Core/shell nanorod arrays of ZnO/CdS have been synthesized with varying shell thickness and their shell thickness dependent photocatalytic properties have been investigated. Core/shell nanorod arrays of core diameter of 100 nm with variable shell thickness (10–30 nm) are synthesized by varying the concentration of the citric acid. XRD analysis reveals that tensile strain is obtained for ZnO nanorods and the compressive strain is obtained for core/shell nanorods. The UV–visible absorption spectra of the core/shell nanorod arrays show a red shift of the band edge of uncoated ZnO with shell growth. Steady-state photoluminescence (PL) spectra of the core/shell nanorod arrays show red shift of emission band with the increase in shell thickness. Decay kinetics indicate that the average lifetime (⟨τ⟩) of the core/shell nanorod arrays is larger than that of the uncoated ZnO nanorods due to charge separation. I–V studies show a 16-fold enhancement in current using the ZnO/CdS core/shell nanorod arrays having CdS shell thickness of 30 nm as compared to bare ZnO nanorods. The photocatalytic studies confirmed that the ZnO/CdS core/shell nanorod arrays exhibit improved degradation efficiency compared to bare ZnO and CdS under simulated solar radiation. The core/shell nanorods having shell (CdS) thickness of 30 nm displays the highest photocatalytic efficiency for the degradation of rhodamine B under simulated solar radiation, indicating efficient separation of electron–hole pairs. The mechanism of the photodegradation of RhB is given to elucidate the efficiency enhancement of ZnO/CdS photocatalysts. These results demonstrate that the ZnO/CdS core/shell nanorod arrays provide a facile and compatible frame for potential applications in nanorod-based solar cells and as efficient photocatalysts.
We report an efficient and environmentally benign onedimensional ZnO/In 2 S 3 core/shell nanostructure to be used as a photocatalyst to overcome the drawback of low photocatalytic efficiency brought by electron−hole recombination and narrow photoresponse range. In 2 S 3 nanoparticles were successfully grown on the ZnO nanorods by the surface functionalization method using citric acid as a surfacefunctionalizing agent. The photocatalytic activity under visible light to degrade rhodamine B (RhB) was enhanced by these core/shell nanostructures due to the formation of heterojunctions, which prolong the separation of photogenerated electrons and holes. The photocatalytic degradation of RhB dye was found to be 83.7, 2.7, and 35.0% for the ZnO/In 2 S 3 core/shell nanorod arrays, ZnO nanorods, and In 2 S 3 nanoparticles, respectively, under visible light irradiation. Photoconductivity studies showed a 6-fold enhancement in photocurrent (compared to the dark current) for the core/shell nanorod arrays. Moreover, these core/shell nanostructures could be reused for degradation of RhB during a three-cycle experiment without significant decrease in the photocatalytic activity which is very important for these ZnO/In 2 S 3 core/shell nanorod arrays to be of practical use in environmental applications.
Expanding the light-harvesting range and suppressing the quick recombination of photogenerated charge carriers are of paramount significance in the field of photocatalysis. One possible approach to achieve wide absorption range is to synthesize type-II core/shell heterostructures. In addition, this system also shows great promise for fast separation of charge carriers and low charge recombination rate. Herein, following the surface functionalization method using 3-mercaptopropionic acid (MPA) as a surface functionalizing agent, we report on designing NaNbO3/CdS type-II core/shell heterostructures with an absorption range extending to visible range and explore the opportunity toward degradation of methylene blue (MB) dye as a model pollutant under visible light irradiation. Characterizations including X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), UV-vis diffuse reflectance spectrum (DRS), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), and Raman spectroscopy support the growth of CdS shell onto NaNbO3 nanorods. The resulting core/shell heterostructures unveiled high surface areas, enhanced light harvesting, and appreciably increased photocatalytic activity toward MB degradation compared to individual counterparts and the photocatalytic standard, Degussa P25, under visible light irradiation. The remarkably enhanced photocatalytic activity of core/shell heterostructures could be interpreted in terms of efficient charge separation owing to core/shell morphology and resulting type-II band alignment between NaNbO3 and CdS, which creates a step-like radial potential favoring the localization of one of the carriers in the core and the other in the shell. A plausible mechanism for the degradation of MB dye over NaNbO3/CdS core/shell heterostructures is also elucidated using active species scavenger studies. Our findings imply that hydroxyl radicals (OH(•)) play a crucial role in dictating the degradation of MB under visible light. This work highlights the importance of core/shell heterostructures in leading toward new paradigms for developing highly efficient and reusable photocatalysts for the destructive oxidation of recalcitrant organic pollutants.
