFavg = 0.0123%, the highest recorded values under natural sunlight conditions. This innovative approach accentuates the exploitation of low-frequency, low-energy photons for the enhancement of photocatalytic CO2 reduction.

Interactions between nano/micro plastics and suspended sediment in water: Implications on aggregation and settling.

Interactions between nano/microplastics and suspended sediment (SS) in natural waters are important for the environmental fate of plastic particles. This study investigated the effect of heteroaggregation between nano/microplastics and SS on the settling of aggregates. In NaCl solutions (0.05-0.5 M), large SS (100-500 µm in diameter) significantly increased the settling ratio of polystyrene nanoplastics (PSNPs) with an average diameter of 100 nm due to the formation of PSNPs-SS aggregates. The settling ratio of the heteroaggregates increased significantly when the NaCl concentration increased from 50 to 200 mM. This was primarily because higher ionic strength reduced the electrostatic repulsion between large SS and PSNPs, and subsequently increased the heteroaggregation rate. No obvious differences in settling ratios were observed in 200 or 500 mM NaCl solutions because the heteroaggregation entered the diffusion-controlled regime. However, in HA solutions (10-50 mg L-1), the surface adsorption of HA on PSNPs and large SS reduced the heteroaggregation of PSNPs-SS and thus led to the low co-settling ratio due to the steric hindrance according to the DLVO theory. In contrast, polyethylene microplastics (PEMPs) with diameters of 1.0-1.2 mm were found to always float on water surface (up to 8 months), even after addition of 500 mg L-1 small SS (<10 µm in diameter). Clearly, the heteroaggregation of PEMPs and small SS had minor effect on the settling of PEMPs due to the overwhelming boyanccy. These results provided new insight into the fate and distribution of nano/microplastics in aquatic environment, which affect the bioavailability of plastic particles in natural waters.

Relative importance of humic and fulvic acid on ROS generation, dissolution, and toxicity of sulfide nanoparticles.

In this study, the effect of natural organic matter (NOM) composition (humic acid (HA) or fulvic acid (FA)) on dissolution, reactive oxygen species (ROS) generation, and toxicity of sulfide nanoparticles (NPs) was investigated under UV irradiation. NOM acted as a UV filter or antioxidant, decreasing ROS (O2-, OH, and 1O2) generation by WS2 and MoS2 NPs. The higher light-absorbing fractions of HA in NP/HA mixtures and the faster reaction rate of HA with ROS resulted in higher inhibition effect of HA than FA on O2- and OH generation by WS2 and MoS2 NPs. Both HA and FA completely inhibited 1O2 generation by WS2 and MoS2 NPs. NOM could transfer electrons to CdS and promote its O2- generation. No measurable amount of OH was generated by CdS with or without NOM. FA decreased 1O2 generation by CdS more significantly than HA due to the higher reaction rate between FA and 1O2. HA showed a higher inhibition effect on the dissolution rate of CdS and WS2 NPs than FA. Both HA and FA played minor roles in the toxicity of CdS toward Escherichia coli but decreased the toxicity of MoS2 and WS2 due to the reduced ROS generation and/or dissolution concentrations.

Comparative toxicity of Cd, Mo, and W sulphide nanomaterials toward E. coli under UV irradiation.

In this study, the phototoxicity of cadmium sulfide (CdS), molybdenum disulfide (MoS2), and tungsten disulfide (WS2) nanoparticles (NPs) toward Escherichia coli (E. coli) under UV irradiation (365 nm) was investigated. At the same mass concentration of NPs, the toxicity of three NPs decreased in the order of CdS > MoS2 > WS2. For example, the death rates of E. coli exposed to 50 mg/L CdS, MoS2, and WS2 were 96.7%, 38.5%, and 31.2%, respectively. Transmission electron microscope and laser scanning confocal microscope images of E. coli exposed to three NPs showed the damage of cell walls and release of intracellular components. The CdS-treated cell wall was more extensively damaged than those of MoS2-treated and WS2-treated bacteria. WS2 and MoS2 generated superoxide radical (O2-), singlet oxygen (1O2), and hydroxyl radical under UV irradiation, CdS produced only O2- and 1O2. CdS and WS2 released ions under UV irradiation, while MoS2 did not. Reactive oxygen species (ROS) generation and toxic ion release jointly resulted in the antibacterial activities of CdS and WS2. ROS generation was the dominant toxic mechanism of MoS2 toward the bacteria. This study highlighted the importance of considering the hazardous effect of sulfide NPs after their release into natural waters under light irradiation condition.

Influence of dissolved organic matter on photogenerated reactive oxygen species and metal-oxide nanoparticle toxicity.

The effect of humic acid (HA) or fulvic acid (FA) on reactive oxygen species (ROS) generation by six metal-oxide nanoparticles (NPs) and their toxicities toward Escherichia coli was investigated under UV irradiation. Dissolved organic matter (DOM) decreased OH generation by TiO2, ZnO, and Fe2O3, with FA inhibiting OH generation more than HA. The generated OH in NPs/DOM mixtures was higher than the measured concentrations because DOM consumes OH faster than its molecular probe. None of NPs/FA mixtures produced O2(-). The generated O2(-) concentrations in NPs/HA mixtures (except Fe2O3/HA) were higher than the sum of O2(-) concentrations that produced as NPs and HA were presented by themselves. Synergistic O2(-) generation in NPs/HA mixtures resulted from O2 reduction by electron transferred from photoionized HA to NPs. DOM increased (1)O2 generation by TiO2, CuO, CeO2, and SiO2, and FA promoted (1)O2 generation more than HA. Enhanced (1)O2 generation resulted from DOM sensitization of NPs. HA did not increase (1)O2 generation by ZnO and Fe2O3 primarily because released ions quenched (1)O2 precursor ((3)HA*). Linear correlation was developed between total ROS concentrations generated by NPs/DOM mixtures and bacterial survival rates (R(2) ≥ 0.80). The results implied the necessity of considering DOM when investigating the photoreactivity of NPs.

Synergistic photogeneration of reactive oxygen species by dissolved organic matter and C60 in aqueous phase.

We investigated the photogeneration of reactive oxygen species (ROS) by C60 under UV irradiation, when humic acid (HA) or fulvic acid (FA) is present. When C60 and dissolved organic matter (DOM) were present as a mixture, singlet oxygen ((1)O2) generation concentrations were 1.2­1.5 times higher than the sum of (1)O2 concentrations that were produced when C60 and DOM were present in water by themselves. When C60 and HA were present as a mixture, superoxide radicals (O2(•­)) were 2.2­2.6 times more than when C60 and HA were present in water by themselves. A synergistic ROS photogeneration mechanism involved in energy and electron transfer between DOM and C60 was proposed. Enhanced (1)O2 generation in the mixtures was partly due to (3)DOM* energy transfer to O2. However, it was mostly due to (3)DOM* energy transfer to C60 producing (3)C60*. (3)C60* has a prolonged lifetime (>4 µs) in the mixture and provides sufficient time for energy transfer to O2, which produces (1)O2. The enhanced O2(•­) generation for HA/C60 mixture was because (3)C60* mediated electron transfer from photoionized HA to O2. This study demonstrates the importance of considering DOM when investigating ROS production by C60.