We report on the oxygen vacancy induced ferromagnetism (FM) at and above room temperature in undoped TiO
2
nanoporous nanoribbons synthesized by a solvothermal route. The origin of FM in ...as-synthesized and vacuum annealed undoped nanoribbons grown for different reaction durations followed by calcinations was investigated by several experimental tools. X-Ray diffraction pattern and micro-Raman studies reveal the TiO
2
(B), TiO
2
(B)-anatase, and anataserutile mixed phases of TiO
2
structure. Field emission scanning electron microscopy and transmission electron microscopy observations reveal nanoribbons with uniform pore distribution and nanopits/nanobricks formed on the surface. These samples exhibit strong visible photoluminescence associated with oxygen vacancies and a clear ferromagnetic hysteresis loop, both of which dramatically enhanced after vacuum annealing. Direct evidence of oxygen vacancies and related Ti
3+
in the as-prepared and vacuum annealed TiO
2
samples are provided through X-ray photoelectron spectroscopy analysis. Micro-Raman, infrared absorption and optical absorption spectroscopic analyses further support our conclusion. The observed room temperature FM in undoped TiO
2
nanoribbons is quantitatively analyzed and explained through a model involving bound magnetic polarons (BMP), which include an electron locally trapped by an oxygen vacancy with the trapped electron occupying an orbital overlapping with the unpaired electron (3d
1
) of Ti
3+
ion. Our analysis interestingly shows that the calculated BMP concentration scales linearly with concentration of oxygen vacancies and provides a stronger footing for exploiting defect engineered ferromagnetism in undoped TiO
2
nanostructures. The development of such highly porous TiO
2
nanoribbons constitutes an important step towards realizing improved visible light photocatalytic and photovoltaic applications of this novel material.
Defect engineered nanoporous TiO
2
nanoribbons exhibiting room temperature ferromagnetism, visible absorption and photoluminescence show promises for spintronics and photocatalytic applications.
The formation of a heterostructure with plasmonic nanoparticles drastically alters the optoelectronic properties of graphene quantum dots (GQDs), resulting in exceptional properties. In the present ...work, we prepare nitrogen-doped GQDs decorated on gold nanoparticles (Au@N-GQDs) by a one-step green reduction method and study its extraordinary fluorescence and photoresponse characteristics. The as-prepared Au@N-GQDs show more than one order of magnitude enhancement in the fluorescence intensity as compared to the bare N-GQDs, which is attributed to hot electron generation and improved absorption in N-GQDs by local field enhancement and the modification of the edge functional groups. Because of the selective coordination to Fe3+ ions, the Au@N-GQDs exhibit extraordinary quenching of fluorescence, with ultrahigh sensitivity for the detection of Fe3+ (<1 nM). A new model for the charge-transfer dynamics is developed involving the Langmuir’s law of adsorption to explain the unusual quenching, which strongly deviates from the known models of static/dynamic quenching. The proposed sensor is successfully implemented for the ultrasensitive detection of Fe3+ ions in human serum and Brahmaputra river water samples, representing its high potential applications in clinical as well as environmental diagnosis. Additionally, because of its high absorption in the UV–vis–NIR region and high charge density with long life excitons, the Au@N-GQDs are utilized as photodetectors with ∼104 times faster response than that of bare N-GQDs. The Au@N-GQD-based photodetector possesses a high responsivity of ∼1.36 A/W and a remarkably high external quantum efficiency of ∼292.2%, which is much superior to the GQD-based photodetectors reported till date. The underlying mechanism of ultrafast photoresponse is ascribed to the transfer of hot electrons along with the tunneling of the electrons from Au NPs to N-GQDs as well as the defect reduction of N-GQDs by the incorporation of Au NPs. Without the use of any charge transporting layer, the outstanding performance of N-GQD-based plasmonic photodetector opens up unique opportunities for future high-speed optoelectronic devices.
