A New Perspective on the Role of A‐Site Cations in Perovskite Solar Cells

As the race toward higher efficiency for inorganic/organic hybrid perovskite solar cells (PSCs) is becoming highly competitive, a design scheme to maximize carrier transport toward higher power efficiency has been urgently demanded. In this study, a hidden role of A‐site cations of PSCs in carrier transport, which has been largely neglected is unraveled, i.e., tuning the Fröhlich electron–phonon (e–ph) coupling of longitudinal optical (LO) phonon by A‐site cations. The key for steering Fröhlich polaron is to control the interaction strength and the number of proton (or lithium) coordination to halide ions. The coordination to I^? alleviates electron–phonon scattering by either decreasing the Born effective charge or absorbing the LO motion of I. This novel principle discloses low electron–phonon coupling in several promising organic cations including hydroxyl–ammonium cation (NH₃OH⁺), hydrazinium cation (NH₃NH₂⁺) and possibly Li⁺ solvating methylamine (Li⁺???NH₂CH₃), on a par with methyl–ammonium cations. A new perspective on the role of A‐site cations could help in improving power efficiency and accelerating the application of PSCs.     

How Methylammonium Cations and Chlorine Dopants Heal Defects in Lead Iodide Perovskites

Lead tri‐iodide methylammonium (MAPbI₃) perovskite polycrystalline materials show complex optoelectronic behavior, largely because their 3D semiconducting inorganic framework is strongly perturbed by the organic cations and ubiquitous structural or chemical inhomogeneities. Here, a newly developed time‐dependent density functional theory‐based theoretical formalism is taken advantage of. It treats electron–hole and electron–nuclei interactions on the same footing to assess the many‐body excited states of MAPbI₃ perovskites in their pristine state and in the presence of point chemical defects. It is shown that lead and iodine vacancies yield deep trap states that can be healed by dynamic effects, namely rotation of the methylammonium cations in response to point charges, or through slight changes in chemical composition, namely by introducing a tiny amount of chlorine dopants in the defective MAPbI₃. The theoretical results are supported by photoluminescence experiments on MAPbI_3?mCl_m and pave the way toward the design of defect‐free perovskite materials with optoelectronic performance approaching the theoretical limits.     

Charge Transport Modulation of a Flexible Quantum Dot Solar Cell Using a Piezoelectric Effect

Colloidal quantum dots are promising materials for flexible solar cells, as they have a large absorption coefficient at visible and infrared wavelengths, a band gap that can be tuned across the solar spectrum, and compatibility with solution processing. However, the performance of flexible solar cells can be degraded by the loss of charge carriers due to recombination pathways that exist at a junction interface as well as the strained interface of the semiconducting layers. The modulation of the charge carrier transport by the piezoelectric effect is an effective way of resolving and improving the inherent material and structural defects. By inserting a porous piezoelectric poly(vinylidenefluoride‐trifluoroethylene) layer so as to generate a converging electric field, it is possible to modulate the junction properties and consequently enhance the charge carrier behavior at the junction. This study shows that due to a reduction in the recombination and an improvement in the carrier extraction, a 38% increase in the current density along with a concomitant increase of 37% in the power conversion efficiency of flexible quantum dots solar cells can be achieved by modulating the junction properties using the piezoelectric effect.     

Lead‐induced DNA damage and cell apoptosis in human renal proximal tubular epithelial cell: Attenuation via N‐acetyl cysteine and tannic acid

This study investigates the exposure of lead‐induced reactive oxygen species (ROS) generation, DNA damage, and apoptosis and also evaluates the therapeutic intervention using antioxidants in human renal proximal tubular cells (HK‐2 cells). Following treatment of HK‐2 cells with an increasing concentration of lead nitrate (0–50 μM) for 24 h, the intracellular ROS level increased whereas the GSH level decreased significantly in a dose‐dependent manner. Comet assay results revealed that lead nitrate showed the ability to increase the levels of DNA strand breaks in HK‐2 cells. Lead exposure also induced apoptosis through caspase‐3 activation at 30 μg/mL. Pretreatment with N‐acetylcysteine (NAC) and tannic acid showed a significant ameliorating effect on lead‐induced ROS, DNA damage, and apoptosis. In conclusion, lead induces ROS, which may exacerbate the DNA damage and apoptosis via caspase‐3 activation. Additionally, supplementation of antioxidants such as NAC and tannic acid may be used as salvage therapy for lead‐induced DNA damage and apoptosis in an exposed person.     

