Showing papers by "P. K. Giri published in 2018"

Interfacial charge transfer in oxygen deficient TiO2-graphene quantum dot hybrid and its influence on the enhanced visible light photocatalysis

Abstract: The present work focuses on understanding the heterojunction formation of graphene quantum dots (GQDs) and oxygen deficient TiO2 nanoparticle hybrid system and its enhanced photocatalytic activity under visible light illumination. We explain the formation of TiO2-GQD heterojunction through the bonding between oxygen vacancy sites in TiO2 and in-plane oxygen functional (epoxy) groups in GQDs possibly via C O Ti bonds. Our FTIR, XPS and Raman results lend support to the proposed mechanism of heterojunction formation. In the TiO2/GQD hybrid, the Raman Eg(1) peak of anatase TiO2 is blue shifted indicating the strong interaction between the GQD and TiO2. The heterojunction formation was simulated through the density functional theory (DFT) calculation to obtain the optical spectrum on the hybrid between oxygen deficient TiO2 and oxygen functionalized GQDs. Interestingly, the calculated results for the hybrid structure show strong optical absorption in the visible to near infrared region, which is in close agreement with the experimental results. The TiO2-GQD heterojunction exhibits enhanced photocatalytic degradation (97%) of MB due to the facile interfacial charge separation, as revealed from the steady state and time resolved photoluminescence studies. Interestingly, the photoluminescence intensity of the TiO2-GQD heterojunction was partially quenched indicating the electron transfer from GQDs to TiO2. The degradation rate constant (first order) for TiO2-GQD hybrid is 5.2 times higher than that of the TiO2. Free radical scavenger test revealed that OH radical played a major role in MB degradation as compared to O 2 ° − radical. These results are significant for the development of metal free catalysts based on carbon nano-materials for ensuing optoelectronic, energy and environmental applications.

Tuning the visible photoluminescence in Al doped ZnO thin film and its application in label-free glucose detection

Abstract: Herein, we study the effect of thermal annealing on the structural and optical properties of Al doped ZnO (AZO) thin film and its application for the label-free detection of glucose based on fluorescence quenching. AZO thin films grown by radio frequency magnetron sputtering are annealed at different temperatures (250–650 °C) in air environment. The post-growth annealing improves the structural quality of the AZO films, as confirmed from the X-ray diffraction, X-ray photoelectron spectroscopy and micro-Raman analyses. The as-grown and annealed samples show strong photoluminescence (PL) in the UV (∼3.33 eV) and visible-NIR (1.6–2.2 eV). The UV PL peak is originated from the near band edge emission of crystalline ZnO, while the broad visible-NIR PL is associated with the radiative transition related to oxygen interstitial (Oi) defects in the ZnO structure. The PL peak intensity is strongly enhanced after annealing due to the partial removal of non-radiative defects. The high intensity visible-NIR PL of the annealed samples is used for the label-free enzyme-based detection of glucose with the help of glucose oxidase based on PL quenching via electron transfer mechanism. Our AZO thin films can efficiently detect ∼20 μM concentration of glucose in presence of glucose oxidase (GOx). We have attempted to quantify the nature of PL quenching based on the Stern-Volmer plot and explained the quenching mechanism as collisional quenching due to charge transfer in presence of a quencher. The Stern-Volmer plot of PL quenching of AZO thin film reveals the linear relationship between the quenching effect and the glucose concentration. Higher sensitivity of the sensor can be achieved by tuning the structure and doping density of the AZO films. This report opens up avenues for the non-destructive, label-free detection of biomolecules with high sensitivity using a low cost ZnO thin film.

Evolution of Nitrogen-Related Defects in Graphitic Carbon Nitride Nanosheets Probed by Positron Annihilation and Photoluminescence Spectroscopy

Abstract: Defects play a pivotal role in the device performance of a photocatalytic, light-emitting, or photovoltaic system. Herein, graphitic carbon nitride (g-C3N4) nanosheets are prepared at different calcination temperatures, and the evolution of defects in the system is studied by positron annihilation spectroscopy (PAS) and photoluminescence (PL) spectroscopy. Steady-state PL spectra show that free and defect-bound excitonic emission peaked at 2.78, 2.58, and 2.38 eV are dominant with above-band-gap excitation. Time-resolved PL studies reveal a significant enhancement of excitonic lifetime from 17.4 ns for free exciton to 27.4 ns in case of defect-bound exciton. We provide a direct correlation between the defects observed by PAS and those of the excitonic lifetime found from PL studies. Below-band-gap excitation activates defect emission, and it is characterized by a short carrier lifetime (∼0.14 ns). An excitation power-dependent PL study with 405 nm laser shows a progressive red shift and narrowing of the e...