Precise control of the thickness of large-area two-dimensional (2D) organometal halide perovskite layers is extremely challenging owing to the inherent instability of the organic component. Herein, a ...novel, highly reproducible, and facile solvothermal route is reported to synthesize and tailor the thickness and optical band gap of the organic–inorganic halide perovskite nanosheets (NSs). Our study reveals that self-assembly of randomly oriented perovskite nanorods leads to the growth of multilayered perovskite NSs at ∼100 °C, while at higher temperature, large-area few-layer to bilayer 2D NSs (CH3NH3PbBr3) are obtained through lattice expansion and layer separation depending precisely on the temperature. Interestingly, the thickness of the 2D NSs shows a linear dependence on the reaction temperature and thus enables precise tuning of the thickness from 14 layers to 2 layers, giving rise to a systematic increase in the band gap and appearance of excitonic absorption bands. Quantitative analysis of the change in the band gap with thickness revealed a strong quantum confinement effect in the 2D layers. The perovskite 2D NSs exhibit tunable color and a high photoluminescence (PL) quantum yield (QY) up to 84%. Through a careful analysis of the steady-state and time-resolved PL spectra, the origin of the lower PL QY in thinner NSs is traced to surface defects in the 2D layers, for the first time. A white light converter was fabricated using the composition-tuned 2D CH3NH3PbBrI2 NS on a blue light-emitting diode chip. The 2D perovskite photodetector exhibits a stable and very fast rise/fall time (24 μs/103 μs) along with high responsivity and detectivity of ∼1.93 A/W and 1.04 × 1012 Jones, respectively. Storage, operational, and temperature-dependent stability studies reveal high stability of the 2D perovskite NSs under the ambient condition with high humidity. The reported method is highly promising for the development of large-area stable 2D perovskite layers for various cutting-edge optoelectronic applications.
Heterogeneous photocatalysis is of overriding significance for emerging energy and environment applications. Nanosized metal–semiconductor heterostructures (HSs) allow extraordinarily tuned and ...intense absorption of light, making it very promising for efficient solar energy harvesting in photovoltaic and photocatalytic applications. Here we report on the ultrahigh rate of photodegradation of organic dye, Rhodamine-B, with two distinct sequential degradation rate processes under visible light illumination on Ag nanoparticle (NP) decorated anatase TiO2 nanorods (NRs) grown by a solvothermal route. HRTEM analysis reveals the uniform decoration of Ag NPs (∼17 nm) over the TiO2 NRs surface for the optimized Ag@TiO2 HS. The defect rich Ag@TiO2 NR HSs grown with an optimal weight ratio Ag:TiO2 = 3:2 (TA32) exhibit very strong optical absorption due to the localized surface plasmon resonance (LSPR) effect over the entire visible region, having an absorption peak at ∼520 nm. The effective band gap of TA32 has been significantly reduced to 2.71 eV from the pure TiO2 band gap of 3.2 eV. Studies on photocatalysis of Rhodamine-B show a very high degradation rate for the HSs due to the LSPR effect in the noble Ag NPs and fast charge transfer at the Ag@TiO2 interface. The optimized heterostructure (TA32) exhibits nearly double plasmonic absorbance than the other HSs, and it shows the highest degradation rate under visible light irradiation. In contrast to the available models, in the present case the degradation kinetics follows a sequential rate process with two distinct exponential decay functions/rate constants. For the optimized HS (TA32), the degradation rate constants are found to be 0.083 (k 2) and 0.033 min–1 (k 1) in the second and first stages of degradation, respectively. The pseudo first order rate constant was >4 times higher in the first stage and ∼10 times stronger in the second stage of degradation for TA32 HS in comparison to the bare TiO2 NRs as well as commercial P25. Our results demonstrate the long-term stability and superiority of the Ag@TiO2 HS over the bare TiO2 NRs and other TiO2 based photocatalysts for detoxification of air/water. This study offers new insights in understanding the mechanism of advanced photocatalysis with multi rate constants by oxygen vacancy enriched Ag@TiO2 nanoheterostructures.
Phytochemicals belonging to the group of alkaloids are signature specialized metabolites endowed with countless biological activities. Plants are armored with these naturally produced nitrogenous ...compounds to combat numerous challenging environmental stress conditions. Traditional and modern healthcare systems have harnessed the potential of these organic compounds for the treatment of many ailments. Various chemical entities (functional groups) attached to the central moiety are responsible for their diverse range of biological properties. The development of the characterization of these plant metabolites and the enzymes involved in their biosynthesis is of an utmost priority to deliver enhanced advantages in terms of biological properties and productivity. Further, the incorporation of whole/partial metabolic pathways in the heterologous system and/or the overexpression of biosynthetic steps in homologous systems have both become alternative and lucrative methods over chemical synthesis in recent times. Moreover, in-depth research on alkaloid biosynthetic pathways has revealed numerous chemical modifications that occur during alkaloidal conversions. These chemical reactions involve glycosylation, acylation, reduction, oxidation, and methylation steps, and they are usually responsible for conferring the biological activities possessed by alkaloids. In this review, we aim to discuss the alkaloidal group of plant specialized metabolites and their brief classification covering major categories. We also emphasize the diversity in the basic structures of plant alkaloids arising through enzymatically catalyzed structural modifications in certain plant species, as well as their emerging diverse biological activities. The role of alkaloids in plant defense and their mechanisms of action are also briefly discussed. Moreover, the commercial utilization of plant alkaloids in the marketplace displaying various applications has been enumerated.
