Maser threshold characterization by resonator Q-factor tuning

Whereas the laser is nowadays an ubiquitous technology, applications for its microwave analog, the maser, remain highly specialized, despite the excellent low-noise microwave amplification properties. The widespread application of masers is typically limited by the need of cryogenic temperatures. The recent realization of a continuous-wave room-temperature maser, using NV− centers in diamond, is a first step towards establishing the maser as a potential platform for microwave research and development, yet its design is far from optimal. Here, we design and construct an optimized setup able to characterize the operating space of a maser using NV− centers. We focus on the interplay of two key parameters for emission of microwave photons: the quality factor of the microwave resonator and the degree of spin level-inversion. We characterize the performance of the maser as a function of these two parameters, identifying the parameter space of operation and highlighting the requirements for maximal continuous microwave emission

Enhanced heterogeneous activation of peroxymonosulfate by Ruddlesden-Popper-type La2CoO4+δ nanoparticles for bisphenol A degradation

The scalable synthesis of stable catalysts for environmental remediation applications remains challenging. Nonetheless, metal leaching is a serious environmental issue hindering the practical application of transition-metal based catalysts including Co-based catalysts. Herein, for the first time, we describe a facile one-step and scalable spray-flame synthesis of high surface area La2CoO4+δ nanoparticles containing excess oxygen interstitials (+δ) and use them as a stable and efficient catalyst for activating peroxymonosulfate (PMS) towards the degradation of bisphenol A. Importantly, the La2CoO4+δ catalyst exhibits higher catalytic degradation of bisphenol A (95% in 20 min) and stability than LaCoO3–x nanoparticles (60%) in the peroxymonosulfate activation system. The high content of Co2+ in the structure showed a strong impact on the catalytic performance of the La2CoO4+δ + PMS system. Despite its high specific surface area, our results showed a very low amount of leached cobalt (less than 0.04 mg/L in 30 min), distinguishing it as a material with high chemical stability. According to the radical quenching experiments and the electron paramagnetic resonance technology, SO4[rad]–, [rad]OH, and 1O2 were generated and SO4[rad]– played a dominant role in bisphenol A degradation. Moreover, the La2CoO4+δ + PMS system maintained conspicuous catalytic performance for the degradation of other organic pollutants including methyl orange, rhodamine B, and methylene blue. Overall, our results showed that we developed a new synthesis method for stable La2CoO4+δ nanoparticles that can be used as a highly active heterogeneous catalyst for PMS-assisted oxidation of organic pollutants

Towards high performance dielectric microwave resonators for X-band EPR spectroscopy

Microwave (MW) resonators in Electron Paramagnetic Resonance (EPR) spectroscopy concentrate the MW magnetic field (B1) at the sample and separate the MW electric field from the sample. There are numerous experimental methods in EPR spectroscopy which all impose different requirements on MW resonators (e.g. high or low quality factor, MW conversion, and B1-field homogeneity). Although commercial spectrometers offer standardized MW resonators for a broad application range, newly emerging and highly-specialized research fields push these spectrometers to or beyond their sensitivity limits. Optimizing the MW resonator offers one direct approach to improve the sensitivity. Here we present three low-cost optimization approaches for a commercially available X-band (9–10 GHz) MW resonator for three experimental purposes (continuous-wave (CW), transient and pulse EPR). We obtain enhanced MW conversion factors for all three optimized resonators and higher quality factors for two optimized resonators. The latter is important for CW and transient EPR. Furthermore, we fabricated a resonator which features an extended area of homogeneous B1-field and, hence, improved pulse EPR performance. Our results demonstrate that small changes to a commercial MW resonator can enhance its performance in general or for specific applications

Synthesis of novel LaCoO3/graphene catalysts as highly efficient peroxymonosulfate activator for the degradation of organic pollutants

Metal leaching in perovskite-based catalysts during peroxymonosulfate activation processes can severely restrict their application in wastewater treatment. Therefore, enhancing the stability of perovskite nanostructures is crucial to improve catalytic performance and broaden applications but has been rarely achieved so far. We developed a scalable method to synthesize novel stable and environmentally-friendly nanocomposites of LaCoO3 and few-layer graphene (consisting of roughly-nine layers) for the removal of organic pollutants from wastewater. With abundant oxygen vacancies and synergistic effects between LaCoO3 and few-layer graphene, the novel LaCoO3/graphene catalyst exhibits outstanding catalytic degradation (>99 %) of diclofenac, metoprolol, carbamazepine, and bisphenol A at a high concentration (40 mg/l) in less than 10 min in the peroxymonosulfate activation system, with mineralization of 57, 55, 61, and 62 %, respectively. The LaCoO3/graphene catalyst exhibited excellent reusability and high catalytic performance within a wide pH range (3–11). The formation of LaCoO3/graphene composites prevents cobalt leaching (0.004 mg/l), stabilizes sub-stoichiometric LaCoO3 and thus increases the content of Co2+ in the structure, leading to much higher catalytic activity than that of pure LaCoO3. Electron paramagnetic resonance and radical quenching experiments revealed that both radical pathways (SO4[rad]−, [rad]OH, and O2[rad]−) and non-radical pathways (1O2) contribute to bisphenol A degradation and the relative contributions of [rad]OH, SO4[rad]−, and 1O2/O2[rad]− were determined to 13.4, 32.6, and 54 % for bisphenol A removal, respectively. Overall, our results indicate that LaCoO3/graphene is a promising material towards peroxymonosulfate activation for environmental remediation