In this research project, we experimentally realized the incorporated AgNP/CQD nanoparticles with regular SiNW arrays as efficient photocatalysts that demonstrated the remarkably improved photodegradation performances. By examining the morphology, microstructure, crystallinity, chemical feature, surface groups, light-emitting and reflection characteristics, these hybrid heterostructures are systematically identified. Moreover, the involving degradation kinetics, band diagram, cycling capability and underlying mechanism of photodegradation are investigated, validating their remarkable and reliable photocatalytic performances contributed from the strongly reduced light reflectivity, superior capability of charge separation and sound wettability with dye solutions. On the basis of such green-formation photocatalytic heterostructures, along with investigations on degradation kinetics, band structures and possessing mechanism, we anticipate that these can further pave many opportunities for practical water treatment, dye detoxification and other functional applications.
All-solution based method is presented for the preparation of highly efficient photocatalysts through the strategy of heterostructure design. Organic dyes, one of the dominant water pollutants, cause the serious destruction to the ecosystems. Degradation of organic dyes using photocatalysts under light illuminations has been considered the efficient and reliable treatment that can ecofriendly degrade the harmful dyes to CO2 and water. To realize the broadband photocatalysts that can activate the dye degradation covering the wide spectral regions from ultraviolet (UV) to visible illuminations, optical management of heterostructures for accomplishing the efficient light absorption of both UV and visible wavelengths turns out to be critically essential. Herein, the well incorporation of Ag nanoparticles (AgNPs) as outer sides with carbon nanodots (CQDs) as inner cores functioning as the broadband absorbers is demonstrated, where the AgNP shells enable to absorb the visible lights centered as 410 nm due to the existence of localized surface plasmon resonance, and the CQD shells essentially facilitate the remarkable light absorption at UV regions. In addition, the as-synthesized hybrid CQD/AgNP nanostructures are uniformly decorated on Si nanowire (SiNW) arrays. Such regulated features allow the well separation of CQD/AgNP nanoparticles rather than the unwanted aggregation, which supports the tailored absorption of broadband incoming lights for sustaining the photodegradation of organic dyes in efficient and reliable way. Moreover, the involving degradation kinetics, cycling capability and underlying mechanism are investigated thoroughly.
Figure 1 Proposed mechanism of photodegradation of MB dyes using heterostructure-based photocatalysts.
Photocatalytic degradation of methylene blue (MB dyes) were tested using a PanChum multilamp photoreactor (PR-2000) under the switchable lamp sources. In each measurement, the samples with fixed size of 1.5 cm by 1.5 cm were immersed in the dye solutions (0.2 mM, 10 ml in volume) with a constant magnetic stir. The solutions were placed in the dark condition for 1.5 h to reach the adsorption equilibrium, and then transferred to the photoreactor for initiating light-excited photocatalytic process. To measure the photodegradation result, 0.1 ml of suspension was withdrawn and the involved concentrations were characterized with a UV/visible spectrophotometer (Shimadzu UV-2401 PC).
Figure 2 (a) Schematic illustrations for the formantion of CQD/AgNP nanoparticles. (b) Correlated TEM image and (c) HRTEM image of CQD/AgNP nanoparticles. The inset of Figure 1b presents the particle distribution with Gaussian fitting.
The degradation efficiency of MB dyes in the presence of SiNW-based photocatalysts with variations of CQD/AgNP contents were examined. In sole SiNWs, the trivial photodegradation efficiency can be found, which is because the involvement of rapid charge recombination appearing in Si crystals that are capable of initiating the oxidative process for dye degradation. To overcome this issue, the incorporation CQD/AgNPs with SiNWs is found to be promising, where the greatly improved photodegradation efficiency of MB dyes is achieved. Foremost is the introduction of 20 mg of CQD/AgNP nanostructures as the optimal condition for preparing the hybrid CQD/AgNP@SiNWs, whereas the introduction of more amount of CQD/AgNPs (24 mg) turn to suppress the degradation performance due to the significant aggregation of CQD/AgNPs on SiNW surfaces that may impede the illumination lights entering the photocatalysts. From kinetic model, we found that that the rate constant of hybrid CQD/AgNP@SiNWs is more than 2.3 times and 21 times than CQD@SiNWs and sole SiNWs, respectively, behaving the superior photocatalysts for treating the MB dyes in the aqueous media.
Figure 3 (a) Photodegradation results of MB dyes in the presence of SiNWs, CQD@SiNWs and CQD/AgNP@SiNWs with three various loading amounts of CQD/AgNP nanoparticles, respectively. (b) Kinetic investigations and (c) rate constant of MB degradation in the presence of CQD/AgNP@SiNWs (20 mg of CQD/AgNP loadings). (d) Cycling tests of CQD/AgNP@SiNW photocatalysts.
Extensive investigations of incorporated CQD/AgNP nanoparticles with regular SiNW arrays made via a facile synthetic method are presented for remarkably improving the photodegradation performances of MB dyes. The low light reflectivity of 2.21%, superior capability of charge separation, sound wettability in a highly stable form with dye solutions of these intriguing CQD/AgNP@SiNW photocatalysts are identified. By adjusting the amount of adding CQD/AgNP nanoparticles, the correlated photodegradation efficiency can be dramatically improved, and these photocatalytic features for dye removal are highly stable, showing the unchanged photodegradation behaviors for four-times cycling utilization.
Figure 4 (a) Scavenger examinations of photodegradation process using CQD/AgNP@SiNWs as photocatalysts. (b) Correlated band diagram of hybrid heterostructures.