Strongly enhanced visible light photoelectrocatalytic hydrogen evolution reaction in an n-doped MoS2 /TiO2(B) heterojunction by selective decoration of platinum nanoparticles at the MoS2 edge sites

Abstract: Herein, we demonstrate strongly enhanced visible light photoelectrocatalytic hydrogen evolution reaction (HER) in few-layer MoS2 grown on a mesoporous TiO2(B) nanobelt (NB) by selective decoration of platinum (Pt) nanoparticles (NPs) on the edge/defect sites of the MoS2 layer. Three catalytically active components are anchored together to increase the photoelectrocatalytic HER activity synergistically, beyond that of commercial Pt/C electrodes (20 wt% Pt). An extremely low concentration of Pt NPs (1.4 wt%) with average size ∼3.8 nm was decorated over the preferentially edge-site-exposed few-layer MoS2, with lateral sizes 130–350 nm, as evidenced from high-angle annular dark-field STEM imaging. During the heterojunction formation, S is doped in the TiO2 layer causing a high density of electrons in TiO2 that migrate to the MoS2 layer inducing n-type doping in it and thus TiO2 acts as an efficient photocathode in photoelectrocatalysis. Quantitative XPS analysis reveals that the catalytically active bridging S22−/apical S2− increases up to ∼72% after the formation of the ternary system Pt@MoS2/TiO2(B). S-enriched MoS2/TiO2(B) selectively loaded with Pt NPs on the edge sites of MoS2 exhibits a giant enhancement in the HER activity in an acidic medium under light. We record a nearly 16 fold higher exchange current density (0.296 mA cm−2) for the ternary system as compared to that of the MoS2/TiO2 binary system under visible light excitation. The marginally Pt loaded ternary system exhibits an extremely low charge transfer resistance (14 Ω) and a low overpotential as well as Tafel slope (−74 mV and 30 mV dec−1, respectively) boosting the overall HER performance under visible light. Chronopotentiometric measurements reveal the high stability of binary and ternary systems to sustain a 10 mA cm−2 cathodic current up to 12 hours. The results show that the marginally loaded Pt NPs activate the inert basal plane, edge sites of MoS2 and porous sites of TiO2, forming an integrated network where the photogenerated electrons can easily be injected from the TiO2 to MoS2 and then to Pt NPs, presenting a feasible approach to boost the HER activity under visible light.

Anomalous fluorescence enhancement and fluorescence quenching of graphene quantum dots by single walled carbon nanotubes.

Abstract: We explore the mechanism of the fluorescence enhancement and fluorescence quenching effect of single walled carbon nanotubes (SWCNTs) on highly fluorescent graphene quantum dots (GQDs) over a wide range of concentrations of SWCNTs. At very low concentrations of SWCNTs, the fluorescence intensity of the GQDs is enhanced, while at higher concentrations, systematic quenching of fluorescence is observed. The nature of the Stern–Volmer plot for the latter case was found to be non-linear indicating a combined effect of dynamic and static quenching. The contribution of the dynamic quenching component was assessed through the fluorescence lifetime measurements. The contribution of static quenching is confirmed from the red shift of the fluorescence spectra of the GQDs after addition of SWCNTs. The fluorescence intensity is first enhanced at very low concentration due to improved dispersion and higher absorption by GQDs, while at higher concentration, the fluorescence of GQDs is quenched due to the complex formation and associated reduction of the radiative sites of the GQDs, which is confirmed from time-resolved fluorescence measurements. Laser confocal microscopy imaging provides direct evidence of the enhancement and quenching of fluorescence at low and high concentrations of SWCNTs, respectively. This study provides an important insight into tuning the fluorescence of GQDs and understanding the interaction between GQDs and different CNTs, which is important for bio-imaging and drug delivery applications.