An enduring impediment in the photocatalysis domain is the rapid recombination of photoinduced charge carriers. One viable strategy to realize efficient separation of photoinduced charge carriers is to design core/shell nanostructures. In this context, our work explains the substantial separation of photocarriers and enhanced light harvesting in TiO 2 nanostructures following the realization of core/shell geometry with CuS. We demonstrate the design of the TiO 2 /CuS core/shell nanostructures, utilizing a surface-functionalizing agent, 3-mercaptopropionic acid, and offering commendable visible light driven photocatalytic performance for degradation of virulent organic pollutants of dye wastewater, like methylene blue (MB). To validate the merits of the TiO 2 /CuS core/shell nanostructures, we have also designed TiO 2 /CuS composite nanostructures under similar conditions (without utilizing the surface-functionalizing agent, 3-mercaptopropionic acid). Successful realization of TiO 2 /CuS nanostructures (core/shell and composite) was concluded from the PXRD, FESEM, TEM, HRTEM, EDS elemental mapping, and DRS studies. The resulting core/shell nanostructures have propitious photocatalytic performance (∼90%) over composite nanostructures (∼58%), which could be scrutinized in terms of core/shell geometry, maximizing the interfacial contact between TiO 2 and CuS and enabling retardation in the recombination rate of the photoinduced charge carriers by confining electrons mainly in one component (core) and holes in the other component (shell).To have the best photocatalytic performance from the TiO 2 /CuS core/shell nanostructures, we also determined the optimum amount of photocatalyst (0.3 g/L) and organic pollutant dye concentration (0.003 g/L) required for visible light driven degradation of MB. A credible mechanism of the charge transfer process and mechanism of photocatalysis supported from trapping experiments in the TiO 2 /CuS nanostructures for the degradation of an aqueous solution of MB is also explicated. Degradation intermediates analysis performed using mass spectroscopy (MS) studies showed that MB dye degradation is initiated by a demethylation pathway. Our work also highlights the stability and recyclability of a core/shell nanostructures photocatalyst and supports its potential for environmental applications. We thus anticipate that our results bear broad potential in the photocatalysis domain for the design of a visible light functional and reusable core/shell nanostructures photocatalyst.
Band gap engineering offers tunable optical and electronic properties of semiconductors in the development of efficient photovoltaic cells and photocatalysts. Our study demonstrates the band gap engineering of ZnO nanorods to develop a highly efficient visible-light photocatalyst. We engineered the band gap of ZnO nanorods by introducing the core/shell geometry with Ag2S sensitizer as the shell. Introduction of the core/shell geometry evinces great promise for expanding the light-harvesting range and substantial suppression of charge carrier recombination, which are of supreme importance in the realm of photocatalysis. To unveil the superiority of Ag2S as a sensitizer in engineering the band gap of ZnO in comparison to the Cd-based sensitizers, we also designed ZnO/CdS core/shell nanostructures having the same shell thickness. The photocatalytic performance of the resultant core/shell nanostructures toward methylene blue (MB) dye degradation has been studied. The results imply that the ZnO/Ag2S core/shell nanostructures reveal 40- and 2-fold enhancement in degradation constant in comparison to the pure ZnO and ZnO/CdS core/shell nanostructures, respectively. This high efficiency is elucidated in terms of (i) efficient light harvesting owing to the incorporation of Ag2S and (ii) smaller conduction band offset between ZnO and Ag2S, promoting more efficient charge separation at the core/shell interface. A credible photodegradation mechanism for the MB dye deploying ZnO/Ag2S core/shell nanostructures is proposed from the analysis of involved active species such as hydroxyl radicals (OH(•)), electrons (e(-)(CB)), holes (h(+)(VB)), and superoxide radical anions (O2(•-)) in the photodegradation process utilizing various active species scavengers and EPR spectroscopy. The findings show that the MB oxidation is directed mainly by the assistance of hydroxyl radicals (OH(•)). The results presented here provide new insights for developing band gap engineered semiconductor nanostructures for energy-harvesting applications and demonstrate Ag2S to be a potential sensitizer to supersede Cd-based sensitizers for eco-friendly applications.
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