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.
In this report, we examine the composition- and size-dependent performance in hierarchical Pd1–x Au x nanowires (NWs) encapsulated with a conformal Pt monolayer shell (Pt∼Pd1–x Au x ). The ultrathin ...Pd1–x Au x NWs are prepared by a solution-based method wherein the chemical composition can be readily and predictably controlled. Importantly, as-prepared Pd9Au NWs maintain significantly enhanced oxygen reduction reaction (ORR) activity (0.40 mA/cm2), as compared with elemental Pd NW/C (0.12 mA/cm2) and Pt nanoparticles (NP)/C (0.20 mA/cm2), respectively. After the deposition of a Pt monolayer, a volcano-type composition dependence is observed in the ORR activity of the Pt∼Pd1–x Au x NWs as the Au content is increased from 0 to 30% with the activity of the Pt∼Pd9Au NWs (0.98 mA/cm2, 2.54 A/mgPt), representing the optimum performance. We note that the platinum group metal activity of the ultrathin 2 nm NWs (0.64 A/mg) is significantly enhanced as compared with that of analogous 50 nm NWs (0.16 A/mg) and commercial Pt NP/C (0.1–0.2 A/mg), thereby highlighting a distinctive size-dependent enhancement in NW performance.
We report on the synthesis, structural characterization, and intrinsic charge transfer processes associated with novel luminescent zero-dimensional (0D) CdSe nanocrystal–one-dimensional (1D) CePO4:Tb ...nanowire composite heterostructures. Specifically, ∼4 nm CdSe quantum dots (QDs) have been successfully anchored onto high-aspect ratio CePO4:Tb nanowires, measuring ∼65 nm in diameter and ∼2 μm in length. Composite formation was confirmed by high-resolution transmission microscopy, energy-dispersive X-ray spectroscopy mapping, and confocal microscopy. Photoluminescence (PL) spectra, emission decay, and optical absorption of these nanoscale heterostructures were collected and compared with those of single, discrete CdSe QDs and CePO4:Tb nanowires. We show that our composite heterostructure evinces both PL quenching and a shorter average lifetime as compared with unbound CdSe QDs and CePO4:Tb nanowires. We propose that a photoinduced 0D–1D charge transfer process occurs between CdSe and CePO4:Tb and that it represents the predominant mechanism, accounting for the observed PL quenching and shorter lifetimes noted in our composite heterostructures. Data are additionally explained in the context of the inherent energy level alignments of both CdSe QDs and CePO4:Tb nanowires.
Doped motifs offer an intriguing structural pathway toward improving conductivity for battery applications. Specifically, Ca-doped, three-dimensional “flower-like” Li4–x Ca x Ti5O12 (“x” = 0, 0.1, ...0.15, and 0.2) micrometer-scale spheres have been successfully prepared for the first time using a simple and reproducible hydrothermal reaction followed by a short calcination process. The products were experimentally characterized by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) mapping, inductively coupled plasma optical emission spectrometry (ICP-OES), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge–discharge testing. Calcium dopant ions were shown to be uniformly distributed within the LTO structure without altering the underlying “flower-like” morphology. The largest lattice expansion and the highest Ti3+ ratios were noted with XRD and XPS, respectively, whereas increased charge transfer conductivity and decreased Li+-ion diffusion coefficients were displayed in EIS for the Li4–x Ca x Ti5O12 (“x” = 0.2) sample. The “x” = 0.2 sample yielded a higher rate capability, an excellent reversibility, and a superior cycling stability, delivering 151 and 143 mAh/g under discharge rates of 20C and 40C at cycles 60 and 70, respectively. In addition, a high cycling stability was demonstrated with a capacity retention of 92% after 300 cycles at a very high discharge rate of 20C. In addition, first-principles calculations based on density functional theory (DFT) were conducted with the goal of further elucidating and understanding the nature of the doping mechanism in this study. The DFT calculations not only determined the structure of the Ca-doped Li4Ti5O12, which was found to be in accordance with the experimentally measured XPD pattern, but also yielded valuable insights into the doping-induced effect on both the atomic and electronic structures of Li4Ti5O12.
One emergent property of ferroelectric nanoparticles is the sized-induced structural distortion to a high-symmetry paraelectric phase at small particle sizes. Finite length scale effects can thus be ...advantageously employed to elucidate ferroelectric transition mechanisms. In this work, we combine infrared spectroscopy with group theory and lattice dynamics calculations to reveal the displacive nature of the ferroelectric transition in BiFeO3, a room temperature multiferroic. Systematic intensity and frequency trends in selected vibrational modes show that the paraelectric phase is Pm3̅m and the lowest frequency A1 feature is the soft mode that drives the first order transition. Finite length scale effects are also evident in the electronic structure with a red-shifted band gap in nanoscale BiFeO3 compared with that of the rhombohedral film, a result that can impact the development of ferroelectric photovoltaics and oxide-based electronics. Taken together, these findings demonstrate the foundational importance of size effects for enhancing the rich functionality and broad utility of transition metal oxides.