Mesoporous Si Nanowire Templated Controlled Fabrication of Organometal Halide Perovskite Nanoparticles with High Photoluminescence Quantum Yield for Light-Emitting Applications

Abstract: We report on the controlled fabrication of CH3NH3PbI3 perovskite nanoparticles (NPs) on a mesoporous Silicon nanowire (NW) template for the first time and study the mechanism of its high photoluminescence (PL) quantum yield. Crystalline perovskite NPs are grown by spin-coating of perovskite precursor on the surface of mesoporous Si NWs fabricated by a metal-assisted chemical etching method. We have tuned the size of the perovskite NPs (5–70 nm) and its photophysical properties by controlling the porosity of the Si NWs and perovskite precursor concentrations. The as-grown perovskite NPs on Si NWs show enhancement in PL intensity by more than 1 order of magnitude as compared to that of the perovskite film on a Si substrate. Depending on the size of the perovskite NPs, the center of the PL peak of the of NPs shows a large blue-shift as compared to that of the perovskite film. A detailed systematic study reveals that decrease in particle size and the quantum confinement in perovskite NPs are primarily respons...

Direct Chemical Vapor Deposition Growth of Monolayer MoS2 on TiO2 Nanorods and Evidence for Doping-Induced Strong Photoluminescence Enhancement

Abstract: Herein, we demonstrate a simple technique to control the population of nonradiative trions and radiative neutral excitons in single-layer MoS2 to enhance its photoluminescence (PL) emission by forming a core–shell heterostructure (HS) of MoS2 on TiO2 nanorods (NRs). The monolayer MoS2 (1L-MoS2) shell is grown directly on a hydrothermally grown TiO2 NR core by chemical vapor deposition, and the HS shows a strong enhancement of PL intensity by about 2 orders of magnitude over the pristine 1L-MoS2 at room temperature. The enhancement of PL in the HS is attributed, first, to the p-doping in the MoS2 lattice through charge transfer from MoS2 to TiO2, and, second, to the radiative recombination of excitons which dominates over the nonradiative ones in the HS, as confirmed by the low-temperature PL analysis. The enhancement of PL because of the p-doping effect in the bare 1L-MoS2 has been confirmed by the oxygen plasma treatment causing the adsorption of oxygen molecules at the defect sites of MoS2, as revealed ...

Trion-Inhibited Strong Excitonic Emission and Broadband Giant Photoresponsivity from Chemical Vapor-Deposited Monolayer MoS2 Grown in Situ on TiO2 Nanostructure.

Abstract: 2D material-based heterostructures often prepared by wet transfer technique suffers from poor interface and contamination issues and it results in inferior device performance. Herein, we report an in situ chemical vapor deposition (CVD) growth of MoS2@TiO2 core-shell heterojunction with single-layer MoS2 (1L-MoS2) as the shell and 3D TiO2 nanoflower (NF) as the core for multifunctional optoelectronic applications. We explore a powerful approach to switch the trions in 1L-MoS2 into neutral excitons by developing a core-shell heterostructure with TiO2 and demonstrate a giant photoluminescence (PL) enhancement in the 1L-MoS2 shell. 3D TiO2 NFs with average diameter ∼1 μm are uniformly coated with 1L-MoS2 shell by in situ CVD technique, resulting in ∼83- and ∼30-fold enhancement in PL intensity at room temperature from the 1L-MoS2 shell on TiO2 NFs as compared to that of 1L-MoS2 grown on Ti and sapphire substrate, respectively. This high PL enhancement is attributed to the migration of excess electrons from MoS2 to TiO2, leading to a heavy p-doping in the MoS2 lattice, as evidenced by the Raman and X-ray photoelectron spectroscopy analyses. Additionally, the formation of the core-shell heterojunction facilitates the suppression of nonradiative recombination of the excitons even at the room temperature, as revealed from the low-temperature PL study. The charge transfer-induced p-doping effect in 1L-MoS2 is verified from the oxygen plasma treatment of the 1L-MoS2@Ti and it shows similar PL enhancement. Further, the 1L-MoS2@TiO2 p-n heterojunction is demonstrated as a high-performance broadband photodetector owing to its favorable band alignment and high absorption in the spectral range of 300-900 nm. The heterojunction photodetector exhibits a record high responsivity and detectivity of ∼35.9 A W-1 and 1.98 × 1013 jones, respectively, in the UV region, and ∼18.5 A W-1 and 1.09 × 1013 jones, respectively, in the visible region. As compared to the 1L-MoS2@Ti and 1L-MoS2@SiO2 with slow photoresponse, 1L-MoS2@TiO2 heterojunction exhibits more than 1 order of magnitude faster photoresponse (rise/fall time ∼33.7/28.2 ms), which is attributed to the fast photogenerated carrier transport at the p-n heterojunction due to the large built-in electric field. This high-performance 1L-MoS2@TiO2 core-shell heterojunction grown by a novel in situ CVD technique is promising for the cutting-edge optoelectronic applications.