We investigate the formation mechanism of graphene quantum dots (GQDs) from a graphene oxide (GO) precursor and study the inter-conversion of edge states during thermal annealing of GQDs through ...various microscopic and spectroscopic tools. Monitoring the early stages of growth of GQDs reveals that the in-plane epoxy (C–O–C) functional groups attached at the defect sites essentially cut the oxidized GOs into small pieces of GQDs during the hydrothermal reaction. By conducting a series of controlled annealing experiments under oxygen and hydrogen gas environments, we monitor and quantify the inter-conversion of edge states and functional groups in GQDs from Raman and photoluminescence (PL) studies. The deconvolution of the Raman spectrum allowed us to monitor the evolution of new Raman bands at ∼1260 and ∼1438 cm −1 , which are assigned to the edge functional groups. Through the fitting of the PL spectra and a quantitative analysis of integrated intensities of PL, we unambiguously assign the blue PL emission bands at ∼407 and ∼440 nm to zigzag and armchair free edge states in GQDs, respectively, for the first time. On the other hand, the green emission bands at ∼490 and ∼530 nm are attributed to COOH/C–OH and CO/C–O edge functional groups, respectively. Our conclusions are corroborated by XPS and FTIR spectroscopic analyses of the as-grown and annealed GQDs.
•3% and 5% Ni doped ZnO nanoparticles are grown by a ball milling technique.•3% doped ZnO nanoparticles exhibit room temperature high magnetic moment that changes with milling time.•Optical and ...magnetic properties of doped ZnO nanoparticles are correlated well.•Mechanism of the magnetic interaction is quantitatively analyzed using a bound magnetic polar model.•Direct correlation between the defects in doped ZnO and magnetic moment is demonstrated.
We report on the room temperature ferromagnetism in the Zn1−xNixO (x=0, 0.03 and 0.05) nanoparticles (NPs) synthesized by a ball milling technique. X-ray diffraction analysis confirms the single crystalline, wurtzite ZnO structure for the 3% Ni doped ZnO NPs for higher milling time. HRTEM lattice image and SAED pattern show that the doped NPs are single crystalline with a d-spacing of 2.47Å corresponding to the (101) plane. Energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy confirm the presence of Ni ions inside the ZnO matrix with 2+ valance state. Room temperature magnetic measurements exhibit the hysteresis loop with saturation magnetization (Ms) of 1.6–2.56 (emu/g) and coercive field (Hc) of 260Oe. Micro-Raman studies illustrate doping/disorder induced additional Raman modes at ∼547, 574cm−1 in addition to 437cm−1 peak of pure ZnO. Photoluminescence (PL) spectra and UV–vis absorption measurements demonstrate some modification in the band edge emission and absorption characteristics, respectively. PL spectra also show defect related strong visible emission, which is believed to play a significant role in the FM ordering. These observations highlight the effect of changing defect density on the observed ferromagnetic moment values for the as synthesized Zn1−xNixO NPs. Magnetic interaction is quantitatively analyzed and explained using a bound magnetic polaron model and expected to arise from the intrinsic exchange interaction of Ni ions and OV, Zni defects. Systemic studies on the structural, magnetic, and optical properties reveal that both the nature of the defects as well as Ni2+ ions are significant ingredients behind attaining high moment as well as high ordering temperature in Ni doped ZnO NPs.
Herein, we investigate the role of Eu
3+
doping on CH
3
NH
3
PbBr
3
nanoplatelets (NPLs) in terms of their optoelectronic properties and photodetection application through a combined experimental and ...theoretical approach. The introduction of EuCl
3
in the CH
3
NH
3
PbBr
3
crystal structure by a facile solvothermal method enabled the tuning of the lateral and vertical dimensions of the NSs to form large-area NPLs and finally monolayer nanocrystals. The appearance of low-angle diffraction peaks with Eu doping, which are observed in layered perovskite structures, confirms the formation of quasi-2D NPLs. The bandgap of the Eu-doped mixed halide perovskite systematically increases from 2.39 eV to 2.94 eV with increasing doping concentration. Interestingly, 10 mol% EuCl
3
doping in the pure CH
3
NH
3
PbBr
3
crystal dramatically enhances its absorbance and photosensitivity, resulting in high-performance photodetection. Under 405 nm excitation, the CH
3
NH
3
Pb
0.9
Eu
0.1
Br
2.7
Cl
0.3
photodetector exhibits self-biased behavior with an on/off ratio >10
3
, which is very significant. The planar device achieves a responsivity as high as 5.29 A W
−
and a detectivity of 1.06 × 10
12
Jones under 405 nm with a power density of 0.14 mW cm
−2
at 5 V. In addition, the device exhibits very fast response time with a rise/fall time of 17.5/38.5 μs, which is ∼4 times faster than the pristine CH
3
NH
3
PbBr
3
counterpart. A linear relationship of photocurrent with light intensity in the CH
3
NH
3
Pb
0.9
Eu
0.1
Br
2.7
Cl
0.3
photodetector signifies low recombination or charge trapping loss. High-performance photodetection in the Eu-doped device is ascribed to the elimination of trap states and the fast charge transfer process. To obtain better insight into the doped system, DFT analysis of the electronic structure of EuCl
3
-doped CH
3
NH
3
PbBr
3
was performed and the results are fully consistent with the experimental findings. It was revealed that EuCl
3
doping increases the density of states near the conduction band along with a blue shift in the bandgap as compared to that of the pristine perovskite, which in turn increases the built-in potential of the fabricated device, resulting in self-biased photodetection. This work paves the way for deeper understanding of lanthanide doping in perovskites and self-biased photodetection applications of a new family of Eu-doped mixed halide perovskite nanostructures.