Single-crystalline perovskite nanostructures with reproducible shape have been prepared using a simple, readily scaleable solid-state reaction in the presence of NaCl and a nonionic surfactant. ...Pristine BaTiO3 nanowires have diameters ranging from 50 to 80 nm with an aspect ratio larger than 25. Single-crystalline SrTiO3 nanocubes with a mean edge length of 80 nm have been produced using a similar procedure.
In this report, we utilize the U-tube double diffusion device as a reliable, environmentally friendly method for the size-controlled synthesis of high-quality, single crystalline Pd nanowires. The ...nanowires grown in 200 and 15 nm polycarbonate template pores maintain diameters of 270 ± 45 nm and 45 ± 9 nm, respectively, and could be isolated either as individual nanowires or as ordered free-standing arrays. The growth mechanism of these nanowires has been extensively explored, and we have carried out characterization of the isolated nanowires, free-standing nanowire arrays, and cross sections of the filled template in order to determine that a unique two-step growth process predominates within the template pores. Moreover, as-prepared submicrometer and nanosized wires were studied by comparison with ultrathin 2 nm Pd nanowires in order to elucidate the size-dependent trend in oxygen reduction reaction (ORR) electrocatalysis. Subsequently, the desired platinum monolayer overcoating was reliably deposited onto the surface of the Pd nanowires by Cu underpotential deposition (UPD) followed by galvanic displacement of the Cu adatoms. The specific and platinum mass activity of the core–shell catalysts was found to increase from 0.40 mA/cm2 and 1.01 A/mg to 0.74 mA/cm2 and 1.74 A/mg as the diameter was decreased from the submicrometer size regime to the ultrathin nanometer range.
In this article, we address two key challenges in the development of electrocatalysts for direct methanol fuel cells by rationally tailoring the morphology and chemical composition of Pd-based ...nanowires (NWs) for enhanced performance. First, we have examined the morphology and composition-dependent performance of Pt1–x Pd x NWs toward the methanol oxidation reaction (MOR). Elemental Pt NWs were found to possess a significant morphology-dependent enhancement of nearly 3-fold in terms of peak MOR-specific activity over that of commercial Pt NP/C. In addition, tailoring the chemical composition in Pt1–x Pd x NWs can lead to measurable increases in MOR kinetics, which can be attributed to improved oxidation of formic acid and, potentially, increased selectivity for a direct, CO-free pathway. Second, we have explored the stability of ORR performance in the presence of measurable concentrations of methanol as a function of chemical composition in Pt1–x Pd x NWs and Pt-free Pd9Au NWs. In the context of the Pt1–x Pd x NWs, a distinctive volcano-type dependence has been noted with respect to chemical composition, and on the basis of the MOR activities and methanol tolerant ORR behavior, Pt7Pd3 NWs have been highlighted as an optimal catalyst architecture. We have also analyzed the methanol tolerance in Pd9Au NWs, which represents a highly active, durable Pt-free alternative to traditional Pt-based nanostructured catalysts. Herein, we have demonstrated that Pd9Au NWs (0.42 mA/cm2) with no effective Pt content can outperform Pt-based nanostructures, such as Pt NWs (0.32 mA/cm2) and nanoparticulate Pt NP/C (0.24 mA/cm2) in the presence of 4 mM methanol/0.1 M HClO4.
Oxidized single-walled carbon nanotubes (SWNTs) have been reacted with cadmium selenide (CdSe) nanocrystals, capped with mercaptothiol derivatives, as well as with titanium dioxide (TiO2) ...nanocrystals, functionalized with 11-aminoundecanoic acid to form nanoscale heterostructures, characterized by transmission electron microscopy and infrared spectroscopy. The reaction with acid-terminated CdSe nanocrystals and acid-terminated tubes was facilitated with the aid of intermediary linking agents, such as ethylenediamine and semicarbazide, in an amide-forming reaction in the presence of 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride, EDC. Based on electronic absorption spectroscopy, charge transfer is thought to proceed from the nanocrystal to the nanotube in the CdSe−nanotube system, whereas in the TiO2−nanotube system, charge transfer is expected to occur from the nanotube to the nanocrystal.
PDF