Adsorption of Small Molecules on Niobium Doped Graphene: A Study Based on Density Functional Theory

Abstract: The letter presents the adsorption properties of CO, NH3, CH4, SO2, and H2S molecules over niobium doped graphene sheet (Nb/G). Using density functional theory, the optimum configuration and orientation of adsorbent molecules over the Nb/G surface are geometrically optimized, and adsorption energy, adsorption distance, Hirshfeld charge transfer, electron localization function, and the work function of Nb/G-molecule systems are calculated. CO and SO2 molecules over Nb/G show chemisorption, hence they have high reactivity towards Nb/G. Adsorption of NH3, CH4, and H2S on Nb/G shows physisorption as they are weakly adsorbed. The adsorption of these molecules indicates the suitability of Nb/G as a sensor. To understand the superiority of Nb/G over pristine graphene, comparison of adsorption properties was made between the two systems. The work function of Nb/G with adsorbed molecule suggests that the Fermi level of Nb/G surface may be controlled by the selection of appropriate adsorbent molecules. Therefore, Nb/G could be a good candidate for gas sensing application.

Tunable and High Photoluminescence Quantum Yield from Self-Decorated TiO2Quantum Dots on Fluorine Doped Mesoporous TiO2Flowers by Rapid Thermal Annealing

Abstract: DOI: 10.1002/ppsc.201800198 past few years for their high aspect ratio (surface-to-volume ratio), distinctive physicochemical properties compared to the conventional nanostructures. Under equilibrium, {001} facet dominated TiO2 nanostructure is rarely observed as it has very high surface energy (average surface energy of {001} facet (0.90 J m−2) is double that of {101} facet (0.44 J m−2)).[14] Thus, during the usual growth process, the less reactive {101} facet is dominated over the highly reactive {001} facet. However, several reports[14–16] have been published on the controlled growth of TiO2 single crystals with exposed {001} facet after the breakthrough work by Yang et al.[17] that demonstrated the hydrothermal synthesis of anatase TiO2 single crystals with 47% exposed {001} facet. Following this work, {001} facet dominated TiO2 microcrystals are being extensively investigated with various doping and photocatalytic applications for environmental cleaning. Recently, mesoporous TiO2 single crystals delivering enhanced mobility and optoelectronic device performance was reported by Crossland et al.[18] Ding et al. reported on mesoporous hollow TiO2 microspheres with enhanced photoluminescence (PL).[19] However, most of these works were carried out using a mesoporous template method and the PL yield was not addressed in these works. To the best of our knowledge, there is no report on fluorine doping of mesoporous TiO2 nanostructure and the extremely high PL quantum yield (QY) of self-grown anatase fluorine doped TiO2 (F-TiO2) nanocrystals (NCs) on mesoporous F-TiO2 flowers. Thus, a detailed study on structural and optical properties of mesoporous F-TiO2 nanostructures is warranted to explore its novel properties including high PL QY. With conventional semiconductor quantum dots, chiral quantum dots based cellular imaging, sensing, and biomedical applications are not so encouraging because of the involvement of toxic elements such as cadmium[20] in the chiral composition. This hinders its applicability in living cells and study for nontoxic analogues of these chiral luminescent nanomaterials is of great interest.[21] Recently, Cleary et al.[22] reported on the highly luminescent TiO2 nanoparticles (NPs) with chiral capping ligand having size 30–50 nm with a PL QY up to ≈3.5%. However, the reported PL yields were measured mostly in colloidal solution, where surface functionalization by the solvents Herein a novel approach is reported to achieve tunable and high photoluminescence (PL) quantum yield (QY) from the self-grown spherical TiO2 quantum dots (QDs) on fluorine doped TiO2 (F-TiO2) flowers, mesoporous in nature, synthesized by a simple solvothermal process. The strong PL emission from F-TiO2 QDs centered at ≈485 nm is associated with shallow and deep traps, and a record high PL QY of ≈5.76% is measured at room temperature. Size distribution and doping of F-TiO2 nanocrystals (NCs) are successfully tuned by simply varying the HF concentration during synthesis. During the post-growth rapid thermal annealing (RTA) under vacuum, the arbitrary shaped F-TiO2 NCs transform into spherical QDs with smaller sizes and it shows dramatic enhancement (≈163 times) in the PL intensity. Electron spin resonance (ESR) and X-ray photoelectron spectroscopy (XPS) confirm the high density of oxygen vacancy defects on the surface of TiO2 NCs. Confocal fluorescence microscopy imaging shows bright whitish emission from the F-TiO2 QDs. Low temperature and time resolved PL studies reveal that the ultrafast radiative recombination in the TiO2 QDs results in highly efficient PL emission. A highly stable, biologically inert, and highly fluorescent TiO2 QDs/flowers without any capping agent demonstrated here is significant for emerging applications in bioimaging, energy, and environmental cleaning. Quantum Dot Photoluminescence