Herein, we investigate the role of Eu
3+
doping on CH
3
NH
3
PbBr
3
nanoplatelets (NPLs) in terms of their optoelectronic properties and photodetection application through a combined experimental and theoretical approach.
Oxygen vacancy engineering in metal oxide-based semiconductors has emerged as an important area of research for sensing applications, such as Surface-enhanced Raman scattering (SERS), gas sensing,
...etc
. It has the potential to replace high-cost and unstable noble metal-based substrates in the near future. However, improving the SERS enhancement factor in semiconductor-based substrates remains a challenge. In the present study, we demonstrate that oxygen vacancy engineering in Niobium pentoxide (Nb
2
O
5
) enables ultrahigh SERS sensitivity. Oxygen vacancies were induced and manipulated in the Nb
2
O
5
nanoparticles
via
a facile high-energy ball milling method and post-growth oxygen annealing. A high enhancement factor (EF) of 5.15 × 10
7
was obtained for the Methylene Blue (MeB) molecule on the oxygen-deficient substrate with the lowest detection limit of 10
−8
M, which is 2 orders of magnitude lower than the pristine substrate. Through a careful analysis of the experimental data and theoretical calculations, we investigated the underlying mechanism behind the high EF in SERS and showed that the SERS performance is directly proportional to the oxygen vacancy concentration in the Nb
2
O
5
nanoparticles. Density functional theory (DFT) calculation suggests a strong coupling of the vibronic states and an increased charge transfer (CT) efficiency in the Nb
2
O
5
-MeB complex mediated through the vacancy-induced trap states in the defective Nb
2
O
5
structure. Finite element method (FEM)-based simulations revealed a field enhancement factor of ∼4.17 × 10
2
that contributed to the SERS EF, while the remaining is contributed to the oxygen vacancy-mediated charge transfer,
i.e.
, a factor of ∼1.23 × 10
5
is due to the high CT efficiency, the highest among the reported values. We believe that these findings offer valuable insights into the fabrication of defect-tailored cost-effective semiconductor-based SERS substrates for ensuing applications, such as trace dye detection.
Oxygen vacancy engineering in Nb
2
O
5
nanoparticles enables high SERS sensitivity through defect mediated charge transfer and electromagnetic enhancement.
Phenylpropanoid pathway provides precursors for numerous secondary metabolites in plants. In this pathway, 4-coumarate-CoA ligase (EC 6.2.1.12, 4CL) is the main branch point enzyme which generates ...activated thioesters. Being the last enzyme of three shared common steps in general phenylpropanoid pathway, it contributes to channelize precursors for different phenylpropanoids. In plants, 4CL enzymes are present in multiple isoforms and encoded by small gene family. It belongs to adenylate-forming enzyme family and catalyzes the reaction that converts hydroxy or methoxy cinnamic acid derivatives to corresponding thioesters. These thioesters are further utilized for biosynthesis of phenylpropanoids, which are known for having numerous nutritional and medicinal applications. In addition, the 4CL enzymes have been characterized from various plants for their role in plant physiology or in biotic and abiotic stresses. Furthermore, specific isoforms are differentially regulated upon exposure to diverse stimuli leading to flux diversion toward the particular metabolite biosynthesis. Evolutionary studies showed that 4CL separately evolved after monocot and dicot segregation. Here, we provide a comprehensive review on 4CL, which includes evolution, function, gene/protein structure, role in metabolite biosynthesis and cellular partition, and their regulation. Based on the available data, we have explored the scope for pathway engineering by utilizing 4CL enzymes.
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