Organic
thermoelectrics are attractive for the fabrication of flexible
and cost-effective thermoelectric generators (TEGs) for waste heat
recovery, in particular by exploiting large-area printing of polymer
conductors. Efficient TEGs require both p- and n-type conductors:
so far, the air instability of polymer n-type conductors, which typically
lose orders of magnitude in electrical conductivity (σ) even
for short exposure time to air, has impeded processing under ambient
conditions. Here we tackle this problem in a relevant class of electron
transporting, naphthalene-diimide copolymers, by substituting the
imide oxygen with sulfur. n-type doping of the thionated copolymer
gives rise to a higher σ with respect to the non-thionated one,
and most importantly, owing to a reduced energy level of the lowest-unoccupied
molecular orbital, σ is substantially stable over 16 h of air
exposure. This result highlights the effectiveness of chemical tuning
to improve air stability of n-type solution-processable polymer conductors
and shows a path toward ambient large-area manufacturing of efficient
polymer TEGs.
EPR and magnetic susceptibility of potassium perchromate (KJCrO s ) have been studied from liquid helium temperatures to 400 K. EPR spectra of dilute aqueous solutions of KjCrO s yielded g iso = 1.9712 ±O.OOOS, and lJA ioo = (18.6±0.S)X 10-' cm-I • The (positive) sign for lJA iW deduced here from the dependence of the linewidths of hyperfine components on nuclear spin quantum numbers, constitutes the first direct experimental measurement of the (negative) sign of the unpaired electron spin density at a Cr'+ nucleus. EPR spectra of powdered K,CrO s doped in (diamagnetic) K,NbO s and of K,CrO s crystals yielded gil = 1.9428±0.OOO5, gl = 1.9851 ±0.OOO5, A II(BCr) = + (36.3±0.5)X 10-' cm-I , andA l('JCr) = + (11 ± 1) X 10-' cm -I, the II direction coinciding with the crystal c axis. These parameters establish that the unpaired electron orbital is the metal d x2 _ y2 orbital, with considerable charge transfer from the peroxy ligands. In concentrated samples, the EPR line shape is Lorentzian, providing direct evidence for electron spin exchange between Cr'+ ions. Temperature dependence of magnetic susceptibility X over 1.4 to 343 K follows the law x(cm'/mol) = [2.7933/(T + 2.7)] + Xd' where Xd = -1.5 X lO-'(cm'/mol) is estimated to be the diamagnetic contribution. From the Curie-Weiss temperature of 2.7 K, an exchange coupling J = 1.35 K between the Cr'+ ions is estimated. The calculated exchange-narrowed EPR linewidth is consistent with the experimental value of 15 G observed for K,CrO s ' No magnetic or structural phase transition was detected in the range of 1.4 to 400 K: hence K,CrO s crystals can serve as useful internal standard for spin concentration measurements over a wide temperature range, especially for free radicals.1916
In this work, a three-layered heterostructure Cu2O/CuO/CuS was obtained through a low-cost and large-area fabrication route comprising electrodeposition, thermal oxidation, and reactive annealing in a sulfur atmosphere. Morphological, microstructural, and compositional analysis (AFM, SEM, XRD, EDS, XPS) were carried out to highlight the surface modification of cuprous oxide film after oxidation and subsequent sulfurization. Impedance, voltammetric, and amperometric photoelectrochemical tests were performed on Cu2O, Cu2O/CuO, and Cu2O/CuO/CuS photocathodes in a sodium sulfate solution (pH 5), under 100 mW cm−2 AM 1.5 G illumination. A progressive improvement in terms of photocurrent and stability was observed after oxidation and sulfurization treatments, reaching a maximum of − 1.38 mA cm−2 at 0 V versus RHE for the CuS-modified Cu2O/CuO electrode, corresponding to a ~ 30% improvement. The feasibility of the proposed method was demonstrated through the fabrication of a large area photoelectrode of 10 cm2, showing no significant differences in characteristics if compared to a small area photoelectrode of 1 cm2.
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