Label-free glucose detection over a wide dynamic range by mesoporous Si nanowires based on anomalous photoluminescence enhancement

Abstract: We report on the label-free glucose detection over a wide dynamic range by mesoporous Si nanowires (NWs) through its photoluminescence (PL) quenching and anomalous PL enhancement caused by glucose (GL) in presence of its conjugate enzyme glucose oxidase (GOx). The GL and GOx induced surface evolution of Si NWs is examined by several spectroscopic tools. At a low concentration of GL, the visible-NIR PL emission from Si NWs is partially quenched due to the electron removal by the H2O2 produced by the GL/GOx solution. At higher concentration of GL, the Si H bonds on the surface of the Si NWs are eliminated and different functional groups, e.g., C C, C C, C OH, C O and COOH are attached. This makes the Si NWs highly photoluminescent as compared to the bare Si NWs. The linear response of the PL enhancement of Si NWs with the logarithm of GL concentration is utilized for the label free sensitive detection of GL. A high dynamic range (0.1–50 mM) for GL detection and a considerable lower limit of detection (1.06 μM) are achieved for the newly developed biosensor. This report opens up possibilities for exploiting the low cost mesoporous Si NW arrays for ultrasensitive biosensing and bioimaging applications.

Ambient condition bias stress stability of vanadium (IV) oxide phthalocyanine based p-channel organic field-effect transistors

Abstract: High bias-stress stability and low threshold voltage (Vth) shift under ambient conditions are highly desirable for practical applications of organic field-effect transistors (OFETs). We demonstrate here a 20-fold enhancement in the bias-stress stability for hexamethyledisilazane (HMDS) treated vanadium (IV) oxide phthalocyanine (VOPc) based OFETs as compared to the bare VOPc case under ambient conditions. VOPc based OFETs were fabricated on bare (non treated) SiO2 and a HMDS monolayer passivated SiO2 layer, with an operating voltage of 40 V. The devices with top contact gold (Au) electrodes exhibit excellent p-channel behavior with a moderate hole mobility for the HMDS-treated device. It is demonstrated that the time dependent ON-current decay and Vth shift can be effectively controlled by using self-assembled monolayers of HMDS on the VOPc layer. For the HMDS-treated case, the bias stress stability study shows the stretched exponential decay of drain current by only ~15% during the longterm operation with constant bias voltage under ambient conditions, while it shows a large decay of >70% for the nontreated devices operated for 1000 s. The corresponding characteric decay time constant (τ) is 104 s for the HMDS treated case, while that of the the non-treated SiO2 case is only ~480 s under ambient conditions. The inferior performance of the device with bare SiO2 is traced to the charge trapping at the voids in the inter-grain region of the films, while it is almost negligible for the HMDS-treated case, as confirmed from the AFM and XRD analyses. It is believed that HMDS treatment provides an excellent interface with a low density of traps and passivates the dangling bonds, which improve the charge transport characteristics. Also, the surface morphology of the VOPc film clearly influences the device performance. Thus, the HMDS treatment provides a very attractive approach for attaining long-term air stability and a low Vth shift for the VOPc based OFET devices.