The chemical modification of polymer nanofiber-based ammonia sensors by introducing dopants into the active layers has been proven as one of the low-cost routes to enhance their sensing performance. Herein, we investigate the influence of different citric acid (CA) concentrations on electrospun polyvinyl acetate (PVAc) nanofibers coated on quartz crystal microbalance (QCM) transducers as gravimetric ammonia sensors. The developed CA-doped PVAc nanofiber sensors are tested against various concentrations of ammonia vapors, in which their key sensing performance parameters (i.e., sensitivity, limit of detection (LOD), limit of quantification (LOQ), and repeatability) are studied in detail. The sensitivity and LOD values of 1.34 Hz ppm⁻¹ and 1 ppm, respectively, can be obtained during ammonia exposure assessment. Adding CA dopants with a higher concentration not only increases the sensor sensitivity linearly, but also prolongs both response and recovery times. This finding allows us to better understand the dopant concentration effect, which subsequently can result in an appropriate strategy for manufacturing high-performance portable nanofiber-based sensing devices.

In this paper, we study the effect of solution pH in the hydrothermal synthesis and post-annealing treatment on the photocatalytic performance of hexagonal prism ZnO grown without a seed layer. By varying the precursor molarity ratio, the solution obtained was 6.88 (ZnO-2), 7.00 (ZnO-1), and 7.58 (ZnO-3). The three samples show hexagonal prism ZnO with wurtzite structures based on scanning electron microscope and x-ray diffraction analysis. ZnO-1 samples could degrade methylene blue as high as 65.9% for a UV irradiation time of 5 h, better than ZnO-2 (51.80%) and ZnO-3 (57.55%). Post-annealing treatment of ZnO-1 with the best photodegradation efficiency was carried out at 200 °C (ZnO-4) and 400 °C (ZnO-5) to alter the structure. The post-annealing treatment changes the domination of crystal orientation from (002) to (100) plane. Also, the morphology of ZnO-5 changed significantly to become smaller rods with a diameter of 2.79 μm, as compared to ZnO-1 (2.83 μm) and ZnO-4 (3.12 μm). It is due to ionic rearrangements occurring at higher temperatures. The ZnO-5 sample reduces methylene blue by 82.91%, which is better than ZnO-1 (65.9%) and ZnO-4 (64.39%). Interestingly, we found a relation between smaller rod diameters and higher photocatalytic activity. The results show the importance of the solution pH and the annealing treatment in improving the photocatalytic performance of hexagonal prism ZnO without the seed layer.

The increasing prevalence of organic pollutants in wastewater poses a significant environmental challenge due to their persistence and harmful effects. Photocatalysis using semiconductor nanoparticles, such as ZnO, has emerged as a promising approach for pollutant degradation, but optimizing the structural and functional properties of these materials remains a critical challenge. In this study, ZnO nanoparticles were synthesized via a hydrothermal method with varying durations (4, 6, and 8 hours) to investigate the impact of synthesis time on their photocatalytic efficiency. The structural and compositional properties were characterized using SEM, XRD, and EDS analyses, revealing that longer synthesis times improve crystallinity and alter the Zn:O atomic ratio, affecting defect density and stoichiometry. Photocatalytic performance was evaluated through the degradation of an organic pollutant under UV illumination. ZnO-6h exhibited the highest rate constant (k=0.017 min−1), outperforming ZnO-4h (k=0.016 min−1) and ZnO-8h (k=0.013 min−1). This superior activity is attributed to an optimal combination of high crystallinity, intermediate morphology, and the presence of oxygen vacancies that enhance charge carrier dynamics. The findings demonstrate that synthesis duration is a critical parameter in tuning the structural and photocatalytic properties of ZnO nanoparticles. This study provides insights into the design of ZnO-based photocatalysts and underscores their potential for environmental remediation. Future research could extend these findings by exploring scalability and pollutant-specific applications, paving the way for more efficient wastewater treatment technologies.