A Novel High Biosorbent of Pb-resistant Bacterium Isolate for the Removal of Hazardous Lead from Alkaline Soil and Water: Biosorption Isotherms In Vivo and Bioremediation Strategy

A novel Pb-resistant bacterium was isolated from aged lead-contaminated alkaline soils, and was identified as Bacillus megaterium via the MIDI protocol. The biosorption isotherms and kinetics of Pb(II) associated with B. megaterium in vivo in the alkaline environment were investigated at the first time. All the batch experiments of biosorption demonstrate that the B. megaterium uptake of lead is pH-dependent, exothermic (ΔH° = ?5224.86 KJ mol^?1), spontaneous, and fits well with the Langmuir isotherm, resulting in different kinetics under different examination temperatures. The maximum biosorption capacity is 503.86 mg g^?1 at optimum conditions, which is much better in comparison to the biosorbent reported at the acidic condition in the literature. The Fourier-transform Infrared spectroscopic analysis of lead-loaded biomass confirms that the biosorption between B. megaterium and lead is the chemical adsorption in vivo. A site test indicates that B. megaterium really increases mobility and bioavailability of lead in Pb-contaminated alkaline soil in terms of chemical fractionation in vivo, which will potentially increase its uptake by hyperaccumulated plants in alkaline soils in arid or semi-arid areas of NW, China. Therefore, the novel isolate of B. megaterium with the highest adsorption capacity is a new promising biosorbent for the lead removal in alkaline water and soil.     

The recovery of heavy metals using encapsulated microbial cells

We prepared capsules containingSaccharomyces cerevisiae andZoogloea ramigera cells for the removal of lead (II) and cadmium ions. Microbial cells were encapsulated and cultured in the growth medium. TheS. cerevisiae cells grown in the capsule did not leak through the capsule membrane. The dried cell density reached to 250 g/l on the basis of the inner volume of the 2.0 mm diameter capsule after 36 hour cultivation. The dry whole cell exopolymer density of encapsulatedZ. ramigera reached to 200 g/L. The capsule was crosslinked with triethylene tetramine and glutaric dialdehyde solutions. The cadmium uptake of encapsulated whole cell exopolymer ofZ. ramigera was 55 mg Cd/g biosorbent. The adsorption line followed well Langmuir isotherm. The lead uptake of the encapsulatedS. cerevisiae was about 30 mg Pb/g biomass. The optimum pH of the lead uptake using encapsulatedS. cerevisiae was found to be 6. Freundlich model showed a little better fit to the adsorption data than Langmuir model. 95 percent of the lead adsorbed on the encapsulated biosorbents was desorbed by the 1 M HCl solution. The capsule was reused 50 batches without loosing the metal uptake capacity. And the mechanical strength of the crosslinked capsule was retained after 50 trials.     

Traffic exposure increases natural 15N and heavy metal concentrations in mosses

Mosses have been used as biomonitors of atmospheric pollution for some years, but few studies have been carried out on the effect of NO_x emissions from traffic on moss tissue N. Eight species of moss (102 samples) growing on walls or roofs next to roads exposed to different traffic densities were collected from urban and rural sites in the UK. The shoots were sampled for total N, their stable isotope ¹⁵N/¹⁴N content (δ¹⁵N) and heavy metal content (Pb, Zn). There was a lack of correlation between tissue total N and traffic exposure, but a very good correlation between traffic exposure and tissue δ¹⁵N. Plants collected near motorways or busy urban roads had δ¹⁵N values ranging between +6 and −1‰, while in rural areas with hardly any traffic these ranged from −2 to −12‰. In a separate survey of mosses, the average δ¹⁵N of shoots from busy roadsides in London was +3.66‰, whereas from samples collected from farm buildings near poultry or cattle pens it was −7.8‰. This indicates that the two main atmospheric N sources, NO_x and NH_x, have different δ¹⁵N signatures, the former tending to be positive and the latter negative. Tissue concentrations of both Pb and Zn show a strong positive correlation with traffic exposure, with Zn in particular being greater than Pb. The results are discussed with regard to the use of moss tissue Zn as a means for monitoring or mapping pollution from vehicles, and of δ¹⁵N as an aid to distinguish between urban (NO_x) and rural (NH_x) forms of N pollution.     

Bioaccumulation of lead by the lichen Acarospora smaragdula from smelter emissions

Accumulation of lead in the crustose lichen Acarospora smaragdula sensu lato is reported in the vicinity of an ore- processing plant where it is subjected to acidification and metal particulate fallout. A combination of light microscopy, X-ray element mapping, field emission scanning electron microscopy (FESEM) and other analytical techniques identifies Pb accumulation within specific fungal tissues derived from smelter particles (PM10s). No Pb was detected within the photobiont layer. Our studies suggest that Pb is highly mobile under the prevailing acidic conditions, and is fixed within the lichen cortex and melanized apothecia. Lead is also accumulated within the medulla and at the rock–lichen interface where it may precipitate as amorphous botryoidal encrustations on medullary hyphae and iron-rich particles. Modern FESEMs and microprobes enable analysis of minute quantities of material, and are important tools in understanding the fate of metals within lichens necessary to develop their use as predictive and sensitive bioindicators of aerial particulate contaminants. We suggest that crustose lichens, hitherto largely ignored in metal pollution studies, may be useful bioindicators of aerial particulate contaminants in polluted areas where macrolichens are absent.     

Chemical speciation of cadmium and lead and their bioavailability to cole (Brassica campestris L.) from multi-metals contaminated soil in northwestern China

A pot experiment was conducted to study the relationship between speciation distribution of cadmium (Cd) and lead (Pb) and their availability to cole (Brassica campestris L.) grown on the Cd–Pb polluted soil in northwest of China. The results showed that Cd in the unpolluted soil was mainly bound to carbonate fraction (F2) and Fe–Mn oxide fraction (F3), and Pb was mainly bound to carbonate fraction (F2) and residual fraction (F5). However, marked change of Cd and Pb fractionation was observed with increasing soil Cd and Pb concentrations, where the concentrations of Cd in F1 (exchangeable fraction), F2 and F3 increased significantly (p < 0.001 for F1, F2 and F3), and Pb in F1, F2, F3 and F4 increased significantly (p < 0.001 for F1, F2, F3 and F4).The correlation analysis between the fraction distribution coefficient of Cd and Pb in the soil and Cd and Pb concentration accumulated in cole showed that both Cd and Pb in F1 fraction in the soil made the greatest contribution on the accumulation of Cd and Pb in cole. Higher bio-concentration factors (BCFs) and translocation factors (TFs) for Cd and lower BCFs and TFs for Pb were observed in the cole, respectively. Cd had higher accumulation in the edible parts of the cole, but Pb had lower accumulation in that. Therefore, Cd has higher risk to human health than Pb when people eat the coles grown in Cd–Pb polluted soil in northwestern China.     

Lead-induced oxidative stress and metabolic alterations in <Emphasis Type="Italic">Cassia angustifolia</Emphasis> Vahl.

Forty-five-days old plants of Indian senna (Cassia angustifolia Vahl.) were subjected to 0–500 μM lead acetate (Pb-Ac) in pot culture. Changes in contents of thiobarbituric acid reactive substances (TBARS), ascorbate, glutathione, proline, sennosides (a+b), and activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), glutathione reductase (GR), and catalase (CAT) were studied at pre-flowering (60 d after sawing, DAS), flowering (90 DAS) and post-flowering (120 DAS) stages of plant development. Compared with the controls, the Pb-Ac treated plants showed an increase in contents of TBARS, dehydroascorbate, oxidized and total glutathione at all stages of growth. However, sennoside yield and contents of ascorbate and reduced form of glutathione declined. Proline content increased at 60 DAS but declined thereafter. Activities of SOD, APX, GR and CAT were markedly increased. Sennoside content was higher at 60 and 90 DAS but lower at 120 DAS, compared to the control.