Synthesis, properties, and formation mechanism of Mn-doped Zn₂SiO₄nanowires and associated heterostructures

Abstract

In this work, we have put forth a facile hydrothermal approach to synthesize an array of one-dimensional (1D) Mn-doped Zn2SiO4 nanostructures. Specifically, we have probed and correlated the effects of controllable reaction parameters such as the pH and Mn dopant concentrations with the resulting crystal structures and morphologies of the products obtained. Based upon our results, we find that careful tuning of the pH versus the Mn dopant level gives rise to opposite trends with respect to the overall size of the resulting one-dimensional nanostructures. Significantly, we have highlighted the role of the Mn dopant ion concentration as a potentially generalizable reaction parameter in solution-based synthesis for controlling morphology and hence, the observed optical behavior. Indeed, such a strategy can be potentially generalized to systems such as but not limited to Mn-doped ZnS, CdS, and CdSe quantum dots (QD), which, to the best of our knowledge, denote promising candidates for a variety of optoelectronic applications. Specifically, we have carefully optimized the synthesis conditions in order to generate a series of chemically well-defined Mn-doped Zn2SiO4 not only possessing Mn concentrations ranging from 3% to 8% but also characterized by highly crystalline, monodisperse wire-like motifs measuring ∼30 nm in diameter and ∼700 nm in length. Optically, the photoluminescence signals associated with the 1D series yielded a volcano-shaped relationship between PL intensities and the Mn dopant level. In additional experiments, we have immobilized CdSe quantum dots (QDs) onto the external surfaces of our as-synthesized Mn-doped Zn2SiO4 nanowires, in order to form novel composite heterostructures. The optical properties of the CdSe QD-Mn:Zn2SiO4 heterostructures have been subsequently examined. Our results have demonstrated the likely co-existence of both energy transfer and charge transfer phenomena between the two constituent components of our as-prepared composites. Specifically, when both components are photoexcited, both energy transfer and charge transfer were found to plausibly occur, albeit in opposite directions. When the CdSe QDs are excited alone for example, charge transfer probably takes place from the CdSe QDs to the dopant Mn2+ ions. We believe that our as-processed heterostructures are therefore promising as a tunable light-harvesting motif. Essentially, these materials have broadened the effective light absorption range for optical 'accessibility', not only through their incorporation of dopant-tunable Zn2SiO4 possessing complementary absorption properties to those of the QDs but also through their integration of CdSe QDs with size-tailorable optical behavior.

Effect of Mn2+ incorporation on the photoelectrochemical properties of BiVO4

“…The oxidation state of Mn 2+ was confirmed using XPS analysis, which can be related to the 4 T 1 to 6 A 1 transitions in Mn 2+ . [46][47][48] The illumination of samples would lead to the separation of charge carriers in BiVO 4 . Since the 4 T 1 state of Mn 2+ is located below the CB of BiVO 4 , 46 it suggests favorable photogenerated electron transfer from BiVO 4 to the 4 T 1 state of Mn 2+ from where the electron will be transferred to ITO and thus will move to the extended circuit.…”

“…[46][47][48] The illumination of samples would lead to the separation of charge carriers in BiVO 4 . Since the 4 T 1 state of Mn 2+ is located below the CB of BiVO 4 , 46 it suggests favorable photogenerated electron transfer from BiVO 4 to the 4 T 1 state of Mn 2+ from where the electron will be transferred to ITO and thus will move to the extended circuit. Mehmood's group 49 have reported the CuInSe 2 /Mn-CdS QDs, where they suggested that the photogenerated electrons stay for a longer time in Mn the d-states and thus help in reducing the recombination of electronhole pairs in the system.…”

Spontaneous Atomic Sites Formation in Wurtzite CoO Nanorods for Robust CO2 Photoreduction

“…Compared with the unlabeled CO 2 , the m/z values of CH 4 and CO produced from 13 C-labeled CO 2 molecules were 17 and 29, respectively, which confirms that the carbon source of CO and CH 4 originates from the photoreduction process of CO 2 . [58] Besides the 13 C isotope labeling experiments, 18 O isotope labeling was also conducted to confirm the half-reaction of oxidation. Shown as Figure S21, Supporting Information, the presence of 16 O 18 O and 18 O 2 in the 18 O labeling experiment verifies that O 2 originated from water oxidation.…”

“…[58] Besides the 13 C isotope labeling experiments, 18 O isotope labeling was also conducted to confirm the half-reaction of oxidation. Shown as Figure S21, Supporting Information, the presence of 16 O 18 O and 18 O 2 in the 18 O labeling experiment verifies that O 2 originated from water oxidation. [59] Further CO 2 photoreduction of all prepared photocatalysts was also performed under simulated sunlight irradiation in a high pressure (10 atm) reactor system.…”

“…[ 12,13 ] To this end, solution‐based thermal decomposition offers powerful opportunities to construct nano‐catalysts with well‐controlled morphologies and sizes, in a “bottom‐up” manner. [ 14–16 ] Nevertheless, when incorporating a dopant precursor into a known host synthetic system, the morphology of the resultant product often changes drastically compared to its host counterparts, [ 17–20 ] and severe phase separation may even occur, due to the more complex thermodynamically controlled process. [ 21–23 ] This poses a huge challenge in using a solution‐based strategy for well‐defined single‐atom catalysts with both surface and volumetric atomic doping, as well as maintaining the nanoscale structure